@article { ISI:000233400500005, title = {Crystallization of polymers from stressed melts}, journal = {Journal of Macromolecular Science-Polymer Reviews}, volume = {C45}, number = {4}, year = {2005}, month = {OCT}, pages = {375-397}, publisher = {TAYLOR \& FRANCIS INC}, type = {Article}, address = {325 CHESTNUT ST, SUITE 800, PHILADELPHIA, PA 19106 USA}, abstract = {

In the last decade, there have been several investigations into the route by which a polymer melt that has been subjected to flow transforms into a crystalline state. Crystallization of polymers from flowing or stressed melts is a technologically important problem since most thermoplastics are subjected to intense flow fields during normal processing operations. The renewed interest in this problem is due to the availability of new experimental tools that, coupled with advances in modeling and simulations, are beginning to yield insights into the molecular characteristics that control the crystallization pathways adopted by a stressed polymer melt. Recent evidence conclusively implicates the high molecular weight tail in promoting crystallization on shearing. It appears that the formation of oriented nuclei in polydisperse melts is controlled by chain orientation of a small fraction of high molecular weight chains in a melt. Thus, it is the theological criteria for {\textquoteleft}{\textquoteleft}critical{\textquoteright}{\textquoteright} chain orientation that govern flow-induced acceleration and orientation in polymer crystallization. This review summarizes the literature on the effect of shearing on polymer crystallization, and focuses on recent advances in understanding the role of molecular parameters that couple polymer crystallization to flow and melt orientation.

}, keywords = {flow, polymer crystallization, processing, shear}, issn = {1532-1797}, doi = {10.1080/15321790500304171}, author = {Kumaraswamy, Guruswamy} } @article { ISI:000231426000052, title = {Layered inorganic-organic clay-like nanocomposites rearrange to form silsesquioxanes on acid treatment}, journal = {Journal Physical Chemistry B}, volume = {109}, number = {33}, year = {2005}, month = {AUG}, pages = {16034-16039}, publisher = {AMER CHEMICAL SOC}, type = {Article}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

The formation of talc-like compounds by the condensation of organotrialkoxy silanes with magnesium hydroxide has been recently reported. These represent layered hybrid nanomaterials that have a layer thickness of around 1 nm, have organic moieties covalently linked to the layer surfaces, and are called {\textquoteleft}{\textquoteleft}organoclays.{\textquoteright}{\textquoteright} We show that such compounds are sensitive to acid treatment. When a phenylclay is treated with hydrochloric acid, magnesium leaches out, destroying the layered structure. The extent to which magnesium is leached out is a function of the time of the acid treatment and the concentration of the acid used. Magnesium leaches out rapidly when the concentration of acid used to treat the phenyl-clay is higher, and the extent of structural magnesium that is leached out is also higher for higher acid concentrations. Removal of the magnesium rearranges the structure of the phenyl-clay to form oligomeric phenylsilsesquioxanes. FTIR and NMR suggest that the silsesquioxanes formed by acid treatment of the phenyl-clay comprise a mixture of ladderlike and cagelike structures.

}, issn = {1520-6106}, doi = {10.1021/jp0526400}, author = {Kumaraswamy, Guruswamy and Deshmukh, Yogesh S. and Agrawal, Vikrant V. and Rajmohanan, P.} } @article { ISI:000231573100016, title = {Polycondensation in liquid crystalline phases of nonionic surfactants. Kinetics and morphology}, journal = {Polymer}, volume = {46}, number = {19}, year = {2005}, month = {SEP}, pages = {7961-7968}, publisher = {ELSEVIER SCI LTD}, type = {Article}, address = {THE BOULEVARD, LANGFORD LANE, KIDLINGTON, OXFORD OX5 1GB, OXON, ENGLAND}, abstract = {

We have investigated acid-catalyzed polycondensation of alkoxysilane monomers in liquid crystalline phases of nonionic CnEm surfactants. The liquid crystalline phase is retained when the monomers polymerize. The high molecular weight molecules formed phase separate from the mesophase and are subsequently organized by it to form micron-sized particles. A variety of particle morphologies are formed by organization of the polymer particles in the mesophase. For condensation of dimethyldimethoxysilane (DMS, with trimethoxysilane, TMS as crosslinker) in hexagonal and lamellar phases, specific reaction conditions, viz. slow condensation kinetics and low crosslink density, give rod-like particles in hexagonal phases and sheet-like structures in lamellar phases. However, when higher acid concentrations are used, the reaction kinetics accelerates and irregular particles form. Irregular particles also form when the fraction of trifunctional crosslinker is increased, and finally complex flower-like structures form for condensation of trimethoxysilane in the hexagonal phase. The particle morphology formed is crucially dependent on the details of the polycondensation rate, crosslinker density and surfactant-monomer/oligomer interactions. (c) 2005 Elsevier Ltd. All rights reserved.

}, keywords = {nonionic, surfactant, templated synthesis}, issn = {0032-3861}, doi = {10.1016/j.polymer.2005.07.001}, author = {Kumaraswamy, Guruswamy and Wadekar, M. N. and Agrawal, Vikrant V. and Pasricha, R.} } @article { ISI:000228760300034, title = {Polymerization in surfactant liquid crystalline phases}, journal = {Chemistry of Materials}, volume = {17}, number = {9}, year = {2005}, month = {MAY}, pages = {2460-2465}, publisher = {AMER CHEMICAL SOC}, type = {Article}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

We report the formation of cross-linked polysiloxane particles having rodlike and sheetlike morphologies by condensation of monomers in organized liquid crystalline mesophases of nonionic surfactants. The characteristic diffraction pattern obtained from the liquid crystalline surfactant assembly is preserved during polymer synthesis. The polymer colloids are rodlike when synthesized in a hexagonal mesophase and are sheetlike when synthesized in a lamellar mesophase. Interestingly, the size of the polymer particles is on the order of micrometers, viz. 3 orders of magnitude larger than the characteristic size of the surfactant mesophase. Thus, the polymers phase separate from the liquid crystalline phase during polymerization, and organize to resemble the mesophase geometry. The polymer particles are organized so as to minimize the elastic distortion of the surfactant mesophase after they phase separate. We speculate that the observed particle morphologies are formed due to the slow kinetics of polycondensation under our experimental conditions.

}, issn = {0897-4756}, doi = {10.1021/cm048240r}, author = {Wadekar, M. N. and Pasricha, R. and Gaikwad, A. B. and Kumaraswamy, Guruswamy} } @article { ISI:000242935400096, title = {Collapse transition in random copolymer solutions}, journal = {Macromolecules}, volume = {39}, number = {26}, year = {2006}, month = {DEC}, pages = {9621-9629}, publisher = {AMER CHEMICAL SOC}, type = {Article}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

We present dynamic Monte Carlo lattice simulations of the coil to globule collapse of single chains of a copolymer comprising monomer units, m and c, wherein there is a net attractive interaction between c-units. As the copolymer is cooled, the solvent quality becomes poorer, and the size of the chain decreases, driven by the net m-m and c-c attractions. The strong c-c attraction increases the overall solvophobicity of the chain relative to a homopolymer and, therefore, copolymers collapse more abruptly and at a higher effective temperature relative to homopolymers. We compare copolymers with homopolymers by rescaling collapse data to the same theta values to account for the effect of overall solvophobicity. This comparison shows that the behavior of copolymers and the corresponding homopolymers is identical as the chain size reduces from high temperatures to the theta value. Beyond theta, copolymers with c-content \< similar to 50\% collapse more abruptly than either homopolymer, after accounting for the difference in overall solvophobicity. Collapse of copolymers containing higher c-content is dominated entirely by the c-c attractions, and these chains behave qualitatively like homopolymers with a higher effective solvophobicity. Analysis of the chain structure during collapse provides a structural reason for the qualitative change in copolymer collapse at low c-content. When such copolymers are cooled below theta, the c-units rapidly aggregate to form an isotropic, compact core surrounded by an anisotropic solvated shell of m-units. The shell densifies as the copolymer is further cooled, but remains anisotropic for the finite chain sizes investigated.

}, issn = {0024-9297}, doi = {10.1021/ma061017q}, author = {Dasmahapatra, Ashok Kumar and Kumaraswamy, Guruswamy and Nanavati, Hemant} } @article { ISI:000251908500042, title = {Pathway to copolymer collapse in dilute solution: uniform versus random distribution of comonomers}, journal = {Journal of Chemical Physics}, volume = {127}, number = {23}, year = {2007}, month = {DEC}, pages = {234901}, publisher = {AMER INST PHYSICS}, type = {Article}, address = {CIRCULATION \& FULFILLMENT DIV, 2 HUNTINGTON QUADRANGLE, STE 1 N O 1, MELVILLE, NY 11747-4501 USA}, abstract = {

Monte Carlo simulations show that copolymers with uniformly (or periodically) distributed sticky comonomers collapse {\textquoteleft}{\textquoteleft}cooperatively,{\textquoteright}{\textquoteright} abruptly forming a compact intermediate comprising a monomer shell surrounding a core of the aggregated comonomers. In comparison, random copolymers collapse through a relatively less-compact intermediate comprising a comonomer core surrounded by a fluffy monomer shell that densifies over a wide temperature range. This difference between the collapse pathways for random and uniform copolymers persists to higher chain lengths, where uniform copolymers tend to form multiple comonomer cores. In this paper, we describe the formation of such an intermediate state, and the subsequent collapse, by recognizing that these arise from the expected balance between comonomer aggregation enthalpy and loop formation entropy dictated by the chain microstructure. (c) 2007 American Institute of Physics.

}, issn = {0021-9606}, doi = {10.1063/1.2802296}, author = {Dasmahapatra, Ashok Kumar and Nanavati, Hemant and Kumaraswamy, Guruswamy} } @article { ISI:000256290100015, title = {Composites of polypropylene with layered Mg-silsesquioxanes show an unusual combination of properties}, journal = {Industrial \& Engineering Chemistry Research}, volume = {47}, number = {11}, year = {2008}, month = {JUN}, pages = {3891-3899}, publisher = {AMER CHEMICAL SOC}, type = {Article}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

We report the synthesis of vinyl modified magnesium silsesqiuoxanes ({\textquoteright}{\textquoteright}vinyl clay{\textquoteright}{\textquoteright}), and the formation of their composites with isotactic polypropylene (iPP) by melt compounding. Vinyl clay is a layered compound with a layer thickness of approximately 1 nm. Vinyl clay does not exfoliate in iPP; rather, it disperses to form a network that exhibits a characteristic low frequency solid-like plateau in the elastic modulus in dynamic melt theological measurements. Strangely, vinyl clay also plasticizes iPP-there is a decrease in the high frequency complex viscosity. The decrease in the complex viscosity is higher at higher frequencies, suggesting the influence of slip at the iPP-vinyl clay interface. The combination of the low frequency elastic plateau and plasticization makes the vinyl clay composite significantly more shear thinning than the matrix iPP. In the solid state, vinyl clay-iPP composites exhibit increased tensile modulus (showing approximate to 50\% increase for a 5\% loading), but surprisingly, no corresponding decrease in the elongation at break. Thus, while microstructural characterization indicates that only a small fraction, if any, of the vinyl clay is exfoliafed, the enhancement in mechanical properties is similar to that observed for iPP-exfoliated montmonillonite nanocomposites. Our compounding protocol is unable to effectively disperse the clay in the iPP at clay loadings greater than about 7.5\%. Therefore, the low frequency plateau in the melt elastic modulus and the solid tensile modulus increase with clay loading until 7.5\% but exhibit a nonmonotonic decrease at higher clay loadings.

}, issn = {0888-5885}, doi = {10.1021/ie071658p}, author = {Kumaraswamy, Guruswamy and Deshmukh, Yogesh S. and Agrawal, Vikrant V. and Nisal, Anuya} } @article { ISI:000254883500012, title = {Gelation of covalently edge-modified laponites in aqueous media. 1. rheology and nuclear magnetic resonance}, journal = {Journal of Physical Chemistry B}, volume = {112}, number = {15}, year = {2008}, month = {APR}, pages = {4536-4544}, publisher = {AMER CHEMICAL SOC}, type = {Article}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

We describe the covalent modification of the edges of laponite with organic groups and the influence of this modification on gelation behavior. We compare three materials: an unmodified laponite, a laponite edge modified with a trimethyl moiety (MLap), and an octyldimethyl moiety (OLap). Gelation is investigated using rheology and NMR T-1 relaxation measurements and nuclear Overhauser enhancement spectroscopy (NOESY). MLap and OLap show qualitatively different gelation. Gelation of MLap is very similar to laponite: MLap gels over the same time scale as laponite and has about the same solid modulus, and the MLap gel is almost as transparent as laponite. In contrast, OLap gels rapidly relative to laponite and forms a weak, turbid gel. We believe that gelation in laponite and MLap results from the formation of a network of well-dispersed platelets (or a few platelets), while in OLap, gelation results from a network of stacks of several platelets. NMR relaxation measurements indicate that gelation does not affect the average relaxation of water protons. However, T, increases marginally for the protons in the organic moieties in MLap and decreases for protons in the organic moieties in OLap. Relaxation measurements, analyses of line width, and NOESY taken together suggest that, in OLap, gelation is a consequence of association of the organic moieties on the laponite edges, and that this association strengthens with time. Thus, the time-dependent changes in NMR suggest a structural origin for the time-dependent changes in the rheological behavior.

}, issn = {1520-6106}, doi = {10.1021/jp710489n}, author = {Patil, Suhas P. and Mathew, Renny and Ajithkumar, Thalasseril G. and Rajamohanan, P. R. and Mahesh, T. S. and Kumaraswamy, Guruswamy} } @article { ISI:000269287200036, title = {Polymer crystallization in the presence of {\textquoteleft}{\textquoteleft}sticky{\textquoteright}{\textquoteright} additives}, journal = {Journal of Chemical Physics}, volume = {131}, number = {7}, year = {2009}, month = {AUG}, pages = {074905}, publisher = {AMER INST PHYSICS}, address = {CIRCULATION \& FULFILLMENT DIV, 2 HUNTINGTON QUADRANGLE, STE 1 N O 1, MELVILLE, NY 11747-4501 USA}, abstract = {

The effect of {\textquoteleft}{\textquoteleft}sticky{\textquoteright}{\textquoteright} additives (viz., those that have attractive interactions with the polymer) on polymer crystallization, has been investigated by dynamic Monte Carlo (DMC) simulations. Additive-polymer attractive interactions result in a slowing down of the polymer chain diffusivity in the melt state. Our results show that with increasing additive stickiness, polymer crystallinity decreases monotonically, and thinner crystallites form, viz., crystallization is inhibited by the presence of sticky additives. Unusually, the observed {\textquoteleft}{\textquoteleft}specific heat{\textquoteright}{\textquoteright} peak at the phase transition shows nonmonotonic behavior with additive stickiness, and exhibits a maximum for intermediate values of additive stickiness. While the origins of this unexpected behavior are not clear, we show that it correlates with a large interchange between crystalline and amorphous states of the monomers, in the vicinity of the additives. At this intermediate additive stickiness, we also find that crystallization follows a qualitatively different route-crystallinity shows a non-Avrami-like evolution, unlike the case at low or high additive stickiness.

}, keywords = {Additives, amorphous state, crystallisation, crystallites, Diffusion, Monte Carlo methods, polymer melts}, issn = {0021-9606}, doi = {10.1063/1.3174449}, author = {Dasmahapatra, Ashok Kumar and Nanavati, Hemant and Kumaraswamy, Guruswamy} } @article { ISI:000264111200022, title = {Self-assembly of silica particles in a nonionic surfactant hexagonal mesophase}, journal = {Journal of Physical Chemistry B}, volume = {113}, number = {11}, year = {2009}, month = {MAR}, pages = {3423-3430}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

We investigate the process of self-assembly, and the resultant structures in composites of silica particles with a hexagonal mesophase of a nonionic surfactant and water. We report a systematic transition in behavior when the particle size is increased relative to the characteristic mesophase spacing. Water dispersible cage-like silsesquioxanes that are molecular analogues of silica particles and are smaller than the mesophase spacing swell the space between the surfactant cylinders. Silica particles comparable to the characteristic hexagonal spacing partition into the hexagonal phase and into strandlike particulate aggregates. Even larger particles phase separate from the hexagonal phase to form particulate strands that organize with a mesh size comparable to the wavelength of visible light. This self-assembly is reversible and the particles disperse by breaking up the aggregates on heating the composite into the isotropic phase. On cooling from the isotropic phase into the hexagonal, the particles are expelled from the growing hexagonal domains and finally impinge to form strandlike aggregates. Unusually, the isotropization temperature is increased in the composites as the particles nucleate the formation of the hexagonal phase.

}, issn = {1520-6106}, doi = {10.1021/jp810769g}, author = {Sharma, Kamendra P. and Kumaraswamy, Guruswamy and Ly, Isabelle and Mondain-Monval, Olivier} } @article { ISI:000281128700008, title = {Adsorption of nonionic surfactant on silica nanoparticles: structure and resultant interparticle interactions}, journal = {Journal of Physical Chemistry B}, volume = {114}, number = {34}, year = {2010}, month = {SEP}, pages = {10986-10994}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

Addition of nonionic surfactant, C(12)E(9), to an aqueous dispersion of charge stabilized silica nanoparticles renders particle aggregation reversible. In contrast, aggregation of the same silica particles in aqueous solutions is irreversible. We use a combination of small-angle X-ray scattering (SAXS) and contrast matching small-angle neutron scattering (SANS) to investigate interparticle interactions and microstructure in dispersions of silica particles in aqueous nonionic surfactant solutions. We show that the silica particles interact through a screened Coulombic interaction in aqueous dispersions; interestingly, this interparticle interaction is hard-sphere-like in surfactant solutions. In surfactant solutions, we show that the final surfactant-particle structure can be modeled as 14 micelles adsorbed (on average) on the surface of each silica particle. This gives rise to the short-range interparticle repulsion that makes particle aggregation reversible, and results in the hard sphere interparticle interaction potential. Finally, we show that adsorption of polyethylene imine on the surface of the silica particles prevents adsorption of surfactant micelles on the particle surface.

}, issn = {1520-6106}, doi = {10.1021/jp1033799}, author = {Sharma, Kamendra P. and Aswal, Vinod K. and Kumaraswamy, Guruswamy} } @article { ISI:000277649500046, title = {Volume transition of PNIPAM in a nonionic surfactant hexagonal mesophase}, journal = {Macromolecules}, volume = {43}, number = {10}, year = {2010}, month = {MAY}, pages = {4782-4790}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

We investigate the volume transition of a thermoresponsive polymer, poly(N-isopropylacrylamide), PNIPAM, in the presence of an aqueous solution of nonionic surfactant, C(12)E(9). We combine turbidimetry with optical microscopy, NMR, and SAXS to follow the volume transition of the PNIPAM and the H(1)-isotropic transition of the surfactant/water system. Nonionic surfactants such as C(12)E(9) are known to interact weakly with PNIPAM. Accordingly, we show that there is only a small change in the volume transition temperature for the PNIPAM in isotropic micellar solutions of C(12)E(9), even for relatively high concentrations of C(12)E(9). Interestingly, once the surfactant forms an H(1) phase, there is a dramatic decrease in the coil globule transition onset temperature. We believe that this behavior results from a competition between C(12)E(9) in the H(1) phase, and PNIPAM to associate with water. When PNIPAM in the H(1) phase is cooled to low enough temperatures so as to be in the coil state, it locally disturbs the hexagonal phase ordering. Thus, we show that for PNIPAM in a weakly interacting surfactant matrix, it is the phase behavior of the matrix rather than the matrix chemistry that governs the coil globule transition. Finally, we show that in a PNIPAM copolymer with a higher LCST we observe an interesting sequence of transitions in the surfactant phase: on cooling from a high temperature free-blowing turbid globular state (similar to 75 degrees C), we enter a free-flowing translucent coil phase (similar to 47 degrees C), then a turbid gel (similar to 25 degrees C) where the copolymer is collapsed in the HI phase, and finally a low-temperature clear gel (similar to 5 degrees C) where the copolymer is in the expanded coil state.

}, issn = {0024-9297}, doi = {10.1021/ma100357h}, author = {Jijo, V. J. and Sharma, Kamendra P. and Mathew, Renny and Kamble, Samruddhi and Rajamohanan, P. R. and Ajithkumar, Thalasseril G. and Badiger, Manohar V. and Kumaraswamy, Guruswamy} } @article { ISI:000292893300014, title = {Assembly of polyethyleneimine in the hexagonal mesophase of nonionic surfactant: effect of pH and temperature}, journal = {Journal of Physical Chemistry B}, volume = {115}, number = {29}, year = {2011}, month = {JUL}, pages = {9059-9069}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

We investigate the dispersion of a pH responsive polymer, polyethyleneimine, PEI, in a hexagonal (H(1)) mesophase of a nonionic surfactant, C(12)E(9), and water, at pH ranging from basic (pH = 12.8) to acidic (pH = 1). While the C(12)E(9)/H(2)O phase behavior is independent of pH, we demonstrate that, in the PEI/C(12)E(9)/H(2)O system, changing the pH influences PEI-C(12)E(9) interactions, and thus, influences the isotropic-H(1) phase transition. With decrease in pH, there is increasing protonation of the PEI chain, and consequently, the chain extends. We show, using a combination of SAXs, optical microscopy and visual experiments, that the inclusion of PEI in a 1:1 surfactant water mixture, lowers the hexagonal-isotropic transition temperature, T. At higher pH = 12.8 T(HI) shows a pronounced decrease from SO to 13 degrees C on addition of PEI, and the PEI/C(12)E(9)/H(2)O system forms a transparent gel. At pH = 1, we observe qualitatively different behavior and an opaque gel forms below T(HI)= 25 degrees C. The isotropic-H(1) transition, in turn, influences the phase separation of PEI chains from the C(12)E(9)/H(2)O system. 2D NMR ROESY data provides evidence that there are strong surfactant PEI interactions at high pH that significantly reduce at lower pH. The NMR data is in accord with molecular dynamics simulations that show that surfactants strongly aggregate with unprotonated PEI chains, but not with fully protonated chains; thus, in this system, the pH controls a cascade of microstructural organization: increasing pH decreases chain protonation and increases polymer-surfactant interactions, resulting in suppression of the isotropic-H(1) transition to lower temperatures, thus, influencing the phase separation of PEI from the surfactant/water system.

}, issn = {1520-6106}, doi = {10.1021/jp202614x}, author = {Sharma, Kamendra P. and Choudhury, Chandan Kumar and Srivastava, Sonal and Davis, Hilda C. and Rajamohanan, P. R. and Roy, Sudip and Kumaraswamy, Guruswamy} } @article { ISI:000295403700010, title = {Influence of DMDBS on the morphology and mechanical properties of polypropylene cast films}, journal = {Polymer Engineering and Science}, volume = {51}, number = {10, SI}, year = {2011}, month = {OCT}, pages = {2013-2023}, publisher = {WILEY-BLACKWELL}, address = {111 RIVER ST, HOBOKEN 07030-5774, NJ USA}, abstract = {

We investigate extrusion film casting (EFC) of polypropylene (PP) and examine the effects of varying the concentration of nucleating agent, dimethyl dibenzylidene sorbitol (DMDBS); melt temperature at the die exit; and the draw ratio. Addition of DMDBS increases the PP crystallization temperature, and consequently the PP lamellar repeat spacing. For the draw ratios explored, we do not observe any substantial orientation of iPP crystals for films of neat iPP. However, addition of DMDBS results in the formation of oriented PP crystals, with greater orientation for higher draw ratios. On cooling from the melt, a network of DMDBS nanofibers forms in the PP. We believe that flow-alignment of this DMDBS network templates the orientation of PP crystals. We observe that the tensile modulus and yield stress increase with PP crystal orientation. However, at high DMDBS concentrations (0.8\%), films have poor mechanical strength due to formation of voids and defects. POLYM. ENG. SCI., 51:2013-2023, 2011. (C) 2011 Society of Plastics Engineers

}, issn = {0032-3888}, doi = {10.1002/pen.21957}, author = {Sreenivas, K. and Pol, Harshawardhan V. and Kumaraswamy, Guruswamy} } @article { ISI:000289028500081, title = {Phase separation of DMDBS from PP: effect of polymer molecular weight and tacticity}, journal = {Macromolecules}, volume = {44}, number = {7}, year = {2011}, month = {APR}, pages = {2358-2364}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

We report an unexpected dependence of DMDBS phase separation temperature on the molecular weight of the matrix isotactic polypropylene (iPP). DMDBS crystallizes out at lower temperatures for iPP with decreasing molecular weight (and correspondingly lower tacticity). All the iPPs in our study are reasonably high molecular weight samples and there is no molecular weight dependence of their solubility parameter. Therefore, the decrease in DMDBS phase separation temperature for lower molecular weights cannot be rationalized using thermodynamic arguments. This molecular weight dependence appears to be unique to isotactic polypropylene and is not observed for either syndiotactic polypropylene or for random copolymers of isotactic polypropylene containing ethylene comonomer.

}, issn = {0024-9297}, doi = {10.1021/ma200035s}, author = {Sreenivas, K. and Basargekar, Rajeev and Kumaraswamy, Guruswamy} } @article { ISI:000288291400015, title = {Self-standing three-dimensional networks of nanoparticles with controllable morphology by dynamic templating of surfactant hexagonal domains}, journal = {Chemistry of Materials}, volume = {23}, number = {6}, year = {2011}, month = {APR}, pages = {1448-1455}, publisher = {AMER CHEMICAL SOC}, type = {Article}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

Assembly of nanoparticles into free-standing three-dimensional networks has implications for a wide range of applications. We show that dynamic templating of surfactant hexagonal domains is a facile technique to organize nanoparticles into a network of particulate strands. Dispersed particles (\> 10 nm), independent of particle chemistry, assemble into networks, when the surfactant matrix cools into the hexagonal phase. We demonstrate assembly of inorganic, polymeric, and protein nanoparticles into networks. Where particle assembly is reversible, particles are coated with polymers that are subsequently cross-linked to stabilize the networks after surfactant removal. This technique involves near ambient temperatures and a benign water wash for template removal. The network mesh size can be varied from submicrometers to tens of micrometers by controlling the cooling rate. Particle networks can be flow-oriented prior to cross-linking, and interpenetrating networks can also be formed.

}, keywords = {hexagonal, macroporous, nanoparticle, self assembly, surfactant}, issn = {0897-4756}, doi = {10.1021/cm102945x}, author = {Sharma, Kamendra P. and Ganai, Anal Kumar and Sen Gupta, Sayam and Kumaraswamy, Guruswamy} } @article { ISI:000295187300051, title = {Synthesis of poly-L-glutamic acid grafted silica nanoparticles and their assembly into macroporous structures}, journal = {Langmuir}, volume = {27}, number = {19}, year = {2011}, month = {OCT}, pages = {12124-12133}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

Polypeptide-coated silica nanoparticles represent an interesting class of organic inorganic hybrids since the ordered secondary structure of the polypeptide grafts imparts functional properties to these nanoparticles. The synthesis of a poly-L-glutamic acid (PLGA) silica nanoparticle hybrid by employing N-carboxyanhydride (NCA) polymerization to synthesize the polypeptide chains and Cu catalyzed azide alkyne cycloaddition reaction to graft these chains onto the silica surface is reported. This methodology enables the synthesis of well-defined polypeptide chains that are attached onto the silica surface at high surface densities. The PLGA-silica conjugate particles are well dispersed in water, and have been thoroughly characterized using multinuclear (C-13, Si-29) solid state NMR, thermogravimetric analysis, Fourier transform infrared, dynamic light scattering, and transmission electron microscopy. The pH-dependent reversible aggregation of the PLGA-silica particles, driven by the change in PLGA structure, has also been studied. Preliminary results on the use of aqueous dispersions of silica-PLGA for the preparation of three-dimensional macroporous structures with oriented pores by ice templating methodology are also demonstrated. These macroporous materials, comprising a biocompatible polymer shell covalently attached to rigid inorganic cores, adopts an interesting lamellar structure with fishbone-type architecture.

}, issn = {0743-7463}, doi = {10.1021/la202036c}, author = {Kar, Mrityunjoy and Pauline, Minois and Sharma, Kamendra and Kumaraswamy, Guruswamy and Sen Gupta, Sayam} } @article { ISI:000307479100016, title = {Lamellar melting, not crystal motion, results in softening of polyoxymethylene on heating}, journal = {Macromolecules}, volume = {45}, number = {15}, year = {2012}, month = {AUG}, pages = {5967-5978}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

We probe temperature-dependent changes in the semicrystalline microstructure of polyoxymethylene using a combination of modulated DSC, SAXS, and solid-state NMR to characterize macroscopic behavior, lamellar-level structure, and molecular environments, respectively, and correlate these with the change in mechanical properties probed using DMA and AFM. Two model samples are investigated: a melt crystallized sample prepared by injection molding and a sample obtained by crystallization from dilute solution. Our investigations reveal that, for both samples, there is an increase in crystalline motions and in the amorphous content on heating. DMA and AFM measurements reveal that the modulus of the molded sample decreases on heating to about 100 degrees C; however, there is a significant difference in behavior of the solution crystals, where we observe no significant decrease in stiffness (from AFM measurements). Thus, in contrast to previous reports, we demonstrate that the decrease in modulus on heating polyoxymethylene does not correlate with chain motions in the crystalline regions. We use SAXS to probe the semicrystalline morphology for the samples on heating and show that, for the molded sample, there is a distribution of lamellar thickness at room temperature and that the thin lamellae in this distribution melt on heating. In contrast to the behavior of the melt crystallized samples, the solution crystals exhibit no change in the lamellar stacking on heating to 150 degrees C. We also demonstrate that, on heating, the amorphous regions in the solution crystals always appear to have restricted mobility while there are mobile and low mobility amorphous regions in the molded samples. Our results suggest that, contrary to conventional belief, the decrease in modulus on heating polyoxymethylene arises not from motions in the crystalline lamellae but primarily from melting of thin lamellae.

}, issn = {0024-9297}, doi = {10.1021/ma3004423}, author = {Kumaraswamy, Guruswamy and Surve, Nikita S. and Mathew, Renny and Rana, Abhimanyu and Jha, Saroj K. and Bulakh, Neelima N. and Nisal, Anuya and Ajithkumar, Thalasseril G. and Rajamohanan, P. R. and Ratnagiri, Ram} } @article { ISI:000303696200040, title = {Multiple topologies from glycopolypeptide-dendron conjugate self-assembly: nanorods, micelles, and organogels}, journal = {Journal of the American Chemical Society}, volume = {134}, number = {18}, year = {2012}, month = {APR}, pages = {7796-7802}, publisher = {AMER CHEMICAL SOC}, type = {Article}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

Glycopolypeptides (GPs) were synthesized by ring-opening polymerization of glycosylated N-carboxyanhydride monomer and attached to hydrophobic dendrons at one chain end by {\textquoteleft}{\textquoteleft}click{\textquoteright}{\textquoteright} reaction to obtain amphiphilic anisotropic macromolecules. We show that by varying polypeptide chain length and dendron generation, an organogel was obtained in dimethylsulfoxide, while nanorods and micellar aggregates were observed in aqueous solutions. Assemblies in water were characterized by electron microscopy and dye encapsulation. Secondary structure of the GP chain was shown to affect the morphology, whereas the chain length of the poly(ethylene glycol) linker between the GP and dendron did not alter rod-like assemblies. Bioactive surface chemistry of these assemblies displaying carbohydrate groups was demonstrated by interaction of mannose-functionalized nanorods with ConA.

}, issn = {0002-7863}, doi = {10.1021/ja300065f}, author = {Pati, Debasis and Kalva, Nagendra and Das, Soumen and Kumaraswamy, Guruswamy and Sen Gupta, Sayam and Ambade, Ashootosh V.} } @article { ISI:000303442500017, title = {Synthesis of functional hybrid silica scaffolds with controllable hierarchical porosity by dynamic templating}, journal = {Chemical Communications}, volume = {48}, number = {43}, year = {2012}, month = {MAR}, pages = {5292-5294}, publisher = {ROYAL SOC CHEMISTRY}, type = {Article}, address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND}, abstract = {

We report a facile one-pot synthesis of hierarchically porous scaffolds, with independent control over nanoparticle mesoporosity and scaffold macroporosity. Our technique combines the chemistry of mesoporous silica nanoparticles with the control afforded by dynamic templating of surfactant mesophases. These materials are readily functionalizable and allow controllable spatial variation in macroporosity.

}, issn = {1359-7345}, doi = {10.1039/c2cc30885a}, author = {Ganai, Anal Kumar and Kumari, Sushma and Sharma, Kamendra P. and Panda, Chakadola and Kumaraswamy, Guruswamy and Sen Gupta, Sayam} } @article { ISI:000326126000033, title = {Exclusion from hexagonal mesophase surfactant domains drives end-to-end enchainment of rod-like particles}, journal = {Journal of Physical Chemistry B}, volume = {117}, number = {41}, year = {2013}, month = {OCT}, pages = {12661-12668}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

Anisotropic rod-like particles assemble end-to-end when the surfactant/water matrix in which they are dispersed is cooled from the isotropic to the lyotropic hexagonal phase. We demonstrate the formation of such end-to-end assemblies for gold nanorods, which are tens of nanometers in size, as well as for micrometer-sized ellipsoidal polystyrene particles. In both cases, the particles are well-dispersed in the low-viscosity surfactant/water phase above the isotropic-H-1 transition temperature. On cooling into the H-1 phase, mesophase domains form and the particles are expelled to the isotropic phase. As the H-1 domains grow and finally impinge, the particles are localized at the domain boundaries where they reorient and assemble end-to-end. Remarkably, we observe the formation of end-to-end assemblies of gold nanorods even for volume fractions as low as 2 x 10(-6) in the initially dispersed state. The extent of particle {\textquoteleft}{\textquoteleft}enchainment{\textquoteright}{\textquoteright} increases with the particle concentration and with the aspect ratio of the particles.

}, issn = {1520-6106}, doi = {10.1021/jp407403a}, author = {Sharma, Kamendra P. and Ganai, Anal Kumar and Sen, Debasis and Bhagavatula L. V. Prasad and Kumaraswamy, Guruswamy} } @article { ISI:000328437300022, title = {Large centimeter-sized macroporous ferritin gels as versatile nanoreactors}, journal = {Chemistry of Materials}, volume = {25}, number = {23}, year = {2013}, month = {DEC}, pages = {4813-4819}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

Organized assemblies of bionanoparticles such as ferritin provides templates that can be exploited for nanotechnological applications. Organization of ferritin into well-defined three-dimensional assemblies is challenging and has attracted considerable attention recently. We have synthesized, for the first time, large (centimeter-sized) self-standing macroporous scaffold monoliths from ferritin bionanoparticles, using dynamic templating of surfactant H-1 domains. These scaffolds comprise three-dimensionally connected strands of ferritin, organized as a porous gel with porosity similar to 55 mu m. The iron oxide inside the ferritin scaffold can be easily replaced with catalytically active monodisperse zerovalent transition metal nanoparticles using a very simple protocol. Since the ferritin is cross-linked in the scaffold, it is significantly robust with enhanced thermal stability and better tolerance toward several organic solvents in Comparison to the native ferritin bionanoparticle. In addition, the scaffold macropores facilitate substrate and reagent transport and hence the monoliths containing active Pd or iron oxide nanoparticles inside apo-ferritin bionanoparticles were used as a recyclable heterogeneous catalyst for the oxidation of 2,3,6-trimethyl phenol to 2,3,6-trimethyl-1,4-benzoquinone (precursor for Vitamin E synthesis) and for Suzuki-Miyaura cross-coupling reaction in both aqueous and organic solvents. The protein shell around the nanoparticles protects them from agglomeration, a phenomenon that otherwise plagues nanoparticles-based catalysis. The presence of macropores allow the ferritin scaffold to act as catalytic monolith for continuous flow reactions having rapid reaction rates, while offering a low pressure drop. Finally, the Pd@apo-ferritin scaffold was immobilized inside a steel cartridge and used for the continuous flow hydrogenation of alkenes to their corresponding alkanes for 15 cycles without any loss of activity.

}, keywords = {catalysis, continuous flow, ferritin, gel, macroporous materials, Self-assembly}, issn = {0897-4756}, doi = {10.1021/cm403240j}, author = {Kumari, Sushma and Kulkarni, Amol and Kumaraswamy, Guruswamy and Sen Gupta, Sayam} } @article { ISI:000323014200005, title = {Nanoparticle size controls aggregation in lamellar nonionic surfactant mesophase}, journal = {Langmuir}, volume = {29}, number = {31}, year = {2013}, month = {AUG}, pages = {9643-9650}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

We show that the size of silica nanoparticles influences the nature of their aggregation in an aqueous solution of a relatively hydrophobic nonionic surfactant, C12E4. We present results for dispersions of silica nanopartides with sizes varying from 8 to 26 nm, in a 75: 25 C12E4/water system, that forms a lamellar phase, L-alpha, at room temperature. Addition of silica particles does not affect the formation of the L-alpha phase. Nanoparticles smaller than about 11 nm aggregate irreversibly in the C12E4/water system. However, nanoparticles larger than about 15 nm aggregate in the L-alpha phase, but are dispersed at temperatures above the L-alpha order-disorder temperature. Thus, in contrast to the smaller particles, aggregation of silica nanopartides larger than about 15 nm is reversible with temperature. We use small-angle neutron scattering (SANS) demonstrate that these results can be explained by the size-dependent wrapping of nanoparticles by surfactant bilayers. Larger particles, above 15 nm in size, are sterically stabilized by the formation of an adsorbed surfactant bilayer. The cost of bilayer bending inhibits adsorption onto the highly curved surfaces of smaller particles, and these {\textquoteleft}{\textquoteleft}bare{\textquoteright}{\textquoteright} particles aggregate irreversibly.

}, issn = {0743-7463}, doi = {10.1021/la4021977}, author = {Venugopal, Edakkal and Aswal, Vinod K. and Kumaraswamy, Guruswamy} } @article { ISI:000343018200079, title = {Enhancing cubosome functionality by coating with a single layer of poly-epsilon-lysine}, journal = {ACS Applied Materials \& Interfaces}, volume = {6}, number = {19}, year = {2014}, month = {OCT}, pages = {17126-17133}, publisher = {AMER CHEMICAL SOC}, type = {Article}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

We report the preparation and characterization of monoolein cubosomes that can be easily surface modified through adsorption of a single layer of cationic poly-epsilon-lysine. Poly-epsilon-lysine coated cubosomes show remarkable stability in serum solution, are nontoxic and, are readily internalized by HeLa cells. The poly-epsilon-lysine coating provides chemical handles for further bioconjugation of the cubosome surface. We also demonstrate that the initial release rate of a hydrophilic drug, Naproxen sodium, from the cubosomes is retarded with just a single layer of polymer. Interestingly, cubosomes loaded with Naproxen sodium, recently shown to have anticancer properties, cause more apoptosis in HeLa cells when compared to free unencapsulated drug.

}, keywords = {bioconjugation, cubosomes, drug delivery vehicle, dual loading, theranostics}, issn = {1944-8244}, doi = {10.1021/am5047872}, author = {Deshpande, Sonal and Venugopal, Edakkal and Ramagiri, Shobha and Bellare, Jayesh R. and Kumaraswamy, Guruswamy and Singh, Neetu} } @article { ISI:000330204700002, title = {Large amplitude oscillatory shear induces crystal chain orientation in velocity gradient direction}, journal = {ACS Macro Letters}, volume = {3}, number = {1}, year = {2014}, month = {JAN}, pages = {6-9}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

Imposition of large amplitude oscillatory shear (LAOS) on crystallizing polymer melts results in lamellar orientation in the shear gradient direction, in contrast to the flow-orientation observed for steady shear. LAOS enhances the formation of plate-like nuclei and orients their normals in the gradient direction. An Arrhenius temperature dependence (with activation energy approximate to 226 kJ/mol) characterizes the relaxation of crystal orientation with annealing.

}, issn = {2161-1653}, doi = {10.1021/mz4005716}, author = {Sreenivas, K. and Kumaraswamy, Guruswamy} } @article { ISI:000341543200034, title = {Soft colloidal scaffolds capable of elastic recovery after large compressive strains}, journal = {Chemistry of Materials}, volume = {26}, number = {17}, year = {2014}, month = {SEP}, pages = {5161-5168}, publisher = {AMER CHEMICAL SOC}, type = {Article}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

Assemblies of inorganic or glassy particles are typically brittle and cannot sustain even moderate deformations. This restricts the use of such materials to applications where they do not experience significant loading or deformation. Here, we demonstrate a general strategy to create centimeter-size macroporous monoliths, composed primarily (\>90 wt \%) of colloidal particles, that recover elastically after compression to about one-tenth their original size. We employ ice templating of an aqueous dispersion of particles, polymer, and cross-linker such that cross-linking happens in the frozen state. This method yields elastic composite scaffolds for starting materials ranging from nanoparticles to micron-sized dispersions of inorganics or glassy lattices. The mechanical response of the monoliths is also qualitatively independent of polymer type, molecular weight, and even cross-linking chemistry. Our results suggest that the monolith mechanical properties arise from the formation of a unique hybrid microstructure, generated by cross-linking the polymer during ice templating. Particles that comprise the scaffold walls are connected by a cross-linked polymeric mesh. This microstructure results in soft monoliths, with moduli similar to O (10(4) Pa), despite the very high particle content in their walls. A remarkable consequence of this microstructure is that the monolith mechanical response is entropic in origin: the modulus of these scaffolds increases with temperature over a range of 140 K. We show that interparticle connections formed by cross-linking during ice templating determine the monolith modulus and also allow relative motion between connected particles, resulting in entropic elasticity.

}, issn = {0897-4756}, doi = {10.1021/cm502643a}, author = {Rajamanickam, Raja and Kumar, Sushma and Kumar, Deepak and Ghosh, Shankar and Kim, Jong Chul and Tae, Giyoong and Sen Gupta, Sayam and Kumaraswamy, Guruswamy} } @article {42675, title = {Ultrathin sheets of metal or metal sulfide from molecularly thin sheets of metal thiolates in solution}, journal = {Chemistry of Materials}, volume = {26}, year = {2014}, month = {MAY}, pages = {3436-3442}, abstract = {

Materials that exist as single molecule thick twodimensional sheets are in great demand because they hold promise as precursors for synthesis of layered functional materials. We demonstrate that metal thiolates, that exist as lamellar assemblies in the neat state, can be disassembled into individual molecular sheets simply by dilution in apolar organic solvents and that these can form ultrathin metallic layers on substrates upon heat treatment. We establish the pathway to the disassembly of metal thiolates in solution using a combination of techniques, including X-ray diffraction, light scattering, FTIR, and TEM. Our results indicate that the lamellar structure of Pd-thiolates is preserved in toluene up to a concentration of 300\% w/v and the average intersheet distance is unchanged. Interestingly, the dynamics of the Pd-thiolate sheets remain correlated even on diluting them up to 30\% w/v, though the disorder within the lamellar stacks increases with a decrease in their coherence length. Finally, at dilutions less than about 5\% w/v, individual sheets of these structures can be accessed that are isolated and directly observed using TEM. Heat treatment of the ultrathin films of metal thiolates deposited on appropriate substrates resulted in the formation of metal or metal sulfides with retention of sheetlike morphologies.

}, doi = {10.1021/cm5006949}, author = {Busupalli, Balanagulu and Kummara, Sreenivas and Kumaraswamy, Guruswamy and Bhagavatula L. V. Prasad} } @article { ISI:000357192200003, title = {Compact polar moieties induce lipid-water systems to form discontinuous reverse micellar phase}, journal = {Soft Matter}, volume = {11}, number = {27}, year = {2015}, month = {MAY}, pages = {5417-5424}, publisher = {ROYAL SOC CHEMISTRY}, address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND}, abstract = {

The role of molecular interactions in governing lipid mesophase organization is of fundamental interest and has technological implications. Herein, we describe an unusual pathway for monoolein/water reorganization from a bicontinuous mesophase to a discontinuous reverse micellar assembly, directed by the inclusion of polar macromolecules. This pathway is very different from those reported earlier, wherein the Fd3m phase formed only upon addition of apolar oils. Experiments and molecular dynamics simulations indicate that hydrophilic ternary additives capable of inducing discontinuous phase formation must (i) interact strongly with the monoolein head group and (ii) have a compact molecular architecture. We present a detailed investigation that contrasts a monoolein-water system containing polyamidoamine (PAMAM) dendrons with one containing their linear analogs. The Fd3m phase forms only on the addition of PAMAM dendrons but not their linear analogs. Thus, the dendritic architecture of PAMAM plays an important role in determining lipid mesophase behavior. Both dendrons and their linear analogs interact strongly with monoolein through their amine groups. However, while linear polymers adsorb and spread on monoolein, dendrons form aggregates that interact with the lipid. Dendrons induce formation of an intermediate reverse hexagonal phase, which subsequently restructures into the Fd3m phase. Finally, we demonstrate that other additives with compact structures that are known to interact with monoolein, such as branched polyethylenimine and polyhedral silsesquioxane cages, also induce the formation of the Fd3m phase.

}, issn = {1744-683X}, doi = {10.1039/c5sm00854a}, author = {Kumar, Manoj and Patil, Naganath G. and Choudhury, Chandan Kumar and Roy, Sudip and Ambade, Ashootosh V. and Kumaraswamy, Guruswamy} } @article { ISI:000357788700014, title = {Phase behaviour of the ternary system: monoolein-water-branched polyethylenimine}, journal = {Soft Matter}, volume = {11}, number = {28}, year = {2015}, month = {JUN}, pages = {5705-5711}, publisher = {ROYAL SOC CHEMISTRY}, address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND}, abstract = {

Addition of a branched polymer, polyethyleneimine, significantly alters the organization of a glycerol monooleate (GMO) lipid-water system. We present detailed data over a wide range of compositions (water content from 10 to 40\%, relative to GMO and PEI fractions from 0 to 4\%) and temperatures (25-80 degrees C). The PEI molecular weight effects are examined using polymers over a range from 0.8 to 25 kDa. Addition of PEI induces the formation of higher curvature reverse phases. In particular, PEI induces the formation of the Fd3m phase: a discontinuous phase comprising reverse micelles of two different sizes stacked in a cubic AB(2) crystal. The formation of the Fd3m phase at room temperature, upon addition of polar, water soluble PEI is unusual, since such phases typically are formed only upon addition of apolar oils. The largest stability window for the Fd3m phase is observed for PEI with a molecular weight = 2 kDa. We discuss how PEI influences the formation and stability of high curvature phases.

}, issn = {1744-683X}, doi = {10.1039/c5sm01082a}, author = {Kumar, Manoj and Kumaraswamy, Guruswamy} } @article { ISI:000374440100005, title = {Colloidal assembly by ice templating}, journal = {Faraday Discussions}, volume = {186}, year = {2016}, month = {OCT}, pages = {61-76}, publisher = {ROYAL SOC CHEMISTRY}, address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND}, abstract = {

We investigate ice templating of aqueous dispersions of polymer coated colloids and crosslinkers, at particle concentrations far below that required to form percolated monoliths. Freezing the aqueous dispersions forces the particles into close proximity to form clusters, that are held together as the polymer chains coating the particles are crosslinked. We observe that, with an increase in the particle concentration from about 106 to 108 particles per ml, there is a transition from isolated single particles to increasingly larger clusters. In this concentration range, most of the colloidal clusters formed are linear or sheet like particle aggregates. Remarkably, the cluster size distribution for clusters smaller than about 30 particles, as well as the size distribution of linear clusters, is only weakly dependent on the dispersion concentration in the range that we investigate. We demonstrate that the main features of cluster formation are captured by kinetic simulations that do not consider hydrodynamics or instabilities at the growing ice front due to particle concentration gradients. Thus, clustering of colloidal particles by ice templating dilute dispersions appears to be governed only by particle exclusion by the growing ice crystals that leads to their accumulation at ice crystal boundaries.

}, issn = {1359-6640}, doi = {10.1039/c5fd00125k}, author = {Kumaraswamy, Guruswamy and Biswas, Bipul and Choudhury, Chandan Kumar} } @article { ISI:000372856600027, title = {Omniphilic polymeric sponges by ice templating}, journal = {Chemistry of Materials}, volume = {28}, number = {6}, year = {2016}, month = {MAR}, pages = {1823-1831}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

Sponges that absorb a large quantity of solvent relative to their weight, independent of the solvent polarity, represent useful universal absorbents for laboratory and industrial spills. We report the preparation of macroporous polymer sponges by ice templating of polyethylenimine aqueous solutions and their cross-linking in the frozen state. The as-prepared monolith is hydrophilic and absorbs over 30 fold its weight in water. Modification of this sponge using valeroyl chloride renders it omniphilic; viz., a modified sponge absorbs over 10-fold its dry weight of either water or hexane. Modification using palmitoyl chloride that has a longer chain length results in the preparation of a hydrophobic sponge with a water contact angle around 130, which retains its oleophilicity underwater. The solvent absorbed in these sponges can be simply squeezed out, and the sponges are stable to several hundred cycles of compression. The large pore sizes of these sponges allow rapid absorption of even high viscosity solvents such as pump oil. Finally, we demonstrate that these sponges are also able to separate apolar oils that are emulsified in water using surfactants. These high porosity sponges with controllable solvophilicity represent inexpensive, high performance universal absorbents for general solvent spills.

}, issn = {0897-4756}, doi = {10.1021/acs.chemmater.5b04988}, author = {Chatterjee, Soumyajyoti and Sen Gupta, Sayam and Kumaraswamy, Guruswamy} } @article {42997, title = {Template determines whether chemically identical nanoparticle scaffolds show elastic recovery or plastic failure}, journal = {Langmuir}, volume = {32}, year = {2016}, month = {NOV}, pages = {11623-11630}, abstract = {

Subtle variations in the preparation of ice-templated nanoparticle assemblies yield monoliths that are chemically identical but exhibit qualitatively different mechanical behavior. We ice template aqueous dispersions to prepare macroporous monoliths largely comprising silica nanoparticles held together by a crosslinked polymer mesh. When the polymer is crosslinked in the presence of ice crystals, we obtain an elastic sponge that is capable of recovery after imposition of large compressive strains (up to 80\%). If, however, the ice is lyophilized before the polymer is crosslinked, we obtain a plastic monolith that fails even for modest strains (less than 10\%). The elastic sponge and the plastic monolith are chemically identical; they have the same organic content, the same ratio of polymer to crosslinker, and the same average crosslink density. Atomic force microscopy (AFM) was used to probe the local mechanical properties of the crosslinked polymer mesh. These measurements indicate that plastic monoliths dissipate significantly more energy and have a larger spatial variation in local mechanical response relative to the elastic sponges. We believe that this behavior might correlate with a wider spatial distribution of crosslinks in plastic scaffolds relative to elastic scaffolds.

}, doi = {10.1021/acs.langmuir.6b03173}, author = {Suresh, Karthika and Patil, Shivprasad and Rajamohanan, Pattuparambil Ramanpillai and Kumaraswamy, Guruswamy} } @article {43871, title = {Capillary uptake in macroporous compressible sponges}, journal = {Soft Matter}, volume = {13}, year = {2017}, month = {SEP}, chapter = { 5731-5740}, abstract = {

The capillarity-driven uptake of liquid in swellable, highly porous sponges is of significant industrial importance. Sponges prepared using polymers and their composites with carbon nanotubes and graphene have been reported, with extraordinary solvent uptake capacities and with the ability to separate oil from water. However, the effect of systematic variation of sponge characteristics on solvent uptake has not been investigated. Here, we report experiments that study capillary uptake in a variety of flexible, centimetre-sized macroporous cylindrical sponges. We used ice-templating to prepare a series of model macroporous sponges in which the porosity, modulus and composition were systematically varied. We investigated two kinds of sponge: (a) those composed purely of cross-linked polymers and (b) those prepared as composites of inorganic particles and polymers. Both kinds of sponge are flexible and exhibit elastic recovery after large compressive deformation. All sponges were characterized thoroughly with respect to their pore microstructure and elastic modulus. When one end of a sponge is plunged into a large reservoir, water rises through capillary action against gravity. We observed a transition from an inertial capillary regime, where the liquid column height rose linearly with time, t, to a viscous capillary regime, where the liquid height rose with time t(0.5). We showed that these results can be rationalized using analyses developed for rigid sponges. We combined differential momentum balance equations for uptake in rigid capillaries with the phenomenological Ergun-Forchheimer relations to account for the effect of the sponge microstructure. This approach works remarkably well in the viscous capillary regime and shows that capillary uptake is governed primarily by the total porosity and pore dimensions of soft sponges.

}, keywords = {aerogels, Building blocks, Catalyst Supports, Contact-angels, Dynamics, flow, Organic Liquids, Porous-Media, Rise}, issn = {1744-683X}, doi = {10.1039/c7sm00826k}, author = {Chatterjee, Soumyajyoti and Doshi, Pankaj and Kumaraswamy, Guruswamy} } @article {43462, title = {Elastic compressible energy storage devices from ice templated polymer gels treated with polyphenols}, journal = {Journal of Physical Chemistry C}, volume = {121}, year = {2017}, month = {FEB}, pages = {3270-3278}, abstract = {Design and fabrication of rechargeable energy storage devices that are robust to mechanical deformation is essential for wearable electronics. We report the preparation of compressible supercapacitors that retain their specific capacitance after large compression and that recover elastically after at least a hundred compression expansion cycles. Compressible supercapacitors are prepared using a facile, scalable method that readily yields centimeter-scale macroporous objects. We ice template a solution of polyethylenimine in green tea extract to prepare a macroporous cross-linked polymer gel (PG) whose walls are impregnated with green tea derived polyphenols. As the PG is insulating, we impart conductivity by deposition of gold on it. Gold deposition is done in two steps: first, silver nanoparticles are formed on the PG walls by in situ reduction by polyphenols and then gold films are deposited on these walls. Gold coated PGs (GPGs) were used as electrodes to deposit poly(3,4-ethylenedioxythiophene) as a pseudocapacitive material. The specific capacitance of PEDOT coated GPGs (PGPG) was found to be 253 F/g at 1 A/g. PGPG could be compressed and expanded over a hundred cycles without any suffering mechanical failure or loss of capacitative performance. The capacitance was found to be 243 F/g upon compressing the device to 25\% of its original size (viz. compressive strain = 75\%). Thus, even large compression does not affect the device performance. This device shows power and energy densities of 2715 W/kg and 22 Wh/kg, respectively, in the uncompressed state. The macroporous nature of PGPG makes it possible to fill the PGPG pores with gel electrolyte. We report that the gel electrolyte filled supercapacitor exhibited a specific capacitance of 200 F/g, which increased by 4\% upon 75\% compression.}, doi = {10.1021/acs.jpcc.6b12822}, author = {Das, Chayanika and Chatterjee, Soumyajyoti and Kumaraswamy, Guruswamy and Krishnarnoorthy, Kothandam} } @article {44110, title = {Fire-retardant, self-extinguishing inorganic/polymer composite memory foams}, journal = {ACS Applied Materials \& Interfaces}, volume = {9}, year = {2017}, month = {DEC}, pages = {44864-44872}, type = {Article}, abstract = {Polymeric foams used in furniture and automotive and aircraft seating applications rely on the incorporation of environmentally hazardous fire-retardant additives to meet fire safety norms. This has occasioned significant interest in novel approaches to the elimination of fire-retardant additives. Foams based on polymer nano composites or based on fire-retardant coatings show compromised mechanical performance and require additional processing steps. Here, we demonstrate a one-step preparation of a fire-retardant ice-templated inorganic/polymer hybrid that does not incorporate fire-retardant additives. The hybrid foams exhibit excellent mechanical properties. They are elastic to large compressional strain, despite the high inorganic content. They also exhibit tunable mechanical recovery, including viscoelastic "memory". These hybrid foams are prepared using ice-templating that relies on a green solvent, water, porogen. Because these foams are predominantly comprised of inorganic components, they exhibit exceptional fire retardance in torch burn tests and are self-extinguishing. After being subjected to a flame, the foam retains its porous structure and does not drip or collapse. In micro-combustion calorimetry, the hybrid foams show a peak heat release rate that is only 25\% that of a commercial fire-retardant polyurethanes. Finally, we demonstrate that we can use ice-templating to prepare hybrid foams with different inorganic colloids, including cheap commercial materials. We also demonstrate that ice-templating is amenable to scale up, without loss of mechanical performance or fire-retardant properties.}, doi = {10.1021/acsami.7b16808}, author = {Chatterjee, Soumyajyoti and Shanmuganathan, Kadhiravan and Kumaraswamy, Guruswamy} } @article {44171, title = {Linking catalyst-coated isotropic colloids into {\textquoteleft}{\textquoteleft}active{\textquoteright}{\textquoteright} flexible chains enhances their diffusivity}, journal = {ACS Nano}, volume = {11}, year = {2017}, month = {OCT}, pages = {10025-10031}, type = {Article}, abstract = {Active colloids are not constrained by equilibrium: ballistic propulsion, superdiffusive behavior, or enhanced diffusivities have been reported for active Janus particles. At high concentrations, interactions between active colloids give rise to complex emergent behavior. Their collective dynamics result in the formation of several hundred particle-strong flocks or swarms. Here, we demonstrate significant diffusivity enhancement for colloidal objects that neither have a Janus architecture nor are at high concentrations. We employ uniformly catalyst-coated, viz. chemo-mechanically, isotropic colloids and link them into a chain to enforce proximity. Activity arises from hydrodynamic interactions between enchained colloidal beads due to reaction-induced phoretic flows catalyzed by platinum nanoparticles on the colloid surface. This results in diffusivity enhancements of up to 60\% for individual chains in dilute solution. Chains with increasing flexibility exhibit higher diffusivities. Simulations accounting for hydrodynamic interactions between enchained colloids due to active phoretic flows accurately capture the experimental diffusivity. These simulations reveal that the enhancement in diffusivity can be attributed to the interplay between chain conformational fluctuations and activity. Our results show that activity can be used to systematically modulate the mobility of soft slender bodies.}, keywords = {active matter, Brownian motion, colloidal assembly, diffusivity, ice templating}, issn = {1936-0851}, doi = {10.1021/acsnano.7b04265}, author = {Biswas, Bipul and Manna, Raj Kumar and Laskar, Abhrajit and Kumar, P. B. Sunil and Adhikari, Ronojoy and Kumaraswamy, Guruswamy} } @article {44000, title = {Living in the polymer age}, journal = {Resonance}, volume = {22}, year = {2017}, month = {APR}, pages = {333-334}, type = {Article}, doi = {10.1007/s12045-017-0472-3}, author = {Kumaraswamy, Guruswamy} } @article {42995, title = {Molecular motifs for additives that retard PEO crystallization}, journal = {Polymer Engineering \& Science}, volume = {57}, year = {2017}, month = {AUG}, pages = {857-864}, type = {Article}, abstract = {

We investigate the influence of several bifunctional phenol additives (resorcinol: RES, hydroquinone: HYD, p-hydroxybenzoic acid: PHBA, and p-nitrophenol: PNP) on the crystallization of matrix polymer, polyethyleneoxide. We employ solid state Nuclear Magnetic Resonance spectroscopy (NMR), Differential Scanning Calorimetry (DSC), optical microscopy, and Small Angle X-ray Scattering (SAXS) to investigate crystallization of the additivated PEO and compare with Density Functional Theory (DFT) calculations of additive-PEO interactions. Additive-polymer interactions are a function of the functional groups on the additive. Temperature-dependent spherulitic growth rate measurements from optical microscopy and SAXS lamellar long spacings indicate a distinct trend in the effect of additives on PEO crystallization. Change in PEO crystallization is most pronounced with PNP, followed by PHBA and finally RES and HYD. This trend correlates qualitatively with the binding energies of additive-PEO interactions from DFT studies. Our results suggest that DFT calculations might be a useful screening tool to evaluate the influence of additives on polymer crystallization. POLYM. ENG. SCI., 2016. {\textcopyright} 2016 Society of Plastics Engineers

}, doi = {10.1002/pen.24462}, author = {Shrivastava, Sudhakar Dinesh and Eldho, Kavalakal Mathai and Rajamohanan, P. R. and Ajithkumar, Thalasseril G. and Vanka, Kumar and Kumaraswamy, Guruswamy} } @article {43718, title = {Nanoparticle assembly: a perspective and some unanswered questions}, journal = {Current Science}, volume = {112}, year = {2017}, month = {APR}, pages = {1635-1641}, type = {Article}, abstract = {In early 2016, the Royal Society of Chemistry arranged a meeting on the topic {\textquoteright}Nanoparticle Assemblies: from Fundamentals to Applications{\textquoteright} which was hosted at IIT-Bombay, Mumbai. The meeting brought several leading nanoscience and nanotechnology researchers to India and is only the second Faraday Discussions meeting to have been held in the country. The papers presented at the meeting and the resulting active discussions have been summarized in a Faraday Discussion issue(1). The broad range of topics discussed at the meeting led to an understanding on where we stand in the field of nanoparticle assembly, and also enunciated some of the outstanding fundamental and practical issues that remain to be resolved before these ideas can be applied to practical situations. Driven by these ideas, here we focus on four topics/questions: (i) Can we achieve function-driven design of nanoparticle assemblies? (ii) What is the minimal information needed to build a desired assembly? (iii) How complex a structure can one build? How can one make it responsive? What are the relative roles of equilibrium versus dynamics in the assembly process, and are we at a point where we can now pursue active assembly as a viable mode for creating complex assemblies? (iv) What are the applications that are being targeted and what are the barriers to implementation? In this perspective, we do not present an exhaustive survey of the vast literature in this area, but indicate overarching themes/questions that require immediate attention, largely based on the discussions at the Mumbai meeting.}, author = {Kumar, Sanat K. and Kumaraswamy, Guruswamy and Prasad, Bhagavatula L. V. and Bandyopadhyaya, Rajdip and Granick, Steve and Gang, Oleg and Manoharan, Vinothan N. and Frenkel, Daan and Kotov, Nicholas A.} } @article {44416, title = {Accelerated in vitro model for occlusion of biliary stents: investigating the role played by dietary fibre}, journal = {BMJ Innovations}, volume = {4}, year = {2018}, month = {NOV}, abstract = {ackground To develop a new accelerated in vitro model that has implications for investigating the mechanism of biliary stent occlusion and help in the development of new materials that can alleviate this problem. Methods We employ a combination of reconstituted animal bile, bacteria and cellulose fibres optimised to reproducibly generate accelerated occlusion of stents, and produce occlusions that closely mimic those found in clinical studies. Results Our model affords repeatable, highly accelerated occlusion (within 2{\textendash}3 days, compared with between about a week to 2 months in previous models). Our results highlight the role of dietary fibre in blockage of stents and demonstrate their importance in the onset of occlusion. Conclusions This accelerated model may have implications for developing biliary stents with enhanced patency.}, doi = {dx.doi.org/10.1136/bmjinnov-2017-000209}, author = {Surwase, Sachin and Balakrishnan, Harishankar and Acharya, Subrat K. and Makharia, Govind K. and Kumaraswamy, Guruswamy and Bhagavatula L. V. Prasad} } @article {44214, title = {Aqueous dispersions of lipid nanoparticles wet hydrophobic and superhydrophobic surfaces}, journal = {Soft Matter}, volume = {14}, year = {2018}, month = {JAN }, pages = {205-215}, type = {Article}, abstract = {

Efficient delivery of aqueous sprays to hydrophobic surfaces is the key technological challenge in a wide variety of applications, including pesticide delivery to plants. To account for losses due to bouncing of pesticide sprays off hydrophobic leaf surfaces, a large excess of pesticide is typically employed, resulting in environmentally hazardous run-offs that contaminate soil and ground water. We demonstrate that aqueous dispersions of glycerol monooleate nanoparticles, called cubosomes, wet hydrophobic and superhydrophobic surfaces and adhere to them. Cubosomes comprise glycerol monooleate lipid molecules self-assembled into a double diamond cubic phase, that form stable aqueous dispersions that are sterically stabilized using amphiphilic block copolymers. We use high speed imaging to monitor the spreading and retraction of aqueous drops impinged on model hydrophobic substrates and on superhydrophobic lotus leaves. We show that cubosomes diffuse to hydrophobic substrates and reorganize to form a thin, approximate to 2 nm adsorbed lipid layer during the millisecond time scales that characterize drop impact. This adsorbed film drastically reduces the water contact angle, transforming the hydrophobic surface to hydrophilic, thus facilitating retention of the aqueous drop on the surface. Aqueous drops of cubosomes impinged at low velocities on inclined natural superhydrophobic lotus leaf surfaces do not roll off, unlike drops of water or surfactant solutions. When sprayed on inclined lotus leaves, corresponding to the case of high velocity drop impingement, cubosome dispersions form a continuous wetting film. Our results have important implications for efficient, environment-friendly delivery of pesticide sprays.

}, issn = {1744-683X}, doi = {10.1039/c7sm01817g}, author = {Kumar, Manoj and Kulkarni, Mayuresh A. and Chembu, Narendiran G. and Banpurkar, Arun and Kumaraswamy, Guruswamy} } @article {44652, title = {Fluorinated nanocellulose-reinforced all-organic flexible ferroelectric nanocomposites for energy generation}, journal = {Journal of Physical Chemistry C}, volume = {122}, year = {2018}, month = {JUL}, pages = {16540-16549}, abstract = {

We report here enhanced ferroelectric crystal formation and energy generation properties of polyvinylidene fluoride (PVDF) in the presence of surface-modified crystalline nanocellulose. Incorporation of only 2-5 wt \% fluorinated nanocellulose (FNC) in PVDF has been found to significantly induce polar beta/gamma-phase crystallization as compared to the addition of unmodified nanocellulose (carboxylated nanocellulose). A device made up of electrically poled PVDF/FNC composite films yielded 2 orders of magnitude higher voltage output than neat PVDF in vibrational energy harvesting. This remarkable increase in energy generation properties of PVDF at such a low loading of an organic natural biopolymer could be attributed to the tailored surface chemistry of nanocellulose, facilitating strong interfacial interactions between PVDF and FNC. Interestingly, energy harvesting devices fabricated from PVDF/FNC nanocomposites charged a 4.7 mu F capacitor at significantly faster rate and the accumulated voltage on capacitor was 3.8 times greater than neat PVDF. The fact that PVDF/FNC nanocomposites still retain a strain at break of 10-15\% and can charge a capacitor in few seconds suggests potential use of these nanocomposites as flexible energy harvesting materials at large strain conditions.

}, issn = {1932-7447}, doi = {10.1021/acs.jpcc.8b03470}, author = {Ram, Farsa and Ambone, Tushar and Sharma, Aakash and Murugesan, Rajarathinam and Kajale, Deepak and Borkar, Vivek and Ali, Shaikh Faruque and Balu, Praveen Kumar and Kumaraswamy, Guruswamy and Shanmuganathan, Kadhiravan} } @article {44513, title = {Large PAMAM dendron induces formation of unusual P4332 mesophase in monoolein/water system}, journal = {Langmuir}, volume = {34}, year = {2018}, month = {MAY}, pages = {6827-6834}, type = {Article}, abstract = {Compact macromolecular dendrons have been shown to induce the formation of discontinuous inverse micellar assemblies with Fd3m symmetry in monoolein/water systems. Here, we demonstrate that a large PAMAM dendron (G5: fifth generation) induces the formation a very unusual mesophase with P4332 symmetry. This mesophase had previously been observed in monoolein/water systems only on addition of cytochrome C. The P4332 mesophase can be considered an intermediate phase between the bicontinuous Ia3d and discontinuous micellar mesophases. In this unusual phase, every third rod junction of the Ia3d mesophase is replaced with a spherical micelle. We present a detailed investigation of the phase behaviour of monoolein/water as a function of G5 concentration and temperature. Addition of 1\% G5 in 85/15 monoolein/water system induces a transition from the L to Ia3d phase. Further increase in G5 concentration to above 2\% induces the formation of the P4332 phase. Thus, incorporation of G5 yields a qualitatively different phase diagram when compared with incorporation of lower generation PAMAM dendrons (G2 {\textendash} G4) in monoolein/water, where the reverse micellar Fd3m phase forms. PAMAM dendrons of all generations, G2 {\textendash} G5, bear terminal amine groups that interact with the monoolein head group. The compact molecular architecture of the dendrons and these attractive interactions induce bending of the monoolein bilayer structure. For smaller dendrons, G2 {\textendash} G4, this results in the formation of the Fd3m phase. However, the large size of the G5 dendron precludes this and a rare intermediate phase between the Ia3d and discontinuous micellar phase, the P4332 mesophase forms instead.}, issn = {0743-7463}, doi = {10.1021/acs.langmuir.8b00551}, author = {Kumar, Manoj and Patil, Naganath Ganapatarao and Ambade, Ashootosh V. and Kumaraswamy, Guruswamy} } @article {44590, title = {Modeling the universal viscoelastic response of polymer fibers}, journal = {Physical Review Materials}, volume = {2}, year = {2018}, month = {JUN}, pages = {062601}, abstract = {

Polymer fibers, including natural silk and synthetic fibers, exhibit universal viscoelastic response. On stretching below yield, they show logarithmic stress decay. On unloading fibers with a glassy amorphous phase, the stress recovers. A simple phenomenological model accurately describes data from independent mechanical experiments and provides insights into the micro structural origins of the fiber response. Counter to intuition, the model indicates that it is the crystalline regions, rather than the amorphous glass, that deform first on stretching fibers at high strain rates. On holding a stretched fiber, stress decays as a consequence of relaxations in amorphous regions. Finally, unloading the fiber transfers stress from the amorphous to crystalline regions resulting in stress recovery. Model parameters correlate well with the fiber microstructure. Crystal and amorphous moduli from the model match those from x-ray diffraction. Activation energies for the temperature dependence of the peak relaxation time are similar to those reported in the literature. Thus, a simple model that invokes only crystal-amorphous coexistence can successfully model the mechanical response of a wide variety of polymer fibers.

}, issn = {2475-9953}, doi = {10.1103/PhysRevMaterials.2.062601}, author = {Sharma, Aakash and Kumaraswamy, Guruswamy and Thakre, Shirish} } @article {44663, title = {Preparation of macroporous scaffolds with holes in pore walls and pressure driven flows through them}, journal = {RSC Advances}, volume = {8}, year = {2018}, month = {JAN}, pages = {24731-24739}, abstract = {

Controlling the pore architecture in macroporous scaffolds has important implications for their use as reactor packings and as catalyst supports. We report the preparation of a macroporous structure, where the pore walls are perforated by holes. These materials are prepared by modification of the ice-templating protocol developed in our group. We freeze a dispersion of colloidal silica, polymer and cross-linker in a water/acetonitrile medium and allow crosslinking to proceed in the frozen state. The presence of a small fraction of acetonitrile (varying between 1.6\% to 6.4\%) results in the formation of holes in the pore walls. Increasing the acetonitrile concentration changes the pore size distribution, and produces smaller pores on average. This also results in an increasing fraction of the wall area being covered by small pores, of the order of a few microns in size. Perforation of the walls by pores does not change the overall porosity or modulus of the scaffolds. However, the introduction of pores leads to a drastic reduction in the pressure drop required to pump liquid through the scaffolds. The observed residence time distribution (RTD) in the scaffolds is represented by two plug flow reactors (PFRs) in parallel. The RTD results indicate that increasing the hole fraction in the pore walls results in increased channelling which explains the aforementioned decreased pressure drop during pressure driven flow.

}, issn = {2046-2069}, doi = {10.1039/c8ra03867h}, author = {Chatterjee, Soumyajyoti and Potdar, Aditi and Kuhn, Simon and Kumaraswamy, Guruswamy} } @article {44572, title = {Single-particle tracking to probe the local environment in ice-templated crosslinked colloidal assemblies}, journal = {Langmuir}, volume = {34}, year = {2018}, month = {APR}, pages = {4603{\textendash}4613}, type = {Article}, abstract = {

We use single-particle tracking to investigate colloidal dynamics in hybrid assemblies comprising colloids enmeshed in a crosslinked polymer network. These assemblies are prepared using ice templating and are macroporous monolithic structures. We investigate microstructure-property relations in assemblies that appear chemically identical but show qualitatively different mechanical response. Specifically, we contrast elastic assemblies that can recover from large compressive deformations with plastic assemblies that fail on being compressed. Particle tracking provides insights into the microstructural differences that underlie the different mechanical response of elastic and plastic assemblies. Since colloidal motions in these assemblies are sluggish, particle tracking is especially sensitive to imaging artifacts such as stage drift. We demonstrate that the use of wavelet transforms applied to trajectories of probe particles from fluorescence microscopy eliminates stage drift, allowing a spatial resolution of about 2 nm. In elastic and plastic scaffolds, probe particles are surrounded by other particles{\textemdash}thus, their motion is caged. We present mean square displacement and van Hove distributions for particle motions and demonstrate that plastic assemblies are characterized by significantly larger spatial heterogeneity when compared with the elastic sponges. In elastic assemblies, particle diffusivities are peaked around a mean value, whereas in plastic assemblies, there is a wide distribution of diffusivities with no clear peak. Both elastic and plastic assemblies show a frequency independent solid modulus from particle tracking microrheology. Here too, there is a much wider distribution of modulus values for plastic scaffolds as compared to elastic, in contrast to bulk rheological measurements where both assemblies exhibit a similar response. We interpret our results in terms of the spatial distribution of crosslinks in the polymer mesh in the colloidal assemblies.

}, issn = {7437463}, doi = {0.1021/acs.langmuir.7b04120}, author = {Suresh, Karthika and Sharma, D. K. and Chulliyil, Ramya and Sarode, Ketan Dinkar and Kumar, V. Ravi and Choudhary, Arindam and Kumaraswamy, Guruswamy} } @article {46084, title = {Characterizing microvoids in regenerated cellulose fibers obtained from viscose and lyocell processes}, journal = {Macromolecules}, volume = {52}, year = {2019}, month = {JUN}, pages = {3987-3994}, type = {Article}, abstract = {

Regenerated cellulose fibers are among the most widely used bio-derived materials. Currently, there is great interest in transitioning from the traditional viscose process to the more environmentally friendly lyocell process for fiber production. Differences between the characteristics of viscose and lyocell fibers can be attributed to microstructural differences that arise due to differences in the processing techniques. Here, we use small-angle scattering to characterize the microvoids in regenerated cellulose fibers that might govern the onset of mechanical failure in these. In regenerated cellulose fibers, scattering of X-rays or neutrons at small angles is largely dominated by scattering from microvoids. We demonstrate that small angle X-ray scattering (SAXS) over the q range that is typical for most commercial instruments arises from Porod scattering from the microvoid surfaces, viz., the scattered intensity scales as q(-4). Therefore, it is not possible to extrapolate this data to lower q to obtain microvoid dimensions and volume fraction. We combine SAXS with medium-resolution small-angle neutron scattering to characterize the microvoids in regenerated cellulose fibers. Specifically, we compare fibers produced using the viscose process with those from the lyocell process. For both viscose and lyocell fibers, microvoids have a high aspect ratio and are elongated in the fiber direction. Also, the volume fraction occupied by the microvoids is comparable for viscose and lyocell fibers (0.04-0.05\%). However, there are differences in the microvoid size: Microvoids are more highly oriented in lyocell fibers and have a larger average length and diameter compared with viscose fibers. This result might have important implications for understanding failure of these fibers.

}, issn = {0024-9297}, doi = {10.1021/acs.macromol.9b00487}, author = {Sharma, Aakash and Sen, Debasis and Thakre, Shirish and Kumaraswamy, Guruswamy} } @article {46245, title = {Critical role of processing on the mechanical properties of cross-linked highly loaded nanocomposites }, journal = {Macromolecules}, volume = {52}, year = {2019}, pages = {5955-5962}, type = {Article}, abstract = {

Polymer nanocomposites are frequently not at equilibrium-therefore, the structure and properties critically depend on the protocol followed in their preparation. Here, we demonstrate that the elastic-brittle transition in highly loaded ice-templated polymer nanocomposites is sensitively determined by the conformation of polymer chains during nanocomposite preparation. Macroporous polymer nanocomposites are synthesized using two preparation pathways, both exhibiting minor modifications of the ice-templating method wherein an aqueous dispersion of nanoparticles, polymers, and cross-linkers is frozen. In one method, the matrix polymer is cross-linked in the presence of ice, whereas in the other method, the ice is removed by freeze-drying before the polymer is cross-linked. Although all measurable structural metrics are practically identical for the two composites, they exhibit qualitatively different mechanical properties. Nanocomposites cross-linked in the presence of ice show exceptional resilience, recovering elastically from large compression for very high nanoparticle loadings-in some instances, above 90\% by weight. Here, the critical particle loading for the elastic- brittle transition is dependent on the molecular weight of the polymer. In comparison, samples cross-linked after freeze-drying appear structurally identical but turn brittle at much lower particle loadings (about 60\% by weight), independent of the matrix polymer molecular weight. We rationalize these differences in terms of the conformational state of the polymer during cross- linking. In the former case, polymer chains are in good solvent during nanocomposite preparation, while after lyophilization, they are in a poor solvent. The large spatial extent of well-solvated chains and chain-chain overlap during cross-linking results in the formation of intermolecular cross-links, and we obtain elastic nanocomposites even at high nanoparticle loadings. In contrast, for chains in collapsed conformations in a bad solvent, chain connectivity during cross-linking is achieved only for much lower particle loadings. Our results reiterate that the sensitivity of polymer conformations to different processing methods can result in large differences in properties, even though their consequences on structural characteristics are effectively indistinguishable.

}, doi = {10.1021/acs.macromol.9b00813}, author = {Suresh, Karthika and Chowdhury, Arindam and Kumar, Sanat K. and Kumaraswamy, Guruswamy} } @article {46013, title = {Effect of electrostatic interactions on structure and mechanical properties of ice templated colloid-polymer composites}, journal = {Journal of Physics D-Applied Physics}, volume = {52}, year = {2019}, month = {MAY }, pages = {214002}, type = {Article}, abstract = {

When an aqueous dispersion of negatively charged colloids, cationic polymer and crosslinker is frozen and the polymer is allowed to crosslink in the frozen state, we obtain a self-standing macroporous composite foam. This material is soft, despite the high concentration of colloids and yet is remarkably elastic to large compressive strains. In these macroporous composites, the pore walls comprise colloidal particles held within a crosslinked polymer network. Here, we investigate the effect of varying colloid- polymer interactions in pre-fabricated macroporous composites on their microstructure and mechanical properties. During preparation of the composite, cationic polymer adsorbs on the negatively charged colloids. We tune the surface charge of particles embedded in a macroscopic monolith by immersing the composites in water maintained at different pH. In this way, we tune polymer-particle interactions in the composite. We observe a sudden increase in interparticle distance and swelling of composite when pH is decreased below the particle{\textquoteright}s isoelectric point. Correspondingly, we observe reduction in Young{\textquoteright}s and shear moduli, compression strength and macroscopic energy dissipation. We did not observe any pH dependent changes in pure polymer sponges (prepared by ice templating and crosslinking a polymer solution that does not contain colloidal particles). Therefore, the pH dependent structural and mechanical property changes arise from the composite structure rather than purely from the crosslinked polymer. We believe that the reduction in mechanical stiffness when the polymer-particle interaction becomes repulsive is because of the reduction in interfacial contacts between particle and polymer. Therefore, the mechanical stiffness of ice templated composite is strongly influenced by interactions between the polymer and particle surface.

}, keywords = {colloid, electrostatic interaction, ice templating, mechanical properties, Polymer}, issn = {0022-3727}, doi = {10.1088/1361-6463/ab0b7b}, author = {Suresh, Karthika and Kumaraswamy, Guruswamy} } @article {45511, title = {On the sensitivity of alginate rheology to composition}, journal = {Soft Matter}, volume = {15}, year = {2019}, month = {JAN }, pages = {159-165}, type = {Article}, abstract = {

The linear response of alginate-phenyl boronic acid (Alg-PBA) esters shows a universal, composition-independent viscoelastic fluid-like behaviour. Reversible association of alginates governs their rheology at all compositions (viz. at all alginate concentrations and solution pH). However, their high strain behaviour is very sensitive to composition. Tuning composition affords liquids that neck to form filaments capable of being drawn to large elongations without failure. We interpret our data by invoking strain-dependent association and dissociation rates for the alginates. High association rates at high strain result in materials with viscoelastic liquid like behaviour.

}, issn = {1744-683X}, doi = {10.1039/c8sm02093k}, author = {Suresh, Karthika and Haering, Marleen and Kumaraswamy, Guruswamy and Diaz Diaz, David} } @article {46151, title = {Structure-property relations in regenerated cellulose fibers: comparison of fibers manufactured using viscose and lyocell processes}, journal = {Cellulose}, volume = {26}, year = {2019}, month = {APR}, pages = {3655-3669}, type = {Article}, abstract = {

Regenerated cellulose fibers are produced using two industrially dominant technologies: the viscose and lyocell processes. Here, we compare commercially available fibers, prepared using the viscose and lyocell processes. Single fibers are subjected to a variety of mechanical deformations to obtain stress-strain, stress relaxation and stress recovery data. These are fitted to a phenomenological model, whose parameters are interpreted in terms of the fiber semicrystalline microstructure. This simple model does not incorporate the complexities of semicrystalline microstructure. Rather, it represents structure in a semicrystalline polymer fiber in terms of an elastic crystalline phase that coexists with a viscoelastic Voigt-like glassy amorphous phase. Lyocell fibers are characterized by higher values of crystalline modulus relative to viscose. Lyocell fibers also have a higher amorphous phase modulus and a wider relaxation spectrum than viscose, suggesting that amorphous and crystalline phases are dispersed in close connectivity in lyocell. Viscose and lyocell fibers exhibit qualitative similarities in their mechanical response. On stretching, there is a transition in the stress-strain curve from a low strain elastic response at a critical value of strain. This critical strain has been incorrectly attributed to yielding of the fiber. We establish that this critical value corresponds to an apparent yield. When subjected to strains higher than this apparent yield point, the fibers develop a memory of the mechanical deformation. This memory decays slowly, logarithmically with time and is lost over about a day as the fiber structure transitions back to the original as spun fiber. Finally, we demonstrate that on wetting the fibers with water, there is an increase in the apparent yield strain for viscose fibers, but not for lyocell. We interpret these results in terms of the semicrystalline microstructure of the fibers.

}, keywords = {mechanical properties, Model, Regenerated cellulose, Yielding}, issn = {0969-0239}, doi = {10.1007/s10570-019-02352-w}, author = {Sharma, Aakash and Nagarkar, Shailesh and Thakre, Shirish and Kumaraswamy, Guruswamy} } @article {47163, title = {Three-dimensional printing with waste high-density polyethylene}, journal = {ACS Applied Polymer Materials}, volume = {1}, year = {2019}, month = {NOV}, pages = {3157-3164}, type = {Article}, abstract = {

Fused filament fabrication (FFF) three-dimensional (3D) printing of semicrystalline polymers such as high density polyethylene (HDPE) is challenging because crystallization-induced shrinkage of the filament, as it cools, results in stresses that warp the printed part and debond it from the print substrate. Here, we demonstrate that waste-derived HDPE can be successfully 3D printed by (i) blending with a small fraction (\<0.5\% by weight) of dimethyl dibenzylidene sorbitol (DMDBS) and (similar to 10\%) linear low density polyethylene (LLDPE) and (ii) printing the object with a thin {\textquoteleft}{\textquoteleft}brim{\textquoteright}{\textquoteright} around it that is adhered to the print substrate using common polyvinyl acetate-based glue. We match our experimental results with FEM simulations that provide insight into the origin of the stresses developed during printing. Because HDPE forms a significant fraction of the plastic waste stream, conversion of waste-derived HDPE to 3D printing filament has important technological implications.

}, keywords = {3d printing polyethylene printing, polymer blend, Recyclability, warpage}, issn = {2637-6105}, doi = {10.1021/acsapm.9b00813}, author = {Gudadhe, Aniket and Bachhar, Nirmalya and Kumar, Anil and Andrade, Prem and Kumaraswamy, Guruswamy} } @article {46764, title = {3D printing of semicrystalline polypropylene: towards eliminating warpage of printed objects}, journal = {Bulletin of Materials Science}, volume = {43}, year = {2020}, month = {JUL}, pages = {171}, type = {Article}, abstract = {

Fused filament fabrication (FFF) is an extrusion-based 3D printing technique for thermoplastic polymers. In this technique, molten polymer is extruded through a print nozzle and is laid down layer by layer to build up the printed object. Currently, FFF is used primarily to print amorphous or low-crystallinity polymers, such as acrylonitrile butadiene styrene copolymer (ABS) or polylactic acid (PLA). Printing of semicrystalline polymers, such as polyethylene or polypropylene remains particularly challenging. During FFF of semicrystalline polymers, large thermomechanical stresses are generated when the polymer solidifies on cooling. These stresses result in warpage of the printed part. Here, we analyse the factors that influence stresses generated during FFF 3D printing of a commercial semicrystalline polymer, isotactic polypropylene. We investigate the effect of height of the printed object on part warpage, as well the effect of infilling during printing. We demonstrate that the stresses generated during FFF can be substantially decreased by incorporation of a {\textquoteleft}brim{\textquoteright}, viz. a thin layer at the base of the printed object, and by adhering the brim to the print substrate using common polyvinyl acetate-based glue. We systematically investigate the effect of the brim size on the warpage of the printed object. We support our experimental findings with finite element method (FEM) simulations that explain the mechanism of stress buildup during printing. The trend in stresses calculated in the FEM simulations parallel the warpage measured in the experiments. Thus, this work represents an important methodological advance towards warpage-free FFF printing of semicrystalline polymers.

}, keywords = {3D printing, polypropylene printing, warpage}, issn = {0250-4707}, doi = {10.1007/s12034-020-02097-4}, author = {Bachhar, Nirmalya and Gudadhe, Aniket and Kumar, Anil and Andrade, Prem and Kumaraswamy, Guruswamy} } @article {46775, title = {Ice templated nanocomposites containing rod-like hematite particles: Interplay between particle anisotropy and particle-matrix interactions}, journal = {Journal of Applied Physics}, volume = {128}, year = {2020}, month = {JUL }, pages = {034702}, type = {Article}, abstract = {

We demonstrate that the mechanical response of ice templated nanocomposite scaffolds prepared from ellipsoidal hematite particles is determined by both the particle aspect ratio and the interaction between the particles and the matrix polymer. We ice template aqueous dispersions of hematite particles, polyethyleneimine, and diepoxy crosslinker and crosslink the polymer in the frozen state. This protocol results in the formation of elastic macroporous monoliths capable of complete recovery from large compressive strains. Hematite particles show an inversion of their surface charge with pH: they are negatively charged at a basic pH and positively charged under acidic conditions. This allows us to change the interaction between hematite particles and crosslinked matrix polymer that they are embedded in, simply by immersing the monoliths in aqueous solutions with different pH{\textquoteright}s. We report that under basic conditions, viz, when polyethyleneimine adsorbs on the particle surface, there is a decrease in the monolith modulus with an increase in the particle aspect ratio. We demonstrate that this correlates with a change in the mechanism of monolith response: from wall compression for isotropic particles to wall bending for anisotropic particles with an aspect ratio of 4. Under acidic conditions (pH=2), where hematite particles show a positive zeta potential, the monolith modulus increases with the aspect ratio of the ellipsoidal fillers. Understanding the interplay between filler aspect ratio and filler-matrix interaction has important implications for the control of nanocomposite mechanical properties.

}, issn = {0021-8979}, doi = {10.1063/1.5144217}, author = {Kumaraswamy, Guruswamy and Suresh, Karthika and Lama, Hisay and Basavaraj, Madivala G. and Satapathy, Dillip K.} } @article {46972, title = {Microstructural differences between Viscose and Lyocell revealed by in-situ studies of wet and dry fibers}, journal = {Cellulose}, volume = {27}, year = {2020}, month = {FEB}, pages = {1195-1206}, type = {Article}, abstract = {

Regenerated cellulose fibers manufactured using Lyocell and Viscose processes exhibit differences in their structural response to mechanical deformation. Here, we study the effect of stretching and stress relaxation on the orientation of crystal and amorphous phases. Our results show that on stretching, orientation in both crystal and amorphous phases increases linearly with strain, correlating with the increase in stress and with the stretching of the crystalline unit cell along the c-axis. On holding after stretching to a particular strain, the stress relaxes logarithmically in time, correlating with a decrease in the strain along the c-axis of the crystal unit cell. The stress relaxation is also correlated with a logarithmic increase in amorphous orientation, while crystalline orientation stays constant. We attribute the stress development during stretching to deformation of the crystal unit cell, while crystal reorientation in the fiber direction results in increase in the crystalline orientation parameter. On holding the fiber at a fixed total strain, the stress relaxes as strain is transferred from crystal to amorphous regions. Thus, the strain on the unit cell c-axis decreases and amorphous orientation increases. There are quantitative differences between the rate of increase in amorphous phase orientation during stress relaxation for Lyocell and Viscose fibers. For dry fibers, Lyocell shows a slower increase in orientation during stress relaxation. On wetting the fibers, their structural response changes qualitatively. We combine wide angle x ray diffraction and birefringence experiments with our model to infer that that on stretching the wet fiber, the crystalline phase is neither strained nor oriented. However, orientation develops in the amorphous phase. During stress relaxation in wet condition, Lyocell fibers shows a faster increase in amorphous orientation than Viscose fibers, in line with the comparison of relaxation time spectra for wet Viscose and Lyocell fibers. Graphic abstract

}, keywords = {birefringence, Crystal-amorphous orientation, In-situ WAXD, Regenerated cellulose}, issn = {0969-0239}, doi = {10.1007/s10570-019-02863-6}, author = {Sharma, Aakash and Thakre, Shirish and Kumaraswamy, Guruswamy} } @article {47387, title = {Colloidal assembly by directional ice templating}, journal = {Soft Matter }, volume = {17}, year = {2021}, month = {APR }, pages = {4098-4108}, type = {Article}, abstract = {

We investigate directional ice templating of dilute aqueous colloidal particle dispersions and examine the nature of the assemblies that result. We coat micron-size polystyrene colloids with cross-linkable polymer (polyethyleneimine, PEI), add cross-linker, and subject this dispersion to unidirectional freezing. We work at sufficiently low colloid concentrations, such that the particles do not percolate on freezing. When the aqueous dispersion freezes, ice crystals force polymer-coated particles and cross-linker into close proximity. This results in the formation of cross-linked clusters of particles at ice crystal boundaries. We vary the particle volume fraction from phi similar to 2.5 x 10(-3) to phi similar to 5.0 x 10(-2) and observe that there is a transition from isolated single particles to increasingly large sized clusters. Most of the clusters formed under these conditions are either linear, two-particle wide chains, or sheet-like aggregates. The probability (P-n) of clusters containing n particles (n \> 2) obeys a power law P-n similar to n(-eta), where eta strongly depends on the particle concentration in the dispersion, varying from 2.10 (for phi similar to 5.0 x 10(-2)) to 3.03 (for phi similar to 2.5 x 10(-3)). This change in eta is qualitatively different from the case of isotropic freezing, where eta is particle concentration-independent and depends only on the ice nucleation density. To understand the differences between isotropic and directional ice templating, we performed lattice simulations of a highly simplified model, where ice crystals grow at a constant rate to force clustering. We ignore hydrodynamic interactions and ice growth instabilities. Despite ignoring these experimental details, the simulations capture the experimental results, nearly quantitatively. As the ice crystals grow and the space available to the colloids {\textquoteleft}{\textquoteleft}closes up{\textquoteright}{\textquoteright} so that the particles cluster to form aggregates, crystallization protocol-induced differences in the geometry of these {\textquoteleft}{\textquoteleft}closed up{\textquoteright}{\textquoteright} spaces determine the scaling behaviour of P-n.

}, issn = {1744-683X}, doi = {10.1039/d0sm02057e}, author = {Biswas, Bipul and Misra, Mayank and Bisht, Anil Singh and Kumar, Sanat K. and Kumaraswamy, Guruswamy} } @article {47357, title = {Elastic piezoelectric aerogels from isotropic and directionally ice-templated cellulose nanocrystals: comparison of structure and energy harvesting}, journal = {Cellulose}, volume = {28}, year = {2021}, month = {JUL}, pages = {6323-6337}, type = {Article}, abstract = {

We report the preparation of highly compressible and elastic piezoelectric aerogels of carboxylated cellulose nanocrystals (CNCs). Aqueous CNC dispersions containing polyethyleneimine and crosslinker were frozen isotropically to yield isotropic aerogels, while oriented aerogels were prepared by directional freezing. These aerogels were highly flexible and porous (similar to 85\% void fraction), exhibiting greater than 90\% recovery at 50\% compressive strain even after 100 compression-decompression cycles. Since such aerogels with low bulk modulus and high anisotropy would be an ideal platform for leveraging the piezoelectric properties of CNCs, we used them to prepare piezoelectric nanogenerator devices and determined their energy transduction behavior. Anisotropic aerogels led to an enhanced open-circuit voltage of 840 mV (at similar to 8 N applied force), which is over 2.6 times higher than isotropic aerogels (320 mV). The energy density of anisotropic aerogels was around 52 nW/cm(2), representing outstanding piezoelectric performance for cellulose-based aerogels. Such aerogels with high compressibility, elastic recovery and exceptional piezoelectric performance could have potential applications in sensors, wearable electronics, etc.

}, keywords = {Aerogel, Anisotropic, Cellulose nanocrystals, energy harvesting, Piezoelectricity}, issn = {0969-0239}, doi = {10.1007/s10570-021-03896-6, Early Access Date = MAY 2021}, author = {Ram, Farsa and Biswas, Bipul and Torris, Arun and Kumaraswamy, Guruswamy and Shanmuganathan, Kadhiravan} } @article {47353, title = {Highly compressible ceramic/polymer aerogel-based piezoelectric nanogenerators with enhanced mechanical energy harvesting property}, journal = {Ceramics International}, volume = {47}, year = {2021}, month = {JUN }, pages = {15750-15758}, type = {Article}, abstract = {

Ceramic piezoelectric materials have orders of magnitude higher piezoelectric coefficients compared to polymers. However, their brittleness precludes imposition of large strains in mechanical energy harvesting applications. We report here that ice templating affords low bulk modulus lead-free aerogel piezoelectric nanogenerators (PENG) with unprecedented combination of flexibility and high piezoelectric response (voltage and power density). A modified ice templating protocol was used to fabricate piezoelectric nanocomposites of surface modified BaTiO3 (BTO) nanoparticles in crosslinked polyethylene imine. This protocol allowed incorporating a significantly high fraction of BTO particles (up to 83 wt \%) in the aerogel, while retaining remarkably high compressibility and elastic recovery up to 80\% strain. The output voltage, at an applied compressive force of 20 N (100 kPa), increased with BTO loading and a maximum output voltage of 11.6 V and power density of 7.22 ?W/cm2 (49.79 ?W/cm3) was obtained for PENG aerogels containing 83 wt\% BTO, which is orders of magnitude higher than previously reported values for foam-based piezoelectric energy harvesters. The BTO/PEI PENGs also showed cyclic stability over 900 cycles of deformation. PENGs with higher porosity showed better elastic recovery and piezoelectric properties than lower porosity and higher BTO content aerogels. To the best of our knowledge, this is the first report to demonstrate the piezoelectric properties of high ceramic content aerogels having very high compressibility and elastic recovery.

}, keywords = {Ceramic polymer aerogels, energy harvesting, ice templating, piezoelectric}, issn = {0272-8842}, doi = {10.1016/j.ceramint.2021.02.147}, author = {Ram, Farsa and Suresh, Karthika and Torris, Arun and Kumaraswamy, Guruswamy and Shanmuganathan, Kadhiravan} } @article {47360, title = {Process-induced microstructure in viscose and lyocell regenerated cellulose fibers revealed by SAXS and SEM of acid-etched samples}, journal = {ACS Applied Polymer Materials}, volume = {3}, year = {2021}, month = {MAY }, pages = {2598-2607}, type = {Article}, abstract = {

Regenerated cellulose fibers represent an important class of bioderived commercial fibers. The traditional viscose process requires the use of environmentally harmful carbon disulfide solvent to produce fibers. Lyocell fibers, produced using a more sustainable recent process, exhibit differences in properties from viscose. These differences arise from their semicrystalline microstructure, formed during fiber spinning. It is widely believed that regenerated cellulose fibers predominantly form fringed fibrillar crystals. We optimize acid etching, followed by SEM as an experimental tool to visualize this fibrillar structure. Acid etching provides sufficient topological contrast to directly visualize similar to O(10 nm) fibrils using field-emission scanning electron microscopy (SEM). We combine SEM with small-angle X-ray scattering (SAXS) to reveal other microstructural details. We observe a Bragg peak, indicating the coexistence of stacked lamellar structure with crystalline fibrils for viscose fibers, but not for lyocell. Viscose and lyocell fibers are characterized by partially oriented semicrystalline microstructure. We present a methodology to calculate the Lorentz correction for such microstructure and employ this to analyze the lamellar scattering from viscose fibers using a 1D correlation function approach. We characterize the lamellar microstructure after swelling viscose fibers with water and observe expansion of the Bragg spacing due to water absorption in the amorphous regions. Our data suggest that the water-induced plasticization of amorphous regions is inhomogeneous. Lamellar stacks that are more misoriented from the fiber direction exhibit lower swelling than those along the fiber direction. The experimental methods described in this work reveal interesting details of semicrystalline microstructure in regenerated cellulose fibers, with important implications for the mechanical response of dry and wet fibers. The methods developed here might find use in investigations of other polymer fibers as well.

}, keywords = {acid etching, crystalline lamellae, fibers, fibrils, Lorentz correction, Regenerated cellulose, SAXS}, issn = {2637-6105}, doi = {10.1021/acsapm.1c00204}, author = {Sharma, Aakash and Wankhede, Parnashri and Samant, Roopali and Nagarkar, Shailesh and Thakre, Shirish and Kumaraswamy, Guruswamy} } @article {47831, title = {Rigidity dictates spontaneous helix formation of thermoresponsive colloidal chains in poor solvent}, journal = {ACS Nano}, volume = {15}, year = {2021}, month = {DEC}, pages = {19702-19711}, type = {Article}, abstract = {The formation of helical motifs typically requires specific directional interactions. Here, we demonstrate that isotropic interparticle attraction can drive self-assembly of colloidal chains into thermo-reversible helices, for chains with a critical level of backbone rigidity. We prepare thermoresponsive colloidal chains by cross-linking PNIPAM microgel-coated polystyrene colloids ({\textquotedblleft}monomers{\textquotedblright}), aligned in an AC electric field. We control the chain rigidity by varying cross-linking time. Above the LCST of PNIPAM, there is an effective attraction between monomers so that the colloidal chains are in a bad solvent. On heating, the chains decrease in size. For the most rigid chains, the decrease is modest and is not accompanied by a change in shape. Much less rigid chains form relatively compact structures, resulting in a large increase in the local monomer density. Unusually, chains with intermediate rigidity spontaneously assemble into helical structures. The chain helicity increases with temperature and plateaus above the collapse transition temperature of the microgel particles. We simulate a minimal model that captures the spontaneous emergence of the helical conformations of the polymeric chain and provides insight into this shape transition. Our work suggests that a purely mechanical instability for semiflexible filaments can drive helix formation, without the need to invoke directional interactions.}, doi = {10.1021/acsnano.1c07048}, author = {Biswas, Bipul and Mitra, Debarshi and KP, Fayis and Bhat, Suresh and Chatterji, Apratim and Kumaraswamy, Guruswamy} } @article {47459, title = {Slip behavior during pressure driven flow of Laponite suspension}, journal = {Physics of Fluids}, volume = {33}, year = {2021}, month = {MAY}, type = {Article}, abstract = {

We investigate pressure driven pipe flow of Laponite suspension, as a model thixotropic fluid. The tendency of the suspension to age is controlled by addition of sodium chloride salt to vary the ionic strength. We use a syringe pump to prescribe the flow and observe that a steady state flow is obtained. Unusually, the steady state pressure drop required to maintain a constant flow rate decreases with an increase in the flow rate, in qualitative contrast to the expectation for Poiseuille flow. We demonstrate that experimental results obtained by varying the flow rate, salt concentration, and flow geometry (pipe diameter and length) can be collapsed onto a single universal curve that can be rationalized by invoking slip of the suspension at the tube walls. The Laponite suspension exhibits plug-like flow, yielding at the tube walls. Our results suggest that the slip length varies linearly with the flow rate and inversely with the tube diameter.

}, doi = {10.1063/5.0051044}, author = {Kamdi, Prophesar M. and Orpe, Ashish V. and Kumaraswamy, Guruswamy} } @article {48759, title = {Polyethylene-grafted sheet-like silsesquioxane nanocomposites with unprecedented adhesion to polar substrates}, journal = {ACS Applied Polymer Materials}, volume = {5}, year = {2023}, month = {JUL}, pages = {5972-5983}, type = {Article}, abstract = {

Polyethylene is a highly apolar polymer with very pooradhesionto most substrates, necessitating the use of tie layers. Therefore,the synthesis of functional polyethylene is a long-standing challengein catalytic ethylene polymerization. Here, we report the preparationof a nanocomposite comprising polyethylene covalently grafted ontosheet-like silsesquioxanes, with unprecedented adhesion to metal andglass. A norbornene-grafted, layered Mg-silsesquioxane is treatedwith Grubbs second-generation catalyst (G-II), and the identity ofcovalently tethered G-II is unambiguously ascertained. Covalentlytethered G-II catalyzes the ring opening metathesis polymerizationof cyclooctene to poly(cyclooctene). The resulting poly(cyclooctene)is catalytically hydrogenated to yield polyethylene. This polyethylenenanocomposite exhibited a bonding strength of the order of 100 MPaon stainless steel and aluminum, 10-fold higher than reported forengineered polyethylene copolymers. The nanocomposite exhibits anincrease in the polar component of surface energy, yet remains compatibleand cocrystallizes with a polyethylene matrix.

}, keywords = {adhesion, clay, Nanocomposite, Polyethylene, silsesquioxane}, issn = {2637-6105}, doi = {10.1021/acsapm.3c00649}, author = {Sharma, Vivek and Paulbudhe, Uday and Bachhar, Nirmalya and Chikkali, Samir H. and Kumaraswamy, Guruswamy} } @article {49563, title = {Thermal properties of polyethylene-grafted sheetlike silsesquioxanes}, journal = {ACS Applied Polymer Materials}, volume = {7}, year = {2025}, month = {MAR}, pages = {4290-4300}, type = {Article}, abstract = {

Polyethylene-grafted layered silsesquioxanes, termed polyethylene-clays (PEC), are nanocomposites comprising polyethylene chains tethered to inorganic sheets with a phyllosilicate-like structure. Here, we report that these nanocomposites show two-stage crystallization on cooling, qualitatively different from previous reports on polyethylene nanocomposites. We employ differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS) to study the melting and crystallization of PEC. End tethering of the polyethylene chains to a nanosheet strongly influences the manner in which PEC crystallizes from the melt on cooling. PEC exhibits two-step crystallization, characterized by a sharp high-temperature exotherm, followed by a broader exotherm at lower temperatures, in contrast to a single sharp exotherm for neat polyethylene. SAXS indicates that lamellar stacks form at high temperatures and that the low-temperature exotherm corresponds to the formation of additional lamellae and their insertion within these stacks. PEC exhibits lower peak melting temperature, lower crystallinity, and a wider melting range relative to polyethylene. We show that the progress of crystallization of PEC is determined by its ultraslow relaxation dynamics. In contrast, PEC in xylene solution exhibits a significantly shorter relaxation time than the melt PEC. Such systems exhibited a single exotherm on cooling and SAXS structure factor peaks with peak positions in a ratio of 1:2. We hypothesize that the high melt viscosity inhibits the crystallization-induced decrease in the specific volume of PEC, resulting in tensile internal stresses that determine the observed thermal behavior.

}, keywords = {clay, Crystallization, Nanocomposite, Polyethylene, silsesquioxane}, issn = {2637-6105}, doi = {10.1021/acsapm.4c04181}, author = {Sharma, Vivek and Paulbudhe, Uday and Gupta, Poonam and Zalte, Akshat Shirish and Chikkali, Samir H. and Kumaraswamy, Guruswamy} }