01972nas a2200193   4500008004100000022001400041245005200055210005200107260008600159300001200245490000800257520134300265653000901608653002801617653001501645653001001660100002701670856008101697       2005                        eng d  a1532-179700aCrystallization of polymers from stressed melts0 aCrystallization of polymers from stressed melts  a325 CHESTNUT ST, SUITE 800, PHILADELPHIA, PA 19106 USAbTAYLOR & FRANCIS INCcOCT  a375-3970 vC453 a<p>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 ``critical'' 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.</p>10aflow10apolymer crystallization10aprocessing10ashear1 aKumaraswamy, Guruswamy  uhttps://library.ncl.res.in/content/crystallization-polymers-stressed-melts-102287nas a2200169   4500008004100000022001400041245005400055210005400109260007100163300001400234490000700248520169500255100003101950700002701981700002102008856008802029       2006                        eng d  a0024-929700aCollapse transition in random copolymer solutions0 aCollapse transition in random copolymer solutions  a1155 16TH ST, NW, WASHINGTON, DC 20036 USAbAMER CHEMICAL SOCcDEC  a9621-96290 v393 a<p>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 &lt; 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.</p>1 aDasmahapatra, Ashok, Kumar1 aKumaraswamy, Guruswamy1 aNanavati, Hemant  uhttps://library.ncl.res.in/content/collapse-transition-random-copolymer-solutions-002369nas a2200181   4500008004100000022001400041245010600055210006900161260007100230300001400301490000700315520164200322100002701964700002501991700002502016700001702041856012902058       2008                        eng d  a0888-588500aComposites of polypropylene with layered Mg-silsesquioxanes show an unusual combination of properties0 aComposites of polypropylene with layered Mgsilsesquioxanes show   a1155 16TH ST, NW, WASHINGTON, DC 20036 USAbAMER CHEMICAL SOCcJUN  a3891-38990 v473 a<p>We report the synthesis of vinyl modified magnesium silsesqiuoxanes (''vinyl clay''), 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.</p>1 aKumaraswamy, Guruswamy1 aDeshmukh, Yogesh, S.1 aAgrawal, Vikrant, V.1 aNisal, Anuya  uhttps://library.ncl.res.in/content/composites-polypropylene-layered-mg-silsesquioxanes-show-unusual-combination-properties-002476nas a2200205   4500008004100000022001400041245009900055210006900154260011000223300001400333490000700347520165000354100001702004700002402021700003002045700001502075700002602090700002702116856012702143       2015                        eng d  a1744-683X00aCompact polar moieties induce lipid-water systems to form discontinuous reverse micellar phase0 aCompact polar moieties induce lipidwater systems to form discont  aTHOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLANDbROYAL SOC CHEMISTRYcMAY  a5417-54240 v113 a<p>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.</p>1 aKumar, Manoj1 aPatil, Naganath, G.1 aChoudhury, Chandan, Kumar1 aRoy, Sudip1 aAmbade, Ashootosh, V.1 aKumaraswamy, Guruswamy  uhttps://library.ncl.res.in/content/compact-polar-moieties-induce-lipid-water-systems-form-discontinuous-reverse-micellar-001913nas a2200169   4500008004100000022001400041245004100055210004100096260011000137300001000247490000800257520133000265100002701595700001801622700003001640856007301670       2016                        eng d  a1359-664000aColloidal assembly by ice templating0 aColloidal assembly by ice templating  aTHOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLANDbROYAL SOC CHEMISTRYcOCT  a61-760 v1863 a<p>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.</p>1 aKumaraswamy, Guruswamy1 aBiswas, Bipul1 aChoudhury, Chandan, Kumar  uhttps://library.ncl.res.in/content/colloidal-assembly-ice-templating02767nas a2200265   4500008004100000022001400041245005700055210005700112260000800169490000700177520201300184653001302197653002002210653002202230653001902252653001302271653000902284653002002293653001702313653000902330100002802339700001802367700002702385856008902412       2017                        eng d  a1744-683X00aCapillary uptake in macroporous compressible sponges0 aCapillary uptake in macroporous compressible sponges  cSEP0 v133 a<p><span style="color: rgb(51, 51, 51); font-family: arial, helvetica, sans-serif; font-size: 13px; background-color: rgb(248, 248, 248);">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.</span></p>10aaerogels10aBuilding blocks10aCatalyst Supports10aContact-angels10aDynamics10aflow10aOrganic Liquids10aPorous-Media10aRise1 aChatterjee, Soumyajyoti1 aDoshi, Pankaj1 aKumaraswamy, Guruswamy  uhttps://library.ncl.res.in/content/capillary-uptake-macroporous-compressible-sponges02461nas a2200181   4500008004100000022001400041245010600055210006900161260000800230300001400238490000700252520181400259100001902073700001702092700002002109700002702129856012302156       2019                        eng d  a0024-929700aCharacterizing microvoids in regenerated cellulose fibers obtained from viscose and lyocell processes0 aCharacterizing microvoids in regenerated cellulose fibers obtain  cJUN  a3987-39940 v523 a<p>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.</p>
1 aSharma, Aakash1 aSen, Debasis1 aThakre, Shirish1 aKumaraswamy, Guruswamy  uhttps://library.ncl.res.in/content/characterizing-microvoids-regenerated-cellulose-fibers-obtained-viscose-and-lyocell04684nas a2200157   4500008004100000245010700041210006900148300001400217490000700231520406800238100002104306700002304327700002104350700002704371856012804398       2019                        eng d00aCritical role of processing on the mechanical properties of cross-linked highly loaded nanocomposites 0 aCritical role of processing on the mechanical properties of cros  a5955-59620 v523 a<p>Polymer <span class="hitHilite">nanocomposites</span> are frequently not at equilibrium-therefore, <span class="hitHilite">the</span> structure and <span class="hitHilite">properties</span> critically depend <span class="hitHilite">on</span> <span class="hitHilite">the</span> protocol followed in their preparation. Here, we demonstrate that <span class="hitHilite">the</span> elastic-brittle transition in <span class="hitHilite">highly</span> <span class="hitHilite">loaded</span> ice-templated polymer <span class="hitHilite">nanocomposites</span> is sensitively determined by <span class="hitHilite">the</span> conformation <span class="hitHilite">of</span> polymer chains during nanocomposite preparation. Macroporous polymer <span class="hitHilite">nanocomposites</span> are synthesized using two preparation pathways, both exhibiting minor modifications <span class="hitHilite">of</span> <span class="hitHilite">the</span> ice-templating method wherein an aqueous dispersion <span class="hitHilite">of</span> nanoparticles, polymers, and cross-linkers is frozen. In one method, <span class="hitHilite">the</span> matrix polymer is <span class="hitHilite">cross-linked</span> in <span class="hitHilite">the</span> presence <span class="hitHilite">of</span> ice, whereas in <span class="hitHilite">the</span> other method, <span class="hitHilite">the</span> ice is removed by freeze-drying before <span class="hitHilite">the</span> polymer is <span class="hitHilite">cross-linked</span>. Although all measurable structural metrics are practically identical for <span class="hitHilite">the</span> two composites, they exhibit qualitatively different <span class="hitHilite">mechanical</span> <span class="hitHilite">properties</span>. <span class="hitHilite">Nanocomposites</span> <span class="hitHilite">cross-linked</span> in <span class="hitHilite">the</span> presence <span class="hitHilite">of</span> ice show exceptional resilience, recovering elastically from large compression for very high nanoparticle loadings-in some instances, above 90% by weight. Here, <span class="hitHilite">the</span> <span class="hitHilite">critical</span> particle loading for <span class="hitHilite">the</span> elastic- brittle transition is dependent <span class="hitHilite">on</span> <span class="hitHilite">the</span> molecular weight <span class="hitHilite">of</span> <span class="hitHilite">the</span> polymer. In comparison, samples <span class="hitHilite">cross-linked</span> after freeze-drying appear structurally identical but turn brittle at much lower particle loadings (about 60% by weight), independent <span class="hitHilite">of</span> <span class="hitHilite">the</span> matrix polymer molecular weight. We rationalize these differences in terms <span class="hitHilite">of</span> <span class="hitHilite">the</span> conformational state <span class="hitHilite">of</span> <span class="hitHilite">the</span> polymer during cross- linking. In <span class="hitHilite">the</span> former case, polymer chains are in good solvent during nanocomposite preparation, while after lyophilization, they are in a poor solvent. <span class="hitHilite">The</span> large spatial extent <span class="hitHilite">of</span> well-solvated chains and chain-chain overlap during cross-linking results in <span class="hitHilite">the</span> formation <span class="hitHilite">of</span> intermolecular cross-links, and we obtain elastic <span class="hitHilite">nanocomposites</span> 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 <span class="hitHilite">the</span> sensitivity <span class="hitHilite">of</span> polymer conformations to different <span class="hitHilite">processing</span> methods can result in large differences in <span class="hitHilite">properties</span>, even though their consequences <span class="hitHilite">on</span> structural characteristics are effectively indistinguishable.</p>
1 aSuresh, Karthika1 aChowdhury, Arindam1 aKumar, Sanat, K.1 aKumaraswamy, Guruswamy  uhttps://library.ncl.res.in/content/critical-role-processing-mechanical-properties-cross-linked-highly-loaded-nanocomposites02665nas a2200193   4500008004100000022001400041245005300055210005300108260000900161300001400170490000700184520208800191100001802279700001802297700002302315700002102338700002702359856008502386       2021                        eng d  a1744-683X00aColloidal assembly by directional ice templating0 aColloidal assembly by directional ice templating  cAPR   a4098-41080 v173 a<p>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 &gt; 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 ``closes up'' so that the particles cluster to form aggregates, crystallization protocol-induced differences in the geometry of these ``closed up'' spaces determine the scaling behaviour of P-n.</p>1 aBiswas, Bipul1 aMisra, Mayank1 aBisht, Anil, Singh1 aKumar, Sanat, K.1 aKumaraswamy, Guruswamy  uhttps://library.ncl.res.in/content/colloidal-assembly-directional-ice-templating