@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 {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 {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 {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 {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 {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 {49575, title = {SANS and SAXS investigation of the melt state structure in disentangled ultrahigh molecular weight polyethylene}, journal = {ACS Macro Letters}, volume = {14}, year = {2025}, month = {MAR}, pages = {349-353}, type = {Article}, abstract = {

Disentangled ultrahigh molecular weight polyethylene exhibits a time-dependent increase in rheology modulus when molten. This originates from its kinetically evolving heterogeneous microstructure consisting of disentangled and entangled regions. We report a quantitative analysis of this microstructure using X-rays and neutrons that capture the signatures of these regions. We analyze the absolute intensities to obtain the volume fraction and size distribution of the disentangled domains in the melt. Employing neutrons, we follow the changes in these parameters with time. The trends are qualitatively similar to those of the previous rheological observations. Our methodology also provides an experimental verification of the theoretical report by McLeish, T. C. B. Soft Matter 2007, 3 (1), 83-87, which predicts the presence of high density disentangled domains in a low density entangled matrix. The analysis presented here is a useful instrument for unveiling the origin of differences in the properties of polymers obtained through different processing routes.

}, doi = {10.1021/acsmacrolett.5c00100}, author = {Sharma, Aakash and Kruteva, Margarita and Willner, Lutz and Romano, Dario and Porcar, Lionel and Dulle, Martin and Zhou, Fuhai and Rastogi, Sanjay and Richter, Dieter} } @article {50079, title = {Dynamic janus hydrogen bond mimicry unlocks tough, flexible supramolecular elastomers for strain sensing}, journal = {Chemistry of Materials}, volume = {38}, year = {2026}, month = {MAR}, pages = {2699-2711}, type = {Article}, abstract = {

Flexible and wearable electronics demand stretchable sensors with polymer elastomers as key matrixes for mechanical flexibility and durability. However, despite their excellent elasticity, their limited mechanical strength remains a challenge. To address this limitation, in this study, we report the rational design of supramolecular polyurethane elastomers (SPUs) incorporating nucleobase-inspired aminopyrimidinedione with DDA-AAD (G-C mimic) reversible triple hydrogen bonds. This dual-domain architecture gives rise to a durable supramolecular network with enhanced mechanical properties, yielding elastomers that are soft, stretchable, and tough. By tuning of the density of dynamic cross-links, mechanical properties were systematically modulated. SPU-0.5 exhibited a maximum tensile strength of 16.14 MPa, representing a 67-fold strength enhancement over that of SPU-0. Although increasing the aminopyrimidinedione (APD) content reduced elongation, SPU-0.2 retained a high elongation of 1060\% and showed the lowest residual strain during cyclic tests. To be of great interest, the activation energy increased with increasing hydrogen bonding content up to SPU-0.1, whereas beyond SPU-0.2 it decreased, likely due to extensive hydrogen bond formation. Furthermore, SPU-0.2-SP, a conductive variant, demonstrated a promising strain-sensing performance even after hundreds of cycles. Overall, the insights gained from this study advance the development of intelligent soft materials and lay the groundwork for next-generation flexible and wearable electronic devices.

}, issn = {0897-4756}, doi = {10.1021/acs.chemmater.5c02829}, author = {Lakshmi, Durga and Wagh, Mahendra A. and Sharma, Aakash and Alam, Md Shafi and Selvan, T. Muthamil and Torris, Arun and Mondal, Titash and Sanjayan, Gangadhar J. and Nair, Kiran Sukumaran} } @article {50049, title = {Simple two-step gelation process to strengthen 3D printed carboxy methyl cellulose gels}, journal = {Cellulose}, volume = {33}, year = {2026}, month = {MAR}, pages = {2015-2030}, type = {Article}, abstract = {

Carboxymethyl cellulose (CMC) is a water-soluble cellulose ether that is widely utilized in hydrogel applications due to its exceptional water absorption and biocompatibility. This study reports a simple two step strategy to make mechanically robust CMC gels without any additional chemical crosslinkers and how it could be leveraged to strengthen 3D printed CMC gels. These hydrogels, prepared through acid-induced gelation followed by freeze-thawing, exhibited a compressive modulus of 12.4 +/- 0.92 kPa and a compressive strength of 491.0 +/- 18.1 kPa. They could be leveraged for strengthening 3D printing hydrogels of intricate shapes by direct ink writing. Small amplitude oscillatory shear (SAOS) tests indicated an order of magnitude higher storage modulus for freeze-thawed gels (FC) as compared to only acid-induced CMC gels (HCMC). Nonetheless, stress relaxation experiments revealed that FC, as well as HCMC gels, relax at similar time scales. FC gels exhibited clear birefringence under crossed polarizers, indicating molecular ordering that is consistent with the presence of ordered/crystalline domains. Such ordered domains likely contribute to the higher elastic modulus and compressive strength observed in FC gels as compared to HCMC gels.

}, keywords = {3D printing, Carboxymethyl cellulose, cellulose, Gels}, issn = {0969-0239}, doi = {10.1007/s10570-026-06963-y}, author = {Rajput, Tejas and Sathe, Asmi and Gopal, Animesh and Sharma, Aakash and Shanmuganathan, Kadhiravan} }