Publications (5) View all
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Article: Thermoreversible Gelation of Poly(vinylidene fluoride-co-chlorotrifluoroethylene): Structure, Morphology, Thermodynamics, and Theoretical Prediction
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ABSTRACT: Thermoreversible gelation of a copolymer, poly(vinylidene fluoride-co-chlorotrifluoroethylene), has been studied in a series of aromatic diesters (phthalates) with varying aliphatic chain length, n. The gelation rate gradually increases with increasing n, but no gelation occurs in dioctyl phthalate (n = 8), giving rise to a solvent dependency. Structures of gels and dried gels have been studied through X-ray diffraction and FTIR studies. Fibrillar morphology is evident for the series of solvents, but its dimension (both lateral and diameter) systematically changes with n. Solvent retention power of gels has gradually been increased with increasing n while the thermal degradation of copolymer occurs at same temperature, reflecting varying interactions between copolymer and various aliphatic chain length phthalates in gels. Phase diagrams of the gels exhibit the formation of two kinds of polymer−solvent complexes and have been predicted theoretically through electronic structure calculation. Both the small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) have been performed to elucidate the structure of fibril indicating gradual changes in lamellar organization. The experimental data have been fitted with different models and power law to check their validity. Molecular modeling has been carried out to understand the nature of interaction between copolymer and the solvent through energy minimization program and found a typical n = 6 value of lowest dipolar distance, indicating strongest interaction. The magic number of six has further been explored through quantum chemical calculations.03/2011; -
Article: Solvent retention, thermodynamics, rheology and small angle X-ray scattering studies on thermoreversible poly(vinylidene fluoride) gels.
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ABSTRACT: Solvent retention power of poly(vinylidene fluoride) (PVDF) gels has been studied for various homologues of phthalate (aromatic diesters). The thermal stability has been examined for gels of varying morphology. Solvent evaporation, gelation, gel melting, and polymer degradation temperatures have increased with increasing aliphatic chain length of phthalates. The thermodynamics and polymer-solvent compound formations in the PVDF-phthalate gels have been explored. The weight fraction of polymer in compound has decreased with increasing aliphatic chain length. SAXS studies have confirmed the lamellar organization inside the fibrils, and interlamellar distance increases with aliphatic chain length of diesters. The scattering patterns follow the power law behavior (I(q) approximately q(-alpha)), and polymer gels consist of high-density mass (fibril), voids, and interlamellar region. Dynamic mechanical properties indicate the splintering and reformation of network structure in gels whose percolation frequency has reduced for higher aliphatic chain length phthalate. Morphology-dependent moduli have been observed, and greater mechanical strength has been verified for thicker fibrillar gels both for steady and dynamic measurements.The Journal of Physical Chemistry B 09/2010; 114(35):11420-9. · 3.70 Impact Factor -
Article: Thermoreversible gelation of poly(vinylidene fluoride-co-hexafluoro propylene) in phthalates.
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ABSTRACT: The thermoreversible gelation of poly(vinylidene fluoride-co-hexafluoro propylene) copolymer have been studied in a series of phthalates, Ph-(COO C(n)H(2n+1))(2) with n = 1-8. The gelation rate increases with increasing aliphatic chain length up to n = 6, and the gelation phenomena does not occur for higher n > 6. The fibrillar morphology is evident for dried gels whose dimension (both lateral and thickness) becomes shorter and thinner with increasing n. The structures of the gels formed in various phthalates have been investigated by small-angle neutron scattering and small-angle X-ray scattering techniques, suggesting sheet-like structure, where the interplanner distance increases with increasing aliphatic chain length. The scattering intensity I(q) decreases with q according to the Ornstein-Zernike model, where q = (4pi/lambda) sin theta (2theta and lambda are scattering angle and wavelength of neutron) and the correlation length, xi, assigned to the average distance between the neighboring crystallites, also increases with increasing aliphatic chain length of diesters. The detailed thermal analyses and phase diagrams of the copolymer gels have been studied in a wide range of phthalates. Further, polymer-solvent complexes leading to the formation of two distinct compounds have been reported. A systematic change of compound composition has also been observed in the whole range of phthalates studied here. On the basis of electronic structure calculation, a model has been proposed to elucidate the conformation of copolymer chain in presence of various phthalates and their complexes, which offer the cause of higher gelation rate for longer aliphatic chain length up to n = 6, no gelation phenomena occurs for n > 6, and formation of two copolymer-solvent compounds. The mechanical properties (storage modulus and viscosity) decrease with increasing aliphatic chain length of phthalates and realignment of fibrils occurs at particular frequency depending on the strength of fibrillar gels.The Journal of Physical Chemistry B 10/2009; 113(41):13516-25. · 3.70 Impact Factor -
Article: Biodegradable nanocomposites of poly(hydroxybutyrate-co-hydroxyvalerate): the effect of nanoparticles.
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ABSTRACT: Copolymer of hydroxybutyrate and hydroxyvalerate, P(HB-HV)/layered silicate or hydroxyapatite nanocomposites were prepared via melt extrusion. The nanostructure, as observed from wide-angle X-ray diffraction and transmission electron microscopy, indicate intercalated hybrids for layered silicates. Hydroxyapatite of nanometer dimension is uniformly distributed in matrix copolymer. The nanohybrids show significant improvement in thermal and mechanical properties of the copolymer as compared to the neat copolymer. The layered silicate nanocomposites exhibit superior mechanical properties as compared to hydroxyapatite nanohybrid. The thermal expansion coefficient is significantly reduced in nanohybrids. The biodegradability of pure copolymer and its nanocomposites were studied at room temperatures under controlled conditions in compost media. The rate of biodegradation of copolymer is enhanced dramatically in the nanohybrids. Hydroxyapatite hybrid shows highest rate of biodegradation. The change in biodegradation is streamlined in terms of nature of nanoparticles used to prepare hybrids.Journal of Nanoscience and Nanotechnology 05/2008; 8(4):1858-66. · 1.56 Impact Factor -
Article: Thermoreversible gelation of poly(vinylidene fluoride) in phthalates: the influence of aliphatic chain length of solvents.
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ABSTRACT: Thermoreversible gelation of poly(vinylidene fluoride) (PVDF) has been studied in a new series of solvents (phthalates), for example, dimethyl phthalate (DMP), diethyl phthalate (DEP), dibutyl phthalate (DBP), and dihexyl phthalate (DHP) as a function of temperature and polymer concentration, both by test tube tilting and dynamic light scattering (DLS) method. The effect of aliphatic chain length (n) of diesters on the gelation kinetics, structure/microstructure and morphology of PVDF gels has been examined. Gelation rate was found to increase with increasing aliphatic chain length of diester. DLS results indicate that the sol-gel transformation proceeds via two-steps: first, microgel domains were formed, and then the infinite three-dimensional (3D) network is established by connecting microgels through polymer chains. The crystallites are responsible for 3D network for gelation in phthalates, and alpha-polymorph is formed during gelation producing higher amount of crystallinity with increasing aliphatic chain length of diester. Morphology of the networks of dried gels in different phthalates showed that fibril thickness and lateral dimensions decrease with higher homologues of phthalates. The scattering intensity is fitted with Debye-Bueche model in small-angle neutron scattering and suggested that both the correlation length and interlamellar spacing increases with n. A model has been proposed, based on electronic structure calculations, to explain the conformation of PVDF chain in presence of various phthalates and their complexes, which offer the cause of higher gelation rate for longer aliphatic chain length.The Journal of Physical Chemistry B 05/2008; 112(15):4594-603. · 3.70 Impact Factor
