Polymer Gels and Networks最新文献

A Monte Carlo study on the degradation of highly branched polymer molecules is presented. The starting molecules were regular dendrimers with 10⁵ units. The algorithm of simulations was similar to that used for modeling degradation of polymer networks described in the first part of the series. In the degradation of the large acyclic molecules, similarly as for networks, an abrupt transition was observed. It resembled a reversed sol–gel transition. Its position on conversion scale as well as other size distribution related parameters of the system depended on the kinetics of bond scission.

Effects of glucose, mannose and konjac glucomannan with different molecular weights on the sol–gel transition in gellan gum aqueous solutions were studied by dynamic viscoelastic measurement and differential scanning calorimetry. Temperature dependence of the loss shear modulus G″ for a 1% gellan gum solution without sugars showed a one-step-like change at a certain temperature, which was attributed to the helix–coil transition temperature T_hc. On addition of sugar, storage shear modulus G′ and loss shear modulus G″ increased and T_hc shifted to higher temperatures. With progressive addition of sugar, the cross-over of G′ and G″ was observed at a lower temperature than T_hc, which was attributed to the sol–gel transition T_sg. Moreover, in the presence of sufficient sugar, both moduli did not decrease remarkably during heating up to 60 °C, although both moduli showed a steep increase around 50 °C during cooling process. In the presence of sugar of fairly low concentration, DSC measurement showed that both exothermic and endothermic peak temperatures, T_s and T_m, slightly shifted to lower temperatures, and that exothermic and endothermic enthalpies, ΔH_s and ΔH_m, significantly decreased. In contrast, rheological measurement showed that elastic moduli and the relaxational strength ΔG significantly increased by the addition of a sugar of fairly low concentration. However, further addition of sugar shifted T_s and T_m to higher temperatures and increased ΔH as well as elastic moduli. In the presence of sufficient sugar, both cooling and heating DSC curves became broader; however, many small peaks were observed at both higher and lower temperatures. Gellan with sufficient sugar formed various junction zones with different thermal stabilities during cooling, and then these junction zones became markedly heat resistant. In the presence of konjac glucomannan with relatively lower molecular weight, the effective concentration of gellan gum increases by immobilising water molecules in gellan gum solutions, so that konjac glucomannan shifts the helix– coil transition temperature in gellan gum molecules to higher temperatures.

Interpenetrating polymer networks (IPNs) were prepared by the sequential mode of synthesis using epoxy resin as the plastomer and poly (n-butyl acrylate) as the elastomer components, respectively. The full and semi IPNs of these two constituents were characterized with respect to their mechanical properties like tensile strength, elongation at break, modulus and toughness. Resistance to thermal degradation of the IPNs was compared to that of pure epoxy resin by conventional thermogravimetric analysis. The glass transition behaviours of some of the IPNs of varying compositions were compared by using differential scanning calorimetry. Scanning electron microscopy (SEM) and polarized light microscopy (PEM) were used to study the fracture mechanics during tensile deformation and the surface morphology, respectively. The mode of changes in the above mentioned properties with changes in IPN compositions was studied. A gradual decrease in modulus and tensile strength with consequent increases in percent elongation at break and toughness was observed as the proportions of elastomeric PBA were increased steadily. The full IPNs however had higher modulus and tensile strength in comparison to the semi IPNs of corresponding compositions. The thermogravimetric results indicated that instead of a stepwise unzipping process of depolymerization as observed with various methacrylates, a C–C bond scisson occurred without formation of free radicals in the case of PBA. DSC tracings exhibited inward shifting of the glass transition temperature and a broadening transition. The morphology suggested by SEM indicates a decrease in shear yielding with consequent increase in crazing mode of failure as the mechanism of fracture. Two particle size system is indicated by the polarized light microscopy.

We have synthesized a range of polymer networks containing varying amounts of the flexible molecule 1,3-divinyltetramethyldisiloxane (B₂) and the rigid mesogenic molecule, di-(4 oxyallyl-(methyl)-phenyl)4-terephthalate. Networks were prepared using hydrosilylation chemistry, by crosslinking the vinyl terminated B₂ and the mesogen, with the tetrafunctional crosslinker tetrakis(dimethylsiloxy) silane (A₄). The elastic modulus (G_e) increased with the incorporation of the rigid mesogen into the network structure, and also increased as the ratio of silane hydrogens (Si–H) to vinyl groups in the preparation was increased from 1 to 1.5. Complementary swelling experiments in toluene showed the soluble fraction (w_sol) decreased with an increase in this ratio. Also, a maximum value of the polymer volume fraction, υ₂, was obtained with the stoichiometry r=1.5. Network properties were analyzed using the Flory–Rehner expression from which values of the average interaction parameter, χ, were extracted.

Dynamic viscoelastic measurements were made on ovalbumin gels prepared by two different thermal denaturation methods for a study of their gel structures. Results are discussed in the light of the fractal concept developed for the critical gel structure of synthetic polymers at the sol–gel transition point. Especially, a role of added salt on association kinetics of denatured proteins as well as on the resulting gel structures are described in detail.

The incorporation of an electronically conducting polyelectrolyte (poly(2-methoxyaniline-5-sulfonic acid)[PMAS]) as the dopant in polypyrrole results in the formation of a new class of materials. The resultant polymer structure has high water content (>90%), reasonable electronic conductivity (5–8 S cm^-1) and multiple switching capabilities. The latter arises from the fact that both the polypyrrole and the PMAS are redox active.