Analysis of chain configuration in semi-crystalline random copolymers
Quantitative analyses of configurational defects and chain distribution in the semi-crystalline random copolymers have been established in order to understand the structure-property relationship of these polymeric materials. Poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)), Ethylene-vinyl acetate (EVA) and poly(methyl methacrylate-co-nbutyl methacrylate) ((P(MMA-nBMA) copolymers have been studied. Structural analyses of P(VDF-HFP) copolymer is motivated by its application in drug-eluting stent as a coating material. The copolymerization of bulky HFP unit into the VDF chains influences the crystallizable segments of PVDF. These configurational defects can be correlated to the structure evolution as the function of storage time and temperature. Using the spectroscopic techniques such as infrared (IR), Raman and nuclear magnetic resonances (NMR), the configuration and conformation of P(VDF-HFP) copolymers have been analyzed. The thermal fractionation called Successive Self-nucleation/Annealing (SSA) method is adopted in order to investigate the configurational defects on the crystallizable chain sequences of P(VDF-HFP) copolymers. From the results, the configurational defects introduced by HFP units have been correlated to the multiple thermal transitions. In addition, it is interesting to find that the thermal fractionation can induce a different crystalline conformation in P(VDF-HFP) copolymers.
Analysis of chain configuration and crystallizable segmental distribution in ethylene-vinyl acetate (EVA) copolymers is motivated by development of hot-melt adhesives composed mainly of EVA-Wax-tackifier multicomponents blend. In this study, the main focus is on understanding co-crystallization between ethylene segments with paraffins. Using the same thermal fractionation technique used for the characterization of P(VDF-HFP) copolymers, EVA copolymers are also studied. Due to the wide distribution of ethylene segments in EVA, it is expected that the n-alkanes of matching length can cocrystallize with EVA. The presence of co-crystallization is observed by the enhancement in crystallization and faster crystallization kinetics in the binary blends. It has been determined by Infrared spectroscopy by observing changes in the crystalline form and intermolecular interaction in the crystalline unit cell. From the results, the mechanism of co-crystallization has been proposed.
Influence of copolymer configuration on the crystallization and phase behavior of ternary blends is also of great interest in order to develop polyurethane-based hot melt adhesives. The phase behavior of various ternary polymer blends containing crystallizable polyester, a non-crystallizable polyether, and an acrylic random copolymer of different chain configuration is investigated. The mean-field Flory-Huggins theory for the free energy of mixing, extended to ternary polymer blends, is adopted for predicting phase diagrams. The differences observed in the rheological processes of various ternary blends with different acrylic copolymers are directly related to changes in miscibility, associated phase behavior and chain configuration.