RELATIONSHIP BETWEEN MICROSTRUCTURE AND PHASE AND RELAXATION TRANSITIONS IN ETHYLENE-(VINYL ACETATE) COPOLYMERS PREPARED BY EMULSION COPOLYMERIZATION

The relaxation and phase transitions in ethylene-(vinyl acetate) (EVA) copolymers prepared by emulsion copolymerization were studied. Substa ntial deviations of the basic characteristics of the transitions (glas s transition temperature, T-g, melting temperature, T-m, and degree of crystallinity, W-c) from those known for commercial products and, to a greater extent, for low-conversion copolymers synthesized in solutio n were found. These deviations were explained by the effect of the mic rostructure and particularly by the configurational heterogeneity of t he copolymer chains. Thus, different fractions of EVA copolymer fracti onated according to the chemical composition revealed values of T-g, T -m and W-c suggesting the existence of two parts in the copolymer chai ns. The comonomer units in one part show a tendency to alternate while the ethylene units in the other part show an enhanced tendency to for m blocks. A similar model of the microstructure of the copolymers stud ied was presented in our previous communication where the distribution of the comonomer units along the polymer chains was studied by means of C-13 n.m.r. spectroscopy. It is now established that the overall ch emical composition of the copolymer and its fractions studied, calcula ted based on the experimental values of T-g, T-m and W-c, fitted fairl y well with the composition found experimentally, thus confirming the correctness of the model of the microstructure proposed. (C) 1998 Else vier Science Ltd. All rights reserved.