SELF-DIFFUSION IN MELTS OF STATISTICAL COPOLYMERS - THE EFFECT OF CHANGES IN MICROSTRUCTURAL COMPOSITION

We have used nuclear reaction analysis to measure diffusion coefficients D in couples consisting of hydrogenated polybutadienes of structure (C2H3(C2H5))(x)(C4H8)(1-x) and their partly deuterated counterparts. The 1,2- and 1,4-olefinic isomers are randomly distributed along the chains and the mean vinyl fraction x varies between 0.38 and 0.94. We find that the effective monomeric mobility D-0 [defined by D = D-0(N-e/N-2) for each copolymer, where N is the backbone length and N, the entanglement spacing] decreases monotonically with increasing vinyl content x. Over the range of microstructures and temperatures T (-14-40 degrees C) investigated we find log(D-0/T) varies smoothly with (T - T-g), where T-g is the glass transition temperature of the respective melts. An analysis of our data in terms of a simple activated rate process model suggests that D-0 is controlled by thermally activated hopping of segments whose effective volume is close to that of the respective statistical segment lengths of the copolymeric chains. (C) 1995 John Wiley and Sons, Inc.