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

We have used nuclear reaction analysis to measure diffusion coefficien ts 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 c hains and the mean vinyl fraction x varies between 0.38 and 0.94. We f ind 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 ent anglement spacing] decreases monotonically with increasing vinyl conte nt x. Over the range of microstructures and temperatures T (-14-40 deg rees 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 melt s. 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 W iley and Sons, Inc.