MISCIBILITY IN BLENDS OF BRANCHED POLYMERS

The effect of branching on miscibility has been studied in model syste ms of chlorinated branched polyethylenes. The branched polymer molecul e was represented as a copolymeric chain composed of end, linear inter ior, and branch-point segmental units, each of which could be either c hlorinated or unchlorinated. An expression was derived for the overall interaction parameter for blends of the branched chlorinated polyethy lenes based on counting intra- and interchain pair interactions betwee n segmental units. Theoretically, when branching is unfavorable for mi xing, the miscibility regime in a branched polymer/branched polymer bl end (as represented in an isothermal copolymer-copolymer composition p lot) is smaller than that of the corresponding linear polymer/linear p olymer blend. When branched polymers are mixed with linear polymers, t he symmetry of the miscibility regime in such a plot found with branch ed/branched or linear/linear blends disappears. The theoretical develo pment was used in an analysis of the phase behavior of 50/50 wt % blen ds of amorphous CPE(B)/CPE(B), CPE(L)/CPE(SB), etc., where CPE(L), CPE (SB), and CPE(B) represent chlorinated linear, short-chain branched, a nd long-chain branched polyethylene, respectively. Miscibility of thes e blends was studied by using standard differential scanning calorimet ry. The results support the theory qualitatively and show that the CPE (L) and CPE(SB) present similar miscibility behaviors because the shor t-chain branches only slightly disfavor mixing. However, CPE(B) genera lly has more limited miscibility; the long-chain branches have a rathe r profound effect on mixing.