DIELECTRIC-SPECTROSCOPY ON DILUTE BLENDS OF POLYISOPRENE POLYBUTADIENE - EFFECT OF THE MATRIX POLYDISPERSITY ON THE DYNAMICS OF PROBE POLYISOPRENE/

We report dielectric normal-mode relaxation of probe cis-polyisoprenes (PI) in dilute blends of PI and polybutadiene (PB) with broad molecul ar weight distributions (MWD). The PB matrixes included two binary PB blends of narrow-MWD PBs and one mixture of seven narrow-MWD PB fracti ons. Results were compared with our previous results on narrow-MWD PI/ PB blends (Macromolecules 1995, 28, 3588) in which the double-logarith mic plot of the longest relaxation time tau of PIs against their molec ular weights, M(I), conformed to a straight line with the slope varyin g from 2 (the Rouse behavior) to 3 (pure reptation) with increasing MW , M(B), of the matrix PBs. The plots for the broad-MWD blends also con formed to straight lines, indicating that the broad-MWD matrix is equi valent to a narrow-MWD matrix of an equivalent molecular weight M(equi v). However, as compared with the same weight-averaged M(B), the slope s are slightly smaller than those of the narrow-MWD blends, suggesting that the small MW matrix chains accelerate the rate of relaxation of the probe chain. The behavior cannot be explained quantitatively by th e constraint-release model proposed by Graessley. We also examined the shape of the dielectric loss epsilon '' versus frequency f curve. All epsilon '' curves observed here are broader than the prediction of th e Rouse and Doi-Edwards theories which predict that the slope of the l og epsilon '' vs log f curve is -0.5 on the high-frequency side of the loss peak.