DYNAMICS OF DIPOLE-INVERTED CIS-POLYISOPRENE CHAINS IN A MATRIX OF LONG, ENTANGLING CHAINS - 2 - EFFECTS OF CONSTRAINT RELEASE ON THE COHERENCE OF THE SUBCHAIN MOTION

For cis-polyisoprene (PI) chains having dipoles parallel along their b ackbone, viscoelastic relaxation reflects orientational anisotropy of subchains (stress-generating units) at respective times while the diel ectric relaxation reflects orientational correlation of the subchains at two separate times. This difference between viscoelastic and dielec tric relaxation processes enables us to examine the short-time coheren ce of subchain motion in individual chains through comparison of these processes. Specifically, for the two extreme cases of perfectly coher ent or incoherent subchain motion, viscoelastic moduli G(coh) and G(i ncoh) are explicitly calculated from the relaxation times tau(p) and eigenfunctions f(p) defined for a local correlation function describin g fundamental features of the dielectric relaxation. On the basis of t hese backgrounds, the G(coh) and G(incoh)* were calculated for a PI c hain (M = 48800) dilutely blended in a high-M entangling polybutadiene (PB) matrix (M = 263000). (The tau(p) and f(p) data necessary for thi s calculation were obtained dielectrically in Part 1 of this series of papers.) The G(coh) was in excellent agreement with the G* data of t he PI chain while G(incoh) was significantly different from the data, meaning that the subchain motion was highly coherent for the PI chain in the high-M PB matrix. In contrast, the subchain motion of the same PI chain was found to be incoherent in an entangling PB matrix of sma ller M (=9240). The constraint release mechanism made a negligible con tribution to the global dynamics of the PI chains in the high-M matrix while it dominated the dynamics in the low-M matrix. These results in dicate that the constraint release is an important factor that determi nes the degree of coherence of the subchain motion.