DIELECTRIC-RELAXATION OF DIPOLE-INVERTED CIS-POLYISOPRENE SOLUTIONS

Dielectric relaxation behavior was examined for solutions of a series of cis-polyisoprene (PI) chains having almost identical molecular weig hts (M congruent to 48K) but differently inverted type-A dipoles paral lel along the chain contour. The solvent used was a dielectrically ine rt butadiene oligomer of M = 0.7K that was a moderately good solvent f or the PI chains. Global motion of the dipole-inverted PI chains induc ed prominent dielectric relaxation at low frequencies, and the dielect ric loss (epsilon '') curves changed their shape (frequency dependence ) with increasing PI concentration c(PI): At c(PI) smaller than the ov erlapping threshold c the epsilon '' curves were rather sharp and rea sonably close to the prediction of the Tschoegl model considering both hydrodynamic and excluded-volume interactions, while for c(PI) > c t he distribution became considerably broader than that predicted from t he Rouse/Zimm/Tschoegl and reptation models. These dielectric changes were quite similar to those found in previous studies for PI solutions in Isopar-G and heptane. The shape of the epsilon '' curves reflected distribution of both relaxation times and intensities of dielectric m odes, while those times and intensities were separately determined by characteristic times tau(p) and integrated eigenfunctions F-p(n) for e igenmodes of a local correlation function, C(n,t;m) = [(u(n,t) . u(m,0 )]/[(u(2)], with u(n,t) being the nth bond vector at time t. For detai led examination of the above dielectric changes with c(PI), the epsilo n '' data of the dipole-inverted PI chains were analyzed to evaluate F -p(n) and tau(p) (for the eigenmode index p = 1-3). At c(PI) less than or equal to c, tau(p) were almost proportional to p(-1.65) and F-p(n ) were nearly sinusoidal with respect to the segment location n. These results were in reasonable agreement with the Tschoegl model. With in creasing c(PI) > c, tau(p) became almost proportional to p(-2) while F-p(n) became nonsinusoidal. This p dependence of tau(p) was still in close agreement with the Rouse and/or reptation models, but the n depe ndence of F-p(n) was considerably different. This result indicated tha t the broadening of the epsilon '' curves with c(PI) was essentially d ue to changes in the eigenfunctions, not due to changes in the p depen dence of tau(p) (relaxation time span). Changes of F-p(n) with c(PI) w ere discussed in relation to coupling of motion pf chains being overla pped at c(PI) > c.