Dielectric Relaxation of Dipole-Inverted cis-Polyisoprene Solutions

Abstract

Dielectric relaxation behavior was examined for solutions of a series of cis-polyisoprene (PI) chains having almost identical molecular weights (M ≃ 48K) but differently inverted type-A dipoles parallel along the chain contour. The solvent used was a dielectrically inert butadiene oligomer of M = 0.7K that was a moderately good solvent for the PI chains. Global motion of the dipole-inverted PI chains induced prominent dielectric relaxation at low frequencies, and the dielectric loss (e) curves changed their shape (frequency dependence) with increasing PI concentration c PI : At c PI smaller than the overlapping threshold c * the e curves were rather sharp and reasonably close to the prediction of the Tschoegl model considering both hydrodynamic and excluded-volume interactions, while for c PI > c * the distribution became considerably broader than that predicted from the 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 e curves reflected distribution of both relaxation times and intensities of dielectric modes, while those times and intensities were separately determined by characteristic times τ p and integrated. eigenfunctions F p (n) for eigenmodes of local correlation function, C(n,t ;m) = / , with u(n,t) being the nth bond vector at time t. For detailed examination of the above dielectric changes with c PI , the e data of the dipole-inverted PI chains were analyzed to evaluate F p (n) and τ p (for the eigenmode indexp = 1-3). At c PI ≤ c * , τ 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 increasing c PI > c * , τ p became almost proportional to p -2 while F p (n) became nonsinusoidal. This p dependence of τ p was still in close agreement with the Rouse and/or reptation models, but the n dependence of F p (n) was considerably different. This result indicated that the broadening of the e curves with c PI was essentially due to changes in the eigenfunctions, not due to changes in the p dependence of τ p (relaxation time span). Changes of F p (n) with c PI were discussed in relation to coupling of motion of chains being overlapped at c PI > c * .