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Articles you may be interested inEffect of pressure on the segmental and chain dynamics of polyisoprene. Molecular weight dependence Dielectric relaxations in undiluted cis-polyisoprene (cis-PI) with narrow molecular-weight distribution were investigated. The normal mode process due to fluctuation of the end-to-end distance and the segmental mode process due to the local segmental motions were observed. The average relaxation time 'Tn for the normal mode process was insensitive to the polydispersity MwlM n , but the width of the loss curve reflecting the distribution of the relaxation times increased with M w / M n' The slope of the double logarithmic plot of 'Tn against M w changed at the characteristic molecular weight Me from 2.0 in the range M w < Me to 4.0 ± 0.2 in the range M w > Me. It was found that although the half-width is nearly independent of M w' the tail of the loss curve in the high frequency side broadens with increasing M w' This indicates that by entanglement effects the distribution of the relaxation time for the normal mode process changes slightly. The segmental mode is found to be almost independent of the molecular weight and the polydispersity factor. the chain is proportional to the endto-end vector r(t).

Dielectric measurements were carried out on ten undiluted cis-polyisoprene (cis-PI) samples with different molecular weights. cis-PI exhibited dielectric relaxation due to the fluctuation of the end-to-end distance, termed the "normal mode process", as well as dielectric relaxation due to the segmental mode process. From the magnitudes of the dispersion, the parallel and perpendicular components of the dipole moment along the chain contour were calculated to be 0.17 ± 0.03 and 0.21 D, respectively. The relaxation time for the normal mode process, rDn, was quite sensitive to the molecular weight M and exhibited an M dependence similar to that for the mechanical relaxation time of cis-PI. Below a characteristic molecular weight Mc (=104), was proportional to M2, in agreement with the Rouse theory, but above Mc, rDn was proportional to M3•7. The latter result was explained by the tube disengagement process proposed by de Gennes. The distribution of rDn became broader with molecular weight, tailing toward the high-frequency side. This behavior was also explained in terms of the tube theory.

Dynamic light scattering in the polarized geometry has been used to investigate the dynamics of composition fluctuations in solutions of two poly(styrene-i>Zocfe-l,4-isoprene) diblock copolymers in a nonselective, good solvent both far from and near to the ordering transition. Four different relaxation mechanisms were identified. One relaxation relates to the cooperative diffusion of copolymer chains, and it behaves similarly to that in semidilute homopolymer solutions. As concentration increases, two additional relaxations gain in amplitude: the wavevector-independent internal or breathing mode, predicted by theory, with amplitude increasing with scattering angle, and the recently established for copolymer melts diffusive relaxation; the relaxation times of both processes increase with copolymer concentration. In addition, another very slow process is evident at low scattering angles and is related to the long-range density fluctuations. A theoretical analysis is presented that predicts the existence and the behavior of the first three relaxations. The new diffusive relaxation is attributed to the composition polydispersity of the diblocks; its intensity is proportional to the degree of polydispersity; its rate is governed by the self-diffusion of the copolymer chains; both its intensity and its relaxation time increase with the copolymer concentration, in contrast to the behavior of the cooperative diffusion.

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