MOLECULAR-DYNAMICS IN BULK CIS-POLYISOPRENE AS STUDIED BY DIELECTRIC-SPECTROSCOPY

Dielectric spectroscopy from 10-1 to 109 Hz was used to investigate bulk amorphous cis-polyisoprenes with different molecular weights having narrow molecular weight distributions. A molecular weight dependent normal-mode relaxation process due to reorientation of the end-to-end vector as well as a molecular weight independent segmental-mode process caused by local chain motions were observed. The relaxation time for the normal-mode process exhibits a molecular weight dependence that can be described below a critical molecular weight, Mc (=104), according to the Rouse theory; but above Mc the relaxation time follows the 3.7 power of Mw, which is explicable with the reptation theory. Both processes were analyzed in terms of dipole-dipole correlation functions and reveal a Kohlrausch-Williams-Watts (KWW) type of relaxation behavior. The nonexponential decay is explained with the cooperative nature of motions in the bulk amorphous state. The stretched exponential /Srww f°r the normal-mode process depends on the chain length whereas it remains constant for the segmental process. The results are in agreement with new theoretical and computational approaches. © 1990, American Chemical Society. All rights reserved.