LOCAL SEGMENTAL AND SUB-ROUSE MODES IN POLYISOBUTYLENE BY PHOTON-CORRELATION SPECTROSCOPY

Photon correlation spectroscopy has been successfully employed to stud y the dynamics of the local segmental and sub-Rouse motion of a monodi sperse polyisobutylene near and above the glass transition temperature . The separation between the maxima of the two peaks corresponding to these two viscoelastic mechanisms is obtained for several temperatures . The temperature dependence of the local segmental relaxation times a s compared to the sub-Rouse modes relaxation times is noticeably stron ger. The segmental correlation function can be well described by a Koh lrausch's stretched exponential function with an exponent beta(alpha) = 0.56. At this time, photon correlation spectroscopy is the only expe rimental means that can determine the correlation of the local segment al motion of polyisobutylene directly. The small value of the coupling parameter of PIB, n(alpha) = 1 - beta(alpha) = 0.44, among amorphous polymers support the interpretation of its unusual viscoelastic proper ties. This value of n(alpha) directly determined from photon correlati on spectroscopy, is consistent with the value deduced indirectly from other experiments. (C) 1997 Elsevier Science Ltd.