DETERMINATION OF THE COUPLING PARAMETER OF LOCAL SEGMENTAL MOTION IN POLY(ISOBUTYLENE) BY PHOTON-CORRELATION SPECTROSCOPY

Many workers in the past have found that poly(isobutylene) (PIB) exhib its viscoelastic properties distinct from other commonly known amorpho us polymers. PIB has perhaps the most compact and symmetric monomer ch emical structure among amorphous polymers, which leads to the expectat ion that it has a smaller capacity for intermolecular coupling and a l ower degree of cooperativity of the local segmental motion. In the fra mework of the coupling model this expectation for PIB implies that it has a smaller coupling parameter which is sufficient to explain its va rious distinct viscoelastic properties as demonstrated in previous wor ks. Up to now there is no direct determination of the coupling paramet er for local segmental motion of PIB to confirm that it is indeed smal ler compared with other amorphous polymers. Dielectric spectroscopy is inapplicable to PIB because the monomer has a negligible dipole momen t. Mechanical spectroscopic measurements performed on PIB have not yie lded unambiguous results because of the difficulty of isolating the co ntribution of the local segmental motion from the experimental data. I n this work we have resorted to photon correlation spectroscopy and su ccessfully obtained the correlation function of the local segmental mo tion of PIB which can be well fit by a Kohlrausch stretched exponentia l function. The stretch exponent, beta, has the value of 0.56. The cou pling parameter, given by 1 - beta, has the value of 0.44 for PIB whic h is indeed significantly smaller compared with other amorphous polyme rs and consistent with the value we deduced indirectly from other expe riments.