Local motion of oligo- and polystyrene chain end studied by the fluorescence depolarization method

Local motion of the oligo- and polystyrene chain end in dilute solution was examined by the fluorescence depolarization method. The molecular weight of the sample varied from 5.1 x 10(2) to 2.5 x 10(4). The relaxation time of local motion, T-m, in benzene solution increased with molecular weight and reached an asymptotic value at MW = 2 x 10(3) with T-m congruent to 0.3 ns. In ethyl acetate, which is a poorer solvent than benzene, T-m became constant at a higher. molecular weight than in benzene, and the asymptotic relaxation time was longer than that in benzene. We proposed that the difference in the relaxation time and in its molecular weight dependence between the two solutions may result from the local potential for the conformational transition of the main chain bond, rather than the segment density. In comparison with the relaxation time for the polystyrene chain center, both the critical molecular weight and the asymptotic relaxation time fdr the chain end were about 1 order smaller than those for the chain center, This indicates that the mobility of a linear polymer chain end is sufficiently different from that of its chain center.