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 o f 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 cons tant at a higher. molecular weight than in benzene, and the asymptotic rela xation time was longer than that in benzene. We proposed that the differenc e in the relaxation time and in its molecular weight dependence between the two solutions may result from the local potential for the conformational t ransition of the main chain bond, rather than the segment density. In compa rison with the relaxation time for the polystyrene chain center, both the c ritical molecular weight and the asymptotic relaxation time fdr the chain e nd were about 1 order smaller than those for the chain center, This indicat es that the mobility of a linear polymer chain end is sufficiently differen t from that of its chain center.