Molecular weight effect on local motion of polystyrene studied by the fluorescence depolarization method

The molecular weight effect on the local motion of polystyrene (PS) was exa mined in dilute solutions by the fluorescence depolarization method. Four P S samples with the fluorescent probe, anthryl group, in the middle of the m ain chain were synthesized by the living anionic polymerization. The molecu lar weight of samples varied from ca. 6.4 x 10(3) to 9.2 x 10(4). Solvents were benzene, a good solvent and ethyl acetate, a poor solvent. In both sol vents, the relaxation time increased with the molecular weight up to MW=10( 4) at which it reached an asymptotic value. The activation energies were al so estimated from the temperature dependence of the relaxation time, and it s molecular weight dependence appeared to be similar to that of the relaxat ion time. It was suggested that the relaxation time of the local motion is determined by the potential for the conformational transition of main chain bonds, rather than by the segment density. Finally, the molecular weight e ffect on the relaxation time for PS was compared with that for poly(oxyethy lene) (POE). The results showed that PS is dynamically stiffer than POE.