Glass Transition Temperature and β Relaxation Temperature around Chain End of Polystyrene Determined by Site Specific Spin Labeling

摘要

A glass transition temperature, T_g, and a /i relaxation temperature, T_β, of spin-labeled polystyrene'(PS) having a number average molecular weight (M_n) of ca. 25 kDa were determined by the microwave power saturation (MPS) method of electron spin resonance (ESR). Spin labeling was selectively carried out at chain ends or midchain segments. This method allowed us to determine the local T_g and the local T_β around the spin-labeled sites, selectively. The T_g determined by the ESR, T_(g,ESR),, was in good agreement with that determined by differential scanning calorimetry, T_(g,DSC); the T_(g,ESR) decreased with decreasing M" with blending oligomers as well as theT_(g,DSC). The T_(g,ESR) for the end-labeled PS (PS-E) was equal to that for the midchain-labeled PS (PS-M) irrespective of the M_n. However, we previously reported that the PS-E showed distinctly higher segmental mobility than the PS-M in the temperature range 423-463 K (above T_g). Therefore, we conclude that the chain ends intrinsically have higher segmental mobihty than midchain segments due to the discontinuity of repeat units; however, the mobilities of chain ends and midchain segments are averaged out in the vicinity of T_g due to the cooperativities with neighboring numerous chain segments. Concerning the β relaxation, the T_β determined by the MPS was in good agreement with those determined by dielectric and dynamic mechanical spectroscopies and dilatometry. The T_β of the PS-E was the same with that of the PS-M within experimental uncertainties; the T_β was insensitive to the M_n in contrast to the T_g. Therefore, we conclude that the effect of chain end is little on the β relaxation of PS due to its local character, In addition, the effect of annealing at 353 K was found to be the same for the T_βs of the PS-E and PS-M.