Glass transition temperature and β relaxation temperature around chain end of polystyrene determined by site specific spin labeling.

A glass transition temperature, T(g), and a β 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(n) with blending oligomers as well as the T(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 mobility 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(β)s of the PS-E and PS-M.