Abstract
Glass transition temperature (Tg) and vibrational properties at the chain end of polystyrene (PS) were selectively determined by a novel method utilizing Fourier transform infrared (FT-IR) spectroscopy. In this method, PSs having almost the same number average molecular weight (Mn) but selectively deuterated at different sites, midchain (M-PS, Mn = 10.7 kDa) or chain end (E-PS, Mn = 10.3 kDa), were synthesized and the asymmetric stretching vibration of the main chain CD2 (νas(CD2)) were analyzed in detail. We found that the temperature dependence of the νas(CD2) frequency shows a clear inflection at the calorimetric Tg of PS. The inflection temperature was defined as Tg,FTIR, which represents the Tg at the local region around the deuterated sites. Although higher segmental mobility at the chain end had been shown above the Tg in our earlier work, this study found that the Tg,FTIRs of the M-PS and E-PS are the same. From this result, it was concluded that the chain ends move cooperatively with numerous neighboring other segments, averaging out those mobilities at temperatures around the Tg. On the other hand, the νas(CD2) frequency of the E-PS was by a few cm−1 higher than that of the M-PS for any temperature. This was explained in terms of the discontinuity of the repeat unit at the chain end resulting in reduced interactions between repeat units along the chain.
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