Effects of molecular weight and chain ends on glass transition of polystyrene

The main glass transition temperatures, T_g of narrow distribution anionic polystyrene samples have been measured by differential thermal analysis (d.t.a.). The d.t.a. data were verified by differential scanning calorimetry. Effects of heating rates were eliminated by extrapolating observed T_g values to a standard heating mode of 1°C/min In a T_g vs. log (heating rate) relation. The variations of T_g peak height and temperature with heating rate and polymer molecular weight (M) agree with conclusions from other reports of d.t.a. work. The T_g values corrected for heating rate effects agree well with figures reported in studies with d.t.a. and other techniques for locating vitrification temperatures.The volumes attributable to end groups differ significantly between anionic and thermally initiated polystyrenes at temperatures above T_g. A comparison of T_g values of both polystyrene types provides a direct test of the iso-free volume model of the vitrification process. A common T_g vs. M relation was found for anionic and thermally initiated polystyrenes, regardless of chain end nature. This comparison is not consistent with the concept that vitrification is governed by free volume defined as the difference between specific volume of the polymer and a reference volume. Our results are compatible with several other mechanisms which have been suggested for the glass transition.