Effect of thermal history on Tg and corresponding Cp changes in PVC of different stereoregularities

Samples of poly(vinyl chloride) polymerized at temperatures from ?60°C to +90°C, and therefore of different degrees of syndiotacticity and crystallinity, have been thermally treated in an attempt to reduce their content of ordered structures. Changes in order have been monitored by measuring Tg and the corresponding specific heat increment Δc_p. Partially syndiotactic samples have higher T_g and lower Δc_p than samples that are nearly atactic. After thermal treatment the differences decrease but do not vanish, even for samples heat-treated to about 250°C. This seems to indicate that some ordered structures are very stable even at relatively high temperatures. A quantitative estimate of the ordered structure content, based on Δc_p data, is proposed.