Glass transition temperatures of stereoblock,isotactic and atactic polypropylenes of various chain lengths

The effect of sample molecular weight on the glass transition temperature has been examined for isotactic, stereoblock and atactic polypropylene samples. Asymptotic values of T₉ (∞) (isotactic) = 272°K, T₉ (∞) (atactic) = 266°K, and T_g (∞) (stereoblock) = 266°K were found. Deviation from T_g (∞) was observed when M?_n was below 10⁴; the dependence of T_g on M?_n has been discussed in relation to the Gibbs-DiMarzio treatment of the glass transition. The possible effects of both tacticity and crystallinity on T_g were examined; comparison of the data obtained with those of other workers was made. It was concluded that molecular order in polypropylene samples could affect T_g significantly and that this was particularly obvious in short chain stereoblock fractions.