Specific heat for atactic and isotactic PPBA

The higher endothermic peak, for the isotactic sample, occurs at a temperature close to the melting point of the atactic polymer, and would appear, at first sight, to be associated with the fusion of atactic polymer present in the isotactic sample. 

This explanation was tested by the study of blends of isotactic polymer (97% isotactic) and atactic polymer. The D.S.C. traces did show two endothermic peaks at 490 K and 540°K. However, upon cooling and reheating, both high and low temperature peaks appeared again without any appreciable change. The explanation that the isotactic sample with a smaller extent of tacticity can be simply regarded as a blend of isotactic and atactic polymer is therefore not sufficient for a complete explanation. 

For both atactic and isotactic sample the initial cast film shows essentially zero birefringence. When the temperature exceeds 

We assume that birefringence can arise from two sources a crystallization (or ordering phenomenon) from the amorphous phase to give birefringent structures and an orienting of macromolec-ular chains either in the crystalline or the amorphous phases. 

The clarification of these effects is best achieved using x-ray diffraction methods, and these results will presently be described. However a correlation of the T.O.A. data with the D.S.C. data can be made. It appears that the original film cast from solution is amorphous and without extensive molecular orientation. At temperatures above Tg the molecular mobility is sufficiently great to induce cold crystallization. 

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