A DTA apparatus for a polymer solution has been constructed. The heating rate was adjusted to approximately 0.50 or 0.42°C/min., a favorable rate for a polymer solution. From the DTA curves obtained by the use of this apparatus, the temperatures of the end points of the melting or dissolution have been determined; their plots give the solubility curves. The solubility of the isotactic polypropylene in decalin system increases monotonously with the increase in the temperature, except for the abnormality in the region from 30 to 50 wt.% in polymer content. In the case of the isotactic polypropylene-nitrobenzene system, on the other hand, the melting curve has a horizontal part in the region with a polymer content of less than 50 wt.%; above this point lies the phase separation curve, which touches the melting curve at the point with a polymer content of approximately 50 wt.%. The double peaks which appear in the DTA curve of the melting of pure isotactic polypropylene, may be due to the paracrystalline smectic modification of isotactic polypropylene, as has been suggested by Natta (Makromol. Chem., 35, 93 (1960)). The heat of fusion for isotactic polypropylene has been estimated to be approximately 1000 cal./unit mol. by measuring the area under the melting peak of the DTA curve and the equation for the melting point depression.
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Tamura et al. (1966) studied this question.