AbstractMeasurements of the density of a large number of experimental isotactic polypropylenes have shown that the specific volume is linearly related to per cent crystallinity as determined from x‐ray data. The density is also linearly related to the absorbance of certain absorption bands in the infrared spectrum of isotactic polypropylene. The infrared data allow a determination of the density of the amorphous polymer which is in good agreement with literature values and with a value determined from the x‐ray data by extrapolation to zero crystallinity. It is found that there is a rough, approximately linear correlation between crystallinity (by density) of unextracted polypropylene and the per cent insoluble residue remaining after extraction with boilingn‐heptane or after successive extraction with other solvents and heptane. In all instances, the amount of residue exceeds the crystalline content. The crystallinity‐insolubility relationship is influenced by molecular weight (as judged by intrinsic viscosity). An increase in crystallinity at constant η corresponds to an increase in insolubility; at constant crystallinity, a larger ηcorresponds to greater insolubility. In terms of density, the unextracted polymer may not be regarded as simply a mixture of two well‐defined components, since the density of insoluble residue increases with the density of the wh
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