The effects of thermal history on the melting behavior of several polypropylene fractions have been studied. The specific fractions investigated include one that has crystallized in the triclinic modification, one that has crystallized in the monoclinic modification, and a third that contained a combination of the two types of crystals. Thermal histories given to these specimens included a series of stepwise annealings at steadily decreasing temperatures, a series of annealings for various times at a given elevated temperature, and a series of stepwise annealings at a set of steadily increasing temperatures. The melting behavior was investigated by means of a differential scanning calorimeter and various DSC peaks are presented. The nature of multiple peaks produced by the different treatments is discussed, and the results are compared with similar data obtained on whole polymer.
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