Origin of Impact Strength in Polycarbonate: I. Effect of Crystallization and Residual Solvent

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Abstract
The high impact strength of glassy polycarbonate has been suggested to be related to an excess free volume, a large low temperature beta transition, or a unique morphology. To determine how crystallization affects free volume, molecular motion and morphology, samples of amorphous and partially crystalline polycarbonate have been studied by means of specific volume and dynamic mechanical measurements, electron microscopy and x-ray diffraction techniques. Previous studies have given conflicting results concerning the effect of crystallization on the low temperature beta transition below Tg . Our results clarify this matter by showing that crystallization and residual solvent give different effects on the beta transition. Crystallization is shown to decrease the intensity of the beta loss peak suggesting that the beta motion occurs in the “non-crystalline” regions of the polymer. Solvent not only decreases the beta peak intensity, but also shifts the peak maximum to lower temperatures and causes an additional loss peak to occur at higher temperatures but still below Tg . The embrittling effect of crystallization can be attributed to a lowering of the beta transition intensity or to a loss of free volume as judged by decreases in specific volume. However, more convincing evidence of the restricting effect crystallites have on amorphous chain segments is given by the observed changes in the alpha transition (Tg ) and sample morphology with crystallization.