Crystallization and melting behaviour of photodegraded polypropylene .2. Re-crystallization of degraded molecules

Isotactic polypropylene bars were exposed to ultraviolet radiation (UV) in the laboratory for periods up to 48 weeks causing a reduction in molecular size and the build-up of chemical groups like carbonyls and hydroperoxides. The specimens were re-crystallized from the melt under isothermal and non-isothermal conditions and investigations were conducted by differential scanning calorimetry (DSC), X-ray diffraction, and light microscopy. At first the fractional crystallinity of the re-crystallized materials increased with exposure as the result of decreasing molecular size, but for longer exposures the fractional crystallinity decreased because of the increase in concentration of chemical impurities. In highly degraded specimens, the presence of gamma-phase crystals was detected. Kinetic studies revealed that the rates of nucleation and growth may be affected differently by photodegradation, and that the degraded molecules crystallized much faster than the unexposed material when their rates of crystallization under the same supercooling are compared. Double melting peaks were observed in DSC thermograms and were shown to be due to re-organization during heating, but in some cases the segregation of highly defective molecules is the major reason for peak doubling. Molecular segregation also changes the spherulite morphology during crystallization, as revealed by polarized light microscopy. (C) 1997 Elsevier Science Ltd.