The Effect of Morphology Changes on Polymer Photodegradation Efficiencies: A Study of Time-Dependent Morphology and Stress-Induced Crystallinity

The effects of time-dependent morphology changes on the photochemical degradation efficiencies of polymers were investigated using the segmented polyurethane 1. A segmented polyurethane was chosen for this study because segmented polyurethanes have distinct hard (crystalline) and soft (amorphous) morphologies in the solid-state. It is known that, as these polymers cure, the hard and soft domains expand in the days and weeks following preparation. Experiments showed that the quantum yields for degradation of thin films of polymer 1 decreased over a period of about 30 days following preparation of the polymer films. Infrared spectroscopy showed an increase in crystallinity in the polymer during this curing period. These results are consistent with the hypothesis that the rate of photodegradation in polymer systems is controlled by the separation rate of the photogenerated radical pair relative to the rate of radical-radical recombination (Scheme 3). An important factor that controls the ratio of these two rates is the crystallinity of the polymer; an increase in crystallinity favors the radical-radical recombination reaction. The effect of tensile stress on photodegradation efficiencies was also studied. Two samples of the same polymer, one more crystalline than the other, were examined. The quantum yields for degradation of the more crystalline sample were less affected by tensile stress than the less crystalline sample. Tensile stress is known to order polymer chains and the results are again interpreted as being consistent with the idea that crystallinity is an important factor in controlling the efficiency of polymer photodegradation.