Origin of tensile stress-induced rate increases in the photochemical degradation of polymers

A novel poly(vinyl chloride)-based polymer (1) containing Mo-Mo bonds along the backbone was prepared by the reaction of carboxylated PVC (ca. 1.8 wt % -COOH) with (HOCH2CH2Cp)(2)Mo-2-(CO)(6). When irradiated with visible light, this polymer photodegrades, even in the absence of oxygen. Infrared spectroscopic analysis demonstrated that the chlorine atoms along the polymer backbone act as built-in traps for Mo-centered radicals formed by photolysis of the Mo-Mo bonds. The effect of stress on the degradation quantum yield of 1 and its plasticized analogue (2), made by addition of 20 wt % dioctyl phthalate (DOP) to 1, was studied. The presence of the internal radical trap permitted the polymer samples to be irradiated in the absence of oxygen, thus eliminating the kinetically complicating effects of rate-limiting oxygen diffusion. Results from both polymers showed that stress initially increased the quantum yields for degradation, but the quantum yields reached a maximum value and then decreased with higher stress. These results support the decreased radical recombination efficiency (DRRE) hypothesis, one of several hypotheses that have been proposed in the literature to explain the effect of stress on polymer photodegradation rates and efficiencies. The DRRE hypothesis proposes that the function of stress is to increase the initial separation of the photochemically generated radical pair, which has the effect of decreasing their recombination efficiency and thus increasing the degradation efficiency. The hypothesis predicts an eventual downturn in degradation efficiency because of polymer chain ordering; the increased order hinders diffusion apart of the radicals and thus increases their probability of recombination. Wide-angle X-ray diffraction and infrared spectroscopy confirmed that chain orientation increased with increasing stress on polymers 1 and 2.