A4 Degradation: Photo-oxidative, Thermal and Hydrolytic

Topic

When exposed to UV radiation, thermal energy sufficient to induce pyrolysis, or water capable of hydrolyzing its bonds, this polymer undergoes random chain scission via any of these three pathways: UV radiation breaks main-chain bonds by generating free radicals that propagate along the chain; thermal energy breaks those same bonds directly through pyrolysis when the temperature is high enough; and water breaks bonds susceptible to hydrolysis wherever they occur along the chain—with none of these three pathways targeting any specific position. Consequently, the more intensely any of these three pathways acts upon the polymer, the more frequently this random chain scission occurs throughout its structure.

Since each instance of random chain scission splits a long polymer chain into two shorter ones, the polymer's molecular weight is reduced; the more frequently this scission occurs along the chains, the shorter they become on average, and the lower the resulting molecular weight. Thus, a higher frequency of random chain scission causes the molecular weight to drop more rapidly. Consequently, this model—characterized at its inception by random chain scission and a reduction in molecular weight—degrades (through that very reduction) all the fundamental properties of the polymer described by the other models in this set, which focus on its structure and physicochemical basis. Each of these properties depends, to a greater or lesser extent, on the polymer chains retaining sufficient length to entangle and intertwine; the reduction in molecular weight shortens these chains, thereby weakening the capacity for entanglement and intertwining upon which those properties rely. The greater the severity of this molecular weight reduction, the more this degradation extends to a larger number of those fundamental properties, and the greater its magnitude becomes for each of them.

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