Some effects of morphology on oxidation and stabilization of polyolefins

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Abstract
Previous work on the effects of crystallinity on the autoxidation of polyolefins is briefly reviewed and shows that polyethylene and polypropylene oxidize exclusively in the amorphous phase. Measurements of the rate of oxidation of poly(4‐methyl pentene‐1) (P4MP) show that the oxidation is first‐order in oxygen pressure over a wide range of film thickness, oxygen pressure, and oxidation temperature. A simple model of diffusion followed by chemical reaction fits the data very well. Values of the diffusion coefficient for oxygen and of its activation energy, calculated from oxidation data, agree well with experiment.It is shown that increased initial crystallinity increases the oxidation rate of P4MP, while the crystallinity of the polymer decreases markedly during oxidation. These results imply that P4MP oxidizes extensively in the crystal phase.Ultraviolet microscopy shows that typical stabilizer molecules are rejected from the crystal phase of P4MP; comparison of oxidation induction periods in polypropylene and P4MP, containing antioxidants, supports the view that the effectiveness of stabilizers in P4MP is markedly reduced by rejection from the crystal phase.