The photo-oxidative degradation of polypropylene monofilaments with xenon-arc irradiation results initially in a drop in elongation-to-break, followed by a decrease in tensile strength on prolonged irradiation. A microscopic examination of fibers that were first peeled to reveal the interior of the filament and then uv-irradiated showed that the interior is partially fibrillar, and that the fibrillar zones are more resistant to photo-oxidative scission than the surrounding nonfibrillar matrix. The formation of cracks in the outer sheath of the fiber and through the nonfibrillar regions correlates with the drop in elongation-to-break. Fibrils are not severed until after long uv exposure times, corresponding to the observed drop in tensile strength. Fiber melting temperatures estimated from DSC endotherms drop steadily during photo-oxidation. This drop and the observed decrease in x-ray crystallinity may be associated with the photo-oxidative cleavage of tie molecules and chain-extended crystalline zones.
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