AbstractLong‐term weathering studies of the subject transparent thermoplastic polymer revealed that sheets extruded from pristine resine gradually became hazy. Electron microscopy of fracture surfaces showed that minute voids were created during outdoor exposure. By the application of the Mie light scattering theory, it was possible to calculate that the average void size was 4 × 103Å and that the concentration of voids numbered about 106/mm3. Yet the total void volume of the poorest sample, which had a haze value of 26, was less than 0.05. The average void size decreased over a three‐year period, but the concentration increased with exposure. A mechanism of void formation was formulated based on photo‐induced moleculàr degradation. Chemical evidence suggests that molecular degradation occurs by complete unzippering of a molecule, resulting in the formation of carbon dioxide and 2,2,4‐trimethylpentene‐3‐al. The aldehyde is believed to plasticize the polymer in the vicinity of a growing void and the carbon dioxide acts as a blowing agent, therby generating a void. Simultaneously, free radicals formed during molecular decomposition initiate further degradation of neighboring molecules. The propensity of this resin to haze can be effectively overcome by suitable cleanup and stabilizat
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