The photooxidation of polymers. III. Photooxidation of polystyrene

Abstract In the photooxidation of polystyrene films at 28°C. under 2537 A. radiation, water and carbon dioxide are the only significant volatile products. The intensity exponent of the reaction is unity under 600 mm. O 2 but zero under 20 mm., the quantum yield for oxygen absorption is 8.73 × 10 −2 , the overall energy of activation is 6.0 kcal./mole, and the rate is independent of molecular weight. In addition, the rate is directly proportional to oxygen pressure, whether or not the total pressure is made up to 600 mm. with nitrogen. Investigation of the effect of film thickness shows that neither the nonuniform distribution of initiating radiation in the film nor the diffusion of oxygen influences the reaction rate abnormally. The free radical inhibitor 2,6‐di‐ tert ‐butyl‐4‐methylphenol has no influence on the reaction, but the ultraviolet absorber 2‐hydroxy‐4‐methoxylbenzophenone accelerates oxygen absorption, having a maximum effect at approximately 3% concentration. It appears that the effect of ultraviolet absorbers is to protect the bulk of the polymer at the expense of the surface layer. Under 3650 A. radiation the oxygen absorption is much slower and the reaction exhibits an induction period. From these results it appears that while the initially formed radicals react with oxygen to form ROO· and HOO· radicals, the next step in the Bolland hydroperoxidation mechanism, RO 2 · + RH → ROOH + R ·, is effectively suppressed by the lack of mobility of the long chain radicals within the solid polymer. The kinetic behavior described above can be accounted for qualitatively in terms of three competing processes, namely: ( a ) direct recombination of the primary radicals, R· and H·, ( b ) reaction of these primary radicals with oxygen, and ( c ) diffusion of the hydrogen atoms from the site of their formation.