In-cage and out-of-cage combinations of benzylic radical pairs in the glassy and melted states of poly(alkyl methacrylate)s

Abstract

Norrish type 1 reactions of 1-(4-methylphenyl)-3-phenyl-2-propanone (ACOB₁) have been used to probe structural and morphological properties of a series of poly(alkyl methacrylate)s (PAMAs, where the alkyl is ethyl, butyl, isobutyl, cyclohexyl, and hexadecyl) below and above their glass transition (or melting) temperatures. The PAMAs investigated cover a wide range of glass transition temperatures and structure types. The ratio of in-cage to the sum of in-cage and out-of-cage recombinations of the triplet benzylic radical-pairs generated upon irradiation of ACOB₁ (F_c) have been calculated from relative photoproduct yields at different temperatures and are compared with the free hole volumes within the polymers as calculated from positronium annihilation lifetime spectroscopy. Laser flash photolysis experiments to follow the growth and decay of the radicals have also been conducted in order to correlate the steady-state irradiation results with the radical pair recombination processes (i.e., in-cage and out-of-cage). The changes in F_c as a function of PAMA type and phase (temperature) can be correlated with chain relaxation rates and the nature of the polymer side chains, but not hole free volumes. These results are compared with those from our previous work, conducted in polyethylenes with differing degrees of crystallinity, where hole free volume was the primary factor in controlling F_c.