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 (ACOB(1)) 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 ACOB(1) (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).

Decay Kinetics of Benzophenone Triplets and Corresponding Free Radicals in Soft and Rigid Polymers Studied by Laser Flash Photolysis

“…Calculation of b F accounts for MFE on G-pairs namely the values of ϕ o,B .This similarity of MFEs [Eqs. (8) and (9)] is in accordance with our hypothesis that the polymer environment in F-and G-cages of EB is practically the same and that HFC is the predominant mechanism of MFE.…”

Cage Effect under Photolysis in Polymer Matrices

“…Nanosecond (ns) laser flash photolysis allowed the observation of the fast generation of benzyl radicals after decarbonylation with their subsequent decay. Decay traces demonstrated slow and fast components [9,10], which is expected for geminate recombination in the liquid phase [16,17].…”

“…Measurements of ϕ at temperatures in the vicinity of T g of the studied poly(alkyl methacrylates) led to smooth changes of ϕ vs. T upon crossing T g [10]. Slight increase of ϕ with temperature was observed for a number of methacrylates [10]. Nanosecond (ns) laser flash photolysis allowed the observation of the fast generation of benzyl radicals after decarbonylation with their subsequent decay.…”

“…One approach is based on product analysis of photodissociation of organic compounds in polymers. It was successfully used by Weiss, Chesta, and their collaborators [8][9][10][11][12][13]. Norrish type I cleavage of 1-(4-alkylphenyl)-3-phenyl-2-propanone (ACOB) was used as a probe of polymer matrices:…”

“…The two latter reactions proceed via singlet RPs [12,13]. It was concluded that polymer free volume V f was the dominant factor controlling the diffusion of the benzyl and other radicals, and, therefore, the kinetics of cage recombination and cage escape [10]. An increase of size (mass) of many atoms radicals leads to a decrease of their cage escape and to an increase of the cage effect [18].…”