Factors Controlling the Rate of Photodegradation in Polymers

Abstract

The effects of the glass transition temperature and of radical trap concentration on the quantum yields of polymer photochemical degradation were studied. Special polymers with metal-metal bonded units incorporated into the polymer backbone were synthesized in order to investigate these effects because these polymers photodegrade in a relatively straightforward reaction involving metal-metal bond photolysis without complicating side-reactions. Using these polymers, it was shown that when polymers are irradiated above their glass transition temperatures (Tg) their quantum yields of degradation are similar to their quantum yields in solution. When irradiated below their glass transition temperatures, the photochemical degradation reactions are much less efficient. When irradiation takes place above the glass transition temperature there is no dependence of the quantum yields on the radical trap concentration. However, when irradiation occurs below the glass transition temperature, the quantum yields are dependent on the concentration of radical trap. These results are explained in terms of polymer chain mobility. It is suggested that, when irradiation takes place above Tg, chain mobility is facile enough that a metalradical trap is encountered before metal radical - metal radical coupling occurs. In contrast, when irradiation takes place below Tg, chain mobility is limited and metal radical - metal radical coupling occurs in many instances before a metal radical encounters a trap. Chain mobility also explains the affect of radical trap concentration on the efficiency of photodegradation. When the irradiation takes place above Tg the reaction of the metal radicals with radical traps is kinetically saturated with trap at the concentrations of trap used in these experiments. In contrast, when irradiation occurs below the glass transition temperature then, because of limited chain mobility, the reaction kinetics are not saturated in trap concentration and the quantum yields are dependent on the concentration of radical trap.