Electron Spin Resonance Studies of Photodegradation in Poly(Methylmethacrylate)

Abstract

The strength of the ESR spectrum produced in poly(methylmethacrylate), PMMA, by exposure of the polymer to ultraviolet light (2537 Å) has been measured as a function of exposure time. The signal strength initially increases and then saturates under continued exposure. It decays when the light is removed. While the time needed to produce saturation depends inversely upon the light intensity it is only slightly affected by changes up to an order of magnitude in the decay rate. It is also observed that, for similar samples, the maximum ESR signal strength obtained is independent of the light intensity. This dynamic behavior strongly suggests that: (1) the ESR spectrum arises from radicals which are produced by the light from a limited number of centers already present in the polymer, and (2) the saturation mainly occurs due to exhaustion of these centers rather than some dynamic equilibrium between the creation and destruction of radicals. Additional experiments were performed in order to identify these centers. In these investigations it was found that the intensity of the ESR signal following irradiation decreases when the amount of residual monomer is descreased. Furthermore, when the monomer of poly (ethyl‐α‐chloroacrylate), PEαClA, is added to a sample of PMMA which has been purified to remove its own monomer and this sample irradiated, the resulting ESR signal is the one normally associated with photodegradation in PEαClA. These results indicate that the residual monomer molecules are the centers which are converted by the radiation into the free radicals which produce the ESR signal. While no attempt was made to obtain the exact form of the radicals, these results could be consistent with previous suggestions that the ESR spectrum arises from radicals which have the form of the propagating polymer radical.