LUMINESCENCE STUDIES OF POLYMER MATRICES .1. PHOSPHORESCENCE OF BENZOPHENONE DISPERSED IN POLY(METHYL METHACRYLATE)

The photophysical behaviour of benzophenone (BP) in poly(methyl methac rylate) (PMMA) has been studied as a function of temperature. Time-res olved phosphorescence data reveal that at no temperature between 77 an d 420 K can the decay behaviour of the triplet state of BP in PMMA be described adequately by a single exponential function. This contrasts with the phosphorescence behaviour of BP in low temperature, low molec ular mass, glassy matrices and that reported previously for BP dissolv ed in PM MA at temperatures below the onset of the beta relaxation of the polymer. The complexity of the phosphorescence decay kinetics of t he BP/PMMA system increases at temperatures greater than that of the b eta transition of the polymer host. This has been rationalized in term s of the onset of enhanced energy transfer interactions between the BP triplet excited state and the PMMA solvent, as proposed earlier. Howe ver, the function derived previously on the basis of diffusion-control led quenching of the triplet state of BP by the PMMA host, did not mod el, adequately, the time-resolved phosphorescence data obtained in the current study. In this respect, the form of the quenching influence e xerted by the polymer matrix upon the photophysical behaviour of the B P solute remains to be explained. Notwithstanding the absence of a com plete rationalization of the influence of the PMMA host upon the photo physical behaviour of the BP guest, it has been shown that both the in tensity of phosphorescence emitted by the BP and its mean decay time p rovide means whereby transitions within the polymer matrix can be dete cted. In this respect the data are not equivalent: the lifetime data ' sense' the presence of the alpha' transition (at similar to 345 K) of the PMMA. This transition is not apparent in the temperature dependenc e of intensity data which, in common with the lifetime data, are sensi tive to the onset of both the gamma and beta transitions of the polyme r. The relevance of the data in respect of establishing phosphor/polym er combinations for application as phosphorescent coding media is disc ussed.