Luminescence studies of polymer matrices: 1. Phosphorescence of benzophenone dispersed in poly(methyl methacrylate)

Abstract

The photophysical behaviour of benzophenone (BP) in poly(methyl methacrylate) (PMMA) has been studied as a function of temperature. Time-resolved phosphorescence data reveal that at no temperature between 77 and 420K 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 molecular mass, glassy matrices and that reported previously for BP dissolved in PMMA at temperatures below the onset of the β relaxation of the polymer. The complexity of the phosphorescence decay kinetics of the BP/PMMA system increases at temperatures greater than that of the β transition of the polymer host. This has been rationalized in terms of the onset of enhanced energy transfer interactions between the BP triplet excited state and the PMMA solvent, as proposed earlier. However, the function derived previously on the basis of diffusion-controlled quenching of the triplet state of BP by the PMMA host, did not model, adequately, the time-resolved phosphorescence data obtained in the current study. In this respect, the form of the quenching influence exerted by the polymer matrix upon the photophysical behaviour of the BP solute remains to be explained. Notwithstanding the absence of a complete rationalization of the influence of the PMMA host upon the photophysical behaviour of the BP guest, it has been shown that both the intensity of phosphorescence emitted by the BP and its mean decay time provide means whereby transitions within the polymer matrix can be detected. In this respect the data are not equivalent: the lifetime data ‘sense’ the presence of the α′ transition (at ∼345 K) of the PMMA. This transition is not apparent in the temperature dependence of intensity data which, in common with the lifetime data, are sensitive to the onset of both the γ and β transitions of the polymer. The relevance of the data in respect of establishing phosphor/polymer combinations for application as phosphorescent coding media is discussed.