INTERMOLECULAR STRUCTURE IN A POLYMER GLASS - ELECTRONIC EXCITATION TRANSFER STUDIES

Electronic excitation transport among interacting polymer molecules li ghtly tagged with chromophore substituents is examined as a function o f tagged polymer concentration in the polymeric solid. The technique o f time-correlated single photon counting is employed to obtain time-re solved fluorescence depolarization data on solid mixtures of poly(meth yl methacrylate-co-2-vinylnaphthalene) in a poly(methyl methacrylate) host. The time-dependent fluorescence anisotropy, the energy transport observable, is compared to a theory developed to model this system. T he theory is based on a first-order cumulant approximation to the tran sport master equation. The model makes use of the Flory ''ideality'' p ostulate by depicting the intramolecular segmental distribution as a G aussian with a second moment that scales linearly with chain size. At low copolymer concentration, the dynamics of excitation transfer depen d only on intramolecular structure. At high copolymer concentration, e xcitation transfer occurs among chromophores on different copolymers i n addition to intramolecular transfer. The only adjustable parameter i n the treatment is the form of the intermolecular radial distribution function, g(r). The sensitivity of the model is analyzed with respect to the behavior of g(r). The theoretical treatment provides a quantita tive description of the time and concentration dependence of the excit ation transfer for the case of g(r) = 1 when r greater-than-or-equal-t o 20 angstrom.