INTERMOLECULAR STRUCTURE IN A POLYMER GLASS - ELECTRONIC EXCITATION TRANSFER STUDIES

Electronic excitation transport among interacting polymer molecules lightly tagged with chromophore substituents is examined as a function of tagged polymer concentration in the polymeric solid. The technique of time-correlated single photon counting is employed to obtain time-resolved fluorescence depolarization data on solid mixtures of poly(methyl 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. The theory is based on a first-order cumulant approximation to the transport master equation. The model makes use of the Flory ''ideality'' postulate by depicting the intramolecular segmental distribution as a Gaussian with a second moment that scales linearly with chain size. At low copolymer concentration, the dynamics of excitation transfer depend only on intramolecular structure. At high copolymer concentration, excitation transfer occurs among chromophores on different copolymers in addition to intramolecular transfer. The only adjustable parameter in 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 quantitative description of the time and concentration dependence of the excitation transfer for the case of g(r) = 1 when r greater-than-or-equal-to 20 angstrom.