The temperature dependent dynamics of polydimethylsiloxane (PDMS) melts are investigated by measuring orientational relaxation of a dissolved probe molecule, 2-naphthyltriethoxysilane (NTES) using time resolved fluorescence depolarization. The temperature dependent viscosity of PDMS is also reported for two molecular weights. The measurements of nonpolar NTES probe dynamics are compared to previous measurements on the polar probe, N-(triethoxysilylpropy1)dansylamide. The activation energies for the orientational relaxation of the two probes arevery different. This is discussed in terms of the influence of the polarity of the solutes on the local structure in the melts. The results have implications for possible modifications of the physical properties of PDMS materials by using solutes or side groups of varying polarity. The synthesis of the NTES probe, which can also be used as a cross-linking reagent for PDMS, is also described.

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(methy1 methacrylate-co-2-vinylnaphthalene) in a poly(methy1 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 L 20 A.

Electronic excitation transport among interacting clusters of chromophores is investigated as a function of chromophore and cluster concentration. The technique of time-correlated single photon counting is employed to obtain time-resolved fluorescence depolarization data on aqueous octadecylrhodamine B/triton X-100 micelle solutions. The time-dependent fluorescence anisotropy, the energy transport observable, is directly compared to a theory developed to model this system. The theory is based on a first-order cumulant approximation to the solution of the transport master equation. The model depicts the micelles as monodisperse hard spheres with chromophores (octadecylrhodamine B) distributed about their surfaces. At low micelle concentration, the dynamics of excitation transfer depend only on internal micelle structure. At high micelle concentration, excitation transfer occurs among chromophores on different micelles in addition to intramicelle transfer. The theoretical treatment provides nearly quantitative descriptions of the time and concentration dependence of the excitation transport. It correctly predicts the concentration at which intermicelle transfer becomes significant. In the low micelle concentration limit (energy transport confined to isolated micelles) the model having a Poisson distribution of chromophores works well for small v ([chromophores]/[micelle]), but progressively worse as v is increased. Following the literature, a chromophore interaction parameter (in the form of a two dimensional second virial coefficient) is used to skew the probe distribution. This enables the transport theory to reproduce the data for all the values of v investigated and provides a determination of the second virial coefficient.

Interfacial behavior of phase separated asymmetric compressible binary polymer blendsTheory of domain boundary effects in a phaseseparated mixture of polymer and liquid crystalThe morphology of nanodomain structures in binary polymer blends of a random copolymer and a homopolymer is determined using electronic excitation transport ͑EET͒ studies. The experimental system employed is a copolymer, 6.5% atactic poly͑methyl methacrylate-co-2-vinyl naphthalene͒ ͓P͑MMA-2VN͔͒, in atactic poly͑vinyl acetate͒ ͑PVAc͒. The naphthalene groups serve as chromophores in the EET experiments. The mixtures are prepared such that initially the P͑MMA-2VN͒ chains are randomly distributed in the PVAc matrix. The nanodomains are formed while low-concentration mixtures of the P͑MMA-2VN͒ in PVAc are held at constant temperature in the melt state (TϾT g ), above the temperature at which phase separation occurs. In the melt the chains diffuse, and P͑MMA-2VN͒ chains aggregate until the temperature is quenched below T g . The structures of the resulting domains are examined with time-resolved fluorescence depolarization measurements, and the data are analyzed using an analytical theory to model EET among interacting polymer chains. The agreement between theory and data is very good. The results of the analysis indicate that the nanodomains correspond to aggregates with a characteristic size equal to the radius of gyration of the copolymer, R g . The number of P͑MMA-2VN͒ chains in aggregates prepared under different conditions is determined.

Poly(styrene-co-2-vinylnaphthalene) with a 1.25% fraction of naphthyl fluorophores is studied in two polymeric hosts, polystyrene and poly(vinyl methyl ether). In the polystyrene host, measurement of the electronic excitation transport-induced fluorescence polarization anisotropy decay, r(t), in conjunction with a previous quantitative statistical theory of electronic excitation transport on lightly tagged polymer chains, allows a determination of the copolymer radius of gyration. Comparison with light scattering measurements from the literature establishes the θ-condition nature of this solid system. Poly(vinyl methyl ether) forms a compatible polymer blend with polystyrene. Analysis of r(t) data shows that the radius of gyration of a copolymer molecule is expanded in poly(vinyl methyl ether) relative to the θ-condition at room temperature. The synthesis of poly(styrene-co-2-vinylnaphthalene) is detailed.