Onder Pekcan

Energy transfer studies on polymer membrane films. Materials with variable apparent dimensionality

Abstract Polymer films composed of poly(vinyl acetate) (PVAc, 91.7 mol%) and poly(2-ethylhexyl methacrylate) (PEHMA, 8.3 mol%) (all the PEHMA picient as a graft copolymer) were prepared by solvent casting. The PEHMA phase was labeled with a low concentration (0.7 mol%/PEHMA) of naphthalene groups. Exposure of the films to a pentane solution of 9-anthrylmethyl pivalate (AMP) transferred the AMP molecules exclusively to the PEHMA phase of the material. Direct energy transfer studies showed that the energy transfer rate was proportional to the AMP concentration, and that the process was characterized by an effective dimensionality d = 1.4. This result can be explained in terms of a crossover phenomenon if the PEHMA is present in the film in the form of interconnected long, thin (≈ 50 A diameter) cylinders. Supporting this idea is our finding that swelling agents for PEHMA such as hexadecane are also taken up by the film from pentane solution. Experiments on these materials give d values that increase with increasing hexadecane until d becomes equal to 3.0.

Phosphorescence from covalently labeled nonaqueous dispersions: Insights into the swelling of microdomains: 2. Luminescence from polymer colloids

Abstract Nonaqueous dispersions of polymer colloids were prepared containing 2 and 10 mol% naphthalene groups covalently attached to the poly(methyl methacrylate) core. The dispersions were sterically stabilized by cillia of polyisobutylene chains that were terminally attached to the PMMA. The phosphorescence spectra and phosphorescence decay times of these particles were studied in various media at 77°K. The freeze-dried powder, suspensions of this powder in methanol, and its colloidally stable dispersions in methylcyclohexane were examined. Two models are considered to explain the variation of the triplet lifetimes with the concentration of naphthalene groups. A mass diffusion model considers diffusion as the origin of phosphorescence quenching. A static quenching model cites particle swelling by solvent. The former model yields unrealistic values of the medium viscosity, whereas the lifetimes permit one to calculate the degree of particle swelling in the presence of different solvents.

Energy transfer studies from polymer bound naphthalene to anthracene in solution: translational and segmental diffusion rates

Abstract A graft copolymer of polyisobutylene with poly(methyl methacrylate) [PMMA] was prepared containing naphthalene [N] groups statistically distributed along the PMMA backbone. Fluorescence studies of dilute solutions of the polymer in ethyl acetate showed small amounts of N-excimer emission when the NMA/MMA ratio was 1:9, and undetectable excimer when the ratio was 2 98 . Energy transfer experiments were carried out between polymer-bound naphthalene and anthracene [A] added to the solution. The rate constant for energy transfer, kET, for the process N∗ + A → N + A ∗ was found to be 1.1 × 1010 M−1 s−1 at 22°, slightly smaller than that (1.2 × 1010 M−1 s−1) for energy transfer from naphthylmethyl pivalate to A. From the value of the rate constants, a value for the segmental diffusion coefficient of polymer bound naphthalene was calculated.

Fluorescence quenching studies of core and stabilizer-labeled non-aqueous dispersions: The nature of the core—stabilizer interface

Abstract Particles ( d = 1 μ m) composed of poly(methyl methacrylate) [PMMA], sterically stabilized by butyl rubber [polyisobutylene, PIB], and labeled with fluorescent dyes were prepared by nonaqueous dispersion polymerization. One set of particles contained phenanthrene [Phe] groups attached to the PMMA chains. A second set of samples contained pyrene groups attached to the PIB component, and a third set was doubly labeled [PIB-Py/PMMA—Phe]. Fluorescence quenching experiments were carried out on dispersions of these particles in cyclohexane using carbon tetrabromide and N,N -dimethylaniline as quenchers. Nonlinear Stern—Volmer plots were obtained. The fluorescence quenching data fit nicely to the fractional quenching model. The quenching experiments and energy transfer measurements on the doubly labeled particles demonstrate the diffuse nature of the PMMA—PIB interface in these materials when the particles are dispersed in aliphatic hydrocarbon media. The interface becomes much sharper when the particles are dried.