Nanoscale confinement and temperature effects on associative polymers in thin films: Fluorescence study of a telechelic, pyrene-labeled poly(dimethylsiloxane)

摘要

This study is the first to report on the tunability of associative polymer behavior via confinement and to model the effect against experimental data. A telechelic, associative polymer, alpha,omega-pyrene-end-labeled poly(dimethylsiloxane) (Py-PDMS-Py), was synthesized and studied via fluorescence spectroscopy in solution at room temperature and in the neat state as a function of confinement and temperature, Emission spectra were characterized by a ratio of excimer to monomer fluorescence intensity, I-E/I-M; excitation spectra were obtained at wavelengths characteristic of monomer and excimer emission. The spectra revealed a strong tendency for static excimer fluorescence originating from ground-state dimer or aggregate formation via pyrene-pyrene association at all concentrations of Py-PDMS-Py in polymeric or oligomeric poly(dimethylsiloxane) (PDMS). The aggregates form due to the insolubility of pyrene in PDMS. In contrast, in very dilute solution in toluene, a good solvent for both pyrene and PDMS, the low excimer fluorescence was due nearly exclusively to dynamic, intramolecular excimer formation rather than to direct excitation of ground-state dimers. A very strong dependence of I-E/I-M on film thickness was observed for neat Py-PDMS-Py, with I-E/I-M being nearly independent of film thickness at thicknesses exceeding 100 to 200 nm but decreasing by more than a factor of 40 relative to bulk at a thickness of 5 nm. The dependence of 1011,1 on film thickness was effectively described via a two-state model. The fit of the model to experimental data leads to the conclusion that the effect of confinement on association may be explained by the presence of two interfacial or surface layers of similar to 10 nm average thickness, each having dramatically reduced physical cross-linking relative to the rest of the film. Investigation of the effect of temperature on I-E/I-M in bulk Py-PDMS-Py found that the energy of pyrene-pyrene ground-state dimer formation is about an order of magnitude greater than thermal energy (kT), making this a model system for strongly interacting, associative polymer. [References: 81]