Deuterium NMR Studies of Bulk and Silica-Absorbed Polystyrene
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Thin polymer films are used in applications such as electronics, biomedical devices and coatings. As the trend to make the devices smaller continues, a film's influence on a material's overall properties becomes more prominent. Previous studies have shown that thin polymer films have characteristics slightly different from those of bulk polymers. Film behavior has been characterized by examining thermal expansion, diffusion, conformation, density and glass transitions. The dynamics of glassy polystyrene has been investigated using a wide variety of theoretical and experimental methods. Schaefer et al., using 13C NMR, reported that ~7% of phenyl rings in polystyrene undergo -flips. Using deuterium NMR on ring-deuterated polystyrene, E. Rossler, et al. determined that a powder pattern of a rigid solid is obtained at room temperature. At 383 K, two low intensity peaks are present in the middle of the powder pattern and, at 433 K, a Lorentzian line was found. While the broad powder pattern was characteristic of deuterated polystyrene with no chain motion, the two low intensity middle peaks were attributed to the phenyl rings undergoing -flips. Kulik and Prins also showed that the deuterium powder pattern for polystyrene contained two weak middle peaks.4 Upon altering the relaxation delay, Kulik et al. determined that, for a partially relaxed sample (short relaxation delay), the two middle peaks were dominant over the 'outer horns' but, when given enough time for complete relaxation, the converse is true. They concluded that the line shapes are superpositions of roughly two contributions from phenyl rings performing relatively slow and fast flipping motions. Zhao et al., using deuterium NMR on ring-deuterated polystyrene, found a strong dependence of the line shape on temperature and echo delay, indicating motion at the microsecond time scale at the temperatures studied. In this study, we investigate the dynamics of polystyrene in bulk and silica-adsorbed samples as temperatures approach the glass transition temperature (Tg).