Effect Of Film-forming Conditions On The Glass Transition Temperature Of Polymer Thin Film Investigated By Fluorescence Spectroscopy

With the advances in nanotechnology, much attention had been paid to the condensedphysical properties of polymer thin film when the thickness reduced to the nanolength scale. Theglass transition temperature of nanoconfined polymer thin film strongly depended on thethickness. The surface morphology, chain conformation, and chain entanglement of polymer thinfilms were influenced by the film preparation conditions, which in turn determined the the glasstransition behavior of polymer thin films. Therefore, it is of great significance to investigate theinfluence of film preparation conditions on the glass transition behavior of polymer thin films,which may help to enhance the fundamental understanding of the molecular motion mechanismof nanoconfined polymer thin films.Fluorescent resonance energy transfer (FRET) fluorescence spectroscopy was widely usedto study the interaction and proximity between the polymer chains, as well as the changing ofchain conformation because of the high sensitivity to the space distance. In this thesis, the effectsof film preparation conditions, such as the film-forming methods, the properties of film-formingsolution, and the surface chemistry of the substrate on the glass transition temperature and chainconformation or chain entanglement of polystyrene thin films were systematically investigatedby ellipsometry and fluorescent resonance energy transfer (FRET) fluorescence spectroscopy,respectively. Some conclusions were obtained as follows:(1) One origin of the thickness-dependence of glass transition temperature of polystyrenethin film was studied in terms of chain entanglement by fluorescence spectroscopy. When thethickness decreased from100nm to12nm, the glass transition temperature decreased from364Kto349K. Meanwhile, the fluorescence quenching ratio decreased from0.58to0.26. It showedthat the average distance between the donor and acceptor became larger indicating a lowerdegree of entanglement between the polymer chains with the decreasing of the film thickness.This may contribute to the lower glass transition temperature of polymer thin film.(2) The effect of film forming methods, spin-coating and solution-casting, on the glasstransition temperature of polystyrene thin film was investigated. It was shown that the glasstransition temperature of spin-coated films was higher than that of solution-cast films, especially for the films with thickness less than30nm. The fluorescence spectroscopy based on the FRETshowed that the ratio of acceptor fluorescence intensity to donor fluorescence intensity (IA/ID) ofspin-coated films was smaller than that of solution-cast films, indicating an expanded chainconformation in spin-coated films. The polymer chains were stretched due to the largecentrifugal force during the spin-coating process. The stretched and nonequilibrium polymerchain provided an additional driving force for the chain mobility because of the entropy effect,thereby a much lower Tg was observed for the spin-coated films.(3) By changing the temperature of decalin, a θ solvent for polystyrene with a θ temperatureof294K, the effect of film-forming solution properties on the glass transition temperature ofpolystyrene thin film was studied. The glass transition temperature of thin film with a thicknessof22nm decreased from361K to349K when the solution temperature changed from333K to291K. The fluorescence spectroscopy confirmed that this was attributed to the changing of thechain entanglement. When the solvent turned from a good solvent to a poor solvent, the IA/IDincreased from0.15to0.26, indicating that the chain contracted and degree of the interchainentanglement reduced, thus a decreasing glass transition temperature was observed.(4) The effect of interfacial interaction on the glass transition temperature of polystyrenethin films was studied. The results showed that the glass transition temperature of polymer thinfilm spin-coated on the substrate modified by PTS was higher than that of the film on the siliconwafer covered with a native oxide layer. This was due to the fact that the PTS changed theinterfacial free energy and enhanced the interaction between the polymer and the substrate byπ-π interaction. Meanwhile, we found that the substrate not only affected the glass transitiontemperature but also the chain conformation. When the interaction between the polymer and thesubstrate was strong, the chain was strectched. While the substrate was removed, the chaincontracted.