Segmental Relaxation And Its Influencing Factors Of Polystyrene Thin Films

A fundamental understanding of the glass transition behavior of polymer thin films isimportant in many applications, including protective and lubrication coatings, adhensives, highperformance composite materials, and microelectronic encapsulants and dielectrics.In this thesis,two kinds of film-formation methods （spin-coating and casting） were employed to preparepolystyrene （PS） thin films. The glass transition behavior and chain segments relaxation of thesethin films were investigated by using the spectral ellipsometry. Th conclusions were obtained asfollows:It was found that the glass transition temperature of spin-coated films and cast filmsdecreased appreciably with decreasing film thickness. It was suggested that the reduction of T_ginthin films is caused by the presence of a liquidlike layer at polystyrene-air surface. However,when the thickness of thin film was below100nm, a larger reduction in T_gfor spin-coated filmsthan cast films was found. And spin-coated films have higher critical thicknesses when T_gbeganto reduce than cast films. We predicted that the films preparation represented a possible cause forthe difference of glass transition temperature between spin-coated films and cast films. Indeed,the fast evaporation of the solvent during spin-coating inevitably leaded to freeze chainsconformation in non-equilibrated. The residual stresses within the spin-coated films enhance themotion of chains.We also measured the T_gvalues for three different molecular weights of polystyrene. Theresults showed that when the film thickness reduced to15nm, the ΔT（gΔT_g=T_g^cast-T_g^spin）for threedifferent molecular weights of PS was22K. It turned out that the dependence of T_greduction onmolecular weight is weak in thin films. But the critical thickness of ΔT_gapproaching to zerodecreased with increasing of PS molecular weights. This result might be caused by theentanglement of chains as the consentrasion increasing to prepared thick films.To shed more light on what kinds of changes took place during preparing of spinning films,we performed detailed studies on the effect of spin speed on film T_gvalues. It turned out that theglass transition temperature decreased while the thermal expansivity （β） increased with spinspeed.Further research has been done on the polystyrene solution. It was found that the T_gof PS thinfilms spin-coated with toluene, cyclohexanone, cyclohexane solution is73℃81℃89℃，respectively. It indicated that the glass transition temperature decreased with the second virialcoefficient （A₂） decreasing. The glass transition temperature of polystyrene films after annealing in difference temperaturewas systematically studied. The T_g of spin-coated films gradually approached to that of the castfilms at the same thickness, after annealing in vacuum to release the residual stresses in thinfilms. It was found that when the film thickness is lower than40nm, both the relaxation time （）and the apparent relaxation activation energy （Ea） inceased with increasing of thickness. Forthick films （>40nm） Eareached to277±7.8KJ/mol. The Eais lower than the polystyrenerelaxation energy. It indicated that chains in thin films move easier than that in thick films.