聚苯乙烯分子链构象与其薄膜的玻璃化转变行为

Investigation on polystyrene chain conformation and glass transition behavior of its thin film

本文利用椭偏仪研究了成膜方式对不同分子量聚苯乙烯(PS)超薄膜玻璃化转变行为的影响.发现PS超薄膜的玻璃化转变温度(Tg)随着厚度降低的幅度与其成膜方式、分子量有关.当PS膜低于一定厚度时,旋涂法制备的PS膜的Tg比相同厚度浇铸法制备的膜低,且二者Tg差值随着厚度的降低而增大.这二种膜Tg的差值和Tg发生偏离时膜的临界厚度随聚苯乙烯分子量的增加而增加.利用非辐射能量转移荧光光谱证实成膜方式主要是影响PS分子链在膜中的构象.旋涂法制备的PS膜相对于本体在近表面区域分子链的形变更大.分子量愈大,分子运动时内摩擦阻力愈大,近表面区域分子的残余应力愈大.由于强运动能力的活性层(空气/PS界面)对PS薄膜Tg的影响占主导,相同厚度下分子链愈伸展,残余应力越大,PS薄膜的Tg越低,导致成膜方式与分子量的影响也愈大.

The effect of film-formation methods on the glass transition behavior of polystyrene thin film was investigated by ellipsometry. It was found that the glass transition temperature（Tg） of thin film prepared by both spin-coating and solution-casting decreased with decreasing of film thickness. The Tg of spin-coated film depressed more dramatically than that of corresponding solution-cast film. What＇s more, the difference in Tg for the two kinds of the films and the critical thickness, at which the Tg of spin-coated films began to deviate from that of the corresponding solution-cast films, increased with increasing of the molecular weight of polystyrene. Nonradiative energy transfer fluorescence spectroscopy was employed to confirm that the chain conformation in polystyrene thin film was affected by the film-forming method. The chain deformed more dramatically at the near-surface region during the spin-coating process and the deformation increased with the increasing of the molecular weight, resulting in larger residual stress that provided additional driving force for the chain mobility at the near-surface region. Because the surface layer（air/PS interface）with high mobility dominated the Tg of the thin film, the larger residual stress resulted in the lower Tg.