Tseng KC, Turro NJ, Durning CJ
Department of Chemical Engineering and Applied Chemistry, Columbia University, New York, New York 10027, USA.
Phys Rev E Stat Phys Plasmas Fluids Relat Interdiscip Topics. 2000 Feb;61(2):1800-11. doi: 10.1103/physreve.61.1800.
Fluorescence recovery after photobleaching was used to measure in-plane dye-probe diffusion coefficients, D, in thin films of monodisperse polystyrene supported on fused quartz substrates. The substrates were prepared with a high density of surface hydroxyl groups which interact favorably with repeat units of the polymer. The effects of temperature and film thickness were investigated, at temperatures above the bulk glass transition of the polymer, T(g), and in the range of film thicknesses from 1-10(2) times the radius of gyration (R(g)) of individual polymer molecules. As the film thickness decreases towards R(g) the value of D increases above the bulk values, with significant effects first appearing in films approximately 20R(g). In the thinnest films studied, about 4R(g), the values of D lie as much as two orders over bulk values. At the same time, the temperature dependence of D becomes much weaker than in bulk. Analysis by free volume theory indicates that apparent values of both T(g) and the thermal expansion coefficient for liquid state, alpha(L), decrease as the film thickness decreases. The possible effects of surface segregation of the dye probe are discussed.
采用光漂白后荧光恢复技术测量了负载在熔融石英基底上的单分散聚苯乙烯薄膜中平面内染料探针的扩散系数D。制备的基底具有高密度的表面羟基，这些羟基与聚合物的重复单元具有良好的相互作用。研究了温度和薄膜厚度的影响，研究温度高于聚合物的本体玻璃化转变温度T(g)，薄膜厚度范围为单个聚合物分子的回转半径(R(g))的1 - 10²倍。随着薄膜厚度朝着R(g)减小，D值增加到高于本体值，大约在20R(g)的薄膜中首次出现显著影响。在所研究的最薄薄膜中，约4R(g)，D值比本体值高出多达两个数量级。同时，D的温度依赖性比本体中的弱得多。自由体积理论分析表明，随着薄膜厚度减小，T(g)和液态热膨胀系数α(L)的表观值均降低。讨论了染料探针表面偏析的可能影响。
