Structural Fluctuations and Crystal Growth in Ultra-thin Polymer Films

1.采用水浇铸法制备了厚度从20 nm到数百nm的超薄聚苯乙烯薄膜。由于分子量为M的聚苯乙烯链的回转半径为0.03*M^1/2；nm，因此分子量大于10万的链将在超薄膜中被挤压，因此链的结构和动力学表现出与本体不同的结构和动力学。2.用PSPC观察到的X射线非晶晕表明薄膜和块体没有差别，薄膜中的大部分链的结构与块体的结构相同。3.原子力显微镜显示非晶态薄膜表面高度的波动约为1 nm。薄膜晶化后在晶面上也观察到了起伏，因此晶体被非晶层所覆盖。4.衬底对一次形核的影响显著。当晶化温度高于2 10ﾟC时，在生长的晶体中心观察到垂直于碳衬底的粒状晶体，云母衬底的中心为螺型位错。在150ﾟC以下，观察到二维球晶，分枝的小柠檬垂直于衬底。5.测量了薄膜厚度随分子量M、晶化温度T_C和衬底材料的变化。对于平行于衬底的片层晶体，生长速率G与厚度D成反比：G/G_*=1-a/D，其中G_*为体相生长速率，a为约5 nm的常数，与T_C、M和衬底材料无关。对于二维球晶，G随D的减小而保持不变。因此，我们得出结论，距离基板小于5 nm的部分高分子链“感觉”到基板上链段的吸收，因此经历了结晶扩散的延迟，导致G以1/D线性下降。

1.Ultra-thin polystirene films from 20 to several hundreds of nm in thickeness were prepared by a water casting method. Since the radius of gyration is given to be 0.03*M^<1/2> nm for a poly-stirene chain with molecular weight of M,the chains with molecular weight of more than 100,000 are to be squeezed in the ultra-thin films ; accordingly chains shows structures and dynamics different from those in bulk. 2.The X-ray amorphous halo observed with use of PSPC has shown no difference between the thin film and bulk ; the structure of the most of chains in the film has the same structure as in bulk. 3.Atomic force microscopy shows that the fluctuation of height on the surface of the amorphous film is about 1 nm. The fluctuation was observed also on the crystal surface after crystallization of the film, and hence crystais are coverd with amorphous layr. 4.The effect of substrate are conspicuous on primary nucleations. Above 210ﾟC of crystallization temperature, lemellar crystals perpendicular to a carbon substrate were observed at the center of grown crystals ; a screw dislocation was at the center for mica substrate. Below 150ﾟC,observed were 2-dimensional spherulites with branched lemellae perpendicular to the substrate. 5.Film thckness dependence of lateral growth rates wrere measured as a function of molecular weight M,crystallization temperature T_C and substrate materials. For the lamellar crystals parallel to the substrate, the growth rate G decease linearly with the inverse of the thickness D : G/G_*=1-a/D where G_* is the growth rate in bulk, a is a constant of about 5 nm, independent of T_C, M and substrate materials. For the 2-D spherulites, G remains unchanged with decreasing D.We therefore conclude that the part of a polymer chain at a distance less than 5 nm from a substrate "feels" the absorption of chain segments on the substrate, and hence suffers the delay of diffusion for crystallization to give rise to G decreasing linearly with 1/D.