Structural Fluctuations and Crystal Growth in Ultra-thin Polymer Films

1.采用水铸法制备了厚度为20 ~数百nm的超薄聚苯乙烯薄膜。由于给定分子量为M的聚苯乙烯链的旋转半径为0.03*M^<1/2> nm，因此分子量大于100,000的聚苯乙烯链将被挤压到超薄膜中；因此，链的结构和动力学特性不同于散装链。2.用PSPC观察到的x射线非晶晕在薄膜和体之间没有区别；薄膜中大部分链的结构与体中的结构相同。3.原子力显微镜显示，非晶膜表面的高度波动约为1 nm。薄膜结晶后，在晶体表面也观察到波动，因此晶体被非晶层覆盖。4.底物对初成核的影响是显著的。在210ºC以上的结晶温度下，生长的晶体中心形成了垂直于碳衬底的lemleleal晶体；云母衬底中心存在螺位错。在150℃以下，观察到的是垂直于衬底的二维球晶。5.测量了薄膜厚度对横向生长速率的依赖关系，作为分子量M、结晶温度T_C和衬底材料的函数。对于平行于衬底的片层晶体，生长速率G与厚度D成反比线性减小：G/G_*=1-a/D，其中G_*为整体生长速率，a为约5 nm的常数，与T_C、M和衬底材料无关。对于二维球粒，G随D的减小而保持不变。因此，我们得出结论，距离底物小于5 nm的聚合物链部分“感受到”底物上链段的吸收，因此遭受结晶扩散的延迟，从而导致G随1/D线性减小。

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.