Relationship between surface friction property and surface structure investigated by surface vibration spectroscopy

聚合物和LB膜等软材料的物理性质和功能在很大程度上取决于它们的表面/界面分子结构。我们的研究主要集中在利用表面敏感的非线性光学振动光谱-和频光谱(SFG)来探索功能有机薄膜表面和/或界面的分子结构，而传统的红外吸收光谱和拉曼散射光谱很难获得这种结构信息。目前的研究主要集中在(1)聚丙烯酸酯及其共聚薄膜，(2)聚酰亚胺薄膜和(3)LB多层薄膜，在不同的环境下。可以清楚地发现，这些聚合物材料中的大多数在表面上都具有有序结构，尽管它们的体相结构完全是随机的。表面结构可以与它们的表面属性相关联。例如，当将聚(2-甲氧基丙烯酸乙酯)(PMEA)浸入水中时，只在其表面观察到羰基与水分子之间的氢键，而在其本体中没有观察到氢键。这种氢键在其他聚丙烯酸酯S中是很难观察到的，因此，有望与PMEA良好的生物相容性有关。另一方面，经摩擦处理后的聚丙烯酸酯S的表面结构几乎相同。这与聚酰亚胺薄膜形成了鲜明的对比。简单摩擦处理后的聚酰亚胺表面呈现出较大的各向异性结构特征。摩擦引起的表面重取向与聚酰亚胺的官能团和π-π堆积有关。从SFG结果分析了聚酰亚胺表面的详细结构变化。此外，基于本课题组最近提出的理论模型，还用SFG测量了多层LB膜中任意层-层界面上的结构。

Physical properties and functionalities of soft materials such as polymer and LB films are significantly dependent on their surface/interface molecular structures. Our researches focus upon exploring the molecular structures on the surface and/or interface of the functional organic thin films, by a surface-sensitive non-linear optical vibration spectroscopy, sum frequency generation (SFG), while such structural information is hard to be obtained by the conventional vibration spectroscopy such as infrared (IR) absorption and Raman scattering.The present research is mainly focused on (1) poly(acrylate) and their co-polymer thin films, (2) polyimide thin films and (3) LB multilayer thin films, in different environments. It is clearly found that most of these polymer materials have ordered structures on the surface although their bulk structures are completely random. The surface structures can be associated with their surface properties. For example, when the poly(2-methoxyethyl acrylate) (PMEA) was immersed into water, a hydrogen bonding between carbonyl group and water molecule was only observed on its surface but not in its bulk. This kind of hydrogen bonding was hard to observe in other poly(acrylate)s and therefore, is expected to related with PMEA's excellent biocompatibility. On the other hand, the surface structures of the poly(acrylate)s are almost identical after rubbing treatment. This is in contrast with polyimide films. A large anisotropic structural feature was observed on the polyimide surface after simple rubbing treatment. The rubbing-induced surface reorientation is dependent on the function groups and π-π stacking of the polyimide. Detailed structural changes on the polyimide surface were analyzed from the SFG results. Furthermore, SFG measurements were also carried out to probe the structure on the any layer-layer interface in a multilayer LB film based on a theoretical model developed by our group recently.