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

软质材料如聚合物和LB膜的物理性能和功能性显著依赖于它们的表面/界面分子结构。我们的研究重点是探索功能有机薄膜的表面和/或界面上的分子结构，通过表面敏感的非线性光学振动光谱，和频产生（SFG），而这些结构信息很难通过常规的振动光谱如红外（IR）吸收和拉曼散射获得（丙烯酸酯）及其共聚物薄膜、（2）聚酰亚胺薄膜和（3）LB多层膜。研究发现，尽管这些高分子材料的整体结构是完全无规的，但它们的表面大多具有有序结构。表面结构可以与它们的表面性质相关联。例如，当将聚（2-甲氧基乙基丙烯酸酯）（PMEA）浸入水中时，仅在其表面而不是在其本体中观察到羰基与水分子之间的氢键。这种氢键在其他聚丙烯酸酯中很难观察到，因此，预计与PMEA优异的生物相容性有关。另一方面，摩擦处理后的聚丙烯酸酯的表面结构几乎相同。这与聚酰亚胺薄膜形成鲜明对比。在简单的摩擦处理后，在聚酰亚胺表面上观察到大的各向异性结构特征。摩擦诱导的表面重取向依赖于聚酰亚胺的官能团和π-π堆积。从SFG结果分析了聚酰亚胺表面上的详细结构变化。此外，基于本课题组最近发展的理论模型，我们还对多层LB膜中任意层-层界面的结构进行了SFG测量。

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.