温敏聚氨酯基纳米原位杂化膜内气体反常传质机理研究

Previous reports on polymeric nanocomposites proposed the possible existence of a higher-free-volume interfacial layer between the bulk polymer and the nanoparticle, which resulted in counterintuitive increase in gas permeability of the composite membranes. However, whether the interfacial layer really exists is still in argument, due to the lack of crucial evidences based on conventional detection technology and non-ideal composite system under consideration. In view of such difficulties, in situ hybridization technique will be employed in this project to prepare thermo-sensitive polyurethane (TSPU) membranes with individually-dispersed SiO2 or TiO2 nanoparticles. Based on such ideal nanocomposite system that contains no nanoaggregates and defects, relationship between the flexibility of macromolecular chains during membrane formation and their packing around nanoparticles will be systematically explored by utilizing: (1) the fact that the flexibility of TSPU chains varies remarkably below and above its switch temperature; (2) characterization methods coupling various detection technologies and molecular simulation. Accordingly, the formation mechanism of higher-free-volume interfacial layer will be elucidated, while crucial evidences supporting its existence in the nanocomposite will be obtained, respectively. These evidences will then be used to rationalize the increased gas permeability of TSPU nanocomposite membranes, which is completely contrary against the expectation of conventional composite theory. Furthermore, a mathematical model will also be established, by which the counterintuitive gas transport behavior through TSPU nanocomposite membranes can be quantitatively predicted. According to the research results in this project, we aim at providing structural insights into verification and development of previous hypothesis, which are capable of broadening our understanding towards the structure as well as the gas transport behavior of polymeric nanocomposites. Simultaneously, these results are also expected to lay foundation for the future application of nanocomposite technology to accurately regulate the gas permeability of polymeric membranes, which thereby are of important theoretical significance and practical value.

前人提出在聚合物基纳米复合膜材料的两相间可能还存在一高自由体积界面层，而这是导致复合膜透气性反常增大的根本原因。但受研究体系和检测技术限制，该界面层是否真实存在尚无法证实。针对这一关键问题，本课题以温敏聚氨酯为连续相，采用原位杂化技术于其中反应生成单分散纳米SiO2或TiO2粒子，拟在排除纳米团聚体和非理想膜缺陷干扰的基础上，利用温敏聚氨酯的温敏特性，并将仪器检测与分子模拟技术相结合，探寻成膜过程中大分子链的运动自由度与其在纳米粒子周围堆积状态间的内在联系，阐明两相间高自由体积界面层的形成机制并获得其存在的关键证据，揭示造成纳米复合薄膜透气性反常增大的根本原因，同时建立可定量预测这种反常传质行为的数学模型。此研究工作旨在验证并发展前人假说，丰富学术界对聚合物基纳米复合膜结构和传质行为的认识；同时也可为未来采用纳米复合技术准确调控聚合物膜材料的透气性能奠定基础，具有重要的理论意义和应用价值。

经典复合物理论认为，无孔、惰性填料的存在会造成气体小分子在聚合物薄膜中的扩散路径曲折化，这是利用外添加无机纳米粉体显著提高聚合物膜材料气体阻隔性的理论基础。. 然而，当人们将纳米SiO2或TiO2与某些玻璃态聚合物共混时意外地观察到：随着纳米填料用量增加，这些复合薄膜的气体渗透系数均显著提高，表现出违背经典复合物理论的透气性变化规律。基于这一反常现象，前人推测，在玻璃态聚合物与纳米填料相间可能还存在一传质阻力较小的高自由体积界面层。然而，由于这一假说涉及两固相间在分子水平的静态结构和气相在皮秒至纳秒水平的动态行为，凭目前的技术又很难直接获取以上空间和时间尺度内的实验数据。因此，前人至今未能提供有力证据，证明这些所谓的高自由体积界面层确实存在，以上假说也并未得到学术界的普遍认同。. 围绕这一高自由体积界面层是否存在及其形成机制，本课题以热力学/动力学特征与玻璃态聚合物类似的温敏聚氨酯为连续相，采用原位杂化技术于其中反应生成单分散纳米TiO2填料，在排除纳米团聚体中包藏孔隙及宏观膜缺陷对实验结果干扰的基础上，利用温敏聚氨酯自身温敏特性，系统研究了成膜过程中大分子链运动自由度差异对杂化膜相界面相互作用方式、自由体积、密度及透气性的影响规律；同时运用分子动力学模拟技术可视量化了温敏聚氨酯在纳米填料周围及主体部分的堆积密度。通过对开关温度上下实验/模拟结果的对比，本课题得出如下结论：成膜过程中运动受限的大分子链受局部收缩应力影响，无法在具有大表面曲率的纳米填料周围紧密堆积，这是导致某些聚合物与纳米填料相间存在高自由体积界面层、复合膜透气性违背传统理论反常提高的根本原因。. 经过努力与探索，本课题完成了既定研究目标，获得了具有代表性和普遍指导意义的理论/应用新成果。在理论上，阐明了成膜过程中聚合物大分子链运动自由度与纳米复合膜材料相界面微区结构、透气传质性能间内在联系，揭示了两相间高自由体积界面层的形成机制并获得了其存在的关键证据，丰富了传统理论对这类材料结构和传质行为的认识。在技术的应用上，本课题首次获得了高自由体积界面层厚度参数及其与纳米填料粒径间定性关系，为建立可定量预测复合膜反常透气性的数学模型及未来采用纳米复合技术实现聚合物膜材料透气性能的按需调控奠定了基础，在皮革/纺织品涂饰、气体分离、建筑气密、医疗卫生等众多膜技术领域具有广泛的应用前景。