壳聚糖分离膜的制备研究：基于凝胶动力学解析的性能优化

生物聚合物由于具有生物相容性和可生物降解性等特性，对于环境友好型分离膜的开发具有重要意义。然而，目前对于以调控分离膜结构为目的的生物聚合物凝胶机理解读仍不够充分。因此，本研究致力于探索新的表征方法以揭示溶解在不同水溶液中的壳聚糖的凝胶动力学，为优化壳聚糖分离膜过滤性能提供关键依据。

本研究通过将微型凝胶装置与光学相干断层成像技术（optical coherence tomography，OCT）相集成，由于OCT在横向和深度方向上均具有微米级分辨率，成功实现了对多种壳聚糖铸膜液凝胶过程的原位表征。除了通过OCT数据集重构凝胶过程的剖面图像外，本研究还基于平行于载玻片-铸膜液界面的各坐标面计算面平均强度和正异常点份率，并以此量化分析了壳聚糖凝胶动力学。凝胶表征结果揭示了，壳聚糖的凝胶速率与凝胶抑制剂（gelation inhibitors，GIs）的移除密切相关，而壳聚糖链的扩散和固化之间存在周期性的竞争（即Liesegang现象）。值得注意的是，针对溶解在碱/尿素水溶液中的壳聚糖的研究表明，由该碱溶法制备的壳聚糖铸膜液的凝胶化速率可以通过改变凝胶浴中的碱度进行更有效的调控。进一步的对比研究揭示了，溶解在酸性水溶液中的壳聚糖的凝胶化可以通过中和GIs（即质子化的氨基）来诱导，且凝胶速度相对更快。这一比较研究还表明，通过改变去除GIs的方式可以显著改变分离膜的不对称性和孔隙连通度，进而导致不同的水渗透性和对颗粒物的截留行为。本研究通过在水溶液相分离（aqueous phase separation，APS）的框架内对表征结果进行深入解读，以建立不同分离膜结构与过滤性能之间的关联。

Biopolymers are of valuable importance for developing environmentally friendly membranes primarily owing to their biocompatible and biodegradable nature. However, mechanisms accounting for the gelation of biopolymers have not been fully understood in the context of regulating the membrane structures. Therefore, this study was aimed at exploring novel characterization methods to resolve the film-formation kinetics of chitosan dissolved in various aqueous solutions, which would be a key to optimizing the filtration performance of chitosan membranes.

Gelation processes of various chitosan dopes were successfully in-situ characterized by integrating a mini-coagulation bath with a system of optical coherence tomography (OCT), which enabled a micron resolution in both the lateral and depth directions. In addition to creating tomographic images from the OCT datasets, quantitative analysis was implemented by averaging the intensities and evaluating the fraction of positive anomalies on each coordinate surface parallel to the dope-glass interface. It was established by the characterization results that the gelation rate could be dominated by removal of the gelation inhibitors (GIs), while there should be a periodic competition between diffusion and solidification of the chitosan chains (i.e., the Liesegang phenomenon). Of special concern is the dissolution of chitosan in an alkali/urea aqueous solution; it was revealed that gelation rate of the alkaline dope could be more effectively regulated by varying the concentration of the alkali in the coagulation bath. A comparative study was implemented to demonstrate that gelation of chitosan dissolved in an acidic aqueous solution could be induced by neutralizing the GIs (i.e., the protonated amino groups) a relatively fast rate. The comparison also indicated that the asymmetry and pore connectivity could be significantly changed by varying the way of removing the GIs, thereby resulting in different water permeability and rejection behavior of particulate species. All the characterization results were rationalized in the framework of aqueous phase separation (APS) to correlate the varied membrane structures with the filtration performance of chitosan membranes.

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