热致相分离(TIPS) 法是制备聚合物膜的一种重要方法，其具有影响因素少、成膜可控性强、膜孔径分布窄等优点。醋酸纤维素(CA)具有成膜性好、来源广泛、可自然降解、应用领域广等特点。近年来，TIPS法制备CA膜开始受到越来越多的关注。选择出高性能的CA稀释剂至关重要。对成膜体系热力学和成膜过程动力学进行深入研究将有助于对CA膜结构进行有效调控，制备具有贯通双连续断面和超薄致密表层的非对称结构。此外，采用TIPS法将亲水性聚合物固定于CA膜结构中可以强化其渗透选择性能并赋予新的功能，有望扩宽TIPS法制膜的应用范围。新型CA分离膜在苦咸水处理、污水处理、生物医药等领域具有应用前景。本文首先以Hansen溶解度参数理论为基础，确定CA稀释剂的选择范围。对比四种稀释剂对TIPS法制备CA膜过程成膜机理的影响。考察稀释剂与凝固浴介质间的质量传递对膜结构和力学性能的影响，选择出新型稀释剂——二苯甲酮(DPK)。CA/DPK体系制备得到膜断面具有贯通双连续孔结构，孔隙率高，力学强度优良。绘制CA/DPK体系热力学相图。液-液相分离区对应最大聚合物浓度达到53 wt%。结合膜结构和力学性能，优选出合适的CA浓度区间。采用溶解度参数分析不同聚合物与CA在DPK中的相容性，考察不同共混聚合物和共混比对膜断面结构的影响，其中聚乙烯吡咯烷酮(PVP)与CA在DPK中相容性好，膜断面无大孔缺陷。考察不同动力学因素——冷却、粗化和蒸发对CA膜致密表层的形成、致密表层厚度以及膜断面孔结构的影响。当铸膜液沿膜厚度方向没有温度梯度和浓度梯度时，冷却速率影响CA膜的断面孔径，制备的CA膜具有各向同性结构。粗化作用可以降低温度梯度，相分离更充分，有效降低致密表层厚度并可以改善膜断面孔结构贯通性。依据Maxwell-Stefan方程和Fourier定律建立的传热传质模型用于描述CA/DPK体系制备平板膜时的蒸发过程，计算结果与实验结果相符，探明了蒸发时间与致密表层厚度的关系。将亲水性添加剂PVP引入CA/DPK体系，制备得到新型CA中空纤维超滤膜。PVP在制膜过程中发生交联反应，有利于PVP分子链固定于CA膜结构中，CA超滤膜透过性能显著提升，并且具有优良的亲水性。膜的韧性和热稳定性得到改善。通过调节CA浓度或采取热处理手段可以使膜表面致密化，实现膜性能从超滤向纳滤的过渡，制备得到的CA中空纤维纳滤膜对一二价电解质溶液具有显著的差异性截留性能。
Thermally induced phase separation(TIPS) is an important method to prepare polymeric membranes. It possesses the advantages of less influencing factors, strong controllability and narrow pore size distribution of the resultant membranes. Cellulose acetate(CA) is a kind of biomacromolecular material which has the characteristics of good membrane formation ability, wide source, natural degradation and wide application. Recently, more and more attention has been paid to the preparation of CA membranes via TIPS method. It is critically important to select a high-performance diluent for CA. The thorough investigation of thermodynamic and kinetics of membrane formation process contributes to the effective control of CA membrane structure in order to obtain a finely tuned asymmetric structure with bicontinuous cross-section and ultrathin skin layer. Besides, fixing a hydrophilic polymer in a CA membrane structure during TIPS process can strengthen the permselectivity and bring some new functions, which is helpful to broaden the application scope of TIPS. The novel CA separation membrane is of great application prospect in the fields of brine and waste water treatment, biomedicine, etc.In this work, the selection scope of CA diluents was determined based on the Hansen solubility parameter theory. Four kinds of diluents were used to study the effect of diluent properties on membrane formation mechanism. The effect of mass transfer between diluent and coagulant medium on membrane structure and mechanical properties was investigated and a novel diluent-diphenyl ketone(DPK) was preferred. The membrane prepared with CA/DPK system had the bicontinuous pore structure, high porosity and excellent mechanical strength.The thermodynamic phase diagram of CA/DPK system was depicted. It showed a broad liquid–liquid phase separation region and the Berghmans point reached the polymer concentration of approximate 53 wt %. The preferred CA concentration region was determined based on membrane structure and mechanical strength. Meanwhile, the compatibilities of different polymers with CA in DPK were analyzed according to solubility parameter. The effects of blend polymer kind and blend ratio on membrane cross-section structure were investigated, which showed that polyvinylpyrrolidone(PVP) had good compatibility with CA and no mcarovoid defect existed in the resultant membranes. Different kinetic factors, including cooling, coarsening and evaporation, were investigated to elucidate the formation and thickness control of dense skin layer as well as the evolution of cross-section pore structure. In the case of no temperature and concentration gradients existed along the casting solution thickness, cooling rate adjusted the pore size of cross-section and the resultant membrane was isotropic. Coarsening decreased the temperature gradient and promoted phase separation. The thickness of skin layer was effectively reduced and the interconnectivity of cross-section pore structure improved. A evaporation model was established based on Maxwell-Stefan equation and Fourier law to depict the flat membrane formation process with CA/DPK system. The calculation results matched well with experimental results, and the relationship between evaporation time and thickness of skin layer was explored. A polymeric additive, PVP, was introduced into CA/DPK system, and a novel CA hollow fiber ultrafiltration membrane with hydrophilic modification was prepared. The cross-linking reaction of PVP was induced which was facilitated to the immobilization of PVP on/in CA membrane. The flux of modified CA ultrafiltration improved obviously and exhibited excellent hydrophilicity. The toughness and thermal stability were also improved. Adjusting CA concentration and conducting thermal treatment densified the skin structure of CA membrane, and membrane performance realized the transition from ultrafiltration to nanofiltration. The resultant CA hollow fiber nanofiltration membranes showed a significant difference on the retention of one and two valent electrolytes.
