Formation of anistripic membrane by the hybrid process with thermally induced phase separation and nonsolvent induced phase separation

采用热致相分离与干法相结合的方法制备了各向异性膜。为了诱导各向异性结构，将稀释剂从熔融共混的聚合物-稀释剂的一侧蒸发，从而在淬灭和诱导相分离之前在新生膜中产生浓度梯度。制备这些膜的体系为等规聚丙烯（iPP）的二苯醚溶液，在不蒸发的情况下，得到了接近各向同性的结构。这表明两个表面处的冷却速率被控制为几乎相等。蒸发过程被发现是有用的，在生产各向异性结构。考察了蒸发时间和聚合物初始浓度对膜结构的影响。随着蒸发时间的延长，膜的上表面孔径逐渐减小。当在空气中冷却时，降低淬火温度带来冷却速率的增加，这导致在底部表面处的平均泡孔直径的减小。热梯度和浓度梯度的结合使用产生了明显的不对称结构，在顶部表面的皮肤层。蒸发过程中通过求解适当的传质和传热方程进行了分析。计算结果与实验结果吻合较好。根据计算得到的膜中聚合物的体积分数分布，详细讨论了膜的各向异性结构。

Anisotropic membranes were produced by the hybrid process with both thermally induced phase separation and dry process. To induce an anisotropic structure, diluent was evaporated from one side of the polymer-diluent melt-blended, thereby creating a concentration gradient in the nascent membrane prior to quenching and inducing phase separation. The system used to prepare these membranes was isotactic polypropylene (iPP) in diphenyl ether.In the case of no evaporation, nearly isotropic structures were obtained. This indicates that the cooling rates at both surfaces were controlled to be nearly equal. The evaporation process was found to be useful in producing anisotropic structures. The effects of evaporation time and initial polymer concentration on the anisotropic membrane structure were investigated. The pore size at the top surface decreased by increasing evaporation time.The effects of thermal history, such as cooling rate and quench temperature on membrane structure were investigated. When cooled in air, decreasing the quench temperature brought about an increase in the cooling rate, which led to a decrease in the average cell diameter at the bottom surface. The combined use of a thermal gradient and concentration gradient produced pronounced asymmetric structures with a skin layer at the top surface.The evaporation process was analyzed by solving appropriate mass transfer and heat transfer equations. The agreement between the calculated results and the experimental data on the membrane weight loss and the membrane thickness was satisfactory. The anisotropic structures were discussed in detail based on the calculated polymer volume fraction profiles in the membranes.