制备面向精密分子筛分纳米微孔膜的表面接枝化学

Molecular filtration and sieving at a nanometer and subnanometer scale is an important engineering problem, with very diverse applications ranging from bio-analytic separation, to chemical processing, pharmaceutical separation, food processing and to industrial water purification. It is well-known that the pore size and its distribution and surface properties are crucial to the size-selective performance of nanoporous membranes. Many current microporous membranes are made by phase inversion and suffered from broad pore size distribution and fouling. Herein we are going to develop a novel method to make antifouling nanoporous membranes targeted at a nanometer and subnanometer separation capacity. The method will be based on the controlled surface grafting on three commercial microporous membranes including regenerated cellulose membrane, polyethersulfone membrane and polyacrylonitrile membrane. In the first step, clickable groups will be introduced onto membrane surface in a controlled manner via facile and tailored chemical reactions. In the second step, a series of different hydrophilic polymers having narrow molecular weight distributions will be tethered onto membrane surface via the highly efficient coupling reaction having characteristics of click chemistry. These "graft to" reactions will include thio-ene addition, copper(I)-catalyzed 1,3-dipolar azide-alkyne cycloaddition, 1,3-dipolar nitrile oxide-alkyne cycloaddition. The polymers used for grafting will include poly(ethylene glycol), poly[oligo(ethylene glycol) methyl ether methacrylate], poly[3-(N,N-dimethylvinylbenzylammonio)propane sulfonate], poly(p-styrene -sulfonate) and poly(vinylbenzyltrimethylammonium chloride) and will be prepared by controlled reaction/polymerizations including atom transfer radical polymerization and reversible addition-fragmentation chain transfer polymerization. In that case, the pore size of the membrane can be narrowed down to a nanometer and subnanometer level and can be tuned by the molecular weights of the grafted polymers. At the same time, a densely grafted polymer brush can be built on the membrane surface forming a hydrophilic layer and a steric/enthalpic barrier to surface fouling. A series of antifouling nanoporous membrane having monomodal dispersed and tunable nanopores, including those of electrically neutral, negatively charged and positively charged, will be obtained. In order to get the knowledge on the relation of membrane process-structure-function and develop a novel method precisely modifying microporous polymeric membranes, the structure of the membranes will be characterized in detail and their size-selective separation capacity for bioseparation and molecular sieving will be tested. The effects of the major structure characteristics on membrane separation and antifouling performance will be addressed. These will include the chemical composition, grafting density and chain length of grafted polymers.

用于生物分离和分子筛分的高分子微孔膜的分离性能与其表面物理化学性质、孔径和孔径分布息息相关。目前的主流纳米微孔膜主要由相转化法制备得到，孔径分布宽，孔径难于调控，耐污染性差。本项目针对再生纤维素、聚醚砜和聚丙烯腈膜三种主流高分子商品微孔膜，通过简单的表面修饰反应直接在膜表面可控引入反应性基团，进一步结合三种高效的点击反应，将一系列不同类型和分子量的、窄分子量分布的、抗蛋白吸附的亲水聚合物接枝到膜表面，最终将微孔膜孔径调节至纳米或亚纳米水平，并实现对孔径和孔径分布的调控；在此基础上，总结表面接枝层化学结构、接枝密度、接枝链长等结构特征与膜孔径、耐污染性能、筛分等性能特征的关系规律，发展一种高分子微孔膜表面精确可控接枝新方法，为简单有效的制备面向分子筛分和生物分离、孔径可调和窄分布的、耐污染的纳米微孔膜提供一种新思路。

项目原计划“基于已经工业化的商品高分子微孔膜, 针对性的提出简单高效的修饰和接枝反应, 发展易于实现的可控修饰高分子微孔膜的化学方法; 针对膜材料需要实现的筛分功能和耐污染性能, 选择要接枝的高分子的类型、结构和长度等特征, 通过模块化调控接枝条件和接枝高分子的分子量、接枝密度、荷电性等多种结构特征, 实现膜材料的多种结构和特定功能, 获得一种可以得到广泛应用的制备面向分子筛分纳米微孔膜的化学方法。”经过四年的研究，项目组顺利完成了上述研究计划，取得了一系列成果：将高分子合成化学的最新进展应用于高分子复合膜的制备，针对主流高分子微孔膜，发展了一系列简便、高效、可控的表面接枝方法，在膜表面引入官能团，改善现有高分子膜的性能，制备了一系列包括纳滤膜、吸附膜、催化膜、油水分离膜在内的新型功能高分子复合膜，实现了膜材料的多种结构和特定功能。提出能有效调控功能层的精细结构（包括接枝聚合物分布、接枝密度、接枝聚合物组成、拓扑结构和分子量）的方法并研究功能层的传质行为，在膜制备条件-功能层精细结构-膜性能之间建立关联的基础上，总结若干膜材料构效关系的内在规律，为进一步提升膜材料的性能提供了理论基础。同时，我们也发现，我们在申请立项时的思路是有局限的。通过项目研究，我们有了两点新认识：1) 利用聚合物接枝调节膜孔径在很大程度上受制于初始膜材料的孔径和孔径分布。利用接枝反应调节膜孔的影响因素很多。利用接枝反应调节膜孔径不是一个科学问题, 而是一个工程问题。接枝聚合物的分子量很多情况下不是主要决定因素。2) 原计划利用膜孔调节实现精密筛分，但经研究发现, 目前关注最多的生物分子的膜分离, 基本在纳滤和紧密超滤(tight UF)范围。在这种情况下, 溶质和膜孔表面的相互作用远大于基于孔径大小的筛分作用。相关工作发表SCI论文18篇，其中包括J. Mater. Chem. A, ACS Appl. Mater. Interfaces和J. Membr. Sci.等膜材料领域国际知名刊物在内的影响因子5.0以上论文14篇，获得授权发明专利7项, 1项已转让。