Supporting data for thesis - Tailoring Morphology of Polyamide Thin Film Composite Membranes with Nanobubble Chemistry for Enhanced Separation Performance in Desalination and Water Reuse

Abstract of thesis:Polyamide-based thin film composite (TFC) reverse osmosis (RO) membranes have been dominantly applied in desalination and water reuse to address worldwide water scarcity. The membrane separation performance is strongly dependent on polyamide roughness features and the associated nanosized voids. However, the formation mechanisms of these features and voids remain poorly understood in literature. The impacts of interfacial polymerization (IP) chemistry and substrate properties on polyamide formation remain controversial. This thesis aims to revisit and explore the exact IP/substrate-morphology-performance correlation based on a novel nanofoaming theory.Inspired by the recent findings that the nanosized voids are formed due to the degassing of CO2 nanobubbles during the IP reaction, we systematically investigated the role of carbonate chemistry, particularly the solubility of CO2 in the aqueous m-phenylenediamine (MPD) solution. “Ridge-and-valley” roughness features were obtained when the pH was between the two acidity constants of the carbonate system (i.e., 6.3 ≤ pH ≤ 10.3). Increasing pH over this range led to both increased water permeability and better rejection of various solutes, thanks to the simultaneously enhanced effective filtration area and crosslinking degree of the polyamide layer. Further increase of pH to 12.5 resulted in more disparate rejection results due to membrane hydrolysis.The impacts of organic solvents on formation of the nanovoids were further investigated. Compelling evidence was found that vaporization of the organic solvent contributes to nanovoids formation during the exothermic IP process. A series of alkane solvents with systematically varying vapor pressure were used to prepare TFC membranes. An organic solvent with higher vapor pressure generated more vapor during the IP reaction, which in turn resulted in larger size of the voids in the polyamide thin film and higher membrane water permeability.The impacts of MPD concentration (0.05-8.0 wt.%) on polyamide formation were subsequently deciphered by adopting a free-interface IP strategy to suppress the nanofoaming effect. The corresponding polyamide nanofilms had negligible nanovoids and monotonously increased film thickness, leading to decreased water permeance at higher MPD concentrations. In contrast, the conventional TFC membranes exhibited optimal water permeance at the intermediate MPD concentration of 2 wt.%, which results from the tradeoff between improved nanovoid formation (which promotes higher permeance) and increased film growth (which limits permeance) at higher MPD concentration.The exact roles of substrates were further dissected. TFC membranes were prepared on a series of polycarbonate substrates with cylindrical track-etched pores (PCTE) of well-defined pore size and several conventional substrates with random pores. Substrate porosity plays a critical role in membrane water permeance, while smaller pores with greater pore density are favored to improve membrane rejection. The TFC membranes prepared on conventional substrates exhibit better performance compared to the PCTE-TFC membranes, thanks to the simultaneously enhanced confinement and MPD storage effects.Overall, this thesis provides new angles to understand the roles of reaction conditions and substrate properties on polyamide. The mechanistic insights can favor better interpretations on some controversial observations in literature. The fundamental framework gained in this thesis further improves the nanofoaming theory, which can guide the future design and optimization of TFC membranes.

论文摘要：以聚酰胺为基础的薄膜复合（TFC）反渗透（RO）膜在海水淡化和水资源再利用领域得到了广泛的应用，以应对全球水资源短缺的问题。膜的分离性能与其表面的聚酰胺粗糙度和相关纳米级孔隙密切相关。然而，这些特征和孔隙的形成机制在文献中尚缺乏深入理解。界面聚合（IP）化学和基底性质对聚酰胺形成的影响仍存在争议。本论文旨在基于一种新颖的纳米发泡理论，重新审视并探索IP/基底-形态-性能之间的确切关联。受近期研究发现，纳米级孔隙是由于IP反应过程中CO2纳米气泡的脱气作用形成的启发，本研究系统地研究了碳酸盐化学，特别是CO2在水性m-苯二胺（MPD）溶液中的溶解度对聚酰胺形成的影响。当pH值位于碳酸盐系统的两个酸度常数之间时（即6.3 ≤ pH ≤ 10.3），可获得“脊谷”状的粗糙度特征。在此pH范围内，pH值的增加既提高了水的渗透性，又改善了各种溶质的排斥效果，这得益于聚酰胺层同时增强的有效过滤面积和交联度。将pH值进一步增加到12.5，由于膜的水解作用，导致排斥效果出现更多差异。进一步研究了有机溶剂对纳米孔形成的影响。有力的证据表明，在放热的IP过程中，有机溶剂的挥发有助于纳米孔的形成。使用一系列具有系统变化蒸汽压的烷烃溶剂制备了TFC膜。在IP反应过程中，具有更高蒸汽压的有机溶剂产生了更多的蒸汽，这反过来又导致了聚酰胺薄膜中孔隙尺寸的增大和膜水渗透性的提高。通过采用自由界面IP策略来抑制纳米发泡效应，随后解析了MPD浓度（0.05-8.0 wt.%）对聚酰胺形成的影响。相应的聚酰胺纳米薄膜几乎不含纳米孔，且薄膜厚度单调增加，导致在高MPD浓度下水渗透率降低。相比之下，传统的TFC膜在2 wt.%的MPD中间浓度下表现出最佳的水渗透性，这是由于在较高MPD浓度下，纳米孔的形成（促进更高渗透性）与薄膜生长（限制渗透性）之间的权衡所致。进一步剖析了基底的确切作用。在一系列具有明确孔径的聚碳酸酯基底（PCTE）和具有随机孔径的几个传统基底上制备了TFC膜。基底孔隙率对膜的水渗透率起着关键作用，而较小孔径和较高孔密度的孔更有利于提高膜的排斥效果。与传统PCTE-TFC膜相比，制备在传统基底上的TFC膜表现出更好的性能，这得益于同时增强的约束和MPD储存效应。总体而言，本论文为理解反应条件和基底性质对聚酰胺的作用提供了新的视角。机制上的洞察力有助于更好地解释文献中的一些有争议的观察结果。本论文中获得的基本框架进一步改进了纳米发泡理论，这可以指导TFC膜的未来设计和优化。