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

<b>Abstract of thesis:</b><br>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.<br>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.<br>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.<br>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.<br>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.<br>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.

<b>论文摘要：</b><br>聚酰胺基薄膜复合（TFC）反渗透（RO）膜已被广泛应用于海水淡化和水资源回用领域，以应对全球水资源短缺问题。膜的分离性能高度依赖于聚酰胺的粗糙结构特征及相关的纳米空隙，但这些结构特征和空隙的形成机制在文献中尚未得到充分理解。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（nanofoaming theory），重新审视并探索IP/基底-形态-性能之间的确切关联。<br>受近期研究发现（即IP反应过程中CO₂纳米气泡脱气导致纳米空隙形成）的启发，我们系统研究了碳酸盐化学的作用，尤其是CO₂在间苯二胺（MPD）水溶液中的溶解度。当pH处于碳酸盐体系的两个酸解离常数之间（即6.3 ≤ pH ≤10.3）时，可获得“脊-谷”型粗糙结构。在此范围内升高pH，聚酰胺层的有效过滤面积和交联度同步提升，从而同时提高水渗透率和对多种溶质的截留性能。pH进一步升高至12.5时，膜发生水解，导致截留结果出现更大差异。<br>研究进一步探讨了有机溶剂对纳米空隙形成的影响。结果发现强有力的证据表明，在放热的IP过程中，有机溶剂的汽化有助于纳米空隙的形成。通过使用一系列蒸气压系统变化的烷烃溶剂制备TFC膜，发现蒸气压更高的有机溶剂在IP反应中产生更多蒸汽，进而导致聚酰胺薄膜中的空隙尺寸更大，膜的水渗透率更高。<br>随后，我们采用自由界面IP策略抑制纳米发泡效应，阐明了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中纳米空隙可忽略不计，且膜厚单调增加，导致MPD浓度升高时水渗透率降低。相比之下，传统TFC膜在MPD浓度为2 wt.%的中间值时表现出最优水渗透率，这是由于MPD浓度升高时纳米空隙形成改善（促进渗透率提升）与膜厚增加（限制渗透率）之间的权衡所致。<br>基底的确切作用得到进一步剖析。我们在一系列具有明确孔径的圆柱形刻蚀孔聚碳酸酯基底（PCTE）和几种具有随机孔隙的传统基底上制备了TFC膜。基底孔隙率对膜的水渗透率起关键作用，而孔径更小、孔密度更高的基底更有利于提升膜的截留性能。与PCTE-TFC膜相比，传统基底上制备的TFC膜表现出更优的性能，这得益于限域效应和MPD存储效应的同步增强。<br>综上，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视角。这些机制见解有助于更好地解释文献中一些有争议的观察结果。本论文获得的基础框架进一步完善了纳米发泡理论，可为未来TFC膜的设计与优化提供指导。