Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Computational Characterization of Ultrathin Polymer Membranes in Liquids Qisong Xu and Jianwen Jiang* Department of Chemical and Biomolecular Engineering, National University of Singapore, 117576 Singapore
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ABSTRACT: Polymer membranes are ubiquitously utilized in a wide variety of technological applications; thus, the characterization and fundamental understanding of their properties in relevant working environment are indispensable. In this study, we develop a methodology to computationally characterize the microscopic structures and properties of polymer membranes in liquids by integrating atomistic simulations and two characteristic lengths; the characteristic lengths are proposed from the concept of Gibbs dividing surface. The methodology is illustrated for the swelling of an ultrathin polymer of intrinsic microporosity (PIM-1) membrane in different solvents (acetone, acetonitrile, methanol, ethanol, and water). Specifically, the swelling dynamics is examined by time-dependent solvent content and radius of gyration in the membrane inner layer. The simulated swelling factors are found to agree well with available experimental data. In the five solvents, the predicted swelling degrees follow the experimentally measured trend. Quantitative relationships are observed for the swelling degree with the Hildebrand solubility parameter and the polymer−solvent interaction energy. Furthermore, the swelling of PIM-1 membrane in seawater is simulated; the ions are completely rejected during swelling, and the swollen membrane is structurally similar to the one in pure water. The computational methodology developed in this study can be applied to other polymer membranes in various solvents and liquids, thereby expanding the scope of potential applications. the physiological microenvironment of the cells.9 In sensing, a chemical sensor can be stimulated due to polymer swelling; particularly, the uptake of certain chemical species may cause unique swelling for precise and rapid detection.10 In different processing media, various membrane structures may form because of polymer swelling.11 A great number of theoretical and experimental efforts have been directed toward fundamental understanding of polymer swelling in liquids. A reptation model was proposed for polymer dissolution by evaluating the free energies and chemical potentials of polymer and solvent, respectively, due to the nonrandom orientational distributions caused by swelling.12 By incorporating the polymer chain disentanglement mechanism into relevant transport equations, a polymer dissolution model was proposed and used to describe the dissolution of polystyrene and poly(methyl methacrylate) in methyl ethyl ketone.13 Narasimhan and Peppas summarized different modeling approaches including phenomenological models and Fickian equations, external mass transfer-control based models, stress relaxation models, anomalous transport models, and scaling laws as well as continuum theories.14 Experimentally, the swelling of polymers can be investigated by
1. INTRODUCTION Polymer membranes have drawn considerable attention in industrial applications. They are broadly classified by dimensions into thin (thicknesses between 100 nm and 1 μm) and ultrathin (thickness
