Nanostructured polymer membranes offer the ability to control pore architecture at the atomic length scale. Lyotropic liquid crystals (LLCs) are a class of nanostructured materials that can be modified, assembled and crosslinked into stable membranes with uniform, straight pores that can perform solute-specific separations. Pore diameters on the order of 1 nm make LLC membranes well-suited for aqueous separations such as desalination and biorefinement. We have matched X-ray diffraction (XRD) patterns generated from equilibrated atomistic molecular dynamics simulation trajectories to experimental 2D wide angle X-ray scattering (WAXS) patterns in order to gain a detailed picture of the LLC membrane nanopores. Using this approach, we created a structural model that is maximally consistent with experimental WAXS by matching reciprocal space motifs to real space configurations. We have identified the density of monomers that pack around pore cylinders. We have evidence to support that aromatic monomer head groups pi-pi stack on top of each other in a sandwiched, rather than parallel displaced, configuration. We show that the long alkane tails of each monomer pack into hexagonal arrays with short range order.  We also observed alternate metastable configurations, stable for 100s of nanoseconds, but whose simulated XRD patterns are distinct from experiment.  By studying the system in real space, we observe that the pores are not ideal<br/>cylinders, but rather consist of a gradient in composition transitioning from hydrophilic cores to hydrophobic tail regions.  We use cross-sections of 3D correlation functions to assess the degree of ordering in our system and to compare it to experimentally measured values. We calculate the nematic order parameter and the correlation length of monomer columns in order to quantify the degree of ordering along the system's pore axis.  Using our detailed model of the membrane pores we will be able to study the transport of small charged and uncharged molecules within them with confidence that the chemical environment is a close match to the real system.

Understanding the nanoscopic pore structure of a hexagonal phase lyotropic liquid crystal membrane

256th National Meeting (2018)

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