Gap Attack Platform 3

Mesoscale Structures to Tailor Fluid Flow

GAP Co-Leads: Rachel Segalman, Venkat Ganesan

GAP Co-Investigators: Glenn Fredrickson, Benny Freeman, Venkat Ganesan, Songi Han, Craig Hawker, Alex Hexemer, Todd Squires, Tom Truskett

Problem Statement

Among the greatest challenges in membrane design is producing porous mesoscale materials with uniform pore sizes and well-defined pore size distributions (PSDs), which are critical in establishing differentiated transport (i.e., separation) of complex mixtures. Water filtration membrane pores (e.g., UF membranes) must necessarily be on the mesoscale (10–100 nm) to achieve component-resolved transport of typical multicomponent water energy fluids. PSDs in commercial membranes (formed via highly non-equilibrium processes) are often modeled as a log-normal distribution (i.e., very broad PSD), and such membranes always exhibit a permeability/selectivity trade-off. Thus, an urgent need is to disrupt this traditional permeability/selectivity tradeoff by employing novel block copolymer membranes from the UMCP platform to tune pore size and distribution and create functionality.

Our ability to design and self-assemble soft matter on the mesoscale is now superb, which suggests the ability to address the fundamental question in mesoscopic membrane design: What pore geometries and distributions are necessary to entrain a targeted component of a complex fluid? Achieving the goal of rational mesoscopic membrane channel design requires fundamental breakthroughs in terms of both materials design and understanding fluid transport:

1)  Equilibrium self-assembly using UMCP materials will provide fundamental information regarding membrane formation, yet equilibrium self-assembly alone cannot give the asymmetric porous structures desired in a working membrane. Thus, ultimately, the desired hierarchical mesostructures will not generally be accessible using standard equilibrium self-assembly routes, but they can be formed via trapping non-equilibrium structures through processing. Novel tools are needed to capture the processing steps, allowing us to harness properties of simple, easily synthesized materials assembled far from equilibrium to create asymmetric, hierarchical pore structures matched to produce energy-relevant, fit-for-purpose water or recover resources.

2)  Component-resolved transport of multicomponent fluids through a complex pore network is poorly understood. The fluid systems of interest are too complex to be described by idealized solution models (e.g., Fick’s law and the Poisson-Nernst-Planck equations for ionic species transport), so understanding structure and flow in heterogeneous confined pore geometries will require new simulation concepts, models, and algorithms as well as experimental tools.

Research Questions

1.  How do we harness self-assembly to make complex, asymmetric pore shapes? Can we fundamentally understand the role of mesoscale and hierarchical structure on multi-component fluid transport, initially leveraging known materials and equilibrium assembly techniques? Can we harness kinetics and thermodynamics of assembly to create asymmetric and non-uniform architectures with pre-determined topologies? Will these structures improve separation in multicomponent systems targeting a particular solute? Can we develop data-driven, inverse-design models to guide mesostructural materials design for separation?

2.  How do we characterize the physicochemical environment of a pore? While we can currently image and scatter from mesoscopic structures, how do we characterize the fluid structure and composition and solid surface chemistry in a buried, mesoscopic architecture?

3.  How can we characterize transport of subtlety different components of multi-component fluids on the mesoscale? As fluid systems of interest are too complex to be described by idealized solution models, can novel simulation concepts, models, and experimental tools be developed to characterize structure and flow in heterogeneous confined pore geometries?

Research Approach

This GAP is fundamentally focused on understanding the role of mesostructure, including associated heterogeneities and asymmetries in structure and topology, in creating differentiated transport rates for a multi-component fluid traversing the pores, and it provides a platform for translating findings from GAPs 1 & 2 into an integrated framework for multicomponent transport in water purification membranes. GAP 3 will utilize experiment, theory, spectroscopy, and modeling to study transport of MFP fluids in simplified hierarchical structures to understand how to harness mesoscopic pore design to favor tailored nonequilibrium transport of specific components to achieve separation. To achieve pore specificity, model architectures will be designed and synthesized using the UMCP. Simultaneously, pore wall chemistry will be tuned to control hydration layers based on input from GAP 1 in parallel with incorporation of pendant alkene and alkyne groups to which specific interactants discovered in GAP 2 will be modularly attached. Water and solute transport will be characterized by bulk measurements, NMR, synchrotron X-ray scattering techniques, and new microfluidic interferometric technique for spatio-temporally evolving concentration profilometry. These measurements in combination with molecular dynamics simulations of transport through the molecular and mesoscale will inform continuum scale models characterizing transport on longer length and time scales. These insights will inform an inverse design framework to optimize flux and selectivity by manipulating pore geometry/surface properties. Non equilibrium processing routes to realize improved designs will be revealed by non-equilibrium simulations. These new materials and processing routes will be used to make and characterize membranes to inform our integrated materials design effort.

Project #1: Connecting Membrane Arch. to Component-resolved Transport/multi-component Fluid Flow

Project Goals:

Design level understanding of how pore topology affects transport and separations of both simple solutions (one solute of known size + water) and complex multi-component fluids.

Development of a design (and physical) library of pore forming copolymers with known and optimized geometry and pore wall chemistry.

Project #2: Inverse Design

Project Goals:

Develop model for transport of fluids (simple ones) in mesoscopic cylindrical pores and use this to develop understanding of effective transport coefficients. The goal is to understand how these parameters depend on pore geometry and chemistry of both proe wall and fluid. This will serve as the training set for the inverse design of the "best" pore.

Designing processing routes that lead to these pore designs.

GAP Attack Platform 3

Research Highlights - GAP 3

Compositional Control of Crosslinked Polyether Networks

Cross-linked polyether networks of varying hydrophilicity have been synthesized using photo-induced cationic polymerizations and will be used to determine how polyether composition can influence water dynamics.

Salt permeation in ligand-functionalized polymer membranes

We show that partitioning dominates over diffusivity to dictate ideal salt permeation in ligand-functionalized polymer membranes.

Connecting Solute Diffusion to Membrane Morphology

Computer simulations reveal that solute diffusion strongly correlates with simple geometric measures of porous triblock copolymer membranes.

Influence of Membrane Architecture on Fluid Transport

Self-Consistent Field and Dissipative Fluid Dynamics simulations and polymer synthesis/membrane characterization probe the impact of pore morphology and pore wall coatings on water transport in a nanoporous membrane.

Role of Side-Chain Architecture in Poly(ethylene oxide)-based Copolymers

The design, synthesis, and characterization of a series of copolymers comprising a poly(ethylene oxide) backbone with discrete linear/cyclic oligo(ethylene oxide) side chains is reported.

Simulations Reveal Asymmetric Membrane Formation Mechanisms in Nonsolvent-Induced Phase Separation

Phase-field simulations demonstrate that mass-transfer-driven spinodal decomposition, thermal fluctuations, and structural arrest are essential to the formation of asymmetric polymer membranes in the Nonsolvent-Induced Phase Separation process.

Versatile Synthetic Platform for Building Membrane Libraries

A robust and efficient synthetic strategy permits development of wide libraries of membrane structures with well-controlled functionality.