Gap Attack Platform C: Fundamental Science of Robust Membrane Manufacturing

Project #1: Design and Preparation of Isoporous Separation Membranes

GAP C research will leverage our prior experience in performing the SNIPS process with PS-b-P4VP and related systems towards the reproducible fabrication of isoporous asymmetric membranes from UMCP materials. Experimental efforts will be directed at understanding and harnessing the nonequilibrium phenomena that will enable the manufacture of isoporous membranes with tunable pore size and size distribution. New UMCP copolymers will be characterized for solution phase ordering via X-ray scattering to narrow the casting solution design space to solvents best suited for inducing self-assembly (Su, Freeman). UMCP variants capable of forming isoporous asymmetric membranes will undergo further characterization of thermomechanical properties via tensile testing, dynamic mechanical analysis, and thermogravimetric analysis of dense films (Sanoja, Freeman) to inform synthetic efforts in improving BCP strength/toughness and thermal stability (Lynd). Isoporous UMCP membrane structures will be identified and characterized via electron microscopy, and their separation performance, namely pure water permeance, molecular weight cutoff, solute rejection, and fouling behavior, will also be measured (Freeman).

The experimental program on SNIPS will leverage a parallel theoretical and computational effort to develop and exploit models of the dry evaporation and wet immersion steps of SNIPS. Phase field models will be constructed (Fredrickson, Ganesan) based on multicomponent fluctuating diffusion equations (Model B dynamics) for the densities of solvent, non-solvent, cosolvent, and individual block species. The Rouse model is used to compute the Onsager mobility tensor entering the dynamical equations. Structural arrest is described by a sigmoidal increase in local friction experienced by solvents and polymer segments when a glass transition threshold is crossed. Such a model produced realistic asymmetric membrane structures in prior NIPS simulations. Unlike that work, where the chemical potential gradients driving multicomponent diffusion were derived from a phenomenological Flory-Huggins-de Gennes free energy functional, the thermodynamic forces will be numerically computed using self-consistent field theory (SCFT). This embeds a rich thermodynamic basis for the model that has a molecular origin faithful to the multi-block polymer architectures deployed. The resulting dynamic SCFT framework (DSCFT) can capture both the macrophase and microphase separation processes involved in SNIPS.

Modeling of the dry and wet steps of SNIPS will take place sequentially. The dry evaporation protocol will follow previous work on modeling solvent evaporation from thin block copolymer films. Key variables to explore are the evaporation time, the solvent volatility, and the block selectivities towards solvent, cosolvent, and air. The morphological output from the dry evaporation step provides the initial condition for wet immersion, wherein the air phase is replaced by a non-solvent bath that is permitted to diffuse into the film and initiate macrophase separation. Future investigations will extend to three dimensions, focus on the UMCP architecture, and include vitrification when the concentration of the matrix-forming block locally exceeds a glass transition threshold.

Since GAPS A and B will incorporate functional groups to the hydrophilic block to impart selectivity to UMCP membranes, the SNIPS/NIPS manufacturing process must be robust to these additional groups. Yet, we have found that the inclusion of a relatively small pendant group has a profound effect on the Flory interaction parameter. An important component of the modeling effort will be to establish a reliable method to estimate the Flory interaction parameters that are inputs to the dynamical theory and describe the pairwise interactions among polymer segments, solvents, co-solvents, and non-solvent. As the chemical variations that will be explored in the project are wide-ranging, an in silico method that requires no experimental input will be pursued. Specifically, structure factors obtained from all-atom molecular dynamics (MD) simulations of short polymer blocks mixed with solvents or low molecular weight polymers will be fit to analytical random phase approximation (RPA) structure factors to extract 𝜒 parameters (Ganesan, Fredrickson). While this procedure has precedent in the literature, we have recently discovered that much more accurate values of 𝜒 result from substituting intramolecular scattering information obtained from the MD simulations for theoretical Debye functions.

The experimental and theoretical components of Project 1 will be closely coordinated, with theory informing experiment of the most promising polymer designs and choices of solvents and casting conditions, and experiment providing a platform for validating the assumptions and parameters of the underlying models.
 

Project #2: Operando Characterization of Assembly Pathways in Integral Asymmetric Membranes

Project 2 of GAP C will unveil the formation process of integral asymmetric UMCP-based membrane. The inherently non-equilibrium nature of SNIPS makes it challenging to predict membrane structure and attain reproducibility. Project 1 (Fredrickson, Ganesan) will provide preliminary estimates of the kinetic evolution of self-assembly and phase separation. Su, Katz, and Crumlin will develop operando scattering tools within the IF which will be used in GAP C to track each stage of the SNIPS process on the submicron scale, addressing knowledge gaps connecting block copolymer chemistry, architecture, and processing to isoporous membrane structure. To complement this sub-micron characterization, Squires will develop microfluidic interferometric techniques that track the spatio-temporal evolution of the evaporating solvent and infiltrating non-solvent under controlled conditions across the thickness of the film, directly visualizing both wet and dry steps.

The main steps of the SNIPS process are block copolymer micelle formation in solution, micelle assembly into a microphase-separated skin during solvent evaporation, and nonsolvent-induced macrophase inversion (i.e., NIPS). In situ spectroscopic tools will be employed for each of these three steps.

Step 1: Micelle formation - In situ scattering techniques will uncover structure formation and kinetics. Block copolymer micelle structure and size in solution dictate the characteristic pore size and spacing within the skin layer. X-ray scattering of block copolymer micelles in solution will track the size of block copolymer micelles and inform optimal solvent compositions. Moreover, by leveraging recent developments at the ALS in in situ liquid resonant soft x-ray scattering (RSoXS), it will be possible to differentiate the spatial extent of the core vs. shell regions of micelles. These findings will also help guide Project 1 experimental efforts in UMCP SNIPS casting solution design.

Step 2: Solvent evaporation - After membrane casting, micelles merge as the solvent evaporates, which templates the morphology of the isoporous skin layer in a non-equilibrium state. Understanding the fundamentals of this process requires operando measurements that reveal the evolving concentration profiles of solvent and polymer (which drive the structural evolution), as well as the evolution of the structure itself. Such measurements provide feedback to guide block copolymer synthesis (Lynd), complementarity and validation for physics-based computation (Fredrickson, Ganesan), and feedforward to full membrane structure, mechanical properties, and separation performance (Freeman, Sanoja). The operando x-ray scattering device will be developed in the IF with appropriate time and length scales to be leveraged in this project to track the morphology evolution during the solvent evaporation step. From our simulations, skin layer ordering and growth are driven by concentration profiles that change both spatially and temporally. These rates will be measured using microfluidic interferometry technique (Squires), to track the solvent profile across precursor films exposed to a flowing gas of controlled composition (e.g., specified vapor pressure of organic solvent or H2O), with ~1 μm spatial and 100 ms temporal resolution.

Step 3: Macrophase inversion - In situ characterization will also include the observation of structure evolution during nonsolvent immersion. Immersion in a nonsolvent produces the open, porous structure below the skin layer due to macrophase separation between the nonsolvent and polymer. In situ x-ray scattering (developed in the IF) will reveal if changes in the self-assembled isoporous layer occur upon nonsolvent immersion. Integral asymmetric isoporous membranes will also be fabricated in a hollow fiber geometry, which is beneficial for continuous manufacturing and large-scale installations. In situ SAXS with the x-ray beam positioned at various distances from the spinning nozzle will provide information on the kinetics of structural rearrangements after extrusion.

Operando characterization of asymmetric membrane formation will be complemented by interrogation of previously formed membranes to connect processing conditions to membrane performance. The UMCP platform will provide a route towards isoporous membranes with functionalizable hydrophilic pore walls. Synchrotron infrared nanospectroscopy (SINS) and resonant x-ray scattering will be used to map nanoscale morphology with chemical sensitivity, through unique IR signatures of specific functional groups and tunable scattering contrast, respectively. This will also provide a route to spatially resolve the distribution of adsorbed solutes in membranes exposed to complex waters.

Project #3: Mechanics and Failure Mechanisms of Isoporous Membranes

Development of next generation isoporous membranes requires co-design for optimized selectivity, permeability, and mechanical properties. The role of pores in the fracture toughness of membranes is of critical importance. Elasto-plastic materials with cracks below 1 mm are generally regarded as behaving as if pristine, suggesting that pore sizes spanning from 10 nm to 1 mm are inconsequential to mechanical properties. However, it is also well known that block copolymer anchoring and other nanometer scale geometrical features strongly impact mechanical properties including the fracture energy, yield stress, and fracture strain.

To understand the role of pore structure on the mechanical properties of membranes, a UMCP series of varying porosity will be produced by SNIPS, equilibrated in water, and inflated until failure with a membrane inflation apparatus (Sanoja, Freeman). The resulting apex stress-strain curves will be used to validate and inform our simulations and determine mechanical properties including the biaxial modulus and work of extension until failure (Dortdivanlioglu). By identifying the critical pore size distribution at which the work of extension until failure of an isoporous membrane matches or outperforms that of a dense membrane, membranes with improved transport properties and excellent mechanical integrity will be prepared.

UMCP membranes will also be cast on deformable substrates (e.g., copper grids), equilibrated in water, and equi-biaxially deformed until failure (Sanoja, Fredrickson). By monitoring the crazing and failure strains using in-situ optical and scanning probe microscopy, and SAXS; fracture mechanisms of membranes will be unveiled, as well as molecular designs that delay craze nucleation and growth under equi-biaxial loads.

Understanding the brittle-to-ductile transition of membranes is important to prevent catastrophic failure, but membrane lifetime is often controlled by creep under fixed sub-critical loads. To evaluate this mechanical degradation, UMCP membranes will be inflated to a fixed apex stress, and the apex strain monitored as a function of time (Sanoja). The resulting creep rate will be correlated with the water permeability and selectivity to gain a better understanding of the relationship between membrane structure, and mechanical and chemical stability under wet operating conditions. We will also explore strategies to improve the creep resistance of UMCP membranes by incorporating some high molecular weight PS homopolymer in the casting solution, and substituting the PS block with a poly(acrylonitrile-co-styrene), PAS, that physically cross-links via dipole-dipole interactions.

We will build size-dependent, continuum-based models for isoporous membranes undergoing large, elastoplastic deformations under membrane inflation (Dortdivanlioglu, Ganesan) to quantify the effect of pore size and size distribution on the overall mechanical and fracture behaviors. The material properties underlying the continuum model will be informed by atomistic simulations on smaller scale nanostructures. The model will further predict fracture nucleation sites and crack propagation through the microstructure to guide experimentation and membrane design (Sanoja, Lynd). Numerical homogenization methods will be developed to model full-sized membranes with gradient porous substrates and isoporous surfaces. The resulting knowledge will provide crucial insights and feedback into the design and casting conditions of Project 1, and ultimately allow for the simultaneous optimization of membrane transport and mechanical properties.