Solution self‐assembly behavior of A‐B‐C triblock polymers and the implications for nanoporous membrane fabrication

Abstract

Self‐assembled block polymer membranes are rising to the fore of next‐generation separations technologies, and a particular fabrication process of significant interest is the self‐assembly and non‐solvent induced phase‐separation (SNIPS) method. This is because the SNIPS process allows for the facile manipulation of independent variables to produce simultaneously size‐ and chemically‐selective membranes in a scalable manner. Despite significant advances in the SNIPS membrane fabrication procedure, the early‐stage solution behavior of many of the polymer–solvent systems employed is still poorly understood. This work addresses the challenges posed by these systems and illuminates guiding relationships for the optimization of SNIPS membrane casting solutions. Specifically, polymer aggregate structures and critical micelle concentration (CMC) values are determined for an A‐B‐C triblock polymer dissolved in 1,4‐dioxane using a combination of scattering and atomic force microscopy techniques. The CMC of this system ranges from 20 to 80 μM and shows a strong dependence on the polymer composition and solution processing method. Additionally, the shape of the self‐assembled structures in solution can be altered by varying these parameters. These studies establish the relationships between polymer solution properties (i.e., the block polymer composition and the processing method) and assembly behavior as they impact membrane fabrication through SNIPS processing. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45531.

A rheometry method to assess the evaporation‐induced mechanical strength development of polymer solutions used for membrane applications

“…The differences in mechanical strength development reported in Figure 5 at the same polymer concentration may be attributed to the possible long-range lattice organization based on the differences in block chemistry of each sample. Sargent, et al 7 previously reported that ISD terpolymer solutions (59 kg mol -1 ) formed individual micelles at low polymer concentrations (<1 wt.%) and long-range lattices at significantly higher polymer concentration (>22 wt.%) using DOX as solvent. Dorin, et al 2 showed that 16 wt.% ISV terpolymer solutions (59 kg mol -1 ) organized into lattices using a 7:3 THF:DOX solvent mixture.…”

“…6 7 8 9 The high solute selectivity as well as the high solvent permeability are key properties of SNIPS membranes that are the direct result of the high density of uniform pores obtained by the assembly of block copolymers. 7 A SNIPS membrane is comprised of a hierarchical tapered structure. 10 The selective layer is the result of block copolymer assembly and exhibits a well-ordered structure with a high density of uniform nanoscale pores, the main function of which is to separate solutes from the solvent.…”

“…6 Importantly, however, one of the major challenges towards the creation of a commercial filtration membrane is to manufacture these revolutionary new materials by continuous casting techniques (e.g., a roll-to-roll process). 7 To increase the scale of membrane production, the SNIPS process must be adapted to a scalable manufacturing environment and, specifically, the film translation steps that occur during roll-to-roll casting. This adjustment involves the introduction of new variables such as convection, shear stresses, and film deformation.…”

“…13 Consequently, elucidating the behavior of the block copolymers and the solvent evaporation at early stages is imperative to accurately translate the SNIPS membrane from the laboratory scale to a commercial-scale product. Mechanisms regarding the self-assembly process and final assembled morphology have been proposed by Dorin, et al, 6 Sargent, et al, 7 Gu, et al, 14 and Rangou, et al 15 Also, the effect of relative humidity over the final assembled morphology have been elucidated by Li, et al 16 However, the roles of block copolymer chemistry and solvent evaporation over the mechanical evolution of SNIPS membranes are still poorly understood.…”

Fit-for-purpose block polymer membranes molecularly engineered for water treatment

“…The evaporation of solvent generates an increased polymer concentration at the solution-air interface, which causes the block polymer to form into ordered nanostructures across the top surface of the film. 56,[58][59][60][61] At this juncture, it is critical to note that these nanostructures are dynamic and driving toward equilibrium such that the instantaneous nanostructure observed at any point during the evaporation step depends upon the solution composition, solvent evaporation rate, and block polymer properties. At the end of the solvent evaporation step, the film is plunged into a non-solvent bath.…”