Tuning substructure and properties of supported asymmetric triblock terpolymer membranes

doi:10.1016/j.polymer.2016.07.076

Polymer

Volume 107, 19 December 2016, Pages 398-405

https://doi.org/10.1016/j.polymer.2016.07.076 Get rights and content

Highlights

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Block copolymer self-assembly based non-solvent induced phase separation is studied.
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Dependence of asymmetric membrane substructure on casting conditions is tested.
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Parameters tested include polymer concentration, evaporation time, and solvent ratio.
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Results provide design guidelines for sponge- and finger-like substructure formation.

Abstract

Asymmetric poly(isoprene-b-styrene-b-4-vinylpyridine) (ISV) block copolymer membranes fabricated via self-assembly and non-solvent induced phase separation (SNIPS) process have drawn significant attention due to the simple processing method and the generation of high-quality isoporous ultrafiltration membranes. With the present study on SNIPS membrane substructure, we systematically varied membrane casting parameters to tune the cross-sectional morphologies of SNIPS membranes while simultaneously preserving top surface structure. Parameters such as polymer concentration, evaporation time, solvent ratio, and coagulation bath temperature were investigated to control transformation of commonly produced sponge-like cross-sectional morphologies into more open and permeable finger-like substructures. Membranes with sponge-like and finger-like substructures were then integrated onto nylon supports for enhanced mechanical properties. Hydraulic permeability tests at various pH conditions gave distinct open-state flux values for SNIPS membranes with different sublayer morphologies, while maintaining pH responsive functionality resulting from the poly(4-vinylpyridine) block.

Graphical abstract

Introduction

Polymeric membranes have widely been used in filtration fields because of their plentiful materials choices, robust mechanical properties, and compatibility with various fabrication techniques such as track-etching, lithography, and solvent based methods [1].

Among these methods, membranes generated through non-solvent induced phase separation (NIPS) attract great interest due to the simple fabrication procedure and accessibility to a range of pore sizes under various casting conditions. The polymer, commonly a homopolymer (e.g. polysulfone [2]), is dissolved into a good solvent, casted on a substrate and immersed into a coagulation bath. As phase inversion occurs with the exchange of solvent and non-solvent (coagulant), membranes are formed by precipitation of the polymer. One of the most useful properties of NIPS membranes is the ability to tailor pore size with casting solution compositions and membrane casting conditions [3]. There are generally two types of cross-sectional membrane morphologies obtained through varying system conditions: 1) dense, sponge-like and 2) open, finger-like structures. Sponge-like and finger-like membranes are used as reverse osmosis and ultrafiltration membranes, respectively, due to their pore size distribution. Although widely used in industry, NIPS membranes are limited due to the trade-off between selectivity and permeability [4]. Membranes with a sponge-like cross section usually have low permeability but are more pressure resistant due to their dense structure. On the other hand, finger-like membranes have high permeability yet low selectivity with respect to solute rejection due to a wide pore size distribution.

Advancing the field from conventional NIPS membranes, Peinemann et al. [5] first reported and demonstrated the fabrication of poly(styrene-b-4-vinylpyridine) (PS-b-P4VP or SV) diblock copolymer derived asymmetric membranes. Block copolymers are an attractive material for the fabrication of membranes due to their ability to self-assemble into well-ordered structures. PS-b-P4VP derived membranes were generated through a hybrid process of block copolymer self-assembly and NIPS (SNIPS) [6]. Membranes were prepared by 1) dissolving the polymer into an appropriate solvent system, 2) casting a thin film with a doctor blade, 3) solvent evaporation for a specified amount of time to allow block copolymer self assembly, and 4) immersion into a non-solvent (water) bath to induce precipitation of the polymer and kinetically trap the structure in a thermally non-equilibrium state. The resultant membranes were characterized by a disordered graded macroporous substructure beneath an ordered selective skin layer. Since then, Phillip et al. [7] applied the hybrid SNIPS process to a triblock terpolymer, poly(isoprene-b-styrene-b-4-vinylpyridine) (PI-b-PS-b-P4VP or ISV) because of the inherent mechanical stability associated with the addition of the rubbery PI block. Membranes made from this triblock terpolymer had triple the toughness compared to its diblock counterpart.

The asymmetry of the SNIPS membranes eliminates the trade-off of permselectivity by enabling selectivity, attributed to the isoporous skin layer, and permeability, associated with the macroporous substructure. Numerous studies have been reported on the tunability of the isoporous self-assembled selective layer. Nunes et al. studied the effect of different solvent system compositions [8], and metal-complexation [9], [10] on their PS-b-P4VP system. Pendergast et al. [11] investigated various parameters including varying polymer concentration, compositions of volatile to less volatile solvent mixtures, and evaporation times on their ISV polymer system. Dorin et al. explored the effect of block copolymer molar mass [12] and the use of small angle x-ray scattering (SAXS) [6] as a screening tool to predict the top surface structure.

In contrast to the past focus on the top separation layer, few studies report on the investigation of parameters controlling the substructure. Current literature details various block copolymer membrane systems with sponge-like [5], [7], [12], [13], [14], [15], comblike [15] and fingerlike [15], [16] morphologies. The current study focuses on the investigation of parameters affecting the substructure to enable its tunability which in turn controls membrane performance (i.e. permeability) without disrupting the self-assembly of the top surface selective layer.

To that end we systematically study several key factors in order to tune the substructure of ISV SNIPS membranes from sponge-like to finger-like. Parameters such as polymer concentration, evaporation time, solvent system, and coagulation bath temperature are shown to affect the substructure morphology. Tunability between sponge-like and finger-like substructures is achieved while preserving the isoporous skin layer responsible for membrane selectively, regardless of substructure morphology. Hydraulic permeability tests at various pH conditions demonstrate the substructure's influence on permeability while leaving pH responsive behavior, associated with the P4VP block [17], [18], unperturbed. Additionally, ISV membranes with both types of substructures are integrated onto nylon supports to further improve mechanical stability.

Section snippets

Polymer synthesis

Four triblock terpolymers, poly(isoprene-b-styrene-b-4-vinylpyridine) (PI-b-PS-b-P4VP, ISV) were synthesized via sequential anionic polymerization as previously detailed by Phillip et al. [7]. ISV terpolymer characteristics were determined by a combination of gel permeation chromatography (GPC) and ¹H NMR. The experimentally determined molar mass (M_n), volume fractions of each block (f) and polydispersity index (PDI) of each triblock terpolymer are listed in Table 1.

Membrane fabrication

ISV terpolymers were

Results and discussion

The formation mechanism of sponge-like and finger-like substructures in NIPS membranes has been extensively studied [3], [19]. One dominating theory in the field attributes the different substructures to the demixing rate. In this theory, upon plunging the thin film into a coagulation bath, the casting dope solution separates into two liquid phases, a polymer-rich phase and a polymer-poor phase, designated as liquid-liquid demixing. The substructure's pore sizes are largely determined by this

Conclusion

In this work, we systematically demonstrated methods to tune the substructure of SNIPS membranes between sponge-like and finger-like morphologies while simultaneously leaving the self-assembled and ordered top surface layer intact. The mechanism of substructure formation was shown to be determined by the demixing rate, which was influenced by kinetic parameters such as concentration, evaporation time, and coagulation bath temperature, as well as by thermodynamic parameters including solvent and

Acknowledgments

The authors would like to acknowledge funding of this work by the Defense Threat Reduction Agency (DTRA) Contract No. HDTRA1-13-C0003. Y.G. was funded by the National Science Foundation (DMR-1409105). This work utilized the Cornell Center for Materials Research (CCMR) facilities supported by NSF MRSEC program (DMR-1120296).

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¹: These authors contributed equally to this work.

View full text

Tuning substructure and properties of supported asymmetric triblock terpolymer membranes

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Polymer synthesis

Membrane fabrication

Results and discussion

Conclusion

Acknowledgments

J. Membr. Sci.

J. Membr. Sci.

Polymer

J. Membr. Sci.

J. Membr. Sci.

J. Membr. Sci.

J. Membr. Sci.

Nanoscale porosity in polymer films: fabrication and therapeutic applications

Soft Matter

Preparation and characterization of membranes formed by nonsolvent induced phase separation: a review

Ind. Eng. Chem. Res.

Materials for next-generation desalination and water purification membranes

Nat. Rev. Mater.