Elsevier

Journal of Membrane Science

Volume 495, 1 December 2015, Pages 334-340
Journal of Membrane Science

Performance study of isoporous membranes with tailored pore sizes

https://doi.org/10.1016/j.memsci.2015.07.041Get rights and content

Highlights

  • Isoporous block copolymer membranes with tailored pore sizes.

  • Detailed water permeance study.

  • Performance study concerning retention and fouling.

Abstract

This performance study deals with isoporous ultrafiltration membranes made through a combination of self-assembly of amphiphilic block copolymers and the non-solvent induced phase separation process (SNIPS). Ten different polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymers were used to prepare membranes with pore sizes increasing with the molecular weight of the polymers. The pore diameters of the membranes vary from 17 to 86 nm. Pure water permeances were studied with respect to pore sizes, P4VP content, thickness of the membranes and flux recovery after protein adsorption. Suitable working conditions were identified and rejection of poly(ethylene glycol) (PEG) molecules with molecular weights between 100 and 1000 kDa were carried out. The characteristics of PS-b-P4VP diblock copolymer membranes were compared with a commercially available polyethersulfone ultrafiltration membrane.

Introduction

Ultrafiltration (UF) membranes are common in the field of size selective biomolecule separation [1], [2], water treatment [3], [4], [5] and controlled drug release [6]. Conventional production processes for commercially available UF membranes include casting, phase inversion, track-etching, anodizing, sintering and film-stretching. Most types of commercially available UF membranes are limited in their separation performances [7] due to huge deviations in pore size, their tendency to adsorb proteins [8], [9], [10], [11], low surface porosity, high production costs or membrane fragility [7], [12]. During the last couple of years a new method for the formation of UF membranes became available utilizing the combination of self-assembly of amphiphilic block copolymers (S) and the non-solvent induced phase separation process (NIPS) called SNIPS. With this method integral-asymmetric membranes with highly ordered, hexagonally arranged pores can be prepared in a fast one-step process [13]. An alternative method to prepare isoporous membranes from block copolymers includes solvent annealing and selective swelling of one block followed by UV-cross-linking and if desired a secondary swelling [14], [15]. Isoporous membranes made from block copolymers can be seen as a new type of membranes promising enhanced performances, namely both high permeability and selectivity, due to their thin selective layer on top consisting of hexagonally arranged cylindrical pores merging in a sponge-like substructure underneath.

In comparison common commercial UF membranes are either limited in pore size distribution leading to low selectivities when made through phase separation techniques [16] or bear uniform size selective pores with low permeability like track-etched membranes. Zydney et al. calculated a critical upper bond for the selectivity–permeability relation analogous to the Robeson Plot [17,18]. When experimental data for the rejection of bovine serum albumin (BSA) and calculated solvent flow and solute rejection is fitted to Zydney's Plot, SNIPS made membranes are superior to commercial ones [19]. Sharper molecular weight cut offs (MWCO) than for commercial membranes made by phase-inversion can be found depending on the block copolymer used for SNIPS made membranes [20], [21].

One of the most used and understood diblock copolymers for the formation of UF membranes through the SNIPS technique is polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) [22], [23], [24], [25], [26], [27], [28]. Recent results show that the pore sizes of integral asymmetric membranes made from block copolymers are tunable [24], [29]. As PS-b-P4VP diblock copolymers can be synthesized by sequential anionic polymerization, it is possible to use such diblock copolymers with defined amounts of poly(4-vinylpyridine) (P4VP) and defined molecular weights for the membrane formations. The pores of the membranes downsizes by either decreasing the molecular weight of PS-b-P4VP or the P4VP content as described before. Therefor it is possible to generate pores in the range of 20–70 nm with this method [24]. In another approach recently published by Radjabian et al. miscible blends of PS-b-P4VP with different compositions were used to tailor the pore sizes of the membranes [30]. This important result shows that membranes with defined pore sizes can be produced even if an exactly matching block copolymer for a given desired pore size is not available. First results concerning the performances of such membranes indicate on the one hand high selectivity through their isoporous surfaces. On the other hand a high permeance is expected due to their high porosity and spongy substructure leading to a low resistance as depicted in Fig. 1.

In this report we describe the performances of PS-b-P4VP membranes with different pore sizes. Pure water permeance was studied with respect to pore sizes, P4VP content, thickness of the membranes and theoretical expectations. In order to verify suitable working conditions of the membranes water permeance of selected diblock copolymer membranes was measured in dependence of the transmembrane pressure and temperature. Rejection of poly(ethylene glycol) (PEG) molecules with molecular weights between 100 and 1000 kDa was measured and MWCO for the membranes were evaluated. In order to study their fouling behavior the flux recovery after BSA adsorption was tested for selected membranes with different pore sizes. Additionally, the performance of PS-b-P4VP membranes is compared with a commercially available polyethersulfone membrane.

Section snippets

Membranes used in this work

Diblock copolymer membranes used in this work were prepared according to a procedure recently published by Rangou et al. [24]. Therefore PS-b-P4VP diblock copolymers differing in molecular weight and P4VP content were dissolved in a solvent mixture of THF and DMF with concentrations between 19 and 35 wt%. The solvent composition was likewise varied according to the total molecular weight and the weight percentage of the P4VP block in the block copolymer. After stirring for 2 days the solution

Membranes used in this work

In order to obtain membranes with pore sizes from 17 to 86 nm different PS-b-P4VP diblock copolymers were used in this work. The molecular characteristics of the polymers and resulting geometrical feature of the membranes are listed in Table 1 and the membrane surfaces measured by SEM are shown in Fig. S1. For the sake of convenience and clarity the membranes are numbered systematically. The letter M is used as a shortcut of the prepared isoporous block copolymer membrane. M is followed by the

Conclusion

This work comprises a detailed examination of the characteristics of isoporous block copolymer membranes made from PS-b-P4VP. It is possible to tune the pore sizes of such membranes using specific molecular weights of the block copolymer and P4VP content as we described before [24]. The results concerning the performances of such membranes can be summarized as follows:

  • Water fluxes increases with rising pore sizes and is competitive to a commercial polyether sulfone membrane.

  • Water flux increases

Acknowledgment

The authors thank Brigitte Lademann for the synthesis of the polymers, Thomas Bucher for the support with the temperature-dependent measurements, Damla Keskin for her help with the membrane fabrication, Maren Brinkmann for the GPC measurements, Anne Schroeder, Sofia Dami and Clarissa Abetz for the discussion and measurements of the SEMs, Jan Pohlmann and Carsten Scholles for the automatic water flux device.

References (31)

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