Self-assembly for membrane formation: influence of solvent quality, hydrogen bonds and coulombic interactions
S. P. Nunesa, P. Madhavana, K. V. Peinemannb
aBiological and Environmental Science and Engineering Division, Water Desalination and Reuse Center,
bPhysical Science and Engineering Division, Advanced Membrane and Porous Materials Center,
King Abdullah University of Science and Technology, 23955-69 Thuwal, Saudi Arabia
The combination of regular phase inversion process for membrane preparation and the self-assembly of block copolymer in solution as a strategy to achieve isoporosity and high flux was demonstrated by Peinemann et al. 1 in 2007 using polystyrene-b-poly(4-vinyl pyridine) copolymer casting solutions. However the development of this technology could only progress years later2, 3, after understanding the mechanism of pore formation. Based on evidences of electron microscopy (cryo transmission2 and scanning3 electron microscopy and focus ion beam), we proposed the key steps are the formation of micelles in solution, their supramolecular assembly and the order immobilization on the top layer of the incipient membrane, during the phase inversion process2. Far from the water-copolymer solution interface a rather disordered gradient of larger pores continues by macroscopic phase separation, following spinodal decomposition or nucleation and growth mechanisms. Since then the process has been explored by different groups using different approaches with analogous copolymer systems4-6.
The thermodynamic interaction between blocks and the solvent mixture guides the assembly and the membrane formation. The assembly order is then kinetically trapped when solvent-non-solvent exchange takes place during phase inversion. A rough estimation of the expected order in solution can be obtained by using dissipative particle dynamics, DPD, and values of Hansen solubility parameters, taking in account van der Waals, polarity and hydrogen bond contributions. By using small angle x-ray scattering, supported by DPD modeling, we demonstrated for polystyrene-b-poly(4-vinylpyridine) how the predominant order varies with the copolymer concentration and the interaction between blocks and solvents, for instance from hexagonal micelle assembly to lamellar6. The order observed in the final membrane structure remarkably reflects the solution assembly. In this way diblock and triblock copolymers solutions with hexagonal and bcc micelle orders led to the corresponding pore arrangement in the final membrane7. The structuration leading to the final membrane morphology could be also followed by time-resolved cryo microscopy and graze incident small angle x-ray scattering8.
A simple way to guide different morphologies is the use of small molecular weight additives in the membrane casting solutions. We have investigated the effect of complexing metal ions 1,2, molecules with hydroxyl and carboxylic groups, able to form hydrogen bonds9 and more recently ionic liquids10.
Block copolymer membranes have been manufactured as flat-sheet, hollow fibers11 and spherical particles12. Their sharp pore size distribution and gate response to pH has allowed their effective application for protein separations with similar molecular weights12, 13.
1. K. V. Peinemann, V. Abetz, P. F. W. Simon, Asymmetric superstructure formed in a
block copolymer via phase separation, Nature Materials 6 (2007) 992-996.
2. S. P. Nunes, R. Sougrat, B. Hooghan, D. H. Anjum, A. R. Behzad, L. Zhao, N. Pradeep, I. Pinnau, U. Vainio, and K.-V. Peinemann, Ultraporous Films with Uniform Nanochannels by Block Copolymer Micelles Assembly, Macromolecules 43 (2010) 8079–8085.
3. S. P. Nunes, A. R. Behzad, B. Hooghan, Rachid Sougrat, Madhavan Karunakaran, Neelakanda Pradeep, Ulla Vainio, and Klaus-Viktor Peinemann. Switchable pH-Responsive Polymeric Membranes Prepared via Block Copolymer Micelle Assembly, ACS Nano 5 (2011) 3516-3522.
4. W. A. Phillip, R. M. Dorin, J. Werner, E. M. V. Hoek, U. Wiesner, M. Elimelech,
Tuning structure and properties of graded triblock terpolymer-based mesoporous and
hybrid films, Nano Lett. 11(2011), 2892–2900
5. S. P. Nunes, M. Karunakaran, N. Pradeep, A. R. Behzad, B. Hooghan, R. Sougrat, H. He, and K.-V. Peinemann, From Micelle Supramolecular Assemblies in Selective Solvents to Isoporous Membranes, Langmuir 27 (2011) 10184-10190.
6. D Marques, Ulla Vainio, Nicolas Moreno Chaparro, Victor Manuel Calo, Ali Reza Bezahd, Jed Pitera, Klaus-Viktor Peinemann and Suzana P. Nunes, Self-assembly in casting solutions of block copolymer membranes, Soft Matter 9 (23) (2013), 5557-5564.
7. R. M. Dorin, D. S. Marques, H. Sai, U. Vainio, W. A. Phillip, K.-V. Peinemann, S. P. Nunes and U. Wiesner, Solution Small-Angle X-ray Scattering as a Screening and Predictive Tool in the Fabrication of Asymmetric Block Copolymer Membranes, ACS Macro Lett. (2012) 1, 614−617.
8. D. S. Marques, R. M. Dorin, U. Wiesner, D. M. Smilgies, A. R. Behzad, U. Vainio, K. V. Peinemann, S. P. Nunes*, Time-resolved GISAXS and cryo-microscopy characterization of block copolymer membrane formation, Polymer 55 (2014) 1327-1
9. P. Madhavan, K. V. Peinemann, S. P. Nunes, Complexation-tailored morphology of asymmetric block copolymer membranes, ACS Applied Materials and Interfaces 5 (2013) 7152-7159.
10. P. Madhavan, R. Sougrat, A. R. Behzad, K. V. Peinemann, S. P. Nunes, Ionic Liquids as Self-Assembly Guide for the Formation of Nanostructured Block Copolymer Membranes, J. Membrane Sci. 2015.
11. R. Hilke, N. Pradeep, P. Madhavan, U. Vainio, A. R. Behzad, R. Sougrat, S. P. Nunes, K. V. Peinemann*, Block copolymer hollow fiber membranes with catalytic activity and pH-response, ACS Applied Materials and Interfaces, 5 (2013) 7001-7006
12. Haizhou Yu, Xiaoyan Qiu, Suzana P. Nunes, Klaus-Viktor Peinemann, Biomimetic block copolymer particles with gated nanopores and ultrahigh protein sorption capacity, Nature Communications, 5 (2014) 4110.
13. X. Qiu, H. Yu, M. Karunakaran, N. Pradeep, S. P. Nunes, K. V. Peinemann, Selective separation of similarly sized proteins with tunable nanoporous block copolymer membranes, ACS Nano 7 (2013) 768-776.
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