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Amphiphilic Graft Copolymer Nanospheres: From Colloidal Self-assembly to CO2 Capture Membranes Harim Jeon, Dong Jun Kim, Min Soo Park, Du Yeol Ryu, and Jong Hak Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01138 • Publication Date (Web): 23 Mar 2016 Downloaded from http://pubs.acs.org on March 24, 2016
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ACS Applied Materials & Interfaces
Amphiphilic Graft Copolymer Nanospheres: From Colloidal Self-assembly to CO2 Capture Membranes Harim Jeon, Dong Jun Kim, Min Soo Park, Du Yeol Ryu*, Jong Hak Kim*
Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, South Korea
*
To whom correspondence should be addressed
E-mail:
[email protected] (D. Y. Ryu) or
[email protected] (J. H. Kim)
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Abstract Colloidal nanosphere self-assembly effectively generates ordered nanostructures, prompting tremendous interest in many applications such as photonic crystals and templates for inverse opal fabrication. Here we report the self-assembly of low-cost, graft copolymer nanospheres for CO2 capture membranes. Specifically, poly(dimethyl siloxane)-graft-poly(4-vinyl pyridine) (PDMS-g-P4VP) is synthesized via one-pot, free radical dispersion polymerization to give discrete monodisperse nanospheres. These nanospheres comprise a surface-anchored highly permeable PDMS layer and internal CO2-philic P4VP spherical core. Their diameter is controllable below the submicrometer range by varying grafting ratios. The colloidal dispersion forms a long-range, close-packed hexagonal array on a substrate by inclined deposition and convective assembly. The array shows dispersion medium-dependent packing characteristics. A thermodynamic correlation is determined using different solvents to obtain stable PDMS-g-P4VP dispersions and interpreted in terms of Flory–Huggins interaction parameter. As a proof-of-concept, the implementation of these nanospheres into membranes simultaneously enhances the CO2 permeability and CO2/N2 selectivity of PDMS-based transport matrices. Upon physical aging of the solution, the CO2/N2 selectivity is improved up to 26, one of the highest values for highly permeable PDMS-based polymeric membranes.
Keywords: self-assembly; colloidal nanosphere; graft copolymer; CO2 capture; membrane.
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1. Introduction Nano/microsphere self-assembly is essential to nanoscience and nanotechnology because it generates well-defined ordered nanostructures from various building blocks, such as small molecules, macromolecules, DNA, proteins, and colloidal particles.1–3 Colloidal nano/microspheres have attracted extensive interest because their size ranges from several micrometers to tens of nanometers, which closely matches the resolution limit of conventional patterning techniques. Highly monodisperse colloidal nanospheres from several substances such as silica (SiO2), polystyrene (PS), and poly(methyl methacrylate) (PMMA) find use in many applications, such as photonic crystals and templates for fabricating inverse opal.4–6 Here we evaluate the application of colloidal nanospheres forming hexagonally closepacked self-assembled structures to CO2 capture membranes. Polymeric membranes are expected to alleviate global energy and environmental crisis because of their high efficiency, low cost, low energy consumption, and easy scaleup.7,8 However, they are usually subject to the trade-off relationship between permeability and selectivity,9 as demonstrated by the Robeson upper bound.10 To solve this problem, mixed matrix membranes using zeolites, metal oxide, metal-organic frameworks, and porous carbon materials have been suggested as substitutes to these materials.11–16 These mixtures comprised a highly permeable continuous bulk phase and a gas-selective dispersed phase. Moreover, many highly permselective membrane materials have been synthesized.17,18 However, high material cost and processing difficulties have limited their commercialization for CO2 capture. Numerous attempts have been made to synthesize polymer colloids with controllable size and narrow molecular weight distribution capable of generating highly regular self3
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assembled colloidal array with tunable light reflectance as scaffolds for three-dimensionally ordered materials.19 However, monodisperse polymer colloids have rarely been utilized in gas separation membranes because common colloids comprise low permeability, low selectivity crosslinked PS or PMMA. Poly(dimethyl siloxane) (PDMS) is the most widely used and successfully commercialized membrane material for air (O2/N2) and olefin/N2 separation. In addition to low cost and facile fabrication procedure, PDMS benefits from high gas permeability derived from its inherent high free volume. However, its low CO2/N2 selectivity (typically < 10) makes it unsuitable for post-combustion CO2 gas sequestration after fossil fuel despite its high CO2 permeability (2,300 Barrer, 1 Barrer = 1 × 10−10 cm3(STP)⋅cm⋅cm−2⋅s−1⋅cmHg−1).17 In contrast, poly(4-vinyl pyridine) (P4VP) presents a potential intrinsic CO2/N2 gas selectivity of 25.5 and a CO2 permeability of 3.30 Barrer.20 Free standing P4VP membranes are usually prepared by casting on a toxic liquid mercury surface because of their strong adhesion to most solid surfaces and brittleness, restricting their application to gas separation. Here we first report the self-assembly of low-cost graft copolymer into long-range, closed-packed three-dimensional (3D) structure displaying dispersion medium-dependent ordering characteristics, and its use in CO2 capture membranes. The amphiphilic PDMS-gP4VP graft copolymer was synthesized through a facile, cheap, one-pot free radical dispersion polymerization in nonpolar solvent. Variations in graft ratios gave PDMS-g-P4VP colloids with adjustable size, narrow size distribution, and well-defined nanostructures. Defect-free, dual-phase composite membranes incorporating these colloids dispersed in PDMS matrix were fabricated and their CO2/N2 separation performance was assessed using a constant pressure–variable volume apparatus. 4
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2. Result and discussion Synthesis of PDMS-g-P4VP graft copolymer Many studies have addressed the colloidal self-assembly of homopolymers, such as PS and PMMA, for solar cells, fuel cells, and catalysis.1,4–6,19 However, few efforts have been deployed to synthesize monodisperse copolymer nanospheres.21,22 Although some papers reported poly(vinyl pyridine) based di- and triblock copolymers exhibiting narrow molecular weight distribution, their synthesis method required multiple tedious steps, high-purity reactants, and energy-consuming conditions, such as low temperature (
