Water and salt transport properties of zwitterionic polymers film
Introduction
Dense polymer films have been widely used in diverse membrane applications for water scarcity alleviation and alternative energy development. These applications include reverse osmosis (RO), forward osmosis (FO), electrodialysis (EDI), pressure retarded osmosis (PRO), reverse electrodialysis (RED) and fuel cells [1], [2], [3]. The ion/salt transport properties of these polymer films play a key role determining the separation performance and energy productivity of these applications [3]. The performance of current polymeric materials must be improved to enhance the performance of water purification and energy technologies, and there is an urgent need to better understand fundamental water and salt transport properties of dense polymeric films.
Zwitterions contain both a positively and a negatively charged moiety on the same pendant group, while maintaining overall charge neutrality [4]. Zwitterionic polymers have received growing attention as a new generation of anti-biofouling desalination materials due to favorable resistance to protein adsorption and bacteria attachment [4], [5], [6], [7], [8]. Chang et al. used interfacial polymerization followed by immobilization of zwitterions by crosslinking to prepare an antifouling NF membrane [4]. Zwitterionic monomers were also directly used in interfacial polymerization to prepare a novel zwitterionic NF membrane [7]. Membrane performance and/or antifouling properties were improved considerably due to the incorporation of zwitterionic moieties in the membrane. However, even as zwitterionic polymers are promising materials for use as antifouling coating layers on membrane surfaces or are of interest as desalting layers of thin film composite membranes, the salt transport properties of zwitterionic polymers are not well understood.
Generally, water and salt transport in non-porous polymers can be described by solution-diffusion theory [9], [10], [11], [12], [13]. Polymer structure has a strong impact on the water uptake of the material [2], [14], [15], [16], [17], [18], which subsequently can influence strongly water and salt sorption, diffusion and permeation properties. In addition, many polymers used as membranes for desalination and energy applications have ionizable or charged functional groups on the polymer backbone [2], [13], [19]. Therefore, it is important to evaluate the impact of charge on transport properties in these films. Charged and uncharged polymers exhibit very different salt and water sorption and diffusion properties [13]. Uncharged polymers, swollen with water and not containing functional groups that can ionize, exhibit water and salt transport properties that follow a simple partitioning mechanism [11], [13], [14], [15], [16]. Conversely, charged polymers containing fixed charge groups, which are covalently connected to the polymer backbone and can ionize when the polymer is swollen with water [20], exhibit different ion sorption and diffusion behavior compared to that of uncharged polymers [3], [13], [20], [21], [22]. Ion sorption in an uncharged polymer proceeds by a simple partitioning mechanism and its dependence of ion diffusion properties on salt concentration in the external solution is mainly related to osmotic de-swelling. On the other hand, ion sorption in a charged polymer proceeds by both ion exchange and simple partitioning mechanisms and ion diffusion is strongly influenced by electrostatic interactions.
Zwitterionic polymer films have a high charge density of both positive and negative charge groups but are overall charge neutral, so it is not clear whether the water and salt transport properties of zwitterionic polymers are similar to charged or uncharged polymers. To study the impact of zwitterionic groups on water and salt transport properties, we synthesized UV crosslinked films containing sulfobetaine and carboxybetaine groups crosslinked with poly(ethylene glycol) diacrylate (PEGDA) and characterized their salt transport properties. A crosslinked poly(ethylene glycol) acrylate (PEGA) film was prepared and used as a control membrane. The structures of the two zwitterionic co-monomers (sulfobetaine methacrylate, SBMA, and carboxybetaine methacrylate, CBMA) and the neutral co-monomer, poly(ethylene glycol) acrylate (PEGA), are shown in scheme 1. Two zwitterionic polymers, poly(sulfobetaine methacrylate) (PSBMA) and poly(carboxybetaine methacrylate) (PCBMA), and a neutral polymer, poly(ethylene glycol) acrylate (PEGA), were obtained using UV photopolymerization where the crosslinking density of the polymers was tuned by controlling the ratio of co-monomer to PEGDA.
Section snippets
Materials and reagents
The photoinitiator 1-hydroxycyclohexyl phenyl ketone (HPK), crosslinker PEGDA (Mn=700, n=13), co-monomer PEGA (Mn=380, n=7) and co-monomer SBMA were obtained from Sigma–Aldrich and used as received. The co-monomer CBMA was synthesized according to the method reported in the literature [23] and the purity of the product was confirmed using 1H NMR (Fig. S1 in Supporting Information). De-ionized (DI) water was obtained from a Millipore MilliQ system (18.2 MΩ cm, 1.2 ppb TOC, pH=6.9). All the other
Synthesis of the zwitterionic films
The synthesis of UV-crosslinked films using PEGDA as a crosslinker have been well documented in literature [16] and sulfobetaine and carboxybetaine are typical zwitterionic co-monomers. Polymer films were synthesized using PEGDA as the crosslinker and using PEGA, SBMA, or CBMA as the co-monomer. Synthesis of the zwitterionic films was first attempted using only the mixture of monomers with HPK. The solid zwitterionic monomers, however, are insoluble in PEGDA. Therefore, 60% water was added to
Conclusions
A series of zwitterionic polymer films were prepared by UV crosslinking of acrylate and methacrylate monomers and their salt /water transport properties were studied. Water and salt transport properties can be correlated with water uptake. Crosslink density strongly influences water uptake and water permeability of UV crosslinked films. Zwitterionic films exhibited similar water and salt transport property dependence on crosslink density. Water permeability increases as the crosslink density
Acknowledgements
The authors gratefully acknowledge the financial support by The National Nature Science Foundation of China (Grant no. 21274108) and the National High Technology Research and Development Program of China (863 Program of China, 2012AA03A602). Prof. MJQ also thanks Prof. Benny Freeman and Mrs. Ni Yan (University of Texas at Austin, USA) for helpful discussions.
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