Characterization of sodium chloride and water transport in crosslinked poly(ethylene oxide) hydrogels

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Abstract

Three series of crosslinked poly(ethylene oxide) (XLPEO) hydrogel materials were synthesized via UV-photopolymerization of aqueous solutions containing (1) poly(ethylene glycol) diacrylate (PEGDA) (n = 10), (2) PEGDA (n = 13), and (3) mixtures of PEGDA (n = 13) and poly(ethylene glycol) acrylate (PEGA) (n = 7), where n is the number of ethylene oxide groups. The water content in the prepolymerization mixture was varied from 0 to 80 wt.% and resulted in XLPEO hydrogels having equilibrium water contents ranging from 0.3 to 0.8 (v/v). These hydrophilic XLPEO hydrogels are highly water permeable. The NaCl transport properties of XLPEO were studied using direct permeation and kinetic desorption methods, and good agreement between these two methods was observed. Generally, NaCl permeability in XLPEO increased from less than 0.1–2 (×10−6 cm2/s) as prepolymerization water content increased from 0 to 80 wt.%. NaCl permeability also increased with increasing PEGDA chain length and was higher in samples prepared with PEGA in the prepolymerization solution, presumably due to decreases in effective crosslink density. There is a tradeoff between water permeability and water/salt selectivity: materials with high water permeability typically exhibit low water/salt selectivity, and vice versa. NaCl permeability and diffusivity were strongly correlated with free volume in the hydrogels. Free volume was characterized based on both equilibrium water content and positron annihilation lifetime spectroscopy (PALS). In these samples, the equilibrium water content was proportional to the fractional free volume from the PALS measurements.

Introduction

Numerous studies have highlighted the application of poly(ethylene oxide) (PEO)-based materials in biomaterials, tissue engineering, drug delivery devices, pharmacy, implanted sensors, etc. [1], [2], [3], [4], [5], [6], [7], and PEO-based materials are often biocompatible, highly hydrophilic, and resistant to protein adhesion. In the last decade, such materials have also gained significant attention for gas [8], [9], [10], [11] and liquid [12], [13], [14] separations. Because of their favorable interaction with acid gases, crosslinked PEO (XLPEO) polymers have interesting performance for separating acid gases (e.g. carbon dioxide and hydrogen sulfide) from light gases and hydrocarbons [8], [9], [10], [11], [15], [16]. In water purification, PEO-based materials have been explored to modify the surface of existing ultrafiltration (UF) membranes, resulting in improved fouling resistance of these UF membranes [12], [13], [17], [18], [19], [20]. For example, applying a coating layer of XLPEO to a polysulfone (PSF) UF membrane simultaneously increases water flux and organic rejection in oil/water emulsion tests, leading to significantly improved fouling resistance against emulsified oil droplets [21], [22]. Additionally, PEO-based coatings enhance fouling resistance of desalination membranes [23].

As we face a global water shortage in the 21st century, a flexible and viable long-term strategy that can efficiently supply clean water is needed [24], [25]. Polymeric membranes are rapidly becoming the technology of choice for water desalination because they are cost-effective, small, and simple to operate and maintain [26]. Commercial reverse osmosis (RO) membranes are capable of rejecting more than 99% of ions such as Na+ and other contaminants to produce water that meets requirements for human consumption and other beneficial uses. Commercial RO membranes (e.g. aromatic polyamides) typically have rough surfaces with high chemical affinities for proteins, oil droplets, and other organic foulants, making these membranes susceptible to surface fouling by organic components [27]. XLPEO polymers are interesting candidates as RO coatings, because they may reduce surface roughness and control surface chemistry, rendering the surface more hydrophilic and endowing it with enhanced fouling resistance towards organic foulants [27], [28]. However, the influence of XLPEO coatings on salt transport properties of RO membranes is not well-known, in part due to a lack of fundamental studies on salt transport in XLPEO.

This study reports salt (i.e. NaCl) solubility, diffusivity, and permeability in three series of XLPEO materials. These polymers were synthesized via UV-photopolymerization of aqueous prepolymerization mixtures containing: (1) PEGDA (n = 10) (XLPEGDA10), (2) PEGDA (n = 13) (XLPEGDA13), and (3) PEGDA (n = 13) and PEGA (n = 7) (XLPEGDA/PEGA), respectively. The prepolymerization water content was varied from 0 to 80 wt.%. For the XLPEGDA/PEGA series, the PEGA monomer has essentially the same ethylene oxide content (about 82 wt.%) as that of the crosslinker, PEGDA [9], [28], thereby maintaining polymer chemical composition essentially constant across this series of materials.

Salt diffusivity and permeability are interpreted using a free volume model, using the equilibrium water content as an estimate of free volume, as suggested in the literature [29], [30], [31]. In addition, free volume in hydrated XLPEO samples was also estimated from positron annihilation lifetime spectroscopy (PALS) measurements, and the PALS results are compared with the equilibrium water content in these materials.

Section snippets

Materials

The crosslinkers, poly(ethylene glycol) diacrylates (PEGDA: n = 10 and 13, where n is the average number of ethylene oxide units in the PEGDA molecule, based on the manufacturer's reported molecular weight), and the monomer, poly(ethylene glycol) acrylate (PEGA: n = 7) were purchased from Sigma–Aldrich (Milwaukee, WI). n-Heptane and 1-hydroxycyclohexyl phenyl ketone (HCPK, photoinitiator) were purchased from Sigma–Aldrich. All chemicals were used as received. All water used in this study was

Results and discussion

Polymer density was characterized in order to estimate the water volume fraction in the samples. The average density values of dry XLPEGDA10, XLPEGDA13, and XLPEGDA/PEGA polymers were 1.205, 1.186, and 1.185 g/cm3, respectively, with a standard deviation of 0.004 g/cm3. The measured density results are consistent with previously reported values [9], [28]. Films prepared at various prepolymerization water contents have essentially the same density, indicating that prepolymerization water content

Conclusions

Three series of XLPEO films were synthesized at various prepolymerization contents, and NaCl transport properties of these films were studied. Increasing prepolymerization water content systematically decreases effective crosslink density, resulting in increasing equilibrium water content in XLPEO films. XLPEO films with higher equilibrium water content generally exhibit higher NaCl and water permeability, but lower permeability selectivity for water over NaCl, indicating a distinct tradeoff

Acknowledgements

We gratefully acknowledge partial support of this research by the National Science Foundation (Grant IIP-0917971 and CBET-0932781/0931761). CSIRO's Water for a Healthy Country Flagship is acknowledged for their support of the internal membrane research program and this international collaboration.

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