Synthesis of Hydrogels with Antifouling Properties As Membranes for Water Purification

Summary

This paper reports practical methods to prepare hydrogels in freestanding films and impregnated membranes and to characterize their physical properties, including water transport properties.

Abstract

Hydrogels have been widely utilized to enhance the surface hydrophilicity of membranes for water purification, increasing the antifouling properties and thus achieving stable water permeability through membranes over time. Here, we report a facile method to prepare hydrogels based on zwitterions for membrane applications. Freestanding films can be prepared from sulfobetaine methacrylate (SBMA) with a crosslinker of poly(ethylene glycol) diacrylate (PEGDA) via photopolymerization. The hydrogels can also be prepared by impregnation into hydrophobic porous supports to enhance the mechanical strength. These films can be characterized by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) to determine the degree of conversion of the (meth)acrylate groups, using goniometers for hydrophilicity and differential scanning calorimetry (DSC) for polymer chain dynamics. We also report protocols to determine the water permeability in dead-end filtration systems and the effect of foulants (bovine serum albumin, BSA) on membrane performance.

Introduction

There is a great need to develop low cost and energy efficient technologies to produce clean water in order to meet the increasing demand. Polymeric membranes have emerged as a leading technology for water purification due to their inherent advantages, such as their high energy efficiency, low cost, and simplicity in operation¹. Membranes allow pure water to permeate through and reject the contaminants. However, membranes are often subjected to fouling by contaminants in the feed water, which can be adsorbed onto the membrane surface from their favorable interactions^2,3. The fouling can dramatically decrease water flux through the membranes, increasing the membrane area required and the cost of water purification.

An effective approach to mitigate fouling is to modify the membrane surface to increase the hydrophilicity and thus decrease the favorable interactions between the membrane surface and foulants. One method is to use thin-film coating with superhydrophilic³ hydrogels. The hydrogels often have high water permeability; therefore, a thin-film coating can increase the long-term water permeance through the membrane due to the mitigated fouling, despite the slightly increased transport resistance across the whole membrane. The hydrogels can also be directly fabricated into impregnated membranes for water purification in osmotic applications⁴.

Zwitterionic materials contain both positively and negatively charged functional groups, with a net neutral charge, and have strong surface hydration through electrostatic-induced hydrogen bonding^5,6,7,8,9. The tightly bound hydration layers act as physical and energy barriers, preventing foulants from attaching onto the surface, thus demonstrating excellent antifouling properties¹⁰. Zwitterionic polymers, such as poly(sulfobetaine methacrylate) (PSBMA) and poly(carboxybetaine methacrylate) (PCBMA), have been used to modify the membrane surface by coating^{11,12,13,14,15,16,17,18} to increase surface hydrophilicity and thus antifouling properties.

We demonstrate here a facile method to prepare zwitterionic hydrogels using sulfobetaine methacrylate (SBMA) via photopolymerization, which is crosslinked using poly(ethylene glycol) diacrylate (PEGDA, M_n = 700 g/mol) to improve the mechanical strength. We also present a procedure to construct robust membranes by impregnating the monomer and crosslinker in a highly porous hydrophobic support before the photopolymerization. The physical and water transport properties of the freestanding films and impregnated membranes are thoroughly characterized to elucidate the structure/property relationship for water purification. The prepared hydrogels can be used as a surface coating to enhance membrane separation properties. By adjusting the crosslinking density or by impregnating into hydrophobic porous supports, these materials can also form thin films with sufficient mechanical strength for osmotic processes, such as forward osmosis or pressure-retarded osmosis⁴.

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Protocol

1. Preparation of the Prepolymer Solutions

1. Preparation using water as a solvent
   1. Add 10.00 g of deionized (DI) water to a glass bottle with a magnetic stir bar.
   2. Measure 2.00 g of SBMA and transfer it to the glass bottle containing the water. Stir the solution for 30 min, until the SBMA is completely dissolved.
   3. In a separate bottle, add 20.00 g of PEGDA (M_n = 700 g/mol).
   4. Add 20.0 mg of 1-hydroxycyclohexyl phenyl ketone (HCPK), a photo-initiator, to the PEGDA solution. Let the solution stir for at least 30 min.
   5. Using a disposable pipette, transfer 8.00 g of the PEGDA-HCPK solution to the SBMA aqueous solution. Continuously stir the mixture until the solution is homogenous.
2. Preparation using water/ethanol mixtures as solvents
   1. Add 6.00 g of DI water and 4.00 g of ethanol to an amber glass bottle with a magnetic stir bar. Stir the solution to allow thorough mixing.
   2. Add 2.00 g of SBMA to the water/ethanol mixture. Stir the solution and allow the SBMA to completely dissolve.
   3. Use a pipette to transfer 8.00 g of the PEGDA-HCPK solution to the SBMA mixture. Stir to mix the solution thoroughly.

2. Preparation of the Freestanding Films

1. Place two spacers with known thicknesses on a clean quartz disc; the thickness of the spacers controls the thickness of the obtained polymer films¹⁹.
2. Transfer a small amount (~1.0 mL) of the prepolymer solution to the quartz disc using a disposable pipette.
3. Place another quartz disc on top of the liquid and ensure that there are no bubbles in the liquid film.
4. Place the sample in an ultraviolet (UV) crosslinker and irradiate for 5 min using UV light with a wavelength of 254 nm¹⁹.
   NOTE: Alternative irradiation times and wavelengths can be used depending on the type of photoinitiator.
5. Separate the polymer film from the quartz discs using a sharp blade. Use tweezers to transfer the film to a DI water bath. Change the water twice during the first 24 h to remove the solvent, unreacted monomer/crosslinker, and sol from the film.
   NOTE: The polymer film should be kept in the DI water to preserve the pore structure, if there is any.
6. Prepare dried films for ATR-FTIR and DSC analysis.
   1. Remove the film from the water bath and allow it to air dry for 24 h.
   2. Place the film in a vacuum oven at 80 °C to dry overnight under vacuum.

3. Preparation of the Impregnated Membranes

1. Place a sheet of porous support onto a quartz disc.
2. Using a foam brush, coat each side of the support twice with the prepolymer solution based on the water/ethanol mixture⁴.
   NOTE: Since the support is hydrophobic, the prepolymer solution containing ethanol can easily wet the support.
3. Place another quartz disc on top of the support.
4. Place the sample in a UV crosslinker and irradiate for 5 min using UV light with a wavelength of 254 nm.
5. To remove the impregnated membrane from the quartz discs, immerse the whole assembly in a DI water bath for 5 min and carefully remove the membrane using a sharp blade and tweezers.
6. Keep the membrane in DI water. Change the water twice to remove the solvent, the unreacted monomer/crosslinker, and the sol from the membrane.
7. Prepare dried, impregnated membranes for ATR-FTIR and DSC analyses.
   1. Remove the membrane from the water bath. Allow the membrane to dry at ambient conditions for 24 h.
   2. Dry the membrane in a vacuum oven overnight at 80 °C under vacuum.

4. Characterization of the Freestanding Films and Impregnated Membranes

1. ATR-FTIR analysis
   1. Prepare a sample of the prepolymer solution, as stated in step 1.1, for FTIR analysis.
   2. Perform a background scan before scanning the sample. Set the wavenumber range from 600 cm^-1 to 4,500 cm^-1 at a 4-cm^-1 resolution of measurement.
   3. Place the sample in the FTIR machine for analysis.
   4. Remove the sample. Clean the crystal and the tip with an appropriate solvent.
   5. Repeat steps 4.1.1 - 4.1.4 for the following samples: porous support, prepolymer solution, dried freestanding films, and dried impregnated membranes.
2. Differential scan calorimetry (DSC)
   1. Place a DSC pan and lid in a weighing balance and record their weight.
   2. Place a small amount of sample (5-10 mg) inside the pan and close it with the lid.
   3. Weigh the pan containing the sample. From the weight difference between the occupied pan and lid and the unoccupied pan and lid, calculate the weight of the sample.
   4. Using a press, hermetically seal the sample inside the pan.
   5. Place the sealed pan inside the DSC cell in which the inert reference is located.
   6. Enter the weight of the unoccupied pan and lid and the weight of the sample in the program.
   7. Scan with the DSC from -80 °C to 160 °C at a heating rate of 10 °C/min.
   8. Perform the DSC analysis using the manufacturer's protocol.
   9. Repeat the DSC experiments for different samples following the aforementioned steps.
3. Measurement of contact angles using a pendant drop method
   1. Cut a rectangular strip of the membrane sample (approximately 30 mm by 6 mm).
   2. Soak this strip in DI water for 10 min and then dry it for 5 min.
   3. Place the dried sample on the sample holder.
   4. Submerge the sample holder in a transparent environmental chamber containing the DI water²⁰.
   5. Using a microliter syringe with a stainless steel needle, dispense drops of n-decane (approximately 1 µL) onto the membrane sample.
   6. Leave the setup undisturbed for 2 min to ensure the stabilization of the droplets.
   7. Use an appropriate image analysis software to determine the contact angle of the samples by measuring the angles of the dispensed droplets on the membrane surface.
   8. Take the average of the contact angle values obtained for various droplets.
4. Characterization of water permeability using a dead-end filtration system
   1. Use a hammer-driven hole punch with an appropriate diameter to cut coupons of freestanding films and impregnated membranes.
   2. Place a prepared coupon on the porous support inside a dead end filtration cell.
   3. Place the O-ring on top of the sample. Screw the two halves of the permeation cell together.
   4. Add approximately 50 mL of DI water to the permeation cell. Screw on the cap and place the permeation cell on a magnetic stirrer. Set the stirring rate between 300 and 900 rpm.
   5. Place a covered beaker on a balance to collect the permeate water. Tare the balance.
   6. Open the valve on the gas cylinder. Turn the pressure regulator valve clockwise until the desired pressure is reached (45 psig for freestanding films and 35 psig for impregnated membranes).
   7. Open the release valve to deliver the pressure to the permeation cell.
   8. Monitor and record the weight of the beaker with time.
   9. Calculate the water permeance (A_w) and permeability (P_w) with the solution-diffusion model shown below^4,21
      
      where A_w is the water permeance (L/m²hbar or LMH/bar), P_w is the water permeability (LMH cm/bar), ρ_w is the water density (g/L), A is the effective area of the membrane (m²), Δm is the change in the mass of water permeate (g) over a time period Δt (h), Δp is the pressure difference across the membrane (bar), and l is the thickness of the swollen film (cm).
   10. Use a BSA solution containing 0.5 g/L BSA in a phosphate-buffered saline (PBS) solution with pH = 7.4 to evaluate the antifouling properties and rejection rates of the membranes.
   11. Repeat steps 4.4.5 - 4.4.10 to determine the water flux in the presence of BSA. Calculate the BSA rejection rate with the following equation²²
       
       where R_BSA is the BSA rejection rate of the membrane (%), C_P is the concentration of BSA in the permeate (g/L), and C_F is the concentration of BSA in the feed (g/L); the concentration of BSA can be determined via UV spectroscopy.

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Results

Freestanding films prepared with the prepolymer solutions specified in steps 1.1 and 1.2 are referred to as S50 and S30, respectively. Detailed information is shown in Table 1. The prepolymer solution specified in step 1.2 was also used to fabricate impregnated membranes, which are denoted as IMS30. Because the porous support is made of hydrophobic polyethylene, only the prepolymer solution containing ethanol can be impre...

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Discussion

We have demonstrated a facile method to prepare freestanding films and impregnated membranes based on zwitterionic hydrogels. The disappearance of three (meth)acrylate characteristic peaks (i.e., 810, 1,190, and 1,410 cm^-1) in the IR spectra of the obtained polymer films and impregnated membrane (Figure 2) indicates the good conversion of the monomers and crosslinker^4,19,21. Additionally, the...

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Materials

Name	Company	Catalog Number	Comments
Poly(ethylene glycol) diacrylate                  Mn = 700 (PEGDA)	Sigma Aldrich	455008
1-Hydroxycyclohexyl phenyl ketone, 99% (HCPK)	Sigma Aldrich	405612
[2-(Methacrloyloxy)ethyl dimethyl-(3-sulfopropyl) ammonium hydroxide, 97%	Sigma Aldrich	537284	Acutely Toxic
Ethanol, 95%	Koptec, VWR International	V1101	Flamable
Decane, anhydrous, 99%	Sigma Aldrich	457116
Solupor Membrane	Lydall	7PO7D
Micrometer	Starrett	2900-6
ATR-FTIR	Vertex 70
DSC: TA Q2000	TA Instruments
Rame’-hart Goniometer: Model 190	Rame’-hart Instruments
Ultraviolet Crosslinker: CX-2000	Ultra-Violet Products		UV radiation
Permeation Cell: Model UHP-43	Advantec MFS
Deionized Water: Milli-Q Water	EMD Millipore