Hydrophilic modification of poly(vinylidene fluoride) ultrafiltration membranes by surface UV photo-grafting with N,N′-methylene-bisacrylamide as monomer and Ce(IV) as initiator

Poly(vinylidene fluoride) (PVDF) separation membranes are widely utilised in water treatment facilities and are extensively studied by researchers to further enhance performance and expand the application of these membranes. However, the weak hydrophilicity of PVDF results in the poor anti-fouling ability of PVDF membranes. Increasing the hydrophilicity of PVDF membranes to enhance their anti-fouling ability is an important research direction (Liu et al. 2011; Kang & Cao 2014).

Grafting several hydrophilic groups or molecular chains onto the surface of membranes through UV photoradiation technology is a convenient method to prepare hydrophilic membranes (He et al. 2009). Many polymer membranes can be easily modified through this technology. However, PVDF can resist UV photoradiation (Liu et al. 2011). Therefore, studies on the hydrophilic modification of pure PVDF membranes through UV photoradiation technology are limited. Hilal et al. (2004) modified a commercial PVDF microfiltration membrane through UV photo-grafting polymerisation with quaternised 2-(dimethylamino) ethyl methacrylate and 2-acrylamido-methyl-propane sulfonic acid as monomers and benzophenon (BP) as the initiator. However, the authors did not discuss the change in the water contact angle or water flux recovery of the membranes. Rahimpour et al. (2009) utilised acrylic acid, 2-hydroxyethylmethacrylate, 2,4-phenylenediamine and ethylene diamine as monomers. BP was also adopted as the initiator (Rahimpour et al. 2009). In their work, the water contact angle decreased by 10°–27°, and the water flux recovery of the modified membranes was 31%–57%. Gu et al. (2013) adopted 4-vinylpyridine and n-butyl chloride as monomers and BP as the initiator to treat PVDF membranes. The water contact angle decreased by 13°–20°, but the water flux recovery was not measured.

Li et al. (2012) employed Ce(IV) (tetravalent cerium ions) as the initiator instead of the common BP to graft N,N′-ethylene bisacrylamide (EBAA) onto polyHEMA-g-PVDF membrane. The water contact angle and water flux recovery of the modified membrane were 22.1° and 96.3%, respectively, but the irradiation duration was extremely long (nearly 5 h). The photocatalytic ability of cerium ions was comprehensively studied by Cheng et al. (2015). The authors confirmed that the photocatalytic ability of Ce(IV) is stronger than that of Ce(III).

Mono- and multi-functional monomers can be utilised to graft monomers. Unlike mono-functional monomers, multi-functional ones (e.g., EBAA or N,N′-methylene-bisacrylamide (MBAA)) produce a cross-linking structure when used; thus, a stable hydrophilic layer is formed on the surface of the matrix (Yang et al. 2005; Deng et al. 2009).

In this study, MBAA was utilised as the monomer and Ce(IV) as the initiator to modify self-made PVDF ultrafiltration membranes. The main objective is to enhance the hydrophilicity and anti-fouling property of these membranes within a short irradiation duration; the modified membranes would have a relatively high water flux recovery (e.g., over 80%). The effects of monomer and initiator concentrations and irradiation duration on water contact angle, water flux recovery, membrane porosity and BSA rejection were comprehensively studied.

PVDF 6010 was purchased from Solvay, Belgium. Polyvinylpyrrolidone (PVP) K30 purchased from BASF, Germany, was utilised as the pore former. Dimethylacetamide (DMAc) purchased from Tianjin Kermel Chemical Reagents Development Centre, China, was used as the solvent. MBAA (AR) was purchased from Maya Reagent Co. Ltd, China. Ce(SO₄)₂ was purchased from Tianjin Bodi Chemical Regent Co. Ltd, China. H₂SO₄ and n-butanol were purchased from Beijing Chemical Reagents Development Centre, China. Bovine serum albumin (BSA; M_w 68,000 g/mol) was purchased from Beijing AoBoXing Bio-tech Co., Ltd, China.

The membranes were prepared through the immersion precipitation phase inversion method, as described by He & Shi (2014). PVDF powder was dried at 80 °C for at least 24 h before use. A casting solution was prepared by blending PVDF, PVP and DMAc. The PVDF and PVP concentrations in the casting solution were 16 and 5 wt%, respectively. After complete dissolution and degassing, the solution was cast on a polyethylene terephthalate nonwoven fabric support (80 g/m²) by using a hand-casting knife with a knife with a 350 μm knife gap. The membrane formed with the nonwoven fabric support was then immersed in a coagulation water bath to form an asymmetric membrane. The membrane was rinsed with distilled water and stored in distilled water for 48 h before the next modification by UV photo-grafting.

Five irradiation durations, namely, 1, 3, 5, 7 and 9 min, were set. After UV irradiation, the grafted membrane was rinsed three times with distilled water within 48 h and then stored in distilled water for the next measurement.

The top surface and cross-section morphologies of the unmodified and modified membranes were observed with a scanning electron microscope (SEM; FEI Sirion, The Netherlands). The top surface groups of the unmodified and modified membranes were analysed through attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) technique performed with a Nicolet FT-IR 360 spectrometer. The surface of the membranes was in contact with a ZnSe crystal with a 45° angle of incidence. The absorption spectra were obtained in the region of 650–4,000 cm⁻¹ with 4 cm⁻¹ resolution. The water contact angle on the top surface of the dried membranes was measured with a contact angle instrument (DSA 100, KRÜSS, Germany). The dried membranes (20 mm × 20 mm) were measured with this equipment with deionised water as the probe. Ten locations were randomly selected for each membrane and the average value was calculated to minimise errors.

Certain key preparation conditions were selected from four studies (Table 1) to compare the changes in the hydrophilicity and flux recovery performance of the modified PVDF membranes prepared at different UV conditions. The water contact angle in this work decreased by about 16°, and flux recovery increased by about 40%. Compared with that in other published studies, the performance enhancement of the modified membrane in the current study was moderate, but the UV irradiation duration (3 min) was significantly shorter. Consequently, the UV modification in the current work generated satisfactory integrative effects in consideration of both irradiation duration and enhanced performance.

When Ce(IV) is used as the initiator and MBAA as the monomer, MBAA can be grafted onto the surface of pure PVDF membranes through the water-phase grafting method under UV photoradiation. At 0.07 mol/L MBAA concentration, 0.04 mol/L Ce(IV) concentration and 3 min irradiation duration, the membrane surface can be grafted with a sufficient amount of monomer under 5.0 mW/cm² UV photoradiation intensity. Compared with that in the pure PVDF membrane, the water contact angle on the surface of the modified membrane decreased by approximately 16°, and flux recovery increased by approximately 40%. With regard to BSA rejection and porosity measurements, no significant changes were observed between the pure PVDF and the graft-treated membranes. Compared with that in other published studies, the enhanced performance of the modified membrane in the current work was moderate, but the UV irradiation duration (3 min) was significantly shorter. This work is significant because the integrative effects of the UV-grafted membranes were satisfactory when both irradiation duration and enhanced performance are considered.

The authors acknowledge financial support by National Natural Science Foundation of China (21376048).