Appl Petrochem Res DOI 10.1007/s13203-016-0158-x ORIGINAL ARTICLE Surface engineering and self-cleaning properties of the novel TiO2/PAA/PTFE ultrafiltration membranes 1,2 1 1 1 2 Lina Chi • Yingjia Qian • Boyu Zhang • Zhenjia Zhang • Zheng Jiang Received: 4 March 2016 / Accepted: 24 May 2016 Ó The Author(s) 2016. This article is published with open access at Springerlink.com Abstract Immobilization of nano-scaled TiO2 onto poly- bonding, as well as the thickness and evenness of the meric ultrafiltration (UF) membrane offers desirable surface functional layers. antifouling and self-cleaning properties to the membrane, which is practical in wastewater purification only if the Keywords PTFE Á TiO2 Á Ultrofiltration membrane Á mechanical strength and long-term self-cleaning durability Ultrafiltration Á Self-cleaning are realized. This paper reported the surface roughness, mechanical properties, thermal stability, and recycling self- cleaning performance of the novel TiO2/PAA/PTFE UF Introduction membranes, which were coated via an innovative plasma- intensified coating strategy. Through careful characteriza- Polytetrafluoroethylene (PTFE) is featured with relatively tions, the enhanced engineering properties and the self- high thermal stability, excellent chemical inertness, low cleaning performance were correlated with the surface surface tension, and small coefficient of friction, which chemical composition and the creative coating technique. endow PTFE with an excellent performance in diverse In the recycling photocatalytic self-cleaning tests in pho- applications including low friction films, seals, electronic todegradation of methylene blue (MB) solution, about and biomedical devices. In particular, porous PTFE mem- 90 % MB photocatalytic capability of TiO2/PAA/PTFE branes have found special importance in water treatment composite membranes could be recovered with simple [1], separators of lithium-ion batteries [1, 2], pervaporation hydraulic cleaning combined with UV irradiation. The [3], and blood purification [4]. However, the presence of mechanical properties and thermal stability of TiO2/PAA/ strong C–F bonding and water repellence makes the PTFE also satisfy the practical application in water and application of PTFE membranes in water treatment field wastewater treatments, despite that the original engineering less competitive because of the rather low water flux. properties were slightly influenced by PAA grafting and Therefore, there is an increasing demand for hydrophilic TiO2 coating. The changed properties of the composite UF modification of PTFE membranes to fulfill the require- membrane relative to PTFE are reasonably attributed to the ments of wastewater treatment. variation of the surface chemical species and chemical Generally, membranes are modified via five principal methods: (1) photo-initiated grafting and miscellaneous grafting; (2) plasma treatment; (3) physical coating/ad- & Lina Chi sorption; (4) chemical reaction modification; (5) surface [email protected] modification of membranes via nanoparticles impregnation & Zheng Jiang [5]. The current trend is to coat or incorporate nano metal [email protected] dioxide particles with the bulk membrane materials to 1 School of Environmental Science and Engineering, Shanghai enhance the performance of the membranes, such as Jiaotong University, Shanghai 200240, China hydrophilicity, permeate flux, photocatalytic activity, 2 Faculty of Engineering and the Environment, University of antifouling, water purification, and pollutant removal [6]. Southampton, Southampton SO17 1BJ, UK Titanium dioxide (TiO2) is one of the most applied 123 Appl Petrochem Res nanoparticles due to its high stability, low cost, non-toxi- layer. However, the practical application of such mem- city to environment and humans, chemical resistance, high brane must fulfill the strict durability requirements to the photocatalytic activity, ability to mineralize organic pol- engineering and mechanical properties of the TiO2 coating, lutants and to kill bacteria [7, 8]. However, owing to the which highly depends on its surface roughness and com- intrinsic non-polar linear molecular configuration of C and position. In this paper, firstly, the physical and chemical F atoms in PTFE, the surface modification of PTFE natures of membrane surface were characterized by AFM membranes via TiO2 nanoparticles faces great challenges and XPS. The mechanical property, the stability of pho- in building strong adherence between TiO2 and PTFE. To tocatalytic self-cleaning in prolonged time and the thermal tackle the challenges, at least two processes should be stability for the adoptability to strict application environ- realized: (1) activating the intrinsic chemical inertia of ment were further evaluated. PTFE via breaking the ultra-stable C–F bonding, (2) pro- viding sufficient bonding sites to fix TiO2 tightly via sur- face functional groups [9–11]. Experiments Among the methods breaking C–F bonding on the sur- face of PTFE membranes, plasma technique has many Materials advantages, because it avoids environmental contamination problems. Plasma assisted cross-linking and functional Bare PTFE porous membranes manufactured by stretching group attachment can also be surface-specified and leave process with mean pore diameter of 0.5 lm, with PET as little damage to mechanical properties of the bulk mem- the substrate and PTFE microfibers to form function sur- branes [5]. It has been reported that N2,NH3,H2O, C2H2, face, were both supplied by Valqua Shanghai Co., Ltd. and H2O/Ar plasmas effectively improve the surface (China). Ti(OBu)4, acrylic acid (AA), potassium persulfate, hydrophilic properties by cross-linking or functionalization and bovine serum albumin (BSA; Mw = 67,000 Da) were mechanisms [12, 13]. The Ar, O2 or Ar/O2 plasmas are purchased from Sinopharm Chemical Reagent Co., Ltd used to improve surface adhesion of PTFE mainly by (China). Analytical grade acetic acid, nitric acid, absolute etching-induced surface activation and roughening effects ethanol, and ethylene glycol were obtained from Shanghai [14]. But, plasma-generated radicals on the polymer sur- Lingfeng Chemical Reagent Co., Ltd (China). P25 was face would react rapidly once exposed to gaseous mono- purchased from Shanghai Aladdin Bio-Chem Technology mers [15]. Plasma-initiated graft polymerization might take Co., Ltd. All reagents were used without further purifica- good use of the short-lifetime surface radicals generated tion. Distilled water was used throughout the study. during the plasma activation and allow the growth of the graft macromolecular chain. Grafting density and length of Membrane modification procedures the grafted chains can be controlled by tuning plasma parameters (power, pressure, sample disposition and so on) The modification methodology was developed in our pre- and polymerization conditions (monomer concentration, vious study [22], including the successive three-step pro- grafting time and so on). Acrylic acid is one of the most cess: plasma pretreatment of PTFE, graft polymerization of frequently applied monomers in membrane surface AA onto plasma-treated PTFE to form PAA/PTFE, and hydrophilic modification [16–18]. The negatively charged TiO2 self-assembly onto the PAA/PTFE to obtain the polyacrylic acid (PAA) layer could reduce the adsorption composite membranes. The mass-gaining rate of the PTFE of negatively charged contaminants due to an enhanced membrane after introducing AA and TiO2 to the surface electrostatic repulsive force between solutes and the mod- was calculated using the following Eq. (1): ified membrane surfaces. Also, the carboxyl groups in PAA Mt À M0 would, in turn, provide sufficient bonding sites for inor- Rð%Þ¼ ð1Þ M0 ganic oxide nanoparticles [19]. This strategy was success- where R is the mass-gaining rate, M is the mass of the fully applied in coating TiO2 on polyvinylidene difluoride t treated and dried membrane, and M is the mass of the (PVDF) membrane using TiO2 nanoparticles [20] or sol– 0 gel [21], however, the mechanical stability and recycling membrane that the treatment is based on. application were not presented therein. Recently, we developed a plasma-intensified coating Membrane characterization process to fabricate TiO2/PAA/PTFE composite membrane [22], and the composite membrane exhibited excellent Atomic force microscopy (AFM) was employed to analyze initial hydrophilicity, high ultrafiltration performance, the surface morphology and roughness of the membranes antifouling ability as well as photocatalytic self-cleaning using Nanoscope Multimode (Digital Instrument, USA). Approximately 1 cm2 of the as-prepared membranes were capability, which closely related to the property of TiO2 123 Appl Petrochem Res cut and glued on the glass substrate before AFM scanning equilibrium on the membrane before light irradiation. (10 9 10 lm) under non-contact mode. A CAM110 con- During photocatalysis, 4-mL reaction solution was taken tact angle-measuring device (Taiwan) was used to analyze out every 15 min and filtered so as to measure the con- contact angle and surface free energy. The surface chem- centration change of MB using a UV–visible spectropho- ical composition and functional groups of the membranes tometer (PerkinElmer, Lambda 650, USA). For were investigated using Kratos AXIS Ultra DLD X-ray comparison, the photodegradation of MB on bare PTFE photoelectron spectroscope (Japan) and a Thermo Fisher membrane was also conducted using the same method. Nicolet 6700 Fourier transform infrared spectrometer (USA), respectively.