Preparation of ion exchange membranes by radiation grafting of acrylic acid on FEP films
Introduction
Modification of polymers is a highly interesting domain to introduce desirable properties in polymers so that the resulting materials may be used for specific applications. A number of interesting polymers exist which show interesting characteristics, such as low cost, high chemical resistance and superior thermal stability but do not have membrane properties, such as low resistivity, aqueous swelling and ion exchange capacity (IEC). The radiation-induced graft polymerization of vinyl and acrylic monomers is the most general and powerful method of effecting modification of polymers in order to develop desired materials (Chapiro, 1962; Chen et al., 2006; Huang et al., 2003; Gupta et al., 1995, Gupta et al., 2002a, Gupta et al., 2002b, Gupta et al., 2006; Tyagi et al., 1993; Okamoto, 1987). In membrane technology, permselective membranes are gaining importance because of their applications in diverse fields, such as desalination of water, liquid mixture separation, enzyme immobilization and fuel cells. (Gupta et al., 1994a; Bozzi and Chapiro, 1988; Elmidaoui et al., 1992; Anjum et al., 2006). Fluorinated polymers, such as Teflon-FEP has been an interesting base material for the membrane development because of the presence of the fluorinated matrix which confers much better resistance to aggressive media, such as hot alkali medium. Several studies have been devoted to the grafting of acrylic acid into FEP films and the influence of reaction and irradiation parameters on the degree of grafting has been visualized (Niemöller et al., 1998). The research group of Chapiro has shown that the initiating species in radiation induced graft polymerization is essentially the fluorinated radical and not the peroxide which initiates the polymerization of a monomer (Bozzi and Chapiro, 1987).
Proton exchange membranes have been prepared by simultaneous radiation grafting of styrene onto Teflon-FEP films and their subsequent sulfonation. Degrees of grafting from 13% to 52% were obtained (Gupta et al., 1994a, Gupta et al., 1994b, Gupta et al., 1994c; Gupta and Scherer, 1993a; Zhili et al., 1993; Hegazy et al., 1981; Gupta and Chapiro, 1989a) Water uptake after swelling of the sulfonated membranes in boiling water was significantly high and specific conductivity was minimum beyond specific graft level. The strong decrease in specific resistivity with increasing degree of grafting is due to increasing proton mobility, reflected by an increasing hydration of the sulfonic acid groups. For degrees of grafting higher than 30%, specific resistivity values as low as 2 Ω cm (20 °C) are measured as compared with a value of 12 Ω cm for Nafion-117 membranes. Cografting of acrylic acid and styrene on FEP films by preirradiation method has been recently reported (Phadnis et al., 2005). The structure–property correlation in these membranes and their application as indicators have been established by these authors.
Our interest is to develop ion exchange membranes by radiation induced graft polymerization of acrylic acid on FEP films using preirradiation method. The aim of this work is to investigate the influence of the nature of the graft levels and the nature of the crosslinker on the membrane characteristics.
Section snippets
Materials
FEP films of 50 μm thickness (supplied by DU PONT) were cut into a size of 2×5 cm2 and washed with methanol to remove any impurity adhering to the surface. The films were dried under vacuum at room temperature before using them for grafting. Acrylic acid was supplied by GS Chemicals, India. Tetraethyleneglycol dimethacrylate, divinylbenzene and methylene bisacrylamide were supplied by Fluka. Ferrous sulfate and methanol were supplied by Qualigens, India. Deionized water was used for all the
Irradiation
Irradiation of FEP films was carried out in air using a 60Co gamma radiation source ‘Gamma chamber 900’ supplied by Bhabha Atomic Research Centre, Mumbai, India, at a dose rate (0.18 kGy/h) to a total dose of 46.7 kGy. After the irradiation, samples were kept at −4 °C under refrigerator prior to the grafting reaction.
Grafting process
The grafting reaction was carried out in glass ampoules under nitrogen atmosphere. The exposed samples were placed in reaction tubes containing acrylic acid, water and ferrous sulfate as homopolymer inhibitor. For crosslinked membranes, the crosslinker content was kept at 4% (v:v) in acrylic acid–crosslinker mixture. Nitrogen was bubbled through the solution to remove air from the tube. The tubes were subsequently placed in a constant temperature water bath at 50 °C for specified period of time.
Swelling measurements
Swelling measurements on membranes were carried out by immersing them into distilled water and sodium hydroxide (0.5 N) solution under constant boiling so that an equilibrium swelling of the samples was reached. After this procedure, excess of water on the film surface was wiped by filter paper and the swollen samples were weighed. The degree of swelling was calculated by the following expression (Gupta et al., 1996):where W0 and Ws are the weight of the dry and swollen
Ion Exchange Capacity (IEC)
IEC of water-swollen membranes was determined by placing them in a 0.5 M potassium chloride solution for 6 h at room temperature. The protons released into the medium during the ion exchange process were titrated with 0.005 N NaOH solution. The IEC was represented as meq/g of the dry membranes (Gupta et al., 1996).
FTIR analysis
FTIR measurements were carried out using Perkin-Elmer, Spectrum BX, in the range of 400–4000 cm−1. The samples were dried in vacuum oven before taking spectra.
X-ray diffraction analysis
X-ray diffraction studies on the samples were carried out on PHILIPS, Holland, Cu Kα X-ray generator to trace the morphological changes in the material. Scanning was carried out in 2θ range of 10° to 35° at wavelength of 1.54 Å.
Thermogravimetric analysis (TGA)
TGA studies were carried out using a Perkin Elmer TGA-7 in the range of 50–650 °C. The heating rate was 10 °C/min. The measurements were made under a constant flow rate (20 ml/min) of nitrogen.
Differential scanning calorimetry (DSC)
DSC analysis of samples were carried out on Perkin Elmer–Pyris system in the temperature range of 50–300 °C. The heating rate was 10 °C/min and the thermograms were run under nitrogen atmosphere.
Specific resistivity measurements
Membrane samples were equilibrated in 0.1 M potassium chloride solution and specific resistivity of samples was measured by placing membranes between two electrodes of 2×2 cm2 (Gupta and Chapiro, 1989b).
Results and discussion
The grafting of acrylic acid on FEP was carried out to prepare ion exchange membranes with optimum degree of grafting. The influence of various reaction parameters on the degree of grafting was evaluated. It has been observed that the degree of grafting is significantly influenced by the reaction time, monomer concentration and the nature of the crosslinker.
The variation of the degree of grafting with reaction time is presented in Fig. 1. The degree of grafting increases with time up to 4 h and
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