Gas permeation properties of ethylene vinyl acetate–silica nanocomposite membranes

Abstract

The effect of silica nanoparticles on the gas separation properties of ethylene vinyl acetate (EVA) copolymer containing 28% vinyl acetate has been investigated. The EVA and hybrid EVA–silica membranes were prepared via thermal phase inversion method. Silica nanoparticles prepared by hydrolysis of tetraethylorthosilicate (TEOS), through the sol–gel mechanism. The prepared membranes were characterized using FT-IR, SEM, DSC and XRD methods. FT-IR and SEM results indicated the nanoscale dispersion of silica particles in polymer matrix. As confirmed by XRD and DSC analyses, increasing the silica content enhances the amorphous regions significantly. Gas permeation of EVA–silica nanocomposite membranes with silica contents of 5, 6 and 10 wt.% was studied for N₂, O₂, CO₂ and CH₄ single gases at pressures of 4, 6 and 8 bar. The obtained results suggest a significant increase in permeability of all gases and an increase in CO₂/N₂ and CO₂/CH₄ gases selectivities upon increasing the silica content. The possible reasons for such behavior were stated and discussed. The pressure dependence of the gas permeabilities of the membranes was also investigated.

Introduction

Organic/inorganic hybrid materials have the potential to combine the desired properties of inorganic and organic systems, improving the mechanical and thermal properties of inorganic ones with the flexibility and ductility of organic polymers. Organic–inorganic nanocomposite membranes have been the subject of growing interest in recent years [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. These nanocomposite membranes can be obtained by adding fillers (e.g., zeolites, silica, alumina, molecular sieving carbons, others) to a polymer matrix or by sol–gel processes. Because of the huge difference between the polymer and inorganic materials in their properties and strong aggregation of the nanofillers, polymer–inorganic nanocomposite membranes cannot be readily prepared by common methods such as melt blending and roller mixing. Solution blending is a simple way to fabricate polymer–inorganic nanocomposite membranes. The polymer is primarily dissolved in a solvent to form a solution, and then inorganic nanoparticles are added into the solution and dispersed by stirring. The nanocomposite membrane will form upon removing the solvent. The other method of adding fillers to the polymer matrix is in situ polymerization. In this method, as the first step, the nanoparticles are completely mixed with organic monomers, and then the monomers are polymerized. There are often some functional groups such as hydroxyl or carboxyl on the surface of inorganic particles, which can generate initiating radicals, cations or anions under high-energy radiation, plasma, UV or other sources of energy to initiate the polymerization of the monomers on the surface. The main disadvantages of the incorporation of fillers to polymers are agglomeration of particles and formation of non-selective voids at the interface of the particles and the polymer matrix [27]. The sol–gel method is the most widely used technique for the preparation of nanocomposite membranes. In this method, organic monomers, oligomers or polymers and inorganic nanoparticle precursors are mixed together in the solution. The inorganic precursors then hydrolyze and condense into the dispersed nanoparticles in the polymer matrix [27]. The advantage of this method is obvious: the reactions will be carried out at moderate conditions—usually at room temperature and ambient pressure, and the concentrations of organic and inorganic components in the solution can be easily controlled. Additionally, the organic and inorganic ingredients are dispersed in the polymer matrix at the molecular or nanometer level, thus making the membranes much more homogeneous. The in situ formation of an inorganic particle within a polymer matrix depends on several parameters, such as temperature, solvent type, water-to-alkoxide ratio, type and concentration of catalyst, alkoxide precursor and PH [5].

The permeability (P) of a gas through a membrane is proportional to the solubility (S) and diffusivity (D) of the gas in the membrane (P = D × S). Therefore, addition of inorganic nanofillers to a polymer membrane may affect its gas separation properties in two ways: firstly, the interaction between polymer-chain segments and nanofillers can disrupt the polymer-chain packing and increase the voids (free volumes) between the polymer chains, thus promoting gas diffusion into the membrane and secondly, the hydroxyl and other functional groups on the surface of the inorganic phase may interact with polar gases such as CO₂ and SO₂, improving the penetrants’ solubility in the nanocomposite membranes [27]. The gas separation properties of numerous combinations of polymers and nanofillers have been explored so far. Amongst various types of such nanocomposite membranes polyimide/silica ones have received considerable attention with respect to the gas permeation studies [5], [9], [10], [11], [12], [16], [17], [18], [25], [26]. Some other combinations such as polyimide/TiO₂ [7], poly(amide-6-b-ethylene oxide) (PEBAX)/silica [28], PEG/silica [24], PAN/silica [19], poly(4-methyl-2-pentyne) (PMP)/silica [13], [14], [15], [20] and poly(1-trimethylsilyl-1-propyne) (PTMSP)/silica [14], [21], [22], [23] have also been proved to be of special interest because of their remarkable contribution in improvement of the permeation properties.

In recent years, ethylene vinyl acetate (EVA) copolymer has been used for membrane preparation in various applications. Marais et al. studied the O₂, CO₂ and H₂O permeabilities of EVA polymer and its composites with PVC [29], [30]. The authors in their previous publications have investigated the effects of membrane preparation conditions on the morphology and gas separation properties of ethylene vinyl acetate membranes [31], [32].

The results obtained in our previous studies, confirm the superior properties of the EVA membrane in gas separation as well as its desirable mechanical properties. However, due to the lack of sufficient information and experience about improvement of the gas separation properties of the EVA block copolymer membranes, we have focused our attention in this work mainly on the enhancement of the gas separation properties by employing the silica nanoparticles in the EVA membrane matrix.

The nanocomposite membranes have been prepared via in situ polymerization of tetraethoxysilane (TEOS) using the sol–gel process. The permeability was measured for N₂, O₂, CO₂ and CH₄. The effect of silica incorporation into EVA with 28% vinyl acetate and corresponding structural changes and permeation characteristics were investigated and thoroughly discussed.