Hydrophilic molecular sieve surface layer formation in hydrophobic poly(1-trimethylsilyl-1-propyne) membranes

Abstract

The transport properties of benzene vapor, water vapor, and nitrogen gas were systematically investigated for vacuum-ultraviolet (VUV)-irradiated highly gas and vapor-permeable poly(1-trimethylsilyl-1-propyne) (PTMSP) membranes by a xenon excimer lamp at 172 nm. The photon was completely absorbed by the surface membrane, and the VUV irradiation did not depend on the color center and the bulk structure of the PTMSP membrane. Moreover, the two-step surface chemical structure change of the PTMSP membrane from the highly hydrophilic C=O bond-rich surface layer to the low hydrophilic SiO₂-rich surface layer was observed after VUV irradiation, in contrast to the effect on general polymer membranes. The two-step surface photooxidation and scission reaction of PTMSP membranes was likewise discovered. The surface-modified layer exhibited gas barrier properties for benzene vapor and nitrogen gas, but not for water vapor, because the benzene vapor and nitrogen permeability decreased as the VUV-irradiation time increased. This gas and vapor permeability reduction strongly depended on the molecular size, whereas the water vapor permeability did not depend on the surface hydrophilicity. This hydrophilic surface layer possessed molecular sieve properties. Thus, the PTMSP membrane in this study was changed by VUV irradiation from a benzene vapor-permselective non-molecular sieve to a water vapor-permselective molecular sieve.

Introduction

The development of a permselective membrane for various gases and vapors is required for separation processes [1]. Separation membrane technology contributes to saving energy by risk reduction. Gas and vapor separation properties are important factors in designing the separation membrane. Benzene vapor separation from air is required in vapor separation because benzene is a dangerous chemical, and volatile organic compounds such as benzene cause serious deleterious effect, as clarified by the law for Pollutant Release and Transfer Register in Japan [2]. On the other hand, water vapor separation properties can explain the important technologies such as natural gas [1] and atmospheric dehumidifiers [3], as well as air-conditioning filters [4].

High gas and vapor permeability is generally required in the development of high-performance separation membranes. Poly(1-trimethylsilyl-1-propyne) (PTMSP) has the highest fractional free volume among the existing membrane polymers, which is attributed to the bulky substituent of its polymer chain. Likewise, PTMSP produces the most permeable non-porous polymer membrane among the existing polymers [5]. Moreover, PTMSP has high heat resistance (i.e., its glass transition temperature is more than 250 °C) and high mechanical strength. Therefore, several studies on the PTMSP-based separation membranes have been conducted [6], [7], [8], [9], [10], [11], [12]. However, these highly permeable PTMSP membranes show low selectivity.

On the other hand, modification technologies have been extensively used in the materials industry, including separation membranes, to improve general materials. Physical blending [13], graft or random copolymerization [14], [15], composite membrane synthesis [16], and surface modification [17] have been used to improve PTMSP membranes. In this study, the surface modification of PTMSP membranes was performed with a 172 nm vacuum-ultraviolet (VUV) excimer lamp. VUV irradiation enables irradiation at atmospheric pressure and at low temperature conditions without any damage to the materials, in contrast to the sputtering and vapor deposition technologies [18]. VUV modification by an excimer lamp is likewise faster than that by a low-pressure mercury lamp in terms of the high capacity to generate oxygen radicals [19], [20]. In addition, VUV irradiation is one of the effective methods for increasing the membrane surface hydrophilicity. The vapor permeability of such surface-modified PTMSP membranes can be measured as the increased vapor selectivity of the modified thin, dense surface layer. However, this property of PTMSP membranes has yet to be studied.

Therefore, the effects of VUV irradiation on the bulk and surface properties, as well as the structures, of PTMSP membranes were analyzed in this study. Moreover, the membrane permeability to vapors of benzene and water, as well as nitrogen gas (i.e., a major component of air) was systematically investigated.