Elsevier

Journal of Membrane Science

Volume 470, 15 November 2014, Pages 132-137
Journal of Membrane Science

Water vapor permeability and competitive sorption in thermally rearranged (TR) membranes

https://doi.org/10.1016/j.memsci.2014.07.024Get rights and content

Highlights

  • Reduced water solubility within a TR poly(benzoxazole) membrane.

  • Changing water clustering behavior within a TR poly(benzoxazole) membrane.

  • Substantial water permeability increase as a result of the TR.

  • Reduction in CO2 permeability through TR membrane as a result of water sorption.

Abstract

Thermal rearrangement (TR) of α-hydroxyl-polyimides (PI) into polybenzoxazoles (PBO) produces membranes with improved gas separation properties. In addition, PBO membranes have high thermal and chemical resistance, which is advantageous for many applications because of the presence of condensable vapors, in particular water in the feed gas. Here, PI and TR produced PBO membranes are studied for their water sorption and permeability. The PI precursor was 3,3′-dihydroxy-4,4′-diamino-biphenyl 4,4′-hexafluoroisopropylidene diphthalic anhydride (HAB-6FDA), and its TR conversion is well characterized. It was observed that water sorption into the PI and TR PBO membranes followed a slightly convex sorption isotherm characteristic of water clustering as the water activity increased. The corresponding water permeabilities through both membranes varied with the water activity. Water permeability through the TR PBO was an order of magnitude greater than the PI membrane, ranging from 35,000 to 43,500 Barrer. It was also observed that the presence of water reduced the permeability of CO2 and CH4 for both membranes due to competitive sorption. However, the CO2/CH4 selectivity of the TR PBO membrane was relatively constant, because of similar decreases in CO2 and CH4 permeability when exposed to water vapor. Conversely, the PI membrane CO2/CH4 selectivity was reduced when exposed to water vapor, because CO2 experienced greater competitive sorption from water than CH4.

Introduction

The high temperature thermal rearrangement (TR) of α-hydroxl-polyimides (PI) produces membranes with exceptional gas separation properties [1], [2]. This is believed to be due to the TR conversion of the PI into polybenzoxazole (PBO), which produces a bimodal cavity size distribution within the membrane structure [3], [4]. One of the perceived advantages of PBO membranes is their thermal and chemical resistance [1], [5], [6], [7]; TR based PBOs are insoluble in all common solvents [8]. This chemical resistance is attractive for membrane gas separation, because in many potential industrial applications condensable vapors such as water can be present. The sorption of water into polymeric membranes is known to alter separation performance [9], through decreasing gas permeability and permselectivity. Water sorption can also cause plasticization of the polymeric material, which in turn can lead to membrane failure or rupture. Here, water permeability through a TR based PBO membrane is reported, as well as the effects of water on both CO2 and CH4 gas permeability through the membrane. The TR PBO membrane investigated is derived from the polyimide precursor based on 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA) and 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB). This polyimide undergoes TR conversion because of the α-hydroxyl group present in the HAB monomer (Fig. 1). Prior research on this TR PBO membrane and the PI precursor by the authors has focused on synthesis and characterization of TR PBO–PI copolymers [10]. We have also considered blended membranes of the TR PBO with a PI that does not undergo TR [11], to alter the CO2/CH4 permselectivity of the resulting membrane. Hence, the TR conversion, as well as CO2 and CH4 sorption and permeability for both the PI and TR PBO membranes studied here have been previously characterized [10], [11]. This paper focuses on the water solubility and permeability of the PI precursor and TR PBO membrane, given that the literature on water permeability for PI membranes is limited [12], and to the authors׳ knowledge no water solubility or permeability data exists in the literature for TR PBO. Furthermore, the competitive sorption influence of water on the permeability of CO2 and CH4 through both the PI precursor and TR PBO membranes is reported under mixed gas conditions. This simulates natural gas processing conditions and provides an indication of membrane performance for natural gas processing or sweetening [13].

For glassy polymeric membranes, gas sorption occurs within both the polymeric matrix (CXA) and micro-voids (CVA) present within the membrane morphology [14]. Standard dual-mode sorption theory accounts separately for sorption within these two regions. Hence, the concentration of a gas within a glassy polymer can be described asCA=CXA+CVA

For simple gases, such as CH4 and CO2, sorption in the polymeric matrix can be characterized by Henry׳s law and sorption to the micro-voids by Langmuir adsorption [14]:CA=kApA+C׳HAbApA1+bApAwhere kA is the Henry׳s Law constant, CHA is the Langmuir capacity, bA is the Langmuir affinity and pA is the partial pressure of Gas A.

In multicomponent gas mixtures, each species will compete for sorption sites and hence reduce their respective concentration within the polymer film. For a gas mixture of A and B, this competitive sorption can be modeled through dual-mode sorption theory, where the concentration of gas A within the polymer film becomes [15]CA=kApA+CHAbApA1+bApA+bBpBwhere bB is the micro-void affinity constant for the component B.

For more condensable vapors such as water, multilayer adsorption and/or vapor clusters can form, for example through hydrogen bonding [12], [16], [17], [18]. In this case, more complex models such as the Brunauer–Emmett–Teller (BET) equation [19] or its modified form, the Guggenheim–Anderson–de Boer (GAB) [20] are more appropriate. In the present case, we use a model based on the multilayer sorption theory underlying the GAB model, but with additional assumptions to reduce the number of adjustable parameters [21]. The concentration of the vapor in the polymer (CB) is dependent upon the activity of vapor in the gas phase (fB), and the relative interactions between the vapor and the polymer matrix as well as the micro-void regions. Hence, the concentration of the vapor can be expressed asCB=CBp¯k׳(fB/f0)1k(fB/f0)+CBp¯(A1)k(fB/f0)1+(A1)k(fB/f0)where CBp¯ is the weighted mean value of the sorption capacity of a polymer to vapor B, k′ indicates the interaction between the vapor molecule and the polymer molecular segment, A′ measures the interaction of the vapor molecule and the micro-voids and f0 is the saturated vapor fugacity. The second term in this equation is directly equivalent to a Langmuir isotherm, where the Langmuir affinity constant for the vapor isbB=(A1)kf0

In addition to impacts on solubility, vapor clusters in the polymer may significantly hinder the transport of other gas species because they provide sizeable obstructions within the membrane morphology, resulting in a fall in diffusivity [22]. Clusters of vapors will also more effectively swell the polymer matrix and enhance plasticization effects [23], [24]. The tendency for a vapor to cluster in a two component system is based on the thermodynamics and molecular distribution function of the mixture, as determined by Zimm and Lundberg [25]. The value of the cluster function is calculated byGν=(1φ)[(a/φ)a]p,T1where G is the cluster integral, ν is the partial molar volume of the vapor, φ is the volume fraction of the vapor in the polymeric film and a is the activity of the vapor (fB/fo). If the cluster function G/ν is greater than −1, it means that the vapor molecular concentration is higher than the average in the neighborhood of a given vapor molecule, indicative of cluster formation. The larger the cluster integral value the stronger the tendency of the vapor molecules to associate with each other in preference to the polymer.

Section snippets

Experimental

Precursor anhydride 6FDA and diamine HAB were purchased from Chriskev Company Inc. (Lenexa USA). The precursor compounds were heated under vacuum to remove any residual solvent and ensure a purified state. All solvents were obtained from Sigma Aldrich. The polyimide was synthesized by two-step polycondensation with thermal imidization. The polyimide structure and high degree of imidization was confirmed by 1H NMR and ATR-FTIR spectroscopy [10]. Evidence of thermal rearrangement in HAB-6FDA was

Water sorption

The water sorption isotherms for HAB-6FDA PI and TR HAB-6FDA PBO membranes are presented in Fig. 2, and water clustering analysis in Fig. 3. Importantly, neither sorption curve shows evidence of being concave to the activity axis, which is usually indicative of dual mode sorption behavior. This behavior suggests that the micro-voids are filled at very low water activities, and so the majority of water sorption is into the polymeric matrix of both membranes. The lack of any hysteresis between

Conclusion

The TR process results in a material that is more hydrophobic and hence sorbs less water for the same water activity. The lower concentrations of water in the PBO polymer means that water clusters only form at higher water activities. The water permeability through both HAB-6FDA PI and TR HAB-6FDA PBO membranes is also reported, and in both systems is found to be dependent on water activity. Importantly, the water permeability through the TR PBO is an order of magnitude greater than through the

Acknowledgments

Colin Scholes would like to thank the Australian–American Fulbright Commission for funding. Colin Scholes and Sandra Kentish would like to thank the Cooperative Research Centres, Australian Government Department of Industry for Greenhouse Gas Technologies (CO2CRC) for support, through the Australian Government Cooperative Research Centre program.

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