Operating temperature effects on the plasticization of polyimide gas separation membranes

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Abstract

Membrane plasticization is the process whereby penetrant dissolution causes membrane swelling or dilation, which in turn, can increase membrane diffusivity and solubility and lead to long time frame polymer relaxation processes. In this work, the effect of temperature upon the plasticization of a rigid polyimide, poly(4,4′-hexafluoroisopropylidene diphthalic anhydride–2,3,5,6-tetramethyl-1,4-phenylenediamine) (6FDA-TMPDA), by carbon dioxide is investigated. It is found that across the full range of temperatures studied, plasticization has little effect on carbon dioxide solubility as all results can be characterized by a standard dual mode sorption model. However, the effect upon diffusivity is significant and this can be described by both an exponential relationship with penetrant concentration and an Arrhenius relationship with temperature. The polymer relaxation processes induced by plasticization are also temperature dependent. However, the total proportion of penetrant sorption associated with such relaxation processes is relatively unaffected by temperature. This paper shows that plasticization effects are dominated by Henry's law dissolution. Conversely, while Henry's law species contribute most to diffusion at high temperatures, at lower temperatures the movement of Langmuir component species also contributes to the total diffusion coefficient.

Introduction

The use of polyimide membranes for the separation of carbon dioxide from other gases is of increasing commercial interest. Of particular concern in this field is the prevention, or at least suppression, of plasticization. This phenomenon is induced by medium to high concentrations of hydrocarbons and polarizable gases, including carbon dioxide. Conceptually, plasticization can be thought of as a swelling and slight solvation of the polymer by the plasticizing gas. This affords greater mobility to the polymer chains, often interpreted as more void space. Current conceptual models of the plasticization/swelling phenomenon suggest that it is primarily due to dissolution of gas into micro-voids that are smaller than the gas molecule diameter, that is, Henry region dissolution [1]. This ‘molecular misfit’ implies that the polymer matrix must swell to accommodate the gas, leading to permanent damage to the matrix. This, in turn leads to larger inter-chain spacing in the polymer, and dramatic increases to gas and vapour diffusion coefficients.

The focus of the present paper is the common glassy polyimide poly(4,4′-hexafluoroisopropylidene diphthalic anhydride–2,3,5,6-tetramethyl-1,4-phenylenediamine) (6FDA-TMPDA). This rigid fluorinated polyimide has previously been the subject of a number of CO2-based gas separation studies [2], [3], [4], [5], [6], [7]. It shows very high permeabilities, while retaining reasonable selectivities. Some work has been done to characterize its thermal properties; including variation of gas permeability and selectivity. Similarly, several works have examined the plasticization phenomenon in polyimide gas separation membranes, with Bos et al. [8], [9], [10], [11], [12] examining primarily BTDA-based polyimides over a narrow temperature range, Okamoto et al. [13] using BPDA systems and both Wind et al. [14], [15] and Wessling et al. [16] focusing on polyimides synthesized from the dianhydride 6FDA. However, the thermal dependence of plasticization in such systems is not well described. It is the purpose of the present paper to study the plasticization performance of the polyimide 6FDA-TMPDA during exposure to carbon dioxide across an extended temperature range.

Section snippets

Theory

Membrane permeability (P) is the well known product of the thermodynamic diffusion coefficient (DT) and the solubility (S). The thermodynamic diffusion coefficient is in turn related to the Fickian diffusion coefficient (D) by [17]:D=DTSdfdCwhere f is the fugacity of the penetrant in the gas phase and C is its concentration within the polymer.

The temperature dependence of the permeability, solubility and diffusivity of a penetrant in a polymeric membrane may be described by two Arrhenius

Experimental

The fluorinated polyimide poly(4,4′-(hexafluoroisopropylidene)diphthalic anhydride–2,3,5,6-tetramethyl-1,4-phenylenediamine), (6FDA-TMPDA) was the polymer of interest in this study, with the structure given in Fig. 1.

The polymer was synthesized via a method described previously [31], [32] and dissolved in dichloromethane (AR grade, used as received from Ajax Finechem) at a concentration of 2.5 wt/vol%. Solutions were filtered through 0.75 μm glass fibre filters (Advantec), before being solution

Solubility

In order to study the thermal dependence of 6FDA-TMPDA plasticization by carbon dioxide, it is necessary to first understand how the solubility of this penetrant in 6FDA-TMPDA varies with temperature. Using the apparatus detailed earlier, solubility was determined gravimetrically, with carbon dioxide uptake by 6FDA-TMPDA measured as a function of penetrant fugacity across a range of pressures and temperatures.

True to most glassy polymers, 6FDA-TMPDA was measured to have a decreasing solubility

Conclusions

The polymer that is the focus of this study, 6FDA-TMPDA is a rigid glassy polyimide with a high glass transition temperature. Subsequently, plasticization has little effect on the total solubility of carbon dioxide across the range of temperatures studied, with a high proportion of the total dissolved penetrant present in large Langmuir voids. Conversely, the effect upon diffusivity is significant. As the concentration of dissolved carbon dioxide increases, the polymer swells through a process

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