Elsevier

Journal of Membrane Science

Volume 389, 1 February 2012, Pages 459-469
Journal of Membrane Science

Crosslinked poly[1-(trimethylsilyl)-1-propyne] membranes: Characterization and pervaporation of aqueous tetrahydrofuran mixtures

https://doi.org/10.1016/j.memsci.2011.11.014Get rights and content

Abstract

To enhance their applicability in a broader range of pervaporation feed streams, poly[1-(trimethylsilyl)-1-propyne] (PTMSP) membranes have been successfully crosslinked, using a 3,3′-diazido-diphenylsulfone crosslinker. Both photochemical and thermal processes were used to activate the bis(azide) and thus initiate the crosslink reaction. The presented photochemical crosslink process, has an insufficient efficiency, due to the unreacted bis(azide) and the formation of by-products such as carboxylic acids. On the other hand, thermal annealing at temperatures of at least 160 °C allows successful crosslinking of PTMSP. In contrast to photochemical crosslinking, the bis(azide) completely decomposes after thermal treatment, rendering the membranes insoluble in solvents that dissolve the uncrosslinked polymer, such as tetrahydrofuran (THF), n-heptane and methyl-tert-butyl ether. All membranes were extensively characterized by means of infrared analysis, solid-state 1H-wideline NMR, positron annihilation lifetime spectroscopy, swelling capacity measurements and pervaporation measurements. These techniques, allowed to gain insight in the crosslink reaction mechanism, crosslinking density of the crosslinked polymer network, changes in the free volume cavity sizes, solvent resistance and pervaporation performance, respectively. The potential of the thermally crosslinked PTMSP membranes in the removal of demanding solvents from aqueous mixtures was illustrated by pervaporation tests on dilute THF/water mixtures. The membrane containing 15 wt.% of crosslinker and treated at 180 °C during 1.5 h showed specific permeation rates that are approximately 4 times higher than those of the commercially available polydimethylsiloxane-based membranes, combined with competitive THF/water separation factors. Feed streams containing 10 wt.% THF could be enriched up to 84 wt.% THF in the permeate.

Highlights

► Characterization of 3,3′-diazido-diphenylsulfone crosslinked PTMSP membranes. ► Crosslinked PTMSP membranes for the removal of THF from aqueous feed streams. ► Benchmarking crosslinked PTMSP membranes against two commercial PDMS membranes.

Introduction

Poly[1-(trimethylsilyl)-1-propyne] (PTMSP) is a substituted polyacetylene, that combines a rigid backbone chain with bulky trimethylsilyl side groups. This particular configuration hampers the polymer chains in their packing, resulting in a high fractional free volume and inherent nanoporosity [1]. Thanks to this intrinsic nanoporosity, glassy nature (Tg > 300 °C), solubility controlled separation and pronounced hydrophobicity, PTMSP has received much attention in literature as a promising membrane material for gas and vapour separations [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Despite the high permeability of PTMSP, its relatively low solvent resistance and physical stability make crosslinking of the membrane often inevitable for selective separations of more demanding feed mixtures.

Several methods have been applied, aiming to improve the chemical and physical stability of PTMSP. Blending PTMSP with rubbery polymers [13], adding nanoparticles [4], [14], [15], bromination [16], [17] and crosslinking [18], [19], [20], [21], [22], [23], [24] are some examples. In the latter, the use of bis(azides) as a successful crosslinker for PTMSP was reported. Both the solvent and physical stability could be increased, resulting in PTMSP films insoluble in commonly used solvents for the manufacturing of PTMSP membranes such as toluene, cyclohexane and tetrahydrofuran (THF) [18], [19].

Crosslinking of PTMSP with bis(azides) capitalizes on the ability of aryl azides (R – N3) to decompose to reactive nitrenes (R – N:) and molecular nitrogen (N2) at elevated temperatures or irradiation [18], [19], [20], [21], [22], [23], [24]. These reactive nitrenes readily add to alkenes to form aziridines or insert into carbon–hydrogen bonds to form substituted amines [18], [19], [20], [21], [22], [23], [24]. During crosslinking with bis(azides) nitrogen gas and reactive nitrenes are exclusively generated. As a result no low volatile and low molecular weight by-products [18], [19], [20], [21], [22], [23], [24] that could contaminate the polymer are formed. Therefore, the bis(azides) are ideal for crosslinking PTMSP.

Despite the great potential of PTMSP membranes in several separation processes like vapour [2], [3], [6], [7], [8], [9], [10], [11], [12], liquid [25], [26] and gas [4], [5], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24] separations, to our best knowledge only the latter has been reported in relation with crosslinked membranes [18], [19], [20], [21], [22], [23], [24]. Crosslinking was conducted here to avoid an unwanted decline in gas permeability as a function of time. The application of azide-crosslinked PTMSP membranes in organophilic pervaporation (OPV), in particular for the separation of more demanding feed streams, has not been reported in literature.

In this work differences between photochemically and thermally crosslinked dense PTMSP membranes are investigated by infrared spectroscopy, proton wideline nuclear magnetic resonance (NMR) relaxometry, positron annihilation lifetime spectroscopy (PALS), determination of the swelling capacity in a wide range of solvents. Pervaporation (PV) tests on a standard aqueous ethanol mixture reveal possible changes in vapour transport characteristics across the membrane due to chemical changes after crosslinking. Subsequently, the PTMSP membranes with the highest degree of crosslinking are applied in the separation of an aqueous THF mixture, allowing to validate the crosslinking method on a demanding and strongly swelling feed. The influence of the feed temperature and THF concentration on the PV performance is studied and the crosslinked PTMSP membranes are benchmarked against two commercially available thin film composite polydimethylsiloxane (PDMS) membranes.

Section snippets

Materials

Poly[1-(trimethylsilyl)-1-propyne] (PTMSP, Mw  400 × 103 g mol−1) was purchased from Gelest, Inc. (USA). The crosslinker, 3,3′-diazido-diphenylsulfone, was supplied by Chemos (Germany). Toluene, methanol (MeOH), ethanol (EtOH), 1-butanol (BuOH), methyl ethyl ketone (MEK), methyl-tert-butyl ether (MTBE), n-heptane, tetrahydrofuran (THF) and dichloromethane (DCM) all of analytical grade, were used as received from Merck (Belgium). Analytical grade ethyl acetate (EA) and N,N-dimethylformamide (DMF)

Characterization of crosslinking

The crosslink mechanism proposed in the literature is presented in Fig. 1. After irradiating or heating the azide reactive nitrenes are formed, which will easily add to alkenes to form aziridines or insert into carbon–hydrogen bonds to form substituted amines [18], [19], [22]. Since allylic C–H bonds are significantly weaker (∼85 kcal mol−1) than the C–H bonds in Si(CH3)3 (∼100 kcal mol−1), reaction at the allylic methyl groups (Cdouble bondC–CH3) along the PTMSP backbone is favoured (Fig. 1) [18], [19], [22].

Conclusion

3,3′-Diazido-diphenylsulfone was added to a PTMSP polymer matrix and activated by photochemical or thermal treatment. The photochemical process produces insufficiently crosslinked membranes, since the crosslinker does not completely react and unwanted carboxylic acids are formed. Complete crosslinking of PTMSP is achieved by annealing the membranes at temperatures above 160 °C. In contrast to photochemical crosslinking, the bis(azide) decomposes completely after thermal treatment. At least 10 

Acknowledgements

S. Claes acknowledges VITO for a grant as doctoral research fellow and F.H.J. Maurer acknowledges the financial support of the Swedish Research Council. B. Bongers, K. Wyns and S. Van Vlierberghe are acknowledged for their help with the pervaporation experiments. L. Heylen and B. Seigers are specially thanked for helping with the preparation and characterization of the membranes. Finally, R. Carleer and A. Riskin are gratefully acknowledged for performing the GPC measurements and TGA analyses,

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