Ultrathin membrane with robust and superior CO2 permeance by precision control of multilayer structures
Introduction
Membrane separation has been acknowledged as a proven technology over the other separation techniques due to its energy efficiency and cost-effectiveness in broad applications, such as ammonia production [1], wastewater treatment [2], solar cells [3], and desalination using electrodialysis and reverse osmosis [4], [5]. Gas separation membranes, in particular, have the potential to selectively separate CO2 gas molecules from mixed gas steams including emissions from fossil fuel-based power stations, cement manufacture and in the future direct capture from the atmosphere [6]. The main mechanisms for membrane gas separation include solution-diffusion and molecular sieving. It is considered the most promising method due to its simple separation with high processability, low energy consumption, and low capital costs [7], [8]. Gas separation membranes are generally prepared as either dense polymeric membranes or ultrathin film composite (UTFC) membranes. Dense polymeric membranes can offer a relatively high selectivity of gas pairs; however, their gas permeabilities are quite poor since this is an inherent measure of the material’s performance and not dependent on the thickness [9], [10]. Instead, UTFC membranes can result in higher gas permeance by reducing their thickness as described by Winston Ho and Kamalesh K. Sirkar [11]. In recent years, the absolute gas permeance of a membrane has been recognized as more important than its gas permeability, as it is a true measure of the throughput a membrane can achieve. Therefore, the development and design of UTFC membranes are crucial for high-performance membranes. UTFC membranes typically consist of multiple layers including a gas selective layer, an intermediate gutter layer, and a highly porous supportive layer [12], [13], [14]. The majority of recent approaches have been successful at improving the selective layer. Although there are recent attempts in improving the gutter layer, it often involves additional approaches or extra additives [15]. The important role of the gutter layer is to offer a flat uniform surface able to accommodate an ultrathin selective layer, overcoming the uneven surfaces of the porous substrate that causes a reduction in gas permeance [16]. However, the gutter layer often is much thicker than the desired target, due to the partial filling of the pores in the supporting layer. Numerous approaches have been explored to improve the gutter layer performance, including investigating different materials [17], incorporating additives [15], [18] and trialling casting techniques such as chemical vapour deposition [19], [20], sputtering [21], dip-coating [22] and spin-coating [23], [24]. All these approaches have been either provided less than ideal performance or involve overly complicated procedures.
Polydimethylsiloxane (PDMS) is commonly used in gutter layer design, due to its high CO2 permeability along with great processability [16]. Yoo et al. used a series resistance model of the CO2 permeability of PDMS dense membranes to simulate the CO2 permeance of PAN/PDMS membranes relative to their thicknesses [25]. According to their modelling, CO2 permeances of up to 14,000 GPU could be achieved for a 200 nm PDMS layer, with thinner layers predicted to offer even higher CO2 permeances [25]. Despite this predicted performance, previous reported laboratory-scale experiments have only achieved PDMS gutter layer CO2 permeances up to 5,000 GPU due to the previously described challenges caused during the fabrication process such as the formation of pin holes and the materials filling the pores of the membrane support [14], [26], [27], [28]. Although we have recently reported a PDMS membrane embedded with metal–organic framework (MOF) nanosheets showing CO2 permeance of 10,450 GPU, currently there is no simple procedure that can produce pure PDMS gutter layers with CO2 permeances of >5,000 GPU [15].
In this study, we report a newly developed method for producing a PDMS gutter layer using a conventional membrane fabrication approach but with precise thickness control by using the chemistry of pre-crosslinking. With this simple and new approach, we can consistently produce super-permeable ultrathin pristine PDMS layers with extremely high CO2 permeance around 15,000 GPU (repeatability of 98.4 %) with the highest value reaching 17,064 GPU. We investigated the crucial factors in the process for achieving such membranes. Using this ultra-permeable gutter layer, we also fabricated a UTFC membrane consisting of a top PolyActive™ (PA) selective layer and achieved superior gas separation performance of the UTFC membrane.
Section snippets
Materials
Aminopropyl-terminated polydimethylsiloxane (Mn 5.0 kDa; PDMS-bis-NH2) was purchased from Gelest and used as received. The crosslinker, 1,3,5-benzenetricarbonyl trichloride (TMC, 98 %, Sigma-Aldrich), was stored and used in an inert N2 atmosphere to avoid hydrolysis of acyl chlorides with atmospheric moisture. Chloroform (AR grade, >99 %) and n-hexane (AR grade, >99 %) were purchased from Chem-Supply. Deuterated chloroform (CDCl3, >99.8 %, Cambridge Isotope Laboratories) was used for 1H NMR
Properties influencing PDMS gutter layer formation
In the conventional preparation of PDMS gutter layers, PDMS macromonomer is mixed with a crosslinker in solution prior to spin coating. Sufficient time in solution is required to ensure a high degree of crosslinking in solution before casting onto a substrate because the high content of flexible siloxane linkages in the PDMS backbone can enhance gas permeability since gases are transported via a solution-diffusion mechanism [22]. Previous publications have reported the thicknesses of these PDMS
Conclusion
A superior method for forming ultrathin crosslinked PDMS gutter layers on a substrate was investigated by control and optimization of the pre-crosslinking time in a solution state. This is the first time the pre-crosslinking time was precisely controlled for membrane formation that resulted in superior gas separation performance. Defect-free ultrathin PAN/PDMS membranes were successfully prepared with excellent CO2 permeation properties within 220 s to 580 s of pre-crosslinking time. The
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank Billy Murdoch at RMIT University’s Microscopy and Microanalysis Facility (RMMF) for the XPS measurement.
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