Perspective Article
Gas permeation properties of highly cross-linked castor oil-based polyurethane membranes synthesized through thiol-yne click polymerization

https://doi.org/10.1016/j.reactfunctpolym.2020.104799Get rights and content

Highlights

  • Castor oil-based polyurethane (COPU) membranes were synthesized via thiol-yne click polymerization.

  • The COPU membranes exhibited promising thermo-mechanical properties.

  • Permeability measurements were carried out for CO2, H2, and CH4 gases.

  • Promising CO2/H2 separation performance was attained.

Abstract

In this paper, a new chemistry approach was developed for synthesis of castor oil (CO)-based polyurethane (PU) membranes to attain improved structural properties for gas separation applications. For this purpose, propyne-terminated CO-based PU prepolymer (PTPU) was synthesized by the reaction of CO and isophorone diisocyanate (IPDI), followed by propargyl alcohol (PrAl) and isocyanate (NCO) terminated prepolymer reaction. The ultimate membranes were prepared through thiol-yne cross-linking reaction of PTPU and pentaerythritol tetrakis (3-mercaptopropionate) (PETMP), in presence of azobisisobutyronitrile (AIBN), as a reaction initiator. It was revealed that mechanical and thermal properties of the prepared membranes were improved owing to the formation of flexible thioether linkages through cross-linking reaction. The gas permeation measurements were carried out in the temperature and pressure ranges of 298–338 K and 200–1200 kPa, respectively. The results exhibited reverse size selective performance, as a typical transport behavior of rubbery membranes. The infinite dilution permeability coefficient of CO2 at 298 K was found identical to 2.78 Barrer, along with the infinite dilution perm-selectivity values for CO2/CH4 and CO2/H2 obtained equal to 25.27 and 7.94, respectively. Furthermore, higher permeation activation energies were found in this work compared to a number of cross-linked rubbery membranes in the literature, such as poly (dimethylsiloxane) (PDMS) and poly(ethylene glycol) diacrylate (PEGDA).

Introduction

Membrane gas separation processes are predominantly conducted by means of non-porous/dense polymeric membranes [1,2], where permeability of the gas penetrants is described by well-known solution-diffusion theory [3]. Nowadays, finding new materials, possessing desirable combination of separation and structural properties for membrane fabrication is of paramount importance. Indeed, desired membranes for gas separation applications, have to present commendable mechanical and film-forming properties, providing possibility of thin film fabrication with the lowest aging [[4], [5], [6]]. In addition, the proposed new materials should offer conspicuous thermal and chemical stability under real operational conditions [7,8].

The onset of membrane gas separation applications was primarily concerned with natural polymers, e.g. gelatin, natural rubber, and gutta percha, followed by developing synthetic polymers, consisting of different rubbery and glassy ones [[9], [10], [11], [12], [13]]. The rubbery polymers, such as silicon rubber are used in reverse size selective applications, e.g. in vapor recovery (C3H6/N2, C2H4/N2, C2H4/Ar, C3+/CH4, and CH4/N2). On the other hand, the glassy ones, e.g. polyimides, polysulfone, polyphenylene oxide, polycarbonates, and cellulose acetates are ordinarily used in size screening applications, e.g. in hydrogen recovery (H2/N2, H2/CH4, and H2/CO), N2 production (O2/N2), and natural gas treatment (CO2/CH4, H2S/CH4, and He/CH4) [14,15]. It is well recognized that the gas transport properties of polymers are essentially governed by their physicochemical characteristic. Hence, their separation performance might be augmented via purposefully altering their network structure. Among different polymer networks, copolymers such as poly(ether-b-amide) (PEBA) and polyurethanes (PUs) are robust candidates for membrane fabrication, owing to their unique structural properties. In these copolymers, the hard segments provide mechanical strength, while soft segments are responsible for gas transport properties [16].

PUs are a subclass of resins obtained as a product of the reaction of an isocyanate, more frequently a polymeric isocyanate, with a polyol. The presence of polar urethane, urea and NH electron donating groups in PUs, enhances the polymer network favorable interaction with CO2 molecules. This characteristic along with their noticeable film forming ability and mechanical properties, make them potential alternatives for CO2 separation from various gas streams [17,18]. Reis et al. [19] prepared dense membranes from aqueous dispersions of poly(urethane-urea) (PUU) based on poly(propylene glycol) (PPG) and a block copolymer composed of poly (ethylene glycol) (PEG) and poly(propylene glycol) (PPG). The fabricated membranes exhibited appropriate performance for CO2/CH4 and CO2/N2 separation. As it was already mentioned, the nature and physicochemical characteristics of applied starting materials for synthesis of PUs, are of critical importance in ultimate structural and separation performance properties [20]. Some investigations involve preparation of PUs from renewable sources starting materials, in particular vegetable oil (VO)-based polyols, due to the economic and environmental concerns [21,22]. Indeed, VOs are the most widely used renewable raw materials, which can be employed as reliable monomers for synthesis of PUs [23,24]. It is expected that these efforts would lead to reduce the dependence on petro-based materials and impart enhanced bio-degradability, cost efficiency and environment sustainability [25,26]. Xia et al. [27] prepared antibacterial soybean oil-based cationic PU coatings from different amino polyols for biomedical application. In another study, Garrison et al. [28] investigated effects of amine ratio and degree of crosslinking on thermo-mechanical and antibacterial properties of antibacterial soybean oil-based cationic PUs. Vegetable oil-based PUs have been applied for vrious applications, including anticorrosive and antibacterial coatings [29], biomedical applications, such as wound dressing [30], and cardiac patches [31,32]. Jiang et al. [33] synthesized fully degradable soybean oil-based waterborne PUs (SWPU) from epoxy soybean oil and ricinoleic acid. The prepared films presented tunable mechanical, thermal, and biodegradation properties [33]. In another research, Raychura et al. [34] developed wood finished PU coatings, using N,N-bis(2-hydroxyethyl) fatty amide precursor derived from peanut oil. They compared performance properties of the coatings with commercially available PUs and perceived that bio-based PUs offer beneficial properties that could successfully replace the petroleum-based materials [35]. Yingbin et al. [35] designed a series of bio-based PUs from polyols obtained by thiol-ene reaction of different plant oils including olive, rice bran, grape seed, and linseed oils. It appeared that the fabricated PU films have tunable thermo-mechanical performance and are promising candidates to find application in coatings, leather, and adhesive [35].

In spite of extensive studies involving synthesis of PUs by VO-based polyols, there are only few studies dealing with this approach for membrane gas separation applications [21]. Moreover, castor oil (CO) has drawn attention of many researchers among different VOs [36], as the only commercially available VO, directly produced from plant sources containing hydroxyl groups (OH), required for synthesis of PUs. It should be noted that except CO, all other VOs call for some further chemical modifications prior to being used for synthesis of PUs [37,38]. It has been ascertained that castor oil-based polyurethane (COPU) membranes, can be extensively used in different applications e.g. coatings, automotive, construction, and adhesives, due to their efficient properties [37].

Nevertheless, in contrast to the aforementioned desirable properties, the VO-based PUs suffer from some drawbacks compared to conventional PUs, synthesized using petro-based polyols. Some natural features of VOs, such as absence of flexible etheric bonds, polar intermolecular interactions, and hydrogen bonding formation sites, results in poor mechanical properties, low flexibility and viscoelastic properties, as well as weak chemical and thermal stability [[39], [40], [41]]. Therefore, different approaches are being followed to overcome the structural weaknesses of COPUs. For instance, Pankhaniya et al. [39] improved structural properties of COPUs, through modification of CO by transesterification reaction with various alcohols, such as glycerol, pentaerythritol, trimethylol propane, and etc. Recently, more attention has been directed to click polymerization, as a facile and versatile method for preparation and modification of polymeric materials [42]. The thiol-yne click reaction, as a subset of click polymerization reactions, is a simple and efficient synthesis route, that has been widely used for preparation of polymeric networks. Hence, in this work the thiol-yne click reaction was applied to prepare COPU membranes, owing to the favorable advantages, such as low shrinkage of the final network, oxygen and moisture insensitivity, and inoffensive byproduct [43]. This type of reaction proceeds rapidly to completion in the bulk, while minimal catalyst or radical initiating species are required. In addition, the polymeric networks formed by this method are expected to have a homogeneous structure [43]. Indeed, preparation of PU networks using direct reaction of isocyanate and CO has been reported in the literature [[44], [45], [46], [47]]. However, in the present study, structural and gas transport properties of COPU membranes, prepared using an alkyne terminated urethane prepolymer with no free isocyanate functional groups, are evaluated. In this way, the necessity of working with moisture-sensitive and toxic isocyanate intermediates is eliminated by transforming the isocyanate groups into alkyne moieties via reaction with propargyl alcohol. Additionally, in contrast to isocyanate-terminated prepolymers, the alkyne terminated PU is stable at ambient temperature and can be easily exposed to thiol-yne click polymerization. The reaction of these alkyne terminated intermediates with a tetrathiol core leads to cross-linked PU networks with flexible thioether linkages that improve both mechanical properties and flexibility of the final networks. The thermal and mechanical characteristics of the prepared membranes were evaluated by different structural analyses. Moreover, gas transport properties of the selected penetrants, including CO2, CH4, and H2 were investigated in at a relatively wide ranges of operating conditions.

Section snippets

Materials

Castor Oil (CO) with hydroxyl number 154.5 mg KOH/g, isophorone diisocyanate (IPDI) (C12H18N2O2, MW = 222.3 g/mol), propargyl alcohol (PrAl) (C3H4O, MW = 56.06 g/mol), pentaerythritol tetrakis (3-mercaptopropionate) (PETMP) (C17H28O8S4, MW = 488.7 g/mol), dibutyltin dilaurate (DBTDL) (C32H64O4Sn, MW = 631.56 g/mol), dibutylamine (C8H19N, MW = 129.24 g/mol), phosphorpentoxid (P4O10, MW = 283.92 g/mol), azobisisobutyronitrile (AIBN) (C8H12N4, 164.21 g/mol), dimethylformamide (DMF) (C3H7NO,

Gas permeability measurements

Gas permeability coefficients of the synthesized samples for CO2, CH4, and H2 gases were determined by constant volume-variable pressure apparatus. The details of experimental set-up is not given here for the sake of brevity [48,49]. The experiments were conducted in the temperature range of 298–338 K and a pressure range of 200–1200 kPa. The permeability coefficients for a penetrant A, (PA), in Barrer (1 Barrer = 10−10 cm3 (STP) cm cm−2 s−1 cmHg−1) were calculated using Eq. (3) [50]:PA=22414Vlp

Characterization of the fabricated membranes

As depicted in the overall synthesis route followed for preparation of PTPU in Scheme 1, the NCO-terminated polyurethane prepolymer was initially prepared through a reaction of one equivalent of CO with three equivalents of IPDI. After completion of prepolymer formation, three equivalents of PrAl were added and the reaction was continued until no free NCO remained in FTIR spectrum taken from the reaction mixture. FTIR and 1HNMR spectroscopic methods were employed to identify the chemical

Conclusions

In this research, CO-based PU membranes were synthesized through thiol-yne click polymerization reaction, named XPU1 and XPU2. The latter exhibited better characteristics and thereby was selected for further investigations. The relatively high gel contents (> 95%), along with low water absorption value (0.03% ± 0.005) were attained for XPU2 membrane. It was revealed that there is no preference in the reaction of thiol with the first or second unsaturated bonds of the yne groups. A single Tg was

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.

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