Comparative assessment of hydrocarbon separation performance of bulky poly(urethane-urea)s toward rubbery membranes

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Among rubbery memberanes, PUs have indicated comparable capability for C 2 H 6 /CH 4 and C 3 H 8 /CH 4 separations (Khosravi and Sadeghi, 2013;Khosravi et al., 2014;Tirouni et al., 2015). Polyurethanes possess complex and multi block structure containing hard and soft domains. Soft segments are usually flexible consisting of polyol (polyether or polyester) while hard domains consist of urethane or urea linkages generated by reacting diisocyante and diol or diamine chain extender. Micro-phase separation occurring in polyurethane affects its physical and gas separation behavior (Halim et al., 2014;Sadeghi et al., 2018;Pournaghshband Isfahani et al., 2017). The results of previous studies ensure that an improved phase separation warrants higher permeability, since the soft domains of PUs dominate the gas transport (Shahrooz et al., 2016). Additionally, the presence of urea linkages in the structure favors the phase separation degree (Sadeghi et al., 2010). Beside phase separation, other factors including microstructure, molecular weight, fractional free volume (FFV), composition, chain flexibility, interchain interactions, and diffusion barriers affect the gas separation properties of polymeric membranes (Khosravi and Sadeghi, 2013;Pournaghshband Isfahani et al., 2016aFakhar et al., 2019aFakhar et al., , 2019bMozaffari et al., 2017;Luo et al., 2016;Dai et al., 2005;Qiu et al., 2007;Nagel et al., 2002). Khosravi et al. (Khosravi and Sadeghi, 2013) studied the influence of different polyols (PTMG, PPG, PCL), various diisocyanates (IPDI, HDI, TDI) and chain extenders (BDO and BDA) on the separation of higher hydrocarbons from methane. Decreased phase separation was obtained for PCL-based PU compared to PPG or PTMG-based PUs. The maximum propane permeability and C 3 H 8 /CH 4 selectivity were 200 barrer and 5.74, respectively. The notable result of this study was the trade-off between permeability and selectivity of some membranes which is the main issue considering gas separation membranes (Khosravi and Sadeghi, 2013;Robeson, 2008;Zhu et al., 2019). Increased rubbery behavior of the membrane was expressed as the main interpretation for enhanced hydrocarbon permeability through higher phase separated PUs (Khosravi and Sadeghi, 2013). The maximum propane permeability and its selectivity over methane were reported as 117.2 barrer and 3.64 through polyurethane membranes by Tirouni et al. (2015).
Separation of hydrocarbons like propane with high condensability from methane are best-done by rubbery sorption-selective membranes. Here, new polyurethane membranes were served for this purpose. We aim to elucidate the structure-property relations of the PUs membranes and their comparative potential for hydrocarbon separation. Hard domains considered as impermeable regions for gas transport and mainly responsible for the mechanical strength of PU (Fakhar et al., 2020). To investigate the influence of hard segment design on gas transport, polyurethanes containing the same soft segment and various hard segments (two types of diisocyanates and two series of side chain based-chain extenders) were studied (Fakhar et al., 2019a(Fakhar et al., , 2019b. The first PUU series contained aromatic side chain substituted chain extenders with different ring numbers, while the second class were based on alkyl side chain substituted chain extenders with different-length aliphatic side chains (Fakhar et al., 2019a(Fakhar et al., , 2019b. This study investigates the novel PUU membranes for separation of heavy hydrocarbons from methane compared to other rubbery membranes.

Polyurethane synthesis
First, a two-step reaction was used to synthesis two categories of diamine chain extenders; one contains aromatic and the other contains aliphatic side chains. As the first step, substitutution of one chlorine atom on the triazine ring of CC by an aromatic or aliphatic-based amines was done. Afterwards, the two residual chlorines of the triazine ring was substituted throught a reaction with hydrazine. Through this procedure, two aromatic side chain-and three aliphatic side chain-based chain extenders were synthesized based on the procedures as mentioned in our previous studies, in details (Fakhar et al., 2019a(Fakhar et al., , 2019b. The chemical structures of all synthesized chain extenders for preparing two series of bulky PUUs are depicted in Scheme S1. A two-step polymerization method was applied to synthesize all polymers. The dried PTMG under vacuum was used with excess of IPDI or HDI (PTMG: diisocyanate 1:3 M ratio) at 80 • C under nitrogen for 2 h with some DMAc as solvent. DBTDL was the catalyst for the reaction. Then, the obtained -NCO terminated pre-polymer was chain extended by adding the pre-synthesized chain extenders to the reaction media at ambient temperature. The temperature was rised to 70 • C and the reaction was kept under nitrogen for 24 h. The post reaction of the product was performed in an oven at 95 • C overnight. The molar ratio of NCO: OH was 1:1 to gain a linear polymer, and 1:3:2 M ratio of PTMG: diisocyanate: chain extender was used.
Different sample notations of the prepared polymers were reported in Table 1. In addition, Scheme S2 depictes the chemical structures of all synthesized bulky PUUs.

Membrane preparation and characterization
A procedure of casting solutions of 10 wt% polymer in DMF followed by solvent evaporation at 60 • C for 24 h in an oven followed by 24 h in vacuum was served for membrane preparation. The thickness of prepared membranes was measured around 100 μm by a digital micrometer. ATR-FTIR and DSC was performed on the membranes and the results were reported in our previous studies (Fakhar et al., 2019a(Fakhar et al., , 2019b. Solubility parameters of soft and different hard segments of various polyurethanes were also reported in the previous studies (Fakhar et al., 2019a(Fakhar et al., , 2019b.
Pure hydrocarbon (CH 4 , C 2 H 6 and C 3 H 8 ) permeation measurements of the membranes were performed at 30 • C and 1 bar. Gas separation tests were done three times for each membrane on three samples of the same polyurethane to report the average results.  Fig. 1 shows the FTIR spectra of the bulkiest and longest side chainbased PUUs of two categories for two types of diisocyanate. The characteristic peaks for polyurethanes are observed in all spectra including C--O stretching at 1600-1800 cm − 1 , NH stretching at around 3300 cm − 1 , the C-O-C ether at 1110 cm − 1 , the CH 2 signals of PTMG at 2940 and 2856 cm − 1 . Additionally, the completion of the reaction was confirmed by the disappearance of the NCO peak at 2250 cm − 1 (Fakhar et al., 2019a(Fakhar et al., , 2019b.

Phase separation extent and FFV values
Two types of hydrogen bonding can occured in polyurethanes including hydrogen bonding between NH and C--O groups of the hard domains which favors phase separation between hard and soft domains, and the interaction between soft segments and the NH groups of hard segments (Chattopadhyay et al., 2005;Miller et al., 1985). Hydrogen bonding index (HBI), the ratio of absorbance of bonded to free carbonyl groups from FTIR spectra, quantifies the phase segregation extent. Higher HBI values indicate higher segregation between soft and hard phases (Pournaghshband Isfahani et al., 2016b, 2016c. The deconvolution data (using OriginPro) of the free and bonded carbonyl groups are presented in Fig. S1 for the bulkiest and longest side chain-based PUUs. HBI and FFV values of the PUUs are depicted in Fig. 2. These values along with glass transition temperature (T g ) of soft segments and solubility parameters of both domains of PUUs are reported in Table S1. Fig. 2 and Table S1 indicate that phase separation (or HBI) in both series of synthesized PUs increase by the size of the side chains (bulkier or longer side chains) attached on hard domains. Additionally, HDIbased PUUs in both series possess a higher phase separation degree compared to IPDI-based ones. The decreased T g of soft segments is another evidence for enhanced phase separation. The solubility parameter difference between soft and hard segments for the aromatic side chain-based PUUs for both types of diisocyanates increased as a bulkier substituent was used which induces stronger phase separation (Miller et al., 1985). For the aliphatic side chain-based PUUs, a lower solubility parameter difference between soft and hard segments was obtained by lengthening the aliphatic side chain. Due to the incompatibility of the aliphatic substituents with the main chain of hard domains (see their solubility parameters in Table S1), phase separation between main and side chain parts in the hard domains would be also propable (Hiller et al., 2004;Hugger et al., 2004). As interpreted in details in our previous study (Fakhar et al., 2019b), the side chains presenting a low surface energy can shield the main chains of hard segments which results in more segregation between soft and hard phases (Tan et al., 2004a(Tan et al., , 2004b(Tan et al., , 2005. This shielding mechanism would decrease accessible NH groups to interact with the soft segment favoring phase separation. Additionally, compatibility of the alkyl side chains with the soft phase is clear based on their similar solubility parameters. This likely give rise to creation of more pure soft and hard phases. Bulking up and lengthening the side chains in both series led to an increase and decrease in FFV, respectively ( Fig. 2 and Table S1). The higher FFV along with more phase separation is a typical observation as seen by others (Wang et al., 2003;Amani et al., 2014). For the second series, longer side chains resulted in lower FFV values, despite the higher HBI values. Reduction in FFV values with longer side chains would strengthen the shielding mechanism, mentioned before.
HDI-based PUUs gives higher FFV values compared to IPDI-based ones with the same chain extender. This is consistent with the usual behavior of higher FFV values for PUs with higher phase segregation.

Gas permeation properties
The CH 4 , C 2 H 6 and C 3 H 8 permeabilities, C 2 H 6 /CH 4 and C 3 H 8 /CH 4 selectivities are reported in Table 1 at 30 • C and 1 bar. To ensure that the selected pressure is below the plasticizing point, N 2 permeability before and after each hydrocarbon transport measurement was measured. No noticeable change was seen in N 2 permeability after every measurement (Khosravi et al., 2014;Tirouni et al., 2015). Based on Table 2, the growth in permeability through aromatic and alkyl side chain substituted PUUs is evident by bulking up and lengthening the side chain. The enhanced permeability of the bulkier PUUs arises from higher phase separation and higher FFV values. More shielded hard domains, higher plasticization effect, higher purity of soft and hard domains along with higher HBI all explain the enhanced permeability by lengthening the aliphatic side chains. Higher phase separation results in a more rubbery character of PU. The increased rubbery behavior gives rise to increased permeability of more condensable gases (Khosravi and Sadeghi, 2013). For all PUUs the permeability increased as the number of carbons in the alkanes increased: C 3 H 8 > C 2 H 6 > CH 4 . Since gas separation through rubbery polymers such as PUs is solubility-controlled, propane with the highest critical temperature among the studied hydrocarbons presents the highest permeability (the critical temperatures Fig. 1. FTIR spectra of the bulkiest and longest side chain-based PUUs. A. Fakhar et al. of the studied alkanes decrease in the following order: propane (231.1 K) > ethane (184.5 K) > methane (111.7 K)) (Semenova, 2004). Considering the obtained C 3 H 8 /CH 4 and C 2 H 6 /CH 4 selectivities the bulkier or longer side chain-based PUUs provide higher selectivities along with higher permeability. The Higher phase separation obtained by the bulkier or longer substituted PUU enhances the rubbery character of polyurethanes favoring easier separation of more condensable molecules from molecules with lower critical temperatures. Enhanced selectivity in C 2 H 6 /CH 4 and C 3 H 8 /C 2 H 6 separations by an increase in rubbery property of polyurethanes was also seen by Khosravi et al. (Khosravi and Sadeghi, 2013). Also, lengthening the side chains with hydrocarbon nature enhances their structural similarity to propane molecules which provides higher C 3 H 8 /CH 4 selectivity. The simultaneous increased permeability and selectivity by longer side chain-based PUUs confirms the higher purity of the domains which facilitates the interaction of aliphatic side chains and soft domains of polyurethane with the alkanes. In addition, HDI-based PUUs presented higher permeability and selectivity compared to IPDI-based ones for the same chain extender due to improved phase separation. The highest hydrocarbon separation was obtained by PUU-O-H as the longest side chain based-PUU containing higher phase separation, higher similarity of side chain with hydrocarbons and more pure domains (166% and 43% improvement in C 3 H 8 permeability and C 3 H 8 /CH 4 selectivity compared to linear PUU).
The increased permeability and selectivity through both series of PUUs is a considerable effect for their potential in hydrocarbon separation.
Diffusivity and solubility coefficients, and diffusivity and sorption selectivities for the prepared membranes are presented in Table 3.
Based on Table 3, bulkier or longer side chain-based PUUs with enhanced phase separation result in higher diffusivity coefficients due to enhanced chain mobility. The increase in gas diffusivity without a decrease in diffusivity selectivity through bulkier side chain-based PUUs is probably due to the screening character of the bulky side chains. For the second series of PUUs, plasticizing effect of aliphatic side chains favors diffusivity and decreased FFV is in favor of diffusivity selectivity through longer side chain-based PUUs. Solubility and solubility selectivity increased by bulkier or longer side chain as a result of increased rubbery behavior of PUs arising from higher phase separated structure. Enhanced similarity between longer side chains and higher hydrocarbons, especially propane is another reason for higher sorption. This  confirms the creation of the domains with higher purity by longer side chain substituted PUUs. The gas diffusivity versus Lennard-Jones diameter (d LJ ) and solubility versus Lennard-Jones potential factor (ε/k) for all the membranes were depicted in Fig. 3a and b, respectively (d LJ and ε/k values were adopted from (Semenova, 2004)). Gas diffusivity decreased with Lennard-Jones diameter. A linear correlation between log D and the d LJ was obtained, as seen by others (Pournaghshband Isfahani et al., 2020). Fig. 3b indicates higher solubility through the membranes for increased ε/k. The normalized solubility through both series of membranes (relative to PUU as the reference membrane) as a function of ε/k is shown in Fig. 3c and d. The higher rise in the gas solubility with ε/k, confirmeds the dominant role of the enhanced rubbery character compared to linear PUU. Fig. 4 compares the selectivity and permeability of the studied PUUs for C 2 H 6 /CH 4 and C 3 H 8 /CH 4 separation compared to other rubbery memberanes. The performance and potential of the studied PUUs is clear from Fig. 4. The novel studied PUUs are more effective for hydrocarbon separation than PUs in the literature and some organosilicon polymers (Khosravi and Sadeghi, 2013;Khosravi et al., 2014;Semenova, 2004). These results insure the PUUs based on the designed hard domains containing side chain are more succesful in higher hydrocarbon separation from methane in comparison with usual polyurethanes studied before in our group and by others. Additionally, the close gas separation Table 3 Diffusivity and solubility coefficients, diffusivity and solubility selectivities for hydrocarbon separations through the synthesized polyurethanes at 1 bar and 30 • C.  properties of these membranes to some organosilicon membranes warrants the PUUs' potential for this application. Tables 4 and 5 indicates the improved percentage for the separation performance by the novel membranes compared to other rubbery membranes (presented in Fig. 4). Since the highest separation performance was obtained by OH-PUU, this comparison was expressed between OH-PUU and other membranes. For organosilicon polymers and various polyurethanes drived from other studies, the mean values of permeability and selectivity were served. Improved C 2 H 6 /CH 4 and C 3 H 8 /CH 4 separation values through OH-PUU is visible compared to almost all samples in Tables 4 and 5 The highest improvement was obtained compared to the other polyurethanes and PDMSTM. Based on Tables 4 and 5, improvement in permeability can be seen in some cases, too.

Conclusions
The intention of this study was to compare the capability of novel PUUs for hydrocarbon separation with other rubbery membranes. Two series of PUUs containing bulky chain extenders based on aromatic and aliphatic side chains were synthesized for this purpose. The main findings indicate a higher phase separation degree between soft and hard domains of the PUUs by bulking up or lengthening the side chains.
Enhanced phase separation was accompanied by higher gas permeability and selectivity. The increased rubbery behavior of PUU membranes due to improved phase separation explains the more efficient separation of more condensable hydrocarbons through rubbery polymers as the gas transport through a rubbery polymer is dominated by solubility. The aliphatic side chain based PUUs provide a better hydrocarbon separation performance due to the increased affinity of hydrocarbons, especially propane, to the aliphatic side chains. The best results for C 3 H 8 /CH 4 separation were 186.49 barrer for C 3 H 8 permeability with a 6.51 selectivity by the longest substituted PUU membrane. This performance is comparable with other rubbery membranes which confirms that these polyurethanes are good candidates for this separation.

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.

Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi. org/10.1016/j.jngse.2021.104356. Fig. 4. a) Performance of the bulky PUU membranes compared to other rubbery membranes for a) C 2 H 6 /CH 4 and b) C 3 H 8 /CH 4 separations.

Table 4
The improved percentage for C 2 H 6 /CH 4 separation through OH-PUU compared to other rubbery membranes.