Highly permeable reverse osmosis membranes incorporated with hydrophilic polymers of intrinsic microporosity via interfacial polymerization

Jing Dou, Shuo Han, Saisai Lin,*, Zhikan Yao,2, Lian Hou,3, Lin Zhang,2

1 Engineering Research Center of Membrane and Water Treatment of MOE, College of Chemical & Biological Engineering, Zhejiang University, Hangzhou 310027, China

2 Research Institute of Ningbo, Zhejiang University, Ningbo 315100, China

3 Xi’an High-Tech Institute, Xi’an 710025, China

Keywords:PIM-1 Intrinsic microporosity Reverse osmosis Interfacial polymerization Trade-off

ABSTRACT Enhancing the water permeation while maintaining high salt rejection of existing reverse osmosis (RO)membranes remains a considerable challenge.Herein, we proposed to introduce polymer of intrinsic microporosity,PIM-1,into the selective layer of reverse osmosis membranes to break the trade-off effect between permeability and selectivity.A water-soluble a-LPIM-1 of low-molecular-weight and hydroxyl terminals was synthesized.These designed characteristics endowed it with high solubility and reactivity.Then it was mixed with m-phenylenediamine and together served as aqueous monomer to react with organic monomer of trimesoyl chloride via interfacial polymerization.The characterization results exhibited that more ‘‘nodule” rather than ‘‘leaf” structure formed on RO membrane surface, which indicated that the introduction of the high free-volume of a-LPIM-1 with three dimensional twisted and folded structure into the selective layer effectively caused the frustrated packing between polymer chains.In virtue of this effect,even with reduced surface roughness and unchanged layer thickness,the water permeability of prepared reverse osmosis membranes increased 2.1 times to 62.8 L·m-2·h-1 with acceptable NaCl rejection of 97.6%.This attempt developed a new strategy to break the trade-off effect faced by traditional polyamide reverse osmosis membranes.

1.Introduction

Water scarcity is one of the major global challenges of the society, and reverse osmosis (RO) technology plays a crucial role in providing alternative water resource by water desalination [1-3].The mostly commercial RO membranes are aromatic polyamides(PA) commonly preparedviainterfacial polymerization (IP)betweenm-phenylenediamine (MPD) and trimesoyl chloride(TMC) [4].These wholly aromatic polyamide RO membranes possess excellent water permeability(＞30 L·m-2·h-1)and salt selectivity (＞99%), recognized as the gold standard for reverse osmosis technology.Although the state-of-art PA membrane already has high water permeability, the RO process is still characterized as high energy consumption [5].The further improvement of water permeability will no doubt reduce the total energy and economic costs.However, because of the inherent trade-off between permeability and selectivity in PA RO membranes, it remains a major challenge to improve the water permeability without the expense of salt rejection.

The water and salt transport mechanism for nonporous polymeric membranes is conventionally modelled by the solutiondiffusion theory [6-8].As depicted by the theory, the penetrants first molecularly dissolve into the polymer matrix and then diffuse through the polymer.The latter process of diffusion is widely considered as the rate limiting step that is controlled by the opening and closing of transient gaps (i.e.free volume elements) in the polymer matrix.Cohenet al.codified the impact of polymer structure on the transport property of penetrants as follows [9]:

The relationship between free-volume and penetrants transport through polymeric membranes has been exploited to improve the water permeability of RO membranes [10-12].For example, Leeet al.[13]fabricated anionomeric reverse osmosis membrane, in which a disulfonatedpoly(arylene ether sulfone) copolymer containing hydrophilic units (BPS-20) in the potassium salt form was blended with poly(ethylene glycol) oligomers (PEG).The freevolume and water permeability of BPS-20_PEG blend films were increased due to the formation of defined and interconnected hydrophilic channels structureviaion-dipole interaction and high coordination.van der Bruggenet al.cross-linked the flexible aliphatic chains with the rigid aromatic backbone and rendered the polyamide thin-film composite membrane with tunable free volume,obtaining high permeability and selectivity in organic solvent nanofiltration process[14].Zhanget al.[15]bonded the polyamide membrane face with copolymers containing zwitterionic and hydroxyl-based anchoring groups by free radical polymerization,thereby reshaped its three-dimensional(3D)structure with higher permeability.The introduction of nanomaterials could also contribute to polymeric free-volume increase [16,17].An increase of free-volume was achieved by adding ZIF-8 nanoparticles into membranes, and the improvement of gas permeability was achieved [18-20].By introduced UiO-66-NH2nanoparticlesviaspray assisted pre-disposition, a ～50% increase was observed in water permeance [21].

Polymers of intrinsic microporosity(PIMs)are a kind of materials with three-dimensional, twisted and folded structure formed by rigid unit such as spirocyclic or triptycene [22-26].These twisted and folded structures cause the polymeric chains to pack ineffectively, resulting in large number of micropores and high free-volume [27,28].Compared to traditional polymers such as polyethylene, polypropylene and polystyrene with free-volume less than 5%, the free-volume of PIMs are generally high to above than 50%[29].PIM-1 is one of the most typical PIMs and can be dissolved in a variety of organic solvents, which is quite beneficial to membrane separation application.A series of PIM-1 membranes of high permeability have been reported.The gas separation membranes based on PIM-1 represent a significant advance across Robeson’s upper bound plot [25,30-34].PIM-1 membranes also show fairly good performance in solution system separation including organic solvent nanofiltration (OSN) and pervaporation[35,36].

Currently,PIM-1 membranes available for molecular separation are mostly preparedviasolution cast accompanied with micronlevel thickness [27,28,37], whose permeability is far from the requirement in water desalination.Nevertheless, the thickness of solution-casted PIM-1 films below 100 nm would unexpectedly result in a decrease rather than an increase in permeance, on account of the structural relaxation of PIM-1 molecules in films[35,38].Recently, Livingstonet al.employed interfacial polymerization with rigid contorted 5,5′,6,6′-tetrahydroxy-3,3,3′,3′-tetrame thyl-spirobisindane (TTSBI) as aqueous monomer and TMC as organic monomer to synthesize thin and cross linked polyarylate films with the thickness of only 20 nm, achieving two orders of magnitude higher permeance [38].Zhuet al.[39]also developed a highly permeable nanofilmsviainterfacial polymerization by incorporating hydrophilic PIM-1 nanoparticles into PA layer.These suggested that interfacial polymerization is a reliable strategy to prepare thin and crosslinked PIM-1 membranes for water desalination.It should be noted that the above PIM-1 membranes were all defined as nanofiltration (NF) membranes with relatively loose selective layer and low rejection of ～50% for monovalent ions.Actually, the atomistic simulation study has predicted that PIM-1 membrane with nanoscale thickness can achieve high water permeation simultaneously with 100% salt rejection [40], indicating that it is also a potential candidate for dense reverse osmosis membrane preparation.However, PIM-1 is the macromolecule with high steric hindrance that only can be dissolved in organic solvents.The limited solubility and low chemical reactivity are the big challenge to prepare a thin and dense PIM-1 based reverse osmosis membraneviainterfacial polymerization.

In this work,we developed a highly permeable reverse osmosis membrane incorporated with PIM-1viainterfacial polymerization.In order to introduce the intrinsic micropores of PIM-1 into the membrane, a water-soluble a-LPIM-1 of low-molecular-weight and hydroxyl terminals was synthesized and served as aqueous monomer to react with TMC.Simultaneously, highly reactivemphenylenediamine was mixed into aqueous solution to guarantee cross-linking degree and obtained a dense RO membrane.The permeability and selectivity of the PIM-1 based RO membranes were tested,and the influence of the incorporated a-LPIM-1 on their surface property was also evaluated in detail.This study may open a new avenue to break the trade-off effect between permeability and selectivity in traditional polyamide reverse osmosis membranes by covalently incorporating polymers of intrinsic microporosity into the selective layer.

2.Materials and Methods

2.1.Materials

Polysulfone (PSf) ultrafiltration membrane, with a molecular weight cut-off of 30,000, provided from Hangzhou Water Treatment Technology Development Center (China) was used as supporting membrane.TMC and MPD were purchased from Sigma-Aldrich Trading Co.Ltd.(Shanghai, China).Triethylamine hydrochloride (TEA-HCl), 1,4-dicyanotetrafluorobenzene (DCTB)and TTSBI were purchased from Aladdin Industry Co.(Shanghai,China).Isopar?-G was obtained from Exxon Mobile Corporation(USA).Other solvents and reagents were purchased from Sinopharm Chemical Reagent Co.Ltd.(China).All reagents mentioned above were of analytical grade.Deionized water was used throughout all experiments.

Once upon a time...One summer s day a little tailor sat on his table by the window in the best of spirits, and sewed for dear life. As he was sitting thus a peasant woman came down the street, calling out: Good jam to sell, good jam to sell. This sounded sweetly in the tailor s ears; he put his frail1 little head out of the window, and shouted: up here, my good woman, and you ll find a willing customer. The woman climbed up the three flights of stairs with her heavy basket to the tailor s room, and he made her spread out all the pots in a row before him.

2.2.Synthesis of a-LPIM-1

The low-molecular-weight, water-soluble a-LPIM-1 was synthesizedviaa two-step reaction described in Fig.1.Firstly, PIM-1 with low-molecular weight, LPIM-1, was prepared by the condensation polymerization between TTSBI and DCTB[41],and the molar ratio TTSBI/DCTB/K2CO3was optimized to 2/1/4.56.TTSBI(20 mmol), DCTB (10 mmol), evenly ground and dried K2CO3(45.6 mmol), dimethylacetamide (DMAc) (30 ml) and toluene(20 ml)were added to a three necked round bottom flask equipped with a reflux condenser system and a magnetic stirrer.Then the device was pre-heated to 150°C and the reactants continued to fully react for 2 h with aeration of nitrogen and reflux.After cooling, the product was washed alternatively by methanol and water through suction filtration and then dried in vacuum at 60°C for 24 h.Secondly, the side chains of cyanogroups in LPIM-1 was hydrolyzed into amide groups by hydrogen peroxide,and obtained the hydrophilic a-LPIM-1.The method for the hydrolysis of the side groups was reported by Yanaranopet al.[42].Dimethyl sulfoxide(DMSO) (60 ml), K2CO3(1 g) and LPIM-1 powder (2 g) was added to a beaker and then stirred for 1 h at room temperature,followed by drop-wide addition of 30% hydrogen peroxide (H2O2) (12 ml).The reaction was carried out for 24 h at room temperature with magnetic stirring.After the reaction, K2CO3was filtered out then the solution was removed by rotary steaming.The remaining solids were dried in vacuum for 24 h.

Fig.1. The synthesis procedure of a-LPIM-1.

2.3.Characterization of a-LPIM-1

The chemical structure of LPIM-1 and a-LPIM-1 was analyzed by fourier transform infrared spectroscopy (FT-IR, Nicolet iS50,USA) and1H NMR (500 MHz NMR spectrometer, BRULCER CO.,Switzerland).Gel permeation chromatography (GPC, Waters-Wyatt, USA) was used to analyze the molecular weight and the molecular weight distribution.The sample was dissolved in tetrahydrofuran(THF)to form a solution of 3 mg·ml-1and then filtered for use.

2.4.Fabrication of RO composite membrane

The PARO membrane was prepared by interfacial polymerization on the PSf support.Before the membrane preparation, the PSf supporting membrane was soaked in 30%(volume)isopropanol for 30 min and then rinsed by deionized water prior to use.The aqueous phase consisted of 2% (mass) MPD, TEA-HCl and a series of different concentration of a-LPIM-1 dissolved in deionized water, while the organic phase consisted of 0.1% (mass) TMC dissolved in Isopar?-G.The PSf support was firstly exposed to aqueous phase for 1 min.Then the excess liquid was drained and residual liquid on the surface was gently removed by rolling a silicone roller.After that, the top surface of the PSf support was allowed to contact with an equal volume of the organic phase for 1 min followed by removal of excess solution.Afterwards, Isopar?-G was used to rinse the membrane surface.Lastly, the prepared membrane was dried at 90°C for 10 min.The prepared membranes were stored in deionized water for later use and marked as a-LPIM-1x_PA, the subscript indicating the mass percent of a-LPIM-1 addition.

2.5.Characterization of RO composite membranes

Attenuated total reflectance fourier transform infrared spectroscopy(ATR-FTIR,Nicolet iS50,USA)was used to analyze the surface chemistry of the prepared RO membranes.Contact angle analyzer (Dataphysics OCA20, Germany) was utilized to measure the static water contact angle.Electrokinetic analyzer (SurPASS Anton Parr, GmbH, Austria) was used to detect the surface potential with KCl (1.0 mmol·L-1) solution as the electrolyte solution.HCl (0.1 mmol·L-1) and NaOH (0.1 mmol·L-1) were used to adjust pH values.Field emission scanning electron microscopy (FE-SEM,ZEISS Ultra 55, Germany)and atomic force microscope (AFM, Bruker ICONs-sys, USA) were used to investigate the surface morphology.

2.6.Evaluation of RO performance

The RO performance of membranes with different proportion of a-LPIM-1 was evaluated at 25°C by using a self-made cross-flow device which had three parallel cells with an effective area of 12.56 cm2.Typically, the membrane was pre-pressured to 1.8 MPa for 1 h and the pressure was adjusted to 1.6 MPa.The test can be performed after the pressure and flow rate were stable.NaCl solution (2 mg·L-1) was used as feed to investigate the RO performance of the prepared membranes.The permeation fluxJ(L·m-2·h-1) was calculated as following [43]:

whereV(L)is the volume of the penetrate during the test timet(h),S(m2) represents the effective area of a test cell.

The salt rejectionR(%)was calculated using following equation[44]:

whereCfandCprepresent the concentration of the feed and the penetrate respectively.The concentration differences between the feed and the penetrate was measured by electric conductometer.

All the data engaged are the average of multiple parallel experiments.

3.Results and Discussion

3.1.Characterization of a-LPIM-1

Fig.2. (a) FTIR spectra of LPIM-1 and a-LPIM-1; (b) Predicted 3D model of a-LPIM-1.

FT-IR was employed to confirm the synthesis of LPIM-1 and a-LPIM-1.As shown in Fig.2(a), the broad band at 3450 cm-1was assigned to the O-H stretching [45].Particularly, the absorption peak at 1251 cm-1ascribed to the stretching vibration of the C-O-C existed both in LPIM-1 and a-LPIM-1, indicating that the reaction between TTSBI and DCTB had successfully occurred [46].When the hydrolysis was implemented on LPIM-1, the absorption peak at 2240 cm-1attributed to the stretch vibration of the nitrile was disappeared [47], while the characteristic absorption peak at 1672 cm-1linked to the stretching vibration of the newborn amide C=O [42]was only observed in a-LPIM-1.Furthermore, the peak strength at 3000-3500 cm-1also became stronger, which may be corresponded to the stretching vibration of the amide N-H.These variations proved that the a-LPIM-1 with abundant amide groups had been successfully formed after the hydrolysis of LPIM-1, and exhibited an obviously enhanced hydrophilicity and water solubility as shown in Fig.S1.The1H NMR results also proved the structural transformation mentioned above, as shown in Fig.S2.

The molecular weight of a-LPIM-1 was measured by GPC.According to the test results shown in Fig.S3,a-LPIM-1 was mainly composed of small oligomers with the number-average molecular weight of 1,837 and 889 g·mol-1, much lower than approximately 300,000 g·mol-1of the reported PIMs [34,41,48].Combined with the FT-IR and GPC results, the possible 3D configuration of a-LPIM-1 was simulated as Fig.2(b).As in the interfacial polymerization process, the aqueous monomer had to diffuse from aqueous phase to organic phase, and then reacted with the organic monomer of TMC.Therefore, the low molecular weight of a-LPIM-1 not only further promoted its dissolution in water, but also reduced its steric hindrance in the phase-transfer process, and participate more in the interfacial polymerization, which had been confirmed by the free-standing experiment (see Fig.S4).

3.2.Surface characterization of membranes

3.2.1.Surface chemistry

ATR-FTIR was employed as well to characterize the surface chemistry of the as-prepared RO composite membranes.Fig.3 showed the ATR-FTIR spectra of the pristine and a-LPIM-10.5_PA RO membranes,respectively.The typical spectra of PA membranes identified as amide II (-C-N-H, 1510 cm-1) modes in secondary amides and aromatic amide band (N-H, 1598 cm-1), originated from the crosslinking of MPD and TMC, were presented in both membranes.Such results were in agreement with the previous reported results [49-53], and confirmed the success formation of PA RO membranes.While compared to the pristine PA membrane,the a-LPIM-10.5_PA membranes particularly presented new adsorption peaks of 1714 and 1238 cm-1linked to the C=O stretching vibration and the C-O-C stretching vibration in ester bond, respectively.The appearance of these peaks indicated the successful crosslinking between the hydroxyl terminal of a-LPIM-1 and the acyl chloride of TMC, proving that a-LPIM-1 was covalently incorporated into the PA layerviathe ester bonds formed during the interfacial polymerization.

Fig.3. ATR-FTIR spectrum of RO membranes.

Fig.4. The contact angle and isoelectric focusing of RO membranes.

3.2.2.Surface hydrophilicity

The water contact angle was measured to investigate the effect of the addition of a-LPIM-1 on membrane surface hydrophilicity.The lower value of water contact angle means the higher surface hydrophilicity.As shown in Fig.4,the water contact angle reduced from 70.0°to 57.9°as the a-LPIM-1 mass content increased from 0 to 1%, indicating the improvement in membrane surface hydrophilicity.Such enhanced hydrophilic capability was mainly attributed to the more carboxyl groups hydrolyzed from acyl chlorides of TMC because of the steric hindrance, as well as the introduction of a-LPIM-1with six amide groups

3.2.3.Surface morphology

Surface morphology of reverse osmosis membranes imposes a significant influence on its separation performance.The top surface morphology of the fabricated RO membranes with various adding amount of a-LPIM-1 was characterized by SEM as shown in Fig.5.The pristine and a-LPIM-1 based membrane both presented a unique and typical surface morphology with ‘‘leaf-nodule” feature for the PA RO membranes prepared by interfacial polymerization [54].However, it was clear that the addition of a-LPIM-1 still caused an obvious change on the surface morphology.With the increase of a-LPIM-1 amount, the nodule structures got increased.Moreover, such nodule structures on the membrane surface became denser and more uniform.It may be due to the fact that a-LPIM-1 with three-dimensional twisted and folded structure entered into the active layer and alleviated the tight packing of polymer segments [38].Furthermore,it was reported that interfacial polymerization for RO membrane preparation intrinsically is a reaction-diffusion process [55].Because of the steric hindrance originated from the twisted and folded structure and relatively large molecular weight of 1837 and 889 g·mol-1, the diffusion of a-LPIM-1 into the organic phase was inevitably slower than that of traditional diamine aqueous monomer commonly with molecular weight around 100 g·mol-1.The reactivity of the terminal hydroxyl with TMC was also lower than that with amino groups.Of necessity, such moderate reaction-diffusion process preferred to form nodule structure rather than leaf structure.

Fig.5. SEM images of RO membranes with different mass content of a-LPIM-1: (a) and (b) pristine RO membranes, (c) and (d) 0.1%, (e) and (f) 0.5%, (g) and (h) 1.0%.

Fig.6. AFM images of RO membranes with different mass content of a-LPIM-1: (a) pristine RO membranes, (b) 0.1%, (c) 0.5% and (d) 1.0%.The x- and y-axis represent the horizontal size, while the z-axis represents the height in the vertical direction.

In addition, AFM images in Fig.6 revealed that the pristine RO membranes exhibited the typical ‘‘leaf-nodule” structure that was consistent with the previously reported literature [56], while the a-LPIM-1 based membranes presented less leaf structure.The result was that the roughness of a-LPIM-1 based RO membranes reduced gradually from 59.0 to 35.6 nm with a-LPIM-1 addition as shown in Table 1.It was reasonable and aligned with the above SEM images as more nodule structures formed.

Table 1Roughness and surface area of the membrane surface obtained from AFM

The permeation resistance of water molecules increases with the thickness of the active layer,which leads to the decrease of permeability [57].Therefore, it is necessary to investigate the thickness of the as-prepared a-LPIM-1 based membranes.The crosssection SEM images were shown in Fig.7.As discussed above,the addition of a-LPIM-1 with twisted and folded structures may alleviate the packing status of polymer segments and the reaction-diffusion rate, the active layer was theoretically to be thinner.However, considering that 2% (mass) of the aqueous monomer was highly reactive MPD, the bulk matrix was mainly determined by the formed polyamide.Therefore, the thickness of the active layer before and after a-LPIM-1 addition did not change significantly as shown in Fig.7, both approximately at 200 nm.Fig.8

Fig.7. SEM cross-section images of RO membranes with different mass concentration of a-LPIM-1: (a) 0 and (b) 0.5%.

3.3.Separation performance of membranes

Water permeability and salt rejection, the two most crucial indexes for RO membranes,were evaluated by a cross-flow system.With the addition of a-LPIM-1,a significant increase in membrane permeability was gained.The water flux increased by about 30%at 0.1% (mass) addition of a-LPIM-1, and was nearly doubled at 0.5%(mass) addition of a-LPIM-1.As for the NaCl rejection, the 0.1%(mass) addition of a-LPIM-1 caused a slight decrease from 98.0%to 97.6%.Then the NaCl rejection kept nearly constant with the addition of a-LPIM-1.However, when the addition amount was up to 1.0% (mass), the salt retention rate suddenly dropped to 96.7%with only a slight increase in water flux.It may be attributed to the fact that the introduction of too much free volume of a-LPIM-1 consequently caused a relatively loose selective layer.

It has been established that in reverse osmosis membranes,the water permeation is positive to the surface roughness as the effective permeation area increased, and negative to the thickness of the active layer as the permeation resistance increased.Therefore as shown in Fig.6 and Table 1, the reduced surface roughness and unchanged layer thickness actually was not beneficial to the water permeation of these a-LPIM-1 membranes.In this case,considering the more nodule structures caused by a-LPIM-1 addition,the significant improvement in membrane permeability was mainly attributed to the uncompacted polymer chains caused by the a-LPIM-1 of three-dimensional twisted and folded structure,which was totally different from the tightly packed selective layer in traditionally polyamide RO membranes.Moreover, the high dihedral angle of around 90° in the spiral ring of a-LPIM-1 would also lead to form the internal space between the polymeric segments, further increasing the membrane permeability.

In order to compare the separation performance with the previously reported RO membranes,we intendedly selected three types RO membranes as reference:commercial membranes,membranes with novel aqueous monomers and mixed matrix membranes, as shown in Fig.9.It can be seen that the introduction of a-LPIM-1 with high micropores and free-volume rendered the obtained RO membranes a superb water permeability.Simultaneously, the improved cross-linking degree induced by the mixture ofmphenylenediamine further assured an acceptable salt rejection.Therefore, it was clear that this work provided a new avenue to break the trade-off effect between permeability and selectivity in PA RO membranes.

Fig.8. Water permeability and NaCl rejection of membranes with different content of a-LPIM-1.All the tests were implemented under 1.6 MPa,25°C with 2 g·L-1 NaCl.

Fig.9. Summary of permeability performance of commercial and previously reported RO membranes [52,58-60].Dow-SW30HR and Dow-BW30 are commercial RO membranes.

4.Conclusions

In summary,a water-soluble a-LPIM-1 of low-molecular-weight was synthesized and served together with MPD as aqueous monomers to form highly permeable reverse osmosis membraneviainterfacial polymerization.With the addition of a-LPIM-1, the well-controlled frustrated stacking between polymer chains and the rise in internal space between segments were accomplished resulting an increase in polymeric free volume.Consequently, the a-LPIM-1 incorporated RO membranes showed remarkable improvement in water permeation with acceptable NaCl rejection.When the addition amount of a-LPIM-1 reached to 0.5%(mass),the optimized RO membranes showed an excellent water permeance of 39.3 L·m-2·h-1·MPa-1, 2.1-folds higher than that of the pristine RO membranes,without obviously comprising NaCl rejection.This study may provide a novel approach to break the trade-off effect between permeability and selectivity in PA RO membranes by covalently incorporating polymers of intrinsic microporosity into the selective layer.

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

This work was supported by Zhejiang Provincial Natural Science Foundation of China (LZ20B060001), National Natural Science Foundation of China (22008208 & 21908192), and China Postdoctoral Science Foundation (2019TQ0276).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.05.002.