Hongyong Zhao ,Lizhong Feng ,Xiaoli Ding *,Xiaoyao Tan ,Yuzhong Zhang

1 State Key Laboratory of Separation Membranes and Membrane Processes/National Center for International Joint Research on Separation Membranes,Tianjin Polytechnic University,Tianjin 300387,China

2 School of Environmental and Chemical Engineering,Tianjin Polytechnic University,Tianjin 300387,China

3 Institute of Separation Material and Process Control,School of Material Science and Engineering,Tianjin Polytechnic University,Tianjin 300387,China

Keywords:Gas separation Polymerization-induced phase separation UV-cross-linked porous substrate Metallic ion-cross-linked PIM-1TFC membrane

A B S T R A C T Metallic ion-cross-linked polymer of intrinsic microporosity(PIM-1)thin- film composite(TFC)membranes supported on an ultraviolet(UV)-cross-linked porous substrate were fabricated.The UV-cross-linked porous substrate was prepared via polymerization-induced phase separation.The PIM-1 TFC membranes were fabricated via a dip-coating procedure.Metallic ion-cross-linked PIM-1 TFC membranes were fabricated by hydrolyzing the PIM-1 TFC membrane in an alkalisolution and then cross-linking it in a multivalent metallic ion solution.The pore size and porous structures were evaluated by low-temperature N2 adsorption–desorption analysis.The membrane structure was investigated by field-emission scanning electron microscopy.The effects of heat treatment and pore-forming additives on the gas permeance of the UV-cross-linked porous substrate are reported.The effects of different pre-coating treatments on the gas permeance of the metallic ion-cross-linked PIM-1 TFC membrane are also discussed.The metallic ion-crosslinked PIM-1 TFC membrane displayed high CO2/N2 selectivity(23)and good CO2 permeance(1058 GPU).

1.Introduction

The great potential for gas separation applications of polymeric membranes has been well known,due to its great potential for gas separation applications since the 1970s[1].Currently,membrane technologies for gas separation are available as independent operating units for industrial applications,such as flue gas treatment(CO2/N2),and natural gas refining(CO2/CH4)[2,3].Synthesizing high performance polymeric materials and asymmetric membranes remains critically important for future investigations[4].

Polymers of intrinsic microporosity(PIMs)have been successfully prepared by Budd and McKeown[5–7]as one of promising membrane materials.The most representative PIMs,PIM-1,displays high gas permeability attributed to its large free volume[7].Subsequently,in 2004,a PIM-1/polyacrylonitrile(PAN)thin- film composite(TFC)membrane was prepared for gas separation and organic solvent nanofiltration by McKeown,Budd and Fritsch in 2004[8].Additionally,a cross-linked PIM copolymer/PAN TFC membrane with excellent solvent resistance was produced by the Fritsch research group in 2012[9].Through spin coating and deposition,a PIM-1/PAN TFC membrane with an ultrathin selective layer(thickness a low as~35 nm)was successfully fabricated by the Livingston research group in 2014 for organic solvent nanofiltration[10].Finally,a high flux PIM-1/polyvinylidene fluoride(PVDF)TFC membrane for 1-butanol/water pervaporation was fabricated by the Budd research group in 2017 using PVDF,a more hydrophobic porous support[11].

However,PIM-1 also exhibits low gas selectivity,which represents an enormous obstacle to its industrial application.The post modification of PIM-1 dense film was recently developed as an efficient means for improving this material's gas-separation performance[12–21].Using a number of different techniques,a variety of post-modified PIM-1dense films have been prepared,including the following:a carboxylated PIM-1 membrane via hydroxide hydrolysis[12]and a PIM-1 membrane containing CO2-philic pendant tetrazole groups[13]prepared by the Guiver research group,an ultraviolet(UV)-cross-linked PIM-1 membrane[14]synthesized by Honeywell UOP,an amidoxime-PIM-1 membrane[15]prepared by the Yavuz research group,a thermally self-cross-linked PIM-1 membrane[16]and a metal ion-modified PIM-1membrane[17]prepared by the Chung research group,a thermo-oxidatively cross-linked PIM-1 membrane[18]prepared by the Sivaniah research group,and a thioamide-PIM-1 membrane[19],amine-PIM-1 membrane[20]and di/mono-ethanol amine-modified PIM-1 membrane[21]prepared by the Budd research group.

In this study,a post-modification step was planned for the PIM-1 TFC membrane to improve its gas selectivity because applying post modification to metallic ion-cross-linked PIM-1 dense films in our previous work significantly improved the films' gas selectivity[22].The post-modification of PIM-1 dense films often includes hydrolysis as the first step[12,18,22],which requires soaking the film in NaOH solution(H2O/ethanol)for hydrolysis.Prior to the post-modification of the PIM TFC membrane,a type of UV-cross-linked membrane prepared via polymerization-induced phase separation(PIPS)and a commercial PAN membrane were soaked in NaOH solution for 2 days to determine their chemical stability.As shown in Fig.1,the UV-cross-linked membrane displayed excellent anti-alkali properties compared to the commercial PAN membrane.Moreover,because of its resistance to most organic solvents,including chloroform and tetrahydrofuran,which are good solvents for PIM-1,the UV-cross-linked porous membrane was determined to be a suitable substrate for the fabrication of PIM-1 TFC membranes in the present study.

PIPS technology is commonly used to synthesize cross-linked porous polymers as ion-exchange or adsorbing materials.The first report focusing on cross-linked polymers with macroporous structures formed by polymerization was published in the1960s[23].The mechanism underlying PIPS was studied and discussed by Okada et al.in 1995[24].Reaction-induced phase separation in amorphous thermo-plastic modified epoxy systems was observed and investigated by Williams et al.in 1997[25].A comprehensive review of PIPS technology and the theoretical models used to predict the PIPS process and total porosity was published by Okay in 2000[26].Cross-linked polyethylene oxide free-standing films with porous morphologies were prepared via UV PIPS by Wu et al.in 2010[27].

In this work,the effects of pore-forming additives and heat treatment on the gaspermeance and porousstructure of the UV-cross-linked porous substrate are investigated and discussed.The effects of pre-coating treatment and the coating solution concentration on the gas permeance of metallic ions-cross-linked PIM-1 TFC membranes are also investigated.

2.Experimental

2.1.Materials

The PIM-1 polymer was synthesized as reported previously[22].The commercial PAN membrane was supplied by Dalian Euro- film Industrial Ltd.Co.Dipentaerythritol hexa acrylate(DPHA,Double bound chemical IND.,Co.,Ltd.)and 1-hydroxylcyclohexyl phenyl ketone(HCPK,98%,J&K Chemicals)were used as received.Methanol,chloroform,polyethylene glycol(PEG,Mw=400 g·mol-1),and γ-butyrolactone(GBL)were supplied by Tianjin Kermel Chemical Reagents Development Center and used as received.

2.2.UV-cross-linked porous substrate

The UV-cross-linked porous substrate membrane was synthesized by a PIPS process.DPHA was employed as the cross-linker,HCPK was used as the UVinitiator,and PEG 400 or GBL was used as the pore-forming additive.The HCPK content was maintained at 1 wt%relative to the total amount of DPHA and additive.The pre-polymerization mixture was sonicated for 30 min to eliminate bubbles(Ultrasonic cleaner,Model7KQ3200DB,YuHua Instrument,GongYi,China).UV lamp with an average wavelength of 312 nm was used.Subsequently,the pre-polymerization mixture was sandwiched between two quartz plates,and the membrane thickness was controlled by Cu spacers.The distance between UV lamp and the two quartz plates is about5 cm.The PIPS process was performed under UVirradiation for 20 min.The resulting membranes were dried for different times to remove the additive in a vacuum oven at 180°C.

2.3.PIM-1 TFC membrane

A certain concentration of PIM-1 coating solution was prepared via dissolving PIM-1 powder in chloroform.Prior to dip-coating procedure,a pre-coating step was performed,which involved dipping the UV cross-linked porous substrate in water or chloroform.A UV crosslinked porous substrate without pre-coating was also investigated to allow for comparison.Then,the porous substrate was withdrawn from the water or chloroform,and the excess water or chloroform on the surface was wiped off quickly with filter paper.The pre-coated porous substrate was attached on a glass plate,dipped into the PIM-1 coating solution for 30 s,and then withdrawn from the coating solution at a controlled speed of 1 m·min-1.In general,the coating thickness decreased as the withdrawal speed decreased[28];however,the PIM-1 TFC membrane developed obvious defects,when the withdrawal speed was too low.Thus,a with drawalspeed of1 m·min-1was chosen.The PIM-1 TFC membrane was dried at room temperature for 72 h.Then,the dried membrane was immersed in methanol to remove residual chloroform from the selective layer and dried again at room temperature for 72 h.

Fig.1.Photographs of the membranes after soaking in 10 wt%NaOH solution(H2O/ethanol=1:9(v/v))for 2 days.(A):commercial PAN membrane;(B):UV-cross-linked membrane prepared in this work.

Fig.2.Synthesis process used to obtain the metallic ion-cross-linked PIM-1 TFC membrane.

2.4.Metallic ion-cross-linked PIM-1 TFC membrane

The PIM-1 TFC membrane was soaked in 10 wt%NaOH solution(H2O/ethanol=1:9(v/v))at room temperature with stirring for 2 days.Subsequently,the membrane was washed with a large amount of water and methanol and soaked in 0.1 mol·L-1AlCl3aqueous solution at room temperature for 24 h with stirring.The resulting membrane was washed with a large amount of water and methanol and dried at room temperature for 72 h.The synthesis process used to obtain the UV-cross-linked porous substrate,PIM-1 TFC membrane,and the metallic ion-cross-linked PIM-1 TFC membrane is shown in Fig.2.

2.5.Membrane morphology

The morphologies of the cross-linked substrate and composite membrane were characterized by field-emission scanning electron microscopy(FESEM)(Hitachi-s-4800,Japan).The samples were sputter-coated with gold to increase their conductivity.

2.6.Characterization of the pore size of the porous substrate membrane

Low-temperature(77 K)N2adsorption–desorption analysis was conducted to determine the pore size and porous structure of the cross-linked porous substrate membrane using a Quanta chrome Autosorb-iQ-C gas sorption analyzer.The samples were allowed to degas for 3 days at 100°C under high vacuum prior to analysis.

2.7.Permeation measurement

The pure gas permeance(P/L,GPU,1 GPU=10-6cm3(STP)·cm-2·s-1·cm Hg-1)of the membrane was measured by a constant pressure/variable volume method in the order of N2and CO2at 25°C.The feed-side and permeate-side pressures were maintained at 0.5 MPa(G)and 0 MPa(G),respectively.The gas permeance was calculated using the following equation:P/L=Q/(Δp A),where Q is the volumetric flow rate of the pure test gas at standard temperature and pressure(cm3·s-1,STP),which was determined by a soap- film flow meter;A is the effective membrane area(cm2);and Δp is the trans-membrane pressure difference(cm Hg).

3.Results and Discussion

3.1.Characterization of the UV-cross-linked porous substrate

3.1.1.Heat treatment

UV-cross-linked DHPA membranes obtained from a prepolymerization mixture containing 50 wt%PEG 400 were prepared with different heating times.As seen in Fig.3,unlike the pristine membrane,which was transparent,the membranes subjected to heat treatment became white and opaque,because of the removal of a large amount of PEG 400.In general,polymers without agglomerates(i.e.,containing nuclei,microspheres and large irregular moieties)or porous structures appear transparent[26].In contrast,polymers with agglomerates and porous structures appear white or opaque.In addition,if the agglomerates are small,the polymer will be translucent.The results obtained here indicated that nuclei(i.e.,nonporous small particles),microspheres(i.e.,agglomerates of nuclei),and large irregular moieties(i.e.,agglomerates of microspheres)[29]formed in the white and opaque PSM-50%-PEG-72 membrane during the PIPS process.Additionally,micropores,mesopores,and macropores developed among the nuclei,microspheres,and large irregular moieties,as illustrated in detail in the section below and Fig.4.

Fig.3.Photographs of the PSM-50%-PEG-(0/24/72)membranes.(The UV-cross-linked DHPA membrane prepared in this work was termed PSM-X%-(PEG/GBL)-(0/24/72),where PSM is the porous substrate membrane,X%is the pore-forming additive content in the pre-polymerization mixture,and the numbers 0,24,and 72 indicate that the membrane was heat treated for 0,24,or 72 h,respectively.)

Fig.4.Surface and cross-sectional FESEM images of the PSM-50%-PEG-72 membrane.

The gas permeance values of the UV-cross-linked DHPA membranes prepared with different heating times are given in Table 1.The gas permeance of the PSM-50%-PEG-0 membrane was too low to be accurately detected by our soap- film flow meter(i.e.,less than approximately 1.4 GPU,calculated based on a 1-ml volume taking more than 10 min of test time with a membrane area of 3.14 cm2).Clearly,the PSM-50%-PEG-24 and PSM-50%-PEG-72 membranes had high gas permeance values coupled with CO2/N2selectivities of less than 1.These findings indicated that the PSM-50%-PEG-24 and PSM-50%-PEG-72 membranes were both porous,and that gas transport within the membranes mainly followed the Knudsen diffusion mechanism[28].In addition,the PSM-50%-PEG-24 membrane displayedrelatively low gas permeance compared to the PSM-50%-PEG-72 membrane,possibly because of the small amount of residual PEG 400 in the membrane.The PSM-50%-PEG membrane obtained after 72 h of heat treatment exhibited high gas permeance and had good potential as a porous substrate for TFC membrane fabrication.

Table 1Gas permeance of the PSM-50%-PEG-(0/24/72)membranes(tested at 0.5 MPa and 25°C)

3.1.2.Additives

UV-cross-linked DHPA membranes were prepared from prepolymerization mixtures with different contents of PEG 400.The PSM-50%-PEG-0 and PSM-70%-PEG-0 membranes obtained with no heat treatment were translucent,as shown in Fig.5,because of a large amount of residual PEG 400 in these membranes.As the PEG 400 content in the pre-polymerization mixture increased,the membranes obtained after the heat treatment changed from translucent to white and opaque.The PSM-10%-PEG-72 membrane was slightly translucent,the PSM-30%-PEG-72 membrane was translucent,and the PSM-50%-PEG-72 and PSM-70%-PEG-72 membranes were white and opaque.As mentioned above,a white and opaque appearance may indicate the presence of a porous structure in the membranes.Moreover,the PSM-70%-PEG-72 membrane was cracked after heat treatment,possibly because too much PEG 400 was removed,resulting in poor mechanical properties.

Fig.5.Photographs of the PSM-X%-PEG-0 membranes(no heat treatment)and PSM-X%-PEG-72 membranes(72 h heat treatment).

Fig.6.FESEM images of the PSM-X%-PEG-72 membranes.

Fig.6 shows FESEM images of the UV-cross-linked DHPA membranes obtained from pre-polymerization mixtures with different PEG 400 contents after 72 h of heat treatment.The membranes prepared from pre-polymerization mixtures containing more than 10 wt%PEG 400 exhibited obvious pores on their surfaces of membranes compared to the PSM-0%-PEG-72 membrane.Additionally,the PSM-10%-PEG-72 membrane displayed a large number of circular pores,the PSM-30%-PEG-72 membrane displayed some circular pores and some slitshaped pores(see the 50000×magnification FESEM image),the PSM-50%-PEG-72 membrane displayed a small number of circular pores and a large number of slit-shaped pores,and the PSM-70%-PEG-72 membrane displayed slit-shaped pores almostexclusively.These results clearly demonstrated that adding PEG 400 to the pre-polymerization mixture led to porous structures on the membrane surface.Additionally,the pores on the surface changed from mainly circular pores to slit-shaped pores as the PEG 400 content in the pre-polymerization mixture increased.The membranes obtained from pre-polymerization mixtures with moderate PEG 400 contents exhibited pores of both types.In addition,the cross-sections of the membrane exhibited loose and porous structures as the PEG 400 content increased.

In this work,only DHPAwas used as a cross-linker to form highly crosslinked polymers that hardly absorb liquid PEG 400.Therefore,the liquidphase PEG 400 was separated from the gel phase(i.e.,polymer phase),which initiated the PIPS process.When the PEG 400 content in the prepolymerization mixture was low,the PEG 400 became a discontinuous phase that existed as smalldroplets inside the highly cross-linked polymer,resulting in the formation ofcircular pores.This process was termed microsyneresis,as proposed by Du?ek[30],and the resulting circular pores were almost disconnected.In contrast,when the PEG 400 content in the prepolymerization mixture was high,small cross-linked particles,which were termed nuclei,formed as the discontinuous phase.Subsequently,these nucleiagglomerated into microspheres,which continually agglomerated to form larger irregular moieties and,eventually,a continuous phase,as shown in Fig.4.Therefore,a system of two continuous phases–the liquid-phase PEG 400 and the polymerphase–was established,which contributed to forming the slit-shaped pores.This process was termed macrosyneresis,as proposed by Du?ek[30],and the resulting slit-shaped pores were commonly interconnected.

The gas permeance values of the PSM-X%-PEG-72 membranes are given in Table 2.The gas permeance values of the PSM-0%-PEG-72 and PSM-10%-PEG-72 membranes were too low to be accurately detected by a soap- film flow meter.Thus,although a large number of circular pores can be found on the surface of the PSM-10%-PEG-72 membrane,this result seems to confirm that these pores were disconnected and did not penetrate the whole membrane.Compared with the PSM-10%-PEG-72 membrane,the PSM-30%-PEG-72 and PSM-50%-PEG-72 membranes displayed much higher gas permeance values because of their high numbers of slit-shaped and interconnected pores,as shown in Fig.6.Additionally,the gas permeance increased as the PEG 400 content in the pre-polymerization mixture increased.Moreover,the CO2/N2selectivities of the PSM-30%-PEG-72 and PSM-50%-PEG-72 membranes were both less than 1,which is consistent with Knudsen diffusion[28].This result indicated that the mesopores and macropores were present in both membranes.

To further investigate the porous structures and the pore size distributions of the PSM-30%-PEG-72,PSM-50%-PEG-72,and PSM-70%-PEG-72 membranes,low-temperature N2adsorption–desorption analysis was performed and the pore size distributions were calculated using density functional theory(DFT).Fig.7 shows that the adsorption–desorption isotherms of all the membranes contain obvious H3 desorption hysteresis loops according to the IUPAC classification term[31].Thus,the slit-shaped mesopores mainly arose in the PSM-30%-PEG-72,PSM-50%-PEG-72,and PSM-70%-PEG-72 membranes.Because of its much lower gas sorption and surface area,the PSM-30%-PEG-72 membrane displayed substantially fewer pores compared to the PSM-50%-PEG-72 and PSM-70%-PEG-72 membranes.This finding explains why the PSM-30%-PEG-72 membrane's gas permeance was lower than that of the PSM-50%-PEG-72 membrane.For the PSM-30%-PEG-72 membrane,the pore size was in the range of 4–15 nm.However,increasing PEG 400 content in the pre-polymerization mixture increased the pore size range slightly to approximately 5–24 nm in the PSM-50%-PEG-72 and PSM-70%-PEG-72 membranes.As shown in Fig.4,the pore sizes both on the surface and inside of the PSM-50%-PEG-72 membrane were on the nanoscale.Fig.4 also shows that most pores were smaller than 50 nm,placing these pores in the range of mesopores and indicating that these pores were formed from microspheres[30].Additionally,a few pores exceeded 50 nm and fell into the range of macropores;thus,these pores were formed from large irregular moieties.

Table 2Gas permeance of the PSM-X%-PEG-72 membranes(tested at 0.5 MPa and 25°C)

Fig.7.Low-temperature N2 adsorption–desorption analyses of the PSM-X%-PEG-72 membranes:(a)PSM-30%-PEG-72,(b)PSM-50%-PEG-72,and(c)PSM-70%-PEG-72.

Fig.8.Photographs and FESEM images of the PSM-X%-GBL-72 membranes.

GBL was also used as a pore-forming additive to produce UV-crosslinked DHPA membranes.As seen in Fig.8,the membranes created from pre-polymerization mixtures containing more than 50 wt%GBL were distorted and broken,and thus,their gas permeance values could not be determined.In contrast,the gas permeance values of the membranes produced from pre-polymerization mixtures containing less than 30 wt%GBL were too low to be accurately detected by a soap- film flow meter.Additionally,the pore structures of the PSM-X%-GBL-72 membranes were similar to those of the PSM-X%-PEG-72 membranes,and as GBL content in the pre-polymerization mixture increased,the surface pores changed from mainly circular pores to slit-shaped pores.The micro-syneresis process occurred when the GBL content was low,and the macro-syneresis process occurred when the GBL content was high,which is consistent with the result found for the PSM-X%-PEG-72 membranes and described above.In summary,the effect of PEG 400 as a pore-forming additive was better than that of GBL for the formation of a cross-linked DHPA porous substrate membrane by the PIPS process.The PSM-50%-PEG-72 membrane was selected as the porous substrate for PIM-1 TFC membrane fabrication.

Additionally,compared with traditional polysulfone,polyetherimide and PAN porous substrates,the UV-cross-linked porous substrate synthesized through the PIPS process did not display very desirable mechanical properties or a porous structure.However,this substrate did exhibit a good ability to resist acid–base environments.This should provide additional choices and research directions to develop gas or water membrane separation process under strongly acidic and alkali conditions.Furthermore,three-dimensional printing technology may have potential to be employed to create UV-cross-linked porous membranes[32].

3.2.Metallic ion-cross-linked PIM-1 TFC membrane

3.2.1.PIM-1 TFC membrane

The gas permeance values of the PIM-1 TFC membranes on the PSM-50%-PEG-72 substrate are given in Table 3.All membranes that were pre-coated with water(CM-W membranes),exhibited fairly high gas permeance and almost no CO2/N2gas selectivity,possibly because of the defects formed through the slight phase separation between water and chloroform.Meanwhile,as shown by Fig.9(a),the selective layer in the CM-W-2%membrane was less than 0.1 μm in thickness.This thin selective layer was caused by the PIM-1 coating solution becoming blocked in the substrate that had been pre-coated with water[33].However,this thin layer was easily broken by the occurrence of a slight phase separation.

Compared with the CM-N-1%membrane,the CM-N-2%and CM-N-3%membranes both displayed higher CO2/N2selectivities that were close to their intrinsic selectivities[22].This is because that higher coating solution concentration led to thicker selective layers and fewer defects,which resulted in better gas selectivity.However,the CM-N-2%and CM-N-3%membranes exhibited relatively low gas permeance values because of their thicker selective layers.A selective layer that was more than 5 μm thick was obtained in the CM-N-2%membrane,as shown in Fig.9(b).

Similar to the CM-N-X%membranes,for CM-S-X%membranes,increasing the coating solution concentration resulted in lower gas permeance and higher gas selectivity.In addition,compared with the CM-N-2%membrane,the CM-S-2%membrane displayed threefold-higher gas permeance coupled with gas selectivity that was close to its intrinsic selectivity[22].As seen in Fig.9c,an interesting structure was found in the CM-S-2%membrane.This layer,which has a fine leaf-vein structure,was termed the solvent-diffusion layer in this study,and may be attributable to the pre-coating of the membrane with chloroform.It is presumed that the porous microspheres were swelled by the chloroform and then became finer particles,when the chloroform was removed,resulting in the formation of new agglomerates with leaf-vein structure.The top layer,which has a cavity structure,was the selective layer and its thickness was approximately 2 μm.These results explain why the CM-S-2%membrane displayed almost threefold-higher gas permeance compared to the CM-N-2%membrane.As for the cavity structure in the selective layer,it is presumed that the PIM-1 coating solution caused the porous microspheres of the selective layer to swell,similar to the effect of chloroform in the solvent-diffusion layer.However,the difference is that the swelled porous microspheres were prevented from getting de-swelling by the presence of the PIM-1 polymer in the porous microspheres after the solvent was removed,eventually resulting in the formation of the cavity structure in the selective layer.Notably,this structure retained sufficient mechanical properties to be tested at a gas pressure of 0.5 MPa.

In summary,the CM-W-Y%membranes had very thin selective layers because of the immiscibility of water and chloroform,and it is difficult to fabricate defect-free selective layers using the method reported here.The CM-N-Y%membranes displayed thicker selective layers than the CM-S-Y%membranes because the PIM-1 coating solution entered the pores of the substrate.The CM-S-2%membrane exhibited good gas permeance coupled with good gas selectivity,and its selective layer was approximately 2 μm thick.

3.2.2.Metallic ion-cross-linked PIM-1 TFC membrane

The gas permeance values of the metallic ion-cross-linked PIM-1 TFC membranes are given in Table 3.Compared with the neat PIM-1 TFC membranes(CM-W-Y%),the metallic ion-cross-linked PIM-1 membranes(PCM-W-Y%)exhibited higher gas permeance but lower gas selectivity(less than 1).It is presumed that the hydrolysis step in the post-modification process damaged the selective layer by forming corroded pores on the surface of the membrane,as shown in Fig.10.

Regarding the PCM-N-Y%membranes,post-modification significantly decreased the gas permeance,which is consistent with the result found for metallic ion-cross-linked PIM-1 dense films[22].Meanwhile,similar to metallic ion-cross-linked PIM-1 dense films,post-modification led to an obvious increase in the CO2/N2selectivity(i.e.,up to 23)[22].Additionally,the corroded pores formed in hydrolysis process did not damage the selective layer,unlike in CM-W-Y%membranes,because of thicker selective layers(＞5 μm)in CM-N-Y%membranes.

The PCM-S-Y%membranes also displayed higher CO2/N2selectivities(i.e.,up to 23),similar to PCM-N-Y%membranes.However,post modification decreased the gas permeance slightly,especially in the PCM-S-2%membrane,unlike in PCM-N-Y%membranes.It is presumed that this difference can be attributed to the presence of corroded pores.Thinner selective layers are more strongly affected by corroded pores,leading to greater increases in the gas permeance.In summary,corroded pores increased the gas permeance but did not penetrate the whole selective layer and,thereby,contributed to maintaining high gas selectivity in the CM-S-2%membrane.Compared with the PIM-1/PAN TFC membrane[8],the PCM-S-2%membrane displayed higher CO2/N2selectivity because of the cross-linking post-modification,but its CO2permeance was lower,although it remained higher than 1000 GPU.In general,membranes with high CO2permeance values(＞1000 GPU)accompanied by moderate CO2/N2selectivity(20–40)are considered attractive for competing with the absorption process[34].

4.Conclusions

We prepared metallic ion-cross-linked PIM-1 TFC membranes supported on a UV-cross-linked porous substrate in this study.The UV-cross-linked porous substrate was synthesized via the PIPS process and exhibited excellent alkali resistance.Additionally,when the additive content in the pre-polymerization mixture was low,the liquid additive existed as a discontinuous phase in the form of small droplets inside the polymer and finally resulted in circular,disconnected pores.In contrast,when the additive content in the pre-polymerization mixture was high,a system of two continuous phases was established,which eventually led to slit-shaped,interconnected pores.Compared with GBL,PEG 400 led to better results.Indeed,after 72 h of heat treatment,the PSM-50%-PEG membrane displayed a suitable pore size and gas permeance,making this material an appropriate substrate for TFC membrane fabrication.

The PIM-1 TFC membrane supported on the UV-cross-linked porous substrate was further modified to improve its gas-separation performance.No pre-coating,water pre-coating,or chloroform pre-coating had been systematically investigated previously.No pre-coating treatment led to a thick selective layer.In contrast,water pre-coating treatment produced a thin selective layer with defects.Finally,chloroform pre-coating generated a defect-free selective layer that was approximately 2 μm in thickness in the CM-S-2%membrane.The corresponding metallic ion-cross-linked PIM-1 TFC membrane(PCM-S-2%)displayed high CO2permeance and CO2/N2selectivity.

Fig.10.FESEM images of the PIM-1 TFC membrane and metallic ion cross-linked PIM-1 TFC membrane.