Effects of Thermal Cross-Linking on the Structure and Property of Asymmetric Membrane Prepared from the Polyacrylonitrile

Improving the thermal and chemical stabilities of classical polymer membranes will be beneficial to extend their applications in the high temperature or aggressive environment. In this work, the asymmetric ultrafiltration membranes prepared from the polyacrylonitrile (PAN) were used to fabricate the cross-linking asymmetric (CLA) PAN membranes via thermal cross-linking in air to improve their thermal and chemical stabilities. The effects of thermal cross-linking parameters such as temperature and holding time on the structure, gas separation performance, thermal and chemical stabilities of PAN membranes were investigated by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), positron annihilation lifetime spectroscopy (PALS), scanning electron microscopy (SEM), thermogravimetic analysis (TGA) and gas permeation test. The thermal cross-linking significantly influences the chemical structure, microstructure and pore structure of PAN membrane. During the thermal cross-linking, the shrinkage of membrane and coalescence or collapse of pore and microstructure make large pores diminish, small pores disappear and pore volumes reduce. The gas permeances of CLA-PAN membranes increase as the increasing of cross-linking temperature and holding time due to the volatilization of small molecules. The CLA-PAN membranes demonstrate excellent thermal and chemical stabilities and present good prospects for application in ultrafiltration for water treatment and for use as a substrate for nanofiltration or gas separation with an aggressive and demanding environment.


Introduction
Membrane-based separation technology as a novel and promising separation technology has been widely used to solve the problems on resources, energy and environment due to its high efficiency, low energy consumption and eco-friendliness [1,2]. The membrane materials, which are currently used in the membrane processes such as microfiltration, ultrafiltration, reverse osmosis and gas separation for the water treatment, desalination, food and biological product purification, and air separation [3][4][5][6][7][8][9][10], are mainly the polymeric membrane materials owing to their relatively simple preparation technics and adequate perm-selectivity. However, the poor thermal and chemical stabilities of classical polymeric chemical and thermal stabilities of PAN membranes. These works will be beneficial to prepare the cross-linking PAN membrane with a well-kept pore structure and good chemical and thermal stabilities, which present a good prospect for application in ultrafiltration for water treatment and for use as a substrate for nanofiltration or gas separation with an aggressive and demanding environment.

Preparation of Cross-Linked PAN Asymmetric Membranes
Dry-wet phase inversion technique was used to prepare the PAN asymmetric membranes. The homogeneous casting solution prepared by dissolving PAN powder into DMAc with continuous stirring was cast onto a glass plate. Then the cast membrane was evaporated in air for 30 s before been immersed in water to exchange the solvent for 1 day, prior to drying in air for 2 days.
The thermal cross-linking of PAN membranes was carried out in a tubular furnace. Figure 1 showed the cross-linking program. In order to investigate the effect of cross-linking temperature on the properties of PAN membranes, the different cross-linking temperatures of 180 °C, 220 °C, 230 °C and 260 °C were selected at heating rate of 2 °C/min with the holding time of 1 h and airflow rate of 500 mL/min. Holding time was kept for 1 h-6 h at 230 °C to investigate the effect of cross-linking holding time on the properties of PAN membranes. The furnace was cooled to room temperature naturally after cross-linking.

Characterization of Membranes
The chemical structure of PAN and CLA-PAN membranes was detected by Attenuated total reflection-Fourier transform infrared spectra (ATR-FTIR) (NICOLET 6700, Thermo Fisher Scientific, Waltham, MA, USA) and the ESCALab220i-XL X-ray photoelectron spectroscopy (XPS, Thermo

Characterization of Membranes
The chemical structure of PAN and CLA-PAN membranes was detected by Attenuated total reflection-Fourier transform infrared spectra (ATR-FTIR) (NICOLET 6700, Thermo Fisher Scientific, Waltham, MA, USA) and the ESCALab220i-XL X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, Basingstoke, UK). In order to better understand the kinetics of cross-linking, the extents of reaction (EOR) following the cross-linking temperature and holding time was introduced to quantitatively show the degree of cross-linking, which was derived from the intensity of the nitrile absorption and the absorption at 1600 cm −1 in the spectra of ATR-FTIR, and the EOR was calculated according to the following equation [39]: where I CN and I 1600 were the intensity of the nitrile absorption and the absorption at 1600 cm −1 , respectively. The microstructure of the as-made CLA-PAN membranes was detected and measured by TF30 transmission electron microscope (TEM, FEI, Hillsboro, OR, USA) and D/Max-2400 X-ray diffraction (XRD, Rigaku Industrial Corporation, Osaka, Japan). The cross-section morphologies of the PAN asymmetric membranes were observed by the NOVA Nano SEM 450 scanning electron microscope (SEM, FEI, Hillsboro, OR, USA).
Positron annihilation lifetime spectroscopic (PALS) experiments were conducted to determine the free-volume in PAN. A conventional fast-fast coincidence spectrometer with a time resolution of 250 ps was used. A radioactive source of 22 Na (0.74 MBq), which was sealed in between 7.5-µm thick Kapton ® Type HN sheets from DuPont Inc. (Calgary, AB, Canada), was sandwiched in two stacks of membrane samples that consisted of several layers of free-standing PAN. Each stack of sample had a total thickness of 1 mm. Positron annihilation lifetimes were recorded using a fast-fast coincidence timing system. A time-to-amplitude converter was used to convert lifetimes and to store timing signals in a multi-channel analyzer (Ortec System, AMETEK ORTEC Inc., Oak Ridge, TN, USA). Two million counts were collected, and all positron annihilation lifetime spectra were analyzed by a finite-term lifetime analysis method using PATFIT and MELT programs [40][41][42].
The membrane pore sizes were also characterized using the method of the molecule weight cut-off (MWCO) by testing the flux and rejection rate of PEG 10000, PEG 20000 and BSA on a dead-end membrane module for PAN membranes and CLA-PAN membranes. During the measurement, membranes were pretreated under 0.9 MPa for 30 min, and permeated the solution of PEG 10000 and PEG 20000 (100 mg/L) and bovine serum albumin (BSA, 1 g/L) under 0.8 MPa at ambient temperature. The rejection R was calculated using the following equation: where C p and C f were the concentration of the permeate solution and feed solution, respectively. All the experiments on rejection were repeated for three times [43]. Permeances of O 2 /N 2 mixtures (21/79 mol. %) for the PAN membranes and CLA-PAN membranes were measured by the variable volume-constant pressure method [44] and represented in GPU (1 GPU = 10 −6 cm 3 (STP)/cm 2 s cmHg), in which the permeated gas at a feed pressure of 5 kPa at 298 K was analyzed by gas chromatography (GC, Zhejiang Fuli analytical instruments Co., Ltd., Wenling, China, GC 7890). The detailed information about the gas permeating test process was in the reference [45].
The thermal stabilities of the PAN membrane and CLA-PAN membranes were detected by the NETZSCH TG 209C thermogravimetic analyzer (TGA, Netzsch, Bayern, Germany) in nitrogen. The chemical stabilities of CLA-PAN membranes were tested by the solvent dissolution, in which the Polymers 2018, 10, 539 5 of 16 membranes were first soaked in different solvents for 4 days and then washed by water. The residual mass fraction W was calculated from the following equation after dried in vacuum, where w 0 was the membrane mass before soaked in solvent; w t was the membrane mass after soaked in solvent, washed by water and dried in vacuum.

Results and Discussions
3.1. Chemical Structure Variations of PAN Asymmetric Membrane during Cross-Linking Figure 2 shows the ATR-FTIR spectra of PAN membrane and CLA-PAN membranes cross-linked at different temperatures and holding time. Compared to the spectra of PAN membrane, in which the typical characteristic peaks of PAN molecular are identified as -C≡N stretching vibration peak (2243 cm −1 ), -C-H stretching vibration peak (2950 cm −1 ) and C-H deformation vibration peak (810 cm −1 ), the intensity of -C≡N and -C-H stretching vibration peaks in the CLA-PAN membranes are reduced gradually, and the intensity of the -C-H deformation vibration peak is enhanced with the increase of thermal cross-linking temperature and holding time. Additionally, the -C=C and -C=N stretching vibration peak (1628 cm −1 ), -N-H stretching vibration peak (3300 cm −1 ) and C-H deformation vibration peak of C=C-H (810 cm −1 ) are observed. These indicate that -C≡N of the PAN molecular structure is ruptured and the -C=N and -N-H are formed during the thermal cross-linking. EOR data that can quantitatively show the degree of cross-linking were calculated from the intensity of the peak at 1628 cm −1 , I 1600 , and that of the nitrile peak at 2243 cm −1 , I CN , are shown in Figure 2c,d. With the increase of cross-linking temperature and holding time, the EOR data increases from 0.14 at 180 • C (1 h) to 0.85 at 230 • C (2.5 h), indicating that the cross-linking degree of PAN membranes gradually increases during the thermal cross-linking. It can also be proved from the XPS spectra of PAN membrane and CLA-PAN membranes shown in Figure 3. In the CLA-PAN membranes, new peaks of -C=N, -C=C and -C-O appear and the intensity of -C≡N peak gradually decrease in the C1s spectrum, and the intensity of -C=O and -C=N peak in O1s and N1s spectra gradually increase as the cross-linking temperature rises, suggesting that -C≡N in PAN decompose and react with oxidative medium to form the -C=N, -C=C, -C-O and -C=O. In addition, the XPS spectra of the PAN and CLA-PAN membranes ( Figure 4) show that the O atomic percentage in the membrane increases with the enhancement of cross-linking temperature, indicating that O 2 has participated in the oxidative cross-linking reaction. where w0 was the membrane mass before soaked in solvent; wt was the membrane mass after soaked in solvent, washed by water and dried in vacuum. Figure 2 shows the ATR-FTIR spectra of PAN membrane and CLA-PAN membranes crosslinked at different temperatures and holding time. Compared to the spectra of PAN membrane, in which the typical characteristic peaks of PAN molecular are identified as -C≡N stretching vibration peak (2243 cm −1 ), -C-H stretching vibration peak (2950 cm −1 ) and C-H deformation vibration peak (810 cm −1 ), the intensity of -C≡N and -C-H stretching vibration peaks in the CLA-PAN membranes are reduced gradually, and the intensity of the -C-H deformation vibration peak is enhanced with the increase of thermal cross-linking temperature and holding time. Additionally, the -C=C and -C=N stretching vibration peak (1628 cm −1 ), -N-H stretching vibration peak (3300 cm −1 ) and C-H deformation vibration peak of C=C-H (810 cm −1 ) are observed. These indicate that -C≡N of the PAN molecular structure is ruptured and the -C=N and -N-H are formed during the thermal cross-linking. EOR data that can quantitatively show the degree of cross-linking were calculated from the intensity of the peak at 1628 cm −1 , I1600, and that of the nitrile peak at 2243 cm −1 , ICN, are shown in Figure 2c,d. With the increase of cross-linking temperature and holding time, the EOR data increases from 0.14 at 180 °C (1 h) to 0.85 at 230 °C (2.5 h), indicating that the cross-linking degree of PAN membranes gradually increases during the thermal cross-linking. It can also be proved from the XPS spectra of PAN membrane and CLA-PAN membranes shown in Figure 3. In the CLA-PAN membranes, new peaks of -C=N, -C=C and -C-O appear and the intensity of -C≡N peak gradually decrease in the C1s spectrum, and the intensity of -C=O and -C=N peak in O1s and N1s spectra gradually increase as the cross-linking temperature rises, suggesting that -C≡N in PAN decompose and react with oxidative medium to form the -C=N, -C=C, -C-O and -C=O. In addition, the XPS spectra of the PAN and CLA-PAN membranes ( Figure 4) show that the O atomic percentage in the membrane increases with the enhancement of cross-linking temperature, indicating that O2 has participated in the oxidative crosslinking reaction.  According to above evidence, the oxidative cross-linking reaction of PAN in air can be speculated as following: the strong polarity of cyano groups in PAN molecule chains makes its α-H more active, and results in that the dehydrogenation more easily reacts to form the C=C among the cross-linking reaction [46]. Figure 5 illustrates the thermal cross-linking mechanism of PAN membrane. The β-C in PAN chains is oxidized to carbonyl, and -C≡N is cyclized to form -C=N [47]. Besides the intramolecular cross-linking reaction, the intermolecular cross-linking reaction might occur between the β-C of the two PAN chains that combine with O to form C-O-C structure, and finally forms a three-dimensional cross-linked network structure with thermal resistance.  Figure 6 presents the XRD patterns of PAN and CLA-PAN membranes prepared at different cross-linking temperatures. The obvious diffraction peak corresponding to the (100) plane of the quasi-hexagonal lattice and another weak diffraction peak corresponding to the (110) crystallographic plane appear at the 2θ value of 17.7° and 25.2°, respectively. As the cross-linking temperature increases, the intensity of (100) diffraction peak firstly increases, and then decreases. It According to above evidence, the oxidative cross-linking reaction of PAN in air can be speculated as following: the strong polarity of cyano groups in PAN molecule chains makes its α-H more active, and results in that the dehydrogenation more easily reacts to form the C=C among the cross-linking reaction [46]. Figure 5 illustrates the thermal cross-linking mechanism of PAN membrane.

Microstructure Variations of PAN Asymmetric membrane during Cross-Linking
The β-C in PAN chains is oxidized to carbonyl, and -C≡N is cyclized to form -C=N [47]. Besides the intramolecular cross-linking reaction, the intermolecular cross-linking reaction might occur between the β-C of the two PAN chains that combine with O to form C-O-C structure, and finally forms a three-dimensional cross-linked network structure with thermal resistance.  According to above evidence, the oxidative cross-linking reaction of PAN in air can be speculated as following: the strong polarity of cyano groups in PAN molecule chains makes its α-H more active, and results in that the dehydrogenation more easily reacts to form the C=C among the cross-linking reaction [46]. Figure 5 illustrates the thermal cross-linking mechanism of PAN membrane. The β-C in PAN chains is oxidized to carbonyl, and -C≡N is cyclized to form -C=N [47]. Besides the intramolecular cross-linking reaction, the intermolecular cross-linking reaction might occur between the β-C of the two PAN chains that combine with O to form C-O-C structure, and finally forms a three-dimensional cross-linked network structure with thermal resistance.  Figure 6 presents the XRD patterns of PAN and CLA-PAN membranes prepared at different cross-linking temperatures. The obvious diffraction peak corresponding to the (100) plane of the quasi-hexagonal lattice and another weak diffraction peak corresponding to the (110) crystallographic plane appear at the 2θ value of 17.7° and 25.2°, respectively. As the cross-linking temperature increases, the intensity of (100) diffraction peak firstly increases, and then decreases. It Cross-linking mechanism of PAN membrane (a) Intramolecular cyclization; (b) Intermolecular cross-linking. Figure 6 presents the XRD patterns of PAN and CLA-PAN membranes prepared at different cross-linking temperatures. The obvious diffraction peak corresponding to the (100) plane of the quasi-hexagonal lattice and another weak diffraction peak corresponding to the (110) crystallographic plane appear at the 2θ value of 17.7 • and 25.2 • , respectively. As the cross-linking temperature increases, the intensity of (100) diffraction peak firstly increases, and then decreases. It reaches the maximum at 180 • C and disappears at 260 • C. Additionally, the intensity of (110) diffraction peak enhances with the increase of cross-linking temperature. This implies that the crystal structure of PAN membrane become ordered at the initial stage of cross-linking process, and then gradually transforms into a disordered structure as the heating temperature rises. It can be further confirmed by TEM image shown in Figure 7. The microstructure of CLA-PAN membranes prepared at 260 • C (CLA-PAN-260) looks more disordered and denser than that of membranes prepared at 230 • C (CLA-PAN-230). The measured lattice distance of the microstructure reduces from 0.46-0.5 nm to 0.37-0.38 nm as the cross-linking temperature increases from 230 • C to 260 • C, which indicated that the thermal cross-linking would make microstructure of membranes more disordered and compact at a high temperature due to the structure shrinkage of membranes.

Microstructure Variations of PAN Asymmetric membrane during Cross-Linking
Polymers 2018, 10, x FOR PEER REVIEW 8 of 16 reaches the maximum at 180 °C and disappears at 260 °C. Additionally, the intensity of (110) diffraction peak enhances with the increase of cross-linking temperature. This implies that the crystal structure of PAN membrane become ordered at the initial stage of cross-linking process, and then gradually transforms into a disordered structure as the heating temperature rises. It can be further confirmed by TEM image shown in Figure 7. The microstructure of CLA-PAN membranes prepared at 260 °C (CLA-PAN-260) looks more disordered and denser than that of membranes prepared at 230 °C (CLA-PAN-230). The measured lattice distance of the microstructure reduces from 0.46-0.5 nm to 0.37-0.38 nm as the cross-linking temperature increases from 230 °C to 260 °C, which indicated that the thermal cross-linking would make microstructure of membranes more disordered and compact at a high temperature due to the structure shrinkage of membranes.

Pore Structure Variations of PAN Asymmetric Membrane during Cross-Linking
The Tables 1 and 2 and Figure 8 present the pore structure variations of the PAN asymmetric ultrafiltration membrane before and after thermal cross-linking, which indicate that the thermal cross-linking greatly affects the pore structure of the PAN ultrafiltration membrane. reaches the maximum at 180 °C and disappears at 260 °C. Additionally, the intensity of (110) diffraction peak enhances with the increase of cross-linking temperature. This implies that the crystal structure of PAN membrane become ordered at the initial stage of cross-linking process, and then gradually transforms into a disordered structure as the heating temperature rises. It can be further confirmed by TEM image shown in Figure 7. The microstructure of CLA-PAN membranes prepared at 260 °C (CLA-PAN-260) looks more disordered and denser than that of membranes prepared at 230 °C (CLA-PAN-230). The measured lattice distance of the microstructure reduces from 0.46-0.5 nm to 0.37-0.38 nm as the cross-linking temperature increases from 230 °C to 260 °C, which indicated that the thermal cross-linking would make microstructure of membranes more disordered and compact at a high temperature due to the structure shrinkage of membranes.

Pore Structure Variations of PAN Asymmetric Membrane during Cross-Linking
The Tables 1 and 2 and Figure 8 present the pore structure variations of the PAN asymmetric ultrafiltration membrane before and after thermal cross-linking, which indicate that the thermal cross-linking greatly affects the pore structure of the PAN ultrafiltration membrane.

Pore Structure Variations of PAN Asymmetric Membrane during Cross-Linking
The Tables 1 and 2 and Figure 8 present the pore structure variations of the PAN asymmetric ultrafiltration membrane before and after thermal cross-linking, which indicate that the thermal cross-linking greatly affects the pore structure of the PAN ultrafiltration membrane.  Table 1 shows the fluxes and rejection rates of PEG 10000, PEG 20000 and BSA for PAN membrane and CLA-PAN membranes, which reflect the changes of pore structure larger than 1 nm. Compared to the PAN membrane, the fluxes of CLA-PAN membranes obviously decrease and the rejection rates increase, which suggest that the pore volumes of PAN membrane reduce and pore sizes (>1 nm) become smaller during thermal cross-linking.
PALS experiments are normally used to determine the free-volume that size is less than 1 nm in polymeric membranes. Table 2 presents the free-volume of the PAN membrane and CLA-PAN membranes measured by PALS, in which the o-Ps lifetime (τ 3 ) and the relative intensity (I 3 ) correspond to the size and the concentration of the free volume, respectively. Compared to the PAN membrane, CLA-PAN membrane has a lower I 3 , bigger τ 3 and R 3 , which indicates that the concentration of free volume is reduced and the average free volume radius of PAN membrane is largened after thermal cross-linking. Figure 8 shows the o-Ps lifetime distribution of PAN membrane and CLA-PAN membrane. After thermal cross-linking, the free volume concentration of PAN membrane diminishes and free volume distribution of PAN membrane becomes narrow. The free volumes which sizes are below 0.25 nm are vanished. That all suggests that the pore structure of PAN ultrafiltration membrane is obviously changed during thermal cross-linking. The large pores diminish, small pores disappear and pore volumes reduce.   Table 1 shows the fluxes and rejection rates of PEG 10000, PEG 20000 and BSA for PAN membrane and CLA-PAN membranes, which reflect the changes of pore structure larger than 1 nm. Compared to the PAN membrane, the fluxes of CLA-PAN membranes obviously decrease and the rejection rates increase, which suggest that the pore volumes of PAN membrane reduce and pore sizes (>1 nm) become smaller during thermal cross-linking.
PALS experiments are normally used to determine the free-volume that size is less than 1 nm in polymeric membranes. Table 2 presents the free-volume of the PAN membrane and CLA-PAN membranes measured by PALS, in which the o-Ps lifetime (τ3) and the relative intensity (I3) correspond to the size and the concentration of the free volume, respectively. Compared to the PAN membrane, CLA-PAN membrane has a lower I3, bigger τ3 and R3, which indicates that the concentration of free volume is reduced and the average free volume radius of PAN membrane is largened after thermal cross-linking. Figure 8 shows the o-Ps lifetime distribution of PAN membrane and CLA-PAN membrane. After thermal cross-linking, the free volume concentration of PAN membrane diminishes and free volume distribution of PAN membrane becomes narrow. The free volumes which sizes are below 0.25 nm are vanished. That all suggests that the pore structure of PAN ultrafiltration membrane is obviously changed during thermal cross-linking. The large pores diminish, small pores disappear and pore volumes reduce.     Figure 9 shows the SEM images of the PAN and CLA-PAN membrane. PAN membrane possesses a typical asymmetric structure with a dense top skin layer followed by the sponge-like and finger-like porous sub-layers. After thermal cross-linking, the thickness of the membrane reduces from 136 µm to 97 µm. The large pores of membrane such as the finger-like holes and some large sponge-like pores are well maintained, and some smaller sponge-like pores disappear. On the top surface of dense skin layer, some pores diminish and some pores seem to vanish after thermal cross-linking, which hints that the coalescence or collapse of pore and microstructure occurs in the CLA-PAN-230 membrane besides the shrinkage of membrane during thermal cross-linking at the temperature of 230 • C. According to SEM images and results obtained from Tables 1 and 2 and Figure 8, it can be concluded that the changes of pore structure of PAN ultrafiltration membrane during thermal cross-linking are attributed to the shrinkage of membrane and the coalescence or collapse of pore and microstructure. The shrinkages of membranes make the large pores smaller and the coalescence or collapse of pore and microstructure might form the dead-end pores or make the small pores disappear which will significantly affect the pore structure of derived CLA-PAN membranes, and even destroy the pore structure. Hence, controlling or reducing the coalescence or collapse of pore and microstructure during thermal cross-linking will be very important in the preparation of thermal cross-linking membrane with well-kept pore structure. finger-like porous sub-layers. After thermal cross-linking, the thickness of the membrane reduces from 136 μm to 97 μm. The large pores of membrane such as the finger-like holes and some large sponge-like pores are well maintained, and some smaller sponge-like pores disappear. On the top surface of dense skin layer, some pores diminish and some pores seem to vanish after thermal crosslinking, which hints that the coalescence or collapse of pore and microstructure occurs in the CLA-PAN-230 membrane besides the shrinkage of membrane during thermal cross-linking at the temperature of 230 °C. According to SEM images and results obtained from Tables 1 and 2 and Figure  8, it can be concluded that the changes of pore structure of PAN ultrafiltration membrane during thermal cross-linking are attributed to the shrinkage of membrane and the coalescence or collapse of pore and microstructure. The shrinkages of membranes make the large pores smaller and the coalescence or collapse of pore and microstructure might form the dead-end pores or make the small pores disappear which will significantly affect the pore structure of derived CLA-PAN membranes, and even destroy the pore structure. Hence, controlling or reducing the coalescence or collapse of pore and microstructure during thermal cross-linking will be very important in the preparation of thermal cross-linking membrane with well-kept pore structure.

Gas Separation Performance of PAN and CLA-PAN Membranes
The mixed gas performances of PAN and CLA-PAN membranes are investigated (shown in Figure 10 and Table 3) to further understand the changes of pore size and structure in the PAN membrane during the cross-linking. Compared to the PAN membrane, the gas permeance of CLA-PAN membrane reduces obviously, and the O2/N2 selectivity slightly increases due to the shrinkage of membrane and the coalescence or collapse of pore and microstructure during cross-linking. As the cross-linking temperature increases (Figure 10a), gas permeance of CLA-PAN membrane enhances and O2/N2 selectivity keeps constant (about 1.2), which is slightly higher than the value of Knudsen diffusion (0.935). And as the cross-linking holding time prolongs (Figure 10b), gas permeance enhances more obviously and the O2/N2 selectivity slightly decreases. The O2 permeance of the asymmetric membrane is 242 GPU at the cross-linking holding time of 6 h at 230 °C, which is higher than the PAN membrane (shown in Table 3). It might be attributed to formation of new gas penetrating channels created by the escape of small molecules existing in the membrane matrix such

Gas Separation Performance of PAN and CLA-PAN Membranes
The mixed gas performances of PAN and CLA-PAN membranes are investigated (shown in Figure 10 and Table 3) to further understand the changes of pore size and structure in the PAN membrane during the cross-linking. Compared to the PAN membrane, the gas permeance of CLA-PAN membrane reduces obviously, and the O 2 /N 2 selectivity slightly increases due to the shrinkage of membrane and the coalescence or collapse of pore and microstructure during cross-linking. As the cross-linking temperature increases (Figure 10a), gas permeance of CLA-PAN membrane enhances and O 2 /N 2 selectivity keeps constant (about 1.2), which is slightly higher than the value of Knudsen diffusion (0.935). And as the cross-linking holding time prolongs (Figure 10b), gas permeance enhances more obviously and the O 2 /N 2 selectivity slightly decreases. The O 2 permeance of the asymmetric membrane is 242 GPU at the cross-linking holding time of 6 h at 230 • C, which is higher than the PAN membrane (shown in Table 3). It might be attributed to formation of new gas penetrating channels created by the escape of small molecules existing in the membrane matrix such as solvent molecules and unstable functional groups during cross-linking at higher temperature and longer holding time. However, the channels formed in membrane matrix can only act as the gas molecular penetrating routes and cannot contribute to improving the filtration of PEG molecule larger than 10,000 Da (Table 1). That suggests that the change of pore structure of PAN ultrafiltration membrane during thermal cross-linking are attributed not only to the shrinkage of membrane and the coalescence or collapse of pore and microstructure but also to the new gas penetrating channels formed in membrane matrix during the cross-linking. In addition, it must be noted that the poor gas selectivity of the derived CLA-PAN membranes suggests that they are not suitable for gas separation but can be used as a substrate for preparing a composite gas separation membrane. as solvent molecules and unstable functional groups during cross-linking at higher temperature and longer holding time. However, the channels formed in membrane matrix can only act as the gas molecular penetrating routes and cannot contribute to improving the filtration of PEG molecule larger than 10,000 Da (Table 1). That suggests that the change of pore structure of PAN ultrafiltration membrane during thermal cross-linking are attributed not only to the shrinkage of membrane and the coalescence or collapse of pore and microstructure but also to the new gas penetrating channels formed in membrane matrix during the cross-linking. In addition, it must be noted that the poor gas selectivity of the derived CLA-PAN membranes suggests that they are not suitable for gas separation but can be used as a substrate for preparing a composite gas separation membrane.

Thermal Stability of PAN and CLA-PAN-230 Membranes
The thermal stabilities of the PAN and CLA-PAN-230 membranes were investigated via TGA ( Figure 11). As the temperature rises at higher than 300°C, the PAN membrane shows a sharp weight loss, and the CLA-PAN-230 membrane is gradually declining (in Figure 11a). The PAN membrane reveals a maximum weight loss rate at the temperature between the 320 °C-390 °C, and the CLA-PAN-230 membrane cannot observed it clearly (in Figure 11b). Suggesting that strong thermal decomposition reaction occurs in the PAN membrane during pyrolysis, but in the CLA-PAN membrane, it seems not to happen during pyrolysis due to its cross-linking structure, which indicates that the thermal cross-linking greatly improves the thermal stability of PAN membrane [50]. Commonly, the temperature at 5% weight loss and weight remaining (char yield) at 600 °C are used to evaluate the thermal stability of membranes at the early stage and the final stage of pyrolysis, respectively [34]. For the CLA-PAN-230 membrane, the temperature at 5% weight loss and the char yield are 354.5 °C and 77.7%, respectively. They are higher than those of PAN membrane (338.5 °C and 47.9%), indicating that the thermal stability of CLA-PAN membranes is improved. It means that the thermal cross-linking greatly enhances the thermal stability of PAN membrane.

Thermal Stability of PAN and CLA-PAN-230 Membranes
The thermal stabilities of the PAN and CLA-PAN-230 membranes were investigated via TGA ( Figure 11). As the temperature rises at higher than 300 • C, the PAN membrane shows a sharp weight loss, and the CLA-PAN-230 membrane is gradually declining (in Figure 11a). The PAN membrane reveals a maximum weight loss rate at the temperature between the 320 • C-390 • C, and the CLA-PAN-230 membrane cannot observed it clearly (in Figure 11b). Suggesting that strong thermal decomposition reaction occurs in the PAN membrane during pyrolysis, but in the CLA-PAN membrane, it seems not to happen during pyrolysis due to its cross-linking structure, which indicates that the thermal cross-linking greatly improves the thermal stability of PAN membrane [50]. Commonly, the temperature at 5% weight loss and weight remaining (char yield) at 600 • C are used to evaluate the thermal stability of membranes at the early stage and the final stage of pyrolysis, respectively [34]. For the CLA-PAN-230 membrane, the temperature at 5% weight loss and the char yield are 354.5 • C and 77.7%, respectively. They are higher than those of PAN membrane (338.5 • C and 47.9%), indicating that the thermal stability of CLA-PAN membranes is improved. It means that the thermal cross-linking greatly enhances the thermal stability of PAN membrane.

Chemical Stability of PAN and CLA-PAN Membranes
The solubility of membranes in DMAc was commonly adopted to characterize the solvent resistance or chemical stability of the membrane materials, because DMAc is a strong polar solvent. Figure 12 and Table 4 exhibit the solubility of CLA-PAN membranes in DMAc. The solubilities of CLA-PAN membranes in other solvents are also shown in Table 5. Compared to PAN membrane, which is completely dissolved in DMAc (Figure 12), the solubilities of CLA-PAN membranes decline rapidly as the increase of cross-linking temperature. The mass residual rates of CLA-PAN membranes cross-linked above 230 °C are 100%, which means that the CLA-PAN membranes are insoluble in DMAc (Table 4). Furthermore, results shown in Table 5 demonstrate that CLA-PAN-230 membrane is also insoluble in NMP, DMF, DMSO, CHCl3, THF, alcohol, H2SO4 (6 mol/L) and NaOH (20%). These indicate that the PAN membrane exhibits a strong chemical resistant stability after cross-linking.

Chemical Stability of PAN and CLA-PAN Membranes
The solubility of membranes in DMAc was commonly adopted to characterize the solvent resistance or chemical stability of the membrane materials, because DMAc is a strong polar solvent. Figure 12 and Table 4 exhibit the solubility of CLA-PAN membranes in DMAc. The solubilities of CLA-PAN membranes in other solvents are also shown in Table 5. Compared to PAN membrane, which is completely dissolved in DMAc (Figure 12), the solubilities of CLA-PAN membranes decline rapidly as the increase of cross-linking temperature. The mass residual rates of CLA-PAN membranes cross-linked above 230 • C are 100%, which means that the CLA-PAN membranes are insoluble in DMAc (Table 4). Furthermore, results shown in Table 5 demonstrate that CLA-PAN-230 membrane is also insoluble in NMP, DMF, DMSO, CHCl 3 , THF, alcohol, H 2 SO 4 (6 mol/L) and NaOH (20%). These indicate that the PAN membrane exhibits a strong chemical resistant stability after cross-linking.

Chemical Stability of PAN and CLA-PAN Membranes
The solubility of membranes in DMAc was commonly adopted to characterize the solvent resistance or chemical stability of the membrane materials, because DMAc is a strong polar solvent. Figure 12 and Table 4 exhibit the solubility of CLA-PAN membranes in DMAc. The solubilities of CLA-PAN membranes in other solvents are also shown in Table 5. Compared to PAN membrane, which is completely dissolved in DMAc (Figure 12), the solubilities of CLA-PAN membranes decline rapidly as the increase of cross-linking temperature. The mass residual rates of CLA-PAN membranes cross-linked above 230 °C are 100%, which means that the CLA-PAN membranes are insoluble in DMAc (Table 4). Furthermore, results shown in Table 5 demonstrate that CLA-PAN-230 membrane is also insoluble in NMP, DMF, DMSO, CHCl3, THF, alcohol, H2SO4 (6 mol/L) and NaOH (20%). These indicate that the PAN membrane exhibits a strong chemical resistant stability after cross-linking.

Conclusions
The CLA-PAN membranes with favorable thermal and chemical stabilities were fabricated via thermal cross-linking in air. The thermal cross-linking parameters such as temperature and holding time obviously affect the structure and the properties of CLA-PAN membranes, including the chemical and microstructure, pore structure and gas separation performance of membrane. During the cross-linking, the -C≡N in the structure of PAN molecular ruptures to form the -C=N and -N-H, and O 2 participates the reaction and forms the cross-linking structure. The crystal structure of the PAN membrane firstly becomes ordered; and then transforms into a disordered structure with the increase of the cross-linking temperature. The pore structure of PAN membrane obviously changed during thermal cross-linking. The large pores diminish, small pores disappear and pore volumes reduce, which attributed to the shrinkage of membrane and the coalescence or collapse of pore and microstructure. Controlling or reducing the coalescence or collapse of pore and microstructure during thermal cross-linking will be very important in the preparation of thermal cross-linking membrane with well-kept pore structure. The gas permeance of CLA-PAN membranes increases gradually with a steady selectivity as the increase of the cross-linking temperature and holding time. In addition, the CLA-PAN membranes exhibit good thermal and chemical stabilities. They are insoluble in various solvents, even though in the strong solvents such as DMAc and NMP. Results derived in this work demonstrate that CLA-PAN membranes have a good prospect for application in ultrafiltration for water treatment and for use as a substrate for nanofiltration or gas separation with an aggressive and demanding environment.