membranes crosslinked with various dianhydrides - Wiley Online Library

Properties and pervaporation performance of poly(vinyl alcohol) membranes crosslinked with various dianhydrides Sheng Xu,1,2 Liang Shen,1,2 Cailian Li,1 Yan Wang

1,2

1

Key Laboratory of Material Chemistry for Energy Conversion and Storage, Huazhong University of Science and Technology,

Ministry of Education, Wuhan 430074, China 2 Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China Correspondence to: Y. Wang (E - mail: [email protected])

In this work, three dianhydrides with similar chemical structures, 3,30 ,4,40 -benzophenone tetracarboxylic dianhydride (BTDA), 4,40 -oxydiphthalic anhydride (ODPA), and pyromellitic dianhydride (PMDA), are employed for the crosslinking modification of poly(vinyl alcohol) (PVA) membranes for ethanol dehydration via pervaporation. The changes in crosslinking degree, surface hydrophilicity, and glass-transition temperature are investigated and compared. Compared to the pure PVA membrane, all crosslinked membranes show higher fluxes but lower separation factors, because of the higher fractional free volume and the lower hydrophilicity by the crosslinking of the PVA matrix, respectively. In addition, all crosslinked PVA membranes exhibit similar flux, and the separation factor presents a decreasing order of PVA/PMDA-2 > PVA/ODPA-2 > PVA/BTDA-2, which is in the reverse order of their hydrophilicity, probably because of the reduction in the swelling resistance. With the PMDA content increasing from 0.01 to 0.04 mol/(kg PVA) in the PVA/PMDA crosslinked membranes, the crosslinking degree is enhanced and the hydrogen bonding is weakened, resulting in a flux increase from 120.2 to 190.8 g m22 h21, but the separation factor declines from 306 to 58. This work is believed to provide useful insight on the chemical modification of PVA membranes for pervaporation and other membrane-based separation C 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46159. applications. V ABSTRACT:

KEYWORDS: crosslinking; membranes; separation techniques

Received 8 August 2017; accepted 5 December 2017 DOI: 10.1002/app.46159

INTRODUCTION

As a commercial membrane-based technology in modern industry, pervaporation provides numerous advantages of high separation efficiency, low energy consumption, and environmental benignity, compared with most other traditional separation processes.1 Depending on the membrane properties, pervaporation can be employed for organic dehydration,2 organic recovery from water,3 or organic separations.4 Based on the solution-diffusion model, a membrane with desirable molecular structure and suitable morphology is the critical factor in obtaining a satisfactory pervaporation performance. Typical membrane materials, such as poly(vinyl alcohol) (PVA),5 chitosan,6 sodium alginate,7 and polyimide,8 have been extensively studied for pervaporation applications. Among them, PVA, as one of the most common materials of pervaporation membranes, is still attractive to membrane researchers because of its high hydrophilicity, excellent membrane-forming property, and presence of easily modified hydroxyl groups.9

PVA is one of the most important water-soluble vinyl polymers, prepared by partial or complete hydrolysis of poly(vinyl acetate) (PVAc). Strong hydrogen bonds can form between intramolecular and intermolecular hydroxyl groups in PVA chains, which endow PVA with a high affinity to water, therefore exhibiting high water selectivity as a pervaporation membrane for organic dehydration.10 However, PVA-based membranes generally exhibit poor mechanical strength and weak stability in aqueous solution due to excessive swelling, thereby leading to a drastic decline in separation performance and low stability in a longterm operation. To solve this issue, various modification techniques are employed for PVA membranes, such as blending,11 hybridization,12,13 grafting,14 and crosslinking.9 Among them, crosslinking modification is a convenient and efficient technique, which can be realized by chemical crosslinkers,15–19 freezing,20 irradiation,21 and so on. For chemical crosslinking, the physicochemical properties and separation performance of the

C 2018 Wiley Periodicals, Inc. V

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J. APPL. POLYM. SCI. 2018, DOI: 10.1002/APP.46159

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crosslinked membranes can be optimized with various crosslinker concentrations and different crosslinking times.22 Typical crosslinkers of PVA membranes include carboxylic acids and their analogs,23–27 various aldehydes,28,29 alkoxysilanes,30 and other chemical crosslinking agents.31–33 Among them, carboxylic acids and their analogs have drawn great attention in the past decades as PVA crosslinkers, which can be realized via an esterification reaction. In general, crosslinkers can be divided into carboxylic acids (including di-, tri-, and polycarboxylic acids),24 anhydrides (including monoanhydride and dianhydride),23,34,35 and acid chlorides.36 To the best of our knowledge, there are only a few studies on PVA crosslinked with dianhydrides. The first was reported by Gimenez et al.23,37 with three different dianhydrides, namely 3,30 ,4,40 -benzophenone tetracarboxylic dianhydride (BTDA), pyromellitic carboxylic acid dianhydride (PMDA), and 7-methyl-5-(tetrahydro-2,5-dioxo-3-furyl)-3,4,5,7tetrahydroiso-benzofuran-1,3-dione (Epiclon B-4400). The obtained crosslinked PVA with tridimensional networks containing polar carboxylic groups has a significant effect on the thermal behavior23 and a moderate ability to absorb water.37 Ruiz et al.25 used a small amount of ethylenediaminetetraacetic dianhydride (EDTDA) as the crosslinker to achieve superabsorbent PVA hydrogels with a low crosslinking degree and a high water uptake. However, no systematic study has been reported yet on the effects of various dianhydrides on membrane physicochemical properties and separation performance. Therefore, in this work, various dianhydrides are employed for the crosslinking modification of PVA membrane. The effects of dianhydride structure and content on membrane properties are studied and characterized by various techniques. The separation performance of crosslinked PVA membranes for ethanol dehydration via pervaporation is also reported and discussed. EXPERIMENTAL

Materials PVA 1799, with a polymerization degree of 1700 (average molecular weight 75,000) and a saponification degree of 99% (mol/mol) (about 0.023 mol OH groups per gram PVA), was purchased from Aladdin and dried overnight at 100 8C under vacuum prior to use. Three dianhydride crosslinkers, 3,30 ,4,40 -benzophenone tetracarboxylic dianhydride (BTDA), 4,40 -oxydiphthalic anhydride (ODPA), and pyromellitic carboxylic acid dianhydride (PMDA), with purity of 96% were purchased from Aladdin (California, United States) and used as received. Ethanol (EtOH) of analytical grade and concentrated sulfuric acid (H2SO4) of 98% were provided by Sinopharm Chemical Reagent Co. (Shanghai, China) and used without further treatment. Dimethylsulfoxide (DMSO) of analytical grade was provided by Sinopharm Chemical Reagent Co. and dehydrated with 4 A˚ molecular sieves before use. Deionized water was supplied by a Wuhan Pin Guan (Wuhan, China) Ultrapure system in our own lab. Preparation of the Dianhydride Crosslinked PVA Membranes A 10 wt % PVA aqueous solution was prepared under magnetic stirring at 90 8C for 12 h and degassed overnight prior to use. The PVA aqueous solution was then cast onto a polyethylene terephthalate (PET) plate with a casting knife of 250 lm

thickness to fabricate the pure, dense PVA membrane. The asfabricated membrane was first dried at room temperature overnight and then in a vacuum oven at 80 8C for 24 h to evaporate the solvent. Subsequently, the membrane was solvent-exchanged with ethanol for 24 h to effectively remove the trapped water and DMSO, and further dried under vacuum at 80 8C for 24 h to remove the residual ethanol completely. Then the resultant PVA membrane (denoted as pure PVA) was peeled off from the PET plate carefully. The dianhydride-crosslinked PVA membranes were fabricated using the same process as the pure PVA membrane. The casting solution was prepared by blending the 10 wt % PVA aqueous solution (100 g) and dianhydride/DMSO (2 mL) solution with a certain amount of dianhydride (0.005 to 0.02 mol/mL), followed by the addition of two drops of sulfuric acid under magnetic stirring at 20 8C (thermostatic water bath) for 12 h. The pH value of the casting solution was 4.2. Membranes with the dianhydride crosslinker are abbreviated as PVA/x-y, where x and y refer to the dianhydride employed and its percentage in PVA (0.01 mol/ kg PVA), respectively. The composition details are listed in Table I. The thickness of the pure and crosslinked PVA membranes was about 16–18 lm as measured by a Mitutoyo micrometer (Tokyo, Japan). All of the dried membranes were stored in a vacuum environment before the various tests and characterizations. Pervaporation Study A static pervaporation cell of laboratory scale was used for the pervaporation test, and its schematic design was depicted elsewhere.38 The membrane was fixed in a stainless steel permeation cell with an effective area of 13.5 cm2. The feed solution (85/15 wt % ethanol/water) was added into the cell contacting the upper surface of the membranes, and the temperature was kept at 40 8C as monitored by a thermometer, while the bottom surface of the membrane was under vacuum (less than 2 mbar). For all pervaporation tests, before permeate measurement, there was 2 h of conditioning for each membrane sample to reach a steady state. Subsequently, at least two permeate samples for all of the tested membranes were collected in a cold trap cooled with liquid nitrogen. The compositions of the feed and the permeate were determined by an Agilent Technologies GC 7890 A equipped with a thermal conductivity detector (TCD). The permeation flux (J) and the separation factor (a) are calculated according to eqs. (1) and (2): Table I. Compositions of the Pure and Crosslinked PVA Membranes Casting solution composition Membrane

Crosslinker

PVA (wt %)

Crosslinker (mol/kg PVA)

Pure PVA

None

10

0.02

PVA/BTDA-2

BTDA

10

0.02

PVA/ODPA-2

ODPA

10

0.02

PVA/PMDA-2

PMDA

10

0.02

PVA/PMDA-1

PMDA

10

0.01

PVA/PMDA-3

PMDA

10

0.03

PVA/PMDA-4

PMDA

10

0.04

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Figure 1. Crosslinking mechanism of PVA by dianhydride.23

J5

Q At

(1)

where Q represents the total mass of component in the permeate (g), A stands for the effective membrane area (m2), and t is the operation time (h). yw;1

a1=2 5

=yw;2 xw;1 = xw;2

(2)

where subscripts 1 and 2 refer to water and ethanol, while yw and xw represent the weight fractions of the component in the permeate and feed, respectively. Membrane Characterizations The gel content was measured to study the solubility of crosslinked, dense PVA membranes.15,34 Samples with a certain weight were immersed in water at 40 8C for 24 h. Then the insoluble parts were dried in a vacuum oven at 80 8C for at least 48 h until no weight change occurred. The membrane samples before and after testing were weighed, and the gel content was evaluated by eq. (3): W1 Gel content5 3 100% (3) W0 where W0 and W1 are the original weight of the crosslinked PVA membrane and the weight of the insoluble fraction, respectively. The chemical structures of the crosslinked PVA membranes were characterized by Fourier transform infrared with attenuated total reflection (ATR-FTIR; Bruker Tensor 27, Ettlingen, Germany) with an average of 16 scans and a resolution of 2 cm21. The thermal decomposition behavior of the crosslinked PVA membranes was recorded on a Perkin-Elmer (Norwalk, CT, United States) TGA7 analyzer at a heating rate of 20 8C/min with the temperature ranging from 50 to 800 8C under an argon atmosphere. Glass-transition temperatures (Tg) were recorded with a differential scanning calorimeter (Perkin-Elmer DSC 7 thermal analyzer). All samples were placed in sealed aluminum pans and heated at a rate of 20 8C/min from 40 to 150 8C in a nitrogen atmosphere; subsequently, they were quenched immediately to

40 8C at a cooling rate of 90 8C/min to remove the previous thermal history; the samples were rescanned to 150 8C at a heating rate of 20 8C/min for the second cycle. The Tg value was taken as the middle of the slope transition in the DSC curve based on the second heating step. The water contact angle (WCA) on the membrane surface was measured by a Kruss ZSA25 Contact Angle Goniometer (Hamburg, Germany) at ambient temperature. A deionized water droplet of 5 lL was dropped by a microsyringe onto the top surface of the membrane samples. For each membrane, at least five points were measured to obtain the average WCA. The mechanical properties of the pure and crosslinked PVA membranes were characterized by an Instron 5542 Material Testing Instrument (Massachusetts, United States) at room temperature. All samples with effective dimensions of 15 3 5 mm2 were stretched vertically at a speed of 10 mm/min until breakage. At least five samples were tested for all membrane samples to get the average value. For the swelling test, the preweighed membrane strips were immersed in a liquid solution of 85/15 wt % ethanol/water at 40 8C for half a month, and the weight of the immersed membrane samples showed no notable change. The samples were then taken out, blotted with tissue paper, and weighed. The swelling degree was calculated by eq. (4): Wwet – Wdry Swelling degree ðmg=g membraneÞ 5 (4) Wdry where Wwet and Wdry represent the weights of the wet membrane samples at equilibrium and the original dry membrane samples, respectively. RESULTS AND DISCUSSION

PVA Membrane Crosslinked with Different Dianhydrides The reaction mechanism of dianhydride crosslinking the PVA was proposed, as shown in Figure 1. That is, hydroxyl groups in the PVA chain react with the anhydride via nucleophilic attack to open the anhydride ring with H2SO4 as the catalyst, resulting in the formation of an ester group and a carboxyl group in the network. Thus, PVA chains can be crosslinked by the dianhydride with two anhydride rings to form the less crosslinked PVA

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Table III. Mechanical Properties of the Pure and Crosslinked PVA Membranes Membrane

Tensile strength (MPa) Young’s modulus (MPa)

Pure PVA

104.4 6 2.2

4152 6 85

PVA/BTDA-2

93.6 6 1.8

4531 6 105

PVA/ODPA-2

93.1 6 2.1

3592 6 112

PVA/PMDA-2 93.6 6 1.2

Figure 2. ATR-FTIR spectra of the pure and crosslinked PVA membranes. [Color figure can be viewed at wileyonlinelibrary.com]

chains. And the produced carboxyl groups may further react with the free hydroxyl groups in the PVA chain in the presence of catalyst to form the highly crosslinked PVA, which requires a higher activation energy. To understand the effects of crosslinker structure on membrane properties and separation performance, three different dianhydrides, namely BTDA, ODPA, and PMDA, were used in this work. As we can see from Table I, the selected dianhydrides have similar chemical structures. The successful crosslinking can be confirmed by the FTIR characterizations. As illustrated in Figure 2, some characteristic peaks of pure PVA chains can be seen at 3276 cm21 (vibration of the hydroxyl groups) and 1417 cm21 (symmetric bending of the methylene groups), which are consistent with those in a previous work.39 For the crosslinked PVA membranes, an obvious difference in the peak intensity can be seen around 1659 cm21, which could be attributed to the generated carboxyl groups as analyzed above. And compared to the pure PVA membrane, the crosslinked PVA membranes show some new IR peaks at 1725 and 1251 cm21 (ester groups) and 1136 cm21 (ether groups). The FTIR results indicate the presence of carboxyl groups and ester groups in the crosslinked PVA membranes; that is, all of the peak changes confirm the proposed reaction mechanism, as shown in Figure 1. Since the crosslinking does not affect the amount of methylene groups in the membrane matrix, the peak intensity of methylene groups can be considered as a constant. Therefore, the crosslinking degree of the crosslinked PVA membranes can be qualitatively determined by the area ratio of the

3809 6 69

peak at 3276 cm21 (hydroxyl groups, A3276) to that at 1417 cm21 (methylene groups, A1417), or A3276/A1417. The results find that the A3276/A1417 value follows the order pure PVA (5.11) > PVA/ BTDA-2 (5.03) > PVA/ODPA-2 (5.01) > PVA/PMDA-2 (4.98). It indicates the decrease in the hydroxyl group number and the increase in the crosslinking degree. This phenomenon should be due to the consumption of hydroxyl groups and the disruption of PVA chains with the dianhydride introduction.39,40 To investigate the different crosslinking degrees of PVA membranes with various dianhydrides, gel content tests of pure and crosslinked PVA membranes were performed, and the results are listed in Table II. There is no insoluble fraction of the pure PVA membrane after the gel content test, since water is a good solvent for PVA. But the gel content increases significantly after the crosslinking modification, confirming the successful crosslinking of PVA by three dianhydrides in this study. And the gel content of the crosslinked PVA membranes shows the order PVA/BTDA-2 < PVA/ODPA-2 < PVA/PMDA-2, indicating the increase in the crosslinking degree. The gel content results are consistent with the FTIR spectra (Figure 1), where both hydroxyl groups and carboxyl groups decrease with the same order. The mechanical property is also investigated and presented in Table III. It is found that all of the crosslinked PVA membranes show lower tensile strength and Young’s modulus (except for PVA/BTDA-2) than the pure PVA membrane, which could be due to the decreased rigidity. In our work, the membrane rigidity is actually affected by both the crosslinking degree and the hydrogen bonding intensity among PVA chains. However, the

Table II. Gel Content Results for the Pure and Crosslinked PVA Membranes Membrane

Gel content (wt %)

Pure PVA

0

PVA/BTDA-2

71.5 6 1.5

PVA/ODPA-2

74.9 6 3.1

PVA/PMDA-2

75.5 6 3.6

Figure 3. TGA curves of the pure and crosslinked PVA membranes. [Color figure can be viewed at wileyonlinelibrary.com]

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Figure 4. DSC curves of the pure and crosslinked PVA membranes. [Color figure can be viewed at wileyonlinelibrary.com]

introduction of crosslinker weakens the hydrogen bonding severely and therefore leads to the lower rigidity of the crosslinked PVA membranes than that of the pure PVA membrane. Similar results have also been reported in a previous work.23 The thermal behavior of the crosslinked PVA is investigated by TGA and DSC characterizations. As shown in Figure 3, there is one main decomposition stage in the pure PVA membrane starting at 270 8C, which should be due to backbone decomposition.41 In comparison, all crosslinked PVA membranes show two obvious degradation steps. The first weight loss in the range of 180–310 8C may be due to breakage of the ester linkages or decarboxylation of free carboxylic acids in the crosslinked PVA matrix, while the second weight loss starting at about 350 8C should be the result of backbone degradation.23,37 An obvious difference also exists in the decomposition rates of the first stage for the crosslinked PVA membranes, which are believed to result from their different crosslinking degrees and the corresponding different amount of ester linkages and carboxyl groups, as aforementioned. The DSC curves in Figure 4 also show that the Tg values of all crosslinked PVA membranes are lower than that of pure PVA membrane (Tg 5 82.9 8C) and follow an order of pure PVA > PVA/BTDA-2 > PVA/PMDA-

Figure 5. WCA results of the pure and crosslinked PVA membranes. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 6. Pervaporation performance of the pure and crosslinked PVA membranes (feed concentration, water/ethanol 85/15 wt %; operation temperature, 40 8C; operation pressure, less than 2 mbar). [Color figure can be viewed at wileyonlinelibrary.com]

2 > PVA/ODPA-2, which should again be due to the lower rigidity and weakened intermolecular hydrogen bonding among polymeric chains in the crosslinked PVA membrane.42 The surface hydrophilicity of the pristine and crosslinked PVA membranes was also studied by the water contact angle test. As shown in Figure 5, all crosslinked PVA membranes have a higher water contact angle than the pure PVA membrane, which indicates the lower hydrophilicity of crosslinked PVA membranes, which is probably due to the fewer hydroxyl groups after the crosslinking reaction. In addition, the water contact angles of the crosslinked membranes show an increasing order of pure PVA < PVA/BTDA2 < PVA/ODPA-2 < PVA/PMDA-2, which may be mainly ascribed to the decrease in the amount of hydrophilic hydroxyl groups and carboxyl groups. Similar results can be found in previous work.43 Effect of Dianhydride Structure on the Pervaporation Performance Figure 6 shows the effect of dianhydride structure on the flux and separation factor of the crosslinked PVA membranes. Compared to the pure PVA membrane, all crosslinked membranes show higher fluxes but lower separation factors. The higher flux may be ascribed to the fact that the dianhydride crosslinking disturbs the crystalline region in PVA by destroying the hydrogen bonding and generates more amorphous regions and therefore a higher fractional free volume (FFV) in the PVA matrix.12 The corresponding lower separation factor should be mainly due to their lower hydrophilicity in spite of the enhanced swelling resistance by the crosslinking. In addition, all crosslinked PVA membranes exhibit a similar flux, and the separation factor presents a decreasing order of PVA/PMDA-2 > PVA/ODPA2 > PVA/BTDA-2, which is in the reverse order of their hydrophilicity, probably due to the reduction in the swelling resistance. In general, for a polymeric membrane, a higher swelling resistance means a higher separation factor.12,40 As shown in Figure 7, the swelling degree of the crosslinked membranes shows a decreasing order of PVA/BTDA-2 > PVA/ODPA2 > PVA/PMDA-2, indicating the increase of the swelling resistance and therefore the separation factor.

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Figure 7. Swelling degree of the pure and crosslinked PVA membranes. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 9. ATR-FTIR spectra of PVA membranes crosslinked with different PMDA contents. [Color figure can be viewed at wileyonlinelibrary.com]

Effect of PMDA Content on the Crosslinked PVA/PMDA Membrane In this study, the effect of dianhydride content on the properties and pervaporation performance of the crosslinked PVA/PMDA membranes is studied. It can be seen from Figure 8 that the gel content increases with the increase of the PMDA content, indicating the increasing crosslinking degree. And this can also be confirmed by the FTIR results in Figure 9, where the A3276/ A1417 value decreases from 5.11 to 4.89 with the increase of the PMDA content in the corresponding crosslinked PVA/PMDA membranes. This is because a higher PMDA (crosslinker) content means more dianhydride groups are introduced into the membrane matrix, resulting in a higher consumption of hydroxyl groups and therefore a higher crosslinking degree. A similar result has also been reported in a previous work.27

dianhydride crosslinking of PVA chains involves two steps: (1) the formation of two ester and two carboxyl groups by one dianhydride molecule in the network and (2) further crosslinking between carboxyl groups and free hydroxyl groups in PVA chains. When the dianhydride content is relatively low (less than 0.02 mol/kg), all carboxyl groups generated in the first step should be involved in the further crosslinking of PVA membranes, leading to the hydrophilicity decrease due to significantly fewer hydroxyl groups.43 When the dianhydride content is relatively high (more than 0.02 mol/kg), the carboxyl groups will only be partially involved in the subsequent crosslinking, resulting in the higher hydrophilicity because of the presence of remaining carboxyl groups.24,44

With an increase in the PMDA content, the surface hydrophilicity of the crosslinked PVA membranes experiences a down-andup trend, as shown in Figure 10, where the highest water contact angle can be found with the PVA/PDMA-2 membrane. The results may be explained as follows. As analyzed in the first section in the Results and Discussion and in Figure 1, the

The effect of PMDA content on the separation performance of the crosslinked PVA membranes is shown in Figure 11. With the increase in the PMDA content, the flux shows an increasing trend, which may be ascribed to the changes of the crosslinking degree and the surface hydrophilicy of the crosslinked membranes. Since the crosslinking degree of the PVA/PMDA membrane increases with the increase in PMDA content, the crystalline region of PVA is disturbed more severely, resulting in

Figure 8. Gel content results of PVA membranes crosslinked with different PMDA contents.

Figure 10. WCA results of PVA membranes crosslinked with different PMDA contents.

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decrease in the separation factor of the crosslinked PVA membranes should be mainly due to the reduced surface hydrophilicity and the loosened chain packing density. ACKNOWLEDGMENTS

Figure 11. Effect of PMDA content on the pervaporation performance of crosslinked PVA membranes (feed concentration, water/ethanol 85/15 wt %; operation temperature, 40 8C; operation pressure, less than 2 mbar). [Color figure can be viewed at wileyonlinelibrary.com]

a looser morphology with lower chain packing density and higher FFV in the PVA matrix, and therefore a higher flux. On the other hand, the separation factor of the crosslinked PVA membranes decreases with the increase in PMDA content, which should be mainly due to the reduced surface hydrophilicity and the loosened chain packing density. CONCLUSIONS

In this work, three different dianhydrides with similar chemical structures are employed for crosslinking modification of PVA membranes for the pervaporation dehydration of ethanol. The effects of the dianhydride structure and content on the physicochemical properties and the pervaporation performance of the crosslinked PVA membranes are explored. The following conclusions can be made from this study: 1. A crosslinking mechanism of PVA membrane by dianhydrides is proposed, which involves two steps: (i) the formation of two ester and two carboxyl groups by one dianhydride molecule in the network and (ii) further crosslinking between free carboxyl groups and hydroxyl groups in PVA chains. The mechanism is further confirmed via various characterization techniques (gel content test, FTIR spectra, tensile testing, TGA, DSC, and water contact angle measurement). 2. Dianhydrides with different structures can have different effects on the properties of crosslinked PVA membranes. The crosslinking degree shows an increasing order of pure PVA < PVA/BTDA-2 < PVA/ODPA-2 < PVA/PMDA-2, while the hydrophilicity and Tg exhibit an opposite trend. Compared to the pure PVA membrane, all crosslinked membranes show higher fluxes but lower separation factors, which may be due to the higher FFV in the PVA matrix and the lower hydrophilicity, respectively. 3. With the increase of PMDA content, the crosslinking degree increases while the surface hydrophilicity shows a down-andup trend. And the crystalline region of PVA is disturbed more severely by PMDA crosslinking, resulting in a looser chain packing density, a higher FFV in the PVA matrix, and therefore a higher flux of the crosslinked PVA/PMDA membrane. The

The authors thank the National Key Technology Support Program of China (2014BAD12B06), Natural Science Foundation of Hubei Scientific Committee of China (2016CFA001) and the Opening Project of Key Laboratory of Biomedical Polymers of Ministry of Education at Wuhan University (20140401) for the financial support. We would also like to thank the Analysis and Testing Center, the Analysis and Testing Center of Chemistry and Chemical Engineering School, and the State Key Laboratory of Materials Processing and Die & Mould Technology, in Huazhong University of Science and Technology, for their help with material characterizations.

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