Cross-Linked Disulfonated Poly(arylene ether sulfone) Telechelic

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Crosslinked Disulfonated Poly(arylene ether sulfone) Telechelic Oligomers. Part 2. Elevated Transport Performance with Increasing Hydrophilicity Benjamin J. Sundell, Euisoung Jang, Benny D. Freeman, Joseph Cook, Judy S. Riffle, and James E. McGrath Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04050 • Publication Date (Web): 11 Jan 2016 Downloaded from http://pubs.acs.org on January 14, 2016

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Crosslinked Disulfonated Poly(arylene ether sulfone) Telechelic Oligomers. Part 2. Elevated Transport Performance with Increasing Hydrophilicity Benjamin J. Sundell,a,c Eui-Soung Jang,b Joseph R. Cook,b Benny D. Freeman,b Judy S. Riffle,a* James E. McGratha a

Macromolecules and Interfaces Institute and Department of Chemistry, Virginia Tech,

Blacksburg, VA 24061, USA b

Department of Chemical Engineering and the Center for Energy and Environmental Resources,

University of Texas at Austin, Austin, TX 78758, USA c

Present Address: Aramco Research Center-Boston, Advanced Materials Team, Cambridge, MA,

02139, USA

Keywords Disulfonated polysulfone; polysulfone; crosslinking; water purification; transport properties

Abstract Disulfonated poly(arylene ether sulfone) copolymer membranes are of interest for water purification by desalination. These negatively charged copolymers exhibit lower fouling and greatly improved resistance to oxidants such as chlorinated disinfectants compared to state of the art highly crosslinked aromatic polyamide, porous polysulfone supported thin film composite (TFC) systems. A systematic series of controlled molecular weight 4,4’-biphenol and bisphenolA based partially disulfonated poly(arylene ether sulfone)s were synthesized with terminal amine functionalities via end-capping with m-aminophenol. The stoichiometric balance of the end1 ACS Paragon Plus Environment

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capping reagent, bisphenols, and two activated dihalides controlled Mn and degree of disulfonation. Oligomers with controlled molecular weights and ionic content were thermally crosslinked with a multi-functional epoxy resin (TGBAM) derived from methylene dianiline. The residual masses from boiling solvent extraction confirmed high gel fractions. The networks had improved salt rejection compared to linear controls due to reduced swelling, and this proved to be a valuable parameter for enhancing transport properties. The crosslinked highly sulfonated copolymers produced the best water purification properties to date observed for disulfonated polysulfone membranes by retaining high salt rejection with enhanced water permeabilities. For example, an epoxy-crosslinked 4,4'-biphenol-based 60% disulfonated polysulfone with an IEC of 1.85 meq/g had a salt rejection of 96.7% and a relatively high hydraulic water permeability of 1.18 (L µm m-2 h-1 bar-1), compared to a linear 4,4'-biphenol-based 40% disulfonated polysulfone with a similar ionic content (IEC=1.78) that only had a salt rejection of 92.5% and a hydraulic water permeability of 0.62 (L µm m-2 h-1 bar-1).

Introduction Crosslinked aromatic polyamides are the state-of-the-art membranes for water desalination, a commercially important method of purifying water in the 21st century.1 These membranes can achieve over 99% solute rejection from seawater at relatively high water fluxes.2 However, despite excellent transport properties, aromatic polyamides are susceptible to biofouling and degradation by chlorinated disinfectants, and this has prompted investigations of new desalination membranes such as sulfonated polysulfones.3 One advantage of the thin film polyamide-porous polysulfone composites is that they allow for separate optimization of the polysulfone support layer and the aromatic polyamide selective layer. For example, very thin aromatic polyamide membranes are highly crosslinked, 2 ACS Paragon Plus Environment


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brittle, and weak, but can survive the high pressure reverse osmosis process because they are synthesized directly on a tough, ductile polysulfone support layer.4 The extent of crosslinking in these polyamide membranes was identified early on as being important for the transport properties.5 Permeate flux and salt rejection rely on membrane density and crosslink density.6 In some cases, the use of diamines bulkier than m-phenylene diamine produced membranes with lower salt rejections, possibly because of decreased membrane density.7

Despite the clear

importance of crosslinking on transport properties, fundamental studies remain elusive because of the difficulty in studying the in situ produced aromatic polyamides as separate entities without the support layers. The aromatic polyamides resist separation from the polysulfone support and may fail mechanically once removed. Recently, crosslink density of a state-of-the-art aromatic polyamide was studied by X-ray reflectivity, which measured a decrease in density upon membrane swelling. The membrane swelled up to 40% in water, yet the distance between crosslinks was small, equal to about five repeat units.8 Because of the demonstrated importance of crosslinking in the state-of-the-art materials, it is of interest to investigate effects of crosslinking in alternative water purification membranes such as disulfonated polysulfones. Part I of this study described numerous examples of crosslinking disulfonated polysulfones and introduced several new reactions for crosslinking telechelic sulfonated polysulfone oligomers.9

One successful crosslinking route produced ductile films with gel

fractions up to 99%, among the highest reported to date for such disulfonated polysulfone copolymers.

The reaction between amine terminated oligomers and an approximately

tetrafunctional epoxy resin was identified as a suitable candidate for further investigation regarding the effects of crosslinking on water purification properties.

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In the present investigation, two systematic series of compositionally varied disulfonated poly(arylene ether sulfone)s were synthesized.

One series was based on the bisphenol-A

(BisAS) monomer structure explored in Part I, and the other series was based on 4,4’-biphenol (BPS). Each oligomeric series was produced with three levels of hydrophilicity, approximately 40, 50 and 60% of disulfonated repeat units relative to non-sulfonated units. Each of the oligomers was crosslinked with the ~tetrafunctional epoxy resin to achieve dense membranes with high gel fractions, and relationships among composition, crosslinking and transport properties were investigated.

Experimental Materials 4,4’-(Propane-2,2-diyl)diphenol

(BisA),

4,4’-biphenol

(BP)

and

4,4’-

dichlorodiphenylsulfone (DCDPS, 99%) were kindly donated by Solvay and recrystallized from toluene before use. 3-Aminophenol (m-AP, 99%) was purchased from Acros Organics. Calcium hydride

(90-95%)

was

purchased

from

Alfa

Aesar.

3,3’-Disulfonated-4,4’-

dichlorodiphenylsulfone (SDCDPS, 98%) was purchased from Akron Polymer Systems, and dried at 160oC for 72 h before use. The monomer was characterized by UV spectroscopy to quantify the amount of residual salt.10 Toluene, N,N-dimethylacetamide (DMAc, 99%) and 2propanol were purchased from Fisher Scientific. The DMAc reaction solvent was dried with calcium hydride, distilled under reduced pressure and stored over molecular sieves. Triphenylphosphine (TPP, 99%), tetraglycidyl bis(p-aminophenyl)methane (TGBAM, calculated functionality of 3.66 epoxy groups per molecule compared to the possible functionality of 4 epoxy groups per molecule11), potassium carbonate (99%), phosphorus pentoxide (P2O5) and



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lithium bromide were purchased from Aldrich. 1-Methyl-2-pyrrolidone (NMP) was purchased from Spectrum Chemicals. Synthesis of an ~5,000 molecular weight (Mn) amine-endcapped 4,4’-biphenol based 50% disulfonated polysulfone oligomer (am-BPS50) The oligomers containing biphenol were synthesized by nucleophilic aromatic substitution using a weak base as described in Part I.9,12 Biphenol (BP, 43.9 mmol, 8.1711 g), dichlorodiphenylsulfone (DCDPS, 25 mmol, 7.1790 g), disulfonated dichlorodiphenylsulfone (SDCDPS, 25 mmol, 12.6611 g), meta-aminophenol (m-AP, 12.2 mmol, 1.3481 g), and DMAc (96 mL) were added to a 250-mL three-necked flask equipped with a mechanical stirrer, nitrogen inlet, and Dean-Stark trap filled with toluene. A stirring, thermally regulated, oil bath was used to heat the reaction mixture to 155oC. After the bath temperature reached 155oC, K2CO3 (70.0 mmol, 9.679 g) was added with toluene (48 mL). The reaction was stirred at 155oC until all of the water was azeotropically removed with toluene. The temperature was increased to 175oC, and water and toluene were drained. After 48 h, the amber opaque mixture was cooled to room temperature and filtered. The resulting transparent solution was precipitated in isopropanol (900 mL) to yield a white solid. The copolymer was filtered, washed with isopropanol (500 mL), and dried in a vacuum oven at 130oC for one day to obtain the product in 79% yield. The 40 and 60% disulfonated oligomers were synthesized in a similar manner, by varying ratios of SDCDPS and DCDPS. Nuclear Magnetic Resonance (1H-NMR) 1

H-NMR analyses of the oligomeric copolymers were performed on a Varian Unity Plus

spectrometer operating at 400 MHz. The spectra of the copolymers were obtained from 32 scans of a 15% (w/v) 1 mL solution in DMSO-d6. 5 ACS Paragon Plus Environment

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Size Exclusion Chromatography (SEC) Molecular weights of the copolymers were obtained by size exclusion chromatography (SEC).

The SEC system consisted of an isocratic pump (Agilent 1260 infinity, Agilent

Technologies, Santa Clara, CA) with an online degasser (Agilent 1260, Agilent Technologies, Santa Clara, CA), autosampler and column oven used for mobile phase delivery and sample injection, and three Agilent PLgel 10 µm Mixed B-LS columns 300×7.5 mm connected in series with a guard column as the stationary phase. A system of multiple detectors connected in series was used for the analysis. A multi-angle laser light scattering (MALLS) detector (DAWNHELEOS II, Wyatt Technology Corporation), operating at a wavelength of 658 nm, a viscometer detector (Viscostar, Wyatt Technology Corporation), and a refractive index detector operating at a wavelength of 658 nm (Optilab T-rEX, Wyatt Technology Corporation, Goleta, CA) provided online results. The system was corrected for interdetector delay and band broadening. The MALLS signals were normalized using a 21,720 g/mol polystyrene standard obtained from Agilent Technologies. Data acquisition and analysis were conducted using Astra 6 software (Wyatt Technology Corporation). The mobile phase of NMP was vacuum distilled over P2O5 before use. The salt, dried LiBr, was added and dissolved in the NMP (0.05 M) before the solvent was degassed and filtered. The sample solutions were prepared in a concentration range of 2~3 mg/mL and were filtered to remove any dust or insoluble particles using 0.22 µm PTFE filters. Molecular weights were measured using light scattering. Specific refractive index increments (dn/dc's) were calculated for each backbone type based on an assumption of 100% mass recovery using Astra 6 software.



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Dense Film Preparation The dense crosslinked films were prepared by adding the oligomers to TGBAM in a 1:2.5 molar ratio, respectively. TPP was added in a 2.5% by weight ratio relative to the weight of TGBAM.

The following steps were completed to prepare a crosslinked film containing

5kamBPS-50. A mixture of amBPS-50 (0.3261 mmol, 1.5000 g), TGBAM (0.8152 mmol, 0.3764 g) and TPP (0.0358 mmol, 9.4 mg) were dissolved in DMAc (12 mL) and stirred until a homogeneous solution was obtained. The solution was syringe filtered through a 0.45 µm PTFE filter into a new vial. The solution was cast on a clean glass plate on a level surface inside a vacuum oven and dried under vacuum for 2 h at 100oC. The vacuum was released, and the oven was heated at the crosslinking temperature (180-200oC) for 90 min. The oven was turned off, and the film was allowed to cool slowly to room temperature overnight. The following day, the film was transferred to a stirring 0.03 M aqueous potassium carbonate bath to aid in detaching the epoxy-cured network from the glass substrate. Films were generally inseparable from the glass plates prior to treatment in the dilute potassium carbonate bath. Once separated, the film was stored in DI water. Films thicknesses approximated 100 µm.

Figure 1. Synthesis of a crosslinked network comprised of tetra-epoxy and 5,000 molecular weight (Mn) amineendcapped 4,4’-biphenol based 50% disulfonated polysulfone oligomer (BPS50)


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Gel fraction measurements Crosslinked films were dried at 120°C under vacuum overnight. After drying, 0.1-0.2 g of the sample was placed in a 20-mL scintillation vial filled with DMAc and stirred at 100°C for ~12 h. The remaining solid was filtered, transferred to a pre-weighed vial, dried at 120°C under vacuum for ~12 h, and then weighed. Two measurements was taken for each film and gel fractions were calculated by Equation 1.

Equation 1:

Water uptake measurements Films (0.1-0.2 g) were dried at 120°C under vacuum overnight, then weighed to obtain the dry weight (Wdry). Next, they were immersed in DI water for at least 48 h. The films were removed, quickly blotted to remove any surface water droplets and weighed to obtain the wet weight (Wwet). Three measurements were taken for each film and water uptake values were calculated by Equation 2.

Equation 2:

Water permeability and salt rejection Water purification properties of the dense crosslinked oligomers were determined at 25oC in a previously described cross-flow filtration system using stainless steel crossflow cells machined at the University of Texas at Austin.13 Permeate samples were collected and analyzed for their mass and conductivity, which were measured to calculate water permeability (L µm m-2



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h-1 bar-1 or cm2 s-1), salt permeability (cm2 s-1), salt rejection (%) and water/NaCl selectivity. The pressure difference across the membrane was 400 psi (27.6 bar).

The aqueous feed

contained 2000 ppm NaCl and the feed solution was circulated past the samples at a continuous flow rate of 3.8 L min-1. The feed pH was adjusted to 6.5 to 7.5 using a 10 g L-1 sodium bicarbonate solution. NaCl concentrations in the feed water and permeate were measured as conductivities with an Oakton 100 digital conductivity meter.

Results and Discussion Fabrication of sulfonated oligomers into crosslinked dense membranes The epoxy-crosslinked amBPS and amBisAS networks were prepared as relatively large (10 cm x 15 cm) membranes according to a previously established procedure.9 All six of the crosslinked copolymers had gel fractions greater than 89.5%. With regard to water purification properties such as water and salt permeability, the crosslink density and molecular weight between crosslinks (Mc) are of interest. The crosslink density affects the free volume available for permeate transport. The Mn’s of the oligomers should correspond to the distance between the crosslinked regions in these networks. Each of the telechelic primary amine endgroups were bifunctional towards the ~tetrafunctional epoxy reagent, and therefore, crosslinks should have occurred at most endgroups. Table 1 lists the gel fractions and water uptakes of the crosslinked dense membranes. The relationship between linear sulfonated polysulfone water uptake and morphology has been previously studied by tapping mode atomic force microscopy.14 In the linear BPS polymers, lower levels of sulfonation result in isolated ionic domains, and at a 60% degree of sulfonation these domains become continuous throughout the polymeric material. This morphological conversion results in a sharp increase in water uptake and possibly uncontrollable swelling. Morphological studies would need to be performed on the crosslinked materials to 9 ACS Paragon Plus Environment

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investigate such a morphological conversion, though they are out of the scope of this paper. The 5kamBPS-60 copolymer does have a significantly higher IEC than the 5kamBisAS-60 copolymer because it has lower molecular weight repeat units, and thus a higher weight percentage of sulfonate groups per repeat unit.

Table 1. Extent of crosslinking, water uptake and IEC in amBisAS and amBPS oligomers crosslinked with TGBAM

Sample

Gel Fraction

Water Uptake (%)

IEC (meq/g)

Fixed Charge Concentration

(%)

(mol/L)a 5kamBisAS-40

90.5 ± 1.5

16.7 ± 1.2

1.20

7.2

5kamBisAS-50

94.8 ± 4.8

25.9 ± 0.5

1.48

5.7

5kamBisAS-60

93.0 ± 3.0

26.6 ± 0.7

1.71

6.4

5kamBPS-40

89.5 ± 2.5

18.0 ± 0.7

1.36

7.6

5kamBPS-50

96.0 ± 3.0

26.6 ± 0.4

1.60

6.0

5kamBPS-60

92.5 ± 1.5

41.3 ± 3.4

1.85

4.5

a

mol/L water in swollen polymer, estimated from IEC/Water Uptake



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Salt rejection, water permeability, salt permeability, and selectivity of crosslinked oligomeric disulfonated polysulfone dense membranes Water and salt transport properties of the terminally crosslinked disulfonated oligomers were evaluated by cross-flow filtration. Table 2 tabulates these various properties, including salt rejection, hydraulic and diffusive water permeability, salt permeability and the permeability selectivity. The salt rejection was quite high (96-98%) for all of the networks, which indicated that crosslinking controlled the high amount of swelling observed with linear analogues of these disulfonated copolymers. Interestingly, the salt rejection did not change appreciably despite considerably different water permeabilities and degrees of disulfonation. Table 2. Water purification properties of amBisAS and amBPS oligomers crosslinked with TGBAM

Sample

Salt Rejection (%)

Water Permeability 2 (L µm/(m h bar))

Water Permeability 2 (cm /s)

Salt permeability 2 (cm /s)

Permeability Selectivity (Pw/Ps)

5kamBisAS-40

96.01 ± 0.99

0.11 ± 0.002

4.12 ± 0.07 -7 x 10

3.32 ± 0.80 -10 x 10

1.32 ± 0.34 3 x 10

5kamBisAS-50

96.57 ± 0.51

0.36 ± 0.01

1.38 ± 0.03 -6 x 10

9.57 ± 1.69 -10 x 10

1.49 ± 0.23 3 x 10

5kamBisAS-60

97.05 ± 0.18

0.83 ± 0.06

3.17 ± 0.24 -6 x 10

1.88 ± 0.26 -9 x 10

1.70 ± 0.10 3 x 10

5kamBPS-40

97.93 ± 0.04

0.12 ± 0.01

4.68 ± 0.46 -7 x 10

1.91 ± 0.15 -10 x 10

2.44 ± 0.04 3 x 10

5kamBPS-50

97.85 ± 0.82

0.43 ± 0.05

1.63 ± 0.20 -6 x 10

6.81 ± 1.33 -10 x 10

2.54 ± 0.70 3 x 10

5kamBPS-60

96.72 ± 0.17

1.18 ± 0.11

4.51 ± 0.43 -6 x 10

2.96 ± 0.13 -9 x 10

1.52 ± 0.08 3 x 10

Salt rejections of the crosslinked amBPS networks are compared with previously reported data on linear disulfonated BPS based disulfonated polysulfone copolymers in Figure 2.15 The 11 ACS Paragon Plus Environment

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numbers adjacent to the data points are ion exchange capacities (IEC) calculated from 1H-NMR. For example, the crosslinked amBPS-50 network had an IEC of 1.6, a salt rejection of 97.9% and a hydraulic water permeability of 0.43 (L µm m-2 h-1 bar-1). Figures 2 and 3 compare the linear and crosslinked copolymers by IEC instead of degree of disulfonation, because the crosslinked copolymers contain approximately 20% by weight of the ~tetrafunctional non-ionic crosslinking reagent.

Figure 2. Salt rejection versus hydraulic water permeability in linear3 and crosslinked BPS copolymers. The numbers next to the data points are the IEC values (meq/g)



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Water content and the degree of fixed charge functionalization are important properties influencing water and salt transport in ion exchange membranes.16-18 In this regard, several recent studies have examined the correlation between transport properties and the degree of sulfonation in linear polysulfone copolymers.1,15,19-24 These studies showed that as the degree of disulfonation was increased, the water content of the copolymers also increased and this corresponded to increase in both water and salt permeabilities.1,15,19-24 The water permeability increased at higher degree of disulfonation due, at least in part, to higher water solubility coefficients. The disulfonated copolymers achieved high levels of salt rejection, mainly because of the fixed anionic sulfonate groups on the polymer backbone. These anionic groups reduced the sorption of dissolved anions such as Cl- by Donnan exclusion.25 However, this repulsive effect was sensitive to water purification operating conditions, including external salt concentration (feed water concentration).20 Despite the reduced salt sorption due to the fixed anionic groups, high water content at large degrees of disulfonation greatly increased salt diffusion throughout the polymer matrix and led to lower salt rejection values. Crosslinking of polymer network suppresses swelling of the polymer while increasing degree of fixed charge functionalization. Figure 2 shows the trend for salt rejection versus water permeability in linear BPS copolymers. The water permeability increased with increasing degree of disulfonation and IEC, but at the cost of decreased salt rejection. On the contrary, networks comprised of terminally crosslinked amBPS oligomers showed higher salt rejection compared to the non-crosslinked, linear BPS series (Figure 2). For example, the crosslinked amBPS-50 membrane (IEC=1.6) had higher salt rejection compared to a linear BPS-35 copolymer (IEC=1.52) with a similar ionic content, but it maintained the high water permeability. Therefore, by introducing crosslinks at the terminal ends of these oligomers, salt rejection 13 ACS Paragon Plus Environment

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improved significantly at a given IEC. Crosslinking with a relatively hydrophobic component such as TGBAM allowed for increasing the ionic content within the oligomeric component without sacrificing mechanical integrity caused by high water swelling. A numerical comparison could not be made between the crosslinked amBPS-60 (IEC=1.85) network and linear BPS-60, because linear BPS-60 swelled so greatly that it lost all mechanical integrity and failed at high pressures in aqueous environments. However, the crosslinked amBPS-60 membrane had the best combination of high salt rejection and high water permeability out of all the networks tested. The water permeability improved substantially with increasing degree of disulfonation and IEC in the crosslinked amBPS series. The crosslinked amBPS-60 had water permeability nearly three times higher than the amBPS-50 network, but similar (only 1% lower) salt rejection. Crosslinked amBPS-60 had similar salt rejection compared to crosslinked amBPS-50, even with the increased water uptake of the amBPS-60 membrane. Yasuda et al. demonstrated that salt permeation increases with rising water uptake in hydrated polymers.26 As water uptake of the polymer increases, salt permeates through hydrated domains resulting in higher overall salt throughput. However, the decrease in salt rejection of crosslinked amBPS-60 was modest, suggesting less difference in swelling between these crosslinked materials and in salt diffusion rates through hydrated domains. An advantage in water permeability and other transport properties might be gained by exploring even higher water solubility coefficients in crosslinked highly disulfonated copolymers. Figure 3 compares the transport properties of epoxy-crosslinked amBisAS networks with linear BisAS copolymers. As expected, the linear BisAS series showed a similar trend to the linear BPS series, that is, a higher degree of disulfonation and IEC led to decreased salt rejection



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and increased water permeability. As described earlier for the BPS series, the transport results for the crosslinked amBisAS oligomers varied widely from the linear control series. In all cases, the crosslinked oligomers had higher salt rejection and lower water permeability than the linear copolymer. For example, crosslinking the amBisAS-40 oligomer raised the salt rejection by approximately 3.5% compared to the linear BisAS-40 copolymer (IEC=1.49), but this increase in rejection was accompanied by an order of magnitude decrease in water permeability. Alternatively, the crosslinking of amBisAS-60 increased the salt rejection by about 8.5% compared to a linear BisAS-60 (IEC=2.05) copolymer, with a smaller decrease in water permeability.

The results emphasized that crosslinking was an effective way to raise salt

rejection. The crosslinked amBisAS series showed a remarkable trend.

While the water

permeability increased substantially with IEC, little change was observed in salt rejection. The increase in water permeability was expected, but the maintenance of high salt rejection was unanticipated. One reason for this phenomenon could be enhanced Donnan exclusion in the higher IEC crosslinked systems, resulting in higher salt rejection: Table 1 lists the Fixed Charge Concentration (FCC) of the crosslinked polymer systems in mols of fixed charged sulfonate groups per liter of water absorbed in the polymer. Because of the high degree of swelling in the crosslinked amBPS series, the concentration of fixed charges in the material decreases even as the IEC of the amBPS material increases; a similar relationship between IEC and FCC exists in the linear materials, and consequently both linear materials as well as the crosslinked amBPS show a similar trend, where selectivity and rejection decrease as IEC increases. In the case of the crosslinked amBisAS series, however, the crosslinking more effectively controls the swelling of the polymer as IEC increases.3 Because the number of fixed charged groups on the polymer


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backbone are able to be increased with less of a rise in water content as compared to the amBPS series, the amBisAS series shows a higher concentration of fixed charged groups than the amBPS series, and this greater concentration of fixed charged groups serves to more effectively control salt transport in the polymer.17,20

Figure 3. Salt rejection versus hydraulic water permeability in linear and crosslinked BisAS copolymers. The numbers next to the data points are the IEC values (meq/g)

Figures 2 and 3 provide useful comparisons between the linear copolymers and the crosslinked networks.

However, both salt rejection and hydraulic water permeability are

dependent on operating conditions. Increased applied pressures will increase the water flux while the salt flux remains relatively constant. Selectivity and diffusive water permeability are 16 ACS Paragon Plus Environment


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material properties that may be more easily compared with material properties of other water purification membranes. Selectivity is the ratio of the diffusive water and salt permeabilities (Pw/Ps). Selectivity is an intrinsic membrane property, similar to diffusive water permeability and salt permeability coefficients, because it does not depend explicitly on operating conditions such as applied pressure and osmotic pressure difference. However, selectivity will still be sensitive to salt concentration if the salt or water permeability depends on salt concentration, which has been observed in other sulfonated polysulfones.15 An “upper-bound” has been identified for water purification membranes by plotting the log of water/salt selectivity versus the log of diffusive water permeability.19 The upper bound is an empirically formulated line that represents the best transport results obtainable for a given combination of selectivity and water permeability. The best water purification membranes are ones on or closest to the upper bound. One focus of material science research is to traverse the upper bound in the upper right direction. Figure 4 presents the crosslinked oligomers on an upper-bound plot along with the linear disulfonated polysulfone copolymers, the state-of-the-art aromatic polyamides, and another water purification membrane. The results in Figure 4 show that crosslinking greatly improved the water purification properties of these disulfonated polysulfones. Crosslinked networks with 59 and 60% degree of disulfonation (IEC=1.71 and 1.85 for BisAS and BPS copolymers, respectively) were much closer to the upper-bound than any of the linear disulfonated polysulfone copolymers. The linear disulfonated BPS copolymers in the graph span a wide range of selectivity and water permeability values; however, the selectivities of the crosslinked sulfonated oligomers remained relatively constant. Similar phenomena have previously been observed in crosslinked materials for use as fuel cell membranes, in which crosslinking simultaneously increased proton


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conductivity and membrane selectivity (conductivity/methanol permeability).27 Similarly, in these studies, crosslinking permitted preparation of sulfonated polysulfones with higher levels of hydrophilicity (i.e., higher degrees of sulfonation) within the oligomers than previously used. Crosslinked sulfonated polysulfone copolymers with 70, 80, 90 and even 100% of the units disulfonated might give higher values of water permeability while still maintaining relatively constant selectivity.

Figure 4. Fundamental transport properties of water/NaCl permeability selectivity, Pw/Ps, and the permeability coefficients, Pw: comparison between crosslinked amBPS membranes (●), crosslinked amBisAS membranes ( ), linear BPS copolymers (▼), a state-of-the-art aromatic polyamide (■), and PBP (polybenzimidazolepyrrolone,♦)19

Synthesis and structural characterization of the sulfonated oligomers The water transport properties of linear BPS and linear BisAS sulfonated polysulfones are significantly different. Linear BisAS copolymers have enhanced water permeabilities and



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slightly lower salt rejections at similar values of IEC. Polymeric composition greatly affects the chain packing and free volume available for transport. With regard to disulfonated polysulfones, BisAS based polymers have lower densities than BPS based polymers, and this has been hypothesized to lead to higher fractional free volumes and enhanced water permeabilities.23 In this study, we sought to determine the effect of composition on the transport properties of crosslinked disulfonated polysulfones.

The synthesis of telechelic amine end-capped

disulfonated polysulfones based on a bisphenol-A backbone structure (amBisAS-50) were described in Part I.9 A similar approach was used to synthesize telechelic amine end-capped disulfonated polysulfones based on 4,4’-biphenol (amBPS-50) as shown in Figure 5. Oligomers with 5,000 g mol-1 Mn's were targeted for consistency with prior studies.9,11 In these prior studies oligomers with a Mn of 10,000 g mol-1 achieved very low gel fractions of 17% and had significantly higher water uptakes and swelling ratios compared to lower molecular weight oligomers. Oligomers with a Mn of 3,000 and 5,000 g mol-1 were not statistically different in regards to the three aforementioned quantities. The transport properties in these systems were not measured, but these quantities indicate that a maximum molecular weight exists for telechelic oligomers before extents of crosslinking are reduced, membrane swelling occurs and salt rejection falls. The differences in transport properties between oligomers with a Mn of 3,000 and 5,000 g mol-1 are not apparent and are of interest for future studies.


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NaO3S x

Cl

O S O

O Cl

+

1-x

Cl

S O

Cl

+

HO

OH

+

HO NH2

SO 3Na

DMAc/Toluene K 2CO3 175 oC 2-3 days KO3 S O H 2N

O S O

O O

O x

S O

O O

SO3 K

O 1-x

S O

O

n

NH 2

Figure 5. Synthesis of amBPS-X oligomers 1

H-NMR spectra, as illustrated in Figure 6, confirmed the compositions of the oligomers.

The aromatic region contains all of the peaks, unlike the amBisAS oligomers that had an isopropylidene peak in the aliphatic up-field region. The backbone peak integrations were compared to the amine endgroup peaks (i) and used to calculate molecular weights of the oligomers (Mn).

Figure 6. 1H-NMR spectrum of an amBPS-50 oligomer

Integration of the sulfonated and non-sulfonated repeat units also established the degree of sulfonation (D.S.), a measure of hydrophilicity calculated by equation 3.



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Equation 3:

Use of equation 3 with the 1H-NMR spectra numerically quantified the levels of disulfonation in both the amBisAS and amBPS sulfonated copolymers. These calculated D.S. values were used to calculate IECs in Figures 2 and 3 and Table 1. Figure 7 shows the increasing extent of sulfonation in the amBisAS series at 38, 48 and 59% targeted D.S. The peaks corresponding to the hydrophilic repeat units (a, b, c) were larger than other peaks that remained constant, such as the bisphenol-A aromatic protons. Increasing D.S. was also displayed in the telechelic amine endgroups (i), which appeared as two singlets since the endgroups were adjacent to either DCDPS or SDCDPS. The relative ratio of these two singlet peaks also corroborated increasing presence of sulfonated groups as the target D.S. was increased from 38 to 59%.


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Figure 7. 1H-NMR spectra of amBisAS-38, amBisAS-48 and amBisAS-59 oligomers

Table 3 summarizes the targeted and measured degrees of sulfonation and molecular weights of the disulfonated oligomers. Success was obtained regarding control over sulfonate group concentration, as the measured degrees of sulfonation were all within 5% of the targeted composition. The Mn's calculated from the 1H-NMR data were also very close to the targeted molecular weights of 5,000 g mol-1, which indicated that the addition of a monofunctional amine end-capping reagent controlled the molecular weight.

The results from SEC were always

somewhat higher than both the targeted molecular weights and the molecular weights calculated from the 1H NMR integrals. This could be a result of analysis using the MALLS light scattering detector that may not have had sufficient signal at the low molecular weight end of these distributions. This discrepancy could also be a consequence of interaction between the rather 22 ACS Paragon Plus Environment


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highly sulfonated oligomers and the SEC column, which may skew the molecular weight analysis. Stoichiometric calculations to prepare the dense crosslinked films used the 1H-NMR results for determining molecular weights of the ~5,000 g mol-1 oligomers. Table 3. Structure and molecular weights of amBisAS and amBPS oligomers

Targeted

Measured

Mn

Mn

D.S.

D.S. a

(g/mol) a

(g/mol) b

5kamBisAS-40

40

38

4,800

5,000

5kamBisAS-50

50

48

4,800

5,200

5kamBisAS-60

60

59

5,400

6,300

5kamBPS-40

40

40

5,200

7,400

5kamBPS-50

50

50

4,600

6,400

5kamBPS-60

60

60

4,600

7,500

Sample

a b

Obtained from 1H-NMR spectra Obtained from SEC

Conclusions Water and salt transport properties of crosslinked disulfonated polysulfone oligomeric membranes were systematically studied by varying polymer composition, crosslinking, and degree of disulfonation/hydrophilicity.

1

H-NMR showed well-defined structures of the

disulfonated oligomers, ensuring that consistent molecular weights and end-group functionalities were obtained. All of the oligomers produced network films with gel fractions exceeding 90%.


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Crosslinking effectively reduced the swelling of the polymer membranes and controlled the salt rejection while increases in the degree of sulfonation improved water permeability. In particular, the crosslinked oligomers at the highest degree of sulfonation studied (60%) achieved superior transport results relative to previously studied linear sulfonated copolymers.

Acknowledgements The authors are grateful for the support of Dow Water & Process Solutions, Inc. This research was also supported by the U.S. National Science Foundation Partnerships for InnovationAccelerating Innovative Research (PFI-AIR, Grant #1237858) and Partnerships for Innovation (PFI) - Partnerships for Water Purification (Grant #0650277) The authors thank Dr. Kwan-soo Lee for his aid in constructing Figure 1.

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326x190mm (96 x 96 DPI)


Cross-Linked Disulfonated Poly(arylene ether sulfone) Telechelic

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