Water and salt transport properties of triptycene-containing sulfonated

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Water and salt transport properties of triptycene-containing sulfonated polysulfone materials for desalination membrane applications Hongxi Luo, Joseph Aboki, Yuanyuan Ji, Ruilan Guo, and Geoffrey M. Geise ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17225 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 13, 2018

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ACS Applied Materials & Interfaces

Water and salt transport properties of triptycene-containing sulfonated polysulfone materials for desalination membrane applications Hongxi Luo,1 Joseph Aboki,2 Yuanyuan Ji,1 Ruilan Guo,2 Geoffrey M. Geise1,*

1

Department of Chemical Engineering University of Virginia 102 Engineers’ Way, P.O. Box 400741 Charlottesville, VA 22904 USA 2

Department of Chemical and Biomolecular Engineering University of Notre Dame 205 McCourtney Hall Notre Dame, IN 46556 USA

*To whom correspondence should be addressed: [email protected] (Tel: +1-434-924-6248, Fax: +1-434-982-2658)

Date: January 5, 2018 Manuscript prepared for submission to: ACS Applied Materials & Interfaces

Keywords: Desalination; Triptycene; Sulfonated polysulfone; Salt transport; Selectivity; Membrane; Ion exchange membrane

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Abstract A series of triptycene-containing sulfonated polysulfone (TRP-BP) materials was prepared via condensation polymerization, and the desalination membrane-relevant fundamental water and salt transport properties (i.e., sorption, diffusion, and permeability coefficients) of the polymers were characterized. Incorporating triptycene into sulfonated polysulfone increased the water content of the material compared to sulfonated polysulfone materials that do not contain triptycene. No significant difference in salt sorption was observed between the TRP-BP membranes and other sulfonated polysulfone membranes, suggesting that the presence of triptycene in the polymer did not dramatically affect thermodynamic interactions between salt and the polymer. Both water and salt diffusion coefficients in the TRP-BP membranes were suppressed relative to other sulfonated polysulfone materials with comparable water content, and these phenomena may result from the influence of triptycene on polymer chain packing and/or free volume distribution, which could increase the tortuosity of the transport pathways in the polymers. Enhanced water/salt diffusivity selectivity was observed for some of the TRP-BP membranes relative to those materials that did not contain triptycene, and correspondingly, incorporation of triptycene into sulfonated polysulfone resulted in an increase, particularly for acid counter-ion form TRP-BP materials, in water/salt permeability selectivity, which is favorable for desalination membrane applications.


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1. Introduction Freshwater is a critical resource for human life, and it plays a key role in economic and societal development.1-6 However, global population growth continues to amplify stress on existing freshwater supplies, and water scarcity increasingly is becoming a worldwide problem.4, 7-10

According to the United Nations, more than one-third of the world’s population is suffering

from either physical water scarcity or economic water shortage.1-2,

4

Numerous efforts are

focused on increasing global freshwater supply, and wastewater treatment11-13 and desalination of saline water2, 4, 12, 14 are among the most common approaches. Wastewater treatment focuses on the removal of contaminants using physical and/or chemical treatment (e.g., filtration, adsorption, flocculation, and photocatalytic degradation),11-13, 15 and desalination aims to produce freshwater by separating salt and water (e.g., using distillation,16-17 electrodialysis,18-19 reverse osmosis,20-24 or solar evaporation25). Membrane-based processes are used in both wastewater treatment and desalination processes. Due to generally high energy efficiency and low cost, membrane-based desalination processes are widely used to desalinate water, and membrane-based desalination processes are expected to continue to play a key role in the desalination industry in the future.2, 4, 12, 14

Current state-of-the-art desalination membranes are based on polyamide chemistry that was introduced in the early 1980s.21, 26 These membranes possess favorable combinations of water and salt transport properties and are very efficient.14, 27 The polyamide linkages in these polymers, however, are susceptible to oxidative degradation that can be accelerated by the use of chlorine-based disinfectants, which are used to prevent biofouling in water purification systems.28-33 Consequently, pretreatment steps (e.g., chlorine removal and pH adjustment) are needed to prolong the useful life of desalination membranes, and pretreatment increases the overall cost of desalination.28-30, 34 Chemically stable desalination membrane polymers would address this issue. Hydrocarbon polymers can be chemically stable but are often too hydrophobic to function effectively as water treatment membranes, so strategies, including sulfonation, have been used to increase the hydrophilicity of otherwise hydrophobic hydrocarbon polymers.4, 23, 28, 35-45 While promising for desalination applications, sulfonated polymer membranes have not yet achieved the desalination selectivity and water permeance characteristics of commercially available

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polyamide membranes.42,

45-46

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Recent recognition of the importance of water/salt selectivity

properties of desalination membrane polymers47 underscores the need for studies that focus on understanding how membrane chemistry and structure can be used to enhance the water/salt selectivity properties of materials, such as sulfonated polysulfone, that are of interest for desalination membrane applications. Incorporating bulky co-monomers into membrane polymers is one strategy used to tune membrane selectivity.48-51 For example, triptycene and pentiptycene based polymer membranes have exhibited promise for gas separation applications,52-58 and their transport properties can exceed the gas permeability-selectivity tradeoff barrier (i.e., the so-called upper bound).59-60 In these materials, the incorporation of bulky, space occupying moieties (i.e. triptycene and pentiptycene) can generate additional free volume and simultaneously narrow the free volume distribution in the polymer.52-54,

61-62

Increased membrane free volume enables higher

permeability, and a narrow free volume element size distribution enhances selectivity.52-54, 61-62 The success of incorporating triptycene in gas separation membranes motivated us to study the influence of incorporating triptycene moieties in sulfonated polysulfone materials, as the mechanism of small molecule transport in gas separation and desalination membranes is similar.4, 34, 45, 47, 63-64

In this study, a series of triptycene-containing sulfonated polysulfone membranes (TRPBP) was prepared, and the fundamental water and salt transport properties of the materials were characterized. The TRP-BP materials sorbed more water compared to sulfonated polysulfones without triptycene. The incorporation of triptycene in sulfonated polysulfone did not appear to influence salt sorption, suggesting that triptycene does not dramatically influence the bulk thermodynamic interactions between salt and the polymer. Both the water and salt diffusion coefficients were lower in the TRP-BP materials compared to other sulfonated polysulfone materials at comparable water content. Many of the TRP-BP materials exhibited improved water/salt permeability selectivity compared to other sulfonated polysulfone materials. In particular, acid counter-ion form TRP-BP materials exhibited greater water/salt permeability selectivity compared to acid counter-ion form sulfonated polymers that did not contain triptycene. Experimental results suggest that incorporation of triptycene into sulfonated polysulfone polymers may be a viable strategy for increasing desalination membrane selectivity.


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2. Experimental Methods 2.1 Materials Triptycene-1,4-hydroquinone (TRP) was prepared, as described previously, and dried overnight at 90°C under vacuum prior to use.52 4,4’-biphenol (BP) and 3,3’-disulfonated-4,4’dichlorodiphenylsulfone (SDCDPS) were purchased from Akron Polymer Systems and dried under vacuum for 24 h at 120°C prior to use. Monomer grade 4,4’-dichlorodiphenylsulfone (DCDPS) and anhydrous potassium carbonate (K2CO3) were purchased from Alfa Aesar and dried under vacuum for 24 h at 110°C prior to use. Anhydrous N,N-dimethylacetamide (DMAc), anhydrous N-Methyl-2-pyrrolidinone (NMP), toluene, and 2-propanol were purchased from Sigma-Aldrich and used as received. 2.2 Polymer Synthesis and Membrane Preparation A series of triptycene-containing di-sulfonated poly(arylene ether sulfone) random copolymers (TRP-BP) with different degrees of sulfonation and varied molar ratios of triptycene1,4-hydroquinone (TRP) to 4,4’-biphenol (BP) were synthesized via nucleophilic step growth polymerization. It should be noted that random copolymers with TRP as the only bisphenol moiety did not have sufficiently high molecular weight for film formation due to the low reactivity of TRP and SDCDPS. The nomenclature used for the copolymers is TRP-BP a:b-X, where a:b is the molar ratio of TRP to BP and X is the molar percentage of sulfonated SDCDPS in the copolymers as shown in Scheme 1. For example, TRP-BP 1:1-35 refers to the copolymer containing 1:1 molar ratio of TRP to BP, 35 mol% sulfonated monomer (SDCDPS), and 65 mol% non-sulfonated monomer (DCDPS). An example polymerization of TRP-BP 1:1-35 is: 1.8621 g (10.0 mmol) of BP, 2.8633 g (10.0 mmol) of TRP, 3.5450 g (7.0 mmol) of SDCDPS, 3.7707 g (13.0 mmol) of DCDPS, and 3.3169 g (24.0 mmol) of anhydrous K2CO3 were charged into a three-necked round bottom flask equipped with a condenser, mechanical stirrer, Dean-Stark trap, and nitrogen inlet. Anhydrous NMP (60 mL) and toluene (30 mL) were then added to the flask, and the reaction was heated, under a N2 purge, to 150°C while stirring. The reaction was refluxed at 150°C for 4 h to azeotropically dehydrate the system. Afterwards, toluene was removed from the reaction by slowly increasing the temperature to 185°C. The reaction was allowed to proceed at 185°C for another 48 h until a viscous solution formed. The polymer solution was then cooled to room temperature, filtered to remove salts, and coagulated in a stirred isopropanol bath. The precipitated fibrous TRP-BP copolymer was collected and dried under vacuum at 120°C for 24 h. 3 ACS Paragon Plus Environment


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Scheme 1. Synthesis of triptycene-containing sulfonated poly(arylene ether sulfone)s

All salt-form (as synthesized) copolymer films were prepared via a solution casting method. The TRP-BP copolymers were dissolved in DMAc (~7% w/v), and the solution was filtered through a 0.45 µm PTFE syringe filter. To form polymer films, the filtered solution was cast onto a clean leveled glass substrate and dried for 24 h under an infrared lamp at ~55°C. The films were subsequently dried under vacuum at 110°C for 24 h to further remove residual solvent. The salt counter ion form (salt-form) membranes were converted to the sulfonic acid form (acid-form), i.e., replacing K with H in the TRP-BP structure shown in Scheme 1, by boiling the films in a 0.5 M sulfuric acid solution for 2 h followed by boiling the films in deionized (DI) water (18.2 MΩ cm) for 2 h. The acid-form TRP-BP membranes were subsequently stored in DI water until use. The suffix “salt” or “acid” was added to the copolymer names to


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indicate whether the films were in the salt-form (as polymerized) or acid-form (converted as described above) before characterization. 2.3 Water Sorption and Polymer Density Measurements Cast TRP-BP membranes were soaked in DI water, and the hydrated membrane mass, mhydrated, was measured periodically until a constant value was obtained. A laboratory wipe was used to remove residual water from the membrane surface before measuring mhydrated. After reaching equilibrium, hydrated membranes were placed in vented plastic petri dishes and dried under vacuum at 25℃ for 7 days.65 After drying, the dry membrane mass, mdry, was measured, and water uptake, W.U., was calculated as: . . =

(1)

Immediately following the mdry measurement, the dry polymer density, , was measured. An analytic balance (XSE204, Mettler Toledo) equipped with a density measurement kit was used, and cyclohexane was chosen as the auxiliary liquid to be consistent with previous studies on sulfonated polysulfone materials.65-66 The temperature of the auxiliary liquid was measured during the density measurement, and Archimedes’ principle was used to determine dry:

= ( − )

(2)

where A and B are the membrane masses in the auxiliary liquid and in air, respectively, is the density of the auxiliary liquid,67-68 which was evaluated at the measurement temperature, and the density of air, , was taken as 0.0012 g/cm3.67 The volume fraction of water in the polymer, , was calculated, assuming additive mixing of water and the polymer, as:65 =

../ ! ../ ! "#/

(3)

where the density of water, , was taken as 1 g/cm3.65 The membrane water sorption coefficient, KW, was defined as the ratio of the membrane water concentration, g(water)/cm3(hydrated polymer), to the water concentration in the external solution, g(water)/cm3(external solution), in equilibrium with the membrane:69-70 %&

$ = ! %'

(4)

!

) where ( was taken as 1 g/cm3 at room temperature.64 The value of ( was calculated as: ( =

*! +! 0(123) .=/ 5 4 (623 89:3) ,!

(5)



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where ; is the molecular weight of water (18 g/mol), =

#

,

?!

2

(7)

where dV/dt is the slope of the linear steady-state permeate volume versus time data and represents the volumetric permeation rate. The effective area of the sample available for water A permeation, APW, was 14.6 cm2. Hydraulic water permeability, @ , was calculated as:64 A @ =

B! :

(8)

∆8

where ∆D is the pressure difference across the membrane and l is the hydrated membrane thickness, which was taken as the hydrated sample thickness measured immediately following E the water permeability measurement.65 Diffusive water permeability, @ , was calculated from A the measured @ data as:45, 70-71 FG #H!

E A @ = @ . , !

I

/

(9) 6


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where R is the gas constant, T is absolute temperature, and the term in square brackets is a correction factor that accounts for both convective frame of reference (i.e., the 1 – KW term) and thermodynamic non-ideality (i.e., J).45 In other sulfonated polysulfone studies, Flory-Huggins theory72-73 was used to estimate the value of J.45, 65, 74 These analyses, however, used a single-point fit to determine the FloryHuggins interaction parameter that is needed to evaluate J (c.f., Supporting Information).45 Given the uncertainty about whether Flory-Huggins theory describes the TRP-BP materials, the value of J in Eq. (9) was set equal to unity during calculation of the diffusive water permeability for the TRP-BP materials and the BPS(H) and BisAS materials considered: FG

E A @ = @ .1 − $ / ,

(10)

!

In the Supporting Information, the single point Flory-Huggins theory fit method was used during calculation of the diffusive water permeability for all of the materials considered, and this alternate approach to the analysis is provided in the Supporting Information with the caveat discussed above about whether Flory Huggins theory is applicable to these materials. After converting the hydraulic water permeability to diffusive water permeability, using Eq. (10), the E average water diffusion coefficient, DW, was calculated from @ and $ as:70

L =

N M!

H!

(11)

2.5 Salt Transport Measurements Salt permeability, PS, was measured at 25℃ using a direct salt permeation technique and a custom dual chamber permeation cell. Prior to the measurement, the sample was soaked in 1 M NaCl solution, and a laboratory wipe was used to remove residual salt solution from the membrane surface. Two rubber gaskets, with 1.59 cm diameter openings, were used to seal the membrane in the cell and define the area available for salt transport, APS. The donor chamber was initially filled with 100 mL of 1 M NaCl solution, and the receiver chamber was initially filled with 100 mL of DI water. The system temperature was maintained at 25℃ using a water bath to circulate water through jackets surrounding the donor and receiver chambers. The solutions in both chambers were mixed by mechanical stirring (CaframoTM BDC250, 350 rpm). A conductivity meter (Inolab® Cond 7310) was used to track the change in the receiver chamber solution conductivity over time, and the solution conductivity was converted to salt concentration via a calibration curve. Salt permeability, PS, was calculated as:75 7 ACS Paragon Plus Environment


% (2)

,:

ln .1 − 2 % R(S)/.− T / = @) U N

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(12)

?'

where CR(t) is the receiver chamber salt concentration at time t, CD(0) is the initial donor chamber salt concentration (1 M), V is the solution volume in each chamber (100 mL), l is the hydrated sample thickness (measured following the experiment), and the effective transport area APS was 1.98 cm2. The pseudo steady-state condition was verified by observing a linear variation of ln .1 − 2

%R (2) %N (S)

,:

/.− T] vs. t, and the slope of this line was taken as @) according to Eq. (12).

The membrane salt sorption coefficient, Ks, was defined as the ratio of membrane salt concentration (mol(salt)/cm3(hydrated polymer)) to salt concentration in the external solution (mol(salt)/cm3(external solution)) in equilibrium with the membrane.69-70 Salt sorption was measured using a desorption method76 where the TRP-BP samples were cut into 1.59 cm diameter discs and soaked in 50 mL of 1 M NaCl solution for two days to reach sorption equilibrium. The time needed for a sample to reach sorption equilibrium was estimated using the salt permeability of the sample as l2/4PS. This time was found to be on the order of 1000 sec and was recognized to be an overestimation of the characteristic time for diffusion, l2/4DS, since PS = DS KS and KS < 1.45 Soaking the samples in salt solution for a factor of 10 greater than this characteristic time for diffusion was taken to be sufficient for reaching sorption/desorption equilibrium.76 After equilibration, the membranes were removed from the salt solution, wiped with a laboratory wipe to remove salt solution from the sample surface, and soaked in 20 mL of DI water for two days to allow the sorbed salt to desorb from the polymer. The desorption solution concentration, which was targeted to be near 1 mg(NaCl)/L by controlling the sample volume and desorption solution volume, was measured using ion-chromatography (Dionex ICS-2100), and Ks was calculated as: $) =

%V ,V %W ,&

(13)

where (X is the desorption solution concentration,

Water and salt transport properties of triptycene-containing sulfonated

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