Engineering Selective Desalination Membranes via Molecular Control

reverse osmosis and nanofiltration.6-10 The selective properties of these desalination membranes. 37 result from the ... sample nomenclature, HEMA:GMA...
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Engineering Selective Desalination Membranes via Molecular Control of Polymer Functional Groups Hongxi Luo, Kevin Chang, Kevin Bahati, and Geoffrey M. Geise Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.9b00351 • Publication Date (Web): 10 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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Engineering Selective Desalination Membranes via

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Molecular Control of Polymer Functional Groups

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Hongxi Luo,+ Kevin Chang,+ Kevin Bahati, Geoffrey M. Geise*

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Department of Chemical Engineering, University of Virginia, 102 Engineers’ Way, P.O. Box

5

400741, Charlottesville, VA 22904, USA

6 7

+Authors

contributed equally

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9

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KEYWORDS. Desalination; Selectivity; Water Purification; Membrane; Dielectric

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ABSTRACT

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Membrane-based desalination processes are used widely to address increasing global demand for

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purified water. To continue to meet increasing demand, advanced and highly selective

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desalination membranes are needed to effectively and efficiently purify water that is increasingly

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contaminated and saline. A general lack of fundamental structure-property relationships

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frustrates the development of these membranes. Advanced polymer synthesis capabilities that

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enable increasingly precise molecular control over the position of functional groups on a

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polymer backbone could be instrumental in engineering advanced desalination membranes, but

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little is known about how the position of functional groups influences water and salt transport

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properties. In this study, we prepared and characterized a series of equivalent water content co-

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polymers to determine how the functional group configuration along the polymer backbone

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influences desalination-relevant transport properties. Shifting the co-polymer composition from

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one rich in vicinal diol groups to one rich in side chains with a single hydroxyl group drove an

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increase in the water/salt permeability selectivity, which is directly related to desalination-critical

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salt rejection. The results suggest that controlled placement of functional groups within a

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polymer could be a viable strategy for preparing advanced and highly selective desalination

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membrane materials.

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Introduction Meeting global demand for clean water is a pressing engineering challenge.1 Membrane-

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based technologies are used widely to desalinate water, and more efficient membranes are

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needed to meet growing demand for purified water from increasingly contaminated and/or saline

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water.2-6 Water purification membranes are generally polymeric, and highly selective membranes

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are needed to meet the growing separation challenges in desalination technologies including

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reverse osmosis and nanofiltration.6-10 The selective properties of these desalination membranes

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result from the preferential transport of water relative to hydrated ions.11 Engineering highly

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selective materials has been frustrated by a lack of fundamental structure-property relationships

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to guide the design of advanced polymer membranes to address global water needs.

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The ability to precisely place functional groups along a polymer backbone, with a high

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degree of molecular control over functional group position, is becoming increasingly viable.12, 13

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This structural control, facilitated by advances in synthesis capabilities, could be important for

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engineering advanced desalination membranes. Little is known, however, about how functional

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group placement within a polymer influences water/salt selectivity.

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We report the desalination (i.e., water/salt) selectivity properties of a series of model

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materials where the distribution of hydroxyl functional groups along the polymer backbone was

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varied from a clustered to a more uniform configuration. Importantly, this series of materials was

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prepared such that the membrane water content did not change as the distribution of the hydroxyl

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groups changed. Membrane water content has a significant influence on water/salt selectivity

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properties,9, 14 so the ability to vary the distribution of functional groups within the material

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without perturbing the water content enabled us to ascribe the observed results to changes in the

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functional group configuration.

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Shifting the functional group configuration to space the hydroxyl groups out more evenly

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(compared to the more clustered configuration) resulted in increased water/salt permeability

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selectivity, which is directly related to desalination-critical salt rejection.11 This increase in

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selectivity was largely driven by sorption, or thermodynamic, effects. The results suggest that an

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even distribution of hydrophilic chemical functionality in polymers may lead to more selective

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membranes to address global demand for desalinated water.

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Materials and Methods

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HEMA:GMA:GMAOH Co-Polymers

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Five co-polymers containing 2-hydroxyethyl methacrylate (HEMA), glycidyl methacrylate

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(GMA), and glycerol methacrylate (GMAOH) co-monomers and cross-linked using

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poly(ethylene glycol) dimethacylate (PEGDMA) were prepared via UV-initiated free radical

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polymerization. These model materials are chemically different from the aromatic polyamide-

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based membranes commonly used in reverse osmosis and nanofiltration membranes.15 The

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structure of these materials (i.e., the choice of co-monomers) was chosen to enable preparation of

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a series of materials where the distribution of hydrophilic functional groups (in this case,

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hydroxyl groups) could be varied by changing the composition of the co-polymer without

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changing the water content of the polymer.

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The hydroxyl group distribution in the co-polymer was varied by changing co-monomer

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composition of the pre-polymerization solution used to prepare the materials (Table 1). The

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sample nomenclature, HEMA:GMA:GMAOH x:y:z, reflects the composition (by mass) of the

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pre-polymerization solution used to prepare that material (e.g., 15:55:30 was prepared from a

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pre-polymerization solution containing 15%, 55%, and 30%, by mass, of HEMA, GMA, and

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GMAOH, respectively). The PEGDMA cross-linker and initiator (1-hydroxycyclohexyl phenyl

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ketone) were added to the co-monomer solution at compositions of 0.1 and 0.01 g/g(total co-

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monomer), respectively, to form the pre-polymerization solution, which was stirred for 30 min at

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room temperature to ensure the solution was well-mixed. The pre-polymerization solution was

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degassed (via sonication for 10 min), confined between quartz plates (separated by spacers to

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control film thickness), and irradiated, for 5 min, with 120 µJ/cm2 of 312 nm light to form

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transparent and homogeneous polymer films that were approximately 100 m thick. The

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chemical structure and additional information about the co-polymers are reported in Section S1

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of the Supporting Information.

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Table 1. Co-monomer mass fraction in the pre-polymerization solution used to prepare the

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HEMA:GMA:GMAOH co-polymers defines the sample nomenclature. The water uptake and dry

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density data were measured at ambient temperature, and the water sorption coefficient, 𝐾𝑤, was

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calculated using the water uptake and dry polymer density. Uncertainty in the measured data is

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one standard deviation from the mean of at least three measurements, and the uncertainty in the

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water sorption coefficient was calculated using standard propagation of error.16 CoPolymer Sample

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0:60:40 15:55:30 30:50:20 45:45:10 60:40:0 aUnits:

Co-Monomer Mass Fraction in the Pre-Polymerization Solution HEMA

GMA

GMAOH

0.0 0.15 0.30 0.45 0.60

0.60 0.55 0.50 0.45 0.40

0.40 0.30 0.20 0.10 0.0

Water Uptakea

Dry Polymer Density (g/cm3)

𝑲𝒘

0.24 ± 0.02 0.24 ± 0.01 0.24 ± 0.01 0.23 ± 0.01 0.23 ± 0.01

1.29 ± 0.02 1.30 ± 0.03 1.27 ± 0.01 1.27 ± 0.01 1.26 ± 0.02

0.23 ± 0.01 0.24 ± 0.01 0.23 ± 0.01 0.23 ± 0.01 0.23 ± 0.01

g(water) / g(dry polymer)

93 94 95 96

Water and Salt Transport Property Characterization To characterize the potential desalination performance of the HEMA:GMA:GMAOH copolymers, we measured water uptake, salt sorption coefficient, and water and salt permeability 5 ACS Paragon Plus Environment

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properties at ambient temperature. Water uptake was combined with dry polymer density to

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calculate the water sorption coefficient, 𝐾𝑤 (Table 1).17 Water (i  w) and salt (i  s) sorption

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coefficients, 𝐾𝑖, were combined with permeability properties, 𝑃𝑖, to calculate, via the solution-

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diffusion model,18 apparent diffusion coefficients, 𝐷𝑖, as:19

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𝐷 𝑖 = 𝑃𝑖 𝐾 𝑖

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At least three measurements, per sample, were made for each property, and the uncertainty in the

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measurement was one standard deviation from the mean. Standard propagation of error was used

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to quantify the uncertainty in calculated quantities.16 Details about the specific methods used to

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measure water uptake, salt sorption, pure water permeability, salt permeability, and hydrated

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polymer dielectric properties are reported in Section S2 of the Supporting Information.

(1)

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Results and Discussion Each composition of HEMA:GMA:GMAOH had equivalent water content (Table 1). Water

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content affects the transport (and, thus, desalination) selectivity of hydrated polymers, and

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increases in water content often correlate with decreases in selectivity.9, 14 Therefore, preparing

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materials with equivalent water content is critical for decoupling the influence of functional

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group configuration on water/salt transport selectivity from the influence of changing water

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content on water/salt transport selectivity.

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The equivalent water content of this series of HEMA:GMA:GMAOH co-polymers likely

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results from subtle changes in the extent of cross-linking and hydroxyl group content across the

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series of co-polymers. These changes were estimated to be reasonably small, as discussed in

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Section S1.1 of the Supporting Information. Ultimately, maintaining equivalent water content in

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the series of materials was prioritized due to the strong influence of water content on the water

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and salt transport properties of hydrated polymers.9, 14

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We compared the HEMA:GMA:GMAOH transport properties to those of cross-linked

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poly(ethylene glycol) diacrylate (PEG) hydrogels, as these materials also preferentially transport

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water over salt via a solution-diffusion mechanism.20 The water content of these hydrogels can

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be manipulated by changing the ethylene glycol chain length or by adding a diluent to the pre-

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polymerization mixture.20 Thus, the PEG materials illustrate how transport properties change

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with water content to contrast the situation in equivalent water content HEMA:GMA:GMAOH.

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The water/salt transport selectivity is defined as the ratio of water to salt permeability, 𝑃𝑤 𝑃𝑠,

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sorption 𝐾𝑤 𝐾𝑠, or diffusion, 𝐷𝑤 𝐷𝑠, coefficients, and the three selectivity values are related via

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the solution-diffusion model:9

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𝑃𝑤 𝑃 = (𝐾𝑤 𝐾 ) × (𝐷𝑤 𝐷 ) 𝑠 𝑠 𝑠

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This water/salt permeability selectivity can be directly related to salt rejection, which is a critical

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characteristic of effective desalination membranes.11 The salt rejection, R, is defined as the salt

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concentration reduction from the bulk solution on the feed side of the membrane, 𝑐𝑠,𝑓𝑒𝑒𝑑, to the

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bulk solution on the permeate (or product) side of the membrane normalized by 𝑐𝑠,𝑓𝑒𝑒𝑑 and

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depends on the water/salt permeability selectivity: 𝑃𝑤𝑉𝑤

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𝐑=

𝑐𝑠,𝑓𝑒𝑒𝑑 ― 𝑐𝑠,𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒 𝑐𝑠,𝑓𝑒𝑒𝑑

=

𝑃𝑠 𝑅𝑇 (∆𝑝

1+

𝑃𝑤𝑉𝑤

― ∆𝜋)

𝑃𝑠 𝑅𝑇 (∆𝑝

(2)

(3)

― ∆𝜋)

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where 𝑉𝑤 is the molar volume of water, ∆𝑝 is the hydraulic pressure difference across the

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membrane, ∆𝜋 is the osmotic pressure difference across the membrane, R is the gas constant, and

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T is the absolute temperature.9

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Water/salt permeability selectivity depends on a combination of both water/salt sorption

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and diffusivity selectivity properties (Equation 2). Functional group configuration significantly

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influenced the water/salt sorption selectivity of HEMA:GMA:GMAOH (Figure 1A). A

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distributed hydroxyl group configuration (HEMA-rich co-polymer) led to higher sorption

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selectivity compared to a vicinal-diol rich configuration (GMAOH-rich co-polymer), and this

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result can be attributed to more effective exclusion of salt from the HEMA-rich co-polymer

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compared to the GMAOH-rich co-polymer.

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A 0

B

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Figure 1. Water/salt sorption selectivity (A) and water/salt diffusivity selectivity (B) as a

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function of water sorption coefficient and apparent water diffusion coefficient, respectively, for

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HEMA:GMA:GMAOH ( ) and PEG ( )20 materials. For HEMA:GMA:GMAOH, the HEMA co-

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monomer composition of the pre-polymerization solution used to prepare the co-polymer is

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reported for each data point. The dashed lines represent reported empirical sorption (left) and

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diffusivity (right) trade-off frontiers for desalination membrane materials,21, 22 and the solid lines

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are least-squares fits to the data.

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Comparing HEMA:GMA:GMAOH water/salt sorption selectivity with that of

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poly(ethylene glycol) diacrylate (PEG) hydrogels, we observed that the HEMA:GMA:GMAOH

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data points move vertically upwards as HEMA content increases in a manner different from the

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PEG materials (Figure 1A). The PEG result is expected if water content primarily drives sorption

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selectivity properties.20 The HEMA:GMA:GMAOH result suggests that polymer chemistry, not

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changing water content, is responsible for the change in sorption selectivity of

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HEMA:GMA:GMAOH. Thus, preparing polymers with a distributed functional group

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configuration may be a viable strategy to increase the water/salt sorption selectivity.

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The HEMA:GMA:GMAOH materials are approximately an order of magnitude more

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water/salt diffusion selective compared to the PEG materials (Figure 1B). This result may be due

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to the glassy nature of hydrated HEMA:GMA:GMAOH compared to the rubbery nature of PEG

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(see Section S1.3 of the Supporting Information).23 More rigid (glassy) polymer backbones are

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often more diffusion selective compared to more flexible (rubbery) polymer backbones.24-26

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The apparent water diffusion coefficient decreases and the water/salt diffusivity selectivity

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increases as the co-polymer composition shifts toward a more HEMA rich (or distributed

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hydroxyl group configuration). This observed trend is similar to that observed for PEG. The

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water content of the PEG materials was varied systematically, and the explanation for the

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observed relationship between water/salt diffusivity selectivity and water diffusion coefficient is

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that reduction in the free volume of the polymer (as water content decreases) causes a general

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reduction in diffusion that influences the larger hydrated ions to a greater extent than water.20 In

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HEMA:GMA:GMAOH, the change in co-monomer composition (and thus, distribution of

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hydroxyl groups) results in a situation where water diffusion slows as HEMA content increases.

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This reduction in water diffusivity is accompanied by an increase in water/salt diffusion

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selectivity that is similar in proportion to that of PEG. Therefore, changes in water diffusivity,

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due to compositional changes in the HEMA:GMA:GMAOH materials, may influence water/salt

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diffusivity selectivity in a manner similar to that in the free volume-based explanation where

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water diffusion depends strongly on free volume.9, 14

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Water/salt diffusivity selectivity is affected to a lesser extent, compared to water/salt sorption

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selectivity, by changing the HEMA content of the pre-polymerization solution. The water/salt

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diffusivity selectivity increases by 26% over the range of HEMA compositions considered. The

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water/salt sorption selectivity, by comparison, increases by 53% over the range of HEMA

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compositions considered. Importantly, the 53% increase in sorption selectivity comes without a

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change in the water sorption coefficient, which as discussed subsequently, means that, as co-

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polymer composition changes, the water permeability will not change because of sorption

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effects.

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The decrease in the apparent water diffusivity as the co-polymer composition changes at

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constant water content could be considered unexpected, as diffusivity in hydrated polymers is

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often a strong function of water content.9, 14, 27-29 Dielectric permittivity properties, however,

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provide insight into the observed water diffusivity. As further discussed in Section S3 of the

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Supporting Information, the dielectric loss spectra can be interpreted in terms of time constants,

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which describe the dipole relaxation dynamics of different modes of water motion, and the

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dielectric strengths of those relaxations, which describe the relative amount of water in the

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material that is participating in each relaxation mode.30

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Dielectric spectroscopy suggests three populations of dipolar water motion: highly restricted

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motion that relaxes on an order 0.1 ns timescale, less restricted motion that relaxes with a time

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constant of approximately 45 ps, and non-restricted (i.e., bulk water) motion that relaxes with a

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time constant of 8.8 ps (see Supporting Information for additional discussion). In general, the

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dielectric strength associated with each mode of motion decreases as HEMA content increases

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(Table S1), though the reduction is more pronounced for the non-restricted relaxation mode

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compared to the other two modes. This decrease in the dielectric strength associated with all

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three relaxation modes, coupled with the statistically equivalent water content of the materials,

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suggests that increasing HEMA content promotes water-polymer interactions that relax at

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frequencies lower than that probed in our experiments.

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An example of such interactions would be water that is very tightly associated with the

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hydroxyl groups. Such (effectively immobile) water will not be detected by dielectric

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spectroscopy in the frequency range considered here.31 As such, the dielectric strength data

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suggests that distributing the hydroxyl groups in the polymer causes more water to tightly

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associate with hydroxyl groups compared to the situation in the vicinal diol-rich materials.

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This molecular picture is consistent with a steric explanation suggesting that water molecules

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may be able to hydrate the hydroxyl group on a HEMA side chain to a greater extent compared

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to the more sterically hindered GMAOH side chain. This explanation is also consistent with

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estimates of the enthalpy of hydration for the hydroxyl groups on the side chains (see Section S4

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of the Supporting Information). Ultimately, this reduction in water motion within the polymer

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appears to have a similar effect on both water and salt diffusion as reducing the water content of

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the polymer (as supported by the similar slopes of the data in Figure 1B).

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Primarily due to the strong increase in water/salt sorption selectivity at constant water

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content, the HEMA:GMA:GMAOH water/salt permeability selectivity increases as the HEMA

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content of the pre-polymerization solution used to prepare the materials increases (Figure 2). A

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distributed hydroxyl group configuration (HEMA-rich) is more selective for water over salt

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transport compared to the vicinal-diol rich configuration (GMAOH-rich). The overall selectivity

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of the HEMA:GMA:GMAOH series of materials is greater than that of the PEG materials due to

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the previously discussed differences in the diffusivity selectivity properties.

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Figure 2. Water/salt permeability selectivity as a function of diffusive water permeability for HEMA:GMA:GMAOH ( ) and PEG ( )20 materials. For HEMA:GMA:GMAOH, the HEMA comonomer composition of the pre-polymerization solution used to prepare the co-polymer is reported for each data point. The dashed line represents a reported empirical permeability tradeoff frontier for desalination membrane materials,21, 22 and the solid lines are least-squares fits to the data.

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The PEG materials exhibit a typical tradeoff relationship whereby water/salt selectivity tends

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to decrease as materials become more permeable to water. This tradeoff is often observed in cases

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where the water content of a series of materials is varied systematically.22 In these cases, the higher

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water content polymers tend to have higher water permeability and lower water/salt selectivity

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compared to the lower water content polymers.

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The HEMA:GMA:GMAOH materials suffer less of a reduction in the diffusive water

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permeability as water/salt selectivity increases compared to the PEG materials (i.e., the slope of

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the HEMA:GMA:GMAOH dashed line is steeper than that of the PEG dashed line in Figure 2).

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This result stems from the equivalent water content nature of the HEMA:GMA:GMAOH series of

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materials and the fact that both water/salt sorption and diffusivity selectivity increase with

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increasing content of HEMA in the pre-polymerization solution.

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Chemical modification of a series of water content equivalent co-polymers, from a vicinal

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diol-rich to a distributed hydroxyl group-rich configuration, increased water/salt permeability

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selectivity with a smaller water permeability penalty compared to that often observed in hydrated

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polymers. These results, obtained using a model series of co-polymers, suggest that controlling

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the spatial arrangement of functional groups in hydrated membrane materials may be important

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for engineering highly selective polymers for desalination applications. The results on these

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model materials represent a step toward establishing general water and salt transport structure-

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property relationships for membrane materials including polymers that are more chemically

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similar to commercial desalination membranes than those materials considered here. The results

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suggest that distributed hydrophilic functional groups may lead to increased selectivity and may

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represent a strategy for improving water/salt selectivity of advanced membrane materials to

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address global water shortages.

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ASSOCIATED CONTENT

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Supporting Information. Additional information regarding experimental methods, material

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properties and characterization (molar composition, hydroxyl group content, FT-IR, and glass

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transition temperature analysis), microwave dielectric spectroscopy data analysis, and hydroxyl

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group hydration analysis. This material is available free of charge via the Internet at

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http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author

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*102 Engineers’ Way, P.O. Box 400741, Charlottesville, VA 22904 USA, [email protected]

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Funding Sources

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This material is based upon work supported in part by the National Science Foundation (NSF)

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Faculty Early Career Development (CAREER) Program (CBET-1752048).

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Notes

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The authors declare no competing financial interest.

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Figure 1. Water/salt sorption selectivity (A) and water/salt diffusivity selectivity (B) as a function of water sorption coefficient and apparent water diffusion coefficient, respectively, for HEMA:GMA:GMAOH (■) and PEG (□)20 materials. For HEMA:GMA:GMAOH, the HEMA co-monomer composition of the pre-polymerization solution used to prepare the co-polymer is reported for each data point. The dashed lines represent reported empirical sorption (left) and diffusivity (right) trade-off frontiers for desalination membrane materials,21, 22 and the solid lines are least-squares fits to the data. 207x191mm (300 x 300 DPI)

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Figure 1. Water/salt sorption selectivity (A) and water/salt diffusivity selectivity (B) as a function of water sorption coefficient and apparent water diffusion coefficient, respectively, for HEMA:GMA:GMAOH (■) and PEG (□)20 materials. For HEMA:GMA:GMAOH, the HEMA co-monomer composition of the pre-polymerization solution used to prepare the co-polymer is reported for each data point. The dashed lines represent reported empirical sorption (left) and diffusivity (right) trade-off frontiers for desalination membrane materials,21, 22 and the solid lines are least-squares fits to the data. 218x207mm (300 x 300 DPI)

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Figure 2. Water/salt permeability selectivity as a function of diffusive water permeability for HEMA:GMA:GMAOH (■) and PEG (□)20 materials. For HEMA:GMA:GMAOH, the HEMA co-monomer composition of the pre-polymerization solution used to prepare the co-polymer is reported for each data point. The dashed line represents a reported empirical permeability trade-off frontier for desalination membrane materials,21, 22 and the solid lines are least-squares fits to the data. 215x205mm (300 x 300 DPI)

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