Full text access provided via ACS AuthorChoice
Energy and the Environment
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
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 20
Environmental Science & Technology Letters
1
Engineering Selective Desalination Membranes via
2
Molecular Control of Polymer Functional Groups
3
Hongxi Luo,+ Kevin Chang,+ Kevin Bahati, Geoffrey M. Geise*
4
Department of Chemical Engineering, University of Virginia, 102 Engineers’ Way, P.O. Box
5
400741, Charlottesville, VA 22904, USA
6 7
+Authors
contributed equally
8
9
10
KEYWORDS. Desalination; Selectivity; Water Purification; Membrane; Dielectric
1 ACS Paragon Plus Environment
Environmental Science & Technology Letters
Page 2 of 20
12
ABSTRACT
13
Membrane-based desalination processes are used widely to address increasing global demand for
14
purified water. To continue to meet increasing demand, advanced and highly selective
15
desalination membranes are needed to effectively and efficiently purify water that is increasingly
16
contaminated and saline. A general lack of fundamental structure-property relationships
17
frustrates the development of these membranes. Advanced polymer synthesis capabilities that
18
enable increasingly precise molecular control over the position of functional groups on a
19
polymer backbone could be instrumental in engineering advanced desalination membranes, but
20
little is known about how the position of functional groups influences water and salt transport
21
properties. In this study, we prepared and characterized a series of equivalent water content co-
22
polymers to determine how the functional group configuration along the polymer backbone
23
influences desalination-relevant transport properties. Shifting the co-polymer composition from
24
one rich in vicinal diol groups to one rich in side chains with a single hydroxyl group drove an
25
increase in the water/salt permeability selectivity, which is directly related to desalination-critical
26
salt rejection. The results suggest that controlled placement of functional groups within a
27
polymer could be a viable strategy for preparing advanced and highly selective desalination
28
membrane materials.
2 ACS Paragon Plus Environment
Page 3 of 20
30 31
Environmental Science & Technology Letters
Introduction Meeting global demand for clean water is a pressing engineering challenge.1 Membrane-
32
based technologies are used widely to desalinate water, and more efficient membranes are
33
needed to meet growing demand for purified water from increasingly contaminated and/or saline
34
water.2-6 Water purification membranes are generally polymeric, and highly selective membranes
35
are needed to meet the growing separation challenges in desalination technologies including
36
reverse osmosis and nanofiltration.6-10 The selective properties of these desalination membranes
37
result from the preferential transport of water relative to hydrated ions.11 Engineering highly
38
selective materials has been frustrated by a lack of fundamental structure-property relationships
39
to guide the design of advanced polymer membranes to address global water needs.
40
The ability to precisely place functional groups along a polymer backbone, with a high
41
degree of molecular control over functional group position, is becoming increasingly viable.12, 13
42
This structural control, facilitated by advances in synthesis capabilities, could be important for
43
engineering advanced desalination membranes. Little is known, however, about how functional
44
group placement within a polymer influences water/salt selectivity.
45
We report the desalination (i.e., water/salt) selectivity properties of a series of model
46
materials where the distribution of hydroxyl functional groups along the polymer backbone was
47
varied from a clustered to a more uniform configuration. Importantly, this series of materials was
48
prepared such that the membrane water content did not change as the distribution of the hydroxyl
49
groups changed. Membrane water content has a significant influence on water/salt selectivity
50
properties,9, 14 so the ability to vary the distribution of functional groups within the material
51
without perturbing the water content enabled us to ascribe the observed results to changes in the
52
functional group configuration.
3 ACS Paragon Plus Environment
Environmental Science & Technology Letters
53
Page 4 of 20
Shifting the functional group configuration to space the hydroxyl groups out more evenly
54
(compared to the more clustered configuration) resulted in increased water/salt permeability
55
selectivity, which is directly related to desalination-critical salt rejection.11 This increase in
56
selectivity was largely driven by sorption, or thermodynamic, effects. The results suggest that an
57
even distribution of hydrophilic chemical functionality in polymers may lead to more selective
58
membranes to address global demand for desalinated water.
59 60
Materials and Methods
61
HEMA:GMA:GMAOH Co-Polymers
62
Five co-polymers containing 2-hydroxyethyl methacrylate (HEMA), glycidyl methacrylate
63
(GMA), and glycerol methacrylate (GMAOH) co-monomers and cross-linked using
64
poly(ethylene glycol) dimethacylate (PEGDMA) were prepared via UV-initiated free radical
65
polymerization. These model materials are chemically different from the aromatic polyamide-
66
based membranes commonly used in reverse osmosis and nanofiltration membranes.15 The
67
structure of these materials (i.e., the choice of co-monomers) was chosen to enable preparation of
68
a series of materials where the distribution of hydrophilic functional groups (in this case,
69
hydroxyl groups) could be varied by changing the composition of the co-polymer without
70
changing the water content of the polymer.
71
The hydroxyl group distribution in the co-polymer was varied by changing co-monomer
72
composition of the pre-polymerization solution used to prepare the materials (Table 1). The
73
sample nomenclature, HEMA:GMA:GMAOH x:y:z, reflects the composition (by mass) of the
74
pre-polymerization solution used to prepare that material (e.g., 15:55:30 was prepared from a
75
pre-polymerization solution containing 15%, 55%, and 30%, by mass, of HEMA, GMA, and
4 ACS Paragon Plus Environment
Page 5 of 20
Environmental Science & Technology Letters
76
GMAOH, respectively). The PEGDMA cross-linker and initiator (1-hydroxycyclohexyl phenyl
77
ketone) were added to the co-monomer solution at compositions of 0.1 and 0.01 g/g(total co-
78
monomer), respectively, to form the pre-polymerization solution, which was stirred for 30 min at
79
room temperature to ensure the solution was well-mixed. The pre-polymerization solution was
80
degassed (via sonication for 10 min), confined between quartz plates (separated by spacers to
81
control film thickness), and irradiated, for 5 min, with 120 µJ/cm2 of 312 nm light to form
82
transparent and homogeneous polymer films that were approximately 100 m thick. The
83
chemical structure and additional information about the co-polymers are reported in Section S1
84
of the Supporting Information.
85 86
Table 1. Co-monomer mass fraction in the pre-polymerization solution used to prepare the
87
HEMA:GMA:GMAOH co-polymers defines the sample nomenclature. The water uptake and dry
88
density data were measured at ambient temperature, and the water sorption coefficient, 𝐾𝑤, was
89
calculated using the water uptake and dry polymer density. Uncertainty in the measured data is
90
one standard deviation from the mean of at least three measurements, and the uncertainty in the
91
water sorption coefficient was calculated using standard propagation of error.16 CoPolymer Sample
92
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
Environmental Science & Technology Letters
Page 6 of 20
97
properties at ambient temperature. Water uptake was combined with dry polymer density to
98
calculate the water sorption coefficient, 𝐾𝑤 (Table 1).17 Water (i w) and salt (i s) sorption
99
coefficients, 𝐾𝑖, were combined with permeability properties, 𝑃𝑖, to calculate, via the solution-
100
diffusion model,18 apparent diffusion coefficients, 𝐷𝑖, as:19
101
𝐷 𝑖 = 𝑃𝑖 𝐾 𝑖
102
At least three measurements, per sample, were made for each property, and the uncertainty in the
103
measurement was one standard deviation from the mean. Standard propagation of error was used
104
to quantify the uncertainty in calculated quantities.16 Details about the specific methods used to
105
measure water uptake, salt sorption, pure water permeability, salt permeability, and hydrated
106
polymer dielectric properties are reported in Section S2 of the Supporting Information.
(1)
107 108 109
Results and Discussion Each composition of HEMA:GMA:GMAOH had equivalent water content (Table 1). Water
110
content affects the transport (and, thus, desalination) selectivity of hydrated polymers, and
111
increases in water content often correlate with decreases in selectivity.9, 14 Therefore, preparing
112
materials with equivalent water content is critical for decoupling the influence of functional
113
group configuration on water/salt transport selectivity from the influence of changing water
114
content on water/salt transport selectivity.
115
The equivalent water content of this series of HEMA:GMA:GMAOH co-polymers likely
116
results from subtle changes in the extent of cross-linking and hydroxyl group content across the
117
series of co-polymers. These changes were estimated to be reasonably small, as discussed in
118
Section S1.1 of the Supporting Information. Ultimately, maintaining equivalent water content in
6 ACS Paragon Plus Environment
Page 7 of 20
Environmental Science & Technology Letters
119
the series of materials was prioritized due to the strong influence of water content on the water
120
and salt transport properties of hydrated polymers.9, 14
121
We compared the HEMA:GMA:GMAOH transport properties to those of cross-linked
122
poly(ethylene glycol) diacrylate (PEG) hydrogels, as these materials also preferentially transport
123
water over salt via a solution-diffusion mechanism.20 The water content of these hydrogels can
124
be manipulated by changing the ethylene glycol chain length or by adding a diluent to the pre-
125
polymerization mixture.20 Thus, the PEG materials illustrate how transport properties change
126
with water content to contrast the situation in equivalent water content HEMA:GMA:GMAOH.
127
The water/salt transport selectivity is defined as the ratio of water to salt permeability, 𝑃𝑤 𝑃𝑠,
128
sorption 𝐾𝑤 𝐾𝑠, or diffusion, 𝐷𝑤 𝐷𝑠, coefficients, and the three selectivity values are related via
129
the solution-diffusion model:9
130
𝑃𝑤 𝑃 = (𝐾𝑤 𝐾 ) × (𝐷𝑤 𝐷 ) 𝑠 𝑠 𝑠
131
This water/salt permeability selectivity can be directly related to salt rejection, which is a critical
132
characteristic of effective desalination membranes.11 The salt rejection, R, is defined as the salt
133
concentration reduction from the bulk solution on the feed side of the membrane, 𝑐𝑠,𝑓𝑒𝑒𝑑, to the
134
bulk solution on the permeate (or product) side of the membrane normalized by 𝑐𝑠,𝑓𝑒𝑒𝑑 and
135
depends on the water/salt permeability selectivity: 𝑃𝑤𝑉𝑤
136
𝐑=
𝑐𝑠,𝑓𝑒𝑒𝑑 ― 𝑐𝑠,𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒 𝑐𝑠,𝑓𝑒𝑒𝑑
=
𝑃𝑠 𝑅𝑇 (∆𝑝
1+
𝑃𝑤𝑉𝑤
― ∆𝜋)
𝑃𝑠 𝑅𝑇 (∆𝑝
(2)
(3)
― ∆𝜋)
137
where 𝑉𝑤 is the molar volume of water, ∆𝑝 is the hydraulic pressure difference across the
138
membrane, ∆𝜋 is the osmotic pressure difference across the membrane, R is the gas constant, and
139
T is the absolute temperature.9
7 ACS Paragon Plus Environment
Environmental Science & Technology Letters
Page 8 of 20
140
Water/salt permeability selectivity depends on a combination of both water/salt sorption
141
and diffusivity selectivity properties (Equation 2). Functional group configuration significantly
142
influenced the water/salt sorption selectivity of HEMA:GMA:GMAOH (Figure 1A). A
143
distributed hydroxyl group configuration (HEMA-rich co-polymer) led to higher sorption
144
selectivity compared to a vicinal-diol rich configuration (GMAOH-rich co-polymer), and this
145
result can be attributed to more effective exclusion of salt from the HEMA-rich co-polymer
146
compared to the GMAOH-rich co-polymer.
147
A 0
B
148 149
Figure 1. Water/salt sorption selectivity (A) and water/salt diffusivity selectivity (B) as a
150
function of water sorption coefficient and apparent water diffusion coefficient, respectively, for
151
HEMA:GMA:GMAOH ( ) and PEG ( )20 materials. For HEMA:GMA:GMAOH, the HEMA co-
152
monomer composition of the pre-polymerization solution used to prepare the co-polymer is
153
reported for each data point. The dashed lines represent reported empirical sorption (left) and
154
diffusivity (right) trade-off frontiers for desalination membrane materials,21, 22 and the solid lines
155
are least-squares fits to the data.
156
8 ACS Paragon Plus Environment
Page 9 of 20
157
Environmental Science & Technology Letters
Comparing HEMA:GMA:GMAOH water/salt sorption selectivity with that of
158
poly(ethylene glycol) diacrylate (PEG) hydrogels, we observed that the HEMA:GMA:GMAOH
159
data points move vertically upwards as HEMA content increases in a manner different from the
160
PEG materials (Figure 1A). The PEG result is expected if water content primarily drives sorption
161
selectivity properties.20 The HEMA:GMA:GMAOH result suggests that polymer chemistry, not
162
changing water content, is responsible for the change in sorption selectivity of
163
HEMA:GMA:GMAOH. Thus, preparing polymers with a distributed functional group
164
configuration may be a viable strategy to increase the water/salt sorption selectivity.
165
The HEMA:GMA:GMAOH materials are approximately an order of magnitude more
166
water/salt diffusion selective compared to the PEG materials (Figure 1B). This result may be due
167
to the glassy nature of hydrated HEMA:GMA:GMAOH compared to the rubbery nature of PEG
168
(see Section S1.3 of the Supporting Information).23 More rigid (glassy) polymer backbones are
169
often more diffusion selective compared to more flexible (rubbery) polymer backbones.24-26
170
The apparent water diffusion coefficient decreases and the water/salt diffusivity selectivity
171
increases as the co-polymer composition shifts toward a more HEMA rich (or distributed
172
hydroxyl group configuration). This observed trend is similar to that observed for PEG. The
173
water content of the PEG materials was varied systematically, and the explanation for the
174
observed relationship between water/salt diffusivity selectivity and water diffusion coefficient is
175
that reduction in the free volume of the polymer (as water content decreases) causes a general
176
reduction in diffusion that influences the larger hydrated ions to a greater extent than water.20 In
177
HEMA:GMA:GMAOH, the change in co-monomer composition (and thus, distribution of
178
hydroxyl groups) results in a situation where water diffusion slows as HEMA content increases.
179
This reduction in water diffusivity is accompanied by an increase in water/salt diffusion
9 ACS Paragon Plus Environment
Environmental Science & Technology Letters
Page 10 of 20
180
selectivity that is similar in proportion to that of PEG. Therefore, changes in water diffusivity,
181
due to compositional changes in the HEMA:GMA:GMAOH materials, may influence water/salt
182
diffusivity selectivity in a manner similar to that in the free volume-based explanation where
183
water diffusion depends strongly on free volume.9, 14
184
Water/salt diffusivity selectivity is affected to a lesser extent, compared to water/salt sorption
185
selectivity, by changing the HEMA content of the pre-polymerization solution. The water/salt
186
diffusivity selectivity increases by 26% over the range of HEMA compositions considered. The
187
water/salt sorption selectivity, by comparison, increases by 53% over the range of HEMA
188
compositions considered. Importantly, the 53% increase in sorption selectivity comes without a
189
change in the water sorption coefficient, which as discussed subsequently, means that, as co-
190
polymer composition changes, the water permeability will not change because of sorption
191
effects.
192
The decrease in the apparent water diffusivity as the co-polymer composition changes at
193
constant water content could be considered unexpected, as diffusivity in hydrated polymers is
194
often a strong function of water content.9, 14, 27-29 Dielectric permittivity properties, however,
195
provide insight into the observed water diffusivity. As further discussed in Section S3 of the
196
Supporting Information, the dielectric loss spectra can be interpreted in terms of time constants,
197
which describe the dipole relaxation dynamics of different modes of water motion, and the
198
dielectric strengths of those relaxations, which describe the relative amount of water in the
199
material that is participating in each relaxation mode.30
200
Dielectric spectroscopy suggests three populations of dipolar water motion: highly restricted
201
motion that relaxes on an order 0.1 ns timescale, less restricted motion that relaxes with a time
202
constant of approximately 45 ps, and non-restricted (i.e., bulk water) motion that relaxes with a
10 ACS Paragon Plus Environment
Page 11 of 20
Environmental Science & Technology Letters
203
time constant of 8.8 ps (see Supporting Information for additional discussion). In general, the
204
dielectric strength associated with each mode of motion decreases as HEMA content increases
205
(Table S1), though the reduction is more pronounced for the non-restricted relaxation mode
206
compared to the other two modes. This decrease in the dielectric strength associated with all
207
three relaxation modes, coupled with the statistically equivalent water content of the materials,
208
suggests that increasing HEMA content promotes water-polymer interactions that relax at
209
frequencies lower than that probed in our experiments.
210
An example of such interactions would be water that is very tightly associated with the
211
hydroxyl groups. Such (effectively immobile) water will not be detected by dielectric
212
spectroscopy in the frequency range considered here.31 As such, the dielectric strength data
213
suggests that distributing the hydroxyl groups in the polymer causes more water to tightly
214
associate with hydroxyl groups compared to the situation in the vicinal diol-rich materials.
215
This molecular picture is consistent with a steric explanation suggesting that water molecules
216
may be able to hydrate the hydroxyl group on a HEMA side chain to a greater extent compared
217
to the more sterically hindered GMAOH side chain. This explanation is also consistent with
218
estimates of the enthalpy of hydration for the hydroxyl groups on the side chains (see Section S4
219
of the Supporting Information). Ultimately, this reduction in water motion within the polymer
220
appears to have a similar effect on both water and salt diffusion as reducing the water content of
221
the polymer (as supported by the similar slopes of the data in Figure 1B).
222
Primarily due to the strong increase in water/salt sorption selectivity at constant water
223
content, the HEMA:GMA:GMAOH water/salt permeability selectivity increases as the HEMA
224
content of the pre-polymerization solution used to prepare the materials increases (Figure 2). A
225
distributed hydroxyl group configuration (HEMA-rich) is more selective for water over salt
11 ACS Paragon Plus Environment
Environmental Science & Technology Letters
Page 12 of 20
226
transport compared to the vicinal-diol rich configuration (GMAOH-rich). The overall selectivity
227
of the HEMA:GMA:GMAOH series of materials is greater than that of the PEG materials due to
228
the previously discussed differences in the diffusivity selectivity properties.
229
230 231 232 233 234 235 236 237
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.
238
The PEG materials exhibit a typical tradeoff relationship whereby water/salt selectivity tends
239
to decrease as materials become more permeable to water. This tradeoff is often observed in cases
12 ACS Paragon Plus Environment
Page 13 of 20
Environmental Science & Technology Letters
240
where the water content of a series of materials is varied systematically.22 In these cases, the higher
241
water content polymers tend to have higher water permeability and lower water/salt selectivity
242
compared to the lower water content polymers.
243
The HEMA:GMA:GMAOH materials suffer less of a reduction in the diffusive water
244
permeability as water/salt selectivity increases compared to the PEG materials (i.e., the slope of
245
the HEMA:GMA:GMAOH dashed line is steeper than that of the PEG dashed line in Figure 2).
246
This result stems from the equivalent water content nature of the HEMA:GMA:GMAOH series of
247
materials and the fact that both water/salt sorption and diffusivity selectivity increase with
248
increasing content of HEMA in the pre-polymerization solution.
249
Chemical modification of a series of water content equivalent co-polymers, from a vicinal
250
diol-rich to a distributed hydroxyl group-rich configuration, increased water/salt permeability
251
selectivity with a smaller water permeability penalty compared to that often observed in hydrated
252
polymers. These results, obtained using a model series of co-polymers, suggest that controlling
253
the spatial arrangement of functional groups in hydrated membrane materials may be important
254
for engineering highly selective polymers for desalination applications. The results on these
255
model materials represent a step toward establishing general water and salt transport structure-
256
property relationships for membrane materials including polymers that are more chemically
257
similar to commercial desalination membranes than those materials considered here. The results
258
suggest that distributed hydrophilic functional groups may lead to increased selectivity and may
259
represent a strategy for improving water/salt selectivity of advanced membrane materials to
260
address global water shortages.
261
13 ACS Paragon Plus Environment
Environmental Science & Technology Letters
Page 14 of 20
262
ASSOCIATED CONTENT
263
Supporting Information. Additional information regarding experimental methods, material
264
properties and characterization (molar composition, hydroxyl group content, FT-IR, and glass
265
transition temperature analysis), microwave dielectric spectroscopy data analysis, and hydroxyl
266
group hydration analysis. This material is available free of charge via the Internet at
267
http://pubs.acs.org.
268
AUTHOR INFORMATION
269
Corresponding Author
270
*102 Engineers’ Way, P.O. Box 400741, Charlottesville, VA 22904 USA,
[email protected] 271
Funding Sources
272
This material is based upon work supported in part by the National Science Foundation (NSF)
273
Faculty Early Career Development (CAREER) Program (CBET-1752048).
274
Notes
275
The authors declare no competing financial interest.
276
REFERENCES
277 278
1.
Mekonnen, M. M.; Hoekstra, A. Y., Four billion people facing severe water scarcity. Sci. Adv. 2016, 2, e1500323.
279 280 281
2.
Tang, C. Y.; Yang, Z.; Guo, H.; Wen, J. J.; Nghiem, L. D.; Cornelissen, E., Potable water reuse through advanced membrane technology. Environ. Sci. Technol. 2018, 52, 1021510223.
282 283
3.
Miller, S.; Shemer, H.; Semiat, R., Energy and environmental issues in desalination. Desalination 2015, 366, 2-8.
284 285
4.
Subramani, A.; Jacangelo, J. G., Emerging desalination technologies for water treatment: A critical review. Water Res. 2015, 75, 164-187. 14 ACS Paragon Plus Environment
Page 15 of 20
Environmental Science & Technology Letters
286 287 288
5.
Amy, G.; Ghaffour, N.; Li, Z.; Francis, L.; Linares, R. V.; Missimer, T.; Lattemann, S., Membrane-based seawater desalination: Present and future prospects. Desalination 2017, 401, 16-21.
289 290
6.
Cohen, Y.; Semiat, R.; Rahardianto, A., A perspective on reverse osmosis water desalination: Quest for sustainability. AlChE J. 2017, 63, 1771-1784.
291 292 293
7.
Werber, J. R.; Deshmukh, A.; Elimelech, M., The critical need for increased selectivity, not increased water permeability, for desalination membranes. Environ. Sci. Technol. Lett. 2016, 3, 112-120.
294 295 296
8.
Park, H. B.; Kamcev, J.; Robeson, L. M.; Elimelech, M.; Freeman, B. D., Maximizing the right stuff: The trade-off between membrane permeability and selectivity. Science 2017, 356, 1-10.
297 298
9.
Geise, G. M.; Paul, D. R.; Freeman, B. D., Fundamental water and salt transport properties of polymeric materials. Prog. Polym. Sci. 2014, 39, 1-42.
299 300 301
10.
Geise, G. M.; Lee, H.-S.; Miller, D. J.; Freeman, B. D.; McGrath, J. E.; Paul, D. R., Water purification by membranes: The role of polymer science. J. Polym. Sci., Part B: Polym. Phys. 2010, 48, 1685-1718.
302
11.
Baker, R. W., Membrane Technology and Applications. 3rd ed.; Wiley: New York, 2012.
303 304 305
12.
Solleder, S. C.; Zengel, D.; Wetzel, K. S.; Meier, M. A. R., A scalable and high-yield strategy for the synthesis of sequence-defined macromolecules. Angew. Chem. Int. Ed. 2016, 55, 1204-1207.
306 307
13.
Lutz, J.-F., Defining the field of sequence-controlled polymers. Macromol. Rapid Commun. 2017, 38, 1700582.
308 309 310
14.
Yasuda, H.; Lamaze, C. E.; Ikenberry, L. D., Permeability of solutes through hydrated polymer membranes Part I. Diffusion of sodium chloride. Die Makromolekulare Chemie 1968, 118, 19-35.
311 312
15.
Petersen, R. J., Composite reverse osmosis and nanofiltration membranes. J. Membr. Sci. 1993, 83, 81-150.
313 314
16.
Bevington, P. R.; Robinson, D. K., Chapter 4: Propagation of errors. In Data reduction and error analysis for the physical sciences, McGraw Hill: New York, 2003; pp 56-65.
315 316
17.
Chang, K.; Luo, H.; Geise, G. M., Water content, relative permittivity, and ion sorption properties of polymers for membrane desalination. J. Membr. Sci. 2019, 574, 24-32.
317 318 319 320
18.
Wijmans, J. G.; Baker, R. W., The solution-diffusion model: A unified approach to membrane permeation. In Materials Science of Membranes for Gas and Vapor Separation, Yampolskii, Y. P.; Pinnau, I.; Freeman, B. D., Eds. John Wiley & Sons, Ltd: West Sussex, 2006; pp 159-189. 15 ACS Paragon Plus Environment
Environmental Science & Technology Letters
Page 16 of 20
321 322
19.
Geise, G. M.; Freeman, B. D.; Paul, D. R., Sodium chloride diffusion in sulfonated polymers for membrane applications. J. Membr. Sci. 2013, 427, 186-196.
323 324 325
20.
Ju, H.; Sagle, A. C.; Freeman, B. D.; Mardel, J. I.; Hill, A. J., Characterization of sodium chloride and water transport in crosslinked poly(ethylene oxide) hydrogels. J. Membr. Sci. 2010, 358, 131-141.
326 327
21.
Zhang, H.; Geise, G. M., Modeling the water permeability and water/salt selectivity tradeoff in polymer membranes. J. Membr. Sci. 2016, 520, 790-800.
328 329 330
22.
Geise, G. M.; Park, H. B.; Sagle, A. C.; Freeman, B. D.; McGrath, J. E., Water permeability and water/salt selectivity tradeoff in polymers for desalination. J. Membr. Sci. 2011, 369, 130-138.
331 332 333
23.
Ju, H.; McCloskey, B. D.; Sagle, A. C.; Kusuma, V. A.; Freeman, B. D., Preparation and characterization of crosslinked poly(ethylene glycol) diacrylate hydrogels as foulingresistant membrane coating materials. J. Membr. Sci. 2009, 330, 180-188.
334 335 336
24.
Chang, K.; Korovich, A.; Xue, T.; Morris, W. A.; Madsen, L. A.; Geise, G. M., Influence of rubbery versus glassy backbone dynamics on multiscale transport in polymer membranes. Macromolecules 2018, 51, 9222-9233.
337 338
25.
Chang, K.; Xue, T.; Geise, G. M., Increasing salt size selectivity in low water content polymers via polymer backbone dynamics. J. Membr. Sci. 2018, 552, 43-50.
339 340
26.
Paul, D. R.; Yampol'skii, Y. P., Polymeric Gas Separation Membranes. CRC Press: Boca Raton, 1994.
341 342 343
27.
Yasuda, H.; Ikenberry, L. D.; Lamaze, C. E., Permeability of solutes through hydrated polymer membranes Part II. Permeability of water soluble organic solutes. Die Makromolekulare Chemie 1969, 125, 108-118.
344 345
28.
Mackie, J. S.; Meares, P., The diffusion of electrolytes in a cation-exchange resin membrane. I. Theoretical. Proc. R. Soc. London, Ser. A 1955, 232, 498-509.
346 347
29.
Meares, P., The mechanism of water transport in membranes. Philos. Trans. R. Soc., B 1977, 278, 113-150.
348 349 350
30.
Lu, Z.; Polizos, G.; Macdonald, D. D.; Manias, E., State of water in perfluorosulfonic ionomer (Nafion® 117) proton exchange membranes. J. Electrochem. Soc. 2008, 155, B163-B171.
351 352
31.
Smith, G.; Duffy, A. P.; Shen, J.; Olliff, C. J., Dielectric relaxation spectroscopy and some applications in the pharmaceutical sciences. J. Pharm. Sci. 1995, 84, 1029-1044.
353
16 ACS Paragon Plus Environment
Page 17 of 20
Environmental Science & Technology Letters
Table of Contents (TOC) Graphic 47x39mm (300 x 300 DPI)
ACS Paragon Plus Environment
Environmental Science & Technology Letters
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)
ACS Paragon Plus Environment
Page 18 of 20
Page 19 of 20
Environmental Science & Technology Letters
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)
ACS Paragon Plus Environment
Environmental Science & Technology Letters
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)
ACS Paragon Plus Environment
Page 20 of 20