Robust Thermoresponsive Polymer Composite Membrane with

Dec 25, 2015 - Citation data is made available by participants in CrossRef's Cited-by Linking service. For a more comprehensive list of citations to t...
2 downloads 16 Views 4MB Size
Subscriber access provided by Weizmann Institute of Science

Article

Robust thermo-responsive polymer composite membrane with switchable superhydrophilicity and superhydrophobicity for efficient oil-water separation Ranwen Ou, Jing Wei, Lei Jiang, George P. Simon, and Huanting Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03418 • Publication Date (Web): 25 Dec 2015 Downloaded from http://pubs.acs.org on December 27, 2015

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 free 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 accessible to all readers and 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.

Environmental Science & Technology 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 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Environmental Science & Technology

Robust thermo-responsive polymer composite membrane with switchable superhydrophilicity and superhydrophobicity for efficient oil-water separation Ranwen Ou, Jing Wei, Lei Jiang, George P. Simon, and Huanting Wang* R. Ou, Dr. J. Wei, Prof. H. T. Wang Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia Email: [email protected] Prof. G. P. Simon Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia Prof. L. Jiang Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia Keywords: polymer membrane, PNIPAM hydrogel, thermo-responsiveness, superhydrophilicity and superhydrophobicity, oil-water separation Abstract

22

In this paper, we report for the first time on the fabrication of a robust, thermo-responsive

23

polymer membrane produced by the combination of an elastic polyurethane (TPU) microfiber

24

web and poly (N-isopropylacrylamide) (PNIPAM). PNIPAM hydrogel is evenly coated on the

25

surface of TPU microfibers, thus, the wettability of TPU-PNIPAM membrane are amplified

26

by taking advantage of the hierarchical structure and increased surface roughness. The TPU-

27

PNIPAM membrane possesses switchable superhydrophilicity and superhydrophobicity as the

28

temperature of membrane changes from 25 to 45 °C. The composite membrane is shown

29

successfully able to separate a 1 wt. % oil-in-water emulsion and 1 wt. % water-in-oil

30

emulsion at 25 and 45 °C, respectively, with a high separation efficiency of ≥ 99.26 %.

31

Furthermore the composite membranes show excellent mechanical properties, and they are

32

highly flexible and mechanically tough. The smart composite membranes reported here have

33

shown great potential for further development for practical high-efficiency oil-water

34

separations.

1 ACS Paragon Plus Environment

Environmental Science & Technology

Page 2 of 25

35 36 37

Abstract art 1. Introduction

38

The ability to separate oil-water mixtures has recently attracted worldwide attention

39

because of increasing amounts of oily wastewater, frequent oil spill accidents and the ongoing

40

development of petroleum industry.

41

separated using conventional separation techniques, such as gravity separation and oil-

42

absorbing materials. In the case of the separation of oil-water emulsions, the conventional

43

separation processes (air flotation, coagulation and flocculation) are costly, energy intensive

44

and are not effective.

45

membranes, have been successfully used for the separation of oil-water emulsions. However,

46

the size-sieving mechanism of UF membrane limits the practicality of this process, due to the

47

resultant high energy consumption, low flux and rapid decrease in permeability with usage. 6-

48

10

49

separation, including polymer-coated copper meshes, hydrogel-coated copper meshes,

50

surface-modified copper meshes and polymer networks,

51

chemistry and roughness.

52

membrane is suitable for separating oil-in-water emulsions, while a superhydrophobic /

53

superoleophilic membrane separates water-in-oil emulsions.

54

process with a superwetting membrane has been proven to be energy-efficient and cost-

55

effective, as the membrane separates the oil-water mixture or oil-water emulsion according to 2

3-5

1, 2

In general, immiscible oil-water mixtures are

Pressure-driven membranes, especially ultrafiltration (UF)

Recently, a series of superwetting membranes have been produced for efficient oil-water

17

11-16

, all taking advantage of surface

It has been shown that a superhydrophilic / superoleophobic

16, 18

ACS Paragon Plus Environment

The oil-water separation

Page 3 of 25

Environmental Science & Technology

56

the different interfacial effects of oil and water on the superwetting surface. For example, the

57

superwetting membrane prepared by Kota et al. allowed for the removal of dispersed-phase

58

droplets that were considerably smaller than the membrane pore size.

59

able to prepare a responsive membrane with switchable superwettability, not only for

60

controllable oil-water separation, but also for fabricating intelligent materials. Stimuli-

61

responsive surfaces have been used to prepare smart membrane for some years, but to date

62

there have been only a few reports on responsive membranes with switchable underwater

63

superoleophilicity and superoleophobicity, and fewer membranes with switchable

64

superhydrophilicity and superhydrophobicity for oil-water separation. In particular, the

65

membranes with such switchable underwater superoleophilicity and superoleophobicity are

66

pH-sensitive;

67

superhydrophobicity are photo-induced or pH-responsive. For instance, the aligned ZnO

68

nanorod arrays-coated mesh film was fabricated by Tian et al., which was switchable between

69

superhydrophilicity and superhydrophobicity, driven by photo-stimulation.

70

membranes with switchable superhydrophilicity and superhydrophobicity for oil-water

71

separation are pH-responsive and can be controlled using solutions with different pH values.

72

25, 26

19-23

12

It is important to be

whereas the membranes with switchable superhydrophilicity and

24

The other

73

Although some responsive membranes exhibit stimuli-responsive ability, they are not

74

capable of switching between superwettabilities. For example, a PDMAEMA hydrogel-coated

75

copper mesh has been reported which was both thermo- and pH-responsive; however, it is

76

solely underwater superoleophobic without switchable superwettability. 27 A PVCL-co-PSF

77

copolymer membrane shows thermo-responsive, but it is hydrophilic with a contact angle of

78

49.5 °C at room temperature.

79

superwetting membranes published in recent years. For example, single-walled carbon

80

nanotube network films and ammonia-modified PVDF membrane were found to be able to

81

separate 99 % oil-water emulsion, and a PAA-g-PVDF membrane was able to efficiently

28

Table S1 (Supporting Information) shows some attractive

3 ACS Paragon Plus Environment

Environmental Science & Technology

2, 16

82

separate a 1 % oil-water emulsion;

83

could separate oil-in-water and water-in-oil emulsions.

84

membranes with a switchable superwettability were applied to separate ≥ 10 % oil-water

85

mixtures, which are not emulsions.

what’s more, a superamphiphilic PVDF membrane 29

However, the responsive

86

Apart from oil-water separation properties, the mechanical properties and durability of

87

membranes are other key factors important for practical application. 2 For instance, polymeric

88

UF membranes are mainly prepared via phase inversion; however, the porous structure

89

formed in the phase inversion process greatly weakens the mechanical properties of the

90

membranes, and thus restricts their applicability. To meet the mechanical requirements for

91

practical ultrafiltration, support materials (glass, metal, polymer, non-woven fabrics) have

92

been applied to improve their mechanical property, although it may decrease the permeation

93

flux. 8, 10, 30, 31

94

Here, we report for the first time a robust, thermo-responsive poly (N-isopropylacrylamide)

95

(PNIPAM) hydrogel-coated thermoplastic polyurethane (TPU) microfiber membrane for

96

efficient oil-water emulsion separation. The membrane is superhydrophilic at room

97

temperature (25 °C) and becomes superhydrophobic at a temperature over the lower critical

98

solution temperature of PNIPAM (LCST, 32°C). TPU was selected for its high elongation,

99

tensile strength, toughness, wear resistance and good flexibility over a wide temperature range.

100

PNIPAM is a thermally responsive polymer that switches between hydrophilicity and

101

hydrophobicity at temperatures lower and higher than its LCST. The force-spinning method

102

was used to produce a non-woven TPU microfiber mesh or web, and is a fast and productive

103

technique to produce a micro- or nanoscale fiber mats in less than a minute. It works by

104

producing a jet of polymer in solution due to a high-speed, rotating nozzle and produces the

105

fiber mesh without the need for a high-voltage power source and the limitation of solution

106

conductivity, as is required for example, by electrospinning. 32-35

4 ACS Paragon Plus Environment

Page 4 of 25

Page 5 of 25

Environmental Science & Technology

107

The TPU-PNIPAM composite membrane has excellent mechanical strength, elasticity and

108

switchable superhydrophilicity and superhydrophobicity, and shows a high efficiency in

109

separating SDS-stabilized oil-in-water and water-in-oil micro-emulsions, but not mixtures, at

110

room temperature and at an elevated temperature, respectively.

111

2. Materials and Methods

112

Materials

113

The following chemicals were used as supplied: thermoplastic polyurethane (TPU,

114

TETON*206) particles were kindly provided by Urethane Compounds Pty. Ltd.

115

Tetrahydrofuran (THF) was purchased from Merck (Kenilworth, NJ, USA). N-isopropyl

116

acrylamide (NIPAM), N, N-methylenebis (acrylamide) (MBA), and ammonium persulfate

117

(APS) were purchased from Sigma-Aldrich (St Louis, MO, USA). Distilled deionized (DDI)

118

water was used in all experiments.

119

Experimental Procedures

120

Fabrication of TPU-PNIPAM membrane: Thermoplastic polyurethane particles (12 g) were

121

dissolved in tetrahydrofuran (THF, 88 g) under stirring to obtain a 12 wt. % polymer solution.

122

N-isopropylacrylamide (NIPAM, 2g, monomer), N, N’-methylenebisacrylamide (MBA, 0.04

123

g, crosslinker) and ammonium persulfate (APS, 0.02 g, initiator) were dissolved in DDI water

124

to form a 16.67 wt. % NIPAM reaction solution. As shown in Figure S1 (Supporting

125

Information), the polymer solution was spun into TPU microfiber mat by a force spinner

126

(Cyclone L-1000M, FibeRio® Technology Corporation, USA), with an operating condition of

127

5000 rpm and 30 seconds. Five microfiber mats were obtained and stacked up to form a TPU

128

microfiber membrane. The as-prepared TPU microfiber membrane was then cut into a certain

129

shape before wetting with NIPAM reaction solution. Free-radical polymerization of the

130

NIPAM monomer solution was applied to coat the PNIPAM hydrogel on the TPU

131

microfibers’ surface. After 2 hours reaction at 70 °C, TPU-PNIPAM membranes were

132

obtained. The PNIPAM hydrogel loading of TPU-PNIPAM membranes was controlled by 5 ACS Paragon Plus Environment

Environmental Science & Technology

133

changing the amount of NIPAM reaction solution added to the TPU microfiber membranes.

134

The TPU-PNIPAM membranes were washed with 25 °C water and 45 °C water more than 10

135

times to rinse the excess monomer before use.

136

Instruments and Characterization: Scanning electron microscopy was undertaken with a

137

Nova NanoSEM 450 FESEM, FEI, USA. Optical microscopy images were taken on an

138

Olympus SZX16 Stereo Microscope. The contact angle of the membranes was determined

139

using a contact angle goniometer (Dataphysics OCA15, Dataphysics, Germany). The average

140

value was obtained from more than 5 measurements per sample. The mechanical properties of

141

the membranes were measured using Mini-Instron (Micro Tester 5848, 100 N load cell,

142

Instron Calibration Laboratory, U.K.). The swelling properties of PNIPAM of composite

143

membrane were tested and calculated according to 36, 37. The swelling ratio was calculated

144

based on the PNIPAM hydrogel amount of the composite membrane, but not the composite

145

membrane.

146

Preparation of oil-water emulsions: 1 wt. % silicone oil-water emulsion was prepared by

147

mixing silicone oil and water in a ratio of 1:99 w/w with 1 mg/mL sodium dedecyl sulfate

148

(SDS) under strong magnetic stirring and shaken to obtain a milky solution. 99 wt. % silicone

149

oil-water emulsion was prepared by mixing silicone oil and water in a ratio of 99:1 w/w with

150

1 mg/mL SDS under strong stirring and shaking. Oil-water emulsions with different oils

151

(hexadecane, paraffin, olive oil and fish oil) were prepared in a similar fashion. The SDS-

152

stabilized oil water emulsions were stable more than a month. 16

153

Oil-water separation Experiment: The as-prepared membrane was fixed between two glass

154

tubes with an inner diameter of 12.86 mm. At room temperature, the 1 wt. % silicone oil-

155

water emulsion was poured into the upper tube, and the separation was achieved by gravity

156

only. At 45 °C, the swollen membrane was immersed in the 45 °C water to deswell and the

157

excess water on the membrane surface was removed using tissue paper. The 99 wt. % silicone

6 ACS Paragon Plus Environment

Page 6 of 25

Page 7 of 25

Environmental Science & Technology

158

oil-water emulsion was heated to 45 °C and was poured into the separation setup in an oven

159

set to 45 °C to achieve separation.

160

The oil concentration in collected water and the water concentration in the collected oil

161

after separation were determined using a UV-Vis spectrophotometer (UV mini 1240) by

162

testing the light scattering data. The standard oil-water emulsion and the collected samples

163

were sonicated for 0.5 hour to obtain a homogeneous solution before test. The separation

164

efficiency was calculated by the following equation (1):

S=

165

C 0 − C1 × 100% C0

(1)

166

where C0 (10000 mg/L) is the original oil concentration or water concentration in the feed. C1

167

is the oil or water concentration in permeate after filtration.

168

3. Results and Discussion

169 170

Figure 1 (a) The preparation process of a TPU-PNIPAM composite membrane. The TPU

171

microfiber membrane immersed with NIPAM reaction solution (16.7 wt. %) is polymerized at

172

70 °C for 2 hours. Photographs of twisting test of (b) TPU microfiber membrane and (c) TPU-

173

PNIPAM membrane with 3.6 wt. % PNIPAM loading (TPU-PNIPAM-3.6). (d) Stress-strain

174

curves of TPU microfiber membrane and swollen TPU-PNIPAM-3.6 membrane. 7 ACS Paragon Plus Environment

Environmental Science & Technology

175 176

The TPU microfiber membranes were prepared using force spinning, as shown in Figure S1

177

(Supporting Information). The PNIPAM-coated TPU microfiber (TPU-PNIPAM) membranes

178

were made using a single step free-radical polymerization (Figure 1a). The schematic diagram

179

of the formation of TPU-PNIPAM composite is shown in Figure S3 (Supporting Information).

180

As shown in Figure 1b-c, the TPU-PNIPAM membrane shows excellent mechanical

181

properties, and can be readily stretched and twisted without failure, and shows significant

182

recovery. The composite membrane has significantly enhanced stretchability, as compared

183

with the TPU membrane. In this study, TPU-PNIPAM membranes with different amounts of

184

PNIPAM (1.4, 3.6, 7.6, 13.4, and 25.3 wt. %) were prepared. As an example of the

185

nomenclature used, a membrane contains 1.4 wt. % PNIPAM hydrogel is denoted as TPU-

186

PNIPAM-1.4.

187 188

The TPU microfibers’ diameter is 1-30 µm, and 92 % of them distributes in 1-15 µm (Figure S4, Supporting Information).

189 190

Figure 2 SEM images of TPU microfiber membrane and TPU-PNIPAM composite

191

membranes (3.6 and 13.4 wt. %). Membrane surfaces of (a) TPU microfiber membrane (at a

192

magnification of 558×), (b) TPU-PNIPAM-3.6 (811×) and (c) TPU-PNIPAM-13.4 membrane 8 ACS Paragon Plus Environment

Page 8 of 25

Page 9 of 25

Environmental Science & Technology

193

(683×); microfiber surfaces of (d) TPU microfibers (71202×), (e) TPU-PNIPAM-3.6

194

(67286×) and (f) TPU-PNIPAM-13.4 (35187×); microfiber cross-sections of (g) TPU

195

microfibers (38448×), (h) TPU-PNIPAM-3.6 (43471×) and (i) TPU-PNIPAM-13.4 (2885×).

196 197

Figure 2 shows the SEM images of TPU microfiber membrane, TPU-PNIPAM-3.6 and

198

TPU-PNINPAM-13.4 membrane, which were taken after 10 times of cyclic heating. The

199

SEM images of microfiber surface and cross-section (Figure 2d-i) indicate that PNIPAM was

200

evenly coated on the TPU microfibers, and its surface roughness increased with increasing

201

PNIPAM loading. These membranes show similar morphologies before the cyclic heating, as

202

shown in Figure S5 (Supporting Information). In addition, in the TPU microfiber membrane,

203

the microfibers were loosely packed whilst in the TPU-PNIPAM membrane the microfibers

204

were bound by the PNIPAM hydrogel phase. In the latter instance, the pore size of the TPU-

205

PNIPAM membranes was greatly decreased, as shown in Figure 2a-c, and the pore size was

206

found to decrease with increasing PNIPAM loading (Figure S5). The pore size distributions of

207

dry TPU-PNIPAM-3.6 and TPU-PNIPAM-13.4 membrane were 3-27 µm, and 2-15 µm,

208

respectively. Figure S6 (Supporting Information) shows the thickness of the composite

209

membranes and their PNIPAM hydrogel coating thickness. The PNIPAM coating thickness of

210

TPU-PNIPAM with a hydrogel loading of 1.4, 3.6, 7.6, 13.4, and 25.3 wt. % were 106, 160,

211

227, 3061, and 10124 nm, respectively. With the hydrogel loading increased, the PNIPAM

212

coating thickness increased. The membrane thickness of TPU-PNIPAM with a hydrogel

213

loading of 1.4, 3.6, 7.6, 13.4, and 25.3 wt. % were 1.47, 0.64, 0.63, 0.54, and 1.13 mm,

214

respectively. It decreased with increasing hydrogel loading when the loading amount was

215

lower than 13.4 wt. %. The results of SEM images, PNIPAM coating thickness, and

216

membrane thickness indicated that TPU microfibers were bound by the PNIPAM hydrogel,

217

and increasing space between microfibers was filled by the hydrogel with greater hydrogel

218

loading. However, there was not enough PNIPAM hydrogel in the TPU-PNIPAM-1.4 to bind

219

the TPU microfibers, resulting in a thick membrane and large pores. 9 ACS Paragon Plus Environment

Environmental Science & Technology

Page 10 of 25

220

The mechanical properties of swollen TPU-PNIPAM membranes increase with increasing

221

hydrogel loading initially, however when the hydrogel loading become greater than 3.6 wt. %,

222

it decreases (Figure 3). The elongation and toughness of the dry composite membranes

223

follow a similar trend, and TPU-PNIPAM-3.6 reaches the maximum as well. However, the

224

tensile strength and Young’s modulus of the dry composite membranes show different trends:

225

TPU-PNIPAM-7.6 exhibits the largest tensile strength, and the Young’s modulus of the

226

composite membranes with the 7.6, 13.4 and 25.3 wt. % hydrogel loading are 10 times higher

227

than those of others. For TPU-PNIPAM-1.4 and TPU-PNIPAM-3.6, the microfibers are

228

coated with the hydrogel uniformly; as the hydrogel loading become greater, the hydrogel

229

coating surface becomes rougher, and thicker hydrogel starts to fill up the space between TPU

230

microfibers (Figure 2i). The dry TPU-PNIPAM-3.6 is flexible and stretchable; whereas the

231

dry TPU-PNIPAM-7.6 becomes hard. Since the dry PNIPAM hydrogel is itself hard and

232

brittle, the dry composite membrane with a hydrogel loading higher than 3.6 wt. % also

233

becomes harder and more brittle with increasing hydrogel loading, resulting in decreased

234

tensile strength and elongation, and high Young’s modulus and low toughness. The swollen

235

TPU-PNIPAM-3.6 membrane is flexible and tough and reaches the optimal values of

236

mechanical properties among the composite membranes. The stress-strain curves of TPU

237

microfiber membrane and swollen TPU-PNIPAM-3.6 membrane (Figure 1d) indicate that the

238

mechanical properties of TPU-PNIPAM-3.6 membrane were significantly enhanced. The

239

elongation, tensile strength, Young’s modulus and toughness of TPU-PNIPAM-3.6 are 2.04

240

times, 2.88 times, 1.71 times and 5.70 times of those of TPU microfiber membrane alone

241

(uncoated). The elongation and tensile strength of TPU-PNIPAM-3.6 are as high as 288 %

242

and 3.90 MPa, which are much better than those of the other reported oil water separation

243

membranes. 2, 36, 37 For example, the ammonia modified PVDF oil water separation membrane

244

prepared by Zhang et al. exhibited an elongation of 29.2% and a tensile strength of 2.0 MPa,

245

which showed good flexibility and mechanical stability in the oil water separation process. 10 ACS Paragon Plus Environment

2

Page 11 of 25

Environmental Science & Technology

246

The toughness of dry TPU microfiber membrane is 1321 kJ/m3; however, the toughness of

247

swollen TPU-PNIPAM-3.6 is 7536 kJ/m3, which is comparable with natural rubber (10000

248

kJ/m3). The images in Figure 1b-c show the twisting test of TPU microfiber membrane and

249

TPU-PNIPAM-3.6 membrane. Both two membranes had good flexibility that could not only

250

withstand bending, but also complex twisting, while TPU-PNIPAM-3.6 showed greater

251

elasticity and recovery.

252 253

Figure 3 (a) elongation, (b) tensile strength, (c) Young’s modulus, and (d) toughness of TPU

254

microfiber membrane (0 wt. %) and TPU-PNIPAM membranes (1.4, 3.6, 7.6, 13.4, and 25.3

255

wt. %).

256 257

The water contact angle of TPU thin film was 82.0°, after spinning the TPU into

258

microfibers, the water contact angle of TPU microfiber membrane increased to 148.3° (Figure

259

S7, Supporting Information) because of the changing of surface roughness of TPU. Figure 4

260

shows the wettability of PNIPAM hydrogel and TPU-PNIPAM membranes. As shown in

261

Figure 4a, compared to swollen PNIPAM, the water contact angles of swollen TPU-PNIPAM 11 ACS Paragon Plus Environment

Environmental Science & Technology

Page 12 of 25

262

membranes were lower at room temperature and increased at 45°C, demonstrating that the

263

TPU-PNIPAM-3.6 membrane exhibited switchable superwettability. At room temperature,

264

with increasing hydrogel loading, the contact angles of composite membranes increase;

265

meanwhile the hydrogel fills the space between TPU microfibers instead of coating on the

266

surface alone (Figure S8, Supporting Information), and the membrane roughness decreased,

267

resulting in a similar wettability to PNIPAM hydrogel. At 45 °C, the contact angles of

268

composite membrane initially increases and then decreases with increasing hydrogel loading,

269

while TPU-PNIPAM-3.6 reaches the highest value. Generally, the thermo-responsive

270

property of PNIPAM can be explained by the reversible formation of intermolecular hydrogen

271

bonding between amide groups of PNIPAM chains and water molecules, and intramolecular

272

hydrogen bonding between amide groups of PNIPAM chains below and above the LCST.

273

38, 39

274

molecules, showing hydrophilicity; while at temperatures above LCST, the collapsed

275

PNIPAM chains are wrapped by isopropyl groups, showing hydrophobicity (Figure 4b).

276

Nevertheless, the synergism of integrating TPU microfibers and PNIPAM amplifies these

277

properties to superhydrophilicity and superhydrophobicity. Figure 4c-f show that TPU-

278

PNIPAM-3.6 membrane was superhydrophilic, with a water drop spreading in a timeframe of

279

less than 0.094 s, and underwater demonstrated oleophobic behaviour (141.3°) at room

280

temperature; but became superhydrophobic (150.2°) and underwater superoleophilic, with an

281

oil drop spreading time of 70 s at 45 °C. TPU-PNIPAM-3.6 submerged under water at room

282

temperature, and floated on the surface of water without a water droplet on the membrane

283

surface at 45 °C (Figure S9, Supporting Information).

17,

At temperatures below LCST, the stretching PNIPAM chains are wrapped by the water

12 ACS Paragon Plus Environment

Page 13 of 25

Environmental Science & Technology

284 285

Figure 4 (a) Water contact angles of swollen TPU-PNIPAM membranes (1.4, 3.6, 7.6, 13.4,

286

and 25.3 wt. %) and the swollen PNIPAM hydrogel (100 wt. %) at room temperature (RT)

287

and 45°C. (b) Schematic diagram of switching wettability of TPU-PNIPAM membrane at

288

different temperatures. (c) A water droplet (1µL) spreading on a swollen TPU-PNIPAM-3.6

289

membrane within 0.094 s at room temperature. (d) Water contact angle (150.2°) of swollen

290

TPU-PNIPAM-3.6 membrane at 45 °C. (e) Underwater oil contact angle (141.3°) of swollen

291

TPU-PNIPAM-3.6 membrane at room temperature. (f) Underwater oil contact angle of

292

swollen TPU-PNIPAM-3.6 membrane at 45°C. Hexadecane was used for testing the

293

underwater oil contact angle.

294

13 ACS Paragon Plus Environment

Environmental Science & Technology

Page 14 of 25

295 296

Figure 5 Swelling properties of the PNIPAM of TPU-PNIPAM-3.6 membrane. (a) Swelling

297

curve of dry TPU-PNIPAM-3.6 membrane. (b) Swelling ratio of TPU-PNIPAM-3.6

298

membrane at different temperatures. (c) Deswelling curve of TPU-PNIPAM-3.6 membrane

299

tested at 45 °C DDI water. (d) Reswelling curve of TPU-PNIPAM-3.6 membrane tested at

300

25°C DDI water.

301 302

Apart from the wettability, the degree of swelling also plays an important role in the

303

potential use of the hydrogel. It has been recognized that PNIPAM hydrogels would possess

304

low reswelling and deswelling rates at temperatures below or over its LCST, which is

305

responsible for the formation of a dense, hydrophobic layer at the surface of the hydrogels due

306

to the occurrence of volume-phase transition (VPT) that retards the collective diffusion of

307

water molecules in the swelled and/or deswelled hydrogels.

308

swelling property of the PNIPAM phase of the TPU-PNIPAM-3.6 membrane. It took 400

309

minutes for dry TPU-PNIPAM-3.6 membrane to achieve a swelling equilibrium in DDI water

310

(Figure 5a). Even though the swelling ratio of the PNIPAM was 5.36 g/g, the appearance of

311

the membrane changed little because of the small amount of hydrogel loading. The LCST of

36, 37, 40, 41

14 ACS Paragon Plus Environment

Figure 5 shows the

Page 15 of 25

Environmental Science & Technology

312

TPU-PNIPAM-3.6 membrane is the same as PNIPAM hydrogel, some 32-33 °C (Figure 5b).

313

Importantly, the deswelling and reswelling equilibrium of TPU-PNIPAM-3.6 membrane

314

could be achieved in less than 3 minutes by immersing it into 45 °C and 25 °C DDI water,

315

respectively, as shown in Figure 5c-d. The rapid deswelling and reswelling properties could

316

be contributed to the morphology of hydrogel nanoparticles on the microfibers surface and the

317

consequent large surface area. The excellent mechanical properties, suitable pore sizes,

318

switchable superhydrophilicity and superhydrophobicity, rapid deswelling and reswelling

319

property make clear the appropriate application of these TPU-PNIPAM membranes for

320

separating oil-in-water emulsion and water-in-oil emulsion at room temperature (25 °C) and

321

45 °C, respectively.

322

To study the oil-water separation ability of TPU-PNIPAM membranes, oil intrusion pressure

323

and water intrusion pressure, the relevant water flux and oil flux were all tested, and the

324

results shown in Figure 6a-b. The intrusion pressure indicates the maximum height of oil /

325

water that the TPU-PNIPAM membrane can support at room temperature / 45 °C, which is

326

calculated by the following equation (2):

327

P = ρgh max

328

where ρ is the density of the oil / water, g is gravitational acceleration, and hmax is the

329

maximum height of oil / water the membranes can support. The fluxes were calculated by

330

measuring the volume of liquid permeated through the membranes over a period of time due

331

to gravity only. Both intrusion pressure and flux were tested more than 5 times to obtain the

332

average value. At room temperature, TPU-PNIPAM membranes are hydrophilic, the oil

333

intrusion pressures of TPU-PNIPAM-1.4, TPU-PNIPAM-3.6, TPU-PNIPAM-7.6, and TPU-

334

PNIPAM-13.4 membrane are 0.35, 1.37, 1.53, and 3.26 kPa, respectively, and the relevant

335

water fluxes are 3288, 3646, 1657, and 41 LMH. With increasing hydrogel loading, the pore

336

size of the composite membranes becomes smaller, which is responsible for the increasing oil

337

intrusion pressure and decreasing water flux. At 45 °C, the TPU-PNIPAM membranes are 15 ACS Paragon Plus Environment

(2)

Environmental Science & Technology

338

hydrophobic, the water intrusion pressure firstly increased and then decreased, while the oil

339

flux decreased with increasing hydrogel loading of the composite membranes. Only TPU-

340

PNIPAM-3.6 exhibited proper water intrusion pressure at 45 °C (0.89 kPa), which was

341

slightly higher than that of TPU microfiber membrane alone (0.80 kPa). TPU-PNIPAM-1.4

342

showed low water intrusion pressure, because the microfibers were loosely piled up with huge

343

gaps among the membrane (Figure S8a, Supporting Information), on the other hand, coating

344

with PNIPAM decreased the water contact angle of TPU microfiber membrane from 148.3 °

345

to 105 °. Coating with PNIPAM hydrogel provides the TPU microfibers with switchable

346

hydrophilicity and hydrophobicity, but only TPU-PNIPAM-3.6 possesses switchable

347

superhydrophilicity and superhydrophobicity (Figure 4a). Increasing hydrogel loading led to

348

decreased pore size and surface roughness, thereby decreasing contact angle and water

349

intrusion pressure. The oil fluxes of TPU microfiber membrane and TPU-PNIPAM-3.6

350

membrane were 4659 and 503 LMH, respectively. The loosened structure of TPU microfiber

351

membrane reduced the water intrusion pressure but enhanced the oil flux. Combining the

352

results of room temperature oil intrusion pressures and 45 °C water intrusion pressures of the

353

membranes, TPU-PNIPAM-3.6 is the membrane that can best be applied to controllable

354

separation of oil-in-water emulsion and water-in-oil emulsion at different temperatures, which

355

is consistent with the contact angle data in Figure 4. The illustration of controllable separation

356

of oil-water mixture with TPU-PNIPAM-3.6 membrane is shown in Figure S10 (Supporting

357

Information). Water selectively permeated through the membrane and oil (hexadecane, red)

358

was retained above the membrane at room temperature. Likewise oil permeated through the

359

membrane, but the permeation of oil was blocked when the membrane surface was covered by

360

water at 45 °C, thus both oil and water were retained. Figure S11 (Supporting Information)

361

shows the variation of flux during 24 hours separation of SDS-stabilized hexadecane-water

362

emulsions. The diameter distribution of oil drops in 1 wt. % hexadecane-water emulsion is 2-

363

15 µm, and that of water drops in 99 wt. % hexadecane-water emulsion is 3-30 µm (Figure 16 ACS Paragon Plus Environment

Page 16 of 25

Page 17 of 25

Environmental Science & Technology

364

S12, Supporting Information). For separating 1 wt. % hexadecane-water emulsion at room

365

temperature, the initial flux was 3700 LMH. It dropped to about 2400 LMH after 1 hour and

366

was remained in the following 23 hours. The flux of separating 99 wt. % hexadecane-water

367

emulsion at 45 °C showed a similar trend. After 2 hours, the flux dropped from 670 to 480

368

LMH, and it remained in the following 22 hours. No external force was employed during the

369

separation process.

370 371

Figure 6 (a) Oil intrusion pressure and water flux of TPU-PNIPAM membranes (1.4, 3.6, 7.6,

372

13.4, and 25.3 wt. %) at room temperature. (b) Water intrusion pressure and oil flux of TPU

373

microfiber membrane and TPU-PNIPAM membranes at 45 °C. Hexadecane was used as the

374

test oil. (c) Separation efficiency of TPU-PNIPAM-3.6 membrane for reversibly oil-water

375

separation at room temperature and 45 °C. The inset shows the 1 wt. % SDS-stabilized

376

silicone oil-water emulsion and the collected water before and after separation at room

377

temperature (left), and the 99 wt. % SDS-stabilized silicone oil-water emulsion and the

378

collected oil before and after separation at 45 °C (right). (d) Separation efficiencies of TPU-

379

PNIPAM-3.6 membrane for separating different 1 wt. % oil-water emulsions at room

380

temperature.

381 17 ACS Paragon Plus Environment

Environmental Science & Technology

382

Silicone oil, with a density approaching that of water, was used to prepare the emulsion

383

with water for testing the oil-water emulsion separation capability of TPU-PNIPAM-3.6

384

membrane. 1 wt. % silicone oil-water emulsion contains 1 wt. % silicone oil, 99 wt. % DDI

385

water and 1 mg/mL sodium dedecyl sulfate (SDS), it is oil-in-water emulsion. Similarly, 99

386

wt. % silicone oil-water emulsion contains 99 wt. % silicone oil, 1 wt. % DDI water and 1

387

mg/mL SDS, and thus is a water-in-oil emulsion. The silicone oil-water emulsions were much

388

more stable than the hexadecane-water emulsions. The diameter of silicone oil drops in water

389

is 2-25 µm, and that of water drops in silicone oil is 5-30 µm (Figure S12, Supporting

390

Information). The TPU-PNIPAM-3.6 membrane was able to demonstrate excellent ability to

391

separate oil-water emulsions. At room temperature, 1 wt. % silicone oil-water emulsion was

392

separated, and the separation efficiency was higher than 99.26 %; at 45 °C, 99 wt. % silicone

393

oil-water emulsion was separated, and the separation efficiency was 99.85 %. TPU-PNIPAM-

394

3.6 showed switchable superhydrophilicity and superhydrophobicity, it separated oil-in-water

395

and water-in-oil emulsions at temperatures below and above LCST respectively. However, the

396

TPU microfiber membrane showed hydrophobicity, it separated a water-in-oil emulsion only.

397

The TPU microfiber membrane was applied to separate 99 wt. % silicone oil-water emulsion,

398

and the separation efficiency was 99.78 %. For separating water-in-oil emulsion, even though

399

TPU-PNIPAM-3.6 was swollen in 45 °C water, it showed similar separation efficiency with

400

the dry TPU microfiber membrane. The separation efficiencies of TPU-PNIPAM-3.6

401

membrane for reversely separating 1 wt. % and 99 wt. % silicone oil-water emulsions at room

402

temperature and 45 °C individually are shown in Figure 6c. It was found that the separation

403

efficiencies were stable both at room temperature and 45 °C, demonstrating that the TPU-

404

PNIPAM-3.6 membrane is stable and shows excellent separation ability under switching

405

temperatures. The inset of Figure 6c shows photographs of oil-water emulsions and the

406

collected liquids, before and after separation, which also indicates that the TPU-PNIPAM-3.6

407

membrane separated emulsions successfully. Other than for silicone oil-water emulsions, the 18 ACS Paragon Plus Environment

Page 18 of 25

Page 19 of 25

Environmental Science & Technology

408

TPU-PNIPAM-3.6 membrane was able to separate other oil-water emulsions with high

409

separation efficiency (Figure 6d). The optical microscopy images of emulsions before and

410

after filtration are shown in Figure S13 and Figure S14 (Supporting Information).

411

To sum up, the TPU microfiber-PNIPAM hydrogel composite membrane (e.g., TPU-

412

PNIPAM-3.63) exhibit excellent oil-water emulsion separation performance: (1) it separates

413

water-in-oil and oil-in-water emulsions effectively and efficiently under switching

414

temperatures, with an separation efficiency of ≥ 99.26 %, while the TPU microfiber

415

membrane without hydrogel coating separates water-in-oil emulsion only; (2) the swollen

416

TPU-PNIPAM-3.6 (45 °C) shows similar separation efficiency with the dry TPU microfiber

417

membrane; and (3) other oil-water emulsions can be separated by TPU-PNIPAM as well, and

418

excellent separation efficiency was achieved.

419

In conclusion, a robust thermo-responsive polymer membrane with switchable

420

superhydrophilicity and superhydrophobicity was successfully fabricated by combining a

421

thermoplastic polyurethane (TPU) microfibers and poly (N-isopropylacrylamide) (PNIPAM)

422

nanoparticles. Among the polymer composite membranes prepared, the TPU-PNIPAM-3.6

423

material performed best, exhibiting excellent mechanical properties with an elongation of

424

288 % and a toughness of 7536 kJ/m3, comparable with natural rubber (10000 kJ/ m3). The

425

water contact angles of TPU-PNIPAM-3.6 membrane switched between 0° (room

426

temperature) and 150.2 ° (45 °C), and the PNIPAM hydrogel of the composite membrane

427

reached deswelling and reswelling equilibrium in 3 minutes. Driven by gravity only, the TPU-

428

PNIPAM-3.6 membrane could separate 1 wt. % silicone oil-water emulsion at room

429

temperature with a separation efficiency of ≥ 99.26 %, and separate 99 wt. % silicone oil-

430

water emulsions at 45 °C with a separation efficiency of ≥ 99.85 %. Our membrane is thus a

431

good candidate for not only oil-water emulsion separation, but also for fabricating smart

432

materials for rapidly responsive drug delivery systems and sensors. In addition, other

19 ACS Paragon Plus Environment

Environmental Science & Technology

433

polymeric micro- or nanofibers membranes could be used as the substrates for fabricating

434

superwetting materials, which broadens the methods of preparation of such materials.

435 436

Acknowledgements

437

This work is in part supported by the Australian Research Council (Project no.:

438

LP140100051). The authors acknowledge the staff of Monash Centre for Electron

439

Microscopy for their technical support of electron microscopes.

440 441

Supporting Information

442

Additional experimental description, diameter distribution of TPU microfiber (Figure S4),

443

pore size distribution of TPU-PNIPAM (Figure S5), membrane thickness and coating

444

thickness results (Figure S6), water contact angle of TPU and TPU-PNIPAM (Figure S7-S8),

445

separation flux (Figure S11), optical microscopy images of oil/water droplet (Figure S12-S14).

446

This information is available free of charge via the Internet at http://pubs.acs.org.

447 448

References

449

(1)

450

Processes; Handbook of Environment Engineering. In The Humana Press Inc.: Totowa, NJ:

451

2006.

452

(2)

453

Superoleophilic PVDF Membranes for Effective Separation of Water‐in‐Oil Emulsions with

454

High Flux. Adv. Mater. 2013, 25, (14), 2071-2076.

455

(3)

456

emulsion by polysulfone membrane. J. Membr. Sci. 2008, 325, (1), 427-437.

457

(4)

458

stable oil-water emulsion by induced air flotation technique. Sep. Technol. 1993, 3, (1), 25-31.

Kajitvichyanukul, P.; Hung, Y.; Wang, L., Advanced Physicochemical Treatment

Zhang, W.; Shi, Z.; Zhang, F.; Liu, X.; Jin, J.; Jiang, L., Superhydrophobic and

Chakrabarty, B.; Ghoshal, A.; Purkait, M., Ultrafiltration of stable oil-in-water

El-Kayar, A.; Hussein, M.; Zatout, A.; Hosny, A.; Amer, A., Removal of oil from

20 ACS Paragon Plus Environment

Page 20 of 25

Page 21 of 25

Environmental Science & Technology

459

(5)

Kwon, G.; Kota, A.; Li, Y.; Sohani, A.; Mabry, J. M.; Tuteja, A., On‐Demand

460

Separation of Oil‐Water Mixtures. Adv. Mater. 2012, 24, (27), 3666-3671.

461

(6)

462

water emulsions. J. Membr. Sci. 1988, 36, 161-177.

463

(7)

464

Hilal, N.; Ismail, A., Membrane technology enhancement in oil–water separation. A review.

465

Desalination 2015, 357, 197-207.

466

(8)

467

nanotechnologies. Energy. Environ. Sci. 2011, 4, (6), 1946-1971.

468

(9)

469

2010, 110, (4), 2448-2471.

470

(10)

471

review of pressure‐driven membrane processes in wastewater treatment and drinking water

472

production. Environ. Prog. 2003, 22, (1), 46-56.

473

(11)

474

and super‐oleophilic coating mesh film for the separation of oil and water. Angew. Chem. Int.

475

Ed. 2004, 43, (15), 2012-2014.

476

(12)

477

membranes for effective oil–water separation. Nat. Commun. 2012, 3, 1025.

478

(13)

479

Separation of Emulsified Oil/Water Mixtures by Ultrathin Free‐Standing Single‐Walled

480

Carbon Nanotube Network Films. Adv. Mater. 2013, 25, (17), 2422-2427.

481

(14)

482

Facile approach in fabricating superhydrophobic and superoleophilic surface for water and oil

483

mixture separation. ACS Appl. Mater. Interfaces 2009, 1, (11), 2613-2617.

Lipp, P.; Lee, C.; Fane, A.; Fell, C., A fundamental study of the ultrafiltration of oil-

Padaki, M.; Murali, R. S.; Abdullah, M.; Misdan, N.; Moslehyani, A.; Kassim, M.;

Pendergast, M. M.; Hoek, E. M., A review of water treatment membrane

Rana, D.; Matsuura, T., Surface modifications for antifouling membranes. Chem. Rev.

Van der Bruggen, B.; Vandecasteele, C.; Van Gestel, T.; Doyen, W.; Leysen, R., A

Feng, L.; Zhang, Z.; Mai, Z.; Ma, Y.; Liu, B.; Jiang, L.; Zhu, D., A super‐hydrophobic

Kota, A. K.; Kwon, G.; Choi, W.; Mabry, J. M.; Tuteja, A., Hygro-responsive

Shi, Z.; Zhang, W.; Zhang, F.; Liu, X.; Wang, D.; Jin, J.; Jiang, L., Ultrafast

Wang, C.; Yao, T.; Wu, J.; Ma, C.; Fan, Z.; Wang, Z.; Cheng, Y.; Lin, Q.; Yang, B.,

21 ACS Paragon Plus Environment

Environmental Science & Technology

484

(15)

Xue, Z.; Wang, S.; Lin, L.; Chen, L.; Liu, M.; Feng, L.; Jiang, L., A Novel

485

Superhydrophilic and Underwater Superoleophobic Hydrogel‐Coated Mesh for Oil/Water

486

Separation. Adv. Mater. 2011, 23, (37), 4270-4273.

487

(16)

488

Fabrication of Superhydrophilic and Underwater Superoleophobic PAA‐g‐PVDF Membranes

489

for Effective Separation of Oil‐in‐Water Emulsions. Angew. Chem. Int. Ed. 2014, 53, (3),

490

856-860.

491

(17)

492

between superhydrophilicity and superhydrophobicity. Angew. Chem. Int. Ed. 2004, 43, (3),

493

357-360.

494

(18)

495

Inorganic Membranes with Superhydrophilicity and Underwater Ultralow Adhesive

496

Superoleophobicity for High‐Efficiency Oil/Water Separation. Adv. Mater. 2013, 25, (30),

497

4192-4198.

498

(19)

499

reversible wetting transition between the underwater superoleophilicity and

500

superoleophobicity. ACS Appl. Mater. Interfaces 2013, 6, (1), 636-641.

501

(20)

502

separation of oil/water/oil ternary mixtures. NPG Asia Mater. 2014, 6, (7), e111.

503

(21)

504

Commun. 2013, 49, (82), 9416-9418.

505

(22)

506

in different surroundings. R. Soc. Chen. Adv. 2014, 4, (28), 14684-14690.

507

(23)

508

superoleophobicity in aqueous media: toward controllable oil/water separation. NPG Asia

509

Mater. 2012, 4, (2), e8.

Zhang, W.; Zhu, Y.; Liu, X.; Wang, D.; Li, J.; Jiang, L.; Jin, J., Salt‐Induced

Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D., Reversible switching

Zhang, F.; Zhang, W. B.; Shi, Z.; Wang, D.; Jin, J.; Jiang, L., Nanowire‐Haired

Cheng, Z.; Lai, H.; Du, Y.; Fu, K.; Hou, R.; Li, C.; Zhang, N.; Sun, K., pH-induced

Ju, G.; Cheng, M.; Shi, F., A pH-responsive smart surface for the continuous

Wang, B.; Guo, Z., pH-responsive bidirectional oil–water separation material. Chem.

Wang, B.; Guo, Z.; Liu, W., pH-responsive smart fabrics with controllable wettability

Zhang, L.; Zhang, Z.; Wang, P., Smart surfaces with switchable superoleophilicity and

22 ACS Paragon Plus Environment

Page 22 of 25

Page 23 of 25

Environmental Science & Technology

510

(24)

Tian, D.; Zhang, X.; Tian, Y.; Wu, Y.; Wang, X.; Zhai, J.; Jiang, L., Photo-induced

511

water–oil separation based on switchable superhydrophobicity–superhydrophilicity and

512

underwater superoleophobicity of the aligned ZnO nanorod array-coated mesh films. J.

513

Mater. Chem. 2012, 22, (37), 19652-19657.

514

(25)

515

Controllable On-Demand Oil/Water Separation on the Switchable Superhydrophobic /

516

Superhydrophilic and Underwater Low-Adhesive Superoleophobic Copper Mesh Film.

517

Langmuir 2015, 31, (4), 1393-1399.

518

(26)

519

coated mesh for oil/water separation based on monolayer electrostatic self-assembly. R. Soc.

520

Chem. Adv. 2014, 4, (93), 51404-51410.

521

(27)

522

responsive materials for controllable oil/water separation. ACS Appl. Mater. Interfaces 2014,

523

6, (3), 2026-2030.

524

(28)

525

cross linked PVCL-co-PSF copolymer for protein separation and easy cleaning. R. Soc. Chem.

526

Adv. 2015, 5, (29), 22609-22619.

527

(29)

528

showing switchable transport performance for oil/water separation. Adv. Mater. 2014, 26,

529

(18), 2943-2948.

530

(30)

531

properties of PVDF hollow fiber membranes from aspects of phase inversion, crystallization

532

and rheology. J. Membr. Sci. 2009, 340, (1), 192-205.

533

(31)

534

review of methods. Prog. Mater. Sci. 2013, 58, (1), 76-150.

Cheng, Z.; Wang, J.; Lai, H.; Du, Y.; Hou, R.; Li, C.; Zhang, N.; Sun, K., pH-

Zhang, W.; Cao, Y.; Liu, N.; Chen, Y.; Feng, L., A novel solution-controlled hydrogel

Cao, Y.; Liu, N.; Fu, C.; Li, K.; Tao, L.; Feng, L.; Wei, Y., Thermo and pH dual-

Sinha, M.; Purkait, M., Preparation of a novel thermo responsive PSF membrane, with

Tao, M.; Xue, L.; Liu, F.; Jiang, L., An intelligent superwetting PVDF membrane

Sukitpaneenit, P.; Chung, T.-S., Molecular elucidation of morphology and mechanical

Zhao, C.; Xue, J.; Ran, F.; Sun, S., Modification of polyethersulfone membranes–a

23 ACS Paragon Plus Environment

Environmental Science & Technology

535

(32)

Badrossamay, M. R.; McIlwee, H. A.; Goss, J. A.; Parker, K. K., Nanofiber assembly

536

by rotary jet-spinning. Nano Lett. 2010, 10, (6), 2257-2261.

537

(33)

538

polymer fiber mats obtained by air flow rotary-jet spinning. Fiber. Polym. 2013, 14, (9),

539

1526-1534.

540

(34)

541

Lozano, K., Electrospinning to forcespinning™. Mater. Today 2010, 13, (11), 12-14.

542

(35)

543

structure and antimicrobial activity of polyvinylpyrrolidone-based iodine nanofibers prepared

544

with high-speed rotary spinning technique. Int. J. Pharm. 2013, 458, (1), 99-103.

545

(36)

546

Nano‐Tracted Poly (N‐isopropylacrylamide) Hydrogels Containing Linear Poly (acrylic acid).

547

Macromol. Biosci. 2006, 6, (11), 959-965.

548

(37)

549

oxide)‐block‐poly (N‐isopropylacrylamide) triblock copolymer to poly

550

(N‐isopropylacrylamide)‐block‐poly (ethylene oxide) hydrogels: Synthesis and rapid

551

deswelling and reswelling behavior of hydrogels. J. Polym. Sci. A Polym. Chem. 2012, 50,

552

(9), 1717-1727.

553

(38)

554

Raman determination of molecular mechanism of poly (N-isopropylacrylamide) volume

555

phase transition. J. Phys. Chem. B 2009, 113, (13), 4248-4256.

556

(39)

557

system for quantitative study of the molecular structure of poly (N-isopropylacrylamide) in

558

water. Polymer 1999, 40, (10), 2619-2624.

559

(40)

560

thermosensitive hydrogels. J. Control. Release 1992, 22, (2), 95-104. 24

Mîndru, T. B.; Ignat, L.; Mîndru, I. B.; Pinteala, M., Morphological aspects of

Sarkar, K.; Gomez, C.; Zambrano, S.; Ramirez, M.; de Hoyos, E.; Vasquez, H.;

Sebe, I.; Szabó, B.; Nagy, Z. K.; Szabo, D.; Zsidai, L.; Kocsis, B.; Zelkó, R., Polymer

Asoh, T. a.; Kaneko, T.; Matsusaki, M.; Akashi, M., Rapid and Precise Release from

Zheng, Q.; Zheng, S., From poly (N‐isopropylacrylamide)‐block‐poly (ethylene

Ahmed, Z.; Gooding, E. A.; Pimenov, K. V.; Wang, L.; Asher, S. A., UV resonance

Lin, S.-Y.; Chen, K.-S.; Liang, R.-C., Thermal micro ATR/FT-IR spectroscopic

Gutowska, A.; Bae, Y. H.; Feijen, J.; Kim, S. W., Heparin release from

ACS Paragon Plus Environment

Page 24 of 25

Page 25 of 25

Environmental Science & Technology

561

(41)

Stenzel, M. H.; Davis, T. P., Star polymer synthesis using trithiocarbonate functional

562

β‐cyclodextrin cores (reversible addition–fragmentation chain‐transfer polymerization). J.

563

Polym. Sci. A Polym. Chem. 2002, 40, (24), 4498-4512.

564

25 ACS Paragon Plus Environment