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Preparation and Characterization of Polymeric Surfactants Based on Epoxidized Soybean Oil Grafted Hydroxyethyl Cellulose Xujuan Huang, He Liu, Shibin Shang, Xiaoping Rao, and Jie Song J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.5b03765 • Publication Date (Web): 29 Sep 2015 Downloaded from http://pubs.acs.org on October 3, 2015

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Journal of Agricultural and Food Chemistry

Preparation and Characterization of Polymeric Surfactants Based on Epoxidized Soybean Oil Grafted Hydroxyethyl Cellulose Xujuan Huang†, He Liu†*, Shibin Shang†‡*, Xiaoping Rao†‡and Jie Song§ †

Institute of Chemical Industry of Forestry Products, Chinese Academy of Forestry, Key

Laboratory of Biomass Energy and Material, National Engineering Laboratory for Biomass Chemical Utilization, Key and Laboratory on Forest Chemical Engineering, State Forestry Administration, Nanjing 210042, Jiangsu Province, China ‡

Institute of New Technology of Forestry, Chinese Academy of Forestry, Beijing 100091,

China §

Department of Chemistry and Biochemistry, University of Michigan-Flint, Flint,

Michigan 48502, United States AUTHOR INFORMATION *Corresponding author: He Liu & Shibin Shang Email: [email protected] & [email protected]. Phone: 086-25-85482452. Fax: 086-25-85482499.

1

ACS Paragon Plus Environment


1

ABSTRACT: Epoxidized soybean oil (ESO) grafted hydroxyethyl cellulose (HEC)

2

was prepared via ring-opening polymerization, in which the hydroxyl groups of HEC

3

acted as initiators, and the polymeric ESO were covalently bonded to the HEC.

4

Hydrolysis of ESO grafted HEC (ESO-HEC) was performed with sodium hydroxide,

5

and the hydrolyzed ESO-HEC (H-ESO-HEC) products were characterized via Fourier

6

transform infrared (FT-IR),

7

spectroscopies, high-temperature gel permeation chromatography (HT-GPC), and

8

differential scanning calorimetry (DSC). The results indicated that ring-opening

9

polymerization of ESO occurred with the hydroxyl groups of HEC as initiators. The

10

molecular weights of the H-ESO-HEC products were varied by adjusting the mass

11

ratio of HEC and ESO. Through neutralizing the carboxylic acid of H-ESO-HEC with

12

sodium hydroxide, novel polymeric surfactants (H-ESO-HEC-Na) were obtained, and

13

the effects of polymeric surfactants on the surface tension of water were investigated

14

as a function of concentration of H-ESO-HEC-Na. The H-ESO-HEC-Na was effective

15

at lowering the surface tension of water to 26.33 mN/m, and the critical micelle

16

concentration (CMC) value decreased from 1.053 to 0.157 g/L with increases in

17

molecular weights of the polymeric surfactants. Rheological measurements indicated

18

that the H-ESO-HEC-Na solutions changed from pseudoplastic property to Newtonian

19

with increasing shear rate.

20

KEYWORDS: hydroxyethyl cellulose; epoxidized soybean oil; polymeric

21

surfactants

1

H and

13

C nuclear magnetic resonance (NMR)

22 2


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23

INTRODUCTION

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Polymeric surfactants are important for a variety of industrial applications. The

25

increasing interest in natural, renewable and biodegradable materials makes natural

26

polymers like cellulose and its derivatives attractive raw materials for the preparation

27

of bio-polymeric surfactants.1 Cellulose-based polymeric surfactants have drawn

28

much attention in the last decades because they present many novel performances

29

such as low cost, biodegradation, associative properties in water, rheological

30

properties, and surface-active properties that can control foaming or emulsion

31

stability.2-7

32

Landoll did pioneering work on cellulose-based polymeric surfactants in

33

1980s.8,9 Since then, various cellulose-based polymeric surfactants have been

34

synthesized

35

Cellulose-based polymeric surfactants were generally synthesized by the modification

36

of hydrophilic cellulose backbones with hydrophobic long alkyl chains. Hydroxyl

37

groups always have chemical handles that can react with hydrophobic chains

38

containing epoxide, halide, acyl halide, isocyanate or anhydride for the preparation of

39

amphiphilic cellulose. Polymeric surfactants based on hydroxyethyl cellulose (HEC)

40

were prepared by the reaction of a long-chain terminal epoxide with the hydroxyl

41

groups of HEC in an alkaline slurry.9 Partial hydrophobization of carboxymethyl

42

cellulose with C12–C18 alkyl halides was carried out in the presence of sodium

43

hydroxide as a catalyst in DMF/H2O (1:1).16 The HEC-based copolymer was prepared

44

via ultraviolet irradiation by copolymerizing HEC with hexadecyl acrylate and

including

nonionic,

ionic,

fluorocarbon,

3


and

amphoteric.10-15


45

co-monomer styrene (St) in aqueous solution.12 The C10–C14 alkyl cellulose ester

46

sulfate surfactants were prepared by hydrophilic sulfonation and hydrophobic

47

esterification.17 Further attempts to substitute the environmentally dangerous

48

petrochemical products as well as schemes to increase the hydrophobicity of cellulose

49

derivatives with fatty acids to synthesize cellulose-based surfactants have been

50

investigated.18,19 Grafting fatty acid chains to cellulose increases the hydrophobicity

51

of the ester derivatives even at a low degree of esterification.

52

Although fatty acid esters are found in nature as triglycerides and inexpensive as

53

acylation reagents, the improvement of surface-active properties of fatty

54

acid-esterified cellulose derivatives is limited.1 According to the literature, the surface

55

tension of water was decreased from 72.8 mN/m to 46 - 65 mN/m by fatty

56

acid-esterified cellulose derivatives.18 However, it is well known that saponified fatty

57

acid esters are an important surfactant and are effective at lowering the surface tension

58

of water from 20 to 40 mN/m.20,21 In addition, the surface tension of polymerized

59

saponified fatty acid ester is 20.5 to 39.6 mN/m.22 This study suggests that, versus

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saponified fatty acid esters, polymerized saponified fatty acid esters have similar

61

utility at lowering the surface tension of water. Saponified fatty acid-grafted cellulose

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surfactants formed via cellulose derivatives as backbones are polymerized saponified

63

fatty acid surfactants. Therefore, cellulose chains grafting saponified fatty acid esters

64

to form ionic surfactants—instead of fatty acid-esterified cellulose surfactants—may

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become an effective method for synthesis of cellulose-based surfactants with highly

66

surface-active properties. Fortunately, the chemical reactivities of some fatty acids 4


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isolated from vegetable oils can achieve this vision.

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Soybean oil is one of the most abundant renewable vegetable oils in the world.23

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It possesses three unsaturated fatty acids, oleic (23%), linoleic (54%) and linolenic

70

(8%). It contains 1, 2, and 3 double bonds, respectively.24 The double bonds can be

71

used in many reactions including addition reaction25, oxidation reaction26, and

72

polymerization reaction27. More significantly, the expoxidation reaction makes the

73

soybean oil become epoxidized soybean oil (ESO). A ring-opening reaction of ESO

74

with different alcohols was carried out in presence of Amberlyst 15 (Dry) as a

75

catalyst.28 Furthermore, the ring-opening polymerization of ESO can be catalyzed by

76

boron trifluoride diethyl etherate (BF3·OEt2) in liquid carbon dioxide and methylene

77

chloride, respectively.26,29 The ESO-based polymeric surfactants in particular were

78

prepared by hydrolysis of polymerized ESO with a base.22 These interesting works

79

suggest that hydroxyl groups of cellulose can initiate ring-opening polymerization of

80

ESO in the presence of catalysts. Moreover, novel saponified fatty acid-modified

81

cellulose-based polymeric surfactants are obtained by saponification of the ESO

82

grafted cellulose products in which fatty acid chain are combined with cellulose by an

83

ether linkage at the middle site of a fatty acid molecular chain.

84

To prove this assumption, the modification of HEC by grafting ESO was

85

investigated and hydrolysis of ESO grafted HEC (ESO-HEC) was performed with

86

sodium hydroxide (Figure 1). The hydrolyzed ESO-HEC (H-ESO-HEC) products

87

were characterized with Fourier transform infrared (FT-IR), 1H and

88

magnetic resonance (NMR) spectroscopies, high-temperature gel permeation 5


13

C nuclear


89

chromatography (HT-GPC), and differential scanning calorimetry (DSC). After

90

neutralizing the carboxylic acid of the H-ESO-HEC with sodium hydroxide, novel

91

polymeric surfactants (H-ESO-HEC-Na) were obtained and the effects of polymeric

92

surfactants on the surface tension of water were investigated as a function of

93

concentration of H-ESO-HEC-Na. In addition, the rheological behaviors of the

94

H-ESO-HEC were investigated.

95

MATERIALS AND METHODS

96

Materials. Hydroxyethyl cellulose (HEC), epoxidized soybean oil (ESO) and

97

stannic chloride (SnCl4) were purchased from Aladdin Industrial Co., Ltd. Dimethyl

98

sulfoxide (DMSO) was purchased from Sinopharm Chemical Reagent Co., Ltd.

99

Sodium hydroxide (NaOH), hydrochloric acid (HCl) and ethyl acetate were obtained

100

from Nanjing Chemical Reagent Co., Ltd. Deionized water was purified to a

101

conductivity of 18.3 MΩ·cm on a Hitech-Sciencetool Master-Q laboratory water

102

purification system (Shanghai Hetai Reagent Co., Ltd). All chemicals were used

103

without further purification.

104

Synthesis of ESO-grafted HEC Derivatives (ESO-HEC). HEC (1.0 g) was

105

dissolved in 40 mL DMSO under stirring at room temperature for 2 h and then

106

transferred into a three-necked round-bottomed flask. ESO (1.0 g) was added to the

107

HEC solution. The mixture was stirred by mechanical agitation at room temperature.

108

A mixture of 10 µL SnCl4 and 10 mL DMSO were then added dropwise into the

109

reactor, and the reaction was maintained for 30 min. The resulting products were

110

washed sequentially with deionized water to remove DMSO and dried under vacuum 6


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at 50 °C. Finally, the ESO-grafted HEC derivatives (ESO-HEC) were obtained.

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Hydrolysis of ESO-HEC (H-ESO-HEC). The ESO-HEC (2.0 g) was added to a

113

flask with 50 mL of 0.6 M NaOH solution. The reactor was heated to 100 °C with

114

agitation for 12 h under a condensing condition. After reaction, the solution was

115

separated by filtration and the aqueous solution was removed by reduced pressure

116

distillation. The solid product was washed with ethyl acetate three times to remove the

117

glycerin. The solid product was then re-dispersed in aqueous solution and the solution

118

was adjusted to pH 5~6 by 0.1 M HCl. The solution became turbid, and the product

119

appeared as a floating oil. The product was extracted by ethyl acetate, and the organic

120

phase was washed with hot distilled water three times. Finally, the H-ESO-HEC

121

product was purified by reduced pressure distillation. The H-ESO-HEC-Na was

122

obtained by neutralizing the carboxylic acid of H-ESO-HEC with sodium hydroxide.

123

FT-IR. Fourier transform infrared (FT-IR) spectra was used to confirm the

124

structure of the H-ESO-HEC. These spectra were obtained on the Thermo Scientific

125

Nicolet IS10 spectrometer with the attenuated total reflectance (ATR) method. The

126

spectra were recorded over the range 4000-500 cm-1 at 4 cm-1 resolution and averaged

127

over 16 scans per sample.

128

NMR. 1H NMR and 13C NMR spectra for H-ESO-HEC were recorded at 40 °C

129

on a BRUKER AV400 spectrometer (Bruker, Rheinstetten, Germany) operating at a

130

frequency of 400.13 and 100.61 MHz, respectively. Deuterated dimethyl sulfoxide

131

(DMSO-d6) and tetramethylsilicane (TMS) were used as the solvent and the internal

132

standard, respectively. The chemical shift values were referenced to the signals of 7



133

DMSO-d6 and TMS.

134

HT-GPC. Molecular weight distribution curves and relative values of number

135

average (Mn) and weight average (Mw) molecular weight of H-ESO-HEC were

136

determined by HT-GPC (HT-GPC Module 350A, Viscotek; GPC equipped with

137

I-MBHMW-3078 HT-GPC column) at 60 °C. The HT-GPC instrument was equipped

138

with refractive index detector, viscosity detector and small-angle light scattering

139

detector. The flow rate of the carrier solvent (HPLC-grade DMSO) was 1.0 mL/min.

140

Samples were filtered over microfilters with a pore size of 0.2 µm (Nylon, Millex-HN

141

13 mm Syringes Filters, Millipore). The results were obtained from OmniSEC

142

software.

143

Conductimetry. The carboxyl content of H-ESO-HEC was determined by

144

conductometric titrations.30 The H-ESO-HEC (30-40 mg) was suspended into 15 mL

145

of 0.01 M hydrochloric acid solution. After 2 h of stirring, the suspensions were

146

titrated with 0.01 M NaOH. The carboxyl content of the sample was determined from

147

the conductivity and VNaOH curve. The carboxyl content is calculated by the following

148

equation:

149

Carboxyl Content (mmol/g) =

c (V2 − V1 ) m

(1)

150

Here, V1 and V2 are the volumes of NaOH (mL) in the first and second inflection point

151

of titration curves, respectively. The c is the concentration of NaOH (mmol/L), and m

152

is the weight of oven-dried sample (mg). To minimize errors, titrations were

153

duplicated at least three times, and the average value was retained for the discussion.

154

DSC. To determine the ring-opening polymerization of ESO, the DSC 8


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155

measurements of the H-ESO-HEC were performed on a Diamond DSC at 40 mL/min

156

dry nitrogen as the purge gas. Typically, ~4 mg of the H-ESO-HEC was accurately

157

weighed in an aluminum pan and sealed with pin-perforated lids. The sample was

158

cooled to -50 °C at a rate of 10 °C/min and a refrigerated cooling system was used to

159

equilibrate the sample at -50 °C for 5 min. Data were recorded while the oven

160

temperature was raised from -50 °C to 0 °C at 10°C /min.

161

Surface Tension. The surface tension of H-ESO-HEC-Na was measured at

162

25 °C using the Wilhelmy plate T107 (width: 19.44 mm; thickness: 0.1 mm; height:

163

65 mm; circumference: 39.08) on a Sigma 701 Automatic Surface Tensiometer. The

164

instrument was calibrated against pure water before measurements were made. The

165

concentration of H-ESO-HEC-Na surfactant was gradually increased by dispersing

166

stock solution into the measurement cell. The software automatically determined the

167

surface tension as a function of the bulk concentration. The surface tension data were

168

plotted against the H-ESO-HEC-Na concentration in water from 0.0003 to 2.0 g/L.

169

Each concentration of aqueous H-ESO-HEC-Na solution was tested in triplicate—this

170

was done automatically by the instrument. The concentration at which the equilibrium

171

surface tension of the polymeric surfactants stopped decreasing with concentration

172

and plateaued is the critical micelle concentration (CMC).22 From the inflection point

173

of the plot, the critical micelle concentration (CMC) and minimum surface tension

174

(γcmc) were derived. All data were obtained from the OneAttention software.

175

Krafft Point (KP). 20 mL 0.5 wt% surfactant aqueous solution (at least

176

quintuplicate the CMC of studied surfactant) was prepared in an alkaline solution and 9



177

added to a beaker. The beaker was then heated and the particular temperature was

178

recorded as KP when the solution transformed from turbid to pellucid.31

179

Hydrophile Lipophile Balance (HLB) Value. The emulsions were prepared

180

using a mixture of the nonionic surfactants Tween 60 and Span 80 to satisfy the

181

proper HLB values for optimum emulsification conditions. The mixed HLB values

182

were calculated with the following equation:

183

HLBmix = HLBT × T % + HLBS × S %

(2)

184

Here, HLBmix, HLBT and HLBS are the HLB values of the mixed surfactants, Tween 60

185

(14.9) and Span 80 (4.3), respectively. The T% and S% are the mass percentages of

186

Tween 60 and Span 80 in the mixed surfactants, respectively.32 All the HLB values

187

used were measured at 25 °C.

188

Rheology. Rheological measurements were performed at a rotational rheometer

189

HAAKE MARS II equipped with a measuring geometry named PP60 Ti (diameter d=

190

50 mm). The H-ESO-HEC-Na-III was selected to evaluate the rheological properties.

191

H-ESO-HEC-Na-III solutions at three different concentrations (0.1 g/L, 1 g/L, 5 g/L)

192

were prepared to measure the shear viscosity that changed with the shear rate ranging

193

from 0.01 to 100 s-1.15 The rheological measurements were carried out at 40 °C. All

194

data were obtained using the HAKKE RheoWin data manager software.

195

Statistical Method. Origin Pro 9.0 was used to calculate the standard deviation of

196

carboxyl content and the surface tension of H-ESO-HEC. The standard deviation was

197

calculated with the following equation:

10


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n

∑(X 198

S=

i

− X )2

i =1

n −1

(3)

199

Here, S is the standard deviation of a sample. X i is the ith value in a sample, X is

200

the average number of a sample, n is the total number of a sample.

201

RESULTS AND DISCUSSION

202

Figure 1 outlines a simplified mechanism for the entire reaction. The ESO-HEC

203

was prepared via ring-opening polymerization, in which the hydroxyl groups of HEC

204

acted as initiators. In addition, the ring-opening polymerization generated new

205

hydroxyl groups that can also act as initiators via the SnCl4 catalyst. The polymeric

206

ESO were covalently bound to the HEC, and hydrolysis of ESO-HEC (H-ESO-HEC)

207

was obtained with sodium hydroxide. As shown in Table 1, H-ESO-HEC products

208

with different molecular weights were obtained by adjusting the mass ratio of HEC

209

and ESO in graft reaction. The graft reaction occurred at room temperature with a

210

short reaction time because of the high-activity of the SnCl4 catalyst. The yields of the

211

H-ESO-HEC were measured and ranged from 1.2 to 2.7 g per g HEC as the amount of

212

ESO increased in graft polymerization reaction. In addition, H-ESO-HEC-Na samples

213

with different molecular weights were obtained by neutralizing carboxylic acid of

214

H-ESO-HEC with sodium hydroxide.

215

Spectroscopic Identification of the Structures of H-ESO-HEC. In Figure 2,

216

the FT-IR spectrum of H-ESO-HEC-III attested to the presence of ESO chains onto

217

the HEC by appearance of the characteristic absorption bands assigned to the carbonyl

218

functions at 1710 cm-1 versus the HEC. The spectral peak at 2920 cm-1 was attributed 11



219

to the –CH3 group of the alkane chains of ESO. Also, peaks at 716 cm-1 indicate the

220

vibration of –(CH2)n– (n ≥ 4).33 No significant change in hydroxyl group signal at

221

3360 cm-1 was noticed because the hydroxyl groups of the HEC acted as initiators and

222

were consumed in the grafting reaction. The ring-opening polymerization generated

223

new hydroxyl groups simultaneously—these are obvious from Figure 1. These results

224

indicated that the H-ESO-HEC was prepared successfully. The introduction of the ESO on HEC chains is further confirmed by the 1H and

225 226

13

C NMR spectra of the H-ESO-HEC-III in Figure 3. In 1H NMR spectrum, the

227

clusters of signal peaks ranged from δ0.8 to δ1.6. These were related to the protons of

228

the alkane chains of ESO on HEC. The peaks at δ2.1 corresponded to the side groups

229

–OH of the HEC. The alkane chains were produced by the ring-open reaction. The

230

clusters of signal peaks in the range of δ3.2 to δ5.0 were attributed to the chemical

231

shifts of anhydroglucose unit (AGU) protons. In the 13C NMR spectrum, the possible

232

assignments of the peaks were found according to the previous literature.34,35 The

233

inserted region at δ176 correspond to carbonyl signals of the carboxylate groups.36

234

The C-1 carbon resonance in AGU was poor at δ101. The clusters of signal peaks in

235

the range of δ62 to δ76 contributed to the chemical shifts of AGU carbons. Table 2

236

offers detailed information. Distinct peaks were observed at δ62. There are attributed

237

to the C-6 substituted by the hydroxyethylation and connected to the ether bond that is

238

obtained by the graft polymerization. The clusters of peaks in the range from δ10 to

239

δ35 corresponding to the carbon signals of ESO chains demonstrated that the graft

240

reaction was successful. The 1H NMR and

13

C NMR spectroscopy confirmed the

12


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241

characteristic groups of H-ESO-HEC-III.

242

Molecular Weight and Carboxyl Content of H-ESO-HEC. The molecular

243

weight value of H-ESO-HEC was determined by HT-GPC. The HT-GPC profile

244

signals of the refractive index detector are shown in Figure 4. This shows that the

245

H-ESO-HEC with different molecular weights had a similar tendency of distribution

246

and moved towards the higher molecular weight regions as the amount of ESO

247

increased in the graft reaction. As listed in Table 1, the Mn of H-ESO-HEC (I-V)

248

ranged from 1.48 × 105 to 4.58 × 105 Da and the Mw of H-ESO-HEC (I-V) ranged

249

from 2.71 × 105 to 8.28 × 105 Da along with the mass ratio of HEC and ESO ranged

250

from 1:1 to 1:5, respectively. The conductometric titration curve to determine

251

carboxyl content of H-ESO-HEC-III was divided into three parts in Figure 5. The first

252

part showed the presence of a strong acid corresponding to excess of HCl. The second

253

part where the conductivity remains constant showed a weak acid corresponding to

254

the carboxyl content. The last part showed that strong base corresponded to an excess

255

of sodium hydroxide. The V1 and V2 are volumes of NaOH (mL) in the first and

256

second inflection points of the titration curves, respectively. Finally, the carboxyl

257

content can be calculated by the equation (1). The carboxyl contents of H-ESO-HEC

258

(I-V) were 1.89-2.80 mmol/g (Table 1). We concluded that the carboxyl content were

259

closely associated with the molecular weight. As the amount of ESO increased in

260

graft reaction, the carboxyl content increased according to the Mn of H-ESO-HEC. In

261

addition, the polydispersity indexes (PDI) of H-ESO-HEC (I-V) ranged from 1.46 to

262

2.1. This indicates that the H-ESO-HEC samples had a wide distribution (figure 4). 13



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263

DSC Analysis. The ring-opening polymerization of ESO grafting on HEC was

264

confirmed by DSC in which the glass transition temperature (Tg) was determined to

265

be the temperature at the inflection point. Figure 6 clearly illustrates that there was no

266

Tg in HEC and H-ESO-HEC-I because the HEC did not exhibit glass transition, and

267

there was no polymerization of ESO on HEC in H-ESO-HEC-I. Previous literature

268

reported that the products obtained by ring-opening polymerization of ESO existed

269

Tg.26

270

H-ESO-HEC-V were -25.81 oC, -23.74 oC, -22.24 oC and -21.48 oC, respectively. As

271

expected, increasing the amount of ESO in the graft reaction resulted in more ESO

272

being grafted on HEC. The ring-opening polymerization reaction took place in the

273

graft reaction—this was in consistent with the GPC results. Furthermore, the Tg of

274

H-ESO-HEC samples became higher as the molecular weights of H-ESO-HEC

275

increased.

The

Tg

of

H-ESO-HEC-II,

H-ESO-HEC-III,

H-ESO-HEC-IV

and

276

Surface Tension of Aqueous H-ESO-HEC Polymeric Surfactants. The

277

H-ESO-HEC products were added to a beaker containing the required amount of

278

NaOH to neutralize all of the acidic protons from carboxylic acid groups. This was

279

then placed in a 60oC water bath and stirred with a glass rod until the samples were

280

dissolved.28, 29 A series of aqueous solutions of the H-ESO-HEC-Na were prepared,

281

and their surface tensions were investigated at room temperature. It is well known that

282

amphiphilic polymers with a suitable hydrophilic-hydrophobic balance can form a

283

micelle structure when exposed to a selective solvent.37 As shown in Figure 7, the

284

surface tension of H-ESO-HEC-Na in water decreased markedly in the low 14


Page 15 of 31


285

concentration at first. When the concentration was higher, the surface tension reduced

286

slightly. Finally, the surface tension was held nearly constant because its

287

concentration was sufficiently high.

288

As shown in Table 3, the minimum surface tensions of H-ESO-HEC-Na (I-V)

289

were almost same and ranged from 26.33 mN/m to 28.88 mN/m. The

290

H-ESO-HEC-Na displayed efficient surface properties versus HEC and the

291

hydrolyzed ESO at 54.68 mN/m and 34.28 mN/m, respectively. In addition, the

292

previous studies reported that the surface tension of fatty acid-esterified cellulose

293

derivatives ranged between 46 mN/m and 65 mN/m.17, 18 Different efficiency of these

294

fatty acid modified cellulose polymeric surfactants in lowering surface tensions of

295

water was attributed to the status of fatty acid in polymeric surfactants. In our study,

296

hydroxyl groups of HEC acted as initiators for ring-opening polymerization of ESO.

297

The fatty acid chains were combined with HEC by ether linkage at the middle site of

298

the fatty acid molecular chain. In this structure, both the hydrophobic alkanes and the

299

hydrophilic carboxyl were introduced into HEC. The simultaneous introduction of

300

hydrophobic

301

hydrophilic-hydrophobic balance of HEC and provided an opportunity to form

302

micelles in water. The CMC values of H-ESO-HEC-Na were decreased from 1.053

303

g/L to 0.157 g/L along with the obviously increasing molecular weight of

304

H-ESO-HEC-Na. As the molecular weights increased, the H-ESO-HEC-Na displayed

305

greater propensity to form micelles because of the flexible molecular chains. This

306

indicated that the efficiency in reducing surface tension was significantly enhanced

alkanes

and

hydrophilic

carboxyl

15


groups

improved

the


Page 16 of 31

307

when the molecular weight of H-ESO-HEC-Na increased. The KP of the

308

H-ESO-HEC-Na ranged from 39 °C to 68 °C. It increased as a function of molecular

309

weight accordingly. The HLB values of the samples ranged from 9.6 to 10.6.

310

Surface

Rheological

Properties.

The

apparent

viscosities

of

the

311

H-ESO-HEC-Na aqueous solution as a function of shear rate at different

312

concentrations were investigated. The results of H-ESO-HEC-Na-III displayed the

313

same properties as the other products (Figure 8). The curves were not smooth at the

314

low shear rate because of instrument error and the complete minor viscosity.

315

Therefore, the curve of the 0.1 g/L H-ESO-HEC-Na-III can largely be considered a

316

straight line. The curve of the 0.1 g/L H-ESO-HEC-Na-III below the CMC

317

demonstrated that the solution behaved like water due to the low number of species

318

H-ESO-HEC-Na-III in solution. The curves of the 1.0 g/L and 5.0 g/L

319

H-ESO-HEC-Na-III above the CMC that formed micelles in aqueous solution can be

320

divided into two parts. First, the viscosity of the H-ESO-HEC-Na-III solution

321

decreased as the shear rate increased, and the H-ESO-HEC-Na-III solution exhibited a

322

pseudoplastic property at the low shear rate.38 This was because micelles in aqueous

323

solution were damaged, and the macromolecules experienced conformational

324

change.39 Its winding structure was separated under the action of shearing force and

325

arranged along the flow direction along with the increasing shear rate.40 Sequentially,

326

the viscosity remained constant when the shear rate reached a certain level. It behaved

327

like a Newtonian property.41 This is because the winding structure was completely

328

destroyed, and the molecular orientation reached at limiting condition. 16


Page 17 of 31


329

In summary, ESO-grafted HEC were prepared via ring-opening polymerization

330

and five novel polymeric surfactants H-ESO-HEC-Na (I-V) were obtained by

331

neutralization of H-ESO-HEC with sodium hydroxide. The molecular weights of

332

H-ESO-HEC products were varied by adjusting the mass ratio of HEC and ESO in the

333

grafting reaction. The synthesized polymeric surfactants H-ESO-HEC-Na exhibited

334

higher activities in reducing water surface tension. These ranged from 26.33 mN/m to

335

28.88 mN/m in comparison to 34.28 mN/m for hydrolyzed ESO and 54.68 mN/m for

336

HEC. In addition, the efficiency in reducing water surface tension was definitely

337

enhanced when the molecular weights of H-ESO-HEC increased. Rheological

338

measurements indicated that the H-ESO-HEC-Na solutions changed from a

339

pseudoplastic state to Newtonian with increasing shear rate. This study may facilitate

340

an increase in the use of renewable and biodegradable materials with improved

341

properties as polymeric surfactants.

342 343 344

Funding

345

The authors express their gratitude for the financial support From Natural Science

346

Foundation of Jiangsu Province of China (BK2012063 and BK20140973); Special

347

Fund for Basic Scientific Research Business of Central Public Research Institutes

348

(CAFINT2012C05); National Natural Science Foundation of China (31200446).

349 350

Notes 17



351

The authors declare no competing financial interest.

352

REFERENCES (1) Tomanova, V.; Srokova, I.; Ebringerova, A.; Sasinkova, V., Surface-Active and Associative Properties of Ionic Polymeric Surfactants Based on Carboxymethylcellulose. Polym. Eng. Sci. 2011, 51, 1476-1483. (2) Nikfarjam, N.; Qazvini, N. T.; Deng, Y. L., Surfactant free Pickering emulsion polymerization of styrene in w/o/w system using cellulose nanofibrils. Eur. Polym. J. 2015, 64, 179-188. (3) Hu, Z.; Ballinger, S.; Pelton, R.; Cranston, E. D., Surfactant-enhanced cellulose nanocrystal Pickering emulsions. J. Colloid. Interf. Sci. 2015, 439, 139-148. (4) Khuman, P.; Singh, W. B. K.; Devi, S. D.; Naorem, H., Viscosity-Temperature Behavior of Hydroxypropyl Cellulose Solution in Presence of an Electrolyte or a Surfactant: A Convenient Method to Determine the Cloud Point of Polymer Solutions. J. Macromol. Sci. A. 2014, 51, 924-930. (5) Durand, A., Synthesis of amphiphilic polysaccharides by micellar catalysis. J. Mol. Catal. a-Chem. 2006, 256, 284-289. (6) Zhang, L. M., Cellulosic associative thickeners. Carbohyd. Polym. 2001, 45, 1-10. (7) Karlberg, M.; Thuresson, K.; Lindman, B., Hydrophobically modified ethyl (hydroxyethyl) cellulose as stabilizer and emulsifying agent in macroemulsions. Colloid. Surface. A. 2005, 262, 158-167. (8) M.L. Landoll, US Patent No. 4,228,277 (1980). (9) Landol, L. M., Nonionic polymer surfactants. J. Polym. Sci. 1982, 20, 443-455. (10) Yang F.; Liu Y.N.; Yu J.L.; Li H.P.; Gang Li, Synthesis, micellization behavior and alcohol induced amphipathic cellulose ﬁlm of cellulose-based amphiphilic surfactant. Appl. Surf. Sci. 2015, 345, 187-193. (11) S. Evani, US Patent No. 4,432,881 (1984). (12) Li, Z. X.; Wang, L. G.; Huang, Y., Photoinduced graft copolymerization of polymer surfactants based on hydroxyethyl cellulose. J. Photoch. Photobio. A. 2007, 190, 9-14. (13) Sardar, N.; Ali, M. S.; Kamil, M.; Kabir-ud-Din, Phase Behavior of Nonionic Polymer Hydroxypropylmethyl Cellulose: Effect of Gemini and Single-Chain Surfactants on the Energetics at the Cloud Point. J. Chem. Eng. Data. 2010, 55, 4990-4994. (14) Thongngam, M.; McClements, D. J., Characterization of interactions between chitosan and an anionic surfactant. J. Agr. Food. Chem. 2004, 52, 987-991. (15) Wei, Y. P.; Cheng, F.; Hou, G.; Sun, S. F., Amphiphilic cellulose: Surface activity and aqueous self-assembly into nano-sized polymeric micelles. React. Funct. Polym. 2008, 68, 981-989. (16) Srokova, I.; Talaba, P.; Hodul, P.; Balazova, A., Emulsifying agents based on 18


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cationic starch derivatives. Carbohyd. Polym. 2008, 74, 673-679. (34) Zhou, J. P.; Xu, Y. L.; Wang, X. L.; Qin, Y.; Zhang, L. N., Microstructure and aggregation behavior of methylcelluloses prepared in NaOH/urea aqueous solutions. Carbohyd. Polym. 2008, 74, 901-906. (35) Tezuka, Y.; Imai, K.; Oshima, M.; Chiba, T., Determination of Substituent Distribution in Cellulose Ethers by Means of a C-13 Nmr-Study on Their Acetylated Derivatives .4. O-Methyl-O-Hydroxyalkylcelluloses. Makromol. Chem. 1990, 191, 681-690. (36) Reuben, J.; Conner, H., Analysis of the carbon-13 NMR spectrum of hydrolyzed O-(carboxymethyl) cellulose: Monomer composition and substitution patterns. Carbohyd. Res. 1983, 115, 1-13. (37) Eastoe, J.; J.S.Dalton, Dynamic surface tension and adsorption mechanisms of surfactants at the air-water interface. Adv. Colloid. Interfac. 2000, 85, 103-144. (38) Hong, P.; Fa, C.; Wei, Y. P.; Sen, Z., Surface properties and synthesis of the cellulose-based amphoteric polymeric surfactant. Carbohyd. Polym. 2007, 69, 625-630. (39) Tam, K. C.; Jenkins, R. D.; Winnik, M. A.; Bassett, D. R., A structural model of hydrophobically modified urethane-ethoxylate (HEUR) associative polymers in shear flows. Macromolecules 1998, 31, 4149-4159. (40) Ji, Y. Q.; Wang, W. S.; Li, G. Z.; Zheng, L. Q., Rheological properties of wormlike micelles formed in the sodium oleate/trisodium phosphate aqueous solution. Chinese. Chem. Lett. 2008, 19, 483-487. (41) Kfistner, U.; Hoffmann, H.; Dönges, R.; Ehrler, R., Interactions between modified hydroxyethyl cellulose (HEC) and surfactants. Colloid. Surface. A. 1996, 209-225.

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Figure captions Figure 1. Synthetic route of H-ESO-HEC polymeric surfactants Figure 2. FT-IR spectra of HEC and H-ESO-HEC-III Figure 3. 1H NMR and 13C NMR spectrum of H-ESO-HEC-III sample in DMSO-d6 Figure 4. GPC profile signal of the refractive index of H-ESO-HEC samples Figure 5. Conductometric titration curve of H-ESO-HEC-III Figure 6. The DSC curves of HEC and H-ESO-HEC samples Figure 7. Effect of H-ESO-HEC-Na concentration in water on equilibrium surface tension of water Figure 8. Apparent viscosity of the H-ESO-HEC-Na-III solution as a function of shear rate

21



Page 22 of 31

Tables Table 1. Effect of reaction condition on yield and carboxyl content and the HT-GPC data of H-ESO-HEC HEC:ESO

Samples

Yielda

Carboxyl Content

(mass ratio)

(g/g)

(mmol/g)

—

—

0

HEC

b

HT-GPC data Mn×10 (Daltons)

Mw×105(Daltons)

PDIc

0.7

2.28

3.20

5

H-ESO-HEC-I

1:1

1.2

1.89±0.13

1.48

2.71

1.80

H-ESO-HEC-II

1:2

1.6

2.12±0.06

1.55

3.31

2.10

H-ESO-HEC-III

1:3

1.9

2.35±0.10

3.77

5.50

1.46

H-ESO-HEC-IV

1:4

2.1

2.72±0.03

4.19

6.61

1.57

H-ESO-HEC-V

1:5

2.7

2.80±0.11

4.58

8.28

1.80

a

Expressed as g of the H-ESO-HEC per g HEC (on dry mass basis)

b

Expressed as mmol of carboxyl per g H-ESO-HEC (on dry mass basis)

c

Determined by Mw/Mn

Table 2. 1H and 13C NMR signals of H-ESO-HEC 1

13

H NMR

Signal

C NMR

δ (ppm)

Signal

δ (ppm)

-OH

2.10

-COOH

176

-(CH2)n-CH3

0.90

-CH3

15

-(CH2)n-CH3

1.30-1.50

-CH2-

27-35

CH2-O-CH2

2.80

CH2-O-CH2

83

CH-OH

3.20

CH-OH

80

AGU-H1

4.75

AGU-C1

101

AGU-H2

3.50

AGU-C2

74

AGU-H3

3.55

AGU-C3

70

AGU-H4

3.30

AGU-C4

76

AGU-H5

3.65

AGU-C5

73

AGU-H6

3.45

AGU-C6

62

Table 3. CMC, γcmc, KP and HLB value of samples Samples

CMC (g/L)

γcmc (mN/m)

KP (oC)

HLB value

HEC

—

54.68±1.20

—

—

H-ESO-Na

1.105

34.28±0.93

18

9.6-10.6

H-ESO-HEC-Na-I

1.053

26.33±0.56

39

9.6-10.6

H-ESO-HEC-Na-II

0.702

28.88±0.99

42

9.6-10.6

H-ESO-HEC-Na-III

0.514

27.83±0.61

45

9.6-10.6

H-ESO-HEC-Na-IV

0.402

26.37±0.50

52

9.6-10.6

H-ESO-HEC-Na-V

0.157

26.94±1.26

68

9.6-10.6

22


Page 23 of 31


Figure graphics Figure 1.

23



Figure 2.

24


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Figure 3.

25



Figure 4.

26


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Figure 5.

27



Figure 6.

28


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Figure 7.

29



Figure 8.

30


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Table of Contents Graphics (For Table of Contents only)

31


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