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Preparation and performance of silica/epoxy groupfunctionalized bio-based elastomer nanocomposite He Qiao, Wenji Xu, Mingyuan Chao, Jun Liu, Weiwei Lei, Xinxin Zhou, Runguo Wang, and Liqun Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03517 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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Industrial & Engineering Chemistry Research

3

Preparation and performance of silica/epoxy group-functionalized bio-based elastomer nanocomposite

4

He Qiao,a,b Wenji Xu,a,b Mingyuan Chao,b Jun Liu,a,b Weiwei Lei,a,b Xinxin Zhou,a,b

5

Runguo Wang, b * Liqun Zhang a, b*

6

a

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Technology, Beijing 100029, China

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b

9

Materials, Beijing 100029, China

1 2

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical

Key Laboratory of Beijing City for Preparation and Processing of Novel Polymer

10 11 12 13 14 15 16 17 18 19

1

ACS Paragon Plus Environment


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KEYWORDS: bio-based elastomer; silica; interfacial interaction

21

ABSTRACT:

22

itaconate-ter-isoprene-ter-glycidyl methacrylate) (PDBIIG) was synthesized via redox

23

emulsion

24

group-included monomer. The silica/PDBIIG nanocomposite was prepared without

25

adding silane coupling agents. Ring-opening reaction, which occurred between the

26

hydroxyl groups on the silica surfaces and the epoxy groups of the PDBIIG chains during

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mixing and vulcanization, was confirmed via bound rubber tests and Fourier transform

28

infrared spectroscopy. This reaction was facilitated through heat treatment at 150 °C

29

effectively. The introduction of covalent bonds significantly improved the interfacial

30

interaction and dispersion of silica, which was indicated by transmission electron

31

microscopy and rubber process analyzer (RPA) results. With the same silica loading and

32

compounding procedure, the inclusion of 3.7 wt.% GMA increased the modulus at 100%

33

strain by 150.0% and the modulus at 300% strain by 152.3%. For the dynamic

34

mechanical properties, the nanocomposite with GMA exhibited higher wet skid resistance

35

and lower rolling resistance than the nanocomposite without GMA.

36

1. Introduction

Epoxy

group-functionalized

polymerization

using

glycidyl

bio-based

methacrylate

elastomer

(GMA)

poly

as

the

(dibutyl

epoxy

37

Elastomers have excellent elasticity and play a significant role in national defense,

38

industries, and daily living. With the dwindling fossil resources, soaring energy demand, 2


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and growing environmental concerns, bio-based elastomers derived from renewable

40

resources have attracted increasing attention1-3. Natural rubber is a typical bio-based

41

elastomer, which is directly acquired from the natural rubber tree Hevea brasiliensis.

42

However, natural rubber faces several serious problems, such as the harsh growth

43

conditions of rubber trees, fungal disease threat to rubber trees, and increasing allergic

44

reactions to the proteins in natural rubber 4; thus, the development of bio-based

45

synthesized elastomers, particularly those for engineering applications, is highly

46

important and urgent 5. Several types of bio-based synthesized elastomers, including polyester elastomers 6-10,

47

11-17

itaconic acid-based elastomers

49

rubber 22, have been developed in recent years. The present study is related to an itaconic

50

acid-based elastomer poly (dibutyl itaconate-co-isoprene) (PDBII) and its relevant

51

nanocomposites. Itaconic acid is a bio-based building block chemical with multiple

52

functional groups; this acid is produced industrially through the fermentation of sugars23,

53

24

54

are promising bio-based monomers for bio-based elastomers 11-15. Comonomer isoprene is

55

used to increase the flexibility of macromolecular chains and to provide crosslink points

56

for elastomers. Bio-isoprene from renewable resources is expected to become available in

57

the market in the near future because of numerous efforts devotedly working on it 22, 25.

58

, polyurethane elastomers

18-21

48

, and the bio-isoprene

. Itaconate esters made through the esterification of itaconic acid and bio-based alcohols

The addition of a filler is necessary to improve elastomer properties 3


26, 27

. Silica is a


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non-petroleum-based filler and is widely used in the rubber industry because it can

60

provide excellent mechanical properties, low rolling resistance, and high wet skid

61

resistance

62

polar silica is prone to agglomerating because of its abundant surface hydroxyl groups,

63

thereby leading to the poor dispersion of silica and the weak interfacial interaction

64

between silica and elastomers 30, 31. The most common method to solve these problems is

65

to use silane coupling agents, such as bis-(γ-triethoxysilylpropyl)-tetrasulfide (Si69) and

66

3-(mercaptopropyl trimethoxysilane (KH590), which can modify the silica surface and

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link silica with elastomer macromolecules

68

(VOCs), such as methanol and ethanol, are released during high-temperature silanization

69

at approximately 150 °C 32, 35, 36; these compounds are harmful to the environment and to

70

operators. The functionalization of the elastomer matrix can also improve the dispersion

71

of silica and the interfacial interaction between silica and elastomer. The hydroxyl groups

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on the silica surface are numerous; hence, if we endow the bio-based PDBII with

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functional groups that can react directly with the hydroxyl groups, then interfacial

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interaction improves by forming covalent bonds at the interfaces. The dispersion of silica

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can also be improved. The ring-opening reaction between the epoxides and hydroxyl

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groups of silica occurs with the help of a mechanical force and increased temperature 37-41.

77

Thus, we intend to functionalize PDBII with epoxy groups. The addition of epoxy

78

group-included comonomer in the polymerization can realize this idea easily. Glycidyl

28, 29

. The present study used silica to reinforce bio-based PDBII. However,

31-34

. However, volatile organic compounds

4


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methacrylate (GMA) is a bifunctional chemical with a double bond and an epoxy group.

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It was chosen as the appropriate third comonomer for PDBII in this study. The terpolymer

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poly (dibutyl itaconate-ter-isoprene-ter-glycidyl methacrylate) (PDBIIG) was designed as

82

the matrix of the silica-filled nanocomposite. The designed schematic is shown in Figure

83

1.

84

In the present work, epoxy group-functionalized bio-based elastomer PDBIIG was

85

synthesized via mild free-radical redox emulsion polymerization, and silica/PDBIIG

86

nanocomposite was prepared via simple mechanical blending. Silica was covalently

87

linked with PDBIIG because of the ring-opening reaction between silica and PDBIIG.

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The direct matrix–filler linkage avoids the addition of any silane coupling agent and

89

inhibits the release of VOCs during processing. The ring-opening reaction between silica

90

and PDBIIG during mixing and vulcanization was discussed and affirmed. The effects of

91

the covalent bonding interfaces on the interfacial interaction between silica and elastomer,

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the dispersion of silica, the dynamic mechanical properties, and the static mechanical

93

properties of the nanocomposite were investigated. Our research on elastomer

94

functionalization through polymerization to improve interfacial interaction between the

95

filler and the matrix provides a new idea for preparing nanocomposite with desired

96

properties.

97

5



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Figure 1. Designed schematic of the nanocomposite with covalent linkage between silica and the elastomer.

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2. Experimental section

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2.1. Materials

103

Dibutyl itaconate (96%) was bought from Sigma-Aldrich. Isoprene (99%) was

104

bought from Alfa Aesar and distilled before use. GMA (97%) was bought from

105

Sigma-Aldrich and passed through a neutral alumina column before use. Sodium dodecyl

106

benzene sulfonate (SDBS, 95%) was purchased from Aladdin. Ferric ethylene diamine

107

tetraacetic acid salt (Fe-EDTA), sodium hydroxymethanesulfinate (SHS), tert-butyl

108

hydroperoxide (TBH), and hydroxylamine (HA) were bought from Sigma-Aldrich and

109

used as received. The precipitated silica (Ultrasil VN3) with a BET specific surface of

110

175 m2/g and a particle size of 20–30 nm was bought from Degussa Chemical. Other

111

materials used were of analytical grade and commercially available.

112

2.2. Synthesis of PDBII and PDBIIG

6


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Figure 2. Polymerization reaction of PDBIIG.

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Bio-based PDBIIG was synthesized via free-radical emulsion polymerization under

116

20 °C based on the formula shown in Table S1. The polymerization reaction is shown in

117

Figure 2. In a three-neck glass flask, deionized water, SDBS solution, Fe-EDTA solution,

118

SHS solution, and a mixture of the monomers (dibutyl itaconate, isoprene, and GMA)

119

were added under nitrogen atmosphere, and the emulsion system was stirred at 400 rpm

120

for 1 h. The initiator TBH was then added, and the stirring rate was reduced to 250 rpm.

121

After 12 h, the polymerization reaction was terminated with HA, and the PDBIIG latex

122

was obtained. The PDBIIG latex was then flocculated with ethanol to acquire the wet

123

PDBIIG elastomer, which was dried in a vacuum oven for 24 h at 60 °C. The PDBII

124

elastomer was also synthesized in the same way except for the addition of GMA to

125

compare with the PDBIIG elastomer.

126

2.3. Preparation of nanocomposites

127

The compounding formulation for silica/PDBII and silica/PDBIIG is shown in Table

128

S2. First, silica was mixed with the elastomer in a Haake internal mixer at 30 °C for 10

129

min. The silica/PDBII (silica/PDBIIG) mixture was taken out and cooled down to room

130

temperature. Second, the other additives were mixed with the silica/PDBII 7



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131

(silica/PDBIIG) mixture at 30 °C to obtain the silica/PDBII (silica/PDBIIG) compound.

132

Finally, the compound was vulcanized at 150 °C under 15 MPa to obtain the silica/PDBII

133

(silica/PDBIIG) nanocomposite. The optimum curing time of the compound was

134

determined using a disc vulkameter. A silica/PDBIIG-150 nanocomposite was also

135

prepared to investigate the reaction between PDBIIG and silica. The silica/PDBIIG

136

mixture obtained at the first step was added to the Haake internal mixer at 150 °C and

137

treated for 5 min to facilitate further reaction between PDBIIG and silica. The obtained

138

silica/PDBIIG-150 mixture was then taken out and cooled down to room temperature.

139

The subsequent steps were the same as those for silica/PDBIIG. The neat PDBII and

140

PDBIIG elastomers were also compounded and vulcanized in the same manner as the

141

silica/PDBII (silica/PDBIIG) nanocomposite except for the addition of silica.

142

2.4. Measurements and characterization

143

1

H nuclear magnetic resonance (1H NMR) measurements were obtained using a

144

Bruker AV400 spectrometer with CDCl3 as the solvent. Fourier transform infrared (FTIR)

145

spectra were collected on a Bruker Tensor 27 spectrometer. Differential scanning

146

calorimetry (DSC) was conducted with a Mettler-Toledo DSC instrument under nitrogen.

147

The sample was heated to 100 °C and kept isothermal for 3 min to remove the previous

148

history. Then, it was cooled to −100 °C and reheated to 100 °C. The heating (cooling)

149

rate was 10 °C/min. Gel permeation chromatography was performed with a Waters

150

Breeze instrument equipped with three columns (Styragel HT3_HT5_HT6E) and a 8


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151

refractive index detector. Tetrahydrofuran was used as the eluent with a flowing rate of

152

1.0 mL/min, and polystyrene standards were used for calibration. The morphologies of

153

the nanocomposites were observed using a Tecnai G2 20 S-TWIN transmission electron

154

microscope (TEM) at an acceleration voltage of 200 KV, and the ultrathin sections were

155

cut with an EM UC7/FC7 microtome at −100 °C. The strain sweep measurements of the

156

compounds and the nanocomposites were analyzed with an RPA 2000 at 60 °C, and the

157

test frequency was 1 Hz. The dynamic mechanical thermal properties were implemented

158

on a 01dB-Metravib VA 3000 dynamic mechanical thermal analyzer at 10 Hz under the

159

strain amplitude of 0.1% in a tension mode. The temperature was scanned from −80 °C to

160

80 °C with a heating rate of 3 °C/min. The mechanical properties were implemented

161

using a SANS CMT 4104 electrical tensile instrument at 25 °C with a tensile rate of 500

162

mm/min according to ASTM D412. The bound rubber tests were measured based on the

163

previously reported methods 42.

164 165

3. Results and discussion

166

3.1. Structure and characterization of PDBIIG

167

168

Table 1 Yields, compositions, and molecular weights of PDBII and PDBIIG. Sample

Yield (%)

GMA content in feed (wt.%)

Actual GMA content in PDBIIG (wt.%)

Mn (g/mol)

PDI

PDBII

89

0

0

475000

2.5

PDBIIG

92

5

3.7

397000

2.2

The bio-based elastomer PDBIIG was successfully synthesized via a mild redox 9



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169

free-radical emulsion polymerization. After polymerization for 12 h, the yields of PDBII

170

and PDBIIG reached 89% and 92%, respectively. Table 1 shows that the number-average

171

molecular weight (Mn) of PDBII is 475000 g/mol with a polydispersity index (PDI) of 2.5,

172

whereas the Mn of PDBIIG is 397000 g/mol with a PDI of 2.2. The actual GMA content

173

of PDBIIG was estimated by NMR. The result shows that the GMA content in the

174

elastomer is lower than that in the feed.

175 176

Figure 3. FTIR spectra of PDBII and PDBIIG.

177 178

The chemical structure of PDBIIG was determined by FTIR and 1H NMR. Figure 3

179

shows the FTIR spectra of PDBII and PDBIIG. The two spectra present similar

180

absorption peaks. The peaks at 1173 cm−1 and 1728 cm−1 belong to the C-O-C and C=O

181

stretching vibrations of dibutyl itaconate, respectively. The peak at 1663 cm−1 is

182

attributed to the C=C stretching vibration of isoprene. The peaks at 844 cm−1 and 1350

183

cm−1 are assigned to the C-H out-of-plane and in-plane deformation vibrations of the 10


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184

double bonds of isoprene, respectively. The peaks at 739 cm−1 and 967 cm−1 correspond

185

to the C-H deformation vibration of the cis-1,4 structures and trans-1,4 structures of

186

isoprene, respectively. The small difference of the two spectra lies on the weak peak at

187

908 cm−1, which corresponds to the ring vibration of the epoxy group. The structure of

188

PDBIIG was further confirmed by 1H-NMR. The spectra and the detailed assignments of

189

each peak to the molecular structure are shown in Figure 4. The small peaks at 2.82, 3.20,

190

and 3.82 ppm originate from the protons of an epoxy group and its adjacent methylene.

191

The results of the FTIR and 1H-NMR verify that GMA has been successfully introduced

192

into the PDBII chains.

193 194

Figure 4. 1H-NMR spectra of PDBII and PDBIIG.

195

The glass transition temperature (Tg) of PDBII and PDBIIG was obtained using DSC

196

thermograms (Figure 5). The introduction of GMA increases the Tg of the elastomer.

197

Moreover, neither of the two curves shows any crystallization peak, which indicates that

198

the elastomers are amorphous. 11



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199 200 201

Figure 5. DSC curves of PDBII and PDBIIG. 3.2. Characterization of nanocomposites

202 203 204 205

Figure 6. Bound rubber contents of silica/PDBII, silica/PDBIIG, and silica/PDBIIG-150 compounds.

206

Bound rubber is the rubber absorbed on the filler surfaces. The interaction between

207

the filler and the rubber leads to the formation of bound rubber; bound rubber

208

significantly affects filler reinforcement

209

silica/PDBIIG, and silica/PDBIIG-150 compounds were measured. Figure 6 shows that

210

the introduction of GMA considerably increases bound rubber content, and the additional

43

. The bound rubber contents of silica/PDBII,

12


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211

heat treatment at 150 °C further increases bound rubber content. Bound rubber

212

measurements were conducted on unvulcanized compounds; hence, we can speculate a

213

ring-opening reaction between PDBIIG and silica during the mixing process, and the heat

214

treatment further facilitates the reaction. The results of the bound rubber content indicate

215

that the interfacial interaction in silica/PDBIIG is stronger than that in silica/PDBII

216

because of the introduction of covalent bonds between silica and the matrix. Moreover,

217

the shear force during mixing with a high temperature facilitates the ring-opening

218

reaction between silica and PDBIIG.

219 220 221 222 223 224 225 226

Figure 7. FTIR spectra of (a) PDBIIG, (b) silica/PDBIIG compound, (c) silica/PDBIIG nanocomposite, (d) silica/PDBIIG-150 compound, (e) silica/PDBIIG-150 nanocomposite, and (f) silica/PDBII compound. (All the test samples were prepared using an identical procedure, as described in Section 2.3, but without the addition of any rubber ingredient to avoid interference peaks.) The

FTIR

spectra the

of

the

silica/PDBIIG

silica/PDBIIG-150

compound,

compound,

and

silica/PDBIIG

227

nanocomposite,

228

nanocomposite were obtained to further investigate the reaction between silica and 13


the

the

silica/PDBIIG-150


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229

PDBIIG during the preparation of the nanocomposites. The corresponding spectra are

230

shown in Figure 7. All the spectra were normalized with the peak of the C=O stretching

231

vibration at 1728 cm−1. The intensity variation of the peak of the epoxy group ring

232

vibration at 908 cm−1 was used to characterize the ring-opening reaction between the

233

epoxy groups of PDBIIG and the hydroxyl groups of silica. The intensity of the peak at

234

908 cm−1 for the silica/PDBIIG compound (b) is slightly lower than that for the neat

235

PDBIIG (a), thereby indicating the ring-opening reaction between PDBIIG and silica

236

during the mixing process. After vulcanization, the intensity of the peak at 908 cm−1 of

237

silica/PDBIIG (c) continuously decreases, which indicates that the ring-opening reaction

238

proceeds during the hot-press process at 150 °C. The comparison between (b) and (d)

239

shows that the intensity of the peak at 908 cm−1 of the silica/PDBIIG-150 compound is

240

lower than that of the silica/PDBIIG compound, thereby indicating that the additional hot

241

treatment with shear force and temperature effectively facilitates ring-opening reaction,

242

which is consistent with the result of the bound rubber content mentioned earlier. The

243

subsequent vulcanization further promotes the reaction, and the peak at 908 cm−1 of the

244

silica/PDBIIG-150 nanocomposite (e) nearly disappears similar to that of the

245

silica/PDBII compound (f), which indicates that the ring-opening reaction between silica

246

and PDBIIG is nearly complete. The FTIR results of all the compounds and the

247

nanocomposites manifest that the reaction between silica and PDBIIG occurs in the

248

mixing and vulcanization processes. The hot treatment at 150 °C will make the reaction 14


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249


comprehensive.

250

The dispersion of the filler in the matrix is an important factor for a composite to

251

achieve high performance. The TEM micrographs shown in Figure 8 exhibit the

252

dispersion of silica in PDBIIG and PDBII. The dark spots in the micrographs are silica

253

particles. Figure 8(a) shows that the dispersion of silica in PDBII is poor with a few

254

aggregates and voids. The introduction of GMA significantly improves the dispersion of

255

silica. Figures 8(b) and 8(c) show that silica disperses uniformly in PDBIIG without any

256

evident aggregates and voids. The significant improvement of dispersion is attributed to

257

the formation of covalent bonds between silica and PDBIIG in the silica-filled PDBIIG

258

nanocomposites.

15



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259 260 261

262 263 264 265 266 267

Figure 8. TEM micrographs of (a) silica/PDBII, (b) silica/PDBIIG, and (c) silica/PDBIIG-150 nanocomposites.

Figure 9. Strain amplitude dependence of (a) G′ of unvulcanized PDBII, PDBIIG, silica/PDBII, silica/PDBIIG, and silica/PDBIIG-150; and (b) tan δ of vulcanized PDBII, PDBIIG, silica/PDBII, silica/PDBIIG, and silica/PDBIIG-150. RPA was used to investigate the effect of covalent bonding interfaces on the filler 16


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268

network, which consists of the filler–filler network and the filler–rubber network, and the

269

interfacial interaction between silica and PDBIIG. Figure 9 shows the strain amplitude

270

dependence of the G′ of the unvulcanized compounds and the tan δ of the vulcanized

271

PDBII, PDBIIG, silica/PDBII, silica/PDBIIG, and silica/PDBIIG-150. Little difference

272

exists between PDBII and PDBIIG in the RPA results. In the filled elastomers, the

273

structure of the filler–filler networks can be reflected by the Payne effect

274

referred to as the strain dependence of G′. G′ decreases sharply when the filler–filler

275

networks are broken after the strain reaches a critical value.

276

initial G′ of the silica/PDBII compound is the highest, and the Payne effect is evident,

277

which indicates that the filler–filler networks are strong in silica/PDBII because of the

278

poor dispersion of silica. The introduction of GMA effectively reduces the Payne effect,

279

and silica/PDBIIG and silica/PDBIIG-150 are less strain-dependent than silica/PDBII,

280

thereby indicating the improved dispersion of silica. Although the shapes of the curves of

281

silica/PDBIIG and silica/PDBIIG-150 are similar with long plateau regions of G′, the G′

282

of silica/PDBIIG-150 is higher than that of silica/PDBIG. This finding can be attributed

283

to the presence of more covalent bonds between silica and PDBIIG in silica/PDBIIG-150;

284

hence, the interfacial interaction between silica and PDBIIG in silica/PDBIIG-150 is

285

stronger than that in silica/PDBIIG. The strong interfacial interaction strengthens the

286

filler–rubber networks and restrains the mobility of the macromolecular chains, which

287

leads to an increase in G′. The tan δ of the vulcanized nanocomposites is presented in 17


44

, which is

Figure 9(a) shows that the


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Page 18 of 34

288

Figure 9(b). The minimal difference is between the tan δ of silica/PDBII and

289

silica/PDBIIG at a low strain value (less than 1%). With increasing strain, the tan δ of

290

silica/PDBII increases sharply, whereas the tan δ of silica/PDBIIG increases gradually.

291

The different tendencies of tan δ with increasing strain between silica/PDBII and

292

silica/PDBIIG is attributed to the different filler network structures. The poor dispersion

293

of silica in silica/PDBII leads to strong filler–filler networks, which are easily destroyed

294

with increasing strain. The destruction of strong filler–filler networks increases filler–

295

filler friction, and the weak interfacial interaction increases filler–rubber friction

296

which leads to the high tan δ at high strain. The improved interfacial interaction and

297

dispersion of silica in silica/PDBIIG contribute to the strong filler–rubber networks and

298

few filler–filler networks. Thus, the tan δ of silica/PDBIIG is lower than that of

299

silica/PDBII at high strain. The interfacial interaction between silica and PDBIIG is

300

stronger in silica/PDBIIG-150 than in silica/PDBIIG. Hence, the filler–rubber friction in

301

silica/PDBIIG-150 is further decreased, and the value of tan δ is lower.

302

18


45

,

Page 19 of 34

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303 304 305 306

Figure 10. Vulcanization curves of the PDBII, PDBIIG, silica/PDBII, silica/PDBIIG, and silica/PDBIIG-150 nanocomposites.

307

Figure 10 shows the vulcanization curves of the PDBII, PDBIIG, silica/PDBII,

308

silica/PDBIIG, and silica/PDBIIG-150 nanocomposites. The corresponding vulcanization

309

characteristics are summarized in Table S3. The minimum torque (ML) and the maximum

310

torque (MH) of the neat PDBIIG are nearly the same as those of the neat PDBII. However,

311

the curing time of PDBIIG is longer than that of PDBII. All the rubber additives and

312

operations are the same; therefore, the increase in curing time is attributed to the presence

313

of the epoxy groups, which may react with the rubber accelerators

314

their accelerating effect on vulcanization. The addition of silica evidently extends the

315

curing time. Delayed vulcanization results from the adsorption of the accelerators on the

316

silica surfaces. The curing time of silica/PDBIIG is longer than that of silica/PDBII

317

because of the probable reaction between the epoxy groups and the accelerators. The

318

curing time of silica/PDBIIG-150 is shorter than that of silica/PDBIIG. The decrease in

319

the curing time of silica/PDBIIG-150 is attributed to the additional mixing at 150 °C, 19


46, 47

, and thus, reduce


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

320

during which more epoxy groups in PDBIIG react with silica, such that fewer epoxy

321

groups react with the accelerators during the vulcanization process. The torque of

322

silica/PDBIIG is significantly lower than that of silica/PDBII because filler–filler

323

networks are less in silica/PDBIIG. The interfacial interaction in silica/PDBIIG-150 is

324

stronger; hence, its torque values are higher than those of silica/PDBIIG. The torque

325

difference (MH−ML) of the vulcanization curve is closely related to crosslink density. The

326

two neat elastomers exhibit similar crosslink densities. In the silica-filled nanocomposites,

327

crosslink densities increase with the increase in interfacial interaction.

328

3.3. Performance of the nanocomposites

329 330 331 332 333 334

Figure 11. Temperature dependence of (a) E′ of the PDBII, PDBIIG, silica/PDBII, silica/PDBIIG, silica/PDBIIG-150, and silica/PDBII-Si69 nanocomposites; and (b) tan δ of the PDBII, PDBIIG, silica/PDBII, silica/PDBIIG, silica/PDBIIG-150, and silica/PDBII-Si69 nanocomposites.

335

The dynamic mechanical thermal properties of the PDBII, PDBIIG, silica/PDBII,

336

silica/PDBIIG, and silica/PDBIIG-150 nanocomposites are shown in Figure 11. With the

337

exception of the slightly higher Tg and lower tan δ at Tg, minimal difference is observed 20


Page 20 of 34

Page 21 of 34

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338

between the two neat elastomers in the temperature dependence of E′ and tan δ. The

339

curves change significantly after silica is added. Figure 11(a) represents the temperature

340

dependence of E′ of the PDBII, PDBIIG, silica/PDBII, silica/PDBIIG, and

341

silica/PDBIIG-150 nanocomposites. The high E′ of silica/PDBII at the rubbery region is

342

attributed to the strong filler–filler networks that result from the poor dispersion of silica.

343

The E′ of silica/PDBIIG-150 is higher than that of silica/PDBIIG because of the

344

improved interfacial interaction between silica and PDBIIG. This finding is consistent

345

with the RPA results. The temperature dependence of the tan δ of the PDBII, PDBIIG,

346

silica/PDBII, silica/PDBIIG, and silica/PDBIIG-150 nanocomposites is shown in Figure

347

11(b) and summarized in Table 2. The Tg and the highest tan δ at Tg increase significantly

348

after the introduction of GMA into PDBII. The increase in Tg is attributed to the

349

strengthened filler–rubber networks resulting from the improved dispersion of silica and

350

interfacial interaction, which confines the mobility of the polymer chains. The confined

351

mobility of the polymer chains also decreases the highest tan δ at Tg. However, the

352

improved dispersion of silica increases the rubber fraction that participates in chain

353

segment relaxation by decreasing the amount of macromolecular chains trapped in the

354

filler–filler networks 45, 48, thereby increasing the highest tan δ at Tg. In silica/PDBIIG, the

355

second factor is dominant, and the highest tan δ at Tg of silica/PDBIIG is higher than that

356

of silica/PDBII. The Tg of silica/PDBIIG-150 is nearly the same as that of silica/PDBIIG,

357

whereas the highest tan δ at Tg of silica/PDBIIG-150 is lower because of the improved 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

358

interfacial interaction between silica and PDBIIG in silica/PDBIIG-150. It is widely

359

accepted in tire industry that the high tan δ at 0 °C indicates good wet skid resistance, and

360

the low tan δ at 60 °C indicates low rolling resistance

361

silica/PDBIIG nanocomposite exhibits significantly better wet skid resistance and lower

362

rolling resistance than silica/PDBII. The wet skid resistance of silica/PDBIIG-150 is

363

slightly lower than that of silica/PDBIIG given the lower tan δ at Tg. Moreover, the

364

silica-filled PDBII nanocomposite which contains 5 phr of Si69 (silica/PDBII-Si69) is

365

also prepared. The Tg and tan δ values of silica/PDBII-Si69 are presented in Table 2. The

366

comparison shows that silica/PDBIIG performs better than silica/PDBII-Si69 in terms of

367

wet skid resistance and rolling resistance. It is worth mentioning that the silica/PDBIIG

368

nanocomposite exhibits excellent wet skid resistance property compared with other

369

silica-filled traditional rubber composites 42, 51-55.

370 371

49, 50

. Table 2 shows that the

Table 2 Tg and tan δ values of the silica/PDBII, silica/PDBIIG, and silica/PDBIIG-150 nanocomposites. Sample

Tg (°C) Tan δ at 0 °C Tan δ at 60 °C

Silica/PDBII

−16.5

0.553

0.155

Silica/PDBIIG

−5.3

1.035

0.123

Silica/PDBIIG-150

−5.4

0.954

0.122

Silica/PDBII-Si69

−12.4

0.546

0.152

22


Page 22 of 34

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372 373 374 375


Figure 12. Stress–strain curves of the PDBII, PDBIIG, silica/PDBII, silica/PDBIIG, silica/PDBIIG-150, and silica/PDBII-Si69 nanocomposites.

376

The stress–strain curves of PDBII, PDBIIG, silica/PDBII, silica/PDBIIG,

377

silica/PDBIIG-150, and silica/PDBII-Si69 are displayed in Figure 12 and summarized in

378

Table S4. The mechanical properties of the neat PDBII and PDBIIG are similar and poor,

379

and the addition of silica significantly improves the mechanical properties of PDBII and

380

PDBIIG. Moreover, silica/PDBIIG exhibits better mechanical properties than

381

silica/PDBII because of the strong interfacial interaction and the good dispersion of silica.

382

With the same silica loading and procedure, the inclusion of 3.7 wt.% GMA increases the

383

modulus at 100% strain by 150% and the modulus at 300% strain by 152.3%. The low

384

elongation at break of silica/PDBIIG results from the strong interfacial interaction

385

because the slippage of the polymer chains is restricted. The interfacial interaction of

386

silica/PDBIIG-150 is stronger than that of silica/PDBIIG. Thus, the elongation at break of

387

silica/PDBIIG-150 is lower and the modulus of silica/PDBIIG-150 is higher. The 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

388

mechanical properties of silica/PDBII-Si69 were also measured. Silica/PDBIIG performs

389

better than silica/PDBII-Si69 in modulus at 100% strain. However, the tensile strength of

390

silica/PDBIIG is slightly lower compared with other traditional rubber composites 31, 42, 48,

391

51-55

.

392 393

4. Conclusions

394

Epoxy group-functionalized bio-based elastomer PDBIIG was synthesized via

395

free-radical redox emulsion polymerization, and silica/PDBIIG nanocomposite was

396

prepared without the addition of silane coupling agents. The dispersion of silica in the

397

elastomer matrix and the interfacial interaction between silica and the elastomer was

398

improved significantly after the introduction of epoxy groups into PDBII. The

399

improvements were attributed to the ring-opening reaction between the hydroxyl groups

400

of silica and the epoxy groups of PDBIIG. Heat treatment at 150 °C facilitated the

401

ring-opening reaction, which further improved the interfacial interaction between silica

402

and PDBIIG. Silica/PDBIIG demonstrated better wet skid resistance, lower rolling

403

resistance, and better mechanical properties than silica/PDBII because of the strong

404

covalent bonding interfacial interaction and the uniform dispersion of silica. Moreover,

405

silica/PDBIIG performed better than silica/PDBII-Si69 in dynamic and static mechanical

406

properties. Silica/PDBIIG nanocomposite may have great potential in tire application

407

after our effort to further improve its comprehensive properties in the future. 24


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408

ASSOCIATED CONTENT

409

Supporting Information

410

The Supporting Information is available free of charge on the ACS Publications website

411

at DOI:

412

Formulation for the emulsion polymerization of PDBII and PDBIIG. Formulation for the

413

silica/PDBII and silica/PDBIIG nanocomposites. Vulcanization characteristics of the

414

PDBII, PDBIIG, silica/PDBII, silica/PDBIIG, and silica/PDBIIG-150 nanocomposites.

415

Mechanical

416

silica/PDBIIG-150, and silica/PDBII-Si69 nanocomposites (PDF)

417

AUTHOR INFORMATION

418

Corresponding Author

419

*

420

Wang)

421

Author Contributions

422

The manuscript was written through contributions of all authors. All authors have given

423

approval to the final version of the manuscript.

424

Funding Sources

425

This work was supported by the National Natural Science Foundation of China

426

(50933001 and 51221002), the Key Program of Beijing Municipal Science and

properties

of

the

PDBII,

PDBIIG,

silica/PDBII,

silica/PDBIIG,

E-mail: [email protected] (Liqun Zhang), [email protected] (Runguo

25



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Page 26 of 34

427

Technology Commission (D14110300230000), and the Joint Development Project of

428

Beijing Municipal Education Commission (JWGJ201602).

429

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