Renewable Epoxy Thermosets from Fully Lignin-Derived Triphenols

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Renewable Epoxy Thermosets from Fully Lignin-Derived Triphenols Shou Zhao, Xiangning Huang, Andrew Whelton, and Mahdi M. Abu-Omar ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00443 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 1, 2018

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ACS Sustainable Chemistry & Engineering

Renewable Epoxy Thermosets from Fully Lignin−Derived Triphenols

1

†

‡

Shou Zhao, Xiangning Huang, Andrew J. Whelton,

2 3

†

4

232, Santa Barbara, California 93106, United States

5

‡

6

Lafayette, Indiana 47907, United States

7

§

8

Indiana 47907, United States

9

ll

‡, §

and Mahdi M. Abu−Omar

†, ll,*

Department of Chemistry & Biochemistry, University of California, Santa Barbara, Building

Lyles School of Civil Engineering, 550 Stadium Mall Drive, Purdue University, West

Division of Environmental and Ecological Engineering, Purdue University, West Lafayette,

Department of Chemical Engineering, University of California, Santa Barbara, Engineering

10

II Building, Santa Barbara, California 93106, United States

11

*

12

ABSTRACT: A series of fully renewable triphenols (TPs) with various number of methoxy

13

group substituents (n = 0−6) were synthesized using lignin−derived phenols (guaiacol and

14

2,6−dimethoxyphenol)

15

syringaldehyde). The structural evolution from TPs to epoxy thermosets was followed by

16

nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy.

17

Thermomechanical properties of the resulting epoxy thermosets were investigated by

18

dynamic mechanical analysis (DMA), tensile analysis (TA) and thermogravimetric analysis

19

(TGA). Increasing the content of methoxy groups decreased the glass transition

20

temperature (132 to 118 °C) and glassy modulus (2.6 to 2.2 GPa). Thermal stability of

21

high−methoxy−content thermosets was reduced due to electron−donating effects and

22

higher oxygen content. Conversions and isolated yields of TPs significantly decreased as

Corresponding author: [email protected]

and

aldehydes

(4−hydroxybenzaldehyde,

1

ACS Paragon Plus Environment

vanillin

and

ACS Sustainable Chemistry & Engineering 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

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23

the number of methoxy substituents increased, which markedly determined the feasibility of

24

TPs as precursors for polymers. This work widens the synthesis route of fully lignin−derived

25

polyphenols, yielding polymers with thermomechanical properties comparable to bisphenol

26

A (BPA) based materials. Evaluation of methoxy substitution provides insight for the

27

selection of lignin−derived monomers.

28

Keywords: Lignin, Epoxy thermoset, Methoxy, Renewable polymer, Thermomechanical

29

properties

30

INTRODUCTION

31

Epoxy thermosets have been extensively used as coatings, adhesives, electronic

32

materials and structural composites because of their excellent thermal and mechanical

33

properties. The most popular epoxy monomers are derived from bisphenol A (BPA), which

34

accounts for more than 90 % of epoxy cross−linked polymers. The suitability of BPA comes

35

from its aromatic structure, which confers good mechanical and thermal properties on the

36

resulting epoxy thermosets. However, as the development of renewable materials has

37

received increased attention, numerous biomass derived molecules have been investigated

38

to replace or supplement the petroleum−based BPA.2−23 Among these feedstocks, lignin is

39

the most promising candidate for making epoxy thermosets, since it is the sole

40

large−volume sustainable source composed of an aromatic skeleton.24, 25

41

1

Recent

catalytic

depolymerization techniques

can

convert lignin

to

various

42

value−added phenolic monomers including phenols and aldehydes.26−31 Lignin−derived

43

phenol monomers (LDPMs) have been extensively studied as BPA alternatives. Since

44

cross−linkable epoxy monomers require at least two epoxides per molecule, special efforts

2


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have been taken to increase the number of functional hydroxyl groups of LDPMs through (1)

46

conversion of other reactive groups like methoxy, double bond, or aldehyde to hydroxyl

47

groups,32−37 and (2) coupling repeated LDPMs using bridging reagents.38−48 Despite the

48

abundance of LDPMs and their coupling methods, lignin−derived aldehyde monomers

49

(LDAMs) are often neglected as renewable bridging reagents (akin to acetone in the

50

preparation of BPA). A few examples include the work of Foyer et al., in which a synthesis

51

route of formaldehyde−free resol resin based on vanillin was reported.

52

hydroxyl group of vanillin had to be protected prior to polymerization because the hydroxyl

53

of vanillin was deprotonated preferentially over phenols. Moreover, it is important to develop

54

building blocks that can yield polymer with tunable properties permitting wider applications.

55

For example, LDAMs can condense with phenols at the para and/or ortho position to yield

56

triphenols (TPs). The TP architecture improves the rigidity compared to conventional

57

bisphenolic systems, while adjusting the number of functional hydroxyl could tune the

58

cross−linking density and therefore mechanical properties of the resulting networks.

59

Furthermore, functionalizing the hydroxyl groups to epoxy, olefin, or cyanate ester results in

60

different types of materials.

49,50

However, the

51

61

Apart from the aromatic nature, lignin is characterized by methoxy substitution of its

62

aromatic rings. There are three types of monolignols (para−coumaryl alcohol, coniferyl

63

alcohol and sinapyl alcohol) existing in the lignin backbone, which form p−hydroxyphenyl

64

(H), guaiacyl (G) and syringyl (S), with varying number of methoxy groups.52,53 In our

65

previous studies, o−demethylation of methoxy groups has been proved as an efficient

66

method to convert lignin−derived phenols into cross−linkable epoxy prepolymer.

3


25,32,43,51


67

However, the presence of methoxy in polymers can alter the properties and should be

68

investigated. For example, methoxy substitution plays an important role in determining the

69

physiochemical properties of lignin. Lignin with high methoxy groups (e.g., hardwood lignin)

70

generally is less thermally stable and produces less char compared to the low methoxys

71

counterparts (e.g., softwood lignin) during lignin pyrolysis.54,55 Several experimental and

72

theoretical studies have shown that oxygen−carbon bond dissociation enthalpy was

73

substantially decreased when ortho or para methoxy substituent was situated on the

74

phenethyl of the dominant β−O−4 linkages.56−58 Inspired by these studies, we set out to

75

investigate the impact of methoxy substituents on the performance of lignin−derived

76

polymers. Hernandez et al. recently reported a bio−based epoxy resin using bis−guaiacol

77

(BG), which was synthesized from vanillyl alcohol and guaiacol.59 To investigate the impact

78

of methoxy groups, BG−based resin was compared to bisphenol F (BPF) based polymer.

79

The methoxy groups lower the glass transition temperature (Tg) and thermal stability of the

80

resulting resins. Harvey et al. reported similar results when making cyanate ester resins with

81

lignin−based bis−creosol, while the significant decrease in thermal stability was attributed to

82

the electron donating effect of methoxy.60 While these studies provided important

83

information on the impact of methoxy substitution, they focused on guaiacol (one methoxy

84

substituent) and ignored syringyl type substrates, which are abundant in hardwood lignin.

85

With the triphenol architecture used in this work, we are able to contrast triphenols with 0−6

86

methoxy substituents. Apart from the thermomechanical properties of thermosets, the

87

feasibility and processability of thermoset precursors (e.g. bisphenol or triphenol) should

88

also be considered. Effect of methoxy on properties of these precursors, including

4


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conversion, difficulty in isolation, yield and melting point needs to be discerned, especially

90

when methoxy substitution is high, 3−6 per molecule.

91

In the present work we report: 1) Synthesis of fully lignin−derived TPs. The synthesis of

92

TPs employs lignin−derived aromatic aldehydes as bridging reagents, which avoids

93

carcinogenic and highly volatile formaldehyde. 2) The impact of methoxy groups on

94

performance of TP−based thermosets. By controlling the starting LDAMs, i.e.,

95

4−hydroxybenzaldehyde (HBA), vanillin (VAN), syringaldehyde (SYA), and LDPMs, i.e.,

96

phenol (PhOH, it was used to compare with other methoxy−bearing phenols, even though it

97

has more petroleum nature), guaiacol (GUA) and 2,6−dimethoxyphenol (DMP), we

98

prepared TPs with 0 to 6 methoxy substituents. Selected TPs were converted to glycidyl

99

ethers and cured to explore their feasibility as precursors to make renewable epoxy resins

100

(Figure 1).

101

EXPERIMENTAL SECTION

102

General.

103

2,6−dimethoxyphenol, epichlorohydrin, tetrabutylammonium bromide, diethylenetriamine

104

(DETA), diglycidyl ether of bisphenol A (DGEBA) and 1,2,4,5−tetrachloro−3−nitrobenzene

105

were purchased from Aldrich Chemical Co. Ethanol (200 proof) was purchased from Decon

106

Labs, Inc. Sulfuric acid (98%) was obtained from Fisher Scientific. All chemicals were used

107

as received without further puriﬁcation.

108

General Procedure for TPs (M0 to M6).

4−Hydroxybenzaldehyde,

vanillin,

syringaldehyde,

phenol,

guaiacol,

109

4−Hydroxybenzaldehyde (2.44 g, 20 mmol) and phenol (7.52 g, 80 mmol) were

110

dissolved in 30 mL absolute ethanol. To this solution, 5.7 g of concentrated sulfuric acid,

5



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111

dissolved in 5 mL of absolute ethanol, were dropwise added while stirred. An ice bath was

112

used to control the temperature below 10 °C. After the addition of sulfuric acid, the

113

temperature was increased to 65 °C and the mixture was gently stirred for 2 days. Then,

114

100 mL of brine was poured into the mixture prior to the addition of 3 × 20 mL of ethyl

115

acetate to extract the product. The extract was dried with MgSO4 and concentrated using a

116

rotary evaporator. Purification of the product using silica gel chromatography (hexane/ethyl

117

acetate, 3:1 to 1:1) gave M0 as an orange solid (4.26 g, 69% isolated yield). H NMR

118

(acetone−d6, 400 MHz, Figure S1−A) δ: 6.91 (d, J = 8.4, 6H, c), 6.73 (d, J = 8.4, 6H, b), 5.30

119

(s, 1H, a).

120

54.26 (Ar3−CH). Anal. calc. for C19H16O3: C 78.06, H 5.52, O 16.42; found: C 77.47, H 5.30,

121

O 17.23. Other TPs (M1 to M6) were prepared using the same methods as M0. Their

122

structures are listed in Figure 2.

1

13

C NMR (acetone−d6, 400 MHz, Figure S1−B) δ: 155.4, 135.9, 130.0, 114.7,

123

M1, prepared from vanillin (3.04 g, 20 mmol) and phenol (7.52 g, 80 mmol), orange

124

solid (3.93 g, 61 % isolated yield). H NMR (acetone−d6, 400 MHz, Figure S2−A) δ: 6.93 (d,

125

J = 5.6, 4H, f), 6.74−6.71 (m, 6H, g+d+e), 6.51 (dd, J = 5.4, 1.2, 1H, c), 5.31 (s, 1H, a), 3.71

126

(s, 3H, b).

127

135.9, 130.0, 121.6, 114.8, 114.4, 112.7, 55.24 (−OCH3), 54.67 (Ar3−CH). Anal. calc. for

128

C20H18O4: C 74.52, H 5.63, O 19.85; found: C 74.23, H 5.44, O 20.33.

1

13

C NMR (acetone−d6, 400 MHz, Figure S2−B) δ: 155.5, 147.1, 144.7, 136.5,

129

M2, prepared from 4−hydroxybenzaldehyde (2.44 g, 20 mmol) and guaiacol (9.92 g, 80

130

mmol), orange solid (3.66 g, 52% isolated yield). 1H NMR (acetone−d6, 400 MHz, Figure

131

S3−A) δ: 6.93 (dd, J = 15.3, 8.5, 2H, d), 6.73 (t, J = 7.8, 6H, g+c+e), 6.53 (dd, J = 8.1, 1.7,

132

2H, f), 5.32 (s, 1H, a), 3.71 (s, 6H, b).

13

C NMR (acetone−d6, 400 MHz, Figure S3−B) δ:

6


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155.5, 147.1, 144.7, 136.4, 135.8, 130.0, 121.6, 114.7, 114.4, 112.7, 55.25 (−OCH3), 55.05

134

(Ar3−CH). Anal. calc. for C21H20O5: C, 71.58; H, 5.72; O, 22.70; found: C 71.47, H 6.33, O

135

20.20.

136

M2’, prepared from syringaldehyde (3.64 g, 20 mmol) and phenol (7.52 g, 80 mmol),

137

orange solid (4.51 g, 64% isolated yield). 1H NMR (acetone−d6, 400 MHz, Figure S4−A) δ:

138

6.94 (d, J = 8.5, 4H, d), 6.73 (d, J = 8.5, 4H, e), 6.38 (s, 2H, c), 5.32 (s, 1H, a), 3.71 (s, 6H,

139

b).

140

125.7, 110.4, 102.5, 51.34 (−OCH3), 50.68 (Ar3−CH). Anal. calc. for C21H20O5: C, 71.58; H,

141

5.72; O, 22.70; found: C 70.95, H 5.77, O 23.28.

13

C NMR (acetone−d6, 400 MHz, Figure S4−B) δ: 151.2, 143.2, 132.5, 131.4, 129.9,

142

M3, prepared from vanillin (3.04 g, 20 mmol) and guaiacol (9.92 g, 80 mmol), orange

143

solid (3.90 g, 51% isolated yield). 1H NMR (acetone−d6, 400 MHz, Figure S5−A) δ:

144

6.77−6.71 (m, 6H, c+e), 6.54 (m, J = 8.1, 1.7, 3H, d), 5.32 (s, 1H, a), 3.71 (s, 9H, b).

145

NMR (acetone−d6, 400 MHz, Figure S5−B) δ: 147.1, 144.7, 136.4, 121.6, 114.4, 112.7,

146

55.44 (Ar3−CH), 55.29 (−OCH3). Anal. calc. for C22H22O6: C, 69.10; H, 5.80; O, 25.10; found:

147

C 68.47, H 5.86, O 25.68.

13

C

148

M4, prepared from syringaldehyde (3.64 g, 20 mmol) and guaiacol (9.92 g, 80 mmol),

149

orange solid (3.79 g, 46% isolated yield). H NMR (acetone−d6, 400 MHz, Figure S6−A) δ:

150

6.78 – 6.69 (m, 4H, f+e), 6.56 (dd, J = 5.4, 1.1, 2H, g), 6.43 (s, 2H, d), 5.32 (s, 1H, a), 3.71 (s,

151

6H, b), 3.69 (s, 6H, c). C NMR (acetone−d6, 400 MHz, Figure S6−B) δ: 147.4, 146.9, 144.7,

152

136.2, 135.5, 121.6, 114.3, 112.7, 106.9, 55.76 (Ar3−CH), 55.67 (−OCH3), 55.28 (−OCH3).

153

Anal. calc. for C23H24O7: C, 66.98; H, 5.87; O, 27.15; found: C 66.69, H 6.53, O 26.78.

154

1

13

M4’,

prepared

from

4−hydroxybenzaldehyde

(2.44

7


g,

20

mmol)

and


Page 8 of 40

155

2,6−dimethoxyphenol (12.32 g, 80 mmol), orange solid (1.48 g, 18% isolated yield). 1H NMR

156

(acetone−d6, 400 MHz, Figure S7−A) δ: 6.95 (d, J = 8.4, 2H, d), 6.74 (d, J = 8.4, 2H, c), 6.43

157

(s, 4H, e), 5.32 (s, 1H, a), 3.71 (s, 12H, b). 13C NMR (acetone−d6, 400 MHz, Figure S7−B) δ:

158

151.2, 143.2, 131.3, 130.9, 129.9, 125.7, 110.4, 102.5, 51.38 (−OCH3 and Ar3−CH). Anal.

159

calc. for C23H24O7: C, 66.98; H, 5.87; O, 27.15; found: C 66.14, H 5.65, O 28.22.

160

M5, prepared from vanillin (3.04 g, 20 mmol) and 2,6−dimethoxyphenol (12.32 g, 80

161

mmol), orange solid (0.62 g, 7% isolated yield). H NMR (acetone−d6, 400 MHz, Figure

162

S8−A) δ: 6.77 (d, J = 1.2, 1H, d), 6.72 (dd, J = 5.3, 2.7, 1H, f), 6.57 (dd, J = 5.4, 1.2, 1H, e),

163

6.45 (s, 4H, g), 5.32 (s, 1H, a), 3.72 (s, 3H, b), 3.70 (s, 12H, c).

164

MHz, Figure S8−B) δ: 143.1, 131.7, 130.8, 129.9, 117.2, 110.0, 108.3, 102.5, 51.71

165

(−OCH3), 51.33 (−OCH3), 50.93 (Ar3−CH). Anal. calc. for C24H26O8: C, 65.15; H, 5.92; O,

166

28.93; found: C 68.30, H 6.47, O 25.23.

1

13

C NMR (acetone−d6, 400

167

M6, prepared from syringaldehyde (3.24 g, 20 mmol) and 2,6−dimethoxyphenol (12.32

168

g, 80 mmol), orange solid (0.47 g, 5% isolated yield). H NMR (acetone−d6, 400 MHz,

169

Figure S9−A) δ: 6.46 (s, 6H, c), 5.32 (s, 1H, a), 3.71 (s, 18H, b). 13C NMR (acetone−d6, 400

170

MHz, Figure S9−B) δ: 147.5, 135.1, 106.8, 56.40 (Ar3−CH), 55.72(−OCH3). Anal. calc. for

171

C25H28O9: C, 63.55; H, 5.97; O, 30.47; found: C 62.70, H 6.19, O 31.11.

172

General Procedure for Glycidyl Ethers of TPs (GEM0 to GEM6).

1

173

GEM0 was prepared by reaction of M0 (2.92 g, 10 mmol) and epichlorohydrin (30 g, 320

174

mmol). Tetrabutylammonium bromide (0.34 g, 1.1 mmol) was used as a phase transfer

175

catalyst. The mixture was heated at 85 °C for 3 h and followed by a dropwise addition of 6 g

176

of 20% w/w NaOH solution. The reaction was kept for another 1.5 h, and the mixture was

8


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177

washed with 15 mL acetone to precipitate the formed NaCl, filtrated to remove NaCl, dried

178

with Na2SO4 and concentrated with a rotary evaporator to yield GEM0 as a yellowish oil

179

(4.33 g, 94% isolated yield). 1H NMR (CDCl3, 600 MHz, Figure S10) δ: 6.91 (6H, f), 6.73 (6H,

180

e), 5.37 (1H, a), 4.15−3.91 (6H, b + b’), 3.32 (3H, c), 2.87−2.71 (6H, d + d’). Other glycidyl

181

ethers (GEM1 to GEM6) were prepared using the same method with 87−96% yields of

182

glycidylation. Epoxy equivalent weights (EEW) of obtained epoxy monomers were

183

determined to be 160−218 g/eq. using the HCl/acetone titration method.

61

184

GEM1, yellowish oil, 94 % isolated yield, EEW = 169 g/eq. 1H NMR (CDCl3, 600 MHz,

185

Figure S11) δ: 6.93 (4H, i), 6.8−6.71 (4H, j+h), 6.25 (1H, g), 6.51 (1H, f), 5.36 (1H, a),

186

4.17−3.91 (6H, b + b’), 3.71 (3H, e), 3.35 (3H, c), 2.88−2.71 (6H, d + d’).

187

GEM2, yellowish oil, 91 % isolated yield, EEW = 179 g/eq. 1H NMR (CDCl3, 600 MHz,

188

Figure S12) δ: 6.93 (2H, g), 6.73 (4H, i+f), 6.64 (2H, h), 6.53 (2H, j), 5.36 (1H, a), 4.18−3.91

189

(6H, b + b’), 3.73 (6H, e), 3.35 (3H, c), 2.86−2.71 (6H, d + d’). 1

190

GEM2’, yellowish oil, 96 % isolated yield, EEW = 181 g/eq. H NMR (CDCl3, 600 MHz,

191

Figure S13) δ: 6.94 (4H, g), 6.73 (4H, h), 6.38 (2H, c), 5.33 (1H, a), 4.18−3.97 (6H, b + b’),

192

3.73 (6H, e), 3.31 (3H, c), 2.86−2.61 (6H, d + d’). 1

193

GEM3, white solid, melting point 115 °C, 87 % isolated yield, EEW = 194 g/eq. H NMR

194

(CDCl3, 600 MHz, Figure S14) δ: 6.77 (3H, i) 6.71 (3H, g), 6.54 (3H, h), 5.34 (1H, a),

195

4.18−3.99 (6H, b + b’), 3.73 (9H, e), 3.36 (3H, c), 2.86−2.71 (6H, d + d’). 1

196

GEM4, yellowish oil, 89 % isolated yield, EEW = 199 g/eq. H NMR (CDCl3, 600 MHz,

197

Figure S15) δ: 6.78 (2H, j), 6.69 (2H, h), 6.56 (2H, i), 6.43 (2H, g), 5.33 (1H, a), 4.18−3.96

198

(6H, b + b’), 3.73 (9H, e), 3.69 (9H, f), 3.36 (3H, c), 2.87−2.61 (6H, d + d’).

9



199

GEM4’, yellowish oil, 91 % isolated yield, EEW = 200 g/eq. 1H NMR (CDCl3, 600 MHz,

200

Figure S16) δ: 6.95 (2H, g), 6.74 (2H, f), 6.43 (4H, e), 5.33 (1H, a), 4.28−3.96 (6H, b + b’),

201

3.73 (12H, e), 3.32 (3H, c), 2.88−2.61 (6H, d + d’). 1

202


203

Figure S17) δ: 6.85 (1H, g), 6.64 (1H, i), 6.58 (1H, h), 6.43 (4H, j), 5.31 (1H, a), 4.19−3.96

204

(6H, b + b’), 3.73 (3H, e), 3.66 (12H, f), 3.35 (3H, c), 2.87−2.61 (6H, d + d’). 1

205


206

Figure S18) δ: 6.29 (6H, f), 5.31 (1H, a), 4.16−3.96 (6H, b + b’), 3.73 (18H, e), 3.36 (3H, c),

207

2.77−2.61 (6H, d + d’).

208

Formation of Epoxy Networks.

209

Four epoxy monomers (GEM0, GEM2, GEM2’ and GEM4) with 0−4 methoxy groups

210

were used to make epoxy networks. GEM6 was not used because of the low isolated yield of

211

M6 (5%), which makes it unreasonable for preparing renewable materials. The monomers

212

were respectively mixed with diethylenetriamine (DETA) with 1:1 molar ratio of epoxy vs.

213

−NH for curing. The mixtures were stirred for 10 min, degassed under vacuum to remove

214

entrapped air and poured into silicone molds for curing with the profile: 65 °C for 8 h, 90 °C

215

for 2 h and 120 °C for 2 h. Cured epoxy networks were expressed as ENM0, ENM2, ENM2’

216

and ENM4, respectively. Degree of cure were monitored using Fourier transform infrared

217

(FTIR). Conventional BPA−based epoxy network (ENBPA) was prepared from DGEBA and

218

DETA using the same method with TP−based materials.

219

Analysis Methods.

220

Chemical structure and conversion of TPs were followed using nuclear magnetic

10


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221

resonance (NMR) spectroscopy. NMR spectra were collected on an Agilent 400−MR DDR2

222

400 MHz spectrometer or a Varian Unity Inova 600 MHz spectrometer. Deuterated acetone

223

or chloroform were used as solvent. 1,2,4,5−Tetrachloro−3−nitrobenzene was used as the

224

NMR internal standard. FTIR analyses were conducted using a Thermo−Nicolet Nexus 470

225

FTIR Spectrometer equipped with an ultra−high−performance, versatile Attenuated Total

226

Reflectance (ATR) sampling accessory. The spectra were scanned over a wavenumber

227

range of 400−4000 cm

228

a capillary melting point apparatus (MEL−TEMP) with a heating rate of 5 °C/min.

−1

−1

with a resolution of 4 cm . The melting point was measured using

229

Crystal of M6 was obtained from slow evaporation of an ethyl acetate solution at room

230

temperature. The crystal was mounted on a cryo−loop and transferred to a Bruker Kappa

231

APEX II diffractometer. The APEX2 program was used to determine the unite cell

232

parameters and data collection (30 sec / frame, 0.5 deg. /frame Omega scan). The data was

233

collected at 100 K using Oxford nitrogen gas cryostream system. The raw frame data was

234

processed using SAINT program. The absorption correction was applied using program

235

SADABS. Subsequent calculations were carried out using SHELXTL program. The

236

structure was solved by direct methods and refined on F2 by full−matrix least−squares

237

techniques. Hydrogen atoms were located theoretically.

238

Differential scanning calorimetry (DSC Q−2000, TA Instruments) was conducted under

239

dry nitrogen atmosphere to monitor exothermic peak temperature. Samples of 5−10 mg

240

were placed in sealed aluminum pans for all DSC runs. Peak temperature and enthalpy

241

were obtained through heating the samples from 10 to 200 °C at a rate of 10 °C/min.

242

Dynamic mechanical properties were characterized using a DMA 2980 (TA

11



243

Instruments). Rectangular specimens with dimensions of 30 mm length, 12.5 mm width and

244

2.5 mm thickness were measured in a single−cantilever mode. The measurements were

245

conducted from 35 to 180 °C at a heating rate of 3 °C/min and a frequency of 1 Hz. The

246

temperature at the maximum in the tan δ curve was taken as Tα (related to Tg).

247

Tensile testing was performed on dog−bone shaped specimens according to the ASTM

248

D638 standard, on a custom−built setup on a vertical TwinRail positioning table (Lintech, CA)

249

with a 100 N load cell. Crosshead speed was set to 0.5 mm/min.

250

Thermal stability studies were carried out on a Discovery Thermo−Gravimetric Analyzer

251

(TGA, TA Instruments) under a nitrogen flow of 40 mL/min. Samples (5−10 mg) were placed

252

in a platinum pan and scanned from 40 to 600 °C at a ramp rate of 20 °C/min.

253

RESULTS AND DISCUSSION.

254

Structure of TPs and Glycidyl Ethers.

255

Triphenylmethane architecture provides a convenient way to manipulate the number of

256

methoxy substituents by employing different starting aldehydes and phenols. To establish

257

the resulting structure, proton and carbon NMR spectra of M0 to M6 are provided in Figures

258

S1−S9 (trace amounts of triphenol isomers with aldehydes coupled at the ortho and/or meta

259

position of phenols are also included in these figures). The proton peak at ~ 5.3 ppm

260

corresponds to the triphenylmethyl group, which indicates successful condensation of the

261

aldehyde and phenol. The characteristic methoxy groups are observed at ~ 3.7 ppm, while

262

aromatic protons are found in the range of 6.3−6.9 ppm. For the carbon NMR, the

263

triphenylmethyl and methoxy groups are observed in the range 50.6−55.7 ppm. While the

264

para position of phenol is the main site for aldehyde coupling, it is noteworthy that certain

12


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265

amount of ortho or meta coupled byproducts are also observed in the proton spectra, with

266

molar ratio below 8 % of the product mixture. The dominant para−coupled products were

267

previously observed, which was related to their higher reactivity and less steric hindrance of

268

the para site.

269

are given in Figure S10−S18. Compared to TPs, new characteristic peaks are identified at

270

3.91−4.19 ppm (−CH2−, b and b’), 3.35 ppm (−CH−, c) and 2.61−2.87 ppm (−CH2− in epoxy

271

ring, d and d’), which are indicative of epoxy groups.

59

Proton NMR spectra of the corresponding glycidyl ethers (GEM0 to GEM6)

272

Because of the structural similarity of TPs, only one crystal structure (M6, prepared

273

from syringaldehyde and dimethoxyphenol) was obtained to confirm the structure and

274

determine the main coupling sites. Figure 3 reveals syringaldehyde couples exclusively at

275

the para sites of both dimethoxyphenol. Coupling at para sites provides TPs with stretched

276

orientation of hydroxyl groups (Figure 3). Together with its rigid architecture and high

277

number of hydroxyl groups (n= 3), TP−based polymers are supposed to exhibit satisfactory

278

mechanical properties.

279

Structure of TPs is further confirmed by IR spectra shown in Figures S19−S27. Starting

280

aldehydes (HBA, VAN and SYA) have an aldehyde peak around 1672 cm−1. After they are

281

condensed with phenols, the aldehyde group becomes triphenylmethyl group. This is

282

consistent with the IR spectra of TPs in which the aldehyde peak disappears (Figures

283

S19−S27, panels a). Other characteristic bands comprise 3058−3586 cm

284

and 1608, 1510 and 1437 cm−1 (aromatic C−C bond). When TPs are converted to glycidyl

285

ethers, the broad hydroxyl band diminishes significantly, while a new epoxy ring band at 912

286

cm

−1

−1

(O−H stretching),

emerges (Figures S19−S27, panels b). This is consistent with the NMR analysis

13



287

confirming the presence of epoxide. This conclusion is further supported by the IR spectra

288

of the cured thermosets in Figures S19−S27, panels c. When epoxy monomers are cured

289

with DETA, epoxy groups are opened by amine while hydroxyl groups are created

290

concurrently. This process is reflected by the IR results, in which the epoxy band at 912

291

cm−1 disappears while the OH band around 3355 cm−1 increases. The absence of epoxy

292

bond is indicative of significant conversion of epoxy groups in our investigated networks.

293

Meanwhile, high reactivity of epoxy/DETA system is confirmed by two cycles of

294

heating−cooling using DSC analysis (Figure S28), as supported by the lack of an exotherm

295

on the second heating cycle. The high conversion of epoxy groups is mainly attributed to the

296

high reactivity of DETA. Meanwhile, at elevated temperature with the catalytic effect of

297

amine, the epoxy could also undergo ring−open polymerization.

298

Effect of Methoxy Substituents on Yields of TPs.

299

To study the conversion of TPs, aldehyde and phenol were reacted under the following

300

conditions: molar ratio of aldehyde: phenol = 1:4, aldehyde: H2SO4 = 1:3, absolute ethanol

301

was used as solvent and the mixture was stirred at 65 °C for 48 hours. Conversions were

302

calculated based on integrals of aldehyde and triphenylmethyl peaks in the proton spectra.

303

Table 1 lists the conversion of TPs prepared from different aldehydes and phenols.

304

Generally, conversions decrease with increased number of methoxy groups (from 83% for

305

M0 to 35% for M6). Comparisons between starting aldehydes and phenols reveal reactivity

306

of aldehydes (HBA > VAN > SYA) and phenols (PhOH > GUA > DMP) decreases with

307

increased number of methoxy substituents (Table 1). The negative effects of methoxy group

308

on conversion could be attributed to: 1) steric effect of methoxy, and 2) the electron donation

14


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309

of the hydroxyl and methoxy substituents increases the nucleophilicity of the ring system.

310

Due to resonance structures, the substituents favor the positions ortho and para to each

311

phenol or methoxy group. Increasing the number of substituents would therefore spread the

312

electron density over multiple sites.

313

Compared to conversion, methoxy groups have even greater impact on isolated yields.

314

For TPs (M0 to M4) that were prepared from aldehydes (HBA, VAN and SYA) and phenols

315

(PhOH and GUA), satisfactory isolated yields of 46−69% were achieved. However, when

316

2,6−dimethoxyphenol (DMP) was reacted with aldehydes, only 18%, 7% and 5% isolated

317

yields were obtained for M4’, M5 and M6, respectively. Low isolated yields of DMP−based

318

TPs could be attributed to: 1) low conversions of these TPs (35−49%), and/or 2) the steric

319

hindrance of the initially formed diphenyl intermediate. The reaction mixtures contain certain

320

amount of the diphenyl intermediate (proton NMR spectrum of the intermediate exhibits no

321

triphenylmethyl peak at 5.3 ppm). These undesired intermediates exhibit similar polarity with

322

TP products and impair the yields of flash column purification. The low yield of DMP−based

323

TPs makes them unsuitable for use in making renewable polymer materials.

324

Effect of Methoxy Substituents on Melting Point.

325

Melting points of TPs with various number of methoxys are listed in Table 2. The

326

highest melting point is found to be 241 °C (M0), while the lowest is 125 °C (M3). This

327

marked difference highlights the role of methoxy. Melting points of TPs from different

328

aldehydes follow the order: HBA > SYA > VAN, while phenols follow the order: PhOH >

329

DMP > GUA (Table 2). The lower melting points of TPs prepared from di−methoxy

330

substituents (SYA and DMP) than corresponding non−methoxy compounds (HBA and

15



Page 16 of 40

331

PhOH) could be explained by the electron donating effect of methoxy. Meanwhile,

332

methoxy−substituted phenols

333

intermolecular H−bonding would be decreased, which leads to lower melting point. As for

334

the mono−methoxy derivatives (VAN and GUA) that exhibit the lowest TP melting points,

335

influence of methoxy may be reflected by its impact on the symmetry of TP units. Symmetric

336

TPs like M0, M2’, M4’ and M6 exhibit melting points above 190 °C. By comparison, when

337

asymmetric units VAN and GUA (only one methoxy at ortho site) are incorporated, melting

338

points of TPs significantly decrease. Especially, M3 with three asymmetric guaiacol units

339

shows the lowest melting point (125 °C) among all TPs. Moreover, impact of unit symmetry

340

can be highlighted when TPs with the same number of methoxy groups are compared. For

341

example, melting point of M2’ (SYA−PhOH, 191 °C) is 27 °C higher than M2 (HBA−GUA,

342

164 °C), while M4’ (HBA−DMP, 204 °C) is 58 °C higher than M4 (SYA−GUA, 146 °C). The

343

impacts of methoxy on melting points could provide insight in the design of more

344

processable lignin−based thermosetting resins beyond epoxy networks.

345

Effect of Methoxy on Properties of TP−Based Epoxy Thermosets.

346

Differential Scanning Calorimetry (DSC) Analysis.

can

participate

in

intramolecular

H−bonding,

while

347

Effect of methoxy group on curing behavior is studied via DSC. Table 3 shows that

348

enthalpy (∆H) values of all curing systems fall in the range 91.5 to 96.4 kJ/ee, which is in

349

62 accordance with the typical value of 90−100 kJ/ee for epoxy/amine reactions. Table 3

350

demonstrates peak curing temperatures are not significantly different for all studied

351

monomers: GEM0 (81 °C), GEM2’ (80 °C), GEM2 (78 °C) and GEM4 (77 °C), which indicates

352

methoxy group has no impact on the epoxy/amine curing reaction. This could be attributed

16


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353

to the inert nature of methoxy, which cannot catalyze the epoxy/amine reaction.63 The

354

degree of curing was determined through two cycles of heating. As demonstrated in Figure

355

S28, all epoxy monomers are mostly cured as supported by the lack of an exotherm in the

356

second heating cycle.

357

Dynamic Mechanical Analysis (DMA) and Tensile Analysis (TA).

358

Figure 4 and Table 4 show that Tα (α−relaxation temperature, related to Tg) of

359

thermosets decreases gradually with increased content of methoxy groups, i.e., ENM0

360

(132 °C) > ENM2’ (125 °C) > ENM2 (120 °C) > ENM4 (118 °C). Similar negative effect of

361

methoxy substitution on thermomechanical performance was reported for cyanate ester

362

resins.60 Even though some methoxys could form hydrogen bonds with hydroxyls that

363

improves the network constraint, reduced cross−link density on a mass/volume basis

364

caused by methoxy groups seems to play a more important role decreasing Tα. Storage

365

modulus (E’) curves of TP−based thermosets are displayed in Figure 4 and values given in

366

Table 4. Similar to Tα, increasing the content of methoxy reduces the storage modulus from

367

ENM0 to ENM4. It is noteworthy that glassy modulus (E30’, storage modulus at 30 °C) values

368

are in the range of 2.2 to 2.6 GPa. To compare the mechanical performance of TP−based

369

polymers

370

diethylenetriamine were cured using the same profiles with TP−based networks. The E30’

371

and Tα of DGEBA/DETA system (ENBPA) were measured to be 2.0 GPa and 103 °C (Figure

372

4 and Table 4), which is lower than our TP−based thermosets. Meanwhile, tensile properties

373

of TP and BPA−based thermosets are illustrated in Figure 5. TP−based thermosets exhibit

374

similar rigidity, i.e., max stress of 74.9 to 80.0 MPa and elongation at break of 2.55 to 2.68%,

with

BPA–based

materials,

diglycidyl

ether

17


of

BPA

(DGEBA)

and


375

while ENBPA is slightly more elastic with stress of 65.2 MPa and strain of 3.56%. These

376

results are attributed to the more rigid structure and higher functionality of TPs over BPA.

377

Moreover, TP−based epoxy networks possess marked mechanical performance that can

378

replace or supplement BPA−based analogues.

379

Thermogravimetric Analysis (TGA).

380

Figure 6 reveals a one−step degradation curve for all TP−based epoxy networks.

381

Figure 6 and Table 4 illustrate that the thermal stability of cured networks decreases with

382

increased content of methoxy (i.e., ENM0 > ENM2’ > ENM2 > ENM4). This is reflected from

383

the significant shift of onset degradation temperature (defined as Td5, temperature at 5%

384

weight loss) from ENM0 (257 °C) to ENM2’ (218 °C), ENM2 (206 °C) and ENM4 (184 °C),

385

respectively. The effect of methoxy substitution is attributed to its electron donation ability.60

386

Meanwhile, as methoxy groups increase, content of oxygen within the polymers increases,

387

which accelerates thermal decomposition. Moreover, the steric effect of methoxy might

388

decrease the degree of glycidylation, while formed side−products or impurities may lower

389

the thermal stability. Td30 (temperature at 30% weight loss) of cured samples reveal the

390

same trend with Td5, which confirms the effect of methoxy substitution. Besides, for M2 and

391

M2’ that have similar methoxy contents, they exhibit close values of Td5, Td30 and Ts (statistic

392

heat−resistant index temperature, calculated using equation 1). This further supports the

393

relationship between methoxy substituents number and thermal performance of the

394

resulting epoxy networks. As for the BPA−based network, it exhibits higher thermal stability

395

than TP−based thermosets as reflected by Td5 (305 °C), Td30 (371 °C) and Ts (169 °C). This

396

could be attributed to the triphenylmethane architecture of TP that is easily oxidized, as well

18


Page 18 of 40

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397

as the electron donating effects of the methoxy groups. When the temperature reaches

398

330 °C, ENBPA exhibits a fast degradation behavior and only 8 wt % char was left when the

399

temperature reaches 600 °C. As for the TP−based networks, they start to exhibit higher

400

stability over ENBPA above 391 °C, with 17−23 wt % char formed at 600 °C.

Ts = 0.49[Td 5 + 0.6(Td 30 − Td 5 )]

401 402

(1)

CONCLUSIONS

403

Preparation of renewable TPs with 0−6 methoxy substituents is described. While the

404

rigid architecture and improved number of hydroxyl of the TP backbone afford epoxy

405

networks with marked properties, the dangling methoxy groups are found to have different

406

levels of impact on the resulting polymers. Because of its chemical inertness, the methoxy

407

group has no impact on the curing process (curing temperature and enthalpy). By

408

comparison, increasing methoxy content results in a decrease in thermomechanical

409

properties (Tα decreases from 132 to 118 °C, and glassy modulus decreases from 2.6 to 2.2

410

GPa). The greater impact of methoxy substitution is related to thermal performance, in

411

which onset degradation temperature decreases significantly from 257 °C for ENM0 (no

412

methoxy substituents) to 184 °C for ENM4 (four methoxy substituents). The greatest impact

413

is on the conversion and especially isolated yields of TPs, which determines the feasibility of

414

using these monomers in the manufacture of renewable polymers. Using harsher conditions

415

like increasing the reaction temperature with the aid of higher boiling solvents or microwave

416

irradiation may increase the yields, especially for TPs with higher content of methoxy. The

417

different impacts of methoxy substitution can guide the selection and/or modification (e.g.

418

deoxygenation) of lignin−derived monomers for making epoxy polymers with desirable

19



Page 20 of 40

419

properies. Decreased Tg and thermal stability as methoxy content increases may also

420

provide insight in the degradation of thermosets.

421

Supporting Information

422

Proton and carbon NMR, and FTIR spectra of all new compounds. DSC curing curves for

423

epoxy/amine systems to examine degree of cure. X−ray structure of M6.

424

ACKNOWLEDGMENTS

425

This research was supported by the Center for direct Conversion of Biomass to Biofuels

426

(C3Bio), an Energy Frontiers Research Center (EFRC) funded by the U.S. Department of

427

Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE−SC000097,

428

National Science Foundation CBET−1624183, the University of California, Santa Barbara,

429

and the Mellichamp Academic Initiative in Sustainability. We acknowledge the use of the

430

MRL Shared Experimental Facilities, supported by the MRSEC Program of the National

431

Science Foundation under award NSF DMR 1121053; a member of the NSF−funded

432

Materials Research Facilities Network.

433

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A. Structure property relationships of biobased n−alkyl bisferulate epoxy resins. Green

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Chem. 2016, 18 (18), DOI 10.1039/C6GC01308B.

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Vanillin−derived high−performance flame retardant epoxy resins: facile synthesis and

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properties. Macromolecules 2017, 50 (5), DOI 10.1021/acs.macromol.7b00097.

569

(45) Hitce, J.; Crutizat, M.; Bourdon, C.; Vivès, A.; Marat, X.; Dalko−Csiba, M.

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Flash−metathesis for the coupling of sustainable (poly) hydroxyl β−methylstyrenes from

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essential oils. Green Chem. 2015, 17 (7), DOI 10.1039/C5GC00759C.

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thermosets

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bran

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pulp:

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epoxy: shape memory, repairing, and recyclability. Macromolecules 2017, 50 (21), DOI

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10.1021/acs.macromol.7b01889.

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Y. Ultrastiff biobased epoxy resin with high Tg and low permittivity: from synthesis to

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synthesis of formaldehyde−free phenolic resins from lignin−based aldehyde precursors. Eur.

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Polym. J. 2016, 74, DOI 10.1016/j.eurpolymj.2015.11.036.

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polyphenols. Macromolecules 2017, 50 (9), DOI 10.1021/acs.macromol.7b00064.

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Synthesis and characterization of bio−based epoxy resins derived from vanillyl alcohol.

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thermal stability of renewable high−temperature cyanate ester resins. Macromolecules

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phosphorus−containing polyol synthesized from cardanol. RSC Adv., 2015, 5, DOI

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622

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623

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624

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modified montmorillonite on the reaction mechanism of epoxy/amine cure. J. Phys. Chem. B

626

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reaction

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development.

627 628 629 630 631 632 633 634 635 636 637 638

29


Materials,

2014,

7,

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639 640 641 642 643 644

Figures and Tables

645 646

Figure 1. Synthesis route of epoxy networks from lignin−derived aldehydes and phenols.

647

648

30


Page 30 of 40

Page 31 of 40 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


649

650

Figure 2. List of TPs with different number of methoxy groups. TPs are abbreviated as Mn,

651

while n indicates the number of methoxy groups. The starting aldehyde and phenol for each

652

TP are also listed in parentheses.

653

654

655

656

657

658

659

31



660 661

Figure 3. Crystal structure of M6. Crystal was collected from the slow evaporation of an

662

ethyl acetate solution at room temperature.

663

664

665

666

667

668

669

670

671

672

673

674

32


Page 32 of 40

Page 33 of 40 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


675 676

Figure 4. DMA curve of epoxy networks derived from TPs with different number of methoxy

677

groups. Temperature at the maximum in tan δ curve is taken as Tα (related to Tg).

678

679

680

681

682

33



683

684

Figure 5. Tensile tests of epoxy networks derived from TPs with different number of

685

methoxy groups.

686

687

688

689

690

691

692

34


Page 34 of 40

Page 35 of 40 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


693 694

Figure 6. Thermogravimetric analysis thermograms of epoxy networks derived from TPs

695

with different number of methoxy groups.

696 697 698 699 700 701 702 703 704 705 706 707 708 709 35



Page 36 of 40

710

Table 1. Conversion (%) of TPs Prepared from Lignin−Derived Aldehydes and Phenols with

711

Different Number of Methoxy Groups

a

Aldehyde Phenol

HBA

VAN

SYA

(0)

(1)

(2)

PhOH (0) b

83 c

72

64

GUA (1)

81

68

64

DMP (2)

49

42

35

712

a

713

for 2 days. Absolute ethanol is used as solvent. b Numbers (in parentheses) next to starting

714

aldehydes or phenols indicate the number of methoxy groups situated on these reagents.

715

Numbers in the table means conversion (%) of TPs prepared from different aldehydes and

716

phenols. For example, 83% is the conversion of TP synthesized from HBA and PhOH.

Reaction conditions: molar ratio of aldehyde: phenol = 1:4, aldehyde: H2SO4 = 1:3, 65 °C

717 718 719 720 721 722 723 724 725 726 727 728

36


c

Page 37 of 40 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


729

Table 2. Melting Point (°C) of TPs with Different Number of Methoxy Groups a Aldehyde Phenol PhOH (0)

b

HBA

VAN

SYA

(0)

(1)

(2)

188

191

241

c

GUA (1)

164

125

146

DMP (2)

204

133

190

730

a

731

a heating rate of 5 °C/min. Numbers (in parentheses) next to starting aldehydes or phenols

732

indicate the number of methoxy groups situated on these reagents. c Numbers in the table

733

mean melting points (°C) of TPs prepared from corresponding aldehydes and phenols. For

734

example, 241 °C is the melting point of TP synthesized from HBA and PhOH.

Melting points were measured using a capillary melting point apparatus (MEL−TEMP) with b

735 736 737 738 739 740 741 742 743 744 745 746 747

37



Page 38 of 40

748

Table 3. DSC Curing Data for Epoxy/DETA Systems Exhibiting Onset Curing Temperature

749

(Ti), Peak Curing Temperature (Tp) and Enthalpy of Reaction (∆H).

750 751 752

753

Ti

Tp

∆H

Theoretic

Measured

(°C)

(°C)

(kJ/ee)

EEW (g/eq.)

EEW (g/eq.)

GEM0

47

81

96

154

160

GEM2

43

78

94

174

179

GEM2’

45

80

95

174

181

GEM4

44

77

92

194

199

sample

754

755

756

757

758

759

760

761

762

763

764

765

766

38


Page 39 of 40 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


767

Table 4. Thermomechanical Data of Tα, E30’ (Glassy Modulus), Td5, Td30 (Temperature at 5%

768

and 30% Weight Loss), Ts (Statistic Heat−Resistant Index Temperature) and Char600 (Char

769

Residue at 600 °C) of Epoxy Networks Derived from TPs and BPA. Tα

E30’

Td5

Td30

Ts

Char600

(°C)

(MPa)

(°C)

(°C)

(°C)

(%)

ENM0

132

2745

257

341

151

20

ENM2

125

2598

206

321

135

17

ENM2’

120

2249

218

324

138

18

ENM4

118

2477

184

296

123

14

ENBPA

100

2042

305

371

169

8

sample

770 771 772 773 774 775 776 777 778 779 780 781 782 783 784

39



785

for Table of Contents use only

786 787 788

Synopsis: Methoxy group affects the thermomechanical properties and feasibility of lignin−derived triphenol−based epoxy thermosets.

789 790 791

40


Page 40 of 40

Renewable Epoxy Thermosets from Fully Lignin-Derived Triphenols

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