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Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Potential Lignin-Derived Alternatives to Bisphenol A in DiamineHardened Epoxy Resins Kaleigh H. Nicastro,† Christopher J. Kloxin,*,†,‡ and Thomas H. Epps, III*,†,‡ †

Department of Chemical and Biomolecular Engineering, University of Delaware, 150 Academy Street, Newark, Delaware 19716, United States ‡ Department of Materials Science and Engineering, University of Delaware, 127 The Green, Newark, Delaware 19716, United States

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/11/18. For personal use only.

S Supporting Information *

ABSTRACT: This work details the synthesis and characterization of potentially lignin-derived bisguaiacols as alternatives to petroleum-derived bisphenol A (BPA) in diamine-cured epoxy resins. The variations in the number of methoxy groups of lignin facilitate the systematic chemical and thermomechanical manipulation of bisguaiacol-based thermosets to achieve desirable properties. Herein, ten bisguaiacols (including structural isomers), differing in the number of methoxy groups and regioisomer content, were synthesized from substituted bioderivable hydroxybenzyl alcohols and phenols by acidcatalyzed electrophilic aromatic substitution approaches and then functionalized with oxirane groups. These bisguaiacol diglycidyl ethers were subsequently cured with a model diamine. All cured bioderivable resins had glass transition temperatures (Tg’s) above 100 °C, 5 wt % loss temperatures above 300 °C, and room-temperature glassy storage moduli above 2.0 GPa, values that were comparable to bisphenol A diglycidyl ether (BADGE/DGEBA) cured resins. Furthermore, final cured resin Tg’s (111−151 °C) and high-temperature rubbery moduli (15−46 MPa) were easily tuned by manipulating the relative number of methoxy moieties and the regioisomer content, demonstrating the versatility and robustness of these bioinspired materials. KEYWORDS: Biobased, Lignin-derived, BPA alternative, Bisguaiacol, Epoxy, Renewable, Lignin

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INTRODUCTION As interest in biobased and bioinspired polymers has grown over the last few decades, a number of natural and sustainable resources have served as alternative feedstocks, including fatty acids, lignocellulosic biomass, and enzymatic products.1−5 For example, aromatic monomers derived from lignocellulose6−10 and other sources9,11 have been reported in the synthesis of novel, biobased polymers. The thermomechanical properties of these polyaromatic biobased materials often are similar to current petroleum-derived macromolecules, such as polystyrene1 and composite resins.12 However, limited feedstock abundance or complex syntheses impede competitive pricing and relegate many biobased polymers to niche markets.1,13 Lignin is the most abundant natural source of renewable, aromatic building blocks for polymer synthesis.14 Through techniques such as pyrolysis15 and depolymerization,16,17 lignin can be deconstructed into smaller polyaromatics or single aromatics possessing various moieties such as alkyl, alkoxyl, carbonyl, and hydroxyl groups.18−20 Carbonyl and hydroxyl groups, in particular, serve as functionalization handles to upgrade single aromatic lignin products to high-value chemicals such as bisaromatics, namely, bisguaiacols.21−24 Upgraded lignin products harness the structural variations of lignin and could facilitate the commercialization of lignin© XXXX American Chemical Society

derived polymers. For example, mixtures of lignin-derived substituted phenols have been used in epoxy resins25 and block polymers17,26 to tune and enhance material properties, such as the glass transition temperature (Tg), solvent resistance, and adhesion strength. Additionally, lignin-derived polyaromatics, including bisguaiacols, have shown promise in thermoset and thermoplastic applications currently dominated by petrochemicals, e.g., bisphenol A (BPA).24,27−33 Routinely, BPA is used to synthesize commercial, highperformance polymers, such as polycarbonates, vinyl esters, dental composites, and epoxies.34 BPA-based epoxies are readily copolymerized with a range of multifunctional monomers, such as amines, anhydrides, acids, or thiols, producing cross-linked materials with exceptional properties, including mechanical integrity at elevated temperatures, chemical resistance, strong adhesion, and electrical insulation.35 Unfortunately, BPA also is a known endocrine disruptor, and there is significant interest in altering this molecular structure across the many materials applications in which BPA is used.36 Thus, there is substantial need to Received: July 12, 2018 Revised: September 24, 2018

A

DOI: 10.1021/acssuschemeng.8b03340 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Scheme 1. Bisguaiacol Synthesis Strategy and Molecular Structures of Epoxy Resin Constituentsa

a

(a) Bisguaiacols were synthesized from substituted hydroxybenzyl alcohols and substituted phenols and subsequently reacted with epichlorohydrin, resulting in bisguaiacol diglycidyl ethers in which n is the extent of oligomerization. (i) Amberlyst 15 catalyst, nitrogen (N2), 50 °C, 50 min; (ii) (1) epichlorohydrin, tetrabutylammonium bromide, N2, 50 °C, 2 h and (2) sodium hydroxide, 0 °C−rt, overnight. (b) Synthesized bisguaiacol diglycidyl ethers, BADGE, and BFDGE were cured with MDA at a 2:1 molar ratio. Structural differences regarding methoxy groups are highlighted in blue.

cured bisguaiacol diglycidyl ethers was decoupled by analyses of regioisomer mixtures and pure regioisomers to develop robust structure−property relationships for these bioderivable materials.

evaluate bioderived BPA alternatives in working toward a more sustainable future.37 Herein, ten biobased bisguaiacol diglycidyl ethers with varying degrees of methoxy substitution and regioisomer content were synthesized, characterized, and subsequently reacted with a model diamine, 4,4′-methylenedianiline (MDA), as illustrated in Scheme 1.24 MDA, an aromatic diamine, was chosen as a model diamine because it is useful in applications that require resin-based materials with greater thermal stability and chemical resistance.38,39 Bisguaiacol regioisomer mixtures were synthesized from lignin-derivable methoxy-substituted hydroxybenzyl alcohols and methoxysubstituted phenols by acid-catalyzed electrophilic aromatic substitution reactions similar to those employed for the largescale production of bisphenols.40,41 Subsequent functionalization of the bisguaiacols with epichlorohydrin generated diglycidyl ethers differing in the number of methoxy groups and regioisomer content. Bisphenol A diglycidyl ether (BADGE/DGEBA) and bisphenol F diglycidyl ether (BFDGE/DGEBF), a nonmethoxy substituted analog of the bisguaiacol diglycidyl ethers, were used as commercial comparisons that exhibit high-temperature mechanical stability.35 The above-mentioned diglycidyl ethers, were cured with MDA, and the resulting networks were characterized by dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA) to assess the potential of bisguaiacol diglycidyl ethers as alternatives to commercial BADGE and BFDGE on the basis of thermal and mechanical performance. Furthermore, the effect of the number of methoxy groups and regioisomer content on Tg, moduli, and thermal stability of

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EXPERIMENTAL SECTION

Materials. Guaiacol (≥98% food grade), vanillyl alcohol (≥98%), and 2,6-dimethoxyphenol (syringol, 99%) were purchased from Sigma-Aldrich. Amberlyst 15 hydrogen form (dry) was purchased from Fluka. Epichlorohydrin (99%), tetrabutylammonium bromide (TBAB, 99+%), and deuterated chloroform (CDCl3, 99.8+% atom D, contains 0.03 v/v% TMS) were purchased from Acros Organics. MDA (97%) was purchased from Alpha Aesar. Sodium hydroxide (NaOH), sodium sulfate (anhydrous, granular), sodium borohydride (>98%), hexanes, ethyl acetate, heptane, acetonitrile, and dichloromethane (DCM) were purchased from Fisher Scientific. Syringaldehyde (>98%) and phenol (>98%) were purchased from TCI America. BADGE (EPON 828) and BFDGE (EPON 862) were purchased from Hexion. n-Butyldimethylchlorosilane was purchased from Gelest, Inc. All chemicals were used as received without further purification. Nitrogen (N2, grade 5) was purchased from Keen Compressed Gas. Synthesis of Syringyl Alcohol. Syringaldehyde (20 g, 0.11 mol) was dissolved in 300 mL of acetonitrile and loaded into a single neck round-bottom flask equipped with a magnetic stir bar, then placed in an ice bath (0 °C) and purged with N2 for 30 min. To the reaction flask, 2 eq. of sodium borohydride (8.31 g, 0.22 mol) was added incrementally over 20 min to control the reaction exotherm. The temperature of the reaction flask gradually increased to room temperature overnight. An off-white precipitate was collected by filtration using a Büchner funnel (grade 1 Whatman filter paper). Once recovered, the precipitate was reacted with 350 mL of distilled water to quench the reaction to produce syringyl alcohol and sodium salts. The inorganic gray salts were collected by Büchner funnel and B

DOI: 10.1021/acssuschemeng.8b03340 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering discarded. The final product was extracted from the aqueous permeate by washing with 300 mL of DCM a total of six times. The DCM was removed under reduced pressure to yield the solid product. Proton nuclear magnetic resonance (1H NMR) spectroscopy confirmed the chemical structure and purity; the spectrum is provided in Figure S1 (yield: 70.7 mol %; purity: 99.5 mol %). General Procedure for the Synthesis of Bisguaiacols. In a typical bisguaiacol synthesis, 1 eq. of substituted hydroxybenzyl alcohol (vanillyl alcohol or syringyl alcohol) and 3.6−4.7 eq. of substituted phenol (phenol, guaiacol, or syringol) were loaded into a single-neck round-bottom flask equipped with a magnetic stir bar. The reaction was heated above the melting point of the reactants (50−80 °C depending on the substituted phenol used), mixed, and purged with N2 for 40 min. Under N2 flow, Amberlyst 15 hydrogen form, dry (30 wt % relative to the substituted hydroxybenzyl alcohol), was added to the reaction flask, and the reaction proceeded for 50 min or until all the substituted hydroxybenzyl alcohol reacted, as determined from 1H NMR spectroscopy analysis. The solid catalyst was removed from the reaction mixture using a Büchner funnel (grade 1 Whatman filter paper). Excess phenol and guaiacol, produced from the synthesis of BGP and BGF, respectively, were removed by vacuum distillation at 110 °C. BGF was crystallized from hot heptane, and all bisguaiacols were purified by column chromatography (Teledyne Isco Combiflash Rf+, gold grade silica gel, 20−40 μm particle size, 60 Å pore size) using a step gradient of ethyl acetate and hexanes as the mobile phase. Bisguaiacols were recovered and dried under vacuum. BGP: HRMS-TOF (LIFDI, m/z): [M + H]+ calcd for C14H14O3 230.0938; found 230.0938. BGF: HRMS-TOF (LIFDI, m/z): [M + H]+ calcd for C15H16O4 260.1043; found 260.1044. BGS: HRMSTOF (LIFDI, m/z): [M + H]+ calcd for C16H18O5 290.1149; found 290.1149. BGM: HRMS-TOF (LIFDI, m/z): [M + H]+ calcd for C17H20O6 320.1254; found 320.1256. The 1H NMR peak assignments that were used to determine regioisomer content, the carbon-13 NMR (13C NMR) spectra, and the two-dimensional (2D) NMR spectra are provided in Figures S2−S15. Nonoptimized bisguaiacol yields after column chromatography were 30−40 mol % with respect to the substituted hydroxybenzyl alcohol. Optimization of reaction conditions (including reactant ratios, catalyst type and loading, reaction time and temperature, and cocatalyst use) should increase bisguaiacol yields, optimize regioisomer ratios, and decrease waste, further increasing the sustainability of bisguaiacols. Regioisomers were isolated separately by column chromatography when pure regioisomers were desired. Bisguaiacol melting points (Tm’s) are provided in Table S1. General Procedure for the Synthesis of Bisguaiacol Diglycidyl Ethers. In a typical bisguaiacol diglycidyl ether reaction, 1.0 eq. of bisguaiacol and 0.4 eq. of TBAB were dissolved in 10 eq. of epichlorohydrin in a single-neck round-bottom flask equipped with a magnetic stir bar and then purged with N2 for 30 min. The reaction mixture was placed in an oil bath (50 °C) for 2 h and then cooled immediately in an ice bath (0 °C). To the cooled reaction vessel, 4 eq. of NaOH in a 50 wt % aqueous solution was added dropwise to the reaction flask, after which the reaction proceeded overnight at room temperature. The reaction mixture was dissolved in 150 mL of DCM, washed with 150 mL of distilled water in a separation funnel until the aqueous layer reached neutral pH, and then washed three times with 150 mL of brine solution. The organic layer was dried over sodium sulfate, reduced under vacuum, and then purified by column chromatography using a step gradient of ethyl acetate and hexanes as the mobile phase. The bisguaiacol diglycidyl ether product was dried under vacuum. Individual bisguaiacol diglycidyl ether yields varied (75−80 mol %) after purification. The 1H NMR with peak assignments, which are used to determine the extent of oligomerization (n) and epoxy equivalent weights (EEW), and the 13C NMR spectra are provided in Figures S16−S25. NMR Spectroscopy. All NMR samples were prepared in CDCl3 with 0.03 vol % TMS as an internal standard and analyzed on a Bruker AVIII 600 Hz spectrometer. 2D NMR techniques, specifically 1 H−13C heteronuclear single-quantum correlation (HSQC) spectroscopy and 1H−13C heteronuclear multiple-bond correlation (HMBC)

spectroscopy, were used to confirm individual regioisomer structures. Quantitative 1H NMR integrations of the methylene bridge and methoxy group(s) (3.0−3.7 ppm) for each bisguaiacol regioisomer were used to calculate the regioisomer content of bisguaiacol regioisomer mixtures. Following the method by Garcia et al.42 using quantitative 1H NMR spectroscopy, the relative integrations of the aromatic protons (6.3−7.2 ppm) to the epoxide protons (2.5−3.5 ppm) were used to determine the n values and EEW of the diglycidyl ethers. Mass Spectrometry (MS). Exact masses of the bisguaiacols were determined by time-of-flight high-resolution mass spectrometry (TOF HRMS) analysis on a Waters GCT Premier mass spectrometer. Samples were dissolved in acetonitrile at a concentration of 0.1 mg/ mL and then loaded using liquid injection field desorption ionization (LIFDI). A voltage field of 1.2 kV and a filament current of 50 mA were employed. Differential Scanning Calorimetry (DSC): Tm’s. Tm’s of synthesized bisguaiacols and bisguaiacol diglycidyl ethers were determined using a Discovery Series DSC (TA Instruments) calibrated using an indium standard. Samples were loaded into hermetically sealed aluminum pans and heated from room temperature to 130 °C at a heating rate of 10 °C/min under continuous N2 flow (50 mL/min). Data from the first heating trace were used for analysis. Sample Preparation for DMA and TGA. Diglycidyl ether DMA and TGA samples were prepared by placing the diglycidyl ether and MDA in a 2:1 diglycidyl ether:MDA molar ratio in a 10 mL polypropylene SpeedMixer plastic cup. The monomer mixtures were placed in an 80 °C oven for 6 min to melt all components and then immediately mixed in a SpeedMixer (DAC 150.1 FVZ-K, FlackTek, Inc.) for 3 min at 3000 rpm. Following mixing, the samples were heated to 80 °C, degassed, and poured into preheated molds. Reusable molds for DMA samples were created by placing 0.8 mm thick rubber gaskets (McMaster-Carr) between two glass slides (Fisher Scientific). The glass slides were surface functionalized with nbutyldimethylchlorosilane to prevent the epoxy samples from bonding to the glass slides.43 Samples were cured in air at 80 °C for 1 h, then 100 °C for 1 h, and subsequently postcured at 170 °C for 1 h. Average sample dimensions were approximately 25 mm × 4 mm × 0.8 mm (length × width × thickness) after being removed from the mold and sanded until smooth. Mixing mix-BGFDGE with MDA resulted in inhomogeneous specimens, owing to rapid reaction kinetics, which were not suitable for mechanical testing. DMA. DMA experiments to determine moduli and Tg of the cured diglycidyl ethers were conducted on a Q800 DMA (TA Instruments) in oscillatory film tension mode with a strain amplitude of 0.1%, a strain frequency of 1 Hz, and a preload force of 0.1 N. Three consecutive temperature sweeps were performed from ∼25 to 200 °C at a heating rate of 3 °C/min. Data from the third heating trace were used for analysis. TGA. The thermal stability of MDA-cured diglycidyl ethers was determined on a Discovery Series TGA (TA Instruments) using 100 μL platinum pans. Experiments were conducted under both N2 and air flow conditions. A continuous flow rate of 50 mL/min N2 or air was passed over the sample and balance pans. For the experiments in N2, samples were heated to 100 °C at a heating rate of 10 °C/min, held at 100 °C for 10 min to facilitate trace water removal, cooled to 25 °C, and then heated to 700 °C at a heating rate of 10 °C/min. For the experiments in air, the temperature ramp profile was identical to the N2 experiments, except the final temperature of the last heating ramp was 600 °C; TGA thermograms and the temperature at 5 wt % loss (T5%) in air are provided in Figure S26 and Table S2. The weight loss of each sample was normalized to the weight of the sample after the isothermal hold at 100 °C. T5% was used to assess the thermal stability of cured MDA-cured diglycidyl ethers.

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RESULTS AND DISCUSSION Synthesis and Characterization of Bisguaiacols and Bisguaiacol Diglycidyl Ethers. Bisguaiacol diglycidyl ethers C

DOI: 10.1021/acssuschemeng.8b03340 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 1. Regioisomer Content, n Values, EEW, and Tm of Diglycidyl Ethers

samplea

regioisomer content (mol %)b p,p′-:m,p′-:o,p′-:o,o′-

nc

EEWc

Tm (°C)d

mix-BGPDGE p,p′-BGPDGE o,p′-BGPDGE mix-BGFDGE mix-BGSDGE p,p′-BGSDGE m,p′-BGSDGE mix-BGMDGE p,p′-BGMDGE m,p′-BGMDGE BADGE BFDGE

81:−e:19:−f 100:−e:0:−f 0:−e:100:−f 80:18:2:−f 21:79:−f:−f 100:0:−f:−f 0:100:−f:−f 24:76:−f:−f 100:0:−f:−f 0:100:−f:−f ∼100:−e:0:0 37:−e:49:1422

0.01 0.02 0.03 0.02 0.00 0.04 0.02 0.02 0.02 0.17 0.11 0.10

172 174 176 189 201 208 204 220 219 249 186 169

44 52 −g 113 −g 64 −g 75 115 −g −g −g

step reaction with epichlorohydrin, Scheme 1.24 First, the bisguaiacols were synthesized by acid catalyzed electrophilic aromatic substitution reactions between vanillyl alcohol and syringyl alcohol (readily synthesized from vanillin and syringaldehyde, respectively),44 phenol, guaiacol, and syringol. Bisguaiacol synthetic techniques were based on industrial scale processes used for the synthesis of bisphenols to highlight the potential of bisguaiacols as drop-in alternatives to bisphenols in current technologies. These reactions resulted in bisguaiacol regioisomer mixtures, Table 1, based on the number and location of methoxy and hydroxyl groups, which could be separated by column chromatography if desired.41,45 Second, the bisguaiacols were reacted with epichlorohydrin at elevated temperature (50 °C) in the presence of a phase transfer catalyst (TBAB) and then ring closed at reduced temperature (0 °C) in the presence of NaOH to yield bisguaiacol diglycidyl ethers. After the bisguaiacol diglycidyl ethers were synthesized (Scheme 1), oligomers were removed using flash column chromatography to eliminate any impact of oligomer content on the thermomechanical properties of cured resins (note: oligomers could provide an additional handle for tuning epoxy resin Tm [i.e., processability] and cured epoxy resin properties in commercial materials). All bisguaiacol diglycidyl ethers had very low n values (Table 1), as determined by 1H NMR spectroscopy (Figures S16−S25) and GPC analysis (Figure S27).42 Bisguaiacol diglycidyl ethers exhibited different Tm’s (Table 1), as the samples ranged from viscous liquids at room temperature (o,p′-BGPDGE, mix-BGSDGE, m,p′-BGSDGE, and m,p′-BGMDGE) to waxy solids with Tm’s between room

a

Regioisomer mixtures are designated with the prefix mix-, and pure regioisomers are designated with the prefix p,p′-, m,p′-, or o,p′-. b Regioisomer connectivity was defined with respect to hydroxyl position. cCalculated by 1H NMR spectroscopy.42 dDetermined by DSC at a heating rate of 10 °C/min. eRegioisomer synthesis is unfavorable. fRegioisomer synthesis is impossible on the basis of the reactant chemical structures. gLiquid at room temperature.

varying in the number of methoxy groups and regioisomer content were synthesized from bisguaiacols in a one-pot, two-

Figure 1. (a) Storage modulus and (b) tan(δ) as a function of temperature for MDA-cured mix-BGPDGE (pink −), mix-BGSDGE (green − − −), mix-BGMDGE (purple - - -), BADGE (orange − - − -), and BFDGE (yellow − - - −) of the third heating DMA trace, in oscillatory film tension mode with a strain amplitude of 0.1%, a strain frequency of 1 Hz, a preload force of 0.1 N, and a heating rate of 3 °C/min. (c) E′200 and (d) Tg as a function of the number of methoxy groups, for BFDGE (0), BGPDGE (1), BGSDGE (3), and BGMDGE (4) regioisomer mixtures (square), pure p,p′-regioisomers (dark gray diamond), and pure o,p′- or m,p′-regioisomers (light gray circle). Error bars indicate one standard deviation. Tg and ′ are dependent on the number of methoxy groups and regioisomer content for bisguaiacol diglycidyl ethers. E200 D

DOI: 10.1021/acssuschemeng.8b03340 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

significantly different across all diglycidyl ethers.39 The storage moduli at 200 °C (E′200) of the cured diglycidyl ethers varied between 15 and 46 MPa as a function of the number of methoxy groups and regioisomer content (Figure 1c). Bisguaiacol diglycidyl ethers with a low number of methoxy groups had higher E′200 than bisguaiacol diglycidyl ethers with a high number of methoxy groups, which is expected because the molecular weight between cross-links (Mc) is inversely proportional to the rubbery modulus (rubber elasticity theory, ′ ).46 Surprisingly, pure p,p′i.e., Mc ∼ 2EEW ∼ 1/E200 regioisomer bisguaiacol diglycidyl ethers had higher E′200 than pure o,p′- and m,p′-regioisomer bisguaiacol diglycidyl ethers independent of the number of methoxy groups. Fourier transform infrared (FTIR) spectroscopy and DSC revealed that all thermosets had similar final conversions and reactivities, respectively, and thus, unreacted functional groups cannot account for the differences in the elastic moduli (Figures S28−S30 and Table S3). The differences in E′200 values between diglycidyl ethers possibly resulted from crosslinking topography differences, such as the formation of elastically ineffective loops (primary, secondary, and so on), influenced by the regioisomer during curing.39,47 Initial room temperature tensile testing (Figure S31 and Table S4) indicated these epoxy resins undergo brittle fracture, as expected for highly cross-linked epoxy resins. The Tg’s of the cured bisguaiacol diglycidyl ethers generally decreased with increasing methoxy content, with additional influences from the regioisomer content, Figure 1d. The increased number of methoxy groups along the polymer backbone is hypothesized to prevent close chain packing, resulting in lower Tg.29 However, the cured p,p′-BGMDGE resin (with four symmetrical methoxy groups across the methylene bridge) had a high T g similar to cured p,p′‑BGPDGE, o,p′-BGPDGE, and BFDGE resins (with one or no methoxy groups across the methylene bridge), possibly as a result of hindered backbone mobility and increased chain packing density. As seen in literature, cured pure p,p′‑regioisomer resins (p,p′-BGPDGE, p,p′-BGSDGE, and p,p′‑BGMDGE) have higher Tg’s (10, 5, and 22 °C higher, respectively) in comparison to the corresponding pure o,p′- or m,p′-regioisomer resins. The difference in Tg’s between the different regioisomer resins is likely due to the increased number of elastically active chains within the p,p′-regioisomer resins relative to the cured o,p′- and m,p′-regioisomer resins, which likely inhibited concerted backbone mobility and contributed to the increase in Tg.38,39 The Tg’s of the cured mixture of regioisomer resins were lower than the corresponding pure regioisomer resins. In literature, mixtures of epoxy monomers with identical connectivity (p,p′- or 1,4-connectivity for single aromatic epoxy monomers) are known to have Tg’s that are intermediate to their pure components.48−50 We hypothesize that ineffective chain packing of cured mixed regioisomer bisguaiacol diglycidyl ether resins led to more free volume, resulting in lower Tg’s as suggested for analogous, nonlignin-derivable systems in the literature.51,52 Thermal Stability of Cured Resins. Cured bisguaiacol diglycidyl ethers were thermally stable at temperatures below 300 °C in N2 and air. In N2, all cured diglycidyl ethers exhibited one distinct decomposition event below 450 °C, with maximum weight losses between 370 and 400 °C (Figure 2), which suggests that all cured diglycidyl ethers, including BADGE and BFDGE, decomposed through similar degradation pathways.53,54 However, the thermal stability (T5%)

Figure 2. TGA thermograms of mix-BGPDGE (pink −), mixBGSDGE (green − − -), mix-BGMDGE (purple - - -), BADGE (orange − - − -), and BFDGE (yellow − - - −) in N2. (a) Sample mass remaining (%) and (b) the first derivative of the mass remaining (%/°C) as a function of temperature for all MDA-cured diglycidyl ethers, with a heating rate of 10 °C/min. All cured diglycidyl ethers were thermally stable at temperatures up to 300 °C in N2. Full thermograms (25−700 °C) are provided in Figure S32.

Table 2. Thermomechanical Properties of MDA-Cured Diglycidyl Ethers samplea mix-BGPDGE p,p′-BGPDGE o,p′-BGPDGE mix-BGSDGE p,p′-BGSDGE m,p′-BGSDGE mix-BGMDGE p,p′-BGMDGE m,p′-BGMDGE BADGE BFDGE

E30 ′ b (GPa) 2.7 2.5 2.8 3.0 2.7 2.4 3.2 2.5 2.0 2.5 2.4

± ± ± ± ± ± ± ± ± ± ±

0.1 0.2 0.1 0.2 0.2 0.4 0.1 0.2 0.5 0.6 0.2

E200 ′ b (MPa) 43 46 22 33 33 28 22 26 15 46 31

± ± ± ± ± ± ± ± ± ± ±

1 4 1 3 2 2 2 3 1 8 1

Tgb,c (°C) 132 151 141 113 134 129 111 141 119 167 138

± ± ± ± ± ± ± ± ± ± ±

3 3 1 4 2 1 2 3 1 2 3

T5%d (°C) 368

359

344

381 375

a Regioisomer contents are reported in Table 1. bDetermined by the third heat DMA trace at a heating rate of 3 °C/min. cTg was defined as the peak in the tan(δ) curve. dDetermined by TGA at a heating rate of 10 °C/min in N2.

temperature and 100 °C (mix-BGPDGE, p,p′‑BGPDGE, p,p′‑BGSDGE, and mix-BGMDGE) to powders with Tm’s above 100 °C (mix-BGFDGE and p,p′-BGMDGE). Thermomechanical Properties of Cured Epoxy Resins. All bisguaiacol diglycidyl ethers that were cured with MDA exhibited glassy thermomechanical properties at room temperature with Tg’s > 100 °C (Figure 1a,b). Cured diglycidyl ethers had storage moduli at 30 °C (E30 ′ ) between 2.0 and 3.2 GPa (Table 1), as expected for glassy thermosets and were not E

DOI: 10.1021/acssuschemeng.8b03340 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

ACS Sustainable Chemistry & Engineering decreased from 375 °C (BFDGE) to 344 °C (mix-BGMDGE) as the number of methoxy groups increased (Table 2). Similar results were obtained for experiments performed under air flow (Figure S26 and Table S2). A small reduction in thermal stability was expected for bisguaiacol diglycidyl ethers relative to BADGE (381 °C) and BFDGE because methoxy groups ortho- to the glycidyl ether linkage reduce the thermal energy required for the homolytic cleavage of the phenoxy-glycidyl oxygen−carbon bond.55 Methoxy groups can be removed from lignin-derived monomers to increase thermal stability, but this option would require further deoxygenation reactions.56 We note that degradation temperatures of MDA-cured bisguaiacol diglycidyl ethers in N2 and air were significantly above potential end-use temperatures, and bisguaiacol diglycidyl ethers would not require deoxygenation for applications such as food can linings or structural adhesives. Ten lignin-derivable bisguaiacol diglycidyl ethers were synthesized to expand the field of high Tg bioderived alternatives to petroleum-derived monomers such as BADGE and BFDGE. All MDA-cured bisguaiacol diglycidyl ethers had Tg’s above 100 °C, T5%’s above 300 °C, E′30’s above 2.0 GPa, and E′200’s between 15 and 46 MPa. The Tg, T5%, and E′200 of MDA-cured bisguaiacol diglycidyl ethers were manipulated by varying the number of methoxy groups and regioisomer content, but bisguaiacol diglycidyl ether E′30’s were independent of the number of methoxy groups and regioisomer content. p,p′-BGPDGE, o,p′-BGPDGE, p,p′-BGSDGE, and p,p′‑BGMDGE had high Tg’s that were similar to BFDGE. Furthermore, the Tm’s of uncured bisguaiacol diglycidyl ethers can be altered for desired applications, ranging from liquids for composite applications to powders for powder coating applications.35,57 Finally, the thermomechanical properties of bisguaiacol diglycidyl ethers are readily tuned to match, or exceed, the desired specifications of petroleum-derived BADGE and BFDGE, making them desirable non-BPAcontaining drop-in alternatives for epoxy resin applications.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by NSF grant DMR-1506623 to C.J.K. and T.H.E. The University of Delaware (UD) NMR facility was supported by the Delaware COBRE program with a grant from NIH NIGMS (1 P30 GM110758-01). The authors thank the UD Advanced Materials Characterization Laboratory for the use of the TA Instruments DSC and TGA. The authors also thank the UD Center for Composite Materials for the use of the Neztsch DSC. The authors acknowledge Dr. Steven Sauerbrunn for helpful discussions regarding DSC curing experiments.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b03340. NMR spectra of bisguaiacols and diglycidyl ethers, GPC traces of diglycidyl ethers, additional TGA thermograms of cured epoxies, FTIR spectra of diglycidyl ether−MDA mixtures before and after curing, and nonisothermal curing exotherms and heats of reaction (HR) for diglycidyl ethers cured with MDA (PDF)

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.J.K.). *E-mail: [email protected] (T.H.E.). ORCID

Kaleigh H. Nicastro: 0000-0003-4587-0053 Christopher J. Kloxin: 0000-0002-1679-0022 Thomas H. Epps, III: 0000-0002-2513-0966 Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acssuschemeng.8b03340 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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