Article pubs.acs.org/Biomac
Biobased Epoxy Nanocomposites Derived from Lignin-Based Monomers Shou Zhao† and Mahdi M. Abu-Omar*,†,‡ †
Brown Laboratory and Department of Chemistry and ‡Forney Hall of Chemical Engineering, School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907, United States
ABSTRACT: Biobased epoxy nanocomposites were synthesized based on 2-methoxy-4-propylphenol (dihydroeugenol, DHE), a molecule that has been obtained from the lignin component of biomass. To increase the content of hydroxyl groups, DHE was o-demethylated using aqueous HBr to yield propylcatechol (DHEO), which was subsequently glycidylated to epoxy monomer. Optimal conditions in terms of yield and epoxy equivalent weight were found to be 60 °C with equal NaOH/phenolic hydroxyl molar ratio. The structural evolution from DHE to cured epoxy was followed by 1H NMR and Fourier transform infrared spectroscopy. The nano-montmorillonite modified DHEO epoxy exhibited improved storage modulus and thermal stability as determined from dynamic mechanical analysis and thermogravimetric analysis. This study widens the synthesis routes of biobased epoxy thermosets from lignin-based molecules. number of reactive units. The first strategy involves introducing an active functional group(s), which promotes further polymerization reactions. For example, a lignin-based bio-oil mimic containing phenol, guaiacol, creosol, 4-ethylguaiacol, 4propylguaiacol, 4-dimethylaminopyridine, etc. was methacrylated via esterification reaction with methacrylic anhydride.13 The methacrylated monomer was then subjected to free radical polymerization condition to make lignin-based vinyl ester resins. The second strategy utilizes the reactive ortho and para sites of phenol for hydroxymethylation or to make Novolac or Resol-type resin using formaldehyde chemistry. A typical example is the synthesis of guaiacol novolac (GCN) and wood−tar creosote novolac (WCN) through the reactions of wood-derived guaiacol and creosote with formaldehyde.15 The resulting lignin-based novolacs were used as hardeners for epoxy networks and showed comparable curing property with their petroleum-based counterparts. The third strategy connects lignin-derived compounds to make oligomers with additional functional groups. For instance, two molecules of ovanillin are combined by p-phenylenediamine via oxidative polycondensation.16 The acquired oligomer with two phenolic hydroxyl groups was further glycidylated with epichlorohydrin to prepare epoxy networks. In another example, vanillin-derived oligomers were synthesized through oligomerization reaction between methoxyhydroquinone and diglycidyl ether of
1. INTRODUCTION Recent decades have witnessed an increasing trend of replacing petroleum-derived materials with sustainable and environmentally friendly biomaterials.1,2 Lignin, an abundant and low-cost biomass waste stream, has been widely recognized as a promising feedstock for synthesizing biobased materials and products.3,4 After the extraction from wood via kraft or organosolv processes, lignin often possesses functional groups like hydroxyl or unsaturated bond. These groups are capable of grafting copolymers, while the aromatic structure of lignin can significantly improve the rigidity and thermal stability of the polymer matrix.5 Lignin-based thermosets, like polyurethane resin,6,7 phenol−formaldehyde resin,8,9 and epoxy resin,10,11 have been reported, and they exhibited satisfactory mechanical and thermal properties. Because of the structural complexity of lignin, increasing attention has been placed on model lignin compounds (like the derivatives of phenol, guaiacol, catechol, etc.12) for thermosets synthesis.13 However, accessibility of lignin derivatives to thermoset polymers is often limited by the deficiency of reactive functional groups. To obtain thermoset-like epoxy, the monomers usually contain at least two epoxide groups.1 As for lignin derivatives, substantial amount of lignin decomposing aromatics possess a structural characteristic of phenol substituted by inert methoxy and alkyl groups (structures like guaiacol or creosol),14 making polycondensation or radical polymerization especially difficult. To overcome these structural defects, three modification strategies have been reported in the literature to increase the © XXXX American Chemical Society
Received: April 5, 2015 Revised: June 16, 2015
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DOI: 10.1021/acs.biomac.5b00670 Biomacromolecules XXXX, XXX, XXX−XXX
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Figure 1. Synthesis route of DHE-based epoxy nanocomposite. Benzodioxane derivative produced during the DHEO glycidylation is a side-product that is not curable with amine curing agent. The appearance of benzodioxane product also decreases the concentration of epoxy groups and impairs the cross-link density of cured epoxy networks. Optimized glycidylation conditions are investigated as stated below to minimize the conversion to benzodioxane. another 3 h, and the mixture was washed with acetone, filtered to remove salt, and concentrated with a rotary evaporator. The reaction between DHEO and epichlorohydrin is likely to cause side reactions like intramolecular cyclization between two adjacent oxiranes. Thus, factors influencing the yield and epoxy content of glycidylation product, including temperature (40, 60, 80, and 100 °C) and NaOH/phenolic hydroxyl molar ratio (0.5, 1, and 2), were investigated to find out optimal conditions. Mass ratio that reflected the conversion of DHEO to glycidylation products was obtained by measuring their weights, while epoxy equivalent weight (EEW) was determined by the HCl/acetone chemical titration method. 2.3. Preparation of Nanoclay Modified DHEO Epoxy Nanocomposite. Dry octadecylamine modified nano-montmorillonite was dispersed directly with certain amount of GEDHEO to give the clay weight proportions as 0 (or expressed as neat), 3, 6, 9, and 12 wt %. The mixture was vigorously stirred for 10 min and ultrasonicated for 30 min to improve dispersion and exfoliation. The clay slurry was then introduced with DETA to obtain stoichiometric ratio of epoxy/−NH, stirred for 10 min, and degassed under vacuum to remove entrapped air. The mixture was then poured into a mold for curing (no catalyst added) according to the profile: 30 °C for 2 h, 65 °C for 2 h, and 95 °C for 2 h, which was determined by differential scanning calorimetry (DSC) analysis as described in the following. 2.4. Analysis Methods. The structural evolution from DHE to final cured epoxy was examined using 1H NMR and Fourier transform infrared (FTIR) spectroscopy. The NMR spectra were performed on a Bruker Avance ARX-400 spectrometer using deuterated chloroform as solvent. FTIR analyses were conducted using a Thermo-Nicolet Nexus 470 FTIR Spectrometer equipped with an ultra-high-performance, versatile attenuated total reflectance (ATR) sampling accessory. The spectra were scanned over a wavenumber range of 400−4000 cm−1 with a resolution of 4 cm−1. Curing profiles and catalytic curing behaviors were determined using a DSC (PerkinElmer Jade DSC 4000) under dry nitrogen atmosphere. Samples of 5−10 mg were placed in sealed aluminum pans for all DSC runs. To determine the curing profile, epoxy/amine mixture was heated from 0 to 200 °C with different heating rates (5, 10, 15, 20, and 25 °C/min). Onset, peak, and offset temperatures under different heating rates were obtained and linearly fitted, respectively. The intercepts of the fitted onset (27.1 °C), peak (64.2 °C), and offset (94.6 °C) were used as temperatures for each curing stage.
methoxyhydroquinone from vanillin. The biobased epoxies from vanillin-derived oligomers were reported to have a good processability property and were suitable for industrial production.17 Recently, we have reported a bimetallic Zn/Pd/C catalytic system that converted lignin in intact lignocellulosic biomass directly into two methoxyphenol products.18,19 In this study, one of the major products, 2-methoxy-4-propylphenol (DHE), was investigated as a monomer in the preparation of ligninbased epoxy networks. This study can be viewed as a continuation of the above-mentioned work, and DHE-based epoxy network is synthesized via a practical route (Figure 1), that is, converting the DHE methoxy into hydroxyl group, and the resulting catechol (DHEO) is glycidylated and cured for fabricating lignin-based well-characterized epoxy nanocomposites. The preparation of epoxy thermosets from DHE is significant because it offers a route to synthesize epoxy (and possible other kinds of thermosets) using lignin-based molecules, the majority of which possesses structures similar to DHE.
2. EXPERIMENTAL SECTION DHE, epichlorohydrin, 48% aqueous hydrobromic acid, octadecylamine modified nano-montmorillonite, tetrabutylammonium bromide, and diethylenetriamine (DETA) were purchased from Aldrich Chemical Co. and used without further purification. 2.1. o-Demethylation of DHE To Make Propylcatechol (DHEO). DHE (16.6 g, 0.1 mol) was added to 83 g of 48% aqueous hydrobromic acid. The reaction mixture was magnetically stirred at 115 °C for 19 h, cooled to ambient temperature, saturated with NaCl, and extracted three times with diethyl ether. The organic layer was dried over MgSO4 and concentrated using rotary evaporation. The obtained DHE o-demethylated product (DHEO) (yield = 94%) was used as a dihydroxyl starting compound for epoxy monomer synthesis. 2.2. Optimized Synthesis Conditions of Glycidyl Ether of DHEO (GEDHEO). GEDHEO was prepared by reaction of DHEO (15 g, 0.10 mol) and epichlorohydrin (150 g, 1.62 mol). Tetrabutylammonium bromide (1.55 g) was used as a phase-transfer catalyst. The mixture was heated at 60 °C for 3 h and followed by a dropwise addition of 50% w/w NaOH solution. The reaction was kept for B
DOI: 10.1021/acs.biomac.5b00670 Biomacromolecules XXXX, XXX, XXX−XXX
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Biomacromolecules The hydroxyl concentration of epoxy monomer was determined by acetylation with acetic anhydride in pyridine solution, and the resulting acetic acid formed both in hydrolysis and acetylation process was titrated with a standard alkali solution. The hydroxyl concentration was measured to be 0.097 eq/100 g. Hydrolyzable and nonhydrolyzable chlorine contents were measured according to ASTM standard (ASTM D1726-03) as previously described,20 and the values were found to be 0.014 mol/100 g and 0.083 mol/100 g, which were comparable to previously reported study.21 The presence of chlorine may be due to many side reactions like dehydrohalogenation. The hydroxyl groups of DHE and DHEO are capable of catalyzing the curing reaction. To highlight this catalytic effect, 10 wt % of DHE (i.e., OH/epoxy = 1:7.5) or DHEO (i.e., OH/epoxy = 1:3.5), taking into account the composition of the mixtures, was respectively incorporated to the epoxy/amine system and cured under both nonisothermal and isothermal conditions. For the nonisothermal condition, the systems were heated from 0 to 150 °C at a rate of 10 °C/min. A decreasing shift of peak temperature compared with noncatalytic system indicates a catalytic effect. Under the isothermal condition, samples were heated at 60 °C for 60 min. Conversions were calculated through the ratio of cumulative exothermic heat evolved at time t (min) versus the total heat of the curing reaction. It is worthy to note that benzodioxane byproduct can also catalyze the curing. To avoid this influence, the concentrations of benzodioxane in epoxy monomer were kept the same in all cases. DSC nonisothermal scans of epoxy/amine/nanoclay systems were also conducted to measure exothermic peak temperature, enthalpy of reaction, and activation energy. Activation energy E is calculated by the Kissinger equation:22
ln(β /Tp2) = E /RTp − ln(AR /E)
Figure 2. Effect of temperature and catalyst amount on EEW and mass ratio of DHEO glycidylation products. Symbols in x-axis represent different experimental conditions. For example, 40C−0.5M means experimental temperature is 40 °C and molar ratio of NaOH to phenolic hydroxyl equals 0.5. Optimized experimental condition (60C−1M) is highlighted.
is likely to cause hydrolysis reactions during the synthesis.24 An overall evaluation of the temperature and catalyst factors reveals the optimized synthesis conditions are 60 °C and equal NaOH/ phenolic hydroxyl molar ratio. This optimized condition, which exhibits highest epoxy content and substantial mass ratio, was employed for further synthesis of GEDHEO. 3.2. Characterization of the Epoxy Monomer. 3.2.1. 1H NMR Spectroscopy. As mentioned previously, to make a crosslinked epoxy polymer, the starting compound is required to possess at least two phenolic hydroxyls for glycidylation. Therefore, the DHE methoxyl group was removed to obtain a dihydroxyl compound. The NMR spectra of DHE and its catechol derivative (DHEO) are shown in Figure 3, panels a and b, respectively. The peak at δ 3.83 corresponds to the −OCH3 group of DHE. After the o-demethylation using 48% aqueous HBr, the −OCH3 peak totally disappeared, while two −OH peaks are observed at δ 5.33−5.44, which confirms complete demethylation. Figure 3, panel c exhibits two products after glycidylation of DHEO with epichlorohydrin. The desired methyloxirane product shows epoxy protons H6a, H6b, and H5 at δ 2.75, 2.88, and 3.37, respectively. Another benzodioxane product, with an intramolecular ring formed through the two ortho− OHs, demonstrates characteristic protons H4a, H5, and H4b at δ 3.99, 4.07, and 4.23, respectively. The NMR chemical shifts of the previously mentioned two products are consistent with those reported in the literature25 for a similar starting compound (4-methylcatechol). Proton peak integrals of methyloxirane and benzodioxane exhibit an overall ratio of 5:1, indicating that the desired methyloxirane is the main glycidylation product. This methyloxirane/benzodioxane ratio is further confirmed by the theoretical and measured EEW values of the mixture. The theoretical EEW of the methyloxirane/benzodioxane = 5:1 system is calculated to be 158.6 g/eq , which is close to the measured value of 172.7 g/eq. Besides, primary alcohol of benzodioxane could also be epoxidized. As can be seen in Figure 3, panel c, small peaks from the benzodioxane primary alcohol epoxidation product are observed at δ 2.68 and 3.13 belonging to the oxirane ring,
(1) −1
where R is the gas constant (8.3144 J K mol ), A is the preexponential factor, β is the heating rate, and Tp is the exothermic peak temperature. Activation energy can be obtained from the slope of the linear relationship between ln (β/Tp2) and Tp−1 under different heating rates of 10, 15, 20, and 25 °C/min. Dynamic mechanical properties were characterized using a DMA 2980 (TA Instruments). Rectangular specimens with dimensions of 30 mm length, 10 mm width, and 2 mm thickness were measured in a single-cantilever mode. The measurements were conducted from 25− 100 °C at a heating rate of 3.00 °C/min and a frequency of 1 Hz. Thermal stability studies were carried out on a TGA Q500 (TA Instruments) under a nitrogen flow of 40 mL/min. Samples (15−20 mg) were placed in a platinum pan and scanned from 30−500 °C at a ramp rate of 20 °C/min.
3. RESULTS AND DISCUSSION 3.1. Synthesis of GEDHEO under Optimized Conditions. Temperature is a key parameter affecting the mass ratio of glycidylation product to DHEO. As can be seen in Figure 2, mass ratio is found to increase gradually as temperature increases from 40 to 100 °C. However, higher temperature for reaction between DHEO with epichlorohydrin gives rise to more side reactions, for example, the formation of benzodioxane by intramolecular reaction from the intermediate monoglycide ether,23 which could diminish the epoxy content of the synthesized product. This phenomenon is confirmed in Figure 2. The overall EEW values tend to increase as temperature increases from 60 to 100 °C. The optimized amount of catalyst is also investigated in terms of mass ratio and EEW. For each synthesis temperature, epoxy monomers catalyzed by NaOH with 2.0 molar ratio to phenolic hydroxyl exhibit higher mass ratios than the 0.5 and 1.0 equiv counterparts. As for the EEW, however, highest epoxy contents are obtained when equal molar ratios of catalyst were used. This may be explained as 0.5 molar ratio catalyst is not sufficient, while excess amount of NaOH (i.e., 2.0 molar ratio) C
DOI: 10.1021/acs.biomac.5b00670 Biomacromolecules XXXX, XXX, XXX−XXX
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Figure 3. 1H NMR spectra of (a) DHE, (b) DHE o-demethylated product (DHEO), and (c) GEDHEO and benzodioxane product after glycidylation of DHEO with epichlorohydrin.
and δ 3.74 and 4.09 that are attributed to the CH2−O protons. The chemical shifts are consistent with those reported for catechin as phenolic prepolymers for making epoxy networks.26 The benzodioxane primary alcohol epoxidation product accounts for about 5 mol % of the total product as calculated from the NMR peak area. 3.2.2. FTIR Spectroscopy. Figure 4 demonstrates the FTIR spectra of DHEO, GEDHEO, and its cured cross-linked networks. The characteristic absorption bands of DHEO appear at 3353 cm−1 (O−H stretching), 2864 cm−1, 2936 cm−1 and 2960 cm−1 (alkyl C−H stretch), and 1602 cm−1, 1508 cm−1, and 1438 cm−1 (aromatic C−C bond) (Figure 4a). When
DHEO is glycidylated with epichlorohydrin, its broad OH band diminishes significantly, and it is accompanied by the appearance of an epoxy ring band at 912 cm−1 and a C−O− C ether linkage at 1027 cm−1 (Figure 4b). This is consistent with the NMR result that confirms the formation of epoxy ring. In the subsequent curing process, in which stoichiometric ratio (epoxy vs −NH) of GEDHEO and DETA (no catalyst added) was cured for 2 h at 30 °C, 2 h at 65 °C, and 2 h at 95 °C, the epoxy ring is opened by the DETA reactive hydrogens, and a hydroxyl group is generated concurrently. This curing process is consistent with the IR spectrum of the resulting networks in Figure 4, panel c, as the epoxy peak is diminished, while a broad OH group at 3353 cm−1 appears. 3.3. Catalytic Curing Behavior. 3.3.1. DSC Analysis. Figure 5, panel a demonstrates the curing reaction between stoichiometric amount of GEDHEO and DETA at 55 °C for 60 min catalyzed by DHE and DHEO, respectively. The curing reaction without DHE or DHEO under the same condition was also compared. All curing systems are nearly cured within 60 min. The epoxy/amine curing system catalyzed by DHE and DHEO exhibits enhanced reaction rates than the system in the absence of DHE or DHEO, which exhibits a 35% conversion within the first 5 min. By comparison, the DHE catalyzed system shows higher conversion of 51% during the same time period (5 min), while the conversion reaches around 59% when DHEO catalyzes the curing process. Within the first 20 min, conversions of the three systems rapidly reach 89, 98, and 98%, respectively. Then the reaction rates gradually decrease until the end of the cure. The evident catalytic effect of DHE and DHEO can be explained by the presence of hydroxyl groups that can catalyze the reaction between GEDHEO and amine cross-linker.27 Specifically, the double hydroxyl groups of
Figure 4. FTIR spectra of (a) DHEO, (b) GEDHEO, and (c) GEDHEO/DTEA cured epoxy networks after 2 h at 30 °C, 2 h at 65 °C, and 2 h at 95 °C. D
DOI: 10.1021/acs.biomac.5b00670 Biomacromolecules XXXX, XXX, XXX−XXX
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peak intensities was observed for the DHE and DHEO catalyzed systems, indicating a well-marked catalytic effect consistent with the DSC studies discussed. Quantitative analysis of the curing process can be reflected through changes of the epoxide peak area.29 The conversion (α) was then calculated using the following equation: α=1−
A 912 (t )/A1508(t ) A 912 (0)/A1508(0)
(2)
where A912(0) and A1508(0) are the areas of the epoxide peak at 912 cm−1 and reference peak at 1508 cm−1 at time zero, while A912(t) and A1508(t) are the areas of the epoxy and reference peaks at time t (min). Similar to the DSC approach, Figure 6, panel a reveals that all curing reactions were complete within 60 min, with curing rate order: DHEO > DHE > no DHE or DHEO. In the first 5 min, the system without DHE or DHEO, DHE, and DHEO curing system demonstrates conversions of 38, 60, and 78%, respectively, while more than 90% are cured for all systems within the first 15 min. The conversion data obtained from FTIR approach are slightly higher than those from the DSC approach. Curing could still occur in the sampling and testing processes in FTIR approach, while this uncontrollable curing could be avoided by the in situ measurement of DSC. The overall conversion curves are consistent with those obtained from the DSC study (Figure 5a), and the catalyzed role of DHE and DHEO could be explained by the presence of acidic hydroxyl groups. 3.4. Effect of Nanoclay on the Performance of DHEBased Epoxy Networks. 3.4.1. Effect of Nanoclay on Epoxy Curing Process. Figure 7, panel A demonstrates a decrease in ΔH as nanoclay content increases from 0 to 12%. This phenomenon could be explained by (1) DETA prefers to be absorbed in or at the surface of nanoclay instead of reacting with epoxy;30 (2) the presence of hydroxyl group in nanoclay that consumes certain amount of epoxy;31 and (3) part of the released heat during the curing is consumed by the exfoliation
Figure 5. (a) DSC data of isothermal conversion at 60 °C of GEDHEO without catalyst versus DHE and DHEO catalytic curing systems. (b) DSC temperature scans of heat release during nonisothermal cures of different curing systems at 10 °C/min. The catalyst-to-epoxy monomer weight ratio was 10 wt % for all runs.
DHEO show slightly higher catalytic efficiency than the single −OH of DHE. This catalytic effect is further supported by the DSC data of heat release in Figure 5, panel b. Compared with the system without DHE or DHEO, the presence of DHEO causes an 8.5 °C decrease of the peak temperature, which indicates an evident catalytic effect.28 The temperature shift of about 2 °C for DHE, although not significant, still provides support for catalytic effect. 3.3.2. FTIR Analysis. The catalytic curing of the epoxy− amine system was also examined following the IR intensity change of the epoxy peak at 912 cm−1. The system was cured at 55 °C, and IR spectra were recorded at 0, 5, 10, 15, 20, 25, 30, and 50 min. Epoxy peaks at 912 cm−1 of the system without DHE or DHEO (Figure 6b), DHE (Figure 6c), and DHEO (Figure 6d) catalyzed curing system are compared. The spectra are normalized to an internal peak at 1508 cm−1 that corresponds to the aromatic C−C bond of the GEDHEO. A rapid decrease of peak intensity is observed in Figure 6, panel b within the first 10 min, and the epoxide group gradually disappears in another 20 min. More progressive diminish of
Figure 6. (a) Conversions of DHE and DHEO catalytic curing systems derived from intensity change of epoxy peak at 912 cm−1. FTIR spectra of epoxy peak (912 cm−1) of system (b) without DHE or DHEO, (c) DHE, and (d) DHEO catalytic systems demonstrates decreasing intensity at 0, 5, 10, 15, 20, 25, 30, and 50 min of cure at 55 °C.
Figure 7. (A) DSC temperature scans, (B) exothermic peak temperature, and (C) activation energy of the epoxy/amine/nanoclay system with varying amounts of nanoclay. The heating rate in panel A is 20 °C/min. Activation energy in panel C is calculated using the Kissinger equation with different heating rates of 10, 15, 20, and 25 °C/min. E
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Biomacromolecules of nanoclay.32 The ΔH of neat curing system is around 43 KJ/ ee, which is lower than the typical value of 90 to 95 kJ/ee for epoxy/amine reactions. One possible explanation would be epoxy groups in ortho position and appearance of benzodioxane byproduct that impair the diffusion and buildup of crosslink networks and further influence the conversion of epoxy groups. Peak temperature of the cure, as seen in Figure 7, panels A and B, is found to decrease slightly as nanoclay content increases. The shift to lower temperature, as discussed, indicates a catalytic effect of nanoclay.28 This catalytic effect of montmorillonite may come both from Brönsted acidity, due to dissociation of intercalated water molecules coordinated to cations, and Lewis acidity located in the octahedral sheets that can be efficiently promoted by cation-exchanged montmorillonite.33 Furthermore, Figure 7, panel C shows an evident decrease in activation energy (from around 55 to 45 kJ/mol) as nanoclay content increases from 0 to 12 wt %, which highlights the catalytic role of nanoclay, as it indicates the catalytic impact of organic clay is stronger than the negative influence of the steric effect of the clay exfoliation.34 3.4.2. Effect of Nanoclay on Dynamic Mechanical Properties. The storage modulus (E′) values of the biobased networks modified with varying clay loadings are presented in Figure 8, panel a. As temperature increases from 25 °C to
carbon tails of DHEO restrict the generation of a highly crosslinked structure; second, the presence of benzodioxane byproduct diminishes the concentration of epoxy groups and weakens the cross-linking; moreover, the addition of nanoclay further decreases the Tα due to clay aggregation and steric effect. 3.4.3. Effect of Nanoclay on Thermal Stability. Figure 9, panel a shows that all of the epoxy thermosets exhibit a one-
Figure 9. Thermogravimetric analysis thermograms of DHEO-based epoxy networks modified with different amounts of nanoclay as a function of temperature. Inset exhibits (b) Tonset (temperature at 5% weight loss), (c) T50 (temperature at 50% weight loss), and (d) char residue at 500 °C of DHEO-based epoxy nanocomposites as a function of clay content.
step degradation profile, which is due to the decomposition of cross-linked polymer network.37 Thermal stability of the epoxy networks improves significantly with an increase in nanoclay loading. This is reflected in change of Tonset (temperature at 5% weight loss) from 190 °C of neat network to 220, 229, 240, and 251 °C as nanoclay loadings increase from 3 to 12 wt % (Figure 9b). This retarded degradation phenomenon is generally attributed to the silicate nanolayers of MMT. These nanolayers with high aspect ratio can serve as a mass transport insulator, which causes a more tortuous pathway for decomposed products to diffuse out and heat to flow into the underlying materials.38,39 Similar reduction in decomposition rates was also observed from the T50 (temperature at 50% weight loss) and char residue (char formed at 500 °C), which increase with increasing nanoclay loadings, as observed in Figure 9, panels c and d.
Figure 8. (a) Storage modulus values of DHEO-based epoxy networks modified with different amounts of nanoclay as a function of temperature. (b) Tα of DHEO-based epoxy nanocomposites as a function of clay content.
around 45 °C, in which all samples are glassy state, their respective storage modulus values decrease evidently, but the modified nanocomposites still exhibit higher E′ than the neat one, which may be attributed to the reinforcement effect of montmorillonite (MMT) on the cross-linked polymeric networks.35 When temperature reaches around 50 °C, all samples lose most of their modulus (only about 11 MPa are left) due to reaching of a rubbery state. As temperature increases gradually to 100 °C, both neat and nanoclay modified networks experience a similar modulus decrease process in which almost all moduli are lost. Figure 8, panel b exhibits alpha transition temperatures (Tα) obtained from tan δ peaks for all the samples are measured to be within the range of 40−51 °C. Tα of neat GEDHEO/DETA epoxy network is 40 °C, which is much lower than 118 °C of the DGEBA/DETA network,36 and this phenomenon could be explained by three reasons. First, the ortho epoxies and three
4. CONCLUSIONS The comprehensive synthesis and characterization of a ligninbased epoxy nanocomposite have been demonstrated. The cleavage of DHE methoxyl gives rise to a dihydroxyl compound, which makes it possible for glycidylation and cross-linking. The whole synthesis process, from demethylation to glycidylation and subsequent cross-linking, was complementarily represented from 1H NMR and FTIR spectra. The curing of epoxy/amine system can be catalyzed by hydroxyl groups in DHE, DHEO, and nanoclay, as inferred from the variation of IR intensity, DSC exothermic peak, and activation energy. The DHE-based epoxy modified with nanoclay exhibits improved thermal and mechanical properties as compared to the neat epoxy polymer. The high-performance and easily prepared lignin-based epoxy nanocomposite from DHE provides a route F
DOI: 10.1021/acs.biomac.5b00670 Biomacromolecules XXXX, XXX, XXX−XXX
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for polymer fabrication using molecules from direct catalytic lignin conversion.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare the following competing financial interest(s): M.M.A.-O. has equity interests with Spero Energy, Inc., a start-up company focused on making specialty chemicals from renewable sources. Activities with Spero Energy have been disclosed to Purdue University in accordance with Purdue Policy on Conflicts of Commitment and Reportable Outside Activities (www.purdue.edu/policies/ethics/iiib1.html).
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ACKNOWLEDGMENTS Funding for this research was provided in part by Spero Energy, Inc. and Purdue Research Foundation Innovation Research Fund (IRF).
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DOI: 10.1021/acs.biomac.5b00670 Biomacromolecules XXXX, XXX, XXX−XXX
