Article pubs.acs.org/Macromolecules
Effects of o‑Methoxy Groups on the Properties and Thermal Stability of Renewable High-Temperature Cyanate Ester Resins Benjamin G. Harvey,*,† Andrew J. Guenthner,‡ William W. Lai,† Heather A. Meylemans,† Matthew C. Davis,† Lee R. Cambrea,† Josiah T. Reams,§ and Kevin R. Lamison§ †
NAWCWD, Research Department, Chemistry Division, US Navy, China Lake, California 93555, United States Rocket Propulsion Division, Air Force Research Laboratory, Edwards AFB, California 93524, United States § Rocket Propulsion Division, ERC, Inc., Air Force Research Laboratory, Edwards AFB, California 93524, United States ‡
S Supporting Information *
ABSTRACT: Renewable phenols derived from biomass sources often contain methoxy groups that alter the properties of derivative polymers. To evaluate the impact of o-methoxy groups on the performance characteristics of cyanate ester resins, three bisphenols derived from the renewable phenol creosol were deoxygenated by conversion to ditriflates followed by palladium-catalyzed elimination and hydrolysis of the methoxy groups. The deoxygenated bisphenols were then converted to the following cyanate ester resins: bis(4-cyanato-2-methylphenyl)methane (16), 4,4′-(ethane-1,1′diyl)bis(1-cyanato-3-methylbenzene) (17), and 4,4′-(propane-1,1′-diyl)bis(1-cyanato-3-methylbenzene) (18). The physical properties, cure chemistry, and thermal stability of these resins were evaluated and compared to those of cyanate esters derived from the oxygenated bisphenols. 16 and 18 had melting points 37 and >95 °C lower, respectively, than the oxygenated versions, while 17 had a melting point 14 °C higher. The Tg’s of thermosets generated from the deoxygenated resins ranged from 267 to 283 °C, up to 30 °C higher than the oxygenated resins, while the onset of thermal degradation was 50−80 °C higher. The deoxygenated resins also exhibited water uptakes up to 43% lower and wet Tgs up to 37 °C higher than the oxygenated resins. TGA-FTIR of thermoset networks derived from 16−18 revealed a different decomposition mechanism compared to the oxygenated resins. Instead of a low-temperature pathway that resulted in the evolution of phenolic compounds, 16−18 had significantly higher char yields and decomposed via evolution of small molecules including isocyanic acid, CH4, CO2, and NH3.
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INTRODUCTION In recent years the development of thermoplastics and thermoset resins from renewable sources has undergone a renaissance.1−9 In large part this remarkable degree of activity has been driven by the need to establish sustainable feedstocks that are not dependent on finite petroleum resources. One key area of study is the generation of renewable phenols that can potentially replace or supplement the ubiquitous bisphenol A.10−20 A number of research groups are exploring this field, not only with the goal of generating full-performance materials, but also with an eye toward developing less toxic alternatives to bisphenol A.21−23 The most logical source of renewable phenolic precursors for thermoplastic and thermoset resins is lignin. This abundant biofeedstock is currently generated by the commercial paper industry and is a significant component of forestry residue and agricultural waste. Crude forms of lignin can be acquired for ∼$0.05/lb,24 and it is estimated that as many as 250 million tons could be sustainably produced annually in the US by 2030.25,26 Despite the promise of ligninderived and other natural phenols, these molecules typically contain additional functional groups (primarily methoxy groups) that can affect the material properties, thermal stability, This article not subject to U.S. Copyright. Published 2015 by the American Chemical Society
and decomposition pathways of derivative thermoplastics and thermosetting resins.27−33 Cyanate esters are an important class of thermosetting resins that can be readily prepared from polyphenols. The high glass transition temperatures, low dielectric constants, and low water uptakes afforded by these resins make them suitable for use in a variety of demanding environments.34−37 The physical properties of the thermosets can be readily modified by altering the position, number, and nature of substituents on the aromatic rings.38−44 Alternatively, similar effects can be obtained by altering the bridging groups between rings.45−56 In a recent paper we described the synthesis and characterization of a series of bis(cyanate) esters derived from the renewable phenol creosol (2-methoxy-4-methylphenol).57 Unlike the majority of modified cyanate ester resins that have aliphatic or aromatic substituents, the resins derived from creosol have methoxy groups ortho to the cyanate ester groups. Although these “oxygenated” cyanate esters exhibit thermal stabilities and water Received: March 8, 2015 Revised: April 30, 2015 Published: May 14, 2015 3173
DOI: 10.1021/acs.macromol.5b00496 Macromolecules 2015, 48, 3173−3179
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temperature and was then filtered through a coarse frit, poured into water (100 mL), and extracted with ether (3 × 75 mL). The organic fractions were combined, washed with water (3 × 50 mL) and brine (3 × 50 mL), and dried over MgSO4. The organic solution was passed through a pad of silica gel, and the solvent was removed under reduced pressure to give 10, 11, or 12 as light tan to off-white solids. These crude solids were suitable for further manipulation, but analytically pure samples were prepared by recrystallization from methanol or chromatography on silica gel eluting with Et2O/hexanes. Bis(4-methoxy-2-methylphenyl)methane (10). Yield: 76.9%. 1H NMR (DMSO-d6) δ: 6.77 (d, J = 2.6 Hz, 2H), 6.73 (d, J = 8.3 Hz, 2H), 6.65 (dd, J = 8.3, 2.6 Hz, 2H), 3.70 (s, 6H), 3.33 (s, 2H), 2.17 (s, 6H). 13C NMR (DMSO-d6) δ: 157.9, 137.8, 131.0, 130.1, 116.1, 111.4, 55.3, 34.8, 19.8. Anal. Calcd for C17H20O2: C, 79.65; H, 7.86. Found: C, 79.45; H, 7.84. 4,4′-(Ethane-1,1′-diyl)bis(1-methoxy-3-methylbenzene) (11). Yield: 74.6%. 1H NMR (CDCl3) δ: 6.92 (d, J = 8.4 Hz, 2H), 6.71− 6.66 (m, 4H), 4.21 (q, J = 7.2 Hz, 1H), 3.69 (s, 6H), 2.15 (s, 6H), 1.39 (d, J = 7.2 Hz, 3H). 13C NMR (CDCl3) δ: 157.1, 136.5, 136.3, 127.1, 115.7, 110.8, 54.8, 21.1, 18.9. Anal. Calcd for C18H22O2: C, 79.96; H, 8.20. Found: C, 80.17; H, 8.17. 4,4′-(Propane-1,1′-diyl)bis(1-methoxy-3-methylbenzene) (12). Yield: 85.4%. 1H NMR (DMSO-d6) δ: 6.98 (d, J = 8.7 Hz, 2H), 6.69 (m, 4H), 3.96 (t, J = 7.4 Hz, 1H), 3.69 (s, 6H), 2.19 (s, 6H), 1.83 (dt, J = 7.4 Hz, 2H), 0.86 (t, J = 7.4 Hz, 3H). 13C NMR (DMSO-d6) δ: 156.9, 137.0, 134.8, 127.6, 115.6, 110.9, 54.8, 42.4, 28.5, 19.2, 12.7. Anal. Calcd for C19H24O2: C, 80.24; H, 8.51. Found: C, 80.52; H, 8.46. General Hydrolysis Process. To a −78 °C solution of bis(methoxy) 10, 11, or 12 (15 mmol) in methylene chloride (30 mL) was added BBr3 (2.1 mL, 21.8 mmol) dropwise. The reaction was stirred at −78 °C for an hour and then allowed to warm to room temperature and stirred for an additional hour. Anhydrous methanol was then added, and the solution was stirred for another hour before being placed under reduced pressure to give crude bisphenol. The bisphenol was recrystallized from ethanol/water to give 13, 14, or 15 as white solids. 4,4′-Methylenebis(3-methylphenol) (13). Yield: 70.5%. 1H NMR (DMSO-d6) δ: 9.03 (s, 2H), 6.59 (d, J = 8.2 Hz, 2H), 6.57 (d, J = 2.6 Hz, 2H), 6.46 (dd, J = 8.2, 2.6 Hz, 2H), 3.62 (s, 2H), 2.09 (s, 6H). 13C NMR (DMSO-d6) δ: 155.3, 137.0, 129.6, 129.0, 116.8, 112.5, 34.3, 19.3. Anal. Calcd for C15H16O2: C, 78.92; H, 7.06. Found: C, 78.79; H, 6.96. 4,4′-(Ethane-1,1′-diyl)bis(3-methylphenol) (14). Yield: 74.5%. 1H NMR (DMSO-d6) δ: 9.02 (s, 2H), 6.80 (d, J = 7.9 Hz, 2H), 6.50 (m, 4H), 4.11 (q, J = 6.9 Hz, 1H), 2.08 (s, 6H), 1.35 (d, J = 6.9 Hz, 3H). 13 C NMR (DMSO-d6) δ: 155.0, 136.2, 134.8, 127.0, 116.9, 112.4, 35.2, 21.4, 18.9. Anal. Calcd for C16H18O2: C, 79.31; H, 7.49. Found: C, 79.23; H, 7.44. 4,4′-(Propane-1,1′-diyl)bis(3-methylphenol) (15). Yield: 70.0%. 1H NMR (DMSO-d6) δ: 9.01 (s, 2H), 6.85 (d, J = 9.2 Hz, 2H), 6.51 (m, 4H), 3.87 (t, J = 7.4 Hz, 1H), 2.12 (s, 6H), 1.79 (dt, J = 7.2 Hz, 2H), 0.84 (t, J = 7.2 Hz, 3H). 13C NMR (DMSO-d6) δ: 154.8, 136.7, 133.3, 127.6, 116.9, 112.5, 42.3, 28.7, 19.2, 12.8. Anal. Calcd for C17H20O2: C, 79.65; H, 7.86. Found: C, 78.94; H, 7.77. General Procedure for Cyanate Ester Synthesis. To a −78 °C solution of bisphenol 13, 14, or 15 (4 mmol) and cyanogen bromide (10 mmol) in anhydrous THF (30 mL), triethylamine (8 mmol) was added dropwise. The reaction was stirred for 1 h and then allowed to warm to 0 °C. A substantial amount of white precipitate was then removed by filtration and washed with ether (3 × 50 mL). The organic fractions were combined, washed with water (3 × 25 mL) and brine (3 × 25 mL), and dried over MgSO4. The solvent was removed under reduced pressure to give 16, 17, or 18. Bis(4-cyanato-2-methylphenyl)methane (16). Yield: 81%. 1H NMR (CDCl3) δ: 7.17 (d, J = 2.8 Hz, 2H), 7.05 (dd, J = 8.5, 2.9 Hz, 2H), 6.90 (d, J = 8.5 Hz, 2H), 3.89 (s, 2H), 2.29 (s, 6H). 13C NMR (CDCl3) δ: 151.6, 139.8, 136.6, 130.9, 117.0, 113.0, 109.1, 35.6, 19.9; mp = 88 °C (DSC). Anal. Calcd for C17H14N2O2: C, 73.37; H, 5.07; N, 10.07. Found: C, 73.36; H, 5.03; N, 10.07.
uptake levels that render them useful for high performance applications, the methoxy groups result in lower Tg’s, lower thermal stability, and increased water uptake compared to conventional resins. In perhaps the first study of its kind, the current work seeks to directly evaluate the impact of o-methoxy groups on the properties and decomposition pathways of renewable cyanate esters by comparing close structural analogues. It is expected that the information generated in this work will provide insight into how o-methoxy groups affect other polymers derived from renewable sources. Further, this study will allow the benefits of deoxygenation to be evaluated by considering thermoset performance in the context of synthetic atom economy and cost.
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EXPERIMENTAL SECTION
General. 5,5′-Methylenebis(2-methoxy-4-methylphenol) (1), 5,5′(ethane-1,1-diyl)bis(2-methoxy-4-methylphenol) (2), and 5,5′-(propane-1,1-diyl)bis(2-methoxy-4-methylphenol) (3) were prepared as previously described.12 All solvents and chemicals were purchased from Sigma-Aldrich and used as received except for triethylamine which was distilled from sodium/benzophenone under nitrogen. NMR spectra were collected with a Bruker Avance II 300 MHz NMR spectrometer (spectra for compounds 7−18 are included in the Supporting Information). 1H and 13C NMR chemical shifts are reported versus the deuterated solvent peak [CDCl3: δ 7.27 ppm (1H), 77.23 ppm (13C). DMSO-d6: δ 2.50 (1H), 39.51 (13C)]. Elemental analysis was performed by Atlantic Microlabs Inc., Norcross, GA. General Procedure for Triflation. A round-bottomed flask equipped with a magnetic stirring bar was charged with bisphenol 1, 2, or 3 (30 mmol) and CH2Cl2 (100 mL). The mixture was cooled in an ice bath under a slow stream of nitrogen, and in one portion pyridine (3 equiv) was added. Then triflic anhydride (2.5 equiv) was slowly added over 20 min with a pressure equalizing addition funnel, and the reaction was stirred at 0 °C for 6 h. The mixture was then washed with H2O several times and finally with brine. The organic phase was dried over MgSO4, and the solvent was removed under reduced pressure, leaving a pale yellow solid. The product was then crystallized from hot MeOH, and a white solid (7, 8, or 9) was obtained by filtration. Methylenebis(2-methoxy-4-methyl-5,1-phenylene) Bis(trifluoromethanesulfonate) (7). Yield: 85.6%. 1H NMR (CDCl3) δ: 6.89 (s, 2H), 6.69 (s, 2H), 3.91 (s, 6H), 3.77 (s, 2H), 2.26 (s, 6H). 13C NMR (CDCl3) δ: 149.2, 137.5, 137.0, 134.8, 120.7, 115.3, 56.1, 28.5, 19.5. Anal. Calcd for C19H18F6O8S2: C, 41.31; H, 3.28. Found: C, 41.62; H, 3.27. Ethane-1,1-diylbis(2-methoxy-4-methyl-5,1-phenylene) Bis(trifluoromethanesulfonate) (8). Yield: 78.2%. 1H NMR (CDCl3) δ: 6.85 (s, 2H), 6.83 (s, 2H), 4.23 (q, J = 7 Hz, 1H), 3.89 (s, 6H), 2.23 (s, 6H), 1.47 (d, J = 7 Hz, 3H). 13C NMR (CDCl3) δ: 149.3, 137.0, 136.9, 136.2, 120.3, 115.3, 56.2, 36.4, 20.7, 19.2. Anal. Calcd for C20H20F6O8S2: C, 42.40; H, 3.56. Found: C, 42.46; H, 3.60. Propane-1,1-diylbis(2-methoxy-4-methyl-5,1-phenylene) Bis(trifluoromethanesulfonate) (9). Yield: 86.3%. 1H NMR (CDCl3) δ: 6.87 (s, 2H), 6.82 (s, 2H), 3.98 (t, J = 7 Hz, 1H), 3.88 (s, 6H), 2.26 (s, 6H), 1.87 (q, J = 7 Hz, 2H), 0.96 (t, J = 7 Hz, 3H). 13C NMR (CDCl3) δ: 149.2, 137.5, 137.0, 134.8, 120.8, 115.3, 56.1, 43.4, 28.5, 19.5, 12.5. Anal. Calcd for C21H22F6O8S2: C, 43.45; H, 3.82. Found: C, 43.53; H, 3.75. General Procedure for Detriflation. To a solution of triethylamine (22 mL, 160 mmol) in anhydrous DMF (45 mL) was added 98% formic acid (6 mL, 26 mmol). The mixture was stirred for 5 min, and then ditriflate 7, 8, or 9 (19 mmol), Pd(OAc)2 (0.17 g, 0.78 mmol), and diphenylphosphinoferrocene (DPPF, 0.84 g, 1.5 mmol) were added to the reaction flask. The resulting solution was stirred at 80 °C for 1 h under a blanket of N2. The solution transitioned from a yellow color to deep red, and after several minutes a yellow precipitate was observed. The reaction mixture was allowed to cool to room 3174
DOI: 10.1021/acs.macromol.5b00496 Macromolecules 2015, 48, 3173−3179
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4,4′-(Ethane-1,1′-diyl)bis(1-cyanato-3-methylbenzene) (17). Yield: 79.6%. 1H NMR (DMSO-d6) δ: 7.21 (m, 6H), 4.40 (q, J = 7.0 Hz, 1H), 2.25 (s, 6H), 1.46 (d, J = 7.0 Hz, 3H). 13C NMR (DMSO-d6) δ: 150.7, 142.5, 139.0, 128.5, 116.9, 112.9, 108.8, 36.0, 20.7, 18.7; mp = 105 °C (DSC). Anal. Calcd for C18H16N2O2: C, 73.95; H, 5.52; N, 9.58. Found: C, 73.82; H, 5.36; N, 9.42. 4,4′-(Propane-1,1′-diyl)bis(1-cyanato-3-methylbenzene) (18). Yield: 95.6% 1H NMR (DMSO-d6) δ: 7.24 (m, 6H), 4.16 (t, J = 7.5 Hz, 1H), 2.29 (s, 6H), 1.89 (dt, J = 7.2 Hz, 2H), 0.88 (t, J = 7.2 Hz, 3H). 13C NMR (DMSO-d6) δ: 150.5, 141.0, 139.5, 129.0, 116.9, 112.8, 108.8, 42.7, 28.1, 19.0, 12.4. Anal. Calcd for C19H18N2O2: C, 74.49; H, 5.92; N, 9.14. Found: C, 73.95; H, 5.88; N, 9.10. Preparation of Networks. Monomers were further purified by dissolution in dichloromethane, followed by passage through a WPrep2XY Yamazen flash chromatography column, and removal of the solvent under reduced pressure. All monomers were first melted at 90 °C or 15 °C above the melting point. The liquid monomer was then either placed directly into a hermetically sealed differential scanning calorimetry (DSC) pan and/or poured into one or more silicone rubber molds having a disc-shaped cavity and cured under flowing nitrogen at 150 °C for 1 h and 210 °C for 24 h using a ramp rate of 5 °C/min, then cooled, and demolded. The resultant orange to red discs measured approximately 12 mm in diameter by 4 mm thick. The densities of the discs prepared from compounds 17 and 18 were 1.19 and 1.16 g/mL, respectively. The disc prepared from 16 was porous, and a density was not determined. Water Immersion Testing. Cured discs were dried to a ±0.0001 g constant weight in a vacuum desiccator, then weighed, and immersed in approximately 250 mL of deionized water maintained at 85 °C for 96 h. After removal from the water, samples were patted dry and weighed to determine the moisture uptake (on a dry weight basis). These samples were used for wet Tg determinations Characterization Techniques for Networks. Differential Scanning Calorimetry (DSC). 5−10 mg pieces of cured resins were removed from the molded discs and hermetically sealed in aluminum DSC pans. Samples were then ramped under 50 mL/min of flowing nitrogen at 10 °C/min, first heating to 350 °C, cooling to 100 °C, and then reheating to 350 °C, using a TA Instruments Q200 differential scanning calorimeter. The melting points for compounds 16 and 17 were obtained from these experiments and are simply the temperatures corresponding to the maximum endothermic heat flows. Oscillatory Thermomechanical Analysis (OTMA). Cured discs were tested via oscillatory thermomechanical analysis (OTMA) with a TA Instruments Q400 series analyzer under 50 mL/min of nitrogen flow. The discs were initially held in place with a compressive force of 0.2 N using the standard ∼5 mm diameter flat cylindrical probe. The force was then modulated at 0.05 Hz over an amplitude of 0.1 N (with a mean force of 0.1 N), and the temperature was ramped twice (heating and cooling) between −50 and 200 °C (to aid in determination of thermal lag) followed by heating to 350 °C, cooling to 100 °C, and reheating to 350 °C, all at 50 °C/min. For samples previously exposed to hot water, the heating rate was decreased to 20 °C/min, and the order of segments was: heating to 350 °C, cooling to 100 °C, two cycles between 100 and 200 °C for thermal lag determination, and finally heating to 350 °C. Thermogravimetric Analysis Fourier Transform Infrared Spectroscopy (TGA-FTIR). Samples were analyzed using a Thermo Nicolet Nexus 870 FTIR interfaced via a heated gas cell and transfer line (held at 150 °C) to a TA Instruments Q50 TGA. The TGA was set to ramp from room temperature to 500 °C at a rate of 10 °C/min. FTIR spectra are an average of 32 scans at 4 cm−1 resolution. A liquid nitrogen cooled MCTA detector was used. Spectra were background corrected with the gas cell heated and under a nitrogen purge. Isothermal TGA. Samples were broken into granular chunks and analyzed with a TA Instruments Q5000 TGA. The temperature was ramped from ambient up to 350 °C at 10 °C/min in air. The samples were then held at 350 °C for an additional 6 h.
Article
RESULTS AND DISCUSSION Cyanate esters derived from the renewable phenol creosol, with the cyanate ester group meta to the bridging group between aromatic rings, have recently been studied57 (compounds 4−6 in Figure 1). Direct analogues of these structures without the o-
Figure 1. Synthesis of deoxygenated cyanate esters from renewable bisphenols.
methoxy group would be difficult to prepare from the parent bisphenol given that the methoxy group would have to be eliminated while protecting the phenol for later conversion to a cyanate ester. A more direct approach to deoxygenated derivatives would involve elimination of the phenol followed by hydrolysis of the methoxy group to a hydroxyl group. Although the resulting compounds would not be direct analogues (i.e., the hydroxyl group would be para to the bridging group between aromatic rings), these structures would still allow for a close comparison between methoxy-functionalized (oxygenated) and methoxy-free (deoxygenated) cyanate ester resins and would also represent the most practical deoxygenated compounds that could be generated from creosol. In regard to the latter, the comparison between the two sets of resins would then allow for a quantitative analysis of the benefits derived from chemically upgrading the renewable resins. Synthesis of Deoxygenated Cyanate Ester Resins. In order to evaluate the deoxygenated cyanate esters, an efficient synthesis of the precursor bisphenols was conceived (Figure 1). Deoxygenation was achieved by conversion of the bisphenols to triflates followed by a palladium catalyzed reductive elimination using HEt3N(CHO2) as the hydride source.58,59 The triflates were readily isolated in recrystallized yields of up to 86%. The ease of recrystallization from methanol solutions made it convenient to utilize crude bisphenols as starting materials which both improved the overall conversion of creosol to the (bis)triflates and simplified the process. The deoxygenation protocol was similarly efficient and resulted in yields of 75− 3175
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provide insight into the molecular design of liquid resins with optimal Tgs. After establishing the effect of methoxy groups on the melting points of the resins, the cure behavior of the deoxygenated resins was evaluated by DSC and IR spectroscopy. The resins had cure exotherms between 109 and 117 kJ/ (mol −OCN). These numbers are in line with the generally accepted value of ∼100 kJ/(mol −OCN).61 To determine the degree of cure, the DSC was cycled up to 350 °C, cooled to 50 °C, and then reheated. In all cases the cure was complete as evidenced by the lack of an exotherm on the second heating cycle (Figures 2 and 3 and Figure S25). As expected, the DSC
85%. Use of a dppf-functionalized palladium catalyst conveniently led to precipitation of the catalyst as an oligomeric species60 after the reaction was complete. This allowed for removal of the catalyst by filtration which eliminated the need for extensive chromatography. Although a relatively high catalyst loading (4 mol %) was used for the detriflation reaction, the amount of catalyst used was selected based on convenience, and significantly lower loadings are likely feasible, particularly because TLC and precipitation of the catalyst showed that the reactions were complete within several minutes. After developing an efficient detriflation protocol, two methods were employed to convert the bis(methoxy) compounds to bisphenols. Reaction with either pyridinium hydrochloride or BBr3 generated the products in 70−75% yields. In the case of BBr3, treatment with methanol yielded a volatile methanol adduct of the residual boron compound that could be conveniently removed under reduced pressure. With the deoxygenated bisphenols in hand, synthesis of the bis(cyanate)esters was straightforward with crude yields of 80− 96%. These products were obtained as analytically pure materials without further purification via crystallization or column chromatography. However, to make certain that trace impurities did not affect the network properties, all of the cyanate ester resins were purified by flash chromatography immediately prior to curing. Comparison of Oxygenated and Deoxygenated Cyanate Ester Resins. The first structure−property relationship studied was the difference in melting points between the two classes of cyanate esters. Low melting or liquid resins are much easier to process than high melting resins and can be easily integrated into conventional composite fabrication methods. As an initial hypothesis it seemed likely that removal of the o-methoxy groups would decrease the melting points of the cyanate esters due to both the decrease in molecular weight and the decreased polarity of the molecules. In general, this was reflected in the results with 16 having a melting point 37 °C lower than 4 and 18 having a melting point at least 95 °C lower than 6 (Table 1). In contrast, 17 actually had a melting point
Figure 2. DSC data for compound 17.
Table 1. Effect of Deoxygenation on the Melting Points of the Resins compound 4 16 5 17 6 18
Tm (°C)a
ΔTm (°C)b
c
125 88 91c 105 120c liquid (RT)
−37
Figure 3. DSC data for compound 18. +14
trace for 18 showed no melting point (Figure 3), providing further evidence that the resin exists as a liquid at room temperature. IR spectra of cured resins (Figures S34−S36) showed the complete disappearance of the CN stretching band, confirming that a high degree of cure had been achieved. To evaluate the glass transition temperatures of resin networks, cured disks of each resin were prepared and subjected to dynamic TMA analysis. Both as-cured Tgs as well as fully cured Tgs were determined for compounds 16−18. To obtain fully cured Tgs, disks were postcured to 350 °C in the TMA. In the case of the oxygenated resins 4−6, heating above 300 °C leads to network degradation and subsequent decreases in the Tg.57 Therefore, the maximum achievable Tgs of both sets of resins were compared (as cured for 4−6 and fully cured for 16−18). Compounds 16, 17, and 18 showed increases in Tg of 12, 30, and 18 °C compared to compounds 4,
>−95
a
Determined by DSC. bChange in Tm due to deoxygenation of the resin. cTaken from ref 57.
14 °C higher than 5. Clearly, there is not always a direct correlation between the presence of o-methoxy groups and an increase in melting point. Instead, other factors including the molecular symmetry, polarity, and lattice energy have a significant impact on the melting points of the resins. In contrast to the other resins, compound 18 is a liquid at room temperature. It is not clear why 18, with a bridging propylidene group, has a melting point 80 °C lower than 17, with a bridging ethylidene group, but the study of how bridging structural groups impact the melting points of cyanate ester resins may 3176
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5, and 6, respectively (Table 2). The Tg of 16 was lower than expected, likely as a result of atypical cure behavior. The sample
Table 3. Effect of Deoxygenation on the Water Uptake and Wet Tgs of the Resins
Table 2. Effect of Deoxygenation on the Glass Transition Temperatures of the Resins
compound
water uptake (%)
4 16 5 17 6 18
2.05c NM 2.61c 2.11 3.21c 1.84
compound
Tg (as-cured, °C)
4 16 5 17 6 18
255 253 271 254 256
a
Tg (fully cured, °C) 248c 267 219c 283 231c 272
a
ΔTg (°C)
b
+12 +30
a c
+18
a
Measured by TMA (loss profile). bDifference in Tg due to deoxygenation. The maximum Tg values (as cured for 4, 5, and 6 and fully cured for 16, 17, and 18 are compared). cTaken from ref 57.
Δ(water uptake) (%)a
wet Tg (°C)b 184c NM 178c 207 161c 198
unknown −19 −43
Δ(wet Tg, °C)a unknown +29 +37
b
Change due to deoxygenation. Measured via TMA (loss profile). Taken from ref 57.
This provides support for the theory advanced by Shimp38,39 and more recently by Guenthner44 that the decreased water uptake in the ortho-substituted resins is due to inhibited access of water to hydrophilic sites and not due to differences in network cross-link density and void space. Given the lower water uptake of the deoxygenated resins, it was also expected that they would have higher wet Tgs. Again, it was not possible to determine a wet Tg for 16, but compounds 17 and 18 had wet Tgs 29 and 37 °C higher than 5 and 6, respectively. Overall, the wet Tg knockdowns of 64 and 58 °C62 for 17 and 18, respectively, are similar to LECy (52 °C).18 Another key property of cyanate ester resins, their thermal stability, has been shown to be significantly affected by the presence of o-methoxy groups.57 This is likely due to electron donation from the methoxy groups into the aromatic rings of the polymer. Defining the onset of thermal degradation (OTD) as the temperature at which 5% weight loss is observed, compounds 16−18 began to degrade at almost identical temperatures between 409 and 414 °C (Figure 4). In contrast,
prepared from 16 was porous and anisotropic and a high quality test article could not be generated using the standard cure protocol. Interestingly, no extraneous exothermic reactions were observed in the DSC data (Figure S25), and cyanate esters are known for curing without the generation of volatiles, so it is not clear what led to the voids in the sample. All of the cyanate esters were prepurified by flash chromatography on silica gel directly before cure reactions, and no significant impurities were observed in either the NMR spectrum of 16 or the elemental analysis results. On this basis it does not seem likely that impurities led to the anisotropic cure. One other possibility is that the high purity of the cyanate ester increased the cure temperature into a regime which resulted in volatilization of the monomer as cure proceeded. It is likely that an altered cure protocol or addition of a catalyst would result in a fully cured and well-formed thermoset material. Despite the difficulties with 16, the surface of the resin disk was well formed and rendered a fragment that was suitable for TMA analysis. Even though 4 has the highest as-cured Tg of the oxygenated cyanate esters (255 °C), the deoxygenated version still has a Tg 12 °C higher. It is also interesting to note that the as-cured Tgs of 6 and 18 are very similar. This suggests that the o-methoxy group does not intrinsically reduce the Tg of the resulting thermoset. Instead, the methoxy group is responsible for the decomposition of the oxygenated resins at the higher temperatures required for complete cure of the resins as represented by reduced fully cured Tgs. After evaluating the impact of o-methoxy groups on the Tgs of the resins, it was of interest to examine how the methoxy groups affected water uptake and the wet Tgs of the materials. These considerations are critical for materials used in maritime environments. Removal of the methoxy group resulted in a decrease in the water uptake of 19% for 17 compared to 5 and 43% for 18 compared to 6 (Table 3). No comparison could be made between 16 and 4 due to the poor quality test article formed from 16. These results were expected based on the ability of the methoxy groups to hydrogen bond with water. In regard to the properties of the deoxygenated resins as compared to conventional cyanate ester resins, a recent paper44 has shown that the presence of methyl groups ortho to the cyanate ester groups results in an up to 50% reduction in water uptake compared to BADCy and LECy, commercial cyanate esters derived from bisphenol A and bisphenol E, respectively. In contrast, the current work shows that the presence of methyl groups in the position meta to the cyanate ester groups does not decrease the water uptake of the resins.
Figure 4. TGA data for compounds 16−18.
we recently reported that compounds 4−6 degrade between 329 and 360 °C.57 The 50−80 °C difference in the OTD reflects the presence of two fundamentally different decomposition pathways. In the case of the o-methoxylated cyanate esters, previous work showed that the primary decomposition products were phenolic fragments and isocyanic acid.57 To explore the thermal decomposition of the deoxygenated cyanate esters, TGA-FTIR was employed. The primary decomposition products at 430 °C were isocyanic acid and CO2. At 490 °C the primary decomposition products were CO2, methane, ammonia, and some isocyanic acid (Figure 5). The lack of electron donation into the aromatic rings appears to increase the stability of the aliphatic bridging groups between 3177
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resins that can be generated from creosol. Thus, this work has shown the degree to which the properties of compounds 4−6 can be altered by deoxygenation of the resins. This in turn allows for an assessment of the amount of manipulation required to generate full-performance resins from renewable phenols. From a renewable and green chemistry perspective, it makes sense to minimize the number of steps required to isolate the final resin. Depending on the application, the renewable materials may exhibit suitable performance. However, when higher performance materials are required, removal of the methoxy groups offers a pathway to improve the properties and increase the thermo-oxidative stability of these materials. In effect, renewable phenols can be upgraded to fullperformance resins. One of the key challenges of this approach is the development of efficient and atom-economic methods for selectively deoxygenating renewable phenols.
Figure 5. Gas phase IR data for the thermal decomposition products of compound 16.
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the rings. This stability then allows the polymer to survive to significantly higher temperatures, which in turn shifts the decomposition pathway to degradation of the triazine ring systems. This reaction pathway is known to generate isocyanic acid, methane, CO2, and NH3, with the latter two molecules dominating at higher temperature.63 On the basis of the evident decomposition pathway, one would expect that the deoxygenated cyanate esters would have higher char yields, not only due to a decrease in thermally labile groups but also due to the greater stability of the resins which allows for char formation to be competitive with decomposition to volatile components. A comparison of char yields at 500 °C (Table 4) shows that the
S Supporting Information *
1
H and 13C NMR spectra of new compounds, DSC, TMA, and TGA data for thermoset networks, IR spectroscopy of resins and thermoset networks, and gas phase IR spectra of decomposition products. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00496.
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char yield (% in N2)
4 16 5 17 6 18
38.6 59.0 30.6 56.8 30.0 51.7
Δ char yield
AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (B.G.H.).
Table 4. Char Yield of Resins at 500 °C compound
ASSOCIATED CONTENT
Notes
% mass (OMe)
The authors declare no competing financial interest.
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18.3 +20.4
ACKNOWLEDGMENTS The authors thank the Strategic Environmental Research and Development Program (SERDP, WP-2214), the Office of Naval Research (ONR), and the Air Force Office of Scientific Research (AFOSR) for financial support of this work.
17.6 +26.2 16.9 +21.7
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deoxygenated resins have char yields 20−26% higher than the oxygenated versions and that these values are higher than can be explained by loss of the methoxy groups. To further characterize the thermal stability of networks derived from compounds 16−18, isothermal TGA experiments at 350 °C in air were conducted (Figures S31−S33). Compound 16 exhibited the best initial thermal stability, with a weight loss of ∼25% after 1 h, but all the resins had similar char yields of 38−41% after 8 h. Not surprisingly, these results indicate more resistance to elevated temperatures than found with epoxy resins but less than other high-temperature resins (polyimides or phthalonitriles, for instance) that are wellknown to be more suitable for long-term oxidation resistance.
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CONCLUSIONS Renewable phenols typically contain oxygenated groups, particularly methoxy groups, which profoundly affect the properties of derivative thermoplastics and thermoset resins. This work has shown that o-methoxy groups have a significant impact on the melting points, glass transition temperatures, thermal stabilities, and decomposition pathways of renewable cyanate ester resins. Although direct deoxygenated analogues with cyanate esters meta to the bridging groups were not studied in this work, compounds 16−18 are the most practical 3178
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