Synthesis, characterization, and cure chemistry of renewable bis(cyanate) esters derived from 2-methoxy-4-methylphenol

Article abstract of DOI:10.1021/bm3018438

A series of renewable bis(cyanate) esters have been prepared from bisphenols synthesized by condensation of 2-methoxy-4-methylphenol (creosol) with formaldehyde, acetaldehyde, and propionaldehyde. The cyanate esters have been fully characterized by infrar

Full text of DOI:10.1021/bm3018438

Article
pubs.acs.org/Biomac
Synthesis, Characterization, and Cure Chemistry of Renewable
Bis(cyanate) Esters Derived from 2‑Methoxy-4-Methylphenol
Heather A. Meylemans,† Benjamin G. Harvey,*,† Josiah T. Reams,‡ Andrew J. Guenthner,*,‡
Lee R. Cambrea,† Thomas J. Groshens,† Lawrence C. Baldwin,† Michael D. Garrison,†
and Joseph M. Mabry‡
†Research Department, Chemistry Division, U.S. Navy, NAWCWD, China Lake, California 93555, United States
‡Air Force Research Laboratory, Aerospace Systems Directorate, Edwards AFB, California 93524, United States
S
* Supporting Information
ABSTRACT: A series of renewable bis(cyanate) esters have been prepared from
bisphenols synthesized by condensation of 2-methoxy-4-methylphenol (creosol)
with formaldehyde, acetaldehyde, and propionaldehyde. The cyanate esters have
1
been fully characterized by infrared spectroscopy, H and 13C NMR spectroscopy,
and single crystal X-ray diffraction. These compounds melt from 88 to 143 °C, while
cured resins have glass transition temperatures from 219 to 248 °C, water uptake
(96 h, 85 °C immersion) in the range of 2.05−3.21%, and wet glass transition
temperatures from 174 to 193 °C. These properties suggest that creosol-derived
cyanate esters may be useful for a wide variety of military and commercial
applications. The cure chemistry of the cyanate esters has been studied with FTIR
spectroscopy and differential scanning calorimetry. The results show that cyanate esters with more sterically demanding bridging
groups cure more slowly, but also more completely than those with a bridging methylene group. In addition to the structural
differences, the purity of the cyanate esters has a significant effect on both the cure chemistry and final Tg of the materials. In
some cases, post-cure of the resins at 350 °C resulted in significant decomposition and off-gassing, but cure protocols that
terminated at 250−300 °C generated void-free resin pucks without degradation. Thermogravimetric analysis revealed that cured
resins were stable up to 400 °C and then rapidly degraded. TGA/FTIR and mass spectrometry results showed that the resins
decomposed to phenols, isocyanic acid, and secondary decomposition products, including CO2. Char yields of cured resins under
N2 ranged from 27 to 35%, while char yields in air ranged from 8 to 11%. These data suggest that resins of this type may
potentially be recycled to parent phenols, creosol, and other alkylated creosols by pyrolysis in the presence of excess water vapor.
The ability to synthesize these high temperature resins from a phenol (creosol) that can be derived from lignin, coupled with the
potential to recycle the composites, provides a possible route to the production of sustainable, high-performance, thermosetting
resins with reduced environmental impact.
bility, and hydrophilicity,8 all important properties for high
performance resins. In contrast to other abundant biopolymers,
INTRODUCTION
■
Over the last several years there has been a renaissance of
such as cellulose or hemicellulose, lignin is not as attractive for
activity directed toward the development of full-performance
polymeric materials derived from renewable feedstocks.1−3
the production of renewable fuels due to its complex structure
and recalcitrance. In addition, aromatics do not burn as cleanly
These efforts have paralleled similar thrusts in the realm of
renewable fuels and chemicals.4−6 Although the general
as linear or branched chain alkanes and have relatively high
melting points. Although methods have been developed to
paradigm is similar in that crude biofeedstocks must first be
deconstructed to tractable materials and then chemically
converted to molecules with the required properties, the
convert lignin to renewable fuels through processes such as
pyrolysis, gasification, and hydroliquefication,9−11 many of
these transformations are hydrogen intensive and are perhaps
less practical than other methods. Instead, within the concept of
a biorefinery, a compelling case can be made for the production
of fuels from the cellulosic and hemicellulosic components of
biomass while utilizing the lignin as a source of aromatics, fine
chemicals, and polymeric synthons such as phenols.12 This
ultimate use of the products dictates the choice of feedstock as
well as the methods used to upgrade the biomass. In the case of
high temperature polymer systems, the most compelling
renewable feedstock is lignin, based on cost, availability, and
chemical structure. The long-term cost of crude biomass is
estimated at $60/dry ton, while the U.S. Department of Energy
has predicted that up to 1.3 billion tons of biomass (∼15−25%
lignin) per year could be sustainably produced by 2030.7 The
aromatic structures present in lignin provide excellent high
temperature stability combined with low reactivity, flamma-
Received: November 29, 2012
Revised: January 10, 2013
Published: January 16, 2013
© 2013 American Chemical Society
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parallel approach takes advantage of the chemical diversity of
biomass and allows for the production of multiple product
streams.
to selectively generate bisphenols linked through the carbon
meta to the hydroxyl group while utilizing stoichiometric
amounts of the phenol.29 Efficient routes to the condensation
products of creosol with formaldehyde, acetaldehyde, and
propionaldehyde were discovered. In addition, a bisphenol
coupled at the position ortho to the phenols was prepared.
With the bisphenols in hand it became of interest to synthesize
high-temperature cyanate ester resins and to conduct some
preliminary experiments to evaluate their suitability for a variety
of applications. Cyanate ester resins have been studied
extensively over the last several decades30−32 and more recently
by our research group due to a number of advantages over
epoxy resins including high glass transition temperatures, low
water uptake, and decreased flame, smoke, and toxicity (FST)
for both monomers and cured resins. These properties make
cyanate ester resins particularly interesting for use in marine
and aerospace environments. Recently they have been used or
proposed for the fabrication of new high performance
components including precision molded nanostructures,33
magnet casings for thermonuclear fusion reactors,34 space
telescopes,35 and interplanetary space probes.36 As an initial
entry into the study of renewable cyanate esters, this paper
discusses the synthesis, characterization, and cure chemistry of
bis(cyanate) esters derived from creosol. These results are
discussed within the context of conventional cyanate esters to
evaluate both the benefits and limitations of the sustainable
materials.
The use of lignin as a significant component of resins and
composite formulations has a rich history.3,9 Lignin has been
investigated as a precursor to low-cost carbon fiber,13
a
component of conducting polymers,14 and a macromonomer
useful for the synthesis of polyesters,15,16 polyurethanes,17 and
epoxy polymers.18−20 Lignin has also been studied as a
replacement for phenol−formaldehyde and urea−formaldehyde
resins, sealants, and adhesives.21 The most straightforward
method for incorporating lignin into composites is as an
oligomeric species. A disadvantage of this approach is the low
degree of functionality per aromatic ring. Although a pure
bisphenol has a hydroxyl/aromatic ring ratio of one, the ratio in
Kraft lignin is approximately 0.5.22 This low degree of
functionality results in a sparse 3-dimensional network upon
cross-linking that is not expected to significantly increase the Tg
beyond that of the native lignin (roughly 150 °C).23 Although
suitable for a variety of commercial applications, this modest
glass transition temperature greatly limits the use of lignin in
high performance composite materials. In addition, the
relatively high melting point of oligomeric lignin reduces the
processability of resins and increases the difficulty of composite
fabrication. Finally, the complex heteroatom containing
structures of the oligomers (Scheme 1) render them susceptible
Scheme 1. Conversion of Lignin to Well-Defined Phenolics
EXPERIMENTAL SECTION
■
General. The starting bisphenols were prepared as previously
outlined.28 Cyanogen bromide and triethylamine were purchased from
Sigma Aldrich and used as received. Anhydrous ether was obtained
from Fischer Scientific and used as received. NMR spectra were
collected on a Bruker Avance II 300 MHz NMR spectrometer.
Samples of the cyanate esters were prepared in CDCl3 and spectra
were referenced to the solvent peaks (δ = 7.26 and 77.16 ppm for 1H
and 13C spectra, respectively). Fourier transform infrared spectroscopy
(FT-IR) was carried out using a Thermo Nicolet Nexuus 6700 FTIR
equipped with the Smart iTr attenuated total internal reflection (ATR)
accessory, single bounce diamond crystal. The detector type was a
liquid nitrogen cooled MCTA. FTIR spectra are an average of 32
scans, at 4 cm−1 resolution, and have been baseline and background
corrected. Melting points were determined with a Mel-Temp
apparatus; temperature values are uncorrected. Elemental analysis
was performed by Atlantic Microlabs Inc., Norcross, GA.
General Procedure for Synthesis of Cyanate Esters. Bisphenol
(25 mmol) was dissolved in 100 mL of ether and cooled to −78 °C.
Cyanogen bromide (63 mmol) was added to the cooled mixture and
allowed to dissolve. Triethylamine (50 mmol) was slowly added
dropwise to the cooled mixture over the course of several minutes.
The reaction was stirred at −78 °C for 30 min and then slowly
warmed up to 0 °C and held at that temperature for the duration of
the reaction. The reaction progress was monitored by TLC and was
complete in ∼3 h. The products were isolated as outlined below based
on the solubility of the final product.
Bis(2-cyanato-3-methoxy-5-methylphenyl)methane (1). The
reaction mixture was filtered and the residual solid washed with an
excess of water to remove HNEt3Br. Yield: 7.62 g (96%) of a white
powder. The solid was further purified by dissolving in a minimum of
ethyl acetate and then reprecipitating with ether. 1H NMR (CDCl3) δ
2.31 (s, 6H), 3.93 (s, 6H), 4.00 (s, 2H), 6.55 (s, 2H), 6.73 (s, 2H); 13C
NMR (CDCl3) δ 21.48, 29.67, 56.29, 110.22, 112.64, 122.51, 129.86,
138.67, 138.80, 149.90; mp (powder), 143−146 °C; (crystals) 156−
158 °C; Elem. Anal. Calcd (%): C, 67.44; H, 5.36; Found: C, 67.17; H,
5.46
to hydrolysis and other cleavage reactions, diminishing the
long-term stability of composite materials based on lignin. One
way to circumvent these issues is to utilize pure or well-defined
mixtures of phenols derived from lignin.8 Among the candidate
phenols, vanillin and 2-methoxy-4-methylphenol (creosol)
show a great deal of promise. Oxidation of lignin has been
shown to yield up to about 14% vanillin,24 which has recently
been studied as a precursor to both polyvanillin25 and
renewable vinyl esters.26 Creosol can also be generated from
lignin or is readily derived from vanillin through catalytic
hydrogenation27 (Scheme 1).
With the goal of producing renewable precursors to high
performance resins, our group recently explored the synthesis
of bisphenols from creosol.28 As a further benefit to its
renewable nature, the use of creosol imparts a number of
synthetic advantages over simple phenol, including the ability
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Scheme 2. Synthesis of Bis(cyanate) Esters and Polycyanurate Thermosets from Creosol
Bis(5-cyanato-4-methoxy-2-methylphenyl)methane (2). This
compound was isolated in an analogous manner to 1. Yield: 8.2 g
(96% yield) of white solid. 1H NMR (CDCl3) δ 2.25 (s, 6H), 3.78 (s,
2H), 3.92 (s, 6H), 6.82 (s, 2H), 6.86 (s, 2H); 13C NMR (CDCl3) δ
19.47, 35.10, 56.30, 109.60, 115.53, 117.51, 130.21, 136.44, 140.28,
146.81; mp 122−124 °C; Elem. Anal. Calcd (%): C, 67.44; H, 5.36;
Found: C, 67.50; H, 5.36.
TGA/FTIR Analysis. Samples were analyzed using a Thermo
Nicolet Nexuus 6700 FTIR interfaced via a heated gas cell and transfer
line (held at 150 °C) to a TA Instruments Q50 TGA. The FTIR
detector type was a liquid nitrogen cooled MCTA. FTIR spectra are an
average of 16 scans, at 4 cm−1 resolution, and have been baseline and
background corrected. The TGA was set to ramp from room
temperature to 400 °C at a rate of 10°/min.
Preparation of Resin Pucks. Cured polycyanurate samples were
prepared by heating the cyanate ester in a 6 mL glass vial to a
temperature just above the melting point of the monomer. Once in the
liquid state, the material was degassed at 300 mmHg for 30 min and
then poured into silicone molds made from R2364A silicone from
Silpak Inc. (mixed at 10:1 by weight with R2364B platinum-based
curing agent, degassed for 60 min at 25 °C and cured overnight at
room temperature, followed by postcure at 150 °C for 1 h). The open
mold and sample were then placed in an oven at 25 °C under flowing
nitrogen and cured following a cure protocol of 150 °C for 1 h and
210 °C for 24 h using a ramp rate of 5 °C/min except for 1, which was
cured at 170 °C for 1 h and 210 °C for 24 h. Void-free, transparent,
yellow-orange vitreous discs with smooth surfaces and no evidence of
shrinkage, bubbles, or phase separation, measuring approximately
11.5−13.5 mm in diameter by 1−3 mm thick and weighing 200−400
mg were obtained by this method. The discs were used for
thermomechanical analysis (TMA) and hot water exposure tests.
Thermoset Characterization. DSC was performed on a TA
Instruments Q200 calorimeter under 50 mL/min of flowing nitrogen.
Samples were subjected to a heat−cool−heat cycle from 40 to 350 °C
with a ramp rate of 10 °C/min. Oscillatory TMA was conducted with a
TA Instruments Q400 series analyzer under 50 mL/min of nitrogen
flow. The discs were held in place via a 0.2 N initial compressive force
with the standard ∼5 mm diameter flat cylindrical probe, while the
probe force was modulated at 0.05 Hz over an amplitude of 0.1 N
(with a mean compressive force of 0.1 N). The polycyanurate samples
were subjected to two heating cycles and a cycle to determine thermal
lag.38 For samples not exposed to water, samples were cycled twice
between −50 and 200 °C at 50 °C/min to determine thermal lag with
5,5′-(Ethane-1,1-diyl)bis(1-cyanato-2-methoxy-4-methyl-
benzene) (3). Volatiles were removed under reduced pressure and
the resulting solid was dissolved in ethyl acetate. The organic layer was
washed three times with water and dried over MgSO4. Most of the
solvent was removed under reduced pressure and then a small amount
of ether was added. The mixture was placed at −15 °C to crystallize:
1
7.93 g (88%) of a white microcrystalline material was obtained. H
NMR (CDCl3) δ 1.48 (d, 3H, J = 7 Hz), 2.20 (s, 6H), 3.89 (s, 6H),
4.22 (d, 1H, J = 7 Hz), 6.80 (s, 2H), 6.98 (s, 2H); 13C NMR (CDCl3)
δ 19.15, 20.75, 36.71, 56.25, 109.74, 115.52, 115.69, 135.80, 136.24,
140.34, 146.61; mp 87−88 °C; Elem. Anal. Calcd (%): C, 68.17; H,
5.72; Found: C, 68.24; H, 5.78.
5,5′-(Propane-1,1-diyl)bis(1-cyanato-2-methoxy-4-methyl-
benzene) (4). This compound was prepared in an analogous manner
to 3. Yield: 4.25 g (46%). 1H NMR (CDCl3) δ 0.92 (t, 3H, J = 7 Hz),
1.89 (q, 2H, J = 7 Hz), 2.24 (s, 6H), 3.90 (s, 6H), 3.98 (t, 1H, J = 7
Hz), 6.79 (s, 2H), 7.01 (s, 2H); 13C NMR (CDCl3) δ 12.56, 19.44,
28.68, 43.66, 56.22, 109.75, 115.63, 115.97, 134.88, 136.28, 140.38,
146.54; mp 115−117 °C; Elem. Anal. Calcd (%): C, 68.84; H, 6.05;
Found: C, 68.65; H, 6.02.
X-ray Diffraction Studies. X-ray intensity data were collected for
omega scans at 296 K on a Bruker SMART APEX II diffractometer
with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å).
Frames were integrated using the Bruker SAINT software package
with a narrow-frame integration algorithm. Data were corrected for
absorption using the empirical multiscan method (SADABS), and the
structures solved by direct methods using SHELXTL and refined by
full-matrix least-squares refinement on F2. X-ray data for compounds
2−4 has been deposited in the Cambridge Structual Database.37
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the exception of 1, which was cycled between −70 and 170 °C. To
determine Tg for 2−4, the temperature was then ramped to 300 °C,
cooled to 100 °C, and ramped again to 380 °C, all at 50 °C/min.
Compound 1 was ramped to 250 °C, cooled to 100 °C, and ramped
again to 300 °C, all at 50 °C/min. Discs that were exposed to water
were ramped from 40 to 350 °C, cycled between 100 and 200 °C to
determine thermal lag and ramped again to 350 °C, all at 20 °C/min.
Density of the cured samples was determined using solutions of
calcium chloride in deionized water. Discs of the partially cured
polycyanurates were placed in a vessel and two solutions, at different
concentrations, were combined until a neutrally buoyant solution was
obtained. The density of the neutral solution was measured by
weighing a 10 mL aliquot of the solution using a volumetric flask. This
value was compared to the expected density of a calcium chloride
solution at the known concentration and ambient conditions.
Thermogravimetric analysis (TGA; without FT-IR) was performed
on a TA Instruments Q5000 analyzer with either nitrogen or air flow
of 25 mL/min. The samples were heated from ambient to 600 °C at
10 °C/min. Moisture uptake experiments were performed using cured
discs of uniform 11.7 mm diameter and 3 mm thickness. Each disk was
placed into ∼300 mL of deionized water maintained at a temperature
of 85 °C for 96 h. The discs were then removed from the water, gently
patted dry with a paper towel, and weighed a minimum of three times
(all weights agreed to within 0.0005 g) and then tested via oscillatory
TMA to measure “wet” glass transition temperatures.
ester groups are in similar environments; this result is in line
with the solid state structure of the parent phenol.
The X-ray structures of the cyanate esters (Figure 1) are
significantly affected by the aliphatic groups on the bridging
Mass Spectrometry. Mass spectra of cured resin pucks were
obtained by the Direct Insertion Probe method (DIP-MS) using a
ThermoFisher DSQII. A small amount of sample was placed in a
quartz-micro tube, and inserted into the MS chamber (∼20 mTorr)
using a direct insertion probe. During analysis the probe was
maintained at 30 °C for 30 s, and the temperature was then increased
to 450 °C at 10 °C/min and held at 450 °C for 5 min. Mass data were
collected for the duration of the temperature program.
RESULTS AND DISCUSSION
■
Synthesis of Bis(cyanate) Esters. The cyanate esters were
readily isolated in good to excellent yields by allowing the
bisphenols to react with cyanogen bromide and triethylamine at
low temperature (Scheme 2). Diethyl ether was selected as a
solvent, and in the case of 1 and 2, the product cyanate esters
precipitated in addition to HNEt3Br. These solids were readily
purified by a water wash followed by recrystallization. In
contrast, 3 and 4 with their additional aliphatic carbons
maintained a significant amount of solubility in ether, but could
be obtained in excellent purity by washing ethyl acetate
solutions of the cyanate esters with water followed by
crystallization from ether/ethyl acetate solutions. Compounds
1−3 were isolated in nearly quantitative yields, whereas 4, with
its greater solubility in ether, was isolated in 46% yield.
Characterization of Bis(cyanate) Esters. Compounds 1−
4 were characterized by 1H and 13C NMR spectroscopy,
attenuated total reflectance Fourier transform infrared spec-
troscopy (ATR-FTIR), elemental analysis, and with the
exception of compound 1, single crystal X-ray diffraction.
NMR spectroscopic analysis of compounds 2 and 3 revealed a
trace of the cyanate esters with one of the rings bridging
through the position ortho to the cyanate ester group. This was
predicted based on the starting phenols,28 but these trace
isomers are expected to have only a minor impact on the
melting point, cure kinetics, and physical properties of the
resins. The IR spectra for 2−4 (Figures S11, S13, and S15)
contained two overlapping CN stretching bands due to
asymmetric environments for the cyanate ester functionalities
in the solid state. In contrast, 1 exhibited only a single broad
cyanate ester peak (Figure S9), suggesting that both cyanate
Figure 1. Solid state structures of bis(cyanate) esters.
carbon atom. In the case of 2, the cyanate esters are nearly
diametrically opposed with a torsional angle between −CN
groups of 157°. This configuration also results in an
intramolecular N−N distance of 8.292(3) Å. In contrast, the
methyl group in 3 results in rotation of the aromatic rings and
reduces the torsional angle to 96° with an N−N distance of
6.745(3) Å. The ethyl group in 4 exerts even more influence on
the structure and although the torsional angle (97°) is similar
to that of 3, the N−N distance is significantly reduced to
5.139(4) Å.
The solid state structures of cyanate esters impart physical
properties to the solids that can have important implications for
the utility of these materials. Compounds with lower melting
points allow for more straightforward fabrication processes that
require less energy input. Additionally, for materials that melt at
<100 °C, hot water can be used as the heat source to generate
molten resins. Although the solid state structures of 3 and 4 are
quite similar, the melting points are significantly different with 3
having a melting point almost 30 degrees lower than 4 (Table
1). This is similar to the difference in melting point between
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cyanate esters (Table 2). Compound 4 crystallizes in the
monoclinic P121/c1 space group (Table 2), whereas 3
crystallizes in the triclinic P-1 space group. The higher degree
of symmetry inherent to the monoclinic space group provides a
possible explanation for the higher melting point of 4.
Compound 2 also crystallizes in a monoclinic space group
(C12/c1), and the similarity between the melting points of 2
and 4 suggests that space group symmetry may be more
important for influencing the melting point than the subtle
differences between hydrogen, methyl, and ethyl groups on the
bridging carbon.
Table 1. Melting Points of Cyanate Esters and Parent
Phenols
compound
Tm (standard)
Tm (DSC)
phenol Tm
1
2
3
4
143−146
122−124
87−88
150.7
125.4
90.8
123−125
131−134
143−146
Liquid at RT
115−117
119.6
Interestingly, the melting point of the cyanate esters shows
the opposite trend compared to the phenols, with the parent
phenol of 3 having the highest melting point. However, in the
case of the phenols, other effects such as hydrogen bonding
play important roles in crystal packing. Also of note, in the case
of 1, the cyanate ester has a significantly higher melting point
than the parent phenol. Due to the lack of hydrogen bonding in
the bis(cyanate) ester, one would expect the opposite result.
Unfortunately, we were unable to isolate X-ray quality crystals
of 1 that would allow for a comparative study of the solid state
structure of 1 with the corresponding phenol.
Cure Chemistry. Initial insight into the cure chemistry of
these resins was obtained from DSC measurements. Com-
pound 1 melted in the DSC at over 150 °C and exhibited a
broad and immediate exotherm upon melting that culminated
in a peak cure exotherm at 226 °C. This lack of a broad
processing window or stable temperature range for the molten
resin may limit the versatility of this resin for the fabrication of
composites. Compound 2, which melted at ∼120 °C, exhibited
a stable processing window but a cure exotherm at the relatively
low temperature of 216 °C. The enthalpy change for the cure
was 211 kJ/(mole of compound 2) or 106 kJ/mol(cyanate
ester), which compares favorably to the widely accepted value
of 100 kJ/mol. Compound 3 is the lowest melting resin of the
four compounds and displays a cure exotherm maximum at 283
Figure 2. Structures of conventional cyanate esters.
the conventional cyanate esters LeCy and BADCy (Figure 2),
with LECy existing as a supercooled liquid at room temperature
(mp = 29 °C) and BADCy having a melting point of 79 °C.
The presence of an unsubstituted methylene linkage, as in 4,4′-
dicyanatodiphenylmethane, yields a resin with an even higher
melting point (108 °C).39
The melting point trend for the conventional cyanate esters
can be explained in terms of a molecular symmetry argument,40
but for 1−4, such an argument is not complete. Compound 1,
which is isolated from a symmetric bisphenol precursor and has
been shown to be symmetric in the solid state by IR, has the
highest melting point. This is followed by 2, which also has a
methylene linkage. Introducing a methyl group at the bridging
carbon disrupts the symmetry of the molecule and, as expected,
results in a lower melting point. However, introduction of an
ethyl group, as in 4, results in a surprising increase in melting
point of nearly 30 °C. Some insight into this trend can be
obtained from a comparison of the space groups of these
Table 2. X-ray Crystallographic Data for Cyanate Esters 2−4
property
cmpd 2
C19H18N2O4
cmpd 3
C20H20N2O4
cmpd 4
C21H22N2O4
empirical formula
formula weight
crystal system
338.35
352.38
366.41
monoclinic
triclinic
monoclinic
space group
C12/c1
P-1
P121/c1
unit cell dimensions
a = 17.5687(12) Å
α = 90°
a = 7.062(2) Å
α = 106.625(4)°
b = 11.734(4) Å
β = 95.476(4)°
c = 12.337(4) Å
γ = 105.337(2)°
928.4(5) Å3
2
a = 9.0462(6) Å
α = 90°
b = 4.7770(3) Å
β = 112.4460(10)°
c = 22.8294(16) Å
γ = 90°
b = 15.0946(10) Å
β = 102.9660(10)°
c = 14.9988(10) Å
γ = 90°
volume
1770.8(2) Å3
1995.8(2) Å3
Z
4
4
density (calcd)
1.269 g/cm3
0.075 × 0.182 × 0.212
24.99°
1.261 g/cm3
0.092 × 0.205 × 0.433
25.00°
1.219 g/cm3
0.148 × 0.179 × 0.340
25.00°
crystal size (mm)
θ (max)
reflections collected
reflections obsd
independent reflections
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I)]
9088
10119
21371
953 [I > 2σ(I)]
1563 [R(int) = 0.0430]
1563/0/127
1.011
2291 [I > 2σ(I)]
3281 [R(int) = 0.0245]
3281/0/256
1.023
2693 [I > 2σ(I)]
3504 [R(int) = 0.0253]
3504/0/270
1.038
R1 = 0.0415
wR2 = 0.0917
R1 = 0.0404
wR2 = 0.0961
R1 = 0.0397
wR2 = 0.1005
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°C with a cure enthalpy of 104 kJ/mol(cyanate ester). Similarly,
compound 4 has a cure exotherm maximum at 285 °C and a
cure enthalpy of 99 kJ/mol(cyanate ester) (Figure 3). Overall,
Table 3. Summary of DSC Data for Renewable Cyanate
Esters
compound
1
2
3
4
processing window (°C)
cure exotherm (max, °C)
cure enthalpy (kJ/mol,
OCN)
∼20
216
∼120
283
∼60
285
226
59
106
104
99
degree of cure
<60%
complete
complete
complete
actually decreased the Tg to 178 °C, likely due to
decomposition reactions at the elevated temperature. TGA
experiments confirmed that 1 experienced significant weight
loss at that temperature. The as-cured Tgs for the meta-
substituted cyanate esters were all essentially the same at 257
°C, while the fully cured Tgs ranged from 248 to 214 °C for
compounds 2−4, respectively (Table 4). Again, the cause of
Table 4. Key Properties of Cyanate Ester Resins (High
Temperature Cure)
Figure 3. DSC curve for compound 4.
compound
1
2
3
4
these data suggest that compounds 2−4 approach complete
cure under the DSC conditions. Further evidence of the degree
of cure can be extracted from a comparison of the IR spectra of
uncured cyanate esters and fully cured thermosets. Figure 4
density (g/cm3)
1.237
2.59
172
1.223
2.56
255
1.198
2.41
253
1.190
2.29
254
a
cyanurate density (mmol/cc)
b
as-cured Tg (LP , °C)
as-cured Tg (tan δ, °C)
181
257
257
257
b
fully cured Tg (LP , °C)
166
243
196
198
fully cured Tg (tan δ, °C)
178
248
214
214
a
b
For the fully cured samples. LP stands for loss profile.
this decrease in Tg was attributed to decomposition reactions.
Compound 2 is anomalous among these resins, as its Tg only
drops 9° upon heating at the elevated temperature, whereas 3
and 4 both have their Tg drop 43°. This result is consistent with
the observation that pucks formed from 3 and 4 were subject to
foaming upon heating to 350 °C. This provided visual evidence
that the thermosets had undergone decomposition that resulted
in significant outgassing. In contrast, the puck prepared from 2
was intact. The more subtle decrease in Tg for compound 2 is
likely attributable to decomposition reactions on a more
modest scale.
In addition to the dry Tg, it was of interest to evaluate the
performance of these thermosets in wet conditions (Table 5).
Table 5. Wet Glass Transition Temperatures and Water
Uptake of Resins
Figure 4. Conversion of compound 3 to a polycyanurate network. The
IR spectrum of 3 is shown in red and fully cured resin in blue. Cure
conditions: 150 °C, 1 h; 210 °C, 24 h.
cmpd
wet Tg (tan δ, °C)
water uptake (%)
1
2
3
4
174
193
185
161
2.05
2.05
2.61
3.21
shows the results of this comparison for compound 3, while the
data for the other resins can be found in the Supporting
Information (Figures S9−S16). These traces show virtually
quantitative conversion of the cyanate esters to cyanurate rings
for all of the resins. The DSC and infrared spectroscopy results,
which are summarized in Table 3, suggest that 3 and 4 are the
most promising cyanate esters on the basis of an acceptable
processing window and high degree of cure.
To measure the glass transition temperatures of the cured
resins, pucks were subjected to TMA analysis. Based on the
DSC data it was expected that 1 would not fully cure, leading to
a modest glass transition temperature. Indeed this was the case
with an as cured Tg of 181 °C. Further heating to 350 °C
To determine a wet Tg, resin pucks were immersed in 85 °C
water for 96 h and then analyzed by TMA. The lowest water
uptake was observed for 1 and 2, while 4 had the highest water
uptake. One possible explanation for this behavior is that 3 and
4 cure more completely than 2. The as-cured Tg is quite similar
for 2−4 even though 3 and 4 would be expected to have lower
Tgs based on their intrinsically more flexible structures. This
higher degree of cure then leads to the formation of more void
space which increases the uptake of water molecules.
Consistent with this hypothesis, the higher uptake of water
molecules leads to a greater extent of network hydrolysis, and
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thus, lower “wet” Tg values for 3 and 4. As an explanation for
the similarity of the dry and wet Tgs exhibited by 1, it has
previously been observed38 that cyanurate networks with lower
cross-linking densities tend to show “wet” Tg values very close
to the dry Tg values, apparently because exposure to water
results in simultaneous trimerization of unreacted cyanate
esters and network hydrolysis.
To minimize any decomposition reactions while allowing the
resins to approach complete cure, a new series of pucks were
prepared and postcured in the TMA at 250 °C (for 1) and 300
°C for 2−4. These low-temperature conditions resulted in fully
cured Tgs comparable to the as-cured Tgs while maintaining the
integrity of the pucks. In contrast to the high-temperature
method, 1 cured completely under these conditions, and all of
the resins had similar Tgs (231−248 °C) with the exception of
3, which had a Tg of only 219 °C (Table 6). The lower Tg
weight loss of about 5% between 180 and 250 °C. After this
initial weight loss, the material maintained a consistent weight
up to 350 °C followed by an additional weight loss of 7% up to
380 °C and rapid weight loss above this temperature to give
55% weight loss by 400 °C . Compound 3 had a weight loss of
7% between 210 and 280 °C, but unlike 2, compound 3 was
stable up to 390 °C and then rapidly degraded. Compound 4
had the highest low temperature weight loss of all the resins
(10% between 200 and 290 °C) and then exhibited similar
thermal stability to 3 with decomposition occurring at 390 °C.
The gas phase FTIR data of volatile decomposition products
collected at low temperature (∼200−290 °C) for compounds
2−4 are consistent with evolution of isocyanic acid and suggest
that decomposition of a partially reacted polycyanurate network
occurs via an unreacted −OCN group that is then converted to
a carbamate followed by decomposition to yield a phenol and
isocyanic acid (Scheme 3).41,42 The progressive increase in low
Table 6. Glass Transition Temperatures of Cured Resins
(Low Temperature Post-Cure)
Scheme 3. Low Temperature Decomposition Pathway for
Incompletely Formed Tricyanurate Networks
a
cmpd
Tg (LP , °C)
Tg (tan δ, °C)
1
2
3
4
236
240
206
219
238
238
248
219
231
244
b
4
a
b
LP = loss profile. Purified by flash chromatography.
observed for 3 suggests that it does not achieve complete cure
under these conditions. Also of interest, the puck made from 4
showed significant degradation and some off-gassing when
heated to 350 °C. To determine if this was due to impurities, a
sample of 4 was purified by flash chromatography on silica gel
and a puck was fabricated. The “high purity” puck had a Tg 13°
higher and was stable at 350 °C. This result highlights the fact
that the purity of the cyanate esters can have a profound impact
on the properties of the resulting thermosets.
To further evaluate the cure chemistry of the cyanate esters,
TGA/FTIR data were collected on uncured samples that were
heated from ambient temperature up to 400 °C under a
nitrogen atmosphere. Gas phase spectra of pyrolysis products
were collected with an in-line spectrometer. Compound 1 was
stable up to ∼350 °C and then rapidly degraded, losing 56% of
its mass by 400 °C (Figure 5). In contrast, compounds 2−4
showed significantly different behavior than 1, and these results
provided insight into both the cure chemistry and decom-
position mechanisms for these resins. Compound 2 exhibited a
temperature weight loss from 2−4 suggests that the molecules
with alkyl groups on the bridging carbon atom cure slower than
2. This result is consistent with the greater steric crowding in
these molecules, which allows for incomplete networks to exist
for longer times at elevated temperatures, resulting in
decomposition of unreacted cyanate ester functionalities.
Despite this correlation, other factors, particularly the purity
of the respective cyanate ester resins, may also play a significant
role in the rate of cure. Although all of the resins utilized in this
work were shown to be pure by conventional analytical
techniques, in other cyanate ester studies, trace impurities
(even those around 0.1%) have been shown to have a
significant effect on the cure rate.43 In spite of their propensity
to cure slower, compounds 3 and 4 appear to cure more
Figure 5. TGA (N2) of compounds 1−4.
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completely, allowing them to have roughly the same Tg as 2,
despite the greater flexibility of the native resins. This higher
degree of cure also imparts greater thermal stability to the
network polymers formed from 3 and 4, as shown by the
delayed onset of high temperature degradation observed from
the TGA studies.
For all of the cyanate esters, gas phase FTIR data of the
pyrolysis products at 390−400 °C showed evolution of
isocyanic acid, methane, and phenolic compounds. The phenols
are characterized by a distinctive O−H stretch at 3550 cm−1 as
well as two overlapping summation bands at ∼1850 cm−1.
Initially we believed that the primary decomposition products
were bisphenols, however, taking compound 3 as an example,
comparison of the IR spectrum of the parent bisphenol with the
gas phase spectrum of the pyrolysis products (Figure 6) showed
observed above 440 °C. Although modest peaks were observed
for the molecular ion at m/z = 302 and loss of a methylene
group at m/z = 288, the peak with the highest intensity in the
mass spectrum had m/z = 138, which corresponds with creosol.
Another significant peak was observed at m/z = 152, which
corresponds to a methyl creosol fragment, ostensibly derived
from cleavage of the ethylidene bridging group (Scheme 4).
Similarly, the molecular ion peaks of the bisphenols were
observed for compounds 1, 2, and 4 in their respective spectra,
along with cleavage products primarily comprised of alkylated
creosols. On the basis of the mass spectrometry results and gas
phase IR data, it is clear that the primary decomposition
products are the parent phenols along with phenolic fragments
generated by cleavage of the aliphatic bridging groups between
aromatic rings. The residual cyanurate rings decompose
primarily to isocyanic acid.
Although these initial TGA/FTIR studies provided insight
into both the cure chemistry and the thermal degradation
pathways of the materials, these experiments did not allow for
an accurate assessment of the stability of fully cured resins. To
remedy this, fragments of cured resin pucks were subjected to
TGA under both a nitrogen and air atmosphere. In contrast to
the TGA results for the uncured samples, no low temperature
weight loss was observed due to the presence of a well-formed
cyanurate ring network. Interestingly, the weight loss results
were similar in both environments up to the decomposition
temperature of the resins. In contrast, char yields were
significantly lower in air due to oxidation and/or hydrolysis
reactions. Although the char yields are somewhat low compared
to conventional cyanate esters, this is to be expected due to the
lower proportion of aromatic carbons in the renewable resins.
Considering loss of all the functional groups except aromatic
carbons, a theoretical maximum for char yield is in the range of
39−43% for these resins. The experimental char yield varies
from 27 to 35% (Table 7) with 2 producing 81% of the
Figure 6. Comparison of FTIR data for the phenolic precursor to 3
(blue) and the pyrolysis products observed from the thermoset
derived from 3 at 400 °C (red).
that in addition to the bisphenol, the evidence points to
cleavage of the bridging group between aromatic rings.
Although there is relatively good overlap of the IR bands
below ∼1620 cm−1, the bands at 1850 cm−1 are particularly
diagnostic and match well with the reported spectrum for
creosol.44 To further characterize the decomposition products,
we analyzed cured resin samples in a mass spectrometer.
Taking compound 3 as an example, the temperature was
ramped from ambient temperature up to 450 °C at 10 °C/min.
Similar to the IR results, onset of decomposition (under
vacuum) was observed at 375 °C and no volatiles were
Table 7. TGA Data for Cured Resin Pucks
T5% loss in N2
(air), °C
T10% loss in N2
(air), °C
char yield at 600 °C in N2
compound
(air), %
1
2
3
4
317 (326)
360 (357)
330 (337)
329 (346)
326 (339)
366 (362)
344 (349)
345 (357)
33 (8)
35 (11)
28 (11)
27 (11)
theoretical char and 4 producing 69% of the theoretical char.
These numbers are in relatively good agreement to BADCy
Scheme 4. Proposed Decomposition Pathway for the Cured Resin Derived from Compound 3
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(78% of theoretical) but deviate significantly from LECy (94%
of theoretical).45 In the case of 1−4, but also for conventional
cyanate esters such as BADCy, some of the loss of char yield is
due to evolution of phenolics during decomposition. For 1−4,
the char yield in air decreases to roughly 10% at 600 °C for all
of the resins, but whether this drop is caused by oxidation
chemistry or hydrolysis reactions is unclear.
ASSOCIATED CONTENT
* Supporting Information
■
S
1H and 13C NMR spectra, IR spectra, DSC data, and TGA data
for compounds 1−4, a combined .cif file for compounds 2−4,
IR spectra, TGA data, and TMA data for the cured resins. This
material is available free of charge via the Internet at http://
pubs.acs.org.
From a renewable standpoint, the evolution of phenols is
quite intriguing and suggests that these resins may potentially
be recycled to phenols that could be utilized as precursors to
future cyanate esters or a host of other industrial applications.
The other main product of the decomposition, isocyanic acid,
can be allowed to react with water to produce CO2 and NH3.
Although beyond the scope of the current work, one could
envision a pyrolitic recycling process (Scheme 5) for out-of-
AUTHOR INFORMATION
Corresponding Author
*To whom correspondence should be addressed. Telephone+1
760 939 0247; e-mail benjamin.g.harvey@navy.mil.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Scheme 5. Proposed Recycling Pathway for Renewable
Cyanate Ester Resins
■
The authors would like to thank Ms. Roxanne Quintana for MS
analysis and the Strategic Environmental Research and
Development Program (SERDP) Project WP-2214 for financial
support of this work.
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