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Polycyanurate Thermoset Networks with HighThermal, Mechanical, and Hydrolytic StabilityBased on Liquid Multifunctional Cyanate EsterMonomers with Bisphenol A and AF Unitsa
Dedicated to Professor H. W. Spiess on the occasion of his 65th birthday
Basit Yameen, Hatice Duran, Andreas Best, Ulrich Jonas,* Martin Steinhart,Wolfgang Knoll
Two cyanate ester monomers (CEMs) based on oligomeric aryl ether (OAE) derivatives ofbisphenol AF and bisphenol A, with multiple reactive cyanate groups, were developed astechnologically highly relevant thermosets. These CEMs are liquids processable at roomtemperature and can be crosslinked by cyclo-trimerization of the cyanate groups to formextended polycyanurate (PC) networks at lowertemperatures (<265 8C) than many existingCEMs. The cured PCs have high Tgs (>280 8C),with excellent thermal, mechanical, and dielec-tric properties. PC nanorods with diameters of65 or 380 nm could be moulded in porousalumina templates from the OAE-CEMs. Thehigh aspect ratio nanorods with a length inthe order of 100 mm were hydrolytically stableupon extended exposure to boiling water.
B. Yameen, H. Duran, A. Best, U. Jonas, W. KnollMax Planck Institute for Polymer Research, Ackermannweg 10,D-55128 Mainz, GermanyFax: 0049 6131 379100; E-mail: jonas@mpip-mainz.mpg.deM. SteinhartMax Planck Institute of Microstructure Physics, Weinberg 2,D-06120 Halle, Germany
U. JonasFORTH/IESL, Voutes Str., P.O. Box 1527, 71110 Heraklion, Crete,GreeceFax: 0030 2810 39 1305; E-mail: ujonas@iesl.forth.gr
a : Supporting information for this article is available at the bottomof the articles abstract page, which can be accessed from thejournals homepage at http://www.mcp-journal.de, or from theauthor.
Macromol. Chem. Phys. 2008, 209, 1673–1685
� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/macp.200800155 1673
B. Yameen, H. Duran, A. Best, U. Jonas, M. Steinhart, W. Knoll
1674
Introduction
Cyanate ester (CE) resin systems are being designated as
the next generation thermosetting polymers following the
widely used epoxy resins. They attract increasing atten-
tion due to their outstanding performance with respect to
resistance to fire and moisture, good mechanical strength
and electric stability at cryogenic and elevated tempera-
tures, high glass transition temperature (Tg), low dielectric
constant, radiation resistance, excellent metal adhesion,
and compatibility with carbon fiber reinforcements. These
unique properties of CE resins make them preferential
candidates as structural materials for high-temperature
applications in aerospace, insulation, microelectronics,
and adhesive industries.[1,2]
The first successful synthesis of aromatic cyanate ester
monomers (CEMs) was developed by Grigat et al. in the
1960s at Bayer AG, which involved the reaction of phenolic
compounds with a cyanogen halide in the presence of a
base. The remarkable aspect of CEMs is their polymerisa-
tion via a cyclotrimerization reaction to form a poly-
cyanurate (PC) thermoset in high yield.[1a,3] The versatility
of the synthetic method developed by Grigat et al. made it
possible to incorporate different aromatic structural entities
into CEMs, offering a control over the chemical, physical,
and thermal properties of CEMs and PCs by careful selection
of the precursor phenols. The development of ambient
temperature processable CEMs, which could produce PC
with good thermal and mechanical properties, is an active
area of current CE resin research. 2,20-Bis(4-cyanatophe-
nyl)-1,1,1,3,3,3-hexafluoropropane (BAFCY, Tm¼ 87 8C) and2,20-bis(4-cyanatophenyl)isopropylidene (BACY, Tm¼ 79 8C)are among the most studied and first commercialized
CEMs.[1a] The PCs derived from these bisphenol A
derivatives have attracted great technological interest as
structural materials due to their high Tg (270 8C for BAFCY-
PC and 289 8C for BACY-PC), high mechanical strength
(Young’s modulus 3.11 GPa/BAFCY-PC and 3.17 GPa/BACY-
PC), good thermooxidative stability (up to 400 8C) and good
moisture resistance. On the other hand, the corresponding
precursor CEMs in their non-crosslinked state suffer from
poor processability due to their crystalline nature at room
temperature.[1a,2a] M. Laskoski et al.[4a,4b] have produced
ambient temperature processable BAFCY- and BACY-based
CEMs by placing oligomeric aromatic ether (OAE) spacers
between the terminal cyanate groups. As the crosslink
density is reduced due to an increased chain length
between the crosslinks, their efforts have produced CEMs
with enhanced processability at the expense of a decrease
in Tg in the corresponding PCs (175 8C/BAFCY-OAE-PC and
140 8C/BACY-OAE-PC). The strategy of Guenthner et al.[5] to
replace in BACY the quaternary carbon center with qua-
ternary silicon produced a CEM which is still a crystalline
solid, but with a lower melting point of 59.9 8C (a 20 K
Macromol. Chem. Phys. 2008, 209, 1673–1685
� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
lower Tm than BACY for improved monomer processa-
bility). At the same time the Tg in the cross-linked PC is
only lowered by 10 K and the cured material essentially
maintains itsmechanical properties, like a tensilemodulus
of 2.8 GPa. These reports present important strategies for
the development of CE resin systems with improved
properties by specifically tailoring the structure of CEMs at
the molecular level. The key physical properties that serve
as a basis for identifying ‘‘improved’’ CE resin systems are
comprehensively outlined by Guenthner et al. and include:
a) ease of uncured CEM processing, b) which produce PCs
with high glass transition temperatures (generally in the
range of 200–300 8C), c) good mechanical properties, d)
good thermooxidative stability, and e) good resistance to
moisture.[5] In particular a high Tg is critical for technical
applications of CE resins, as a low Tg in PCs commonly results
in poor mechanical properties at elevated temperatures.[1e]
Based on these criteria wewere interested in developing
CEMswhich are processable at ambient temperaturewhile
possessing a high Tg, good thermo-oxidative, mechanical,
and hydrolytic stability in the cured PC state. To achieve
this goal we present here a strategy based on OAE deri-
vatives of bisphenol AF and A with pendant and terminal
cyanate groups. The flexible and mixed oligomeric nature
of the OAE spacers will provide CEMs with a low
processing temperaturewhereasmultiple reactive cyanate
groups will improve the thermo-mechanical stability of
the cured PCs by an increased crosslinking density. The
thermal, mechanical, dielectric and hydrolytic properties
of developed CE resins are discussed and compared with
the existing structurally related PCs.
Experimental Part
Materials and Methods
1,10-Phenanthroline �99%, copper (I) iodide 98%, N,N-dimethyl-
formamide (DMF) (anhydrous) 99.8%, toluene (anhydrous) 99.8%,
bisphenol A 97%, bisphenol AF (purum) �98.0% (Fluka), and
cyanogen bromide (reagent grade) 97% were obtained from Sigma-
Aldrich, Schnelldorf, Germany. Boron tribromide 99.9% was
obtained fromAcros Organics, Geel, Belgium. Potassiumcarbonate
was dried overnight at 120 8C prior to use. Acetone was refluxed
overnight with potassium carbonate and calcium oxide before
distilling and stored under argon. Triethylamine was refluxed
overnight with calcium hydride, distilled and stored under argon.
1,3-Dibromo-5-methoxybenzene was synthesized from 1,3,5-
tribromobenzene and potassium methoxide according to the
literature procedure.[6] IR spectra were recorded as neat films
using a Nicolet FT-IR 730 spectrometer. 1H-NMRwas performed on
a Bruker Spectrospin 250 MHz NMR spectrometer (Fallanden,
Switzerland). Differential scanning calorimetric (DSC) analyses
were performed on a DSC 822 (Mettler-Toledo, Greifensee,
Switzerland) at a heating rate of 10 8C �min�1 under a nitrogen
purge of 30 cm3 �min�1. Thermogravimetric analyses (TGA) were
DOI: 10.1002/macp.200800155
Polycyanurate Thermoset Networks with High Thermal . . .
performed on TGA 851 (Mettler-Toledo, Greifensee, Switzerland) at
a heating rate of 10 8C �min�1 under a nitrogen or air purge of
30 cm3 �min�1. Rheological measurements were performed on an
advanced rheometric expansion system (ARES, Rheometric Scien-
tific Inc., New Jersey NJ 08854, United States). Torsion deformation
was applied on rectangular samples (50� 10�1 mm3) under
conditions of controlled deformation amplitude, which was always
remaining in the range of the linear viscoelastic response. A
temperature ramp of 2 8C �min�1 was used to determine tempera-
ture dependent storage (G0) and loss (G00) moduli and damping
factors (tan d) of PCs at a frequency of 10 rad � s�1. Tensile tests
were performed on neat cured thermosets using a universal
material testing machine (Instron 6022, Instron Co., Buckingham-
shire, UK) equipped with a 10 kN load cell. Samples were drawn
with a rate of 0.5 mm �min�1 at room temperature. A strain gauge
extensometer with an initial gauge length of 12.5mmwas used to
follow the extension. The sample width and thickness were about
5 and 1mm respectively. The dependence of nominal stress versus
drawing ratio was recorded. The Young’s modulus (E) was deter-
mined from the linear slope of this dependence at small strain.
Pressure-volume-temperature (PVT) measurements were per-
formed using a fully automated high-pressure dilatometer (GNO-
MIX, Gnomix Inc., Boulder, Colorado, USA). With this technique the
specific volume as a function of pressure and temperature can be
determined. A detailed description of the apparatus and themethod
can be found elsewhere.[7] Each runwas performed by varying the
pressure from 10 to 200 MPa in steps of 10 MPa at constant
temperatures. The isothermal measurements were performed in
the range from 25 to 300 8C in steps of 5 K. Absolute densities at
room temperature used to derive the thermal expansion
coefficients were measured in ethanol using a Mettler-Toledo
AG204 (Greifensee, Switzerland) balance equipped with aMettler-
Toledo (Greifensee, Switzerland) density determination kit.
Temperature dependent dielectric measurements were performed
with an experimental setup of Novocontrol GmbH (Hundsangen,
Germany). The system was equipped with an Alpha high-
resolution dielectric analyzer and temperature controller Quatro
version 4.0. The samples weremilled down to a thickness of about
200 mm and sandwiched between two brass discs with diameters
of 10 mm, forming a flat parallel plate capacitor. An AC voltage of
1 V was applied to the capacitor. The temperature was controlled
using a nitrogen gas cryostat and the temperature stability at
the sample was better than 0.1K. The dielectric constant
e�(v)¼ e0(v)� ie00(v) was measured at a frequency of 1 MHz
between�100 to 150 8C. Scanning electron microscopy (SEM) was
performed with a LEO Gemini 1530 SEM with 3.5 nm resolution.
The electron acceleration voltage was around 6 kV.
Synthesis of Oligomeric Aromatic Ether with Pendant
Methoxy and Terminal Hydroxy Groups 1a and 1b
Bisphenol AF (10.09 g, 30 mmol), 1,3-dibromo-5-methoxybenzene
(3.98 g, 15 mmol), 1,10-phenanthroline (0.240 g, 1.33 mmol),
toluene (7 mL) and DMF (55 mL) were added to a 250 mL three-
necked round bottom flask fitted with a thermometer, a Dean-
Stark trap with condenser, and an argon inlet. The resulting
mixturewas degassed thoroughlywith argon for 10min, followed
by the addition of copper(I) iodide (0.227 g, 1.19 mmol). After
Macromol. Chem. Phys. 2008, 209, 1673–1685
� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
filling the Dean-Stark trap with toluene, the mixture was heated
for 30 min to 1 h at 135–145 8C in order to completely dissolve all
the starting materials. The mixture was cooled to 100 8C and
potassiumcarbonate (3.1 g, 22.43mmol)was added in one portion.
The resulting mixture was again heated at 135–145 8C for 3 h and
the water formed in the reaction was removed by azeotropic
distillation. After this time the reaction mixture was cooled again
to 100 8C and another portion of potassium carbonate (3.1 g, 22.43
mmol) was added. The reactionmixture was again heated to 135–
145 8C for 12–14 h until no further water deposited in the Dean-
Stark trap. The remaining toluene was then removed by
distillation and the reaction mixture was cooled to ambient
temperature. Water was added (200 mL) to the reaction mixture
which was made acidic by the addition of 2 M HCl (200 mL) and
extracted with ether (3� 100 mL). The combined ether extracts
were washed with water until neutral and dried over anhydrous
MgSO4. The solvent was evaporated after passing through a short
silica plug to yield a brown semisolid, which was vacuum dried at
80 8C over night to yield a pure oligomeric mixture with pendant
methoxy and terminal hydroxyl groups (16.31 g, 70%).
IR (film): 3 378 (O–H), 3018 (C––CH), 1 597, 1 506, 1 448 (arom. C––
C), 1 241 (C–F), 1 207, 1 168, 1 143 (C–O), 1 000, 966 (C–OH),
829 cm�1 (arom.). FD-MS: m/z¼ 336 (n¼0), 777.5 (n¼1), 1 218.1
(n¼2), 1 658.8 (n¼3), 2 099.3 (n¼4), 2 540.3 (n¼ 5), 2 982.2 (n¼ 6).1H NMR (250 MHz, CDCl3): d¼7.16–7.29 (8 arom. H flanking CF3groups of bisphenol A6F, br.), 6.90–6.94 (4 arom. H next to –O– of
diarylether groups, br.), 6.71–6.75 (4 arom. H next to –OH groups,
br.), 6.30–6.33 (3 arom.c H of the ring with pendant –OCH3 groups,
br.), 5.11 (2 H of –OH, s), 3.69 (3 H of pendant –OCH3, s).
1b was synthesized in the same manner only bisphenol A was
used instead of bisphenol AF (76%). IR (film): 3 362 (OH), 3 021 (C––
CH), 2 966 (CH3), 1 588, 1 503, 1 463 (arom. C––C), 1 363 (CH3), 1 210,
1 171, 1 143, 1 119 (C–O), 1 003, 951 (C–OH), 829 cm�1 (arom.). FD-
MS: m/z¼ 228 (n¼0), 561 (n¼1), 1 227.8 (n¼2), 1 557.9 (n¼ 3),
1 891.7 (n¼4), 2 224.8 (n¼5). 1H NMR (250 MHz, CDCl3): d¼ 6.99–
7.12 (8 arom. H flanking CH3 groups of bisphenol A, br.), 6.81–6.86
(4 arom. H next to –O– of diarylether groups, br.), 6.63–6.67
(4 arom. H next to –OH groups, br.), 6.19 (3 arom. H of ring with
pendant –OCH3 groups, s), 4.65–4.70 (2 H of –OH, s), 3.65–3.71 (3 H
of pendant –OCH3, s), 1.54–1.58 (6 H of CH3 groups of bisphenol A).
Deprotection of Pendant Methoxy Groups
to Yield 2a and 2b
Borontribromide (12.96 g, 51.75 mmol) was added to a solution of
1a (8.93 g, 11.5 mmol) in dry dichloromethane (100 mL). The
reaction mixture was stirred under reflux for 12 h. The reaction
mixture was cooled down to room temperature and hydrolyzed
with 5% aq. HCl solution (Caution: care should be taken while
adding HCl solution. A very slow dropwise addition with ice
cooling is recommended) and extracted with diethyl ether
(3� 100mL). The organic phase was washed with dilute sodium
bicarbonate (1�100 mL) and water until neutral and dried over
MgSO4. The solvent was removed after passing through a short
silica plug to yield a brown solid, whichwas vacuumdried at 80 8Cfor overnight to yield the pure oligomeric mixture of 2a with
pendant and terminal hydroxyl groups. (8.77 g, quantitive yield).
IR (film): 3 338 (O–H), 3 051 (C––CH), 1 597, 1 509, 1 457 (arom.
C––C), 1 241 (C–F), 1 204, 1 171, 1 134 (C–O), 1 006, 970 (C–OH),
www.mcp-journal.de 1675
B. Yameen, H. Duran, A. Best, U. Jonas, M. Steinhart, W. Knoll
1676
829 cm�1 (arom.). FD-MS: m/z¼ 336 (n¼0), 761.9 (n¼1), 1 188.2
(n¼2), 1 613.2 (n¼3), 2 040.8 (n¼ 4), 2 468.4 (n¼5). 1H NMR (250
MHz, CDCl3): d¼7.19–7.31 (8 arom. H flanking CF3 groups of
bisphenol AF, br.), 6.92–6.95 (4 arom. H next to –O– of diaryl ether
groups, br.), 6.73–6.76 (4 arom. H next to terminal OH groups, br.),
6.24–6.30 (3 arom. H of the ring with pendant –OH groups, br.),
4.82–4.90 (3 H of –OH, s).
2b was also synthesized in the same manner as above in
quantitive yield. IR (film): 3 335 (OH), 3 036 (C––CH), 2 966 (CH3),
1 596, 1 503, 1460 (arom. C––C), 1 363 (CH3), 1 216, 1171, 1 137, 1 119
(C–O), 1 003, 906 (C–OH), 829 cm�1 (arom.). FD-MS: m/z¼546.9
(n¼1), 866.3 (n¼ 2), 1 184.4 (n¼ 3). 1H NMR (250 MHz, CDCl3):
d¼ 6.99–7.11 (8 arom. H flanking CH3 groups of bisphenol A, br.),
6.81–6.85 (4 arom. H next to –O– of diarylether groups, br.),
6.62–6.67 (4 arom. H next to –O– of diarylether groups, br.), 6.06–
6.16 (3 arom. H of the ring with pendant –OH groups, br), 4.7–5.06
(3H of –OH, s), 1.54–1.56 (6 H of CH3 groups of bisphenol A, s).
Synthesis of Oligomeric Aromatic Ether with Pendant
and Terminal Cyanate Groups 3a
2a (8.77 g, 11.5 mmol) and cyanogen bromide (4.26 g, 40.22 mmol)
were dissolved in dry acetone (50 mL) and were transferred under
argon to an oven dried 100 mL three-necked round bottom flask
equipped with a thermometer, magnetic stirrer and argon inlet.
The solution was stirred and cooled to �20 to �30 8C. Dry
triethylamine (4.42 g, 43.68mmol) dissolved in acetone (5mL)was
added dropwise over a period of 1 h while maintaining the
temperature of the reaction mixture below �20 8C. After the
additionwas complete the reactionmixturewas further stirred for
1 h below�20 8C and 1 h at room temperature while Et3NþBr� salt
precipitated. The solvent was removed in vacuo. The resulting
residue was stirred with 250 mL of a hexane/dichloromethane
mixture (1:1). The mixture was then filtered through a short silica
plug to remove the Et3NþBr� salt. The solvent was removed in
vacuo to yield the oligomeric aromatic ether with pendant and
terminal cyanate ester groups 3a (8.67 g, 90%) as a yellow oil.
IR (film): 3067 (C––CH), 2277, 2241 (CN), 1 594, 1506, 1457 (arom.
C––C), 1 241 (C–F), 1207, 1171, 1134 (C–O), 1 012, 966, 927 (C–OCN),
829 cm�1 (arom.). 1H NMR (250MHz, CDCl3): d¼ 7.28–7.49 (12 arom.
H, br.), 6.98–7.01 (4 arom. H next to terminal OCN groups, br.), 6.62–
6.70 (3 arom. H of the ring with pendant –OCN groups, br.).
3b was also synthesized in the same manner as above in 92%
yield as light yellow oil. IR (film): 3 036 (C––CH), 2 969 (CH3), 2 262,
2 235 (CN), 1 591, 1 500, 1 463 (arom. C––C), 1 363 (CH3), 1 213, 1 195,
1168, 1143 (C–O), 1012, 936, 912 (C–OCN), 829 cm�1 (arom.). 1HNMR
(250MHz, CDCl3): d¼ 7.11–7.25 (12 arom.H, br.), 6.86–6.89 (4 arom.
H next to OCN, br.), 6.47 (3 arom. H of the ring with pendant –OCN
groups, br.), 1.60 (6 H of CH3 groups, s).For the1HNMR spectra of 3a
and 3b see Figure S2, Supporting Information.
Curing of CEMs to PC Thermosets
The CEMs 3a and 3bwere neat cured by the following temperature
program in a Teflon mold with a cavity dimension of 70� 20�20 mm3 in a tube furnace under argon to yield the corresponding
PCs. After degassing at 80 8C for 1 h, the curingwas induced accord-
ing to the program 180 8C/2 h! 260 8C/8 h! 290 8C/1 h! cooling
Macromol. Chem. Phys. 2008, 209, 1673–1685
� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
to room temperature. The PCs obtained were cut and sanded
accordingly for TGA, rheometric, dielectric, Young’s modulus (E)
and coefficient of thermal expansion measurements.
Monitoring the kinetics of thermal curing of CEMs 3
Thin films of 3a and 3bwere prepared via neat spin coating (3 000
rpm, 60 s) of liquid CEMs without solvent on silicon substrates.
The film sampleswere placed in a tube furnace purgedwithN2 gas
and subsequently cured at 260 8C for 8 h and at 290 8C for 1 h. An
FT-IR spectrum was taken every hour in order to monitor the
polymerisation kinetics. The reduction in the IR absorbance of the
cyanate groups between 2325–2190 cm�1 for 3a and between
2360–2113 cm�1 for 3b and the appearance of the triazine ring
signal at around 1360 cm�1 were monitored to determine the
extent of curing. Since the number of CH3 and CF3 groups remains
constant before and after the thermal curing, each spectrum was
normalized by division with the factor obtained from dividing the
area of the CH3 and CF3 group absorption at a particular curing
time by the area of the CH3 and CF3 group absorption in the
monomers 3a and 3b prior to thermal treatment (t¼0). The
percent fraction of residual cyanate groups a(t) at a given time (t)
was calculated from the normalized area of cyanate absorbance
before (AOCN)t0 and after (AOCN)t the thermal treatment according
to the following equation
aðtÞ ¼ ðAOCNÞtðAOCNÞt0
� 100 (1)
In a similar way the triazine ring formation was quantified
from the IR spectra.
Template Assisted Fabrication of Polycyanurate
Nanorods (PCNs) and Determination of their
Hydrolytic Stability
Self-ordered nanoporous alumina templates with a pore diameter
of 65 and 380 nm and a pore depth of 100 mm were fabricated by
anodization of aluminum according to the procedure reported
elsewhere.[8] CEMs 3a and 3b were applied on top of nanoporous
alumina templates via neat spin coating (3 000 rpm, 2 min). In
order to ensure complete pore wetting and degassing, the
nanoporous alumina templates with CEMs were additionally
kept under vacuum at 80 8C for tube morphology or 120 8C for rod
morphology for 12 h, before subjecting to the curing program as
mentioned above for the neat material. Individually dispersed
polycyanurate nanorods were obtained by selective dissolution of
the alumina template using NaOH (6M) solution at room
temperature for 1 h. The suspended PCNs were centrifuged
(20000 rpm, 15min) and the supernatant liquidwas removed. The
isolated PCNs were redispersed in deionized water and again
centrifuged. This procedure was repeated until the supernatant
liquid became neutral. Finally, the PCNs were collected and dried.
For hydrolytic stability assessment a small portion of the PCNs
was kept in boiling water for 100 h for accelerated hydrolysis
measurements and stored inwater for threemonths for long-term
hydrolysis measurements. After drying at 120 8C for 5 h the PCNs
were investigated by SEM measurements.
DOI: 10.1002/macp.200800155
Polycyanurate Thermoset Networks with High Thermal . . .
Results and Discussion
Synthesis and Characterization
The synthesis of CEMs 3a and 3b is depicted in Scheme 1. In
the first step 1,3-dibromo-5-methoxybenzene was reacted
with bisphenol AF or bisphenol A in a modified Ullmann
condensation reaction catalyzed by a soluble Cu(I) com-
plex,[9] generated in situ from copper(I) iodide and 1,10-
phenanthroline. K2CO3 was used as base. The reaction was
carried out at 135–145 8C in a mixture of DMF and toluene,
with the latter being used to remove water formed during
the reaction by azeotropic distillation. The formed OAE
Scheme 1. Synthesis of the CEMs 3 with pendant as well asterminal cyanate groups and their curing to the PC thermoset 4.
Macromol. Chem. Phys. 2008, 209, 1673–1685
� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
mixture 1a or 1b with terminal hydroxyl and pendant
methoxy groups contained chain lengths of n� 0–5, as
determined by FD mass spectrometry. The pendant
methoxy groups were then cleaved by refluxing a
dichloromethane solution of 1a or 1b in the presence of
BBr3 to give the OAEmixture 2a or 2bwith terminal aswell
as pendant hydroxyl groups in quantitative yield. The
following reaction with cyanogen bromide in the presence
of triethylamine in dry acetone at a temperature between
�20 and �30 8C afforded the CEM 3a or 3b as oil in more
than 90% yield. All the oligomeric intermediates and
products were purified by simple precipitation and
washing with high yields, which facilitates scale up to
large-scale synthesis for potential commercialization. The
CEMs 3a and 3b are viscous oils with room temperature
viscosities of 288 and 3 Pa � s respectively. The higher
molecular weight of 3a, higher steric hindrance and
rigidity induced by the CF3 substituents at the quaternary
carbon center might account for a higher viscosity of CEM
3a compared to 3b, with CH3 groups instead. A similar
effect is also observable in the parent CEMs, BAFCY and
BACY. The melting temperature for BAFCY (87 8C) is 8 K
higher than the melting temperature of BACY (79 8C) andthe only difference between these two CEMs is the CF3(BAFCY) versus CH3 (BACY) substituents at the quaternary
carbon center. Consequently, the nature of substituents at
the quaternary carbon center is an important factor to be
considered when designing a CEM based on bisphenol A
derivatives.
The formation of CEMs 3a and 3b from 2a and 2b can be
well observed by IR spectroscopy (Figure 1 for compounds
of series a, see Figure S1 in the Supporting Information for
compounds of series b) with the disappearance of the
hydroxyl stretching band (at 3 338 cm�1 for CEM 3a and
3 335 cm�1 for CEM 3b) and the appearance of the -OCN
bands (located at 2 277–2 241 cm�1 for CEM 3a and 2 262–
2 235 cm�1 for CEM 3b). These -OCN bands vanish while
curing the CEMs 3 to PCs 4with the concurrent appearance
of a characteristic stretching band for the triazine ring (at
1 360 cm�1). These experiments demonstrated that IR
spectroscopy is a very convenient tool for the rapid che-
mical characterization and determination of functional
group conversion in these oligomer mixtures without the
need for separation and tedious purification of the
individual oligomeric components. As the curing reaction
is sensitive to residual phenolic groups, it is important to
note that within the resolution of the characterization
methods (NMR and IR) no free phenolic groups were
detectable in the CEMs 3a and 3b.
Figure 2 shows corresponding time dependent FT-IR
absorption spectra for the –OCN and triazine ring
vibrations of CEM 3a and 3b during the thermal curing
process according to the temperature program described in
the experimental section. The arrow on each plot indicates
www.mcp-journal.de 1677
B. Yameen, H. Duran, A. Best, U. Jonas, M. Steinhart, W. Knoll
Figure 1. FT-IR spectra of 1a to 4a. With the conversion of 2a to CEM 3a the –OH stretching band at3 338 cm�1 disappears and the –OCN bands appear at 2 277–2 241 cm�1. During the thermal curing ofCEM 3a to PC 4a the characteristic stretching band for the triazine ring appears at around 1 357 cm�1
while –OCN bands vanish. For series 1b to 4b see Figure S1, Supporting Information.
1678
the curing time direction. Based on the normalization of
the spectra to the persistent CF3 and CH3 groups, we
quantify the conversion by the peak area of the respective
functional group (–OCN or triazine ring) for the given
curing time. The prominent feature of the –OCN group
signal in Figure 2a is a rapid reduction of peak intensity
within the first hour of curing for CEM 3a. The residual
peak intensity vanishes at a much lower rate over the
remaining curing period of 7 h. An essentially analogous
behaviour is found for CEM 3b as visible in Figure 2c.
Interestingly, the signal for the concurrent triazine ring
formation shows a different time dependence, with a lag
time of about 2 h before a strong signal appears (Figure 2b)
in PC 4a. In the following curing time the signal only
slightly increases. In contrast, compound 3b immediately
shows a triazine signal without a lag time, see Figure 2d.
In order to get a more detailed picture of the curing
kinetics, the a(t) factor [Equation (1)] for the -OCN group
conversion and triazine ring formation were plotted
against curing time in Figure 3. The reported data for
each thermal curing step is an average of three separate
film samples. The cyanate group conversion followed a fast
kinetic rate during the early stages of curing, and Figure 3a
shows almost 80% conversion of the cyanate groups for
both CEMs 3a and 3b within the first hour of curing. This
high conversion at the initial curing stage resulted in a
rapid vitrification of the CEM 3a and 3b film samples. The
conversion of the remaining cyanate groups was slow and
a diffusion controlled curing process dominated the later
Macromol. Chem. Phys. 2008, 209, 1673–1685
� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
stages due to vitrification. The
complete conversion of the cya-
nate groups took 5 h for the
slightly more reactive CEM 3a
and 7 h for CEM 3b (see discus-
sion ofDSC analysis and Figure 4
further below for reactivity com-
parison of CEM 3a and 3b). On
contrary, the triazine ring for-
mation followed slower kinetics
from the very beginning of the
curing process (Figure 3b). This is
explained by the rather complex
reaction sequence of the cyanate
moieties via various intermedi-
ates leading to the tria-
zine structure, instead of a
direct concerted ring formation
in a single step involving three
cyanate groups, as discussed
further below. In the case of
CEM 3a an induction period of
2 hwas observed before a signi-
ficant triazine ring formation
occurred, while no induction
period was found for CEM 3b. Interestingly, despite the 2 h
induction period for CEM 3a, the a(t) factor of 60% for the
triazine ring formation was approximately the same for
both CEMs 3a and 3b after 3 h of curing. An analogous
trend for a(t) was followed by both CEMs 3a and 3b during
the remaining curing period, but the a(t) factor for the
triazine ring formation in CEM 3a was always slightly
smaller than in CEM 3b at any particular curing time. The
completion of triazine ring formation took 8 h in total for
both CEMs. The different kinetics are probably due to the
restricted chain mobility of CEM 3a in the vitrified state,
induced by a higher steric hindrance of the CF3 substituent
at the quaternary carbon center compared to CEM 3b
with CH3 groups. The comparison of Figure 3a and 3b
reveals that the a(t) factors for triazine ring formation and
for cyanate group conversion at any particular curing time
are not proportional. The cyanate group conversion is always
higher than the triazine ring formation. For instance, after 3 h
of curing the a(t) factor for cyanate group conversion in both
CEMs 3a and 3b corresponds to 90% conversion, whereas
the a(t) factor for triazine ring formation at the same
curing time corresponds to �60% triazine ring formation.
Considering a direct trimerization reaction, a linear rela-
tionship between the cyanate groups decrease and the
triazine ring formation would be expected at any point in
time, which is not observed here. An essentially similar
behaviour was reported by Grenier-Loustalot et al., where
NMR, HPLC, and IR spectroscopy was used to study the
mechanism and kinetics of non-catalyzed CEMs curing.[10a,b]
DOI: 10.1002/macp.200800155
Polycyanurate Thermoset Networks with High Thermal . . .
Figure 2. FT-IR spectra recorded every hour during thermal curingat 260 8C for 8 h: (a) Disappearance of –OCN bands between2 325–2 190 cm�1 and (b) appearance of stretching band fortriazine ring at 1 360 cm�1 for CEM 3a. (c) Disappearance of–OCN bands between 2 360–2 113 cm�1 and (d) appearance ofstretching band for triazine ring at 1 360 cm�1 for CEM 3b.
Figure 3. (a) Kinetic profile from IR measurements for CEM 3a andCEM 3b for the percent fraction of residual –OCN groups and(b) the triazine ring formation during 8 h at 260 8C and 1 h at290 8C (average of three measurements).
Macromol. Chem. Phys. 2008, 209, 1673–1685
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They proved that the conversion of cyanate groups into the
triazine ring is not a direct trimerization step but passes
through 4-membered ring- and carbamate intermediates.
This explains the substantially higher conversion factor
a(t) for the cyanate group conversion compared to the
triazine ring formation at a particular curing time. In order
to ensure complete conversion, the samples of CEM 3a and
3bwere further kept at 290 8C for 1 h, but no changes were
observed for the IR signals.
Thermal Properties
Since the synthesized CEMs 3a and 3b are viscous oils
(room temperature viscosities: 288Pa � s/3a and 3Pa � s/3b)under ambient conditions, they consequently do not show
endothermic melting transitions in their DSC thermo-
grams above room temperature (Figure 4a and 4b). Only
one exothermic transition was observed, which relates to
the curing reaction leading to the PCs 4a and 4b by formal
cyclotrimerization of the cyanate groups. In the case of
CEM 3a the exothermic transition starts at around 170 8C
www.mcp-journal.de 1679
B. Yameen, H. Duran, A. Best, U. Jonas, M. Steinhart, W. Knoll
Figure 4. DSC thermograms of CEMs 3a and 3b in a nitrogenatmosphere. The exothermic transitions for the cyclotrimeriza-tion of the cyanate groups peak at 227 8C for CEM 3a (a) and 265 8Cfor CEM 3b (a).
1680
and peaks at 227 8C while for 3b it begins with a small
shoulder around 160 8C, but rapidly increases at around
230 8C and peaks at 265 8C. It is worthwhile to note that
these curing peak temperatures are about 40 K lower than
BAFCY- and BACY-based OAE CEMs with only terminal
cyanate groups, which show curing temperatures as
exothermic transitions peaking at 270 and 310 8C, respec-tively. This indicates a comparatively higher reactivity of
CEMs 3, which allows for lower processing temperatures in
technical applications.[4a,4b] The higher reactivity of CEMs
3 is attributed to the electron withdrawing effect of the
two m-phenoxy groups on the pendant cyanate moieties,
which is in accord with previous findings of higher
reactivity in aromatic CEMs having electron withdrawing
groups at the m-position to the cyanate groups.[10c] The
terminal cyanate groups, on the other hand, are attached
to the bisphenol A subunits with the electron-donating
quaternary carbon center in p-position, reducing their
Macromol. Chem. Phys. 2008, 209, 1673–1685
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reactivity. When comparing the bisphenol AF with the
bisphenol A derivatives, CEM 3a with CF3 groups is curing
about 40 K below its structural analogue 3b with CH3
groups. Apparently, the high electronegative fluorine atoms
have a long-range electronic effect on the cyanate groups
mediated through the conjugated system of the aromatic
rings. Such an electronic effect of the F substituents is
supported by the distinct features in the 1H NMR spectra
(Figure S2 in the Supporting Information), where the
aromatic protons in the resorcin substructure of 3a
(assigned as protons ‘‘a’’ in the NMR spectra of Figure
S2a) split into two lines at about d¼ 6.6 ppm compared to
the single line for 3b (protons ‘‘b’’ in Figure S2b). Also, the
protons of the bisphenol AF subunit in 3a (protons ‘‘c’’ and
‘‘d’’ in Figure S2a) show a distinct splitting compared to
those in 3b (protons ‘‘d’’ in Figure S2b). The concept of
higher reactivity in CEMs with molecular arrangements,
that render the cyanate group carbonmore electrophilic, is
also supported by the initiation mechanism of cyanate
group cyclotrimerization by traces of water or residual
phenol.[10a,b]The higher electrophilicity of the cyanate
carbon in CEMs 3 favors the nucleophilic attack bywater or
residual phenol resulting in the formation of carbamate
intermediates that autocatalyse the reaction, ultimately
leading to lower curing temperatures. The commonly high
curing temperatures in other technically applied CEMs
usually demand a catalyst for curing within reasonable
times and temperatures. A major disadvantage of the
catalyst, which will remain in the thermoset PC, is its
activation of hydrolysis reactions and hence accelerated
ageing.[11] With the lower curing temperatures found for
the present CEMs 3 it was possible to produce PCs under
comparable conditions, but without using a curing
catalyst. The DSC of the fully cured PCs 4a and 4b did
not reveal any features and no Tg could be deduced from
such thermal analysis (for Tg determination see rheological
measurements further below).
The catalyst-free cured PC 4a was investigated for its
thermal stability by TGA, as shown in Figure 5a. Noweight
loss was observed up to 400 8C irrespective of the gaseous
environment (air or nitrogen) being used during the TGA
measurements, which is very similar to BAFCY and BAFCY-
OAE without pendant cyanate groups. The main degrada-
tion occurred between 400 and 600 8C. A char yield of 52%
was obtained after heating to 900 8C under N2, analogously
to the PC of BAFCY, but slightly higher than the BAFCY-
OAE-PC without pendant cyanate ester groups (47% char
yield).[2a,4a] In comparison, PC 4b is thermally less stable
and started to decompose above 350 8C in both N2 and air
atmosphere (Figure 5b). A char yield of 44% was observed
when heating to 900 8C under N2, which is higher than the
BACY-OAE-PC without pendant cyanate groups (32% char
yield) and the BACY-PC (41% char yield).[2a,4b] The higher
char yield indicates a better flame resistance with less
DOI: 10.1002/macp.200800155
Polycyanurate Thermoset Networks with High Thermal . . .
Figure 5. TGA thermograms of PC thermosets 4a and 4b under N2and air, reflecting thermo-oxidative stability of 4a up to 400 8C(a) and of 4b up to 350 8C (b).
volatile polymer pyrolysis fragments and is thus desirable,
as in the case of PCs 4a.[12]
Rheological Measurements
Rheological measurements by torsional deformation were
performed on the neat cured PCs 4a and 4b under a dry
nitrogen atmosphere over a temperature range of 50 to
330 8C. At 50 8C the PCs 4a and 4b show storage moduli (G0)
of 1.252 and 1.362 GPa, respectively. During heating to
330 8C, a sharp reduction in these moduli is observed
(Figure 6), which is correlated to the alpha-relaxation at a
Figure 6. Storage modulus (G0), loss modulus (G00) and damping factorand 4b as a function of temperature.
Macromol. Chem. Phys. 2008, 209, 1673–1685
� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
measurement frequency of 10 rad � s�1 and characteristic
of a Tg. As the exact value of a measured Tg depends
critically on the experimental conditions (like technique,
heating or cooling rates) we refer here in accordance with
the literature to ‘‘rheologically determined Tgs’’.[4a,b] These
rheologically determined Tgs may be shifted to higher
temperatures compared to Tgs obtained by DSC measure-
ments, but only small shifts between the DSC and rheo-
logical values were found for other CEMs and all values
cited below for comparison are obtained in the same
way.[4a,b] The rheologically determined Tg values of the PCs
correspond to the midpoint of the sharpest decrease in the
storage moduli curves and peak maximum in the tan d
plots (G0/G00). They were found at 286 8C for 4a and 287 8Cfor 4b, which is 111 K higher for 4a and 147 K higher for
4b than the reported Tgs of BAFCY-OAE-PC and BACY-OAE-
PC without pendant cyanate groups.[4a,b] In comparison to
the Tg of PCs derived fromparent BAFCY (Tg¼ 270 8C) the Tgof 4a is 16 K higher, while the Tg of 4b is only 2 K lower
than the PC derived from parent BACY (289 8C).[2a] The highTgs determined by rheology validate the concept of
pendant cyanate groups to increase the Tg by enhancing
the crosslinking density in the cured thermoset.
Young’s Moduli and Coeffecients of ThermalExpansion
Figure 7 shows the nominal stress as a function of drawing
ratio for PC thermosets. The Young’s moduli (E), as a
measure of stiffness, were calculated from the linear slope
at small strains (solid lines). They were found to be 3.47
GPa for PCs 4a and 3.46 GPa for 4b with a slightly higher
stiffness than the PCs derived from parent BAFCY and
BACY (3.11 and 3.17 GPa).[2a] The PC 4a possesses a higher
density (1.4493 g � cm�3) than 4b (1.2045 g � cm�3) and
shows a slightly lower coefficient of thermal expansion
(113� 10�6 K�1) in comparison to 4b (121� 10�6 K�1). Most
of the PCs derived from commercial CEMs show coeffi-
cients of thermal expansion in the range of 60� 10�6 to
70� 10�6 K�1.[13] The higher coefficients of thermal
(tan d) of PC thermosets 4a
expansion for PCs 4a and 4b
are due to the higher free
volume inherent with the diaryl
ether spacers incorporated in
CEMs 3a and 3b. Due to the
limited number of tensile tests
performed and the defects incor-
porated in the test specimens,
which affects the break point,
we can only roughly state that
the elongation at break of PCs 4
was about 1 to 2%. The small
elongation at break reflects
www.mcp-journal.de 1681
B. Yameen, H. Duran, A. Best, U. Jonas, M. Steinhart, W. Knoll
Figure 7. Stress as a function of drawing ratio for PC thermosets4a (a) and 4b (b), with the Young’s moduli calculated from thelinear slops of these plots at small strains (solid line).
Figure 8. Dielectric constants of PC thermosets 4a and 4b as afunction of temperature measured at 1 MHz between �100 to150 8C. The hump in the first heating scan shows the contributionof absorbed moisture to the dielectric constants which vanishesupon drying (straight line).
1682
brittleness, which is a common feature of cyanate ester
resins and can be counteracted by using additives.[14a]
When comparing the two types of PC 4a and 4b, it is
interesting to note that they have very similar mechanical
properties (G0, E, rheologically determined Tg) despite their
chemical differences, which is partially to be explained by
the very high crosslink density averaging out these
chemical differences. The elastic modulus E in the glassy
state, which depends on the cohesive energy density and
the intensity of sub-glass transitions, is almost the same
for both 4a and 4b PC networks.[14b] At the very high cross-
link densities of our systems (one crosslink per repeating
unit) it may indirectly also depend on this crosslink den-
sity, as its increase leads to a higher number of covalent
bonds per volume element and thus may contribute to the
cohesive energy density being a sum over all covalent
(strongest) and non-covalent (weaker) interactions in the
system.
Dielectric Measurements
The relative permittivity, or relative dielectric constants
(e0) at room temperature of 4a and 4b were measured at
1 MHz frequency as 3.41 and 3.75, respectively after being
stored at ambient conditions (Figure 8). During heating of
Macromol. Chem. Phys. 2008, 209, 1673–1685
� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
the samples a contribution of absorbed moisture to e0
became apparent by a non-linear temperature depen-
dence and e0 decreased to 3.33 for 4a and 3.65 for 4b at
150 8C. In the absence of moisture e0 showed a weak linear
temperature response from 3.30 at �100 8C to 3.33 at
150 8C for 4a and from 3.63 at �100 8C to 3.65 at 150 8C for
4b.[15] The PC 4a exhibits a comparatively lower e0 due to
the fluorinated isopropylidene linkage in the BAFCY
subunit. In summary the e0 of 4a and 4b are not as low as
those of other members of the CE resin family, which
generally lie between 2.5–3.1, and also fall at the upper
margin of the required e0 range (2.5–3.6) for microelec-
tronic applications. Still, these values are lower than the e0
of common epoxy resins cured with active hydrogen
converters (generally in the range of 3.9–4.2),[1a] which
make them competitive substituents in epoxy-based
electronics. The higher e0 of PCs 4a and 4b are thought
to be due to the aryl ether linkages, which increase the
polarizability of the thermosets.[16]
Template Assisted Fabrication of PolycyanurateNanorods and their Hydrolytic Stability
PCs are considered as resin matrix for multi-layer electric
circuit boards in microelectronics industry due to many
favorable properties, but a poor long term hydrolytic
stability, which ultimately causes blistering of the circuit
board assemblies, often limits such a PC application.[11] In
order to assess the hydrolytic stability of the PCs derived
from developed CEMs 3, we have produced polycyanurate
nanorods (PCN) by curing CEMs 3 in the channels of
DOI: 10.1002/macp.200800155
Polycyanurate Thermoset Networks with High Thermal . . .
Figure 9. Schematic representation of the nanomolding process to produce polycyanuratenanorods (PCN) in nanoporous alumina templates.
Figure 10. SEM images of template molded PCNs of 4a before (a) and after (b) storage inwater for three months. PCNs of 4b before (c) and after (d) 100 h boiling watertreatment.
nanoporous alumina templates by
following the reported method for
the templated synthesis of nanos-
tructures (Figure 9).[17] The geo-
metric dimensions of these PCNs
with the length of around 100 mm
and diameters of 65 and 380 nm
correspond to the shapes of the
template pores (Figure 10). The
PCNs shown in the SEM images
demonstrate the high processabil-
ity of the liquid CEMs 3, which
have completely penetrated such
small dimensions of the nanopor-
ous template and fully replicated
the template structure. These
nanostructures with their large
surface-to-volume ratio are ideal
test objects to assess the hydrolytic
stability of the PC material, since a
very large fraction of material is
directly exposed to the surround-
ing water, diffusion paths within
the bulk are comparably short
(essentially limited by the rod
radius), and any substantial hydro-
lysis would have a clearly obser-
vable influence on the PCN shape.
The PCNs composed of PC 4 pre-
sented good hydrolytic and dimen-
sional stability when subjected to
accelerated (100 h boiling water)
or long-term (three months at
room temperature in water)
hydrolytic conditions and no blis-
tering or shape changes were
observed (Figure 10b and d)
The final architecture of the
PCNs with either compact rod- or
hollow tube morphology (see Fig-
ure S3 in the Supporting Informa-
tion) can be conveniently con-
trolled by the temperature during
the template pore filling process.
When the more viscous CEM 3a
was subjected to a higher initial
pore wetting temperature (i.e.,
120 8C), complete pore filling
resulted in a rod-like structure,
while at lower pore wetting tem-
perature (i.e., 80 8C) tubular struc-
tures were favoured by only sur-
face wetting of the template. Since
CEM 3b has a lower viscosity, it
Macromol. Chem. Phys. 2008, 209, 1673–1685
� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mcp-journal.de 1683
B. Yameen, H. Duran, A. Best, U. Jonas, M. Steinhart, W. Knoll
1684
formed already under the low temperature condition (80 8C)a compact rod morphology.
Conclusion
The convenient synthesis route presented here provides
bisphenol A- and AF-based CEMs in high yields, with the
simple purification steps bearing large potential for scale-
up towards commercialization. The specific features due to
their chemical structure are:
1. H
Ma
� 2
igh functional group density of reactive cyanate ester
units along the main chain.
2. T
he OAE units impart high chain flexibility in theuncured state, which together with
3. t
he formation of oligomeric mixtures in the synthesishinders the crystallization and may only lead to
vitrification below room temperature, which substan-
tially facilitates processing of the liquid thermoset
under ambient conditions.
4. T
he large number of cyanate groups in the oligomersa) allow rapid vitrification in the curing process, desi-
rable for fast setting and mechanical stability in the
early curing stage (no flow of the thermoset), and
b) lead to high crosslink density in the polycyanurate
network with very high Tg desirable for high-
crom
008
temperature applications.
Replacing the CH3 substituents in the bisphenol A
subunit (CEM 3b) by CF3 groups of bisphenol AF (CEM 3a)
has substantial consequences on the properties of the
uncured CEM as well as the final PC material:
1. C
EM 3a has an about 100-times higher viscosity at roomtemperature, and
2. c
onsequently a slightly slower rate of triazine ringformation (from IR kinetics),
3. w
hile the cyanate groups react already at lowertemperature (from DSC measurements).
4. T
he cured PC 4a shows a substantially higher thermalstability in air (up to 400 8C in the TGA),
5. a
higher char yield (as indicator for good flameresistance), and
6. a
lower dielectric constant.These effects result most probably from:
1. t
he conformational restriction in the bisphenol AFsubunit due to the larger space requirement of the CF3groups compared to the CH3 moieties, and
ol. Chem. Phys. 2008, 209, 1673–1685
WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
2. b
y an electronic effect of the highly electronegative Fatoms.
Interestingly, besides these distinct differences due to
the F substituents, both PC 4a and 4b have very similar
mechanical properties, like elastic modulus G0, Young’s
modulus E, and Tg. Apparently, the differences in chemical
structure are overcompensated by the very high crosslink
density (one crosslink per repeating unit) with short and
comparably rigid chain segments interconnecting the
network points.
Due to the combination of many positive aspects in this
material class, the novel CEMs presented here show great
potential as high-performance thermosets for a large range
of technical application, like metal-to-polymer adhesives
or as matrix in fiber reinforced composites.
Acknowledgements: B. Y. gratefully acknowledges financialsupport from Higher Education Commission (HEC) of Pakistanand Deutscher Akademischer Austauschdienst (DAAD) (Code # A/04/30795). H. D. thanks European Union for the Marie Curie Intra-European Fellowship (MEIF-CT-2005-024731). Authors alsoacknowledge Dr. Lenz from Siemens for technical discussions.Thanks to Dr. K. Koynov for dielectric constant measurements andAndreas Hanewald for the mechanical measurements.
Received: March 18, 2008; Revised: May 28, 2008; Accepted: May30, 2008; DOI: 10.1002/macp.200800155
Keywords: high performance polymers; high temperature mate-rials; molding; nanotechnology; networks; thermosets
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