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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: [email protected]. 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: [email protected]

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


� 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



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


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



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


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.



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



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



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


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).



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



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



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


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.



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



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



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


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


� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mcp-journal.de 1683


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 the
uncured state, which together with

3. t
he formation of oligomeric mixtures in the synthesis
hinders 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 oligomers
a) 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 room
temperature, and

2. c
onsequently a slightly slower rate of triazine ring
formation (from IR kinetics),

3. w
hile the cyanate groups react already at lower
temperature (from DSC measurements).

4. T
he cured PC 4a shows a substantially higher thermal
stability in air (up to 400 8C in the TGA),

5. a
higher char yield (as indicator for good flame
resistance), and

6. a
lower dielectric constant.
These effects result most probably from:

1. t
he conformational restriction in the bisphenol AF
subunit 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 F
atoms.

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

[1] [1a] I. Hamerton, ‘‘Chemistry and Technology of Cyanate EsterResins’’, Blackie Academic, Glasgow 1994; [1b] R. Prokopec, K.Humer, R. K. Maix, H. Fillunger, H. W. Weber, J. Phys. : Conf.Ser. 2006, 43, 739; [1c] M. W. Hooker, P. E. Fabian, S. D.Grandlienard, D. E. Codell, M. J. Lizotte, AIP Conf. Proc.2006, 824, 306 (Advances in Cryogenic Engineering, Vol.52A); [1d] S. W. Feucht, P. E. Fabian, N. A. Munshi, IEEE Trans.Appl. Supercond. 2003, 13, 1516; [1e] C. P. R. Nair, D. Mathew,K. N. Ninan, Adv. Polym. Sci. 2001, 155, 1.

[2] [2a] T. Fang, D. A. Shimp, Prog. Polym. Sci. 1995, 20, 61; [2b] J.Fan, X. Hu, C. Y. Yue, J. Polym. Sci., Part B: Polym. Phys. 2003, 41,1123;

[3] I. Hamerton, J. N. Hay, Polym. Int. 1998, 47, 465.[4] [4a] M. Laskoski, D. D. Dominguez, T. M. Keller, J. Mater. Chem.

2005, 15, 1611; [4b] M. Laskoski, D. D. Dominguez, T. M.Keller, Polymer 2006, 47, 3727.

[5] A. J. Guenthner, G. R. Yandek, M. E. Wright, B. J. Petteys, R.Quintana, D. Connor, R. D. Gilardi, D. Marchant, Macromol-ecules 2006, 39, 6046.

[6] S. Tisserand, R. Baati, M. Nicolas, C. Mioskowski, J. Org. Chem.2004, 69, 8982.

[7] P. Zoller, D. J. Walsh, ‘‘Standard Pressure-Volume-TemperatureData for Polymers’’, Technomic, Lancaster, PA 1995.

DOI: 10.1002/macp.200800155


[8] [8a] H. Masuda, K. Fukuda, Science 1995, 268, 1466; [8b] H.Masuda, F. Hasegawa, S. J. Ono, Electrochem. Soc. 1997, 144,L127; [8c] K. Nielsch, J. Choi, K. Schwirn, R. B. Wehrspohn, U.Gosele, Nano Lett. 2002, 2, 677.

[9] [9a] M. Wolter, G. Nordmann, G. E. Job, S. L. Buchwald, Org.Lett. 2002, 4, 973; [9b] A. Klapars, J. C. Antilla, X. Huang, S. L.Buchwald, J. Am. Chem. Soc. 2001, 123, 7727; [9c] A. A. Kelkar,N. M. Patil, R. V. Chaudhari, Tetrahedron Lett. 2002, 43, 7143.

[10] [10a] M. F. Grenier-Loustalot, C. Lartigau, P. Grenier, Eur.Polym. J. 1995, 31, 1139; [10b] M. F. Grenier-Loustalot, C.Lartigau, J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 1245;[10c] J. Bauer, L. Hoeper, M. Bauer, Macromol. Chem. Phys.1998, 199, 2417.

[11] [11a] R. J. Yeh, P. W. Lin, K. F. Lin, J. Polym. Res. 2002, 9, 31;[11b] L. J. Kasehagen, I. Haury, C. W. Macosko, D. A. Shimp,J. Appl. Polym. Sci. 1997, 64, 107.



[12] J. W. Gilman, S. M. Lomakin, T. Kashiwagi, D. L. VanderHart, V.Nagy, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1997,38, 802.

[13] M. L. Stuart, ‘‘International Encyclopedia of Composites’’, Vol. 1,VCH, NewYork 1990, p. 552.

[14] [14a] T. J. Wooster, S. Abrol, D. R. MacFarlane, Macromol.Mater. Eng. 2005, 290, 961, and refs. therein; [14b] M. L. Arias,P. M. Frontini, R. J. J. Williams, Polymer 2003, 44, 1537, andrefs. therein.

[15] K. R. Carter, R. A. DiPietro, M. I. Sanchez, S. A. Swanson, Chem.Mater. 2001, 13, 213.

[16] J. O. Simpson, A. K. St. Clair, Thin Solid Films 1997, 308–309,480.

[17] [17a] C. R. Martin, Science 1994, 266, 1961; [17b] C. R. Martin,Adv. Mater. 1991, 3, 457; [17c] M. Steinhart, R. B. Wehrspohn,U. Gosele, J. H. Wendorff, Angew. Chem., Int. Ed. 2004, 43, 1334.


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