Thermal and mechanical properties of azomethine functionalized cyanate ester/epoxy blends

Article abstract of DOI:10.1039/c4ra16004e

A series of azomethine functionalized diols were synthesized by the condensation reaction of aromatic diamines with 4-hydroxy benzaldehyde. Aromatic diamines with different alkyl chain lengths have been used to prepare the bisphenols. These bisphenols wer

Full text of DOI:10.1039/c4ra16004e

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DOI: 10.1039/C4RA161004E
Thermal and Mechanical Properties of Azomethine Functionalized
Cyanate Ester/Epoxy Blends
C.P. Sakthidharan§‡, Pudupadi R. Sundararajan‡ and Muthusami Sarojadevi§*
§ Department of Chemistry, CEG, Anna University, Chennai, India – 600 025
‡ Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Canada, K1S
5B6
*Corresponding author: msrde2000@yahoo.com
Abstract
A series of azomethine functionalized diols were synthesized by condensation reaction of
aromatic diamines with 4ꢀhydroxy benzaldehyde. Aromatic diamines with different alkyl chain
lengths have been used to prepare the bisphenols. These bisphenols were converted to their
corresponding cyanate esters by treating with cyanogen bromide (BrCN) in the presence of
triethyl amine (Et3N). The curing temperature was measured by using differential scanning
calorimetry (DSC). The maximum curing temperature of these cyanate esters are in the range of
203ꢀ242 ºC. Thermal properties of the cured cyanate ester were studied by using thermo
gravimetric analysis (TGA). The polymers show excellent thermal stability (T5 was found to be
in the range of 358 to 465 ºC and T10 was found to be in the range of 508 to 525 ºC) and the
percentage of char yield at 800 ºC was found to be in the range of 37.06 to 57.26. The flame
retardancy of the cyanate ester resins have been evaluated using limiting oxygen index value
which is in the range of 32.32 to 40.40 at 800 ºC. Cyanate ester/epoxy blends were prepared and
the morphology of the blends was studied by scanning electron microscopy. The mechanical
properties and glass transition temperature of cyanate/epoxy ester blends were studied by using
dynamic mechanical analysis (DMA) and the Tg was found to increase with increasing cyanate
ester content (1 to 5%).
Keywords
Azomethine containing cyanate esterꢀepoxy blends
Thermal and mechanical properties

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Introduction
Cyanate esters are one type of thermosetting resins. On heating or irradiation either in the
presence or absence of catalyst, three units of cyanate ester monomer undergo cyclotrimerization
and form symmetric triazine ring with nitrogen and carbon atom in which the carbon atoms are
linked with oxygen atom.1 Heat resistant character of cured cyanate esters (CE) make them more
competing in high thermal applications.2 The polycyanurates exhibit low dielectric constant and
low dissipation factor. This character is well suited for making printed circuit boards in
electronic industries.3ꢀ5 Radar transparency, low moisture absorption,6 very good adhesive
character towards metal and high mechanical strength7ꢀ10 make cyanate esters more reliable for
high end applications in aerospace industries. However, the cured CE is inherently brittle in
nature and easily undergoes micro cracking under service loads. The fracture toughness of
cyanate esters can be improved by blending with thermoplastic tougheners, rubber,11 organic and
inorganic fillers,6, 12 POSS13ꢀ15 and thermosetting resins such as epoxy16ꢀ20, imides21ꢀ24 and
benzoxazines.25ꢀ26
Commercially available cyanate ester, Bisphenol A dicyanate ester (BADCY) is being
widely used in various fields, but further modification is required to enhance its properties in
performance and reduction of cost in production. The thermo oxidative stability and moisture
resistance were improved by introducing silicon atom in BADCY.27 On the addition of epoxy
resin into the BADCY resin, mechanical and hot–wet properties were improved when the
concentration ranges from 5 to 30 wt %20. Epoxy resins also come under thermosetting resins
category and they are widely used for making advanced composites because of their excellent
adhesive nature towards metals, chemical resistance and outstanding mechanical properties27
with superior dimensional stability. They undergo micro cracking, which makes them unfit for
their requirement for high toughness applications (in engineering industries). Another
disadvantage of epoxy resins is their moisture absorption.
Azomethine functionalized cyanate ester with shorter alkyl chain was prepared earlier, 28,
29
in this study, dicyanate esters with azomethine linkages and different alkyl chains with ether
linkages were prepared and the effect of alkyl chain length between two aromatic groups on
thermal stability has been studied. The prepared cyanate esters were blended with epoxy resin in

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three different weight percentages. Thermal and thermo mechanical properties of the CE/epoxy
blends were investigated by using thermo gravimetric analyzer (TGA) and dynamic mechanical
analyzer (DMA). The morphology of the system was studied by scanning electron microscopy
techniques (SEM).
2. Experimental Procedure
2.1. Materials
4ꢀAcetamidophenol was purchased from Aldrich, USA. Acetone, 1, 4ꢀdibromo butane, 1,
5ꢀdibromo pentane, 1, 6ꢀdibromo hexane, potassium carbonate (K2CO3), cyanogen bromide
(CNBr) and 4ꢀ hydroxy benzaldehyde were purchased from Sisco Research Laboratories (SRLꢀ
India). Diglycidyl ether of bisphenolꢀA (DGEBA) was purchased from Huntsman (India).
Diaminodiphenylmethane was purchased from Acros Organics (India). Ethanol and
triethylamine (Et3N) were purchased from Merck (India). All reagents were used as received.
Triethylamine and acetone were distilled prior to use.
2.2. Measurement
Fourier transform infrared (FTꢀIR) spectra were recorded in ABB Bomem MB series FTꢀ
1
IR spectrometer using KBr pellet. H and 13C nuclear magnetic resonance measurements were
carried out with a Bruker AV III spectrometer (500 MHz). Samples were prepared in CDCl3 and
DMSOꢀd6 solution. Melting temperatures were determined with a melting point apparatus IA
6304 at a heating rate of 4 °C/ min. Differential scanning calorimetric analysis (DSC) for curing
studies was carried out using Perkin Elmer DSC 7 model at a heating rate of 5 °C/min under air
atmosphere using aluminum pans. Thermo gravimetric analysis (TGA) was performed using a
Perkin Elmer Diamond series model at a heating rate of 10 °C/min under flowing air in the range
of 25ꢀ800 °C.
2.3. Synthesis of 1, 4-Bis (4-acetamidophenoxy) butane
4ꢀacetamidophenol (5.0g, 0.0330mol) and 10.26 g of potassium carbonate (0.0742mol) in
100 mL of acetone were taken in a 250 mL round bottomed flask. To this 1, 4ꢀdibromobutane
(3.5708g, 0.0165mol) in 50 mL of acetone was added drop wise. The reaction was initiated by
adding trace amounts of potassium iodide. Then, it was refluxed at 80 °C overnight. The mixture
was cooled and poured into water (1000 mL). The white precipitate formed was separated and

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recrystallized from ethanol.30 Yield 89%, M.P.: 190 °C. IR (KBr, cmꢀ1): 3336 (CONH), 1053 (ꢀ
CꢀOꢀCꢀ), 1675 (C=O), 824 (pꢀsubstituted aromatic).

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Scheme 1. Synthesis of dicyanate ester monomer
2.4. Synthesis of 1, 4-Bis (4-aminophenoxy) butane
1, 4ꢀBis (4ꢀacetamidophenoxy) butane (4.0g, 0.0112 mol) in 100mL ethanol was taken in
a 250 mL round bottomed flask. To this, 10.0g of sodium hydroxide (0.25mol) dissolved in
water (35 mL) was added drop wise. After completion of the addition, the mixture was refluxed
at 80 °C for 8 hr, cooled and poured into crushed ice. The resulting pale brown precipitate was
filtered, washed with distilled water twice and recrystallized from ethanol. Yield 93%, M.P.: 140
°C. FTꢀIR (KBr, cmꢀ1): 3395, 3371 (aromatic NH2), 2945 (OCH2), 835 (pꢀsubstituted aromatic).
1HꢀNMR (500 MHz, CDCl3, ppm): 3.7 (s, 4H, a (ArꢀNH2)), 6.6 (d, 4H, b), 7.2 (d, 4H, c), 3.8 (t,
4H, d (OCH2CH2)) 1.8 (m, 4H, e (OCH2CH2)). 13CꢀNMR (CDCl3, ppm): C1ꢀ139.9, C2ꢀ115.7, C3ꢀ
115.2, C4ꢀ149.3, C5ꢀ67.9, C6ꢀ28.8. Other two diamines were prepared in a similar way.
2.5.1. Synthesis of 1, 4-Bis [4(4-hydroxyphenylazomethyl)phenoxy] butane
1,4ꢀBis (4ꢀaminophenoxy) butane (2.750g, 0.01 mol) was taken in a 250 mL round
bottomed flask with 100 mL of ethanol and refluxed until it gets completely dissolved in ethanol.
To this 2.4662g of 4ꢀhydroxy benzaldehyde (0.0201 mol) in 50 mL of ethanol was added drop
wise with constant stirring. The reaction was initiated by adding trace amounts of acetic acid.
The mixture was refluxed for a further period of 3 hr. The mixture was then cooled and poured
into cold water (1.0 L). The resulting yellow precipitate was collected and recrystallized from
ethanol. Yield: 91%, M.P.: 83 °C. FTꢀIR (KBr, cmꢀ1): 3380 (OH), 2943 (OCH2), 1680 (HC=N),
1
834 (pꢀsubstituted aromatic). HꢀNMR (DMSOꢀd6, ppm): 9.4 (s, 2H, a), 6.9 (d, 4H, b), 7.7 (d,
13
4H, c), 8.7 (s, 2H, d), 7.1 (d, 4H, e), 6.9 (d, 4H, f), 4.1 (t, 4H, g), 1.9 (m, 4H, h). CꢀNMR (500
MHz, DMSOꢀd6, ppm): C1ꢀ161.1, C2ꢀ116.8, C3ꢀ130.9, C4ꢀ130.1, C5ꢀ159.3, C6ꢀ145.9, C7ꢀ122.3,
C8ꢀ116.8, C9ꢀ155.8, C10ꢀ66.3, C11ꢀ27.8.

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2.5.2
.
1,5-Bis[4(4-hydroxyphenylazomethyl)phenoxy]pentane
Yield: 83%, M.P.: 90 °C. FTꢀIR (KBr, cmꢀ1): 3414 (OH), 2945 (OCH2), 1688 (HC=N),
1
836 (pꢀsubstituted aromatic). HꢀNMR (DMSOꢀd6, ppm): 9.2 (s, 2H, a), 6.8 (d, 4H, b), 7.8 (d,
4H, c), 8.6 (s, 2H, d), 7.2 (d, 4H, e), 7.0 (d, 4H, f), 4.0 (m, 4H, g), 1.8 (m, 4H, h), 1.8 (m, 2H, i).
13CꢀNMR (500 MHz, DMSOꢀd6, ppm): C1ꢀ160.5, C2ꢀ115.8, C3ꢀ131.2, C4ꢀ129.1, C5ꢀ159.9, C6ꢀ
145.5, C7ꢀ122.5, C8ꢀ115.7, C9ꢀ158.5, C10ꢀ67.3, C11ꢀ29.8, C12ꢀ22.8.
2.5.3. 1,6-Bis[4(4-hydroxyphenylazomethyl)phenoxy]hexane
Yield: 91%, M.P.: 109 °C. FTꢀIR (KBr, cmꢀ1): 3342 (OH), 2932 (OCH2), 1676
1
(HC=N) 839 (pꢀsubstituted aromatic). HꢀNMR (DMSOꢀd6, ppm): 9.2 (s, 2H, a), 6.9 (d, 4H, b),
7.9 (d, 4H, c), 8.6 (s, 2H, d), 7.2 (d, 4H, e), 6.9 (d, 4H, f), 4.1 (t, 4H, g), 1.7 (m, 4H, h), 1.5 (m,
2H, i). 13CꢀNMR (500 MHz, DMSOꢀd6, ppm): C1ꢀ160.8, C2ꢀ116.1, C3ꢀ131.1, C4ꢀ128.9, C5ꢀ
160.1, C6ꢀ144.8, C7ꢀ121.5, C8ꢀ115.9, C9ꢀ157.8, C10ꢀ68.9, C11ꢀ29.6, C12ꢀ25.8.
2.6. Synthesis of cyanate ester monomers
A 250 mL, threeꢀnecked round bottomed flask, equipped with a magnetic stirring device
and
a
nitrogen inlet was charged with 3.5g of the bisphenol (1,4ꢀBis[4(4ꢀ
hydroxyphenylazomethyl)phenoxy]butane) (0.0114mol). The flask was maintained at ꢀ15 °C and
then a solution of 2.3g (0.0228mol) of CNBr in acetone (50 mL) was added with constant
stirring. To this mixture, 3.1 mL (0.0228mol) of triethyl amine was added in drops with
continuous stirring. After complete addition of triethyl amine, the reaction mixture was stirred
for a further period of 2 hr, while maintaining the bath temperature at ꢀ15 °C. The temperature
was then raised to room temperature. The mixture was poured into cold distilled water to
precipitate the cyanate ester. A light brown product (yield: 91%) was obtained. All other
prepared bisphenols were converted into their corresponding dicyanate ester monomers in a
similar way.
2.6.1. 1,4-Bis[4(4-cyanatophenylazomethyl)phenoxy]butane (CE1)
Yield: 91%, M.P.: 91 °C. FTꢀIR (KBr, cmꢀ1): 2198, 2212 (OCN), 2945 (OCH2), 1681
(HC=N), 834 (pꢀsubstituted aromatic). 1HꢀNMR (DMSOꢀd6, ppm): 6.9 (d, 4H, a), 7.6 (d, 4H, b),
13
8.6 (s, 2H, c), 7.1 (d, 4H, d), 6.9 (d, 4H, e), 4.0 (t, 4H, f), 1.9 (m, 4H, g). CꢀNMR (500 MHz,

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DMSOꢀd6, ppm): C1ꢀ109.1, C2ꢀ156.8, C3ꢀ115.9, C4ꢀ131.1, C5ꢀ135.9, C6ꢀ159.9, C7ꢀ143.3, C8ꢀ
122.6, C9ꢀ116.8, C10ꢀ156.3, C11ꢀ67.8, C12ꢀ28.7.
2.6.2. 1,5-Bis[4(4-cyanatophenylazomethyl)phenoxy]pentane (CE2)
Yield: 87%, M.P.: 99 °C. FTꢀIR (KBr, cmꢀ1): 2226, 2240 (OCN), 2942 (OCH2), 1688
(HC=N), 835 (pꢀsubstituted aromatic). 1HꢀNMR (DMSOꢀd6, ppm): 7.1 (d, 4H, a), 7.7 (d, 4H, b),
13
8.7 (s, 2H, c), 7.2 (d, 4H, d), 6.9 (d, 4H, e), 4.1 (t, 4H, f), 1.8 (m, 4H, g), 1.6 (m, 2H, h). Cꢀ
NMR (500 MHz, DMSOꢀd6, ppm): C1ꢀ109.5, C2ꢀ155.8, C3ꢀ116.5, C4ꢀ131.1, C5ꢀ135.8, C6ꢀ161.1,
C7ꢀ144.1, C8ꢀ121.9, C9ꢀ115.1, C10ꢀ157.1, C11ꢀ68.9, C12ꢀ29.6, C13ꢀ22.1.
2.6.3. 1,6-Bis[4(4-cyanatophenylazomethyl)phenoxy]hexane (CE3)
Yield: 93%, M.P.: 123 °C. FTꢀIR (KBr, cmꢀ1): 2219, 2242 (OCN), 2938 (OCH2), 1680
(HC=N), 838 (pꢀsubstituted aromatic). 1HꢀNMR (DMSOꢀd6, ppm): 7.0 (d, 4H, a), 7.7 (d, 4H, b),
13
8.7 (s, 2H, c), 7.2 (d, 4H, d), 6.9 (d, 4H, e), 4.0 (t, 4H, f), 1.7 (m, 4H, g), 1.4 (m, 4H, h). Cꢀ
NMR (500 MHz, DMSOꢀd6, ppm): C1ꢀ109.4, C2ꢀ156.3, C3ꢀ116.2, C4ꢀ131.9, C5ꢀ135.1, C6ꢀ160.1,
C7ꢀ143.8, C8ꢀ121.8, C9ꢀ115.7, C10ꢀ157.9, C11ꢀ68.5, C12ꢀ29.6, C13ꢀ25.9.
2.7. Preparation of cyanate ester /epoxy blends
1 wt. % of cyanate ester (0.200g) was taken with 99 wt. % of epoxy resin (19.80g)
(cyanate ester: epoxy = 1:99) and stirred at 80 °C until the mixture turns homogeneous. 5.346 g
(27 g of DDM taken for 100 g epoxy resin) amount of DDM (hardener) was added to the
homogeneous mixture of cyanate/epoxy resin and immediately, the mixture was transferred to
the mold and cured at 120 °C for 1 hr, 170 °C for 1 hr and post cured at 210 °C for 1 hr. The
same procedure was adopted for the preparation of Cyanate ester + epoxy blends with 3% and
5% of cyanate ester.
3. Results and Discussion
The synthetic procedure for the preparation of dicyanate esters are presented in Scheme
1. The bisphenol precursors were prepared by the condensation reaction between aromatic
1
aldehydes and diamine. The structure of the bisphenols was confirmed by FTꢀIR, HꢀNMR, and
13CꢀNMR spectroscopy. The dicyanate esters were prepared from their precursor diols and CNBr
in the presence of triethylamine. Fig.1 shows the FTꢀIR spectra of dicyanate esters. The

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appearance of new bands at around 1680 cmꢀ1 and disappearance of amine bands (around 3300ꢀ
3400 cmꢀ1) confirmed the formation of azomethine. The appearance of new bands at 2174ꢀ2240
cmꢀ1 and disappearance of the band at 3300ꢀ3500 cmꢀ1 due to hydroxyl group in the FTꢀIR
spectrum confirms the conversion of hydroxyl group into dicyanate ester. The formation of ꢀCꢀ
OꢀC linkage is confirmed by the presence of band at 1050 to 1070 cmꢀ1. 30 The signal at 3.7ppm
due to the amino protons disappears and a new signal at 8.7 ppm due to azomethine proton is
formed indicating the conversion of the amino group to azomethine group. This is further
supported by FTꢀIR spectrum. The strong absorption band due to the amino is missing and a new
band is found at 1681 cmꢀ1.
The conversion of the hydroxyl group to the cyanate ester is confirmed by the
disappearance of the hydroxyl proton signal at 9.4 ppm and the appearance of the signal due to
cyanate ester carbon at 109.1 ppm13 from the NMR spectra. The chemical shifts of all the
aromatic protons appear in the range of 6.9 to 7.6 ppm. The chemical shift at 4.0 ppm may be
attributed to the methylene proton which is attached to aromatic ring through oxygen atom. The
chemical shift at 1.9 ppm may be assigned to methylene protons which are present in the spacer
group between the two aromatic rings. In the 13CꢀNMR spectrum, the carbon atoms in
azomethine linkage of the three cyanate esters show the chemical shift in the range of 115ꢀ116
ppm. Aromatic carbons appear in the range of 115.8ꢀ156.3 ppm. Hence, the proposed structure
of the dicyanate ester is confirmed.

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Figure 1. FTꢀIR spectrum of cyanate ester CE1, CE2 and CE3.

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Figure 2. 1HꢀNMR spectrum of cyanate ester (4,4'ꢀ(butaneꢀ1,4ꢀdiylbis(oxy))bis(Nꢀ(4ꢀ
cyanatobenzylidene)aniline)) (CE1)

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Figure 3. 13CꢀNMR spectrum of cyanate ester (4,4'ꢀ(butaneꢀ1,4ꢀdiylbis(oxy))bis(Nꢀ(4ꢀ
cyanatobenzylidene)aniline)) (CE1)
3.1. DSC analysis
The cure behavior of the dicyanate esters was studied using differential scanning
calorimetry. The DSC curves of the dicyanate esters CE1, CE2 and CE3 are shown in Fig. 4a, 4b
and 4c and the data are given in Table 1. The melting points of the dicyanate esters were found to
be 91, 99 and 123 oC respectively. The initial (Ti) and maximum curing temperatures (Tmax) were
found to be in the range of 143 to 181 oC and 202 to 242 oC respectively. The cyanate ester CE3
(1, 6ꢀBis [4(4ꢀcyanatophenylazomethyl)phenoxy] hexane) shows the peak maximum curing
o
temperature. The end cure temperatures were found to be in the range of 246 to 285 C. The
dicyanate ester CE1 (1, 4ꢀBis [4(4ꢀcyanatophenylazomethyl)phenoxy] butane) showed the
highest end cure temperature of 285 oC. The curing temperature of BADCY is found to be at 281
oC 2 and all the three synthesised cyanate ester were cured at lower temperature when compared

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13
with BADCY. The cyanate ester (CE1) with shortest alkyl chain between the aromatic groups
was found to cure at the lowest temperature. Glass transition temperature of the neat cured
dicyanate ester was decreases with increase the alkyl chain length between two aromatic groups.
No liquid crystallinity was observed for all the dicyanate esters.
Figure 4a. DSC thermogram data of cyanate ester (CE1)

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Figure 4b. DSC thermogram data of cyanate ester (CE2)

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Figure 4c. DSC thermogram data of cyanate ester (CE3)
Table 1. DSC data of the cyanate ester
Sample Code
Melting Point (oC)
Onset of Cure (oC)
Maximum Cure (oC)
End of Cure (oC)
CE1
CE2
CE3
91
148
143
181
203
216
242
285
246
269
102
123

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3.2. Thermogravimetric Analysis
Thermo gravimetric analysis (TGA) is the preferred technique for the evaluation of the
thermal stability of the polymeric materials. The thermal stability of the cured cyanate esters was
evaluated with its 5% weight loss (T5%), 10% weight loss temperatures (T10%) and the char yield
at 800 ºC. The TGA curves of various cured cyanate esters are given in the Fig. 5 and the data
are given in Table 2. T5% and T10% of all the three neat cured cyanate esters found to be in the
range of 358 to 465 ºC and 508 to 525 ºC respectively. The cured cyanate ester (CE2) having
alkyl chain length of five shows the least thermal stability (T5% ꢀ 358 ºC) and it lost 40% weight
between 520 to 593 ºC. The char yield was found to be in the range of 37.06 to 57.26 %. The
cyanate ester having even number (CE1 and CE3) of alkyl chain length between two aromatic
groups shows more stable than the dicyanate ester having odd number (CE2) of spacer length
between two aromatic groups from char yield and LOI values. Char yield of BADCY is found to
2
be 42.5 at 800 ºC , but char yield of CE1 and CE3 show higher char yield of 57.26 and 48.08
compared to commercially available dicyanate ester (BADCY). The cured cyanate ester having
least alkyl chain length between two aromatic groups shows 5% weight loss at 465 ºC. This is
greater than that of other two cured cyanate ester. The 5 weight % of CE/epoxy blends show the
10% (T10%) weight loss temperature in the range of 392ꢀ410 ºC and char yield in the range of
24.11 to 31.26 %. There are also reports by other authors that the thermal stability and char yield
of cyanate ester/epoxy blend increases with increase of cyanate ester concentration in epoxy
blends which is similar to our case13.

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Figure 5. TGA thermogram data of cyanate ester
Table 2. TGA data of Polycyanurates
Sample Code
T5 (oC) T10 (oC) Char Yield %
LOI
CE1
CE2
CE3
465
358
399
508
520
525
57.26
37.06
48.08
40.40
32.32
36.73

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Neat Epoxy
363
393
378
381
369
410
392
395
18.0
31.26
24.11
27.80
24.7
30.00
27.14
28.62
5% CE1 + 95% Epoxy
5% CE2 + 95% Epoxy
5% CE3 + 95% Epoxy
T5
=
=
=
5% weight loss temperature
10% weight loss temperature
Limiting oxygen index
T10
LOI
3.3. SEM Analysis
The crossꢀsection (fracture surface) of CE/epoxy blend was studied by Scanning electron
microscopy. The fractured surface of the neat CE and CE/epoxy blends was sputtered with gold
to deionize the electrons on the surface. Figs. 6 (a, b and c) display the SEM images of cured
neat cyanate ester CE1, 1% cyanate ester + 99 % epoxy and 5% cyanate ester + 95 % epoxy
respectively. The SEM images of the fractured surface of the CE2 and CE3 are given in figures 6
(d) and (e) respectively. CE1 shows the curved hollow structure, CE2 and CE3 shows layered
structure (on both lower and higher magnification). SEM images b and c illustrate the uniform
distribution of the cyanate ester in the epoxy blends without any void. The DMA results show
that toughening of epoxy was effected by adding CE. The neat CE2 have similar morphology to
poly cyanurate of bisphenol A dicyanate ester. 31

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Figure 6. SEM images of (a) neat cyanate ester CE1, (b) 1% Cyanate ester CE1 + 99 %
epoxy sytem and (c) 5% Cyanate ester CE1 + 95 % epoxy sytem, (d) neat cyanate ester CE2
and (e) neat cyanate ester CE3.
3.4. Dynamic Mechanical Analysis (DMA)
Dynamic mechanical properties (Stress relaxation behavior) were measured for the
CE/epoxy blend systems in order to find out the effect of increase of cyanate ester content. The
storage modulus and tan delta of the blend with 5% cyanate ester (CE1 CE2 and CE3) and 95%
epoxy are given in the figures 7 and 8 respectively. The data obtained from DMA scans are
furnished in Table 3. Storage modulus of the CE/epoxy blend increases with increase in the
cyanate ester content (from 1 to 5% except 1% of CE1/epoxy blend). The increase in storage
modulus may be due to the increased crossꢀlinking density of the blends as a result of increase in
the CE content. Glass transition temperature of pure epoxy system was found to be 161 oC 13. In
o
the case of 1% CE1 epoxy blend, the glass transition temperature was decreased to 158 C from
161 oC for the neat cured epoxy. But on further addition of 5% CE1 to the epoxy resin, the glass
o
transition temperature was increased to 180 oC from 161 C. The glass transition temperature of
the blends slightly increases with increase in the cyanate ester content. This observation proves
the fact that the coꢀreaction of cyanate ester with epoxy resin is almost complete with
homogeneous state 14 and the resultant oxazolidinone network 25 which enhance the thermal and
mechanical property of the CE/epoxy blends. But some of the authors reported that the addition
of POSSꢀCY in epoxy resin, leads to a decrease in the glass transition temperature from 161 to
155 due to the formation of the free volume in cyanate/epoxy blend system.13

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Figure 7. DMA analysis showing storage modulus (E´) of the CE/Epoxy blends: a) 5% of
CE1 + 95 % epoxy, b) 5% of CE2 + 95 % epoxy, c) 5% of CE3 + 95 % epoxy.

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Figure 8. DMA analysis showing loss modulus of the CE/Epoxy blends: a) 5% of CE1 + 95
% epoxy, b) 5% of CE2 + 95 % epoxy, c) 5% of CE3 + 95 % epoxy.
3.5. Crosslink density
The crosslink density or concentration of network chain (νe), is the number of moles of
network chains per unit volume of the cured polymers. Crosslink density of highly crosslinked
thermosets can be determined by modulus measurements in the rubbery plateau by using the
equation of state for rubber elasticity32 as
νe
=
E´/3RT
where,
E´
T
=
=
=
tensile storage modulus in the rubbery plateau
temperature in K corresponding to the storage modulus value
gas constant
R

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Table 3 shows the crosslink density of the CE/Epoxy blends. On addition of cyanate ester
to the epoxy resin, the cross link density was increased. The CE2 epoxy blends show the highest
cross link density. The storage modulus of CE2 blends (3.48 GPa, having odd number of alkyl
chain length between aromatic groups) is higher compared to CE/epoxy blends (having even
number of alkyl chain length between two aromatic groups) of CE1 (2.52 GPa) and CE3 (2.09
GPa).
Table 3. Values of crosslink density of CE + Epoxy blends
(from DMA values).
Sample Code
CE %
E´
(GPa)
Temp
k
(273+50)
323
Cross link density
νe×105
(mol mꢀ3)
3.13
5
5
5
2.52
3.48
2.09
CE1 + Epoxy
CE2 + Epoxy
CE3 + Epoxy
323
323
4.32
2.59
3.6. Flame Retardancy
The resistance towards fire of CEs and CE/epoxy blends are expressed in terms of
limiting oxygen index (LOI). LOI may be defined as the minimum fraction of oxygen in a
mixture of O2 and N2 during fire. The LOI value should be higher than that of threshold value
(value 26) for the polymer to behave as a selfꢀextinguishing material or selfꢀextinguisher. The
LOI value is calculated by using Krevelen’s equation.33
LOI = 17.5 + 0.4 (σ).
where σ is the percentage of char yield. The LOI of the neat CE and CE/epoxy blends
were found to be in the range of 32.32 to 40.40 and 27.14 to 30.00 respectively. The cured
cyanate ester with least alkyl space length (CE1) shows the highest LOI value and the cyanate
ester (CE2) with alkyl chain having five carbons shows the lowest LOI value. The LOI values of
all the cured dicyanate esters indicate that they are very good fire retardant materials. From the

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LOI values, it was found that the cured cyanate ester CE1/epoxy blends show the best flame
retardance.
Conclusions
Dicyanate esters with azomethine linkage were successfully synthesised. The structure of
the monomers and precursors were characterized by FTꢀIR, 1HꢀNMR and 13CꢀNMR
spectroscopy, which clearly confirms the molecular structure. The morphology of the cured
cyanate ester and CE/epoxy blends reveal the uniform distribution of cyanate ester in the epoxy
resin. The cured cyanate ester and BADCY show the similar morphology. The DMA studies
o
revealed that the glass transition temperature of epoxy was increased from 161 to 180 C and
storage modulus of the epoxy resin increases with increase in cyanate ester content. The
crosslink density of the cyanate epoxy blends are calculated from the storage modulus value,
which increases (from 3.98 to 4.32mol/m3 for CE2) with increase in the weight ratio of CE in the
blends. TGA results show that the char yield values of neat cyanate esters are in the range of
32.32 to 40%. The char yield increases with increase of CE content in the epoxy blend from 18
% to 30.0 %. This increase in char yield further suggests that the CE/epoxy blends can be used in
high flame retardancy applications. The microwave curing and the effect of odd and even alkyl
chain length (because CE1 and CE3 having even number of alkyl chain length between two
aromatic rings showing better char yield and LOI values compared to CE2 which is having odd
number of alkyl chain between two aromatic rings.) between the two aromatic groups will be
studied in future.
Acknowledgements
This work was supported by University Grants Commission (UGC), India and the
Natural Sciences and Engineering Research Council (NSERC) of Canada.
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