Shape memory effect on the formation of oxazoline and triazine rings of BCC/DGEBA copolymer

Article abstract of DOI:10.1039/c5ra10373h

The shape memory polymer was developed by the copolymerization of varying weight percentages (30, 40 and 50 wt%) of 1,3-bis(4-cyanatophenyl) cyclohexane cyanate ester (BCC) and diglycidyl ether of bisphenol A through the formation of oxazoline and triazin

Full text of DOI:10.1039/c5ra10373h

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Shape memory effect on the formation of oxazoline and triazine rings of
BCC/DGEBA copolymer
Mathivathanan Ariraman, Ramachandran Sasi kumar and Muthukaruppan Alagar*
*Polymer Composites Lab, Department of Chemical Engineering, A.C.Tech, Anna University,
Chennai-600 025, India. E-mail: mkalagar@yahoo.com
Graphical abstract

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Shape memory effect on the formation of oxazoline and triazine rings of
BCC/DGEBA copolymer
Mathivathanan Ariraman, Ramachandran Sasi kumar and Muthukaruppan Alagar*
*Polymer Composites Lab, Department of Chemical Engineering, A.C.Tech, Anna University,
Chennaiꢀ600 025, India. Eꢀmail: mkalagar@yahoo.com
Abstract
The shape memory polymer was developed by the copolymerization of varying weight
percentages (30, 40 and 50 wt%) of 1, 3ꢀbis (4ꢀcyanatobenzyl) cyclohexane cyanate ester (BCC)
and diglycidyl ether of bisphenol A through the formation of oxazoline and triazine ring without
using any external flexibilizer/plasticizer. The copolymer samples were characterized by FTIR
spectroscopy, TGA, UVꢀVis and XPS analysis. Thermoꢀmechanical and rheological
characterizations were carried out using dynamic mechanical analyzer (DMA). The changes of
shape memory behavior and storage modulus were tuned by changing the concentration of
oxazoline and triazine ring in the resulting copolymer using varying weight percentages of BCC.
The shape fixity behavior increases with increase in weight percentages of BCC up to 50 wt%
and beyond this weight percentage of BCC the brittleness behavior of the polymer was noticed.
Key words: Shape memory polymer, copolymer, cyanate ester, DGEBA epoxy
1. Introduction
In the last century, shape memory alloys and polymers had become important class of
materials in the area of material chemistry because of their versatile applications in biomedical,
aerospace, electronics, etc.,1ꢀ6. Shape memory polymers (SMP) are of special interest in the field
of polymers, since they regain its original shape after the removal of external stimuli including
heat, light, solvents, electricity and magnetic fields7,8. The carbon black and carbon nanotube
based hybrid SMPs have significant heat absorbance property. Besides, azobenzenes,
triphenylmethane leuco derivatives and cinnamates are act as a light sensitive SMPs3. SMPs play
a significant role in industrial and academic research areas due to their cost effectiveness,
superior shape recoverability and lower density9. In addition, the thermo responsive SMPs are

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gaining attention due to their amenable processing behaviors with wide range of transition
temperatures10.
Recently, number of SMPs based on polyurethane11, PMMA12, poly(caprolactone)13,14
,
PNIPAAm15, carbon nanotubesꢀpolyurethane were developed16. Lower cross linked polymers
possess lower shape fixity; hence, increasing the cross link density of the shape memory polymer
is an important in this field. Accordingly, Wang Kun et al reported that the cross link density of
the polymer with effective shape memory effect has been increased by react the cyanate ester
with epoxies to obtain hybrid network5. The cyanate ester resins are considered as an important
material in the field of high performance materials17. The high thermal stability, superior
mechanical strength and high performance widen their applications in printed circuit boards,
rocket, missile structures, thermonuclear fusion reactor and highꢀenergy radiation17ꢀ21. In
addition, they also find specialized applications in the areas like low moisture permeable space
craft materials due to their cyclotrimerization of cyanate ester monomers (ꢀOCN) like
polycyanurate networks.
Epoxy polymers are widely used in the field of adhesives, sealants and coating
application22,23 but their thermal and mechanical properties are inferior when compared with
those of cyanate ester polymers. However there are some aliphatic epoxies and aliphatic diamine
curatives which increase the shape memory effect of the polymer composition22. The aliphatic
groups present in the epoxy terminal plays a vital role in enhancing the flexibility of the
polymer24 by losing their thermal properties resulting in low glass transition temperature, thermal
stability and also thermo mechanical properties22.
Hence to overcome such inferior thermoꢀmechanical properties, in the present work, an
attempt has been made to develop thermally stable BCC/DGEBA copolymer by exploiting the
concept of shape memory effect. The structurally modified cyclohexyl bridged cyanate ester
(BCC) was synthesized and subsequently varying weight percentage of BCC were reinforced
into the DGEBA to form BCC/DGEBA copolymer. Then the thermoꢀmechanical and shape
memory behaviors of copolymers were characterized, discussed and reported.
2. Experimental
2.1. Materials
Analytical grade 4ꢀhydroxybenzaldehyde, cyclohexanone, cyanogen bromide (CNBr),
triethylamine (TEA), acetic acid, sulfuric acid and solvents were purchased from SRL, India.

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DGEBA and 10% Pd/C were purchased from javanthee enterprises and SigmaꢀAldrich and were
used as received without further purification.
Scheme 1. Synthesis of BCC cyanate ester monomer.
2.2 Synthesis of 1, 3ꢀbis(4ꢀcyanatobenzyl) cyclohexane (BCC)
The precursor 1, 3ꢀbis (4ꢀcyanatobenzyl) cyclohexane (BCC) was synthesized as the
previously reported procedure18. Three steps were involved the synthesis of BCC, (i) BHC (2,6ꢀ
bis(4ꢀhydroxybenzylidene)cyclohexanone) was synthesized from cyclohexanone and 4ꢀ
hydroxybenzaldehyde (ii) BHC was reduced to form cyclohexyl branched aliphatic chain
bridged phenolic diol (4, 4'ꢀ(cyclohexaneꢀ1,3ꢀdiylbis(methylene))diphenol (CDD) (iii) CDD was
dissolved in dry acetone under nitrogen atmosphere, subsequently the solution of cyanogen
º
bromide in acetone was added, followed by the slow addition of triethyl amine at ꢀ15 C. Then
the reaction temperature was slowly raised to room temperature and stirred for 1h. The progress
of the reaction was monitored by TLC and after the completion of reaction (monitored by TLC),
the resulting product was filtered and the filtrate was quenched with cold water to obtain the final
product which was washed with water for several times and further dried for 12h at 40ºC to yield
88% pale yellow solid as a product.
2.3. Synthesis of BCC/DGEBA copolymer sample
The BCC/DGEBA copolymer with different weight percentages of BCC [30 (0.01 mol),
40 (0.02 mol), and 50 (0.03 mol) wt%] were prepared as follows (Scheme 2); to the solution of

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º
DGEBA in DMF, the BCC was added and stirred for 15 minutes at 30 C to obtain a clear
solution. The resulting viscous solution was poured into a respective glass mold and allowed to
evaporate the solvent at 50 ºC for 3h and then the temperature was raised slowly to 210 ºC at the
º
heating rate of 20 C /h. After that the cured films were removed and preserved for further
characterization.
2.4. Characterization
FTIR spectroscopic technique was used to confirm the functional group present in the
monomer and copolymer, analysis of samples were carried out by Bruker (TENSOR 27) using
KBr pellet method. Thermogravimetric analyses of copolymers were measured using Q500
HiꢀRes thermo gravimetric analyzer (TGA). The samples (about 10 mg) were heated from
ambient temperature to 750 ºC under continuous flow of nitrogen (20 mL/min), at a heating rate
of 10 ºC/min. Surface morphology of copolymer were studied from scanning electron
microscope (SEM) measurements were performed using VEGA 3 TESCAN scanning electron
microscope. The piece of a film fixed in the surface of doubleꢀsided adhesive tape and the film
was sputtered with gold prior to SEM images are observation. Transparences behaviors of the
samples were characterized by shimadzu UVꢀ2450 UV−visible spectrometer. The optical images
were observed using a Euromex polarized optical microscopy (POM) equipped with a Linkem
HFS91 heating stage with a TPꢀ93 temperature programmer and photographs were taken Nikon
Coolpix 4500 camera. Samples were placed in between two thin glass cover slips and melted
with heating at the rate of 2 ºC/minꢀ1.
Dynamic mechanical analyses (DMA) of the copolymer samples were performed with
model Qꢀ800 TA Instruments to study their viscoelastic properties. The copolymer samples were
cut into 20 mm X 5 mm X 1 mm dimension and then clamped on to the tension clamp of the
instrument and scanned from 30 ºC to 200 ºC at a scanning rate of 3 ºC/min. The loss modulus
(E”) and tan δ values were recorded at a constant linear frequency of 1 Hz and preload force of
0.01 N. The Binding energy were measured using Xꢀray photoelectron spectroscopic (XPS),the
analysis were carried out using a JEOL JPSꢀ9200 photoelectron spectrometer with a monoꢀ
chromatized AlꢀKα Xꢀray source operated at 12 kV and 20 mA.

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Scheme 2. Polymerization process of BCC/DGEBA
3. Results and discussion
The molecular structures of synthesized monomer were confirmed by FTIR and NMR
spectral analysis18. The varying weight percentages of BCC were copolymerized with DGEBA
through the formation of oxazoline ring which were confirmed by FTIR spectra. Fig. 1 shows the
disappearance of band at 2263 cmꢀ1 associated with the OCN group of cyanate ester and the
appearance of oxazoline ring peak at 1618 cmꢀ1 infers the presences of oxazoline ring and in turn
confirms the successful incorporation of BCC into DGEBA. Further it also confirms the
oxazoline ring formation through covalent bonding between OCN group of cyanate ester and
glycidyl group of DGEBA. The incorporation of 50 wt% of BCC that is beyond the optimum
percentage level a new absorption peak corresponding to the triazine ring was observed at
1357 cmꢀ1 due to the selfꢀpolymerization of cyanate ester and further the peak intensity increases
with an increase in the weight percentage of BCC cyanate ester18.

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Fig. 1 FTIR spectra of BCC/DGEBA copolymer.
XPS spectrum (Fig. 2) was used to confirm the various types of bonds present in the
cured copolymer. The overall survey spectrum of 50 wt% BCC/DGEBA copolymer shows the
binding energies related to C1s, O1s, and N1s elemental peaks at 284.5 eV, 535 eV, and 401 eV
respectively. Fig. 2b and c represent the deconvolution peaks and the corresponding binding
energies of CꢀC, C=C, CꢀN and N=CꢀO which coincides with the earlier reports18. Fig. 2b shows
the deconvolution of C1s signal; the major peak at 283.9 eV corresponds to C–C and C=C bond,
the second major peak at 285.6 eV is associated with the CꢀC bonds of aliphatic (core
cycloaliphatic groups) carbon atoms25. Subsequently, the third peak at 286.1 eV represents N=C–
O bonds of the oxazoline ring indicating the successful epoxy and cyanate ester coꢀreaction and
the final peak at 288.1 eV confirm the CꢀC=N bond formation26. Fig. 2c shows the
deconvolution of N1s signal into two peaks; the peak appeared at 402.5 eV was assigned to the
presences of C=N–C bonds of triazine ring in the polymer network and the second peak at
399.5 eV corresponds to the formation of CꢀN bond. From the binding energy it is ascertained
that the cyanate esters were both selfꢀpolymerized to form polycyanurate and also coꢀreacted
with epoxy resin with the formation of oxazoline ring. FTIR spectrum (Fig. 1) also supports the
presence of both triazine and oxazoline rings in the copolymer system. The data resulted from
FTIR and XPS confirms the copolymerization of cyanate ester and epoxy resin.

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Fig. 2 XPS spectra of 50 wt% BCC /DGEBA copolymer a) over all survey spectrum b) C1s
deconvolution spectrum and c) N1s deconvolution spectrum
Thermal stability of BCC/DGEBA copolymers has been ascertained by
thermogravimetric analysis (TGA). Fig. 3 shows the TGA curve of different weight percentages
of BCC/DGEBA copolymer. It is clear that the thermal stability of the resulting copolymer
increases with increasing weight percentages of BCC. The maximum of 50 wt% BCC/DGEBA
copolymer composition exhibits better thermal stability than that of DGEBA/30 wt% BCC
copolymer and this may be due to the existence higher amount of rigid triazine ring. In detail, the
º
initial weight loss occurred below 200 C is probably due to the removal of residual solvent and
º
adsorbed moisture. The major weight loss observed above 300 C is associated with the
º
decomposition of polymer network and finally yield a residual chair at 750 C and the data
obtained are presented Table 1. Moreover, the higher decomposition temperature and the higher
char yield indicate the higher thermal stability of the copolymer. In addition, the combination of
higher concentration of oxazoline ring and aliphatic chain significantly lowers the value of Tg
which contributes to modify the shape of the polymer at lower temperature (Fig .5). Where as
that of higher concentration of triazine ring significantly increases the value of Tg and resist the

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modification of shape of the polymer matrix, due to the brittle behaviors of the of a polymer even
at higher temperature.
Fig. 3 TGA thermograms for BCC/DGEBA cyanate ester.
DMA was used to investigate the viscoꢀelastic properties of BCC/DGEBA
copolymer. Fig. 4a and 4b shows the storage modulus and loss tangent (tan δ) curves for
different compositions of BCC/DGEBA with respect to temperature. From the Fig. 4a it is
evident that the storage modulus decreased rapidly when the polymer is heated through the
switching transition regime and this decrease enabled by the mobility of segments within the
polymer thereby inducing the shape memory actuation. The transition temperature (Ttrans) can be
seen in Fig. 4b as the temperature at the maximum value of tan δ. The shape fixity of a material
mainly depends on the glassy state modulus (Eg), high Eg provides higher shape fixity27. High
glassy state modulus of a material indicates the existences of high cohesive energy which is
mostly due to the presence of highly cross linked network structures which leads to minimized
propensity for creep related shape change. High glassy modulus is not always mandatory for
high shape fixity of the polymer but it is desirable, whereas the higher rubbery modulus (Er)
implies high elastic recovery at high temperature.

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Fig. 4 Thermo mechanical property of the BCC/DGEBA copolymer.
The formation of chemical structures of the components significantly contributes to the
shape memory properties of the polymers and it is very clear from the data obtained that the
concentration of BCC plays an important role as a switching segment in the shape memory
copolymers. Among the various weight percentages of BCC, 50 wt% of BCC incorporated
DGEBA possesses excellent shape memory behavior with good thermoꢀmechanical properties.
Hence, the formation of oxazoline ring between BCC and DGEBA plays crucial role towards the
contribution to the shape memory behavior of a polymer in two ways. Firstly the existence of
higher concentration of oxazoline ring significantly reduces the cross link density (Table 1) of
the polymers which were calculated using the equation.1,28, 29
ꢀꢁ ꢂ ꢃꢄ/3ꢅꢆꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ (1)
Where, ꢀꢁ= Cross link density,
Temperature.
ꢃ
ꢄ= Storage modulus, R= Gas constant and T=

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Table.1 Thermal and mechanical property of BCC/DGEBA copolymers
Samples
Cross link density
υe × 10ꢀ4 (mol mꢀ3)
Storage
modulus
(MPa) at 30 ºC
2743±3
Char
yield at
750 ºC
6.65
Glass transition
temperature
@ Tg+40 ºC
(ºC)
110
135
145
5.0
7.4
DGEBA/30 wt% BCC
DGEBA/40 wt% BCC
DGEBA/50 wt% BCC
4586±3
11.87
11.3
6262±3
14.77
Secondly, the increasing weight percentages of BCC in DGEBA polymer significantly
increases the self polymerization of cyanate esters to form triazine ring which in turn increases
the cross link density of the copolymer which also contributes to the shape fixity behaviors of the
copolymer. The storage modulus and glass transition temperature of the copolymers are
increased with increasing the concentrations of BCC from 30 to 50 wt% which are listed in
Table 2. Despite the fact that the 50 wt% BCC/DGEBA possesses excellent shape fixity
properties with enhanced thermal stability the 50 wt% of BCC reinforced DGEBA also possess
high rigidity improved strength behaviors due to the formation of higher concentration of
polycyanurate network.
Fig. 5 Bending test of 50 wt% BCC/DGEBA copolymer.

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Table 2. Elastic modulus ratio, Loss tangent and shape memory properties of BCC/DGEBA
copolymers
Samples
Eg/Er
Ttrans ºC
% shape
% shape
fixity
recovery Ttrans
+20ºC
95
Ttrans ꢀ10 ºC
90
66
45
42
115
140
150
DGEBA/30 wt% BCC
DGEBA/40 wt% BCC
DGEBA/50 wt% BCC
96
96
98
100
Fig. 6 50 wt% BCC/DGEBA copolymer reversibility of Tswitch during the heating and cooling
cycles (heatingꢀcooling range: 60ꢀ200ºC, heating rate: 3 ºC/min).
Bending test was carried out to determine the shape memory behavior of the copolymer
samples between the temperatures (Ttrans+25) and (Ttransꢀ10). The shape changes were made with
respect to the glass transition temperature. 50 wt% BCC/DGEBA was used to demonstrate the
shape memory behavior as changing the temperature of the copolymer as shown in Fig. 5. The
original rectangular planar shape (a) was heated at Ttrans+25 and the sample was immediately
deformed into different shapes through bending and twisting, upon cooling under load, these
deformed temporary shapes (b) were fixed, the shape was retained until the material was heated
to higher temperature30. The sample recovered its original rectangular shape (c) on reheating at

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above Ttrans. The recovered shape was indistinguishable from the original shape, confirming the
excellent shape fixity and recovery. The shape recovery time was noted at various temperatures,
and the original shape was obtained at above the Tg value. The percentage of shape recovery and
retention are calculated using equations 2 and 310.
Shape recovery (%) =[ꢇꢈꢉꢊ − ꢇꢋ/ꢇꢈꢉꢊ] ∗ 100ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ (2)
Shape retention (%) =(ꢇꢌꢋꢊꢍꢎ − ꢇꢈꢉꢊ) ∗ 100 ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ (3)
The polymer was heated to a temperature Ttrans + 25 ºC and deformed to an angle
(θmax). Then the deformed polymer was cooled to Ttrans ꢀ10 ºC to fix the deformation and the
resultant deformation was measured by angle (θfixed). Then the deformed polymer was reheated to
Ttrans+25 ºC and the change in angle (θi) was recorded10.
The SMP is heated to a deformation temperature (Td) (above Tg temperature), which
leads to the material soft and flexible. After applying a force deformation was noted (i.e.
loading). Then decreasing the temperature (below Tg) under the same loading condition, upon
unloading the force, the deformed temporary shape is fixed, which marks the completion of the
shape fixing2. The shape recovery time varies with varying weight percentages of BCC and this
indicates the contribution of formation of chemical structures. 50 wt% of BCC reinforced
DGEBA has 98% shape recovery at 120 sec with 100% shape fixity. The percentages of
recovery have been determined by reheating the samples gradually with increasing the
temperature and the deformation was measured at different temperatures as shown in Fig. 4.
30 wt% and 50 wt% recovery were obtained at 115 ºC and 150 ºC respectively. When the sample
was attaining at Tswitch (175 ºC), 98% recovery was obtained and above the Tswitch (Ttrans+25)
100% recovery of sample was obtained. In addition, the presence of aliphatic chain10, 27in the
cyanate ester (BCC) significantly contributes to the effect of the shape memory behaviors of
copolymers. Moreover, the shape recovery time was mainly influenced by the presence of higher
concentration of triazine ring because of the crosslink density which significantly hindered the
segmental chain mobility in copolymer matrix. The switching temperature (Tswitch) during the
heating process was observed at 145 ºC. Tswitch during cooling process strongly induces the delay
in reorganization of segment. The reproducibility (Fig. 6) in repeated cycle shows that the system
regains its shape memorizing capability on the heating–cooling cycle 23.

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Fig. 7 DRSꢀVis spectrum of BCC/DGEBA copolymers
Transparent behavior of BCC/DGEBA polymers is shown in Fig. 7. On increasing the
BCC concentration, the transparent behaviors of the polymer was decreased due to the formation
of triazine by cyanate ester. Mostly the cyanate ester based polymers are used in space
applications because of their radiation resistance, but it was limited due to their poor
transparency19,20,31. On the other hand, epoxy polymers possess limited radiation resistance with
good transparent behaviors19,20,32. Hence, when epoxy and cyanate ester monomers were
copolymerized, the synergistic properties of epoxy and cyanate ester together improve the
radiation resistance and transparency of the polymer to a significant extent. 30 wt% BCC
incorporation BCC/DGEBA copolymer exhibits 73.6% @ 800 nm transparences behaviors
where as that 50 wt% BCC possess 66.2% @800 nm due to the influencing effect of epoxy
compound in the copolymer.
4. Conclusions
With a view to develop the shape memory polymer, cyanate ester based thermosetting
polymer was synthesized by the copolymerization of BCC and DGEBA. The different weight
percentages of BCC/DGEBA were developed and their thermoꢀmechanical and shape memory
properties were examined. It was inferred that the formation of oxazoline and triazine rings play

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a key role in the storage modulus and shape fixity behavior. A shape memory polymer is
developed through the reaction between BCC and DGEBA where, the oxazoline ring formation
is influenced by the BCC monomer in the copolymer system and in addition to this, they also
undergo a self polymerization to form triazine ring thereby increasing the crosslink density of the
copolymer. Higher loading beyond 50 wt% of cyanate ester increases the brittleness and reduces
the transparent behaviors of the copolymer. 50 wt% BCC/DGEBA copolymer have better shape
memory behavior with higher shape fixity, higher storage modulus, better shape recovery with
improved thermal stability.
Acknowledgment
Dr. Tushar Jana and Miss. Shuvra Singha, School of Chemistry University of Hyderabad,
Hyderabad 500046, India, providing DMA facility. The authors thank to Dr. Gowtham
Department of Bioꢀtechnology, Anna University for is constant support and encouragement.
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