Thermal properties and cure kinetics of a liquid crystalline epoxy resin with biphenyl-aromatic ester mesogen

Article abstract of DOI:10.1016/j.tca.2010.11.016

A liquid crystalline epoxy resin diglycidyl ether of 3,3′,5,5′- tetramethylbiphenyl-4,4′-diyl bis(4-hydroxybenzoate) (DGE-TMBPBHB) was synthesized and its thermal properties were studied by differential scanning calorimetry (DSC) and polarized optical mic

Full text of DOI:10.1016/j.tca.2010.11.016

Thermochimica Acta 513 (2011) 88–93
Contents lists available at ScienceDirect
Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Thermal properties and cure kinetics of a liquid crystalline epoxy resin with
biphenyl-aromatic ester mesogen
Yin-Ling Liu, Zhi-Qi Cai∗, Xiufang Wen, Pihui Pi, Dafeng Zheng, Jiang Cheng, Zhuoru Yang
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 August 2010
Received in revised form 7 November 2010
Accepted 8 November 2010
Available online 16 November 2010
A
liquid crystalline epoxy resin diglycidyl ether of 3,3ꢀ,5,5ꢀ-tetramethylbiphenyl-4,4ꢀ-diyl bis(4-
hydroxybenzoate) (DGE-TMBPBHB) was synthesized and its thermal properties were studied by
differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Thermal history
markedly affected liquid crystalline (LC) phase behaviors. The peak values of the isotropized temper-
ature (Ti), crystallized temperature (Tc) and enthalpy change (ꢀHi) caused by the transition from the
LC phase to the isotropic state of DGE-TMBPBHB decreased slightly during repetitive heat–cool cycles.
Two liquid crystalline thermosets were prepared based on the epoxy monomer with two different cur-
ing agents, 4,4ꢀ-diaminodiphenyl methane (DDM) and 4,4ꢀ-diaminodiphenyl sulfone (DDS), respectively.
Cure kinetics of mixtures of DGE-TMBPBHB/DDM and DGE-TMBPBHB/DDS were investigated by DSC
under nonisothermal processes.
Keywords:
Liquid crystalline epoxy resin
Cure kinetics
Thermal property
Differential scanning calorimentry (DSC)
Thermoset
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
The level of order of a cured thermoset network can be
strongly affected by the nature of a liquid crystalline epoxy com-
In the past few years, widely-held interests have been focused
on liquid crystalline epoxy resins (LCERs) [1–3]. In general, LCER
is composed of an aromatic mesogenic group and a pair of reac-
tive oxirane rings [4], which can react with a curing agent by
a ring-opening reaction. Ordered, anisotropic network structures
have brought LCERs into many potential applications such as
wave-guides, electronic packaging, matrix materials and optional
switches for high performance composites [5].
Cure kinetics and thermal properties of different LCERs have
been performed by various researchers [6–10]. Vyazovkin proposed
an advanced isoconversional method (AICM) conveniently to inves-
tigate phase behaviors of epoxy resin at different curing stages
[11–14]. AICM provides the probabilities to the researchers to
reveal a variation of the effective activation energy of the monomer
at different conversion and to avoid using explicit kinetic models.
Lee et al. synthesized a series of LCERs based on biphenol mesogen
and studied the effect of mesogenic length on the curing behavior
and properties of LCERs [15–18].
pound and physical properties of the cured material. In order to
obtain LCER with high thermal properties, Sun et al. synthesized
a novel rigid-rod epoxy monomer containing the long meso-
genic unit of biphenyl and aromatic ester group, diglycidyl ether
of 3,3ꢀ,5,5ꢀ-tetramethylbiphenyl-4,4ꢀ-diyl bis(4-hydroxybenzoate)
(DGE-TMBPBHB) recently [7]. DGE-TMBPBHB has a higher decom-
position temperature than flexible epoxies and better thermal
resistance than other rigid-rod epoxies. They further investigated
the liquid crystalline phase behavior and curing kinetics of DGE-
TMBPBHB with curing agent diaminodiphenylsulfone (DDS) by
AICM [6].
In this study, an efficient and economic synthetic pathway to
synthesize DGE-TMBPBHB is described which required only two
reaction steps. Differential scanning calorimetry (DSC) and polar-
ized optical microscopy (POM) were used to study the thermal
properties and LC phase transition behavior of DGE-TMBPBHB.
Interestingly, LC phase transition of DGE-TMBPBHB was different
from the observation described earlier in the scientific litera-
tures [6,7]. We found that thermal history of the sample had
a sound influence on the thermal properties. In order to study
this phenomenon more thoroughly, we used two different aro-
matic diamine curing agents 4,4ꢀ-diaminodiphenyl methane (DDM)
and DDS to examine the cure kinetics of DGE-TMBPBHB/DDM
and DGE-TMBPBHB/DDS mixtures. The thermal properties of
two different thermosets prepared from the mixtures were also
compared.
∗
Corresponding author at: School of Chemistry and Chemical Engineering, South
China University of Technology, Rm. 511B, Bd #16, 381 Wushan Rd., Tianhe District,
Guangzhou 510640, Guangdong Province, China.
Tel.: +86 20 8711 4639x606; fax: +86 20 8711 2057x804.
E-mail address: cezqcai@scut.edu.cn (Z.-Q. Cai).
0040-6031/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2010.11.016

Y.-L. Liu et al. / Thermochimica Acta 513 (2011) 88–93
89
2. Experimental
2.5. Theory of the cure kinetics
2.1. Materials
The curing of DGE-TMBPBHB/DDM and DGE-TMBPBHB/DDS
mixtures was studied by DSC. The DSC curves were analyzed on
the basis of the following assumptions: the area under the curves
was proportional to the conversion ˛, whereas the extent of reac-
tion during the mixing of epoxy resin and diamine was neglected.
The conversion ˛ at any time could be defined as:
4-Hydroxybenzoic acid (HBA), p-toluene sulfonic acid (p-TSA)
and epichlorohydrin (EC) were purchased in analytical grade from
the chemical agent company in Guangzhou, China. Sulfolane,
4,4ꢀ-diaminodiphenyl methane (DDM), and 4,4ꢀ-diaminodiphenyl
sulfone (DDS) were purchased in analytical grade from Aladdin
(Shanghai, China). 3,3ꢀ-5,5ꢀ-Tetramethyl-4,4ꢀ-dihydroxybiphenyl
(TMDHBP) was a gift from Ao Kai Chemical Engineering Co.
(Lanzhou city, Gansu province, China).
Ht
a =
(1)
Htotal
where Ht is the partial heat of reaction at time t, and Htotal is the
total heat of curing reaction. For a thermosetting resin, the rate of
curing da/dt is usually obtained by:
2.2. Measurement
da
dt
dH/dt
Htotal
=
(2)
1H NMR and 13C NMR spectra were recorded by a Bruker
Avance-400 spectrometer with CDCl3 or dimethyl sulfoxide
(DMSO-d6) as a solvent and tetramethylsilane (TMS) as the internal
standard. Infrared spectra were obtained with a Nicolet 380 Fourier
transform infrared (FT-IR) spectrophotometer. Mass spectra were
recorded with a Bruker Esquire HCT plus mass spectrometer.
Elementary analysis was recorded on Elemetar Vario EL III. Calori-
metric studies were performed using Netzsch DSC 204 F1 Phoenix
with N2 as purge gas with the flow rate of 70 ml min−1 by dynamic
and isothermal scanning. Thermogravimetric analysis (TGA) was
performed on a Netzsch STA 449 C thermogravimetric analyzer
under N2 atmosphere at a heating rate of 10 ◦C min−1 up to 800 ◦C.
The textures of the mesophase were observed with a polarized opti-
cal microscopy Olympus BX41 equipped with a THMS 600 hot stage
and a TMS 94 Temperature Controller (Scheme 1).
where dH/dt is the heat flow at time t in curing reaction.
3. Results and discussion
3.1. LC phase transition behavior of DGE-TMBPBHB
Thermal programs consisting of repetitive ramps – a ramp heat-
ing scan and a 2-min isothermal step, followed by cooling and a
2-min isothermal step – were used to study the thermal properties
of the epoxy monomer DGE-TMBPBHB. Fig. 1a shows the thermo-
grams of DGE-TMBPBHB with a constant heating rate of 20 ◦C min−1
in the temperature range between 100 ◦C and 280 ◦C. The thermal
transition temperatures and the corresponding enthalpy changes of
the liquid crystalline monomer collected from repetitive heat–cool
scans are summarized in Table 1. As shown in Fig. 1a, an endother-
mic peak at 245.1 ◦C was observed upon the first heating which
could be attributed to the isotropization (Ti = 245.1 ◦C). During the
first cooling run, a blunt peak was observed at around 220.4 ◦C,
which indicated the phase behavior of DGE-TMBPBHB from the
isotropic phase to liquid crystal phase. With the reduction of tem-
perature, the sample began to crystallize at 164.3 ◦C (Tc = 164.3 ◦C).
When we ran the heat–cool cycle a second and a third time, the
peak values of the Ti and Tc slightly shifted towards lower temper-
atures. Enthalpy change caused by the transition from solid phase to
isotropic state (ꢀHi) decreased during repetitive heat–cool cycles,
which changed from 70.9 to 65.4, and to 56.5 J g−1. Fig. 1b shows
the thermograms when heating of the sample was conducted at a
heating rate of 10 ◦C min−1 in each heat–cool ramp. A decrease of
Ti, Tc and ꢀHi was also observed. The small blunt peak practically
disappeared on the third cooling. Obviously, the liquid crystalline
behavior changed at different heating rates.
In order to corroborate that the decrease of enthalpy in DSC
thermograms were not due to the degradation of the monomer,
thermogravimetric analysis of DGE-TMBPBHB was performed. The
result revealed a very small (1.2%) weight loss of the DGE-TMBPBHB
at 280 ◦C. Thus, degradation did not occur at the tempera-
ture used in the DSC experiment [21,22]. Actually, the enthalpy
decrease and the non-reproducibility of DSC thermograms during
repetitive heating and cooling ramps were also observed by some
other authors [22–24]. Percec and coworkers [25,26] found that the
heat and temperature of isotropization, ꢀHi and Ti, were remark-
ably affected by the preceding thermal treatment. We assumed that
two main factors were responsible for the non-reproducibility. For
one thing, thermal history markedly affected LC phase behaviors.
The experimental conditions, specifically heating and cooling rates
used in DSC had a significant impact on enthalpic relaxation [27]
since enthalpic signal were believed to arise from a competition
among kinetic and thermodynamic factors. For another, the cure
behavior and thermal properties of LCERs were affected by the
2.3. Synthesis of DGE-TMBPBHB
A mixture of TMDHBP (0.05 mol, 12.1 g), HBA (0.2 mol, 27.6 g),
sulfolane (50 ml) and p-TSA (5 mmol, 0.9 g) was stirred at 180 ◦C
for 4 h. The crude product was washed with cool water and
ethanol to give 19.3 g of 3,3ꢀ,5,5ꢀ-tetramethylbiphenyl-4,4ꢀdiyl
bis(4-hydroxybenzoate) (TMBPBHB) [19,20]. Then, a mixture of
TMBPBHB (10.5 mmol, 5.1 g), EC (0.42 mol, 39.2 g) and isopropyl
alcohol (0.32 mol, 19.1 g) was stirred at 50 ◦C for 0.5 h. After that,
the aqueous solution of NaOH (30 wt%, 8.5 ml) was added drop-
wise into the mixture in 1 h. The mixture was again stirred at 60 ◦C
for 4 h. After cooling, a white precipitate was recrystallized sev-
eral times with CH2Cl2/CH3OH to give 4.59 g of DGE-TMBPBHB as
white powder. Yield: 73.5%. FT-IR (KBr, ꢁ, cm−1): 2922 (C–H), 1729
(C O), 1510 cm−1 (C–H), 1480 (C–H), 1090 cm−1 (C–O), 1258, 1167,
917 and 841 (oxirane). 1H NMR (400 MHz, CDCl3, ı, ppm): 8.20
(d, J = 8.8 Hz, 4H, Ar), 7.26 (s, 4H, Ar), 7.02 (d, J = 8.8 Hz, 4H, Ar),
4.36–4.33 (m, 2H, CH), 4.04–4.00 (m, 2H, CH), 3.40–3.39 (m, 2H,
CH), 2.94 (t, J = 4.8 Hz, 2H, CH), 2.80–2.78 (m, 2H, CH), 2.22 (s, 12H,
CH3). 13C NMR (100 MHz, CDCl3, ı, ppm): 164.12, 162.78, 147.83,
138.45, 132.38, 130.64, 127.43, 122.10, 114.53, 68.95, 49.95, 44.61,
16.61. MS (APCI) calcd for C36H34O8 594.2, found 595.7. Anal. Calc.
for C36H34O8: C, 72.71; H, 5.76. Found: C, 72.50; H, 5.79.
2.4. Curing processes of DGE-TMBPBHB
DGE-TMBPBHB and a curing agent, DDM or DDS, were pestled
in an agate mortar and mixed together in a stoichiometric ratio.
The DSC experiments were conducted with different heating rates
of 5, 10, 15 and 20 ◦C min−1 under N2 for DGE-TMBPBHB/DDM and
DGE-TMBPBHB/DDS mixtures.

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Y.-L. Liu et al. / Thermochimica Acta 513 (2011) 88–93
Scheme 1. Synthetic pathway of liquid crystalline thermosets.
structure of the aromatic mesogenic group and the length of the
flexible spacer which decouples the reactive end-functional group
from the rigid-rod mesogenic group [28]. Due to the steric hin-
drance between the hydrogen atoms in the two benzene rings in
DGE-TMBPBHB, the biphenyl moiety had greater effect on the liq-
uid crystal phase formation than the phenyl group [21]. From this
point, we assumed that the long mesogenic unit in DGE-TMBPBHB
andmolecularinteractions played amajorrolein thermaltransition
temperature and the corresponding enthalpy change [24,29].
Polarized optical microscopy was applied to identify the
mesophases of DGE-TMBPBHB. The samples were prepared by
sandwiching a tiny powder between two glass plates. Fig. 2 shows
the POM photographs of DGE-TMBPBHB at different temperatures
(200× magnification). When DGE-TMBPBHB was heated to 250 ◦C,
the sample began to melt. The texture color changed with increas-
ing temperature and disappeared at 265 ◦C (Fig. 2a). With the
reduction of temperature, the isotropic state changed to LC phase
and the texture of DGE-TMBPBHB exhibited obvious birefringence
(Fig. 2b, 224 ◦C). The sample began to crystallize when the tem-
perature dropped further. Fig. 2 shows the isotropic state, liquid
crystalline state, and the crystalline state of the DGE-TMBPBHB,
respectively. Birefringence was only observed during the cool-
ing process and it confirmed that DGE-TMBPBHB is a typical
monotropic liquid crystal. This was consistent with the results
obtained by DSC.
3.2. Cure kinetics of DGE-TMBPBHB with DDM and DDS
The curing behavior of DGE-TMBPBHB/DDM was examined
using DSC. Fig. 3 shows the curing curves of DGE-TMBPBHB/DDM
system at the heating rates of 5, 10, 15 and 20 ◦C min−1, respec-
tively. It exhibits typical DSC curves for the curing reaction of
DGE-TMBPBHB and DDM as a function of heating rate. Before the
beginning of the cure, there was an endothermic peak around
91 ◦C in 5 ◦C min−1 curve which was assigned to the melting
of DDM. Following the endothermic peak, an exothermal curing
peak was observed at about 161.9 ◦C. With increasing the heat-
ing rate, the endothermic peak shifted slightly towards a higher
temperature. At the same time, the temperature range of DGE-
TMBPBHB/DDM curing reaction became broader and the initial
Table 1
Thermal transitions and corresponding enthalpy changes in repetitive heat–cool cycles.
Heat –cool cycles
Heating rates (◦C min−1
)
Heating scan
Cooling scan
Enthalpy changes (J g−1
)
Transition temperature (◦C)a
Enthalpy changes (J g−1
)
Transition temperature (◦C)a
Fig. 1a – 1
Fig. 1a – 2
Fig. 1a – 3
Fig. 1b – 1
Fig. 1b – 2
Fig. 1b – 3
20
20
20
10
10
10
70.9
65.4
56.5
73.0
65.9
56.1
245.1
245.3
243.4
243.4
242.8
240.9
−3.4, −54.8
−3.6, −48.4
−2.1, −43.8
−3.6, −49.9
−2.0, −42.3
−38.5
220.4, 164.3, 148.9
207.4, 143.8
200.4, 137.1
218.8, 158.9,150.2
205.1, 144.3
139.8
a
Peak temperatures were taken as phase transition temperatures.

Y.-L. Liu et al. / Thermochimica Acta 513 (2011) 88–93
91
Fig. 3. DSC curves for the curing reaction of DGE-TMBPBHB with DDM.
If one assumes that the curing rate is only a function of conver-
sion and temperature T, the da/dt can be also described as:
ꢀ
ꢁ
da
dt
−Ea
= k(T)f (a) = A exp
f (a)
(3)
RT
or:
d ln(da/dt)
−Ea
=
(4)
dT−1
R
where A is the frequency factor; Ea is the effective activation energy;
f(a) is the reaction model; R is the gas constant. From Eq. (4), a
plot of ln(da/dt) versus 1/T at the same time ˛ from a series of DSC
curves with various heating rates would result in a straight line
with a slope of −Ea/R. Repeating this procedure, the relationship
between Ea and ˛ can be obtained, which is called isoconversional
method (ICM). This method has been successfully applied to study
the curing process since it allowed for evaluating Ea as a function
of the extent of reaction ˛ without assuming a particular form of
the reaction model [14].
Fig. 1. DSC curves of repetitive heat–cool cycles for DGE-TMBPBHB: (a) 20 ◦C min−1
(b) 10 ◦C min−1
,
.
The values of Ea obtained at different conversions are shown
in Fig. 5. In DGE-TMBPBHB/DDM system, the initial curing stages
had an Ea around 46 kJ mol−1. Then the Ea increased in the range of
55–75 kJ mol−1 of typical epoxy-amine curing reaction. The value
of Ea decreased quickly from ≈67 to ≈30 kJ mol−1 at conversions
above 0.8. A decrease in Ea associated with the diffusion control has
been reported in a number of papers following the original paper
by Vyazovkin and Sbirrazzuoli [31]. Generally, kinetic-controlled
reaction dominated the polymerization in the early stage, while as
the reaction proceeded towards gelation and vitrification, diffusion
control became dominative and the Ea value gradually decreased
[13]. The values of Ea obtained for the curing reaction of DGE-
TMBPBHB/DDS system are also plotted in Fig. 5.
curing temperature (Ti), the peak temperature (Tp) and the fin-
ishing temperature (Tf) were all increased with the increasing
heating rate. The mean thermal exothermic effect, which charac-
terized the reaction between DGE-TMBPBHB and DDM, might be
due to the errors connected with methods used to estimate the
baseline [30].
To study the cure kinetics of the curing systems, the relation
of the extent of conversion (˛) and the curing time under dif-
ferent heating rates was obtained from DSC. Fig. 4 presents the
plots of the extent of conversion versus the temperature of DGE-
TMBPBHB/DDM system at different heating rates.
Fig. 2. POM photographs of DGE-TMBPBHB at different temperatures (200×).

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Y.-L. Liu et al. / Thermochimica Acta 513 (2011) 88–93
ation of the polymer chains under a fast heating rate [32]. From
Table 2 it can be learned that the Tg values of DGE-TMBPBHB/DDS
systems were higher than those of cured DGE-TMBPBHB/DDM
thermosets since the melting point of DDS was a little higher than
DDM.
4. Conclusions
A liquid crystalline epoxy resin DGE-TMBPBHB was synthesized
by a highly efficient and economical method. The epoxy monomer
could be obtained from several common primary materials and
thus the procedure was much more effective when the mesogen
was used for mass production in industry. Then, we studied the
LC phase transition behavior of DGE-TMBPBHB and found that
thermal history had a significant influence on the thermal prop-
erties of the epoxy monomer. The peak values of the Ti and Tc in
DGE-TMBPBHB decreased slightly during the second and the third
heat–cool cycle. Enthalpy change caused by the transition from the
LC phase to the isotropic state (ꢀHi) also decreased during repet-
itive heat–cool cycles. The cure kinetics of DGE-TMBPBHB/DDM
and DGE-TMBPBHB/DDS mixtures was studied. The Tg values of
DGE-TMBPBHB/DDS systems were higher than those of cured DGE-
TMBPBHB/DDM thermosets.
Fig. 4. ˛–T curves of DGE-TMBPBHB/DDM cured system.
Acknowledgements
We are appreciating the DSC measurement for Mr Jiwen Chen
and Mr Guoming Zhong from Guangdong Test Center of Product
Quality Supervision (Shunde, Guangdong Province). The authors
also thank Dr. Yanbin Jiang for the POM observation in this work.
Financial support from the Natural Science Foundation of South China
University of Technology (Grant No. x2hge5090490) is gratefully
appreciated.
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