Synthesis and curing behavior of a novel liquid crystalline epoxy resin

Article abstract of DOI:10.1007/s10973-010-1086-0

This article described the synthesis and mesomorphic behavior transition of a novel liquid crystalline (LC) epoxy resin 4-(2,3-epoxypropoxy)biphenyl, 4″-(2,3-epoxypropoxy)phenyl-4′carboxylate (EBEPC), which combined a hydroxyl benzoic aromatic ester and b

Full text of DOI:10.1007/s10973-010-1086-0

J Therm Anal Calorim (2011) 103:1031–1037
DOI 10.1007/s10973-010-1086-0
Synthesis and curing behavior of a novel liquid crystalline
epoxy resin
•
Hai-Mei Wang Yue-Chao Zhang Li-Rong Zhu
•
•
•
Bao-Long Zhang Yu-Ying Zhang
Received: 5 August 2010 / Accepted: 30 September 2010 / Published online: 13 October 2010
´
´
Ó Akademiai Kiado, Budapest, Hungary 2010
Abstract This article described the synthesis and meso-
morphic behavior transition of a novel liquid crystalline
(LC) epoxy resin 4-(2,3-epoxypropoxy)biphenyl,400-(2,3-
epoxypropoxy)phenyl-40carboxylate (EBEPC), which com-
bined a hydroxyl benzoic aromatic ester and biphenol
rigid-rod group. EBEPC showed a clear nematic schlieren
texture under curtain conditions. The reaction kinetics of
EBEPC cured by 4,40-diaminodiphenyl-methane (DDM)
was studied by using an isoconversional method under
isothermal conditions with differential scanning calorime-
try (DSC). The isothermal DSC data can be fitted reason-
ably by an autocatalytic curing model. Smectic phases had
been observed in the EBEPC/DDM curing system. The
results of DSC showed that the formation of the LC phase
had pronounced influence on the curing reaction.
LCERs exhibit excellent properties suitable for versatile
practical applications such as high heat resistance, high
mechanical properties, low coefficient of thermal expan-
sion, low dielectric constant, and good dimensional sta-
bility [5]. During the curing process, LCERs undergo little
shrinkage, and they can be used as high performance
materials for technological applications such as micro-
electronics or optical devices, etc. [6]. Over the past dec-
ades, many reports have been dedicated to the synthesis of
novel LCERs with emphasis on their LC characteristics
and specific curing properties [7–9]. The LCERs studied so
far can be divided into four basic rigid-rod group types:
stilbene, hydroxyl benzoic aromatic ester, azine, and
biphenol (BP). Moreover, there are only few investigations
on LCERs with mesogenic groups which combine different
rigid-rod mesogenic units [10–12].
Keywords Curing kinetics ꢀ Liquid crystal ꢀ
Epoxy resins ꢀ Phase behaviors
It is well known that properties of epoxy polymeric
materials intensely depend on the curing process, so the
studies of cure kinetics for epoxy resins become important,
and the kinetics of curing epoxy resins has been widely
studied using isothermal or dynamic experiments with
differential scanning calorimetry (DSC) [13–16]. Curing of
LC epoxy monomers leads to the formation of nematic and
smectic networks that are important for the above-men-
tioned practical applications [8]. The curing behavior and
thermal properties of LCERs are dependent on the structure
of the aromatic mesogenic group and the length of the
flexible spacer, which decouples the reactive end-func-
tional group from the rigid-rod mesogenic group [17]. So
the difference between LC and isotropic epoxies systems,
in terms of the curing reactions, has attracted much interest
recently.
Introduction
Epoxy resin is one of the most important polymeric
materials widely used in the polymer industry as coatings,
structural adhesives, insulating materials, and polymeric
composite materials, etc. [1–3]. The properties of epoxides
can be much enhanced if liquid crystalline (LC) structures
are incorporated into the epoxy networks [4]. Such com-
bination results in the so-called LC epoxy resins (LCERs).
H.-M. Wang ꢀ Y.-C. Zhang ꢀ L.-R. Zhu ꢀ B.-L. Zhang ꢀ
Y.-Y. Zhang (&)
Department of Chemistry, Nankai University, No. 94,
Weijin Road, Tianjin 300071, China
e-mail: zhangyuying@nankai.edu.cn
In this study, we described the synthesis and charac-
terization of a new type of LCER, 4-(2,3-epoxyprop-
oxy)biphenyl,400-(2,3-epoxypropoxy)phenyl-40carboxylate
123

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H.-M. Wang et al.
1
(EBEPC). The LC behavior of EBEPC was investigated.
The curing behaviors and LC phase transition of the EB-
EPC/DDM system were studied in detail. Finally, we dis-
cussed the curing kinetics based on the isoconversional
method and Kamal model of the EBEPC/DDM system.
1 269 cm-1 (Ar–O–C). H NMR (D-DMSO): d = 6.87,
6.96, 7.30, 7.53, 7.65, 8.02 (12H, aromatic), 9.58 (2H, –OH).
Synthesis of 4-(2,3-epoxypropoxy)biphenyl,400-
(2,3-epoxypropoxy)phenyl-40 carboxylate
(LCER or EBEPC) (Scheme 1)
Experimental part
P2 (1.53 g, 0.005 mol), EC (14 g, 0.15 mol), and isopropyl
alcohol (16 g) were added into a three-necked reaction
flask fitted with a stirrer, condenser, and temperature con-
trol equipment. When the flask was heated to 80 °C, the
phase transfer agent, BTMAC was added, and then the
aqueous solution of NaOH (30 wt%, 8 mL) was dropped
out in 15 min. The reaction was continued at 80 °C for 3 h
and cooled down to room temperature, then the mixture
was put into petroleum ether (50/250, v/v), and the
resulting white precipitate was separated and recrystallized
from a mixture of chloroform and petroleum ether. Yield:
43.7%; mp: 256.3 °C. IR (KBr): 3 002, 2 929 (Ar–H),
1 727 (C=O), 1 604, 1 496 (–Ar–), 1 262 (Ar–O–C),
Materials
4,40-Dihydroxybiphenol was purified by recrystallization
from ethanol. Para-hydroxybenzoic acid, concentrated
sulfuric acid, acetic anhydride, epichlorohydrin (EC),
benzyltrimethylammonium chloride (BTMAC), and 4,40-
diaminodiphenyl-methane (DDM) were used in analytical
grade without further purification. All the solvents were
freshly distilled before use.
Synthesis of 4-(hydroxy)biphenyl,400-(acetoxy)phenyl-
40carboxylate (P1)
1
1 020 cm-1 (C–O–C). H NMR (CDCl3): d = 2.78, 2.92
(4H, epoxy-CH2), 3.38, 4.0 (4H, glycidyl-CH2), 4.25, 4.35
(2H, epoxy-CH), 6.98, 7.02, 7.22, 7.50, 7.57, 8.15 (12H,
aromatic). (C25H22O6)n (418.44)n: Calcd. C 71.76, H 5.30;
Found: C 71.35, H 5.35.
4-Acetoxybenzoyl chloride was synthesized according to a
method reported in the literature [18]. 4,40-Dihydroxybi-
phenol (7.0 g 0.038 mol) and 60 ml THF were added into a
reaction flask equipped with a stirrer. 4-Acetoxybenzoyl
chloride (7.5 g 0.038 mol) was diluted with anhydrous
dichloromethane (100 mL) and poured into the reaction,
and then 5-mL pyridine was added as the catalyst. The
reaction proceeded in an ice bath for 1 h and was kept
stirring at room temperature for 16 h. The mixture was
filtered and the filtrate was poured into petroleum ether
(150/500, v/v). Finally the resulting white precipitate was
separated and recrystallized from a mixture of chloroform
and petroleum ether, and the white acicular powders were
obtained. Yield: 45.3%; mp: 191.0 °C. IR (KBr): 3 416
(-OH), 3 107 (Ar–H), 1 752 (C=O), 1 598 (–Ar–), 1 448,
1 374 (CH3), 1 272 cm-1 (Ar–O–C). 1H NMR (D-DMSO):
d = 2.35 (3H, CH3), 6.87, 7.34, 7.41, 7.52, 7.67, 8.20
(12H, aromatic), 9.58 (1H, –OH).
Curing process of EBEPC
A stoichiometric ratio of EBEPC and the curing agent
DDM were dissolved in proper amounts of acetone. Then,
the solvent was allowed to evaporate at room temperature.
For the isothermal and dynamic curing studies, the
COCI
HO
OH
H3COO
Pyridine
THF
P1
OH
COO
H3CCOO
.
Synthesis of 4-(hydroxy)biphenyl,400-(hydroxy)phenyl-
3
2
Acetone
NH H O
40carboxylate (P2)
P2
COO
HO
OH
O
P1 (1.74 g, 0.005 mol) was added into a flask with 100-mL
acetone. The flask was cooled in an ice bath, and then
20 wt% AL (10 mL) was added dropwise in 20 min. The
mixture was stirred at room temperature for another 15 h and
then poured into ice water (100/300, v/v). Brown precipitates
were obtained and purified with a mixture of acetone and
petroleum ether. Yield: 61.8%; mp: 231.0 °C. IR (KBr): 3
433 (–OH), 3 064 (Ar–H), 1 732 (C=O), 1 605, 1 498 (–Ar–),
EC BTMA
O
O
O
COO
EBEPC
Scheme 1 The synthetic scheme of EBEPC
123

Synthesis and curing behavior
1033
LC behaviors of EBEPC
homogeneous samples were loaded into the DSC and
heated at varied temperatures and heating rates.
The optical micrograph of EBEPC is shown in Fig. 2. The
sample on the glass slide was heated with a scanning rate of
10 °C min-1. Only an isotropic phase was observed
260 °C, A nematic schlieren LC phase could be observed
clearly while cooling to 190 °C with a cooling rate of
0.5 °C min-1 (Fig. 2a). When cooling down to 140 °C, the
system entered into a crystalline phase (Fig. 2b).
Measurements
IR spectra were recorded on a Bio-Rad FTS-6000 Fourier
1
transform infrared (FTIR) spectrometer. H NMR spectra
were obtained with a UNITY Plus-400 (400 MHz) spec-
trometer with CDCl3 or dimethyl suloxide-d6 as the solvent
and tetramethylsilane (TMS) as the internal standard.
Elemental analyses (EA) were carried out on an Elementar
Vario EL C and H analyzer. Thermal kinetic studies were
carried out on a NETZSCH DSC 200 F3 thermal analyzer
with N2 as a purge gas by isothermal and non-isothermal
scanning. The textures of the LC phase were observed with
an OLYMPUS BX51 polarized light microscope equipped
with a Linkam THMSE600 hot stage.
Curing behaviors of the EBEPC/DDM system
The curing DSC curve for EBEPC/DDM system at dif-
ferent isothermal temperatures is shown in Fig. 3. The Htot
is calculated from the total area under the dynamic scan of
curing curve at 10 K min-1, and the corresponding value
for EBEPC/DDM system is 299.5 J g-1. The curing rate of
the system grows dramatically with the increase of the
reaction temperature. In addition, the autocatalytic kinetic
behavior is shown from the non-zero initial reaction rate.
To investigate the kinetics of the curing reaction, the
Results and discussion
Structural characterization of EBEPC
EBEPC was structurally characterized with spectroscopic
techniques. Figure 1 shows the 1H NMR spectrum of
EBEPC, and the chemical shifts of all protons corre-
sponding to the structure have been assigned clearly. The
protons in the aromatic ring were found at 6.98–8.15 ppm.
The protons of aliphatic groups were found at 2.78–
4.35 ppm, respectively.
100 µm
100 µm
(a)
(b)
8
6
7
4
9
5
Fig. 2 Polarized optical micrographs of EBEPC: a nematic phase at
190 °C and b crystalline phase at 140 °C
3
O
3
O
O
O
COO
1
2
1
2
CDCI
TMS
3
0.5
0.4
150 °C
0.3
6
140 °C
130 °C
0.2
1,1
3,3
4,5
120 °C
9
7
H O
2
0.1
1,1
8
2,2
3
3
0.0
15
0
10
5
Time/min
8
6
4
ppm
2
0
Fig. 3 Isothermal DSC curve for EBEPC/DDM system at different
reaction temperatures
Fig. 1 1H NMR spectra of EBEPC
123

1034
H.-M. Wang et al.
relationship between conversion a and the curing time at
different curing temperatures are obtained and is shown in
Fig. 4.
Fig. 6. It is clear that Ea is not a constant and its depen-
dence on the conversion has the characteristics of the
autocatalytic reaction which often occurs in this system. At
the initial curing stage, there was a high Ea value at low
conversion (64.8 kJ mol-1, a = 0.1), which is due to the
fact the phenol hydroxyls addition is predominant at this
stage. At the final curing stage, the activation energy
deceased deeply, indicating the curing reaction started to
shift from the mechanism controlled by the chemical
kinetics to that controlled by diffusion. This later process is
due to the phenomena of gelification, vitrification, and high
viscosity existed in the reaction medium [20]. However,
there was some difference between the current system and
the conventional bisphenol A epoxy/amine system [21] at
the final curing stage (a [ 0.6) where the activation energy
started to decrease deeply. This difference may be due to
Kinetics analysis
All kinetics studies start with the basic equation as follows:
ꢀ
ꢁ
da
dt
Ea
¼ kðTÞfðaÞ ¼ A exp ꢁ
fðaÞ
ð1Þ
RT
where T is the temperature, a is the reaction conversion,
A is the pre-exponential factor, R is the gas constant, and
f(a) is a model function that depends on the reaction
mechanism.
Assuming the heat flow is proportional to the change in
the extent of curing reaction:
da 1 dH
dt Htot dt
¼
ð2Þ
0.9
0.8
6.0
5.5
5.0
4.5
4.0
3.5
0.7
0.6
0.5
0.4
0.3
where dH/dt represents the rate of heat generated during
curing reaction. Htot is the overall heat of reaction.
Under the isothermal condition, we can rearrange Eq. 1
as follows using integral method [19]:
0.2
ꢂ
ꢃ
Aa
Ea
0.1
ꢁ ln ta;i ¼ ln
ꢁ
ð3Þ
gðaÞ
RTi
R
a
0
where gðaÞ ¼ ½fðaÞꢂꢁ1da is the integral form of the
reaction model; and ta;i is the time required to reach a
specified conversion a at temperature Ti. So from Eq. 3, Ea
can be evaluated from the slope of the plot of -ln ta,i
3.0
2.4
2.5
1000/T/K–1
2.6
against Tiꢁ1
.
The plots of -ln ta,i versus 1000/T of the EBEPC/DDM
system is shown in Fig. 5. The activation energy Ea of
EBEPC/DDM system at varied conversions is shown in
Fig. 5 Isoconversional plots of EBEPC/DDM system from different
reaction temperatures
1.0
0.8
70
60
50
40
30
1
2
3
4
0.6
0.4
0.2
0.0
1–120 °C
2–130 °C
3–140 °C
4–150 °C
20
EBEPC/DDM
10
0
0.0
0
10
15
5
0.6
0.4
0.8
1.0
0.2
Time/min
Conversion
Fig. 4 Conversion as a function of the reaction time at different
reaction temperatures for EBEPC/DDM system
Fig. 6 The relationship between activation energy and the fractional
conversion
123

Synthesis and curing behavior
1035
Fig. 7 Comparison of
experimental data with
0.5
0.5
120 °C
Experiment
130 °C
Experiment
0.4
0.3
0.4
0.3
autocatalytic model for EBEPC/
DDM system from different
reaction temperatures
Autocatalytic
Autocatalytic
0.2
0.1
0.0
0.2
0.1
0.0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
0.5
0.5
140 °C
150 °C
Experiment
Autocatalytic
Experiment
Autocatalytic
0.4
0.3
0.4
0.3
0.2
0.1
0.0
0.2
0.1
0.0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
the formation of the LC phase in the EBEPC/DDM system.
In previous works, it had been shown that the LC phase
facilitates the reduction of the viscosity [22].
activation energy Ea1 and Ea2 were calculated from k1 and k2
at different temperatures using Arrhenius relationship,
respectively. Regardless of the temperature, the maximum
reaction rates located around the conversion of 0.2–0.4,
and there was a good fit with the above model for the
experimental data from the start of reaction up to 95%
conversion. The deviation at the final stage is due to the
fact that the diffusion effect dominates at this stage. This
result is consistent with the variety of the effective activation
energies.
We can also use an autocatalytic model proposed by
Kamal [23], which gives a description of curing up to the
vitrification point as follows:
da
n
¼ ðk1 þ k2amÞð1 ꢁ aÞ
ð4Þ
dt
Where k1 is the externally catalyzed rate constant, and k2 is
the autocatalyzed rate constant with Arrhenius temperature
dependency. And the constant k1 can be obtained using the
initial reaction rate at t = 0 from the intercept of the iso-
thermal curve.
Morphologies of the cured blends of EBEPC/DDM
The fitting curves from the experimental data to the
autocatalytic kinetic model are shown in Fig. 7. The kinetic
parameters confirmed are listed in Tables 1 and 2. The
The polarized optical morphology of the EBEPC/DDM
system in the curing process is shown in Fig. 8. The sample
was heated to 160 °C with a scanning rate of 10 °C min-1
.
Table 1 Kinetics parameters of EBEPC/DDM curing reaction of
different temperatures
When the mixture started to melt, a birefringence pattern
(Fig. 8a) could be observed. The LC phase was a smectic
one. When the sample was cured at 160 °C for an hour, the
fan smectic LC phase (Fig. 8b) could be observed clearly
during the curing process. After this process, the sample
was cooled down to room temperature with a scanning rate
of 1 °C min-1, the LC phase now showed a twisted texture
(Fig. 8c).
Temperature/°C
k1/min-1
k2/min-1
m
n
m ? n
120
130
140
150
0.083
0.118
0.212
0.368
0.546
0.648
0.9
1.31
1.05
1.17
1.24
1.05
1.20
1.27
1.45
2.36
2.25
2.44
2.69
1.398
Table 2 Kinetics parameters of EBEPC/DDM curing reaction
k1/min-1
k2/min-1
mavg
navg
Ea1/kJ mol-1
Ea2/kJ mol-1
1.43 9 108 exp(-8.39/RT)
r = 0.9786
2.91 9 105 exp(-5.21/RT)
r = 0.9341
1.19
1.24
69.7
43.3
123

1036
H.-M. Wang et al.
Fig. 8 Polarized optical
micrographs of EBEPC/DDM
system: a 160 °C for 1 min,
b 160 °C for 60 min, and
c room temperature
(a)
(b)
100 µm
100 µm
(c)
100 µm
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Conclusions
A novel LC epoxy resin, EBEPC, was synthesized and
characterized. POM studies revealed that EBEPC had a
nematic schlieren texture. Curing kinetics of EBEPC/DDM
system was studied using an isoconversional method and
Kamal model. The results showed that the effective acti-
vation energy was dependent on the conversion, and the
effect of diffusion on the curing reaction for EBEPC/DDM
system was found at 95% conversion and the formation of
LC phase had much influence on the curing reaction. The
curing EBEPC/DDM system displayed a smectic texture
which became twisted at room temperature.
`
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