Synthesis and characterization of novel liquid crystalline epoxy resin with low melting point

Article abstract of DOI:10.1080/15421406.2013.823363

A liquid crystalline epoxy resin (LCE) having methylstilbene as a mesogenic unit and an ethylene-oxy unit as a spacer (DGE(C2-MS-C2)) was synthesized and characterized. DGE (C2-MS-C2) has a lower melting point (MP) compared to the diglycidylether of 4,4′-dihydroxy-α-methylstilbene (DGEDHMS). The curing of DGE (C2-MS-C2) with diaminodiphenylethane (DDEt) in a mesophase generated a liquid crystalline (LC) network, which have a more highly layered structure than DGEDHMS. The LC network showed extensively large fracture energy on a tensile test. Introducing spacers outside the mesogen unit promotes the mesogen unit to form a highly ordered structure, which enhances the versatility of LC epoxy resins.

Full text of DOI:10.1080/15421406.2013.823363

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Synthesis and Characterization of Novel
Liquid Crystalline Epoxy Resin with Low
Melting Point
Hisanao Yamamoto a b , Akira Fujita a , Miyuki Harada a & Mitsukazu
Ochi a
a Faculty of Chemistry, Materials and Bioengineering , Kansai
University , Suita-shi , Osaka , Japan
b Asahi-Kasei E-materials Co., Ltd. , Fuji-shi , Shizuoka , Japan
Published online: 30 Dec 2013.
To cite this article: Hisanao Yamamoto , Akira Fujita , Miyuki Harada & Mitsukazu Ochi (2014)
Synthesis and Characterization of Novel Liquid Crystalline Epoxy Resin with Low Melting Point,
Molecular Crystals and Liquid Crystals, 588:1, 41-50, DOI: 10.1080/15421406.2013.823363
To link to this article: http://dx.doi.org/10.1080/15421406.2013.823363
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Mol. Cryst. Liq. Cryst., Vol. 588: pp. 41–50, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.823363
Synthesis and Characterization of Novel Liquid
Crystalline Epoxy Resin with Low Melting Point
HISANAO YAMAMOTO,1,2 AKIRA FUJITA,1
MIYUKI HARADA,1 AND MITSUKAZU OCHI1,∗
1Faculty of Chemistry, Materials and Bioengineering, Kansai University,
Suita-shi, Osaka, Japan
2Asahi-Kasei E-materials Co., Ltd., Fuji-shi, Shizuoka, Japan
A liquid crystalline epoxy resin (LCE) having α-methylstilbene as a mesogenic unit and
an ethylene-oxy unit as a spacer (DGE(C2-MS-C2)) was synthesized and characterized.
DGE (C2-MS-C2) has a lower melting point (MP) compared to the diglycidylether of
4,4ꢀ-dihydroxy-α-methylstilbene (DGEDHMS). The curing of DGE (C2-MS-C2) with
diaminodiphenylethane (DDEt) in a mesophase generated a liquid crystalline (LC)
network, which have a more highly layered structure than DGEDHMS. The LC network
showed extensively large fracture energy on a tensile test. Introducing spacers outside
the mesogen unit promotes the mesogen unit to form a highly ordered structure, which
enhances the versatility of LC epoxy resins.
Keywords Fracture toughness; layered structure; LC thermosets; liquid crystalline
epoxy resin
1. Introduction
It has been reported that liquid crystalline (LC) epoxy resins (LCEs) have excellent thermal
and mechanical properties, such as a high Tg, high fracture toughness, and increased bonding
strength [1–7,8]. These characteristics would have a good potential to expand and create
new use of epoxy resins, which have already been widely used for electronics, composites,
adhesives, etc. The origin of these unique properties of LCEs, such as fracture toughness,
was interpreted in terms of their highly oriented or layered structure [3–6]. Q. Lin et al. [9]
researched the relationship between the curing temperature and resulting network structure
of DGDHMS. They demonstrated that as the DGEDHMS is cured at a lower temperature,
a more high-layered structure is formed. This research result implies the possibility that
LCEs would have even more remarkable its unique properties if the resins are cured under
milder conditions such as below 100◦C.
However, it is not very easy to cure LCEs at moderate temperature because curing at
a lower temperature would lead to the generation of a segmental crystalline monomer or
oligomers in the matrices during or before the curing reaction. Some researchers showed a
molecular design that decreases the clearing temperature by introducing spacers between
∗Address correspondence to Mitsukazu Ochi, Faculty of Chemistry, Materials and Bioengineer-
ing, Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka, 564-8680, Japan. Tel.: 06-6368-0811;
Fax: 06-6380-8032. E-mail: mochi@kansai-u.ac.jp
41

42
H. Yamamoto et al.
two mesogenic groups [2,9,10]. Another approach is introducing spacers between the
mesogenic group and also outside of the glycidyl groups [11,12].
In this article, the latter approach was taken using the 4,4ꢀ-dihydroxy-α-
methylstilbene (DHMS) moiety, and then the LCE was synthesized and characterized.
2. Experimental Part
2.1. Materials
DHMS and the diglycidylether of DHMS (DGEDHMS) were synthesized according to the
procedures described in a patent [7]. The 1,3-Diaminopropane (DAP) was purchased from
Tokyo Chemical Industry Co., Ltd. Ethylencarbonate (EC), K2CO3 and other reagents and
solvents were purchased from Wako Pure Chemical Industries, Ltd. These reagents were
used without further purification.
2.2. Measurements
The molecular weight distribution was measured by Gel Permeation Chromatography
(GPC, Shimazdu CBM-20A) using Shimadzu RID-10A reflex indicator and ShodexTM
1
LF804 × 2 columns at 40◦C. The H NMR spectra were recorded using a JNM-EX400
(JEOL Ltd.) spectrometer working at 400 MHz using tetramethylsilane (TMS) as the
internal standard. DMSO-d6 and CDCl3 were used as the solvents for the C2-MS-C2 and
DGE(C2-MS-C2), respectively.
Thermal analysis of the synthesized materials was carried out using a DSC (DSC7020
Seiko Instrument, Inc. Chiba, Japan) at the heating and cooling rate of 5◦C min−1. Optical
anisotropy of the epoxy was observed using an optical microscope (BH-2, Olympus Cor-
poration, Tokyo, Japan) under cross-polarized light. The observation under the polarized
microscope was employed for the sample film mounted on a hot stage. The XRD was
measured using a RINT-Ultima III (Rigaku Corporation).
Tensile test was conducted using an Instron-type tensile machine (Shimadzu Auto-
graph, AGS-J) according to ISO 527-1 and 527-2. The tensile strength and elongation of
the epoxy resin systems were measured with the dumb-bell-shaped specimen according to
ISO 37 type 3, at a crosshead speed of 2 mm min−1. Fracture energies of the cured epoxy
resin systems were evaluated from the area under the stress–strain curves in the tensile test.
2.3. Sample Preparation
2.3.1. Synthesis of C2-MS-C2. The synthesis pathways of (C2-MS-C2) and DGE(C2-MS-
C2) are shown in Scheme 1.
Scheme 1. Pathway for the synthesis of DGE (C2-MS-C2).

LC Epoxy Resin with a Low Melting Point
43
EC (59.4 g, 0.675 mol) and DHMS g (70.7 g, 0.313 mol) were placed in a separate flask
equipped with a stirrer blade, thermometer and cooling condenser. The reaction media was
heated to 120◦C and then 2.04 g of 50 wt% K2CO3aq was added. The reaction was carried
out at 170◦C for 1 hr. The obtained crude C2-MS-C2 was purified twice by crystallization
1
from an MEK solution. The purified C2-MS-C2 (yield 65%) was confirmed by H NMR
(DMSO-d6): 2.18 ppm (3H, s, CH3 of stilbene), 3.72 ppm (4H, m, CH2 of ethylene-oxy),
4.00 ppm (4H, m, CH2 of ethylene-oxy), 4.87 ppm (2H, m, OH), 6.76 ppm (1H, s, CH
stilbene), 6.96 ppm (4H, dd, aromatic), 7.30 ppm (2H, d, aromatic), and 7.48 ppm (2H, d,
aromatic).
2.3.2. Synthesis of DGE (C2-MS-C2). C2-MS-C2 (50.2 g, 0.16 mol), epichlorohydrin
(178 g, 1.91 mol), and dimethylsulfoxide (DMSO, 124 g) were added to a reactor and
heated at 50◦C with stirring. A 50 wt% NaOHaq. (64 g) solution was added dropwise to the
reactor over 30 min. One hour after completion of the 50 wt% NaOHaq. addition, toluene
(250 g) and water (250 g) were added to the reaction mixture, then the stirring was stopped
and the aqueous layer was discharged. The separated organic layer was washed five times
with water (250 g) at 70◦C, then rotary evaporated under vacuum for 30 min. at 80◦C, then
1 hr at 120◦C. The product (70.8 g) underwent the same treatment as above using 250 g
of epichlorohydrin, 175 g of DMSO and 20.0 g of 50 wt% NaOHaq. for completion of the
glycidylation. The obtained crude DGE (C2-MS-C2) (75 g) was purified by crystallization
from a methylisobutylketone solution, yielding 58 g (85%), mp 72◦C (DSC). According to
the GPC, the obtained DGE (C2-MS-C2) contained 7% of other components with a higher
1
molecular weight, some of which were the result of the step-growth polymerization. H
NMR (CDCl3): 2.25 ppm (3H, s, CH3 of stilbene), 2.66 ppm (2H, dd, CH2 of glycidyl),
3.83 ppm (2H, t, CH2 of glycidyl), 3.22 ppm (2H, m, CH of glycidyl), 3.52 ppm (2H, dd,
CH2 of glycidyl), 3.86–3.96 ppm (6H, m, CH2 of ethylene-oxy, CH2 of glycidyl), 4.18 ppm
(4H, t, CH2 of ethylene-oxy), 6.73 ppm (1H, s, CH stilbene), 6.93 ppm (4H, dd, aromatic),
7.29 ppm (2H, d, aromatic), and 7.45 ppm (2H, d, aromatic).
2.4. Curing of Epoxy Resins to Prepare LC Network
DGE (C2-MS-C2) and DGEDHMS were cured with a stoichiometric amount of amine. The
mixture of the epoxy resin and amine was once melted to make a homogeneous system in an
oil bath, then immediately cured in an oven. For the purpose of preparing LC network, the
oven temperature was regulated as low as possible within the range of no crystal generation
in the reaction system.
3. Results and Discussion
3.1. Characterization of DGE (C2-MS-C2)
A DSC thermogram of DGE (C2-MS-C2) is shown in Fig. 1. The endothermic peak at
72◦C during the heating process indicates a crystal-isotropic transition because no clear
mesophase texture was observed on the POM. On the other hand, one small exothermic
peak at 38◦C, and two large ones at 33◦C and 28◦C are observed during the cooling process.
Some of them are associated with LC transitions, because the POM observation indicates
the existence of a nematic phase in the range of 48◦C to 40◦C (Fig. 2). Taking into account
the fact that the sample on the hot stage of the POM is exposed to air, it’s natural that
the temperature region where the birefringence was observed is slightly lower range than

44
H. Yamamoto et al.
Figure 1. DSC thermogram of DGE (C2-MS-C2).
Figure 2. POM photograph of DGE (C2-MS-C2).

LC Epoxy Resin with a Low Melting Point
45
Figure 3. XRD patterns of (a) DGE (C2-MS-C2) and (b) DGEDHMS.
48◦C–40◦C. So, small exothermic peak at 38◦C on DSC corresponds to an isotropic-nematic
transition.
The thermal transitions of DGE (C2-MS-C2) and DGEDHMS are summarized in
Table 1. Because the clearing temperature of DGEDHMS during the heating process is
128◦C, introducing an ethylene-oxy group to DGEDHMS has the effect of decreasing the
clearing temperature by 56◦C.
This effect is rather remarkable as considered by the report of P. Castell et al. [12];
introducing an ethylene-oxy group between the imine type mesogen unit and digycidyl
group decreased the clearing point from 257◦C to 240◦C.
The XRD patterns at room temperature of DGE (C2-MS-C2) and DGEDHMS are
shown in Fig. 3.
Figure 3 shows that DGE (C2-MS-C2) maintains a high crystallinity, and also shows
a longer layer spacing than DGEDHMS. The difference in the layer spacing between DGE
(C2-MS-C2) and DGEDHMS, which is 5A, approximately corresponds to the length of two
oxyethylene units. Therefore, it is natural to consider that oxyethylene units extend along
either side of the DHMS units. The increased volume fraction of the flexible unit, including
the oxyethylene units and glycidyl unit, in the DGE(C2-MS-C2) molecule effectively
decreased the melting point (MP).
3.2. Characterization of DGE (C2-MS-C2) Cured with DDEt
Lowering MP makes it possible to cure the epoxy resin at a lower temperature without
crystal generation of the epoxy resin monomer, oligomer nor curing agent during the
curing process. The curing of DGE (C2-MS-C2) with DDEt at 90◦C for 3.5 hr followed
170◦C for 20 min served to generate matrix having layered structure without crystals
(Fig. 4(a)).
On the other hand, curing DGEDHMS with DDEt generates crystals in the matrix
at the same temperature. Therefore, DGEDHMS was cured with DDEt at 110◦C for 2.5

46

LC Epoxy Resin with a Low Melting Point
47
Figure 4. XRD patterns of (a) DGE (C2-MS-C2) cured with 4,4ꢀ-diaminodiphenylethane and (b)
DGEDHMS cured with 4,4ꢀ-diaminodiphenylethane.
hr followed by 160◦C for 30 min, then an opaque matrix without crystal was obtained
(Fig. 4(b)).
Figures 4(a) and (b) show that the cured DGE (C2-MS-C2) with DDEt has a more
distinct layered structure than the cured DGEDHMS. The possible reason for this would
be (1) curing at a lower temperature of DGE (C2-MS-C2) suppresses the thermal mobility
of the molecule and molecular chain during the curing reaction, (2) extended flexible units
consisting of an oxyethylene unit and glycidyl group on both sides of mesogen unit prohibit
unbound movement of the mesogen unit at an earlier stage, and (3) the longer flexible unit
of DGE(C2-MS-C2) makes the mesogen units easily align and stack on each other during
later curing stage.
Tensile test was conducted on cured DGE (C2-MS-C2) and DGEDHMS. In order to
clarify an effect of a chemical structure and that of a layered structure, each epoxy resin
was also cured at higher temperature to prepare isotropic networks. For this purpose, DGE
(C2-MS-C2)/DDEt system was cured at 170◦C for 1.5 hr, DGEDHMS/DDEt system was
cured at 160◦C for 3.0 hr. Figure 5 and Table 2 show both of cured DGE (C2-MS-C2)s
Figure 5. Stress–strain curve of DGE (C2-MS-C2)/DDEt and system DGEDHMS/DDEt. (a) LC
network of DGE (C2-MS-C2)/DDEt. (b) Isotropic network of DGE (C2-MS-C2)/DDEt. (c) LC
network of DGEDHMS/DDEt. (d) Isotropic network of DGEDHMS/DDEt.

48
H. Yamamoto et al.
Table 2. Tensile test data for DGEDHMS/DDEt and DGE(C2-MS-C2)/DDEt system
Stress Strain Fracture energy
Curing system
Curing condition
[MPa] [%]
[kJ m−2
]
Phase
LC
DGE(C2-MS-C2)/ 90◦C 3.5 hr + 170◦C 43.1
54.5
211
DDEt
20 min
170◦C 1.5 hr
42.2
71.9
38.3
24.9
143
134
Isotropic
LC
DGEDHMS/DDEt 110◦C 2.5 hr +
160◦C 0.5 hr
160◦C 3.0 hr
66.6
19.8
91
Isotropic
perform larger strain and increased fracture energy compared with cured DGEDHMSs.
This result is natural to consider DGE (C2-MS-C2) have flexible unit. Furthermore, in
both epoxy resin system, matrix with layered structure shows increased fracture energy
than isotropic networks. We consider that networks which have layered structure include
local region consisted with predominantly flexible units without rigid moieties such as
methylstilbene unit and phenylene unit. Such a region without rigid moiety contributes to
larger deformation compared with a region comprised both of flexible units and rigid units.
Other possible reason is that curing at a lower temperature served to generate a network
with less defect point on crosslink.
3.3. Curing with Aliphatic Amine
A lower MP makes it possible to formulate the resin with curing agents with a high
reactivity with an epoxy group such as aliphatic amines. Conventional LCEs are usually
difficult to cure with such curing agents, because most LCEs have a higher clearing point.
A mixture of an epoxy resin and a curing agent has to once form a homogeneous system in
molecular order before the curing reaction initiates. However, when a curing agent with a
high reactivity is used, the LCEs would not make a macroscopically homogeneous system
because the reaction initiates before the LCEs completely melt at elevated temperature.
In this study, 1,3-DAP was used as the curing agent with a high reactivity with an
epoxy group.
Indeed, DGEDHMS with a stoichiometric amount of DAP immediately generates a
nonhomogeneous gel during the melting operation before the curing process. The DGE
(C2-MS-C2) makes a homogeneous clear liquid system in a minute with DAP at 90◦C
as expected. After cooling the mixture to room temperature, the DSC of the mixture was
measured (Fig. 6).
Figure 6 shows that the curing reaction initiates below 60◦C.
When the POM observation is carried out, a homogeneous liquid mixture generated
in an oil bath at 90◦C was put on a hot stage preheated at 60◦C. The result of the POM
observation during isothermal curing at 60◦C is shown in Fig. 7. At the beginning, the
system is isotropic (Fig. 7(a)). An emerging birefringence is then observed at 17 min.
(Figure 7(b)), and at 26 min., formation of the oriented structure is confirmed (Fig. 7(c)).
Next, DGE (C2-MS-C2) was cured with DAP at 60◦C for 3 hr followed by curing at 100◦C
for 1 hr, then an XRD analysis of the cured material was completed (Fig. 8). Figure 8
suggests the existence of a smectic-like structure in the resulting network.

LC Epoxy Resin with a Low Melting Point
49
Figure 6. DSC thermogram of DGE (C2-MS-C2) formulated with DAP.
Figure 7. POM photographs of DGE (C2-MS-C2) and DAP system during isothermal curing at
60◦C.
Figure 8. XRD pattern of DGE (C2-MS-C2) cured with DAP.

50
H. Yamamoto et al.
4. Conclusions
An LCE of DGE (C2-MS-C2), which has spacers outside the mesogenic unit, was syn-
thesized and characterized. It was found that DGE (C2-MS-C2) has a low clearing point.
DGE (C2-MS-C2) cured with DDEt produced a more distinct layered structure com-
pared to DGEDHMS without spacers, nevertheless DGE (C2-MS-C2) has a rather limited
mesophase. DGE (C2-MS-C2), especially having layered structure, showed large fracture
energy on a tensile test.
We have also demonstrated the DGE (C2-MS-C2) is cured with DAP at a low tem-
perature and forms an ordered network. The cured properties of DGE (C2-MS-C2) with a
versatile curing agent are currently being investigated.
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