Comparative study of two twin liquid-crystalline diglycidyl ethers containing azomethine moieties

Article abstract of DOI:10.1080/15421406.2011.556449

Two different twin liquid-crystalline diglycidyl ethers containing azomethine moieties were synthesized and characterized and their mesomorphic behaviors, thermal properties, and mechanical properties were compared. It was found that 4,4'-di(2,3-epoxyprop

Full text of DOI:10.1080/15421406.2011.556449

Mol. Cryst. Liq. Cryst., Vol. 537: pp. 128–140, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.556449
Comparative Study of Two Twin
Liquid-Crystalline Diglycidyl Ethers
Containing Azomethine Moieties
R. RATNAMALAR,1 A. M. ISSAM,2 AND
A. BAHARIN2
1School of Materials and Mineral Resources Engineering,
Universiti Sains Malaysia, Nibong Tebal, Penang, Malaysia
2School of Industrial Technology, Universiti Sains Malaysia, Minden,
Penang, Malaysia
Two different twin liquid-crystalline diglycidyl ethers containing azomethine
moieties were synthesized and characterized and their mesomorphic behaviors, ther-
mal properties, and mechanical properties were compared. It was found that 4,40-
di(2,3-epoxypropoxy)-N-benzylidene-o-tolidine-cured systems gave higher thermal
and mechanical properties than 3,30-dimethoxy-4,40-di(2,3-epoxypropoxy)-N-
benzylidene-o-tolidine-cured systems. All liquid-crystalline behaviors exhibited by
the products were found to be in the nematic phase. In addition, the higher curing
ratios used resulted in higher thermal stability as well as mechanical properties in
terms of pendulum and pencil hardness but lower impact properties.
Keywords Azomethine; diglycidyl ether; liquid crystal; mechanical
Introduction
Liquid-crystalline thermosetting polymers possess excellent properties stem from the
preservation of molecular organization in the mesophase of liquid crystal (LC) mono-
mers. Epoxy resins as thermosetting polymers are commercially important because of
their excellent adhesive strength [1], chemical resistance [2], and mechanical and elec-
trical properties. However, epoxy resins exhibit low fracture toughness and poor
wear and crack resistance in real application due to their rigid and brittle nature
[3–5]. To overcome these problems, considerable amounts of work have been carried
out in the direction of toughening epoxies, with some research focused on introducing
appropriate amounts of rubbery and flexible components into epoxy networks [6,7].
Another alternative is by the incorporation of an LC structure into epoxy network.
Liquid-crystalline epoxy resins can be synthesized from mesogens such as rigid rod
molecules with epoxide groups (oxirane rings) directly attached to the mesogenic core,
which were cured by reaction with aromatic amines thereafter. Liquid-crystalline
Address correspondence to R. Ratnamalar, School of Materials and Mineral Resources
Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia. E-mail:
ratmalar@yahoo.com
128

Synthesis and Properties of Two Twin LC Diglycidyl Ethers
129
epoxy (LCE) networks are an important area of research given their potential use in a
number of applications such as electronics, advanced composites, nonlinear optics,
etc. The synthesis, development of texture, mechanical properties, and influence of
curing conditions have been examined for a number of LCEs [8–10]. Azomethine
linkage in the backbone of polymers showed many desirable properties such as
thermal [11], liquid crystal [12], optoelectronic properties [13], etc., due to the reson-
ance of the poly-Schiff’s base unit [14]. Incorporation of azomethine groups in various
liquid-crystal polymers has been extensively studied, such as in polyesters [15,16],
polyurethanes [17,18], polyethers [19], etc. This article mainly aims to compare the
properties of two twin liquid-crystalline diglycidyl ethers containing azomethine
moieties. The route of synthesis along with the characterization, mesomorphic,
thermal, and mechanical properties of the two compounds is presented.
Experimental
Materials
All chemicals were purchased in analytical grade without further purification or
distillation: vanilin, o-tolidine, p-hydroxybenzaldehyde, epichlorohydrin, tetrahexy-
lammonium bromide (THABr; Fluka, Steinheim, Germany), absolute ethanol
(Systerm, Selangor, Malaysia), 1-butanol, 4,40-diaminodiphenyl sulfone (DDS),
and p-phenylenediamine (PDA; Merck, Darmstadt, Germany; Figure 1), diethyl
ether, and toluene (Lab Scan, Bangkok, Thailand).
Instrumentation
Fourier transform infrared (FTIR) spectra of the samples were recorded using
potassium bromide (KBr) pellets with a Perkin-Elmer 2000 spectrophotometer for
monomers and polymers ranging from 4000 to 400 cmꢀ1. 1H-nuclear magnetic reson-
ance (NMR) and 13C-NMR spectra were obtained using a Bruker 400-MHz NMR
spectrometer with dimethyl sulfoxide (DMSO-d6) and deuterated chloroform
(CDCl3) as the respective solvents and tetramethylsilane (TMS) as the internal refer-
ence. Carbon, hydrogen, and nitrogen (CHN) microanalyses were performed using
=
Perkin Elmer 2400 LS Series CHNS O Analyzer. The optical textures of the samples
were studied with a Carl Zeiss Axioskop polarized optical microscope (POM)
equipped with 40 Linkam LTS350 hot stage, Linkam TMS94 temperature controller,
and Linkam LNP cooling system (pump). Differential scanning calorimetry (DSC)
Figure 1. Curing agents.

130
R. Ratnamalar et al.
measurements were conducted using a Perkin-Elmer Pyris Series 7 thermal analyzer
under nitrogen purge at a heating rate of 10ꢁC=min. Thermogravimetric analysis
(TGA) was carried out under a nitrogen atmosphere with a heating rate of 10ꢁC using
a Perkin Elmer Pyris Series 6 thermal analyzer. Mechanical properties for different
ratios of samples in coating thickness of 60 mm were carried out in accordance with
standards [20] for pendulum hardness and pencil hardness, whereas impact tests were
conducted using a tubular impact tester.
Preparation of Bisphenols, 3,30-Dimethoxy-4,40-dihydroxy-N-benzylidene-o-tolidine
(I) and 4,40-Dihydroxy-N-benzylidene-o-tolidine (II). The preparation and
characterization of azomethine bisphenol I was described in previous article [11].
Yield was 93%. Elemental analysis (%): Found: C, 75.18; H, 5.89, N, 5.79,
C30H28N2O4 Calcd.: C, 75.14; H, 5.89; N, 5.84 (Table 1). IR (KBr): 3384 cmꢀ1
1
(OH stretch), 1620 cmꢀ1 (C N stretch). H-NMR (DMSO-d6), ꢀCH Nꢀ,
8.30 ppm. A similar reaction procedure was adopted with replacement of vanilin
to p-hydroxybenzaldehyde to prepare azomethine bisphenol II. Yield was 95%.
Elemental analysis (%): Found: C, 79.46; H, 5.83, N, 6.61, C28H24N2O2 Calcd.: C,
=
=
79.98; H, 5.75; N, 6.66 (Table 1). IR (KBr): 3416 cmꢀ1 (OH stretch), 1623 cmꢀ1
(C N stretch). H-NMR (DMSO-d6), ꢀCH Nꢀ, 8.32 ppm.
1
=
=
Preparation of Twin Diglycidyl Ethers 3,30-Dimethoxy-4,40-di(2,3-epoxypropoxy)-N-
benzylidene-o-tolidine (III) and 4,40-Di(2,3-epoxypropoxy)-N-benzylidene-o-tolidine
(IV). The general reaction is illustrated in Scheme 1. Bisphenols I and II
obtained were separately reacted with epichlorohydrin at 112ꢁC–120ꢁC aided with
tetrahexylammonium bromide (THABr) as catalyst for 1 h. The products III and
IV, respectively, were filtered and washed with diethyl ether and recrystallized
using toluene. Finally, the products III and IV were dried in a vacuum oven for
24 h at 90ꢁC. For twin diglycidyl ether III: Yield was 64%. Elemental analysis (%):
Found: C, 72.54; H, 6.11, N, 4.36, C36H36N2O6 Calcd.: C, 72.95; H, 6.12; N, 4.73
ꢀ1
(Table 1). IR (KBr): 1622 cmꢀ1 (C N stretch), 914 cm
=
(epoxy). 1H-NMR
=
(CDCl3): 8.45 ppm (CH N), 4.05 ppm (ꢀOCH3), 4.15–4.45 ppm (CH2 glycidyl),
3.52 ppm (CH epoxy), 2.7–3.0 ppm (CH2 epoxy). 13C-NMR (DMSO): 159.7 ppm
=
(CH N), 56 ppm (ꢀOCH3), 44 ppm (CH2 in oxirane ring), 47.5 ppm (CH in
oxirane ring), 50 ppm (terminal O-CH2). For twin diglycidyl ether IV: Yield was
73%. Elemental analysis (%): Found: C, 76.18; H, 5.91, N, 4.97, C34H32N2O4
Calcd.: C, 76.67; H, 6.06; N, 5.26 (Table 1). IR (KBr): 1624 cmꢀ1 (C N stretch),
=
1
915 cmꢀ1 (epoxy). H-NMR (CDCl3), 8.2 ppm (CH N), 3.80–4.50 ppm (CH2
=
Table 1. Yield and CHN elemental analyses
Elemental analysis (%)
Calculated
H
Found
H
Compound
Yield (%)
C
N
C
N
I
II
III
IV
93
95
64
73
75.14
79.98
72.95
76.67
5.89
5.75
6.12
6.06
5.84
6.66
4.73
5.26
75.18
79.46
72.54
76.18
5.89
5.83
6.11
5.91
5.79
6.61
4.36
4.97

Synthesis and Properties of Two Twin LC Diglycidyl Ethers
131
Scheme 1. Synthesis route.
glycidyl), 3.50 ppm (CH epoxy), 2.78–2.95 ppm (CH2 epoxy). 13C-NMR (DMSO):
=
159.8 ppm (CH N), 44.5 ppm (CH2 in oxirane ring), 51 ppm (CH in oxirane ring).
Results and Discussions
Characterization
The molecular structures of the epoxy monomers showed in Scheme 1 were evalu-
ated by means of FT-IR and NMR techniques. From the FT-IR spectra obtained,
the ꢀCH3 and ꢀCH2 stretching vibrations were found at 2919 and 2874 cmꢀ1
,
respectively, for twin diglycidyl ether III and at 2921 and 2872 cmꢀ1 for twin digly-
=
cidyl ether IV. Azomethine groups (ꢀCH Nꢀ) of twin diglycidyl ethers III and IV
were separately attributed at 1622 and 1624 cmꢀ1 [21], followed by the absorption of
C C stretching vibrations of aromatic rings at 1511 and 1578 cmꢀ1 for III as well as
=
at 1508 and 1572 cmꢀ1 for IV. Furthermore, the C-O stretching of the chain was
located at 1139 and 1165 cmꢀ1 for III and IV, respectively. The presence of a meth-
oxy substitute (ꢀOCH3) in III was verified at 1024 cmꢀ1. Absorption bands at
914 cmꢀ1 of III and 915 cmꢀ1 of IV were attributed to the terminal oxirane rings.
Figure 2 shows the 1H-NMR spectrum of twin diglycidyl ether III, including the
peak assignments corresponding to the structure. As shown in the figure, the azo-
=
methine moiety (ꢀCH Nꢀ) appeared as a singlet at 8.45 ppm, whereas protons of
the epoxy ring were found at 2.9–3.05 ppm as two doublets and at 3.49–3.57 ppm
as a multiplet, whereas the CH2 glycidyl were given by peaks in the range
4.15–4.45 ppm. Obtained from the raw material vanilin, the proton of the ꢀOCH3

132
R. Ratnamalar et al.
Figure 2. 1H-NMR spectrum of twin diglycidyl ether III in CDCl3.
group was presented by a peak at 4.05 ppm. At frequency 2.5 ppm, the singlet peak
was due to the proton of methyl group that attached to the benzene ring (o-tolidine).
Finally, protons of the aromatic rings were found at frequencies 7.05–7.75 ppm.
Based on the 13C-NMR spectrum of twin diglycidyl ether IV in Figure 3, chemical
shifts in the range of 159.80 ppm where present, which indicated the assignment of
=
the azomethine group (ꢀCH Nꢀ). Carbons of the oxirane ring were present at
frequency ranges of 41.52–50.63 ppm.
Mesomorphic and Thermal Properties
Mesomorphic phase transition of the twin diglycidyl ethers were studied by DSC and
POM. From DSC thermograms, both curves showed two endothermic peaks; one at
lower temperature due to melting (Tm) and the other at higher temperature due to
isotropization (Ti). By POM, a nematic mesophase showing Schlieren textures was
observed for both III and IV in the melt between Tm and Ti (Figure 4). The
liquid-crystalline transition temperatures for melting and isotropization obtained
from DSC thermograms as well as the temperature interval between Tm and Ti
(DT ¼ Ti ꢀ Tm) are presented in Table 2.
As seen from the results, twin diglycidyl ether III melted at lower temperature than
twin diglycidyl ether IV, indicating that the substitution of a methoxy (ꢀOCH3) group
lowered the melting temperature of the monomer. This was due to the fact that

Synthesis and Properties of Two Twin LC Diglycidyl Ethers
133
Figure 3. 13C-NMR spectrum of twin diglycidyl ether IV in DMSO.
substituents could act to reduce the coplanarity of adjacent mesogenic groups and
increasethe diameterordecrease the axialratio ofthemesogens[22]. In thepast, research-
ers have used different substituents to reduce the Tm and Ti of LC polymers [23–25].
Curing Properties
Both twin diglycidyl ethers were cured with DDS and PDA. The cured samples were
prepared from mixing stoichiometric amounts of epoxy monomers with DDS and
Figure 4. Optical textures of twin diglycidyl ethers: (a) for III taken at 134ꢁC (magnification ¼
250ꢂ); (b) for IV taken at 139ꢁC (magnification ¼ 250ꢂ).

134
R. Ratnamalar et al.
Table 2. Thermal transition of twin diglycidyl ethers III and IV
LC monomers
Tm (ꢁC)
Ti (ꢁC)
DT (Ti ꢀ Tm) (ꢁC)
III
IV
130
135
176
220
46
78
PDA, respectively, in ratios of twin diglycidyl ether: diamine of 2:1 and 4:1. The
conversion of epoxy groups into hydroxy groups was confirmed using FT-IR
spectroscopy. In all cases, the characteristic bands of epoxy groups at 914 cmꢀ1 disap-
peared almost completely [26,27]. Figure 5 shows the representative FT-IR spectra of
the cured III:PDA and IV:PDA at a ratio of 4:1.
Representative DSC profiles of the curing behavior of the products are shown in
Figures 6 and 7. Endothermic peaks at 132ꢁC and 148ꢁC in Figure 6 were attributed
to the melting (Tm) and isotropization (Ti) temperatures, respectively. By increase of
temperature, the exothermic curve is attributed to the cross-linking reaction, which is
at 163ꢁC. From Figure 7, the DSC curve shows Tm at 135ꢁC for twin diglycidyl ether
IV and 174ꢁC for DDS as well as Ti at 228ꢁC. The representative optical textures of
cured products illustrated in Figure 8 show nematic phases for III:DDS of 2:1 at
141ꢁC, whereas for IV:DDS of 4:1 it was 156ꢁC.
Figure 5. Representative FT-IR spectra of (a) III:PDA, ratio of 4:1, and (b) IV:PDA, ratio
of 4:1.

Synthesis and Properties of Two Twin LC Diglycidyl Ethers
135
Figure 6. DSC curve of III:DDS ratio of 2:1.
From the representative TGA curves shown in Figure 9, one-step degradation at
280ꢁC and 305ꢁC was found for III:DDS and IV:DDS at a ratio of 4:1, respectively,
for 10% weight loss. Twin diglycidyl ether IV exhibited marginally better thermal
stability than twin diglyidyl ether III-cured systems. This was due to the presence of
O-CH3 (methoxy substituent) in twin diglycidyl ether III-cured system. Furthermore,
for the PDA system, thermal degradation was found to begin at 272ꢁC for the
III:PDA system and at 293ꢁC for the IV:PDA system. Hence, it is interesting to note
that products cured with DDS showed better thermal stability than that of products
Figure 7. DSC curve of IV:DDS ratio of 2:1.

136
R. Ratnamalar et al.
Figure 8. Optical textures of cured products: (a) for III:DDS ratio of 2:1 taken at 141ꢁC
(magnification ¼ 10ꢂ) and (b) for IV:DDS ratio of 4:1 taken at 156ꢁC (magnification ¼ 10ꢂ).
cured with PDA due to the biphenyl structure present in DDS compared to phenyl
structure in PDA [28]. Also, the higher the cross-linking ratio, the higher the thermal
stability of the products. The decomposition of the cured products probably occurred
through pyrolytic cleavage of the azomethine ether group and the linkage between
mesogenic units [29].
Mechanical Properties
Pendulum hardness evaluates the coating’s hardness by measuring the damping time
(represented by the number of counts) of an oscillating pendulum that rests with two
stainless steel balls on the coating surface. The amplitude of oscillation of the pen-
dulum on the coating surface decreases more rapidly on a softer coating because
of the high degree of damping oscillation, that is, the lower the value of counts,
Figure 9. TGA thermograms of III:DDS and IV:DDS ratio of 4:1.

Synthesis and Properties of Two Twin LC Diglycidyl Ethers
137
Figure 10. Pendulum hardness.
the softer is the coating. Results of the pendulum hardness test are presented in
Figure 10. It was observed that twin diglycidyl ether IV possessed higher pendulum
hardness properties than twin diglycidyl ether III, with the utilization of DDS as the
curing agent. In addition, the higher the cross-linking ratio, the higher the surface
hardness of the cured film. This is due to the fact that higher number of the
cross-linking sites produces a more rigid film.
Good hardness of the coating is directly related to the good cross-linking of the
resin [30]. The cross-linked films will produce a high-molecular-weight polymer, thus
creating a hard, tough, and durable polymer that is suitable in various applications.
Figure 11. Pencil hardness properties of the cured film (rating used for pencil hardness: 1:6B;
2:5B; 3:4B; 4:3B; 5:2B; 6:B; 7:HB; 8:F; 9:H; 10:2H; 11:3H; 12:4H; 13:5H; 14:6H).

138
R. Ratnamalar et al.
Figure 12. Scratch hardness properties of the cured film (rating used for scratch hardness:
1:6B; 2:5B; 3:4B; 4:3B; 5:2B; 6:B; 7:HB; 8:F; 9:H; 10:2H; 11:3H; 12:4H; 13:5H; 14:6H).
Coating hardness was evaluated using a pencil hardness test in which the coating
hardness was rated by the highest lead hardness that could not scratch through
the coating. Results of pencil hardness are reported in two different terms, namely,
pencil hardness and scratch hardness, in Figures 11 and 12, respectively, indicating
that with DDS as the curing agent, twin diglycidyl ether IV exhibited higher pencil
and scratch hardness than that of twin diglycidyl ether III. The trend showed in both
Figure 13. Impact properties of the cured film.

Synthesis and Properties of Two Twin LC Diglycidyl Ethers
139
figures also confirmed that a higher ratio resulted in greater pencil and scratch hard-
ness of the cured film. The high ratio gave higher properties due to the intense degree
of cross-linking.
In impact tests (Figure 13), twin diglycidyl ether IV exhibited higher impact
strength than twin diglycidyl ether III, especially with DDS as the curing agent.
The ability of the LC epoxy resin films to resist the blow of force is directly
influenced by the flexibility of the molecular chains in the films. A highly flexible
coating will therefore prevent the films from cracking and flaking, thus creating
an enamel that possesses low stress values. In other words, the lower curing ratio
used, the more flexible the molecular chains, and thus higher impact strength is
obtained.
Conclusions
Two twin liquid-crystalline diglycidyl ethers have been successfully synthesized.
1
Their structures were confirmed by FT-IR, H-NMR, 13C-NMR, and CHN techni-
ques. It was found that twin diglycidyl ether III yielded lower thermal and mechan-
ical properties compared to twin diglycidyl ether IV due to the presence of an O-CH3
(methoxy group). Furthermore, high curing ratios also enhanced the mechanical
properties in terms of pendulum and pencil hardness but reduced impact resistance.
Acknowledgment
The authors thank Universiti Sains Malaysia for the RU Research Grant (No. 1001=
PTEKIND=811017).
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