Films based on new methacrylate monomers: Synthesis, characterisation and electro-optical properties

Article abstract of DOI:10.1080/15421406.2011.570154

A number of liquid crystal monomers based on methacrylate derivatives were synthesised by novel mild and solventless procedures under microwave irradiation. Their photo- and thermo assisted homo- and co-polymerizations with glycidyl methacrylate were investigated. The resulting polymers were characterised by NMR, GPC, DSC, and SEM to determine their structures and properties. The effects of the structures of the monomers on the electro-optical properties were correlated. Copyright Taylor & Francis Group, LLC.

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Films Based on New Methacrylate
Monomers: Synthesis, Characterisation
and Electro-Optical Properties
Ana Mouquinho a , Mara Saavedra a , Alexandre Maiau a , Krasimira
Petrova a , M. Teresa Barros a , J. L. Figueirinhas b & João Sotomayor
a
a REQUIMTE, Departamento de Química, Faculdade de Ciências e
Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, Caparica,
Portugal
b CFMC-UL and Departamento de Física, RoviscoPais, Lisboa,
Portugal
Version of record first published: 14 Jun 2011.
To cite this article: Ana Mouquinho , Mara Saavedra , Alexandre Maiau , Krasimira Petrova , M. Teresa
Barros , J. L. Figueirinhas & João Sotomayor (2011): Films Based on New Methacrylate Monomers:
Synthesis, Characterisation and Electro-Optical Properties, Molecular Crystals and Liquid Crystals,
542:1, 132/[654]-140/[662]
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Mol. Cryst. Liq. Cryst., Vol. 542: pp. 132=[654]–140=[662], 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.570154
Films Based on New Methacrylate
Monomers: Synthesis, Characterisation
and Electro-Optical Properties
ANA MOUQUINHO,1 MARA SAAVEDRA,1
ALEXANDRE MAIAU,1 KRASIMIRA PETROVA,1
M. TERESA BARROS,1 J. L. FIGUEIRINHAS,2 AND
1
˜
JOAO SOTOMAYOR
1
´
REQUIMTE, Departamento de Quımica, Faculdade de Ciencias e
Tecnologia, Universidade Nova de Lisboa, Quinta da Torre,
ˆ
Caparica, Portugal
2
´
CFMC-UL and Departamento de Fısica, RoviscoPais, Lisboa, Portugal
A number of liquid crystal monomers based on methacrylate derivatives were synthe-
sised by novel mild and solventless procedures under microwave irradiation. Their
photo- and thermo assisted homo- and co-polymerizations with glycidyl methacry-
late were investigated. The resulting polymers were characterised by NMR, GPC,
DSC, and SEM to determine their structures and properties. The effects of the
structures of the monomers on the electro-optical properties were correlated.
Keywords Electro-optical properties; green chemistry synthesis; liquid crystal
polymer; PDLC
1. Introduction
The effects of the composition of the mixtures, the curing temperature, and the UV
light intensity are known to influence on the microstructure of the polymer network
in the PDLC films. Different liquid crystal domains imply different responses to an
applied electrical field and thus the microstructure of the polymer network seriously
affects their electro-optical properties.
The published synthesis works with a view for new PDLC devices focus mainly
to the preparation of new mesogenic compounds to be incorporated in the polymer
matrix, and new polymer synthesis have not the same attention.
As an approach, PDLC materials may be formed using a liquid crystalline
bifunctional molecule, such as an acrylate, as the monomer (LC monomer) [1].
The LC monomers have an advantage over conventional acrylate monomers due
´
Address correspondence to Joa˜o Sotomayor, REQUIMTE, Departamento de Quımica,
Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre,
ˆ
2829-516 Caparica, Portugal. Tel.: þ351-212949648; Fax: þ351-212948385; E-mail: jcs@
dq.fct.unl.pt
132=[654]

Films Based on New Methacrylate Monomers
133=[655]
Figure 1. Components of E7 liquid crystal from Merck.
to their high compatibility with the low molecular weight nematic LC materials,
thereby facilitating formation of high concentrations of low molecular weight LC
and yielding a sample with high optical quality. The presence of higher concentra-
tions of low molecular weight LCs in the PDLC material greatly lowers the switching
voltages.
The stabilization of the liquid crystal domains is one important aspect in design-
ing new polymeric materials for PDLC devices and this is related with the miscibility
and compatibility between the LC molecules and the monomer(s) molecules. One
possible approach for this stabilization can be achieved using a comonomer with
the same mesogenic unit as the LC. Therefore, we synthesised a number of photo-
and thermo-polymerisable monomers, mainly mono-methacrylates containing aro-
matic systems and nitrile group, mimicking some structural elements of a LC, a
multicomponent nematic mixture E7 from Merck [2] (Fig. 1, Table 1) with a view
for better miscibility and compatibility during preparation of PDLC films, possible
control on the LC microdroplets size, and hence on the electro-optical properties of
the resulting materials.
New mild and solvent-free procedures have been developed [3], with short reac-
tion times, using microwave (MW) irradiation and which, in many cases, appears to
be a more selective affording higher yields compared to classical thermal heating [4,5].
Table 1. Components and mass composition of E7 liquid crystal from Merck
Molecular Molecular
Designation formula weight g=mol
IUPAC name
Composition w=w
a) 5CB
b) 7CB
c) 8OCB
d) 5CT
C18H19N
C20H23N
C21H25NO
C24H23N
249.36
277.41
307.44
325.45
4-cyano-40-n-pentyl-1,10-
byphenyl
51%
4-cyano-40-n-heptyl-1,10-
byphenyl
25%
16%
8%
4-cyano-40-n-octyloxy-1,10-
byphenyl
4-cyano-400-n-pentyl-
1,10,100-terphenyl

134=[656]
A. Mouquinho et al.
The selected monomers were aromatic methacrylates because they are highly
reactive due to the presence of aromatic ring and thus form an interesting class of
polymerizable monomers. They were polymerised and copolymerised with glycidyl
methacrylate by photo-, thermo-, and microwave assisted polymerization, and the
resulting materials were fully characterised in here and elsewhere [3]. Five-carbon
spacer arm was introduced into the structures between the aromatic ring and the
methacrylate group to increase the flexibility of the molecule. The effects of the rigid-
ity and flexibility of the polymerizable monomers on the microstructure of the
polymer network were investigated.
2. Experimental Methods
2.1. Materials and Methods
Reagents and solvents were purified before use.The nematic liquid crystals (LC) used
were E7 obtained by Merck.
Average molecular weights of the prepared polymers were determined by gel per-
meation chromatograph (GPC), Knauer Equipment with Pump Smartline 1000, UV
detector Smartline2500, IR detector Smartline 2300, ELS detector PL-ELS 1000 and
Autosampler Smartline 3800. The results were obtained using light scattering and
refractive index detectors in a temperature controlled oven (set at 85ꢀC) coupled to
a integrated solvent and sample delivery module (degasser, pump and auto sampler),
at a flow rate 1 mL=min, using DMF as eluent, and with an injection volume of 150 ml.
The detectors alignment and instrument sensitivity parameters were previously cali-
brated using a low molecular weight distribution PMMA standard. Glycidyl methacry-
late have been chosen as a co-monomer (50% w=w) because of its well-known action as
a reactive solvent in polymerization systems in order to solubilise the mixtures [6].
2.2. General Procedure for Photo-Polymerization
Photo-polymerisations of the monomers and their co-polymerisations with GMA
were carried out by mixing the monomers with DMPA (1% by weight with respect
to the monomer mixture). The mixtures were flushed with Ar, sandwiched between
two KBr-plates and irradiated by UV-light (Oriel Hg lamp, 366 nm filter). The poly-
mer films were obtained by dissolving the KBr-plates in distilled water, washed
several times with distilled water, and dried under vacuum.
For SEM studies of PDLC films, mixtures of monomer=E7 (30=70 w=w) were
photo-polymerised with DMPA between two KBr-plates. After dissolving the
KBr-plates in distilled water, the films were dipped in acetonitrile for three times
to dissolve the E7 and then dried under vacuum for 24 h. SEM studies were
performed in a Hitachi, model S-2400.
For electro-optical studies of PDLC films, mixtures of monomer=E7 (30=70 w=w)
were introduced by capillarity into a ITO 20 mm cell spacing optical cell and
photo-polymerised with DMPA.
2.3. General Procedure for Thermo-Polymerization
Thermo-polymerisations of the monomers and their co-polymerisations with GMA
were carried out by mixing the monomers with AIBN (1% by weight with respect to

Films Based on New Methacrylate Monomers
135=[657]
the monomer mixture). The mixtures were flushed with Ar, sandwiched between two
KBr-plates and heated at 74ꢀC in an oven for several hours. The polymerization of
the material was followed by FTIR measuring the amount of the methacrylate car-
bon double bonds present. By the end of the polymerization, the set were cooled to
room temperature. For SEM and electro-optical studies, the procedure was similar
to the one reported above.
2.4. General Procedure for DSC Analysis
DSC measurements were performed in a Setaram, model DSC131, with heating and
cooling scanning rates of 10 K=min.
2.5. General Procedure for Electro-Optical Analysis
The electro-optical studies were performed using a laser (He-Ne 632.8 JDS
Uniphase) equipped optical bench, with an AC voltage programmable pulse gener-
ator Agilent 33220A for sample excitation and a photo diode detection system for
light transmission analysis.
3. Results and Discussion
3.1. Monomer Syntheses
A number of photo- and thermo-polymerisable monomers have been designed for
mimicking some structural elements of E7. The synthesis details of monomethacry-
lated monomers are reported in Table 2.
Spectroscopic characterization data (NMR and FTIR) of the synthesised mono-
mers is presented elsewhere [3].
Table 2. Synthesis of monomethacrylated monomers
R
X
O
Reaction conditions at MW Yield [%]
PhM
200 W, 1 min
200 W, 1 min
200 W, 2 min
200 W, 1 min
95
71
64
88
biPhM
O
O
O
NCPhM
NCbiPhM

136=[658]
A. Mouquinho et al.
Table 3. Synthesis of monomethacrylated monomers with a 5-C spacer arm between
the aromatic system and the methacrylate group
R
X
O
Step 1 Yield [%] Step 2 Yield [%]
200 W,
3 min
200 W,
1 min
biPhOSpM
PhOSpM
52
76
38
65
70
47
57
57
250 W,
5 min
250 W,
5 min
O
O
O
200 W,
5 min
200 W,
5 min
NCPhM
200 W,
2 min
200 W,
1 min
NCbiPhOSpM
The synthesis of monomethacrylated monomers with a 5-C spacer arm between
the aromatic system and the methacrylate group are reported in Table 3.
Spectroscopic characterization data (NMR and FTIR) of the synthesised mono-
mers is presented elsewhere [3]. The structures of the synthesised monomers are pre-
sented in Table 4. We included the commercial monomer phenyl methacrylate
(PhM), acquired from Sigma-Aldrich andit was used as received.
3.2. Polymer Structures and Molecular Weights Evaluation
The average polymer molecular weights were determined by GPC using a 50=50 w=w
monomer mixture with glycidyl methacrylate in order to solubilise the solid mono-
mers. The results for thermal- and photo-polymerization are shown in Tables 5
and 6, respectively.
Typical GPC results for a mixture monomer=glycidyl methacrylate (50=50 w=w)
for thermal- and photo-polymerisation are shown in Figure 2.
Generally, the polymers with lower molecular weight possess a lower polydisper-
sion, but no correlation is observed with the molecular structure. For the
thermal-polymerization, the polymers have a number average molecular weight,
Mn, around e8 or e13, and for photo-polymerization polydispersions are higher
and Mn is around e12. All chromatograms and SEM images are available in the
Supporting Information file, accessed via Internet.
3.3. Polymer Calorimetric Characterisation
The PhM and PhOSpM monomers, liquids at room temperature and both with only
one aromatic ring, show the same calorimetric behaviour. For the PhOSpM mono-
mer, a ꢁ3.6ꢀC melting temperature (Tf) with an enthalpy of fusion of DfH ¼ 115.5 J
gꢁ1, a Tg ¼ ꢁ86ꢀC and a cold crystallization at Tcc ¼ ꢁ40ꢀC with an enthalpy of

Films Based on New Methacrylate Monomers
Table 4. Monomers chemical structures
137=[659]
IUPAC name
Monomers chemical structure
Reference
NCbiPhM
40-isocyanobiphenyl-4-yl
methacrylate
5-(40-cyanobiphenyl-
4-yloxy)pentyl
methacrylate
NCbiPhOSpM
PhOSpM
5-phenoxypentyl
methacrylate
Biphenyl-4-yl
methacrylate
biPhM
4-isocyanophenyl
methacrylate
NCPhM
5-(4-cyanophenoxy)
pentyl methacrylate
NCPhOSpM
biPhOSpM
5-(biphenyl-
4-yloxy)pentyl
methacrylate
Phenyl methacrylate
PhM
Table 5. Average polymer molecular weights by GPC for thermal-polymerization
with (50=50 w=w) glycidyl methacrylate
Monomer reference
PD
Peak=min
Ln MW
Ln Mn
biPhM
NCPhM
1.04
1.04
1.04
1.05
1.91
1.4
20.51
20.81
20.47
20.14
13.86
14.97
13.57
8.13
8.02
8.16
8.22
12.80
12.51
13.05
8.09
7.99
8.12
8.17
12.15
12.16
12.69
NCPhOSpM
biPhOSpM
NCbiPhM
NCbiPhOSpM
PhOSpM
1.44

138=[660]
A. Mouquinho et al.
Table 6. Average polymer molecular weights by GPC for photo-polymerization with
(50=50 w=w) glycidyl methacrylate
Monomer reference
PD
Peak=min
Ln MW
Ln Mn
biPhM
NCPhM
1.24
1.08
1.38
1.36
1.51
1.47
1.35
15.78
14.37
15.63
22.36
14.28
14.68
15.06
11.67
12.30
11.72
6.99
12.80
12.36
12.04
11.45
12.23
11.39
6.68
12.39
11.98
11.74
NCPhOSpM
biPhOSpM
NCbiPhM
NCbiPhOSpM
PhOSpM
Figure 2. A typical GPC result for a mixture monomer=glycidyl methacrylate (50=50 w=w) for
thermal- and photo-polymerisation. (Figure appears in color online.)

Films Based on New Methacrylate Monomers
139=[661]
Figure 3. DSC heating and cooling scans at 10 k=min for PhOSpM monomer.
ꢁ97.4 J gꢁ1, indicate a partial amorphous material when a cooling rate of 10 K=min
is used, Figure 3.
For the PhM, a Tf ¼ 18.9ꢀC with DfH ¼ 109.0 J gꢁ1, a Tg ¼ ꢁ89ꢀC and a cold
crystallization at Tcc ¼ ꢁ55.1ꢀC with an enthalpy of ꢁ107.7 J gꢁ1 can be observed.
For the other monomers, the inclusion of nitrile groups or one more aromatic
ring moved the heating and the cooling curves to higher temperatures, and the inser-
tion of one 5-C spacer group moved the curves to lower temperatures, as can be seen
in Table 7. After one heating cycle, a 10 K=min cooling rate was fast enough to avoid
crystallization and no phase transition on cooling was observed. It was detected on
cooling as well that different textures from the initial one appear. This polymor-
phism was detected with 8OCB molecule and its homologues, with similar structure
to these monomers [7,8].
For NCbiPhM and NCbiPhOSpM monomers, more rigid than the others, both
methacrylates with one nitrile group and two aromatic rings, the crystalline form
melts and partially crystallises on cooling. The NCbiPhOSpM melts at Tf ¼ 78.7ꢀC
with DfH ¼ 102.9 J gꢁ1 and crystallises on cooling at 96.9ꢀC with an enthalpy of
ꢁ5.9 J gꢁ1. The partial amorphous solid formed melts at 100.7ꢀC with an enthalpy
of 5.5 J gꢁ1 and partially crystallises again on cooling at 98.5ꢀC with an enthalpy
of ꢁ5.5 J gꢁ1
.
Table 7. Calorimetric data from DSC for the synthesised
monomers
Monomer reference
Tf=ꢀC
DfH=Jgꢁ1
biPhM
NCPhM
107.7
85.0
50.0
35.9
123.2
78.7
ꢁ3.6
18.9
91.8
129.4
129.7
48.2
NCPhOSpM
biPhOSpM
NCbiPhM
NCbiPhOSpM
PhOSpM
PhM
53.5
102.9
115.5
109.0

140=[662]
A. Mouquinho et al.
All the DSC results are available in the Supporting Information file, accessed via
Internet.
3.4. Electro-Optical Properties
All these polymer=E7 (30=70 w=w) show a very poor electro-optical response. This is
due to the low mobility of the mesogenic parts of the polymeric matrix compared
with the high mobility of the E7 molecules. Thus, with application of an external
electric field there is only the reorientation of E7 molecules along the direction of
electric field. Because of the high birefringence of liquid crystal molecules, it will
strongly scatter incident light when mesogenic parts of the polymeric matrix are
not oriented with the electric field but E7 molecules are.
4. Conclusions
In conclusion, we presented a set of new functionalized monomers that were used
with E7 to obtain PDLC films. The selected monomers were aromatic methacrylates
and they were thermal- and photo-polymerised. The resulting materials were charac-
terised by several structural, calorimetric and optical methods. Although the chemi-
cal similarity between the E7 used as liquid crystal and the matrix network, no
electro-optical response was detected for these PDLC films. This was due to the lack
of mobility of the mesogenic part of the polymeric matrix and it’s no reorientation
along the direction of applied electric field.
Acknowledgment
ˆ
This work was supported by Fundac¸a˜opara a Ciencia e Tecnologia through the
project PTDC=CTM=69145=2006.
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