Synthesis and mesophase characterization of novel methacrylate based thermotropic liquid crystalline monomers and their polymers

Article abstract of DOI:10.1039/c3nj01414b

A new series of side chain methacrylate monomers with a three phenyl ring core connected by an ester with a terminal alkoxy chain and hexamethylene spacer are synthesized by a multistep synthetic route. The corresponding polymers are realized by free radical solution polymerization. Two structurally similar mesogens are also synthesized for use as models to study the influence of the methacrylic unit on the mesophase characteristics. The molecular structure of the intermediates, monomers as well as the polymers is unambiguously confirmed by Fourier-transform infrared (FT-IR), solution ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy and by elemental analysis. The mesophase characteristics of all of the monomers and polymers are determined by hot-stage optical polarized microscopy (HOPM) and differential scanning calorimetry (DSC). These investigations revealed the existence of enantiotropic nematic (N), smectic A (S_A) and smectic C (S_C) mesophases. Furthermore, for a representative monomer and polymer, the presence of the smectic phase is confirmed by variable temperature X-ray diffraction (XRD) where a characteristic layer ordering is noticed. The molecular weight of the polymers is determined by gel permeation chromatography (GPC) and the values are found to be typically in the range of 2.0 × 10³ to 3.7 × 10³. The mesogenic polymers are also found to be stable up to 320 °C by thermogravimetric analysis (TGA). the Partner Organisations 2014.

Full text of DOI:10.1039/c3nj01414b

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Synthesis and mesophase characterization of
novel methacrylate based thermotropic liquid
crystalline monomers and their polymers†
Cite this: New J. Chem., 2014,
38, 4357
G. Siva Mohan Reddy,‡*a T. Narasimhaswamyb and K. Mohana Raju*a
A new series of side chain methacrylate monomers with a three phenyl ring core connected by an ester
with a terminal alkoxy chain and hexamethylene spacer are synthesized by a multistep synthetic route.
The corresponding polymers are realized by free radical solution polymerization. Two structurally similar
mesogens are also synthesized for use as models to study the influence of the methacrylic unit on the
mesophase characteristics. The molecular structure of the intermediates, monomers as well as the
1
polymers is unambiguously confirmed by Fourier-transform infrared (FT-IR), solution H and 13C nuclear
magnetic resonance (NMR) spectroscopy and by elemental analysis. The mesophase characteristics of all
of the monomers and polymers are determined by hot-stage optical polarized microscopy (HOPM) and
differential scanning calorimetry (DSC). These investigations revealed the existence of enantiotropic
nematic (N), smectic A (SA) and smectic C (SC) mesophases. Furthermore, for a representative monomer
and polymer, the presence of the smectic phase is confirmed by variable temperature X-ray diffraction
(XRD) where a characteristic layer ordering is noticed. The molecular weight of the polymers is
determined by gel permeation chromatography (GPC) and the values are found to be typically in the
range of 2.0 ꢀ 103 to 3.7 ꢀ 103. The mesogenic polymers are also found to be stable up to 320 1C by
thermogravimetric analysis (TGA).
Received (in Montpellier, France)
13th November 2013,
Accepted 22nd May 2014
DOI: 10.1039/c3nj01414b
www.rsc.org/njc
the glass transition temperature (Tg) and mesophase tempera-
tures have been reported.16 For example, the Tg and mesophase
Introduction
Thermotropic side chain liquid crystalline polymers (SCLCPs) transitions have been found to vary from a very low temperature
were discovered in the 1970s by various research groups.1–3 (0 1C) to a high temperature (200 1C) depending on the chemical
These are usually designed to have a polymeric group as the structure of the polymer backbone.17 Furthermore, extensive
back bone and a mesogenic core as a side chain. Among the investigations covering the influence of the molecular weight
polymer backbones utilized for constructing SCLCPs, acrylic/ on the mesophase transitions, the evolution of birefringence,
methacrylic and siloxane units are most popular.4–6 Later, side differential scanning calorimetry and variable temperature X-ray
chain liquid crystalline polymers based on the vinyl group also studies for mesophase corroboration have also been reported on
came in to existence.7 As far as the side chain core is concerned, SCLCPs to establish the structure–property correlation.18–21
mostly two ring systems either with ester, imine or azo links etc.
The development of SCLCPs further paved the way for
are used for constructing SCLCPs.8–15 Extensive studies on thermotropic liquid crystalline elastomers (LC elastomers) in
these polymers examining the influence of the backbone on which the mesogenic monomers are one of the key compo-
nents.22 LC elastomers are often prepared by reacting a meso-
genic monomer with a non-mesogenic cross-linking agent.23,24
a Department of Polymer Science & Technology, Sri Krishnadevaraya University,
However, studies have also shown mesogenic bifunctional
Anantapur 515003, India. E-mail: smreddyg@gmail.com,
monomers serving as cross-linking agents.25,26 It is well estab-
kmohanaraju2020@gmail.com
b Polymer Laboratory, CSIR-Central Leather Research Institute, Adyar,
lished from the studies of low molecular weight liquid crystals
that the size of the core (aspect ratio) plays a crucial role not
only in influencing the mesophase stability but also in deter-
mining the nature of the phase.27 Conversely, research invol-
ving the variation of core length in liquid crystalline polymers is
very limited.28,29 In recent years there has been growing interest
in methacrylate based side chain liquid crystals as they are key
Chennai 600 020, India. E-mail: tnswamy99@hotmail.com
† Electronic supplementary information (ESI) available: General procedure for
the synthesis of intermediates, monomers and polymers and DSC data of model
mesogens. See DOI: 10.1039/c3nj01414b
‡ Current address: Chemical, Metallurgical and Material Engineering, Polymer
Technology Division, Tshwane University of Technology, Pretoria, South Africa.
ReddySM@tut.co.za.
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components for the development of liquid crystalline elasto- calorimetry (DSC) traces were recorded on a DSC Q200 instrument
mers which are poised for use as artificial muscles.30 These with a heating rate of 10 1C per minute in a nitrogen atmosphere.
elastomers are also expected to find application in actuators, Thermogravimetry analyses of the polymer samples were carried
membranes and sensors.31 Interestingly, the liquid crystalline out on a Q-10 series instrument in a nitrogen atmosphere at a
elastomers make use of three phenyl ring based monomers heating rate of 10 1C minꢁ1, in the temperature range of 30–
either as the main component or a cross-linking agent.24,25 800 1C. The weight average and number average molecular
In view of these current developments, in this work, focus is weights of the polymers were determined using a WATERS gel
placed on the incorporation of a three phenyl ring core in the permeation chromatograph model BREEZE Million-3 fitted with a
side chain to investigate its influence on the mesophase DRI detector. Tetrahydrofuran was used as the mobile phase at a
transition temperatures. Furthermore, to establish the role of flow rate of 1 mL minꢁ1 at 30 1C. The molecular weights were
a methacrylate back bone on the mesophase characteristics, a calculated from the calibration curve plotted, using linear poly-
three ring model mesogen with a similar core is also examined. styrene standards, 1000–60 000. Small-angle X-ray scattering peaks
The investigation mainly focuses on the synthesis of mesogenic were measured with an evacuated high performance SAXS instru-
monomers, polymers and model compounds and their meso- ment ‘‘SAXSess’’ (Anton Paar KG, Graz, Austria). The ‘‘SAXSess’’
phase as well as their structural characterization. It should be was attached to a conventional X-ray generator (Philips, Holland)
mentioned that much of the work reported in the literature equipped with a sealed X-ray tube (Cu anode target type, produ-
deals with two phenyl ring based side chain liquid crystal cing CuKa X-rays with a wavelength of 0.154 nm), operating at
polymers.1–6 In contrast to them, SCLCPs with a three ring 40 kV and 50 mA. The samples were measured using the paste
core show better mesophase stability and as a consequence cell method. The scattered X-ray intensities were detected with a
tolerate non-mesogenic cross-linkers in the preparation of 2D-imaging plate detection system, Cyclone (Packard, A Packard
elastomers which are then used for the design of liquid crystal Bioscience Company), with a spatial resolution of 50 ꢀ 50 mm per
elastomers.32–35
pixel at a sample to detector distance of 265 mm. Scattering data
read from the imaging plate were first corrected for the absorption
of the X-rays in the sample and transformed to the q scale (q = 4p/
lꢂsin y/2; program SAXS Quant; Anton Paar KG, Graz, Austria). The
SAXS measurements were further corrected with the empty cell
measurements.
Experimental
Materials
4-Hydroxy methyl benzoate, benzyl bromide, 6-chlorohexanol, 4-
ethoxy phenol, 4-butoxy phenol, 4-hexyloxy phenol, 4-benzyloxy
phenol, methacrylic anhydride, palladium on charcoal (Pd/C,
10%), N,N0-dicyclohexylcarbodiimide (DCC) and N,N-dimethyl-
aminopyridine (DMAP) were purchased from Aldrich, USA and
used without further purification. N,N0-Dimethylformamide
(DMF), tetrahydrofuran (THF), ethanol and methanol were
purchased from SD Fine, Mumbai. Dichloromethane (DCM), ethyl
acetate, diethyl ether, n-hexane, ethyl methyl ketone (MEK), acet-
one, acetonitrile, isopropanol, 1,4-dioxane, triethylamine, potas-
sium hydroxide (pellets), Celite-540, anhydrous potassium
carbonate, anhydrous sodium sulphate, silica gel (100–200 mesh)
were obtained from Merck, India and used as received.
Results and discussion
Synthesis of monomers and polymers
The mesogenic monomers and polymers are synthesized by a
multistep strategy as depicted in Scheme 1. The synthetic
protocols and the analytical data of the intermediates, model
mesogens, monomers and polymers are given in the ESI.† The
methacrylate based hexamethylene spacer containing mono-
mers and polymers with a three phenyl ring core and terminal
alkoxy groups at one end are synthesized as depicted in
Scheme 1. Initially, the 4-hydroxyl methyl benzoate is O-alky-
lated with 6-chlorohexanol in a basic medium. The resulting
product (4) is hydrolyzed with a base to produce hydroxyhex-
Instrumental details
FT-IR spectra of all of the compounds were recorded by an ABB yloxy benzoic acid (5). Upon the reaction of methacrylic anhy-
BOMEM MB3000 spectrometer using KBr pellets. 1H and 13C dride with 5 in the presence of DMAP, the key intermediate (6)
NMR spectra of the compounds in CDCl3 were run on a JEOL was produced with a methacrylate unit at one end. Finally, the
500 MHz instrument at room temperature using tetramethyl- esterification of the methacrylate intermediate with two ring
silane as an internal standard. The resonance frequencies of 1H alkoxy phenols yielded mesogenic monomers 7(a–f). The
and 13C were 500.15 and 125.76 MHz, respectively. The EI mass synthesised monomers differ in their terminal chain length
spectra were recorded using a JEOL DX-303 spectrometer. and consist of even carbons from C2–C12. All of the mesogenic
Elemental analyses were recorded using an Elemental Analyzer monomers are polymerized in solution using benzoyl peroxide
for CHNS, Model-Euro EA 3000, Euro Vector S.P.A. The nature as an initiator at 70 ꢃ 1 1C for 24 h under a N2 atmosphere. The
of the mesophase and the temperature of the occurrence of polymers are recovered from their solution in the form of a
various phases were examined using an Olympus BX50 optical white precipitate by pouring into excess methanol. The model
polarizing microscope equipped with a Linkam THMS 600 stage mesogens, 9(a,b), are also prepared without a methacrylic
with a TMS 94 temperature controller. The photographs were taken group as per Scheme 2, to examine the influence of the
using an Olympus C7070 digital camera. Differential scanning methacrylic group on the mesophase properties.
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Scheme 1 Synthetic strategy of the three ring monomers and polymers. (A) DMF, K2CO3, 90 1C; KOH, EtOH, H2O, reflux; (B) TEA, MEK, 0 1C, 3 h; (C) Pd/
C, H2, THF, RT, 24 h; (D) Cl-(CH2)6-OH, DMF, K2CO3, 90 1C; (E) KOH, EtOH, H2O, reflux; (F) methacrylic anhydride, DMAP, TEA, DCM, 0 1C; (G) DCC,
DMAP, DCM/THF, 0 1C, 24 h, and (H) BPO, 1,4-dioxane, 70 1C, 24 h.
Fig. 1 FT-IR spectrum of (A) monomer 7b and (B) polymer 8b.
Scheme 2 Synthetic strategy of the model mesogens.
symmetric C–O–C stretchings of the ether and ester groups
present in the monomer. These features are in line with the
molecular structure of the monomer. The FT-IR spectrum of the
Spectral characterization
The FT-IR spectra of a representative monomer and polymer polymer (Fig. 1B), on the other hand, shows similar features to
are shown in Fig. 1. The C–H stretching of the aromatic rings at the monomer except the absorption at 1632 cmꢁ1 which is due
3071 cmꢁ1 and those of the aliphatic units are noticed at 2942 to the CQC bond in the methacrylate unit, a clear indication of
and 2968 cmꢁ1 (Fig. 1A). The high intensity absorption peak at the formation of the polymer.
1733 cmꢁ1 corresponds to the characteristic CQO stretching
Fig. 2 shows the H and 13C NMR spectra of the representa-
1
of the ester carbonyls in the monomer. The CQC of the tive monomer and polymer. The assignment of each signal to the
methacrylate unit is seen at 1634 cmꢁ1 as a low intensity peak corresponding proton and carbon is carried out by comparing
whereas the CQC skeletal vibrations of the phenyl rings are the spectrum with the iterated spectrum obtained using the ACD
noticed at 1604 and 1587 cmꢁ1. The absorptions observed at chemsketch software, the results are presented in the ESI.† The
1267, 1244, 1193 and 1162 cmꢁ1 are due to the asymmetric and mesogenic monomer contains four sets of protons arising from
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Mesophase transitions by HOPM and DSC investigations
The model mesogens, monomers and their polymers are subjected
to HOPM investigation to identify the presence of a mesophase,
their transitions as well as the mesophase stability. The results are
summarized in Table 1 along with the DSC data. From Table 1, it is
evident that all of the synthesized compounds exhibit enantiotro-
pic mesophases. In the case of monomer 7a, nematic and SA
phases are observed during heating and cooling cycles in HOPM.
The sample, on cooling from the isotropic melt, exhibited sphe-
rical birefringent droplets and threaded textures (Fig. 3A) which
are typical of a nematic phase. Upon further cooling, homeotropic
domains with fan textures typical of the SA (Fig. 3B) phase are
noticed. For the monomers 7b, 7c, 7d and 7e, in addition to
nematic and SA mesophases, a SC phase is also observed during
the cooling cycle (Fig. 3C and D). These mesophases are confirmed
by noticing birefringent droplets, focal conic fans and schlieren
textures typical of nematic, SA and SC phases, respectively. A similar
phase sequence is also noticed for monomer 7f (Fig. 4A–C). Upon
continuous cooling of the sample, prior to crystallization, transition
bars on the focal conic fans are noticed which are a characteristic of
a crystal E phase (Fig. 4D).
The synthesized monomers only differ in their terminal
chain length, which varies from C-2 to C-12 carbons (even
carbons only). When increasing the terminal chain length of
the monomer from C-2 to C-12, a gradual decrease in the
isotropic temperature is observed. The nematic, SA phases are
Table 1 HOPM and DSC data of monomers upon heating
Transition
Sample observed
Fig. 2 (A) 1H NMR spectrum of monomer 7a and polymer 8a; (B) 13C NMR
spectrum of monomer 7a and polymer 8a.
Transition
DH
Phase observed
by HOPM
code
by DSC
temperature (1C) (kJ molꢁ1
)
7a
Cr–SA
SA–N
N–I
94.0
130.0
223.0
35.18
0.18
0.31
Smectic A
Nematic
Isotropic
phenyl rings, aliphatic groups including the methyl group of the
methacrylate unit, the oxymethylene group and the methylene
group of the methacrylate unit, which are clearly identified in the
7b
7c
7d
7e
7f
Cr–SC
SC–SA
SA–N
N–I
66.8
101.0
140.0
180.0
42.0
0.47
0.31
—
Smectic C
Smectic A
Nematic
1
spectrum. Indeed, the H NMR spectrum shows signals in the
Isotropic
region of 6.9–8.3 ppm for aromatic protons, 3.9–4.1 ppm for
oxymethylene protons, 0.9–2.0 ppm for aliphatic protons and
two singlets at 6.1 and 5.4 ppm for the methylene protons of the
methacrylate unit. The spectral features such as chemical shift
values, multiplicity and peak integrations are very much in line
with the structure of the monomer. For the mesogenic polymer,
the disappearance of the two singlets at 5.4 and 6.1 ppm
associated with the methylene group of the methacrylate unit
and the appearance of new signals at 1.8 ppm for a polymeric
back bone (–CH2–) are distinctly seen. Furthermore, the broad-
ening of signals in contrast to the monomer spectrum, owing to
the viscous nature of the solution as well as configurational
heterogeneity, confirms the formation of the polymer. These
trends are also supported by the 13C NMR spectral data and
accordingly, the disappearance of two signals for the CQC of the
methacrylic unit at 125 and 136 ppm and the appearance of a
new low intensity signal at 45.9 ppm for the polymeric back bone
methylene carbon are observed.
Cr–SC
SC–SA
SA–N
N–I
72.0
101.7
145.5
172.0
31.8
0.05
0.65
1.03
Smectic C
Smectic A
Nematic
Isotropic
Cr–SC
SC–SA
SA–N
N–I
69.2
101.5
149.9
166.0
34.8
0.15
1.07
0.90
Smectic C
Smectic A
Nematic
Isotropic
Cr–SC
SC–SA
SA–N
N–I
61.0
93.6
152.03
162.0
26.58
0.03
1.65
0.99
Smectic C
Smectic A
Nematic
Isotropic
Cr–SC
SC–SA
SA–N
N–I
63.0
101.3
153.2
154.5
56.05
0.20
4.85
Smectic C
Smectic A
Nematic
Isotropic
I: isotropic, N: nematic, SA: smectic A, SC: smectic
crystallization.
C and Cr:
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Fig. 3 HOPM photographs of the monomers upon cooling from the
isotropic phase (A) threaded nematic phase (7a) at 202 1C, (B) smectic A
phase (7a) at 136.4 1C, (C) transforming to smectic A phase from nematic
phase (7b) at 142 1C and (D) smectic C phase (7b) at 49 1C.
Fig. 5 HOPM photographs of polymers on cooling from the isotropic
phase (A) threaded nematic phase of 8a at 242.3 1C, (B) broken fan texture
of smectic C phase of 8a at 170.5 1C, (C) batonnets of smectic A phase of
8d at 220 1C and (D) broken fan texture of smectic C phase of 8d at 214 1C.
The HOPM examination of the mesogenic polymers also
revealed nematic and smectic phases with the variation in
the transition temperatures. The mesophase identification is
carried out upon cooling the sample from the isotropic melt.
During the texture examination, the cover slip of the sand-
wiched sample is pressed at regular intervals to determine the
fluidity. In the cooling cycle, except for the polymer 8b, crystal-
lization is not observed and instead, the samples showed frozen
mesophase textures typical of a glassy nature. In the case of
polymer 8a, 8b and 8c, birefringent threaded and broken fan
textures are noticed for nematic (Fig. 5A) and SC (Fig. 5B)
phases, respectively. For all of the three cases, the broken fan
texture is retained down to room temperature (25 1C). However,
upon pressing the cover slips, the fluidity could only be
observed over 100 1C and below that it is not noticed. For the
polymer 8d, only SA and SC phases are seen in contrast to the
lower homologues where the nematic phase is also observed.
These polymers also showed batonnets (Fig. 5C) and broken fan
textures (Fig. 5D), these textures are retained even at room
temperature. In the case of the polymer 8f, the disappearance of
Fig. 4 HOPM photographs of 7f upon cooling from the isotropic phase
(A) transition from nematic to focal conic smectic A at 156.2 1C, (B) focal
conic smectic A phase at 154.2 1C, (C) focal conic smectic A phase at 95 1C
and (D) crystal E at 30.2 1C.
observed for the ethoxy homologue, for the butoxy homologue, the SA phase is observed and it only showed a schlieren and
the appearance of a SC phase is additionally seen. The same broken fan texture typical of the SC phase. These observations
trend continues for higher homologues in the series. Interest- indicate that improved mesophase stability is noticed for the
ingly, in contrast to the other homologues, the C-12 exhibited a polymers in contrast to the monomers. Furthermore, the
monotropic crystal E phase. These trends are commonly disappearance of the nematic phase for the higher homologues
observed for calamitic mesogens in which the core length is is noted for mesogenic polymers whereas for monomers, the
kept constant and the terminal chain length is increased. For nematic phase is seen even for higher homologues.
the model mesogens (9a and 9b) the appearance of nematic, SA
The mesophase assignment of monomers and polymers are
and SC phases is also confirmed (table given in the ESI†). In confirmed by DSC (Fig. 6) where in addition to the transition
contrast to the model mesogens, the monomer homologues temperatures, a change in the enthalpy (DH) values is also
exhibited a drastic decrease in their melting and clearing determined. Initially, the DSC experiments of the mesogenic
temperatures with
a
similar mesophase sequence. This monomers are carried out with a heating rate of 10 1C minꢁ1
indicates that the incorporation of a methacrylic unit favors a (Fig. 6A). This led to broad exothermic peaks in the heating
reduction in the melting temperature without affecting the scans suggesting an onset of polymerization owing to the
mesophase sequence.
presence of a reactive double bond. Subsequently, the heating
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Table 2 HOPM and DSC data of the polymers on cooling
Transition
Sample observed
Transition
DH
Phase observed
by POM
code
by DSC
temperature (1C) (kJ molꢁ1
)
8a
I–N
N–SC
Tg
255
175
42
1.11
0.12
Nematic
Smectic C
Glass transition
8b
8c
8d
8e
8f
I–N
N–SC
Cr
236
186
54
1.21
0.14
1.03
Nematic
Smectic C
Crystallization
I–N
N–SC
Tg
218
192
54
1.19
0.87
Nematic
Smectic C
Glass transition
I–SA
SA–SC
Tg
218
214
55
4.97
4.87
—
Smectic A
Smectic C
Glass transition
I–SA
SA–SC
Tg
228
221
55
Smectic A
Smectic C
Glass transition
Fig. 6 Differential scanning calorimetry traces of (A) the monomers
during a heating cycle (B) the polymers during a heating cycle and
(C) the polymers during a cooling cycle.
I–SC
Tg
225
54
Smectic C
Glass transition
rate increased to 30 1C minꢁ1 to overcome this problem. Hence,
the transition temperatures and the associated enthalpy of the
1st heating cycle are considered for the discussion. Table 1 lists
the transition temperatures, phase stability and transition
enthalpy values of the monomers, determined by the DSC
investigation. The appearance of multiple signals in the 1st
heating cycle for all of the monomers suggests the occurrence
of polymesomorphism which supports the HOPM observations.
The transition enthalpy values of the melting temperatures are in
the range of 30–40 kJ molꢁ1, whereas the nematic to isotropic (I)
transition value was found to be B1 kJ molꢁ1. However, for the SA
to SC transition, a very low enthalpy value is observed indicating a
second order transition associated with this phase change.
Fig. 7 X-ray diffraction pattern of the (A) monomer 7f and (B) polymer 8b.
HOPM and DSC. For a representative mesogen 7f and polymer
For the mesogenic polymers which show a glass transition 8b, the diffraction studies are carried out. Fig. 7 shows the
when cooling down from the mesophase, the identification of the diffraction patterns of 7f as a function of temperature in
phase change is difficult in HOPM. In DSC, on the other hand, it is different mesophases. The spectrum recorded at 40 1C (spec-
possible to identify both the first order transition (mesophase trum not given) showed sharp peaks at small angle and wide
transition) as well as the glass transition temperature (second angle regions, which confirms the crystalline nature. At 70 1C,
order transition). Accordingly, upon cooling the polymers from the the diffractogram exhibits a very sharp reflection at 2y = 1.391
mesophase, glass formation is detected. In the present investiga- and a broad peak at the wide angle region. The disappearance
tion, the polymers are subjected to DSC (Fig. 6B and C) experi- of a large number of peaks and the presence of a very sharp
ments with a heating rate of 10 1C minꢁ1. The data obtained from intense reflection at 2y = 1.391 indicates lamellar ordering
the 2nd cooling cycle are considered for discussion. Table 2 lists typical of smectic mesophases.36,37 The broad peak at the wide
the transition temperatures and the transition enthalpy values angle region indicates the liquid-like ordering of terminal
obtained from the DSC measurements. It is clear from Table 2 that chains. The spectrum obtained at 135 1C shows a similar
the enthalpy values of the I–N transitions are B1.1 kJ molꢁ1 diffraction pattern to the spectrum at 70 1C with a small change
whereas the same values for the I–SA transitions are found to be in the ‘d’ value. The ‘d’ value for the 70 1C measurement is
B4.9 kJ molꢁ1. Furthermore, the glass formations in these poly- 63.48 Å, whereas for 135 1C the value is found to be 61.27 Å.
mers are found in the range of 42–54 1C.
Furthermore, the length (L) of the monomer is found to be 44 Å
from the space-filled model. Thus the d/L ratio is B1.4 which
indicates the interdigitation of the molecules in the smectic
Powder XRD studies
The powder X-ray diffraction technique is also employed to phases.38,39 Upon increasing the temperature to 160 1C, the
confirm the existence of smectic mesophases as observed by disappearance of a small angle sharp reflection is noticed while
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Table 4 Molecular weight (g molꢁ1) and dispersity data of the polymers
the broad peak at a wide angle is intact suggesting an isotropic
phase. The XRD scan of the polymer 8b obtained at 180 1C shows
a sharp reflection at the small angle region and a broad hump at
the wide angle region similar to 7f. These features confirm the SC
phase as observed from the HOPM. Thus the XRD data are very
much in line with the HOPM and DSC observations.
Sample Weight average
Number average
code
molecular weight (Mw) molecular weight (Mn) Polydispersity
8b
8c
8d
8e
8f
30 200
21 800
31 900
37 700
35 400
16 500
12 000
18 900
22 900
22 700
1.83
1.81
1.68
1.64
1.55
Thermogravimetric analysis
The thermal stabilities of the synthesized polymers (Fig. 8) are
evaluated by thermogravimetric analysis where the samples are
subjected to a heating rate of 10 1C minꢁ1. The temperature
corresponding to the initial degradation (IDT), 50% decompo-
sition and final degradation (FDT) are listed in Table 3. The IDT
temperatures of the polymers are in the range of 320–370 1C,
while the typical 50% decomposition is at B450 1C and the FDT
values are in the range of 460–495 1C. Since the structure of the
polymer only differs at the terminal chain, the variation in the
decomposition temperatures of the polymers is not very high.
A comparison of the DSC and TGA data indicates that the
clearing temperatures of the polymers are far below the initial
decomposition temperature. The observed thermal stabilities
of the polymers suggest that the lengthy core with ester
connecting units favors high thermal stability.
GPC in order to determine the molecular weight and the
dispersity values (Table 4). The weight average molecular
weight hMwi is in the range of 21 800–37 700 g molꢁ1, while
the dispersity index is found to be in the range of 1.55–1.83.
This indicates that the three ring monomers are more reactive,
due to the presence of the a-methyl group in the backbone. The
presence of an electron releasing methyl group in the monomer
unit enhances the electron density around the double bond,
thereby facilitating efficient polymerization. The dispersity
index values obtained are in line with those typically observed
for polymers made by free radical initiated polymerization.40
Conclusion
A series of methacrylate monomers with three phenyl ring
systems, their polymers and their model mesogens are synthe-
sized in order to establish a structure–property relationship. All
of the synthesized monomers, polymers and the models exhib-
ited enantiotropic liquid crystalline phases. The polymers with
short terminal chains exhibited nematic and smectic phases
whereas the increase in terminal chain length resulted in the
disappearance of the nematic phase and stabilization of the
smectic phases only. An important observation was made with
respect to the melting and clearing temperatures where a
substantial reduction was noticed after the introduction of a
methacrylic unit in the monomer. Interestingly, the mesophase
sequence was not altered despite the reduction in the melting
and clearing temperatures. The hot stage optical polarized
microscopy and differential scanning calorimetry findings were
further confirmed by X-ray diffraction studies where layer
ordering typical of smectic mesophases was observed. Further-
more, the d/L ratio indicated the interdigitation of molecules in
the smectic mesophases. The molecular weights of the poly-
mers were in the range of 2.1 ꢀ 103 to 3.7 ꢀ 103. Thermogravi-
metric analysis revealed that the polymers are thermally stable
up to 320 1C and the clearing temperatures are far below the
initial degradation temperature.
Gel permeation chromatography
Except polymer 8a, which is not soluble in THF at room
temperature, all of the other polymers are characterized by
Fig. 8 TGA thermograms of polymers 8a–8e.
Table 3 TGA data of polymers
Acknowledgements
Sample code
IDT (1C)
50% DT (1C)
FDT (1C)
GSMR and TNS sincerely thank Dr A. B. Mandal, Director, CSIR-
CLRI for his keen interest and constant support in this work.
GSMR also wishes to thank the Department of Science and
Technology (India) for financial support, in the form of a
fellowship. The authors are also thankful to Dr B. V. N.
Phanikumar, CSIR-CLRI, for his help in NMR measurements.
8a
8b
8c
8d
8e
8f
339
359
319
355
369
354
419
418
404
418
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493
486
477
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