Synthesis of Novel Liquid Crystalline Polyesters of Cholesterol: Thermal, Electrical Conductivity and Dielectric Properties

Article abstract of DOI:10.1080/15421406.2014.990681

Novel liquid crystalline polyesters of cholesterol, Polycholesteryl-3,3-dimethylacrylate (PCHDMA) and Polycholesterylcrotonate (PCHC) are synthesized by free radical polymerization and characterized by FT-IR, ¹H-NMR, and GPC analysis. The thermal properties are studied by TGA and DSC techniques. The liquid crystalline phases are detected under hot stage optical microscope. Electronic conduction in the polymers is determined by Wagner polarization technique. There is about 5000 fold decrease in the impedance values of both PCHDMA and PCHC on increasing the frequencies from 40 Hz to 1 MHz at 30°C. Amperometric analysis of the polymers are also carried out at different temperatures and frequencies.

Full text of DOI:10.1080/15421406.2014.990681

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis of Novel Liquid Crystalline Polyesters of
Cholesterol: Thermal, Electrical Conductivity and
Dielectric Properties
Priyanka Dutta, Porimita Saikia, Samiul Hoque, Narendra Nath Dass &
Neelotpal Sen Sarma
To cite this article: Priyanka Dutta, Porimita Saikia, Samiul Hoque, Narendra Nath Dass &
Neelotpal Sen Sarma (2015) Synthesis of Novel Liquid Crystalline Polyesters of Cholesterol:
Thermal, Electrical Conductivity and Dielectric Properties, Molecular Crystals and Liquid
Crystals, 616:1, 112-122, DOI: 10.1080/15421406.2014.990681
To link to this article: http://dx.doi.org/10.1080/15421406.2014.990681
Published online: 25 Sep 2015.
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Date: 17 October 2015, At: 00:16

Mol. Cryst. Liq. Cryst., Vol. 616: pp. 112–122, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.990681
Synthesis of Novel Liquid Crystalline Polyesters
of Cholesterol: Thermal, Electrical Conductivity
and Dielectric Properties
PRIYANKA DUTTA,1 PORIMITA SAIKIA,1 SAMIUL HOQUE,2
NARENDRA NATH DASS,1 AND NEELOTPAL SEN SARMA1,∗
1Physical Sciences Division, Polymer Section, Institute of Advanced Study in
Science and Technology, Guwahati, Assam, India
2Department of Chemical Science, Gauhati University, Guwahati, Assam, India
Novel liquid crystalline polyesters of cholesterol, Polycholesteryl-3,3-dimethylacrylate
(PCHDMA) and Polycholesterylcrotonate (PCHC) are synthesized by free radical poly-
1
merization and characterized by FT-IR, H-NMR, and GPC analysis. The thermal
properties are studied by TGA and DSC techniques. The liquid crystalline phases are
detected under hot stage optical microscope. Electronic conduction in the polymers is
determined by Wagner polarization technique. There is about 5000 fold decrease in the
impedance values of both PCHDMA and PCHC on increasing the frequencies from
40 Hz to 1 MHz at 30◦C. Amperometric analysis of the polymers are also carried out at
different temperatures and frequencies.
Keywords Conductivity; dielectric constant; hot stage microscope; liquid crystalline
polymers; transport number
1. Introduction
Liquid crystalline polymers (LCPs) are prospective candidates for photonic materials to
be used in integrated circuit as color switch. For fast electronic response, high frequency
photonic switching becomes essential now. Since the transition temperature of liquid crys-
talline phase is composition specific, the thermo switch can also be visualized. Cholesteric
LCPs have significant applications in optical electronics since they can selectively reflect
different frequency ranges of incoming polarized light [1]. Thus they can exhibit differ-
ent colors in the visible region and this property can be utilized in developing different
electro-optical devices and paint technology [2–7]. Cholesteric LCPs also find applica-
tions in biotechnology because of their significant mechanical properties. Biodegradable
cholesterol polyesters can be used as membranes in various biological applications [8–10].
Synthesis of various side chain cholesteric liquid crystal polymers have been reported
recently [11–14]. Thermotropic liquid crystalline phases are shown by various esters of
∗Address correspondence to Neelotpal Sen Sarma, Physical Sciences Division, Polymer Section,
IASST, Paschim Boragaon, Guwahati- 781 035, Assam, India. E-mail: dr.neelot@gmail.com
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
112

Liquid Crystalline Polyesters of Cholesterol
113
cholesterol in specific temperature range [15]. Liquid crystals also exhibit interesting vis-
coelastic properties which is useful for selection of suitable materials for display devices.
Liquid crystals because of their anisotropic optical and dielectric properties are used in
developing liquid crystal displays (LCDs) at low voltages. LCDS are the lowest electrical
power consuming devices which are applied in the development of various devices like lap-
top computer screens, fax machines, calculators, screens for color TVs, portable compact
disc players, [16] and so on.
Earlier, some sulfonated cholesteryl derivatives were successfully synthesized and
characterized in our lab [17]. In the present study, we have synthesized and characterized
some cholesteric liquid crystal polyesters from their mesogenic monomers Cholesteryl-
3,3-dimethylacrylate and Cholesterylcrotonate. The thermal properties are studied with the
help of DSC and TGA studies and their mesogenic phases are detected under a hot stage
fitted with polarizing microscope. The main aim of this study is to determine the effect of
different frequency and temperatures on the conductivity and dielectric properties of these
liquid crystalline polyesters. Such study may be useful in selecting suitable application of
these polymers as liquid crystal displays or in various other electronic and optical devices.
2. Experimental Details
2.1. Materials
Cholesterol (Merck), 3,3-dimethylacrylic acid (Merck), crotonic acid (Merck) Ethyl-
methylketone (Merck), Chloroform (Ranbaxy), methanol (Ranbaxy), AIBN (Merck) were
used without further purification.
2.2. Techniques
The FT-IR spectra was recorded using Brucker Vector 22 Spectrophotometer in the range
of 400–4000 cm−1. Samples for FT-IR measurements were prepared in the form of pellets
by mixing about 20 mg of IR spectroscopic grade potassium bromide with about 2 mg of
dried samples. The spectra were recorded in transmission mode over 64 scans. The proton
NMR spectra were recorded using Varian 400 MHz FT-NMR in CDCl3. The molecular
weight of the polymers was determined in THF by GPC system (WATERS 2414).
The Differential Scanning Calorimetric and Thermo Gravimetric Analysis of the com-
pounds were carried out using Perkin Elmer DSC 6000 and TGA 4000, respectively, at
heating rate 10◦C per min with constant N2 flow rate of 20 ml/min. The liquid crystalline
range was detected by the phase change with the help of Mettler Toledo F90 Hot stage
fitted with polarizing optical microscope and camera.
The bulk electrical conductivity of the compounds was evaluated from the complex
impedance admittance plots recorded at different temperatures and frequencies using a
HIOKI 3532-50 LCR HiTESTER. The plots were recorded in the frequency range of 42 Hz
to 1 MHz at temperature range of 35◦C to 70◦C. I–V analysis of the polymers were carried
out in the HIOKI 3532-50 LCR HiTESTER in the frequency ranges 10 kHz, 100 kHz, and
1 MHz and in the temperature range 40◦C–70◦C.
The total ionic transport number, tion was evaluated by the standard Wagner polar-
ization technique [11]. The cell SS/(PCHDMA/PCHC)/SS (SS stands for Stainless Steel)
was polarized by a step potential (about 1V) and the resulting potentiostatic current was
monitored as a function of time. The stainless steel acts as blocking electrodes for the above

114
cell. The tion was evaluated using the formula:
tion = (iT − ie)/iT
P. Dutta et al.
where iT and ie are total and residual current, respectively.
2.3. Synthesis
2.3.1. Preparation of Acid Chlorides. The acid chlorides were prepared by distillation
of 3,3-dimethylacrylic acid/crotonic acid and benzoyl chloride (in 1:2 mole ratios) with
0.05 g of hydroquinone at its boiling point in a 25 cm long distillation column. The receiver
contains hydroquinone and it was kept inside an ice bath. Redistillation gave 65%–72% of
3,3-dimethylacryloyl chloride/crotonyl chloride [18].
2.3.2. Preparation of Monomers (CHDMA/ CHC). One mole of both cholesterol and 3,3-
dimethylacryloyl chloride/crotonyl chloride were dissolved in ethyl methyl ketone (MEK)
and then stirred for 5–6 hrs inside a round bottom flask in the presence of triethyl amine and
small amount of hydroquinone in an ice bath. The reaction mixture was then filtered and
washed several time with distilled water. The organic layer was separated and distilled to
remove ethyl methyl ketone (MEK). The viscous organic liquid on cooling gave cholestryl-
3,3-dimethylacrylate/cholesteryl crotonate which was recrystallized and finally purified by
column chromatography. The product yields were found to be 85.45% and 86%.
2.3.3. Preparation of Polymers (PCHDMA/PCHC). One mole of the mesogenic ester,
CHDMA/CHC and 1 mole% of AIBN were dissolved in chloroform in a flask. The reaction
flask was evacuated by vacuum pump and then heated for 24 hrs at 70◦C. When the reaction
was completed, the reaction mixture was poured in to an acidified methanol solution. The
polymers were then recrystallized from hot methanol containing traces of chloroform.
Finally the residue was washed several times with MEK and dried over dessicator. The
above reactions are represented schematically in Scheme 1.
3. Results and Discussions
3.1. FT-IR Analysis
The IR spectra of CHDMA as shown in Fig. 1A indicates strong peaks at 1720 cm−1
and 1179 cm−1 for symmetric stretching frequency of C O and C O of ester group. The
cholesteryl C C bond is present at 1662 cm−1 both in the monomer and in the polymer.
The ester C O and C-O symmetric stretching band for polymer molecule appears at the
same position at 1720 cm−1 and 1179 cm−1 as is observed in the monomer but the peak for
3,3-dimethylacrylate C C present at 1622cm−1 in the monomer is absent in the polymer
which is the indicative of complete polymerization of the monomer. A comparative study
of the FT-IR spectra of CHDMA and PCHDMA is shown in Fig. 1B. Similarly, the major
peaks observed in CHC are 1727 cm−1, 1662 cm−1, 1625 cm−1, and 1154 cm−1 among
which the crotonate C C bond at 1625 cm−1 is absent in the polymer assuring complete
polymerization [19].

Liquid Crystalline Polyesters of Cholesterol
115
Scheme 1. Schematic representation of all the reactions involved.
1
3.2. H-NMR Analysis
More strong evidence of formation of the polymers is obtained from 1H-NMR spectroscopy
at 500 MHz in CDCl3. The 1H-NMR spectra, as shown in Fig. 1B, of CHDMA gives two
sharp signals at 5.4 ppm and 5.5 ppm due to = CH of the cholesteryl C C and the ester
C C bond. There are two signals at 2.4 ppm and 2.5 ppm due to the two terminal–CH3
groups, respectively of ester residue. The peak at 5.5 ppm due to the C C bond of the ester
group is absent in the PCHDMA which indicates complete polymerization of the monomer
[20].
1
The H-NMR spectra of CHC gives sharp signals at 5.4 ppm and at 6.1 ppm due
to CH of the cholesteryl C C and the ester C C–HCOO bond. The terminal –CH3 group
of the crotonate group gives a sharp signal at 1.7 ppm. The CH proton of the Me–CH C
group gives a sharp signal at 6.5 ppm. In the polymer PCHC, the signals at 6.5 ppm and
6.1 ppm disappears indicating complete polymerization of the monomer. The signal due to
the cholesteryl C C at 5.4 ppm remains as it is in both the polymers indicating that the
cholesteryl double bond remains unaffected during polymerization.

116
P. Dutta et al.
Figure 1. (A) IR spectra of CHDMA and PCHDMA (B) 1H-NMR spectra of CHDMA and
PCHDMA.
3.3. Molecular Weight Determination by GPC
The number average molecular weight (Mn), Weight average molecular weight (Mw) and
the Polydispersity of the polymers is measured by Waters GPC using HPLC grade THF as
the solvent and polystyrene as the standard. All together three standards are used for the
measurement with molecular weight 0.27 × 104, 0.64 × 104, and 480 × 104 g/mol. From
the experimental data, it is found that both the polymers have very high molecular weights
with uniform chain length as the polydispersity is almost 1 in both the cases [21].
For PCHDMA: Mn ≈ 57,400 Daltons, Mw ≈ 69,300 Daltons and PDI = 1.206
For PCHC: Mn ≈ 68,800 Daltons, Mw ≈ 84,000 Daltons and PDI = 1.215
3.4. Thermal Studies by DSC and TGA Thermograms
The DSC plots (Fig. 2A and 2B) seem to consist of two distinct phase transitions. The
glass transition temperature (Tg) for the PCHDMA is at 105◦C and that of PCHC is 96◦C.
The low values of transition enthalpies further confirm the formation of LC order in both
the polymers. The mesophase for the PCHDMA is 105–130◦C and that for PCHC is at
96–125◦C is observed at a lower temperature for the polymer as compared to the monomer.
The phase transition at the higher temperature in the DSC curve is for the isotropic melt,
as observed under the polarizing microscope. The thermal stability of the polyesters are
determined with the help of thermogravimetric analysis. The TGA thermograms in Fig. 2C
show that PCHDMA and PCHC are thermally stable up to about 300◦C with a weight loss

Liquid Crystalline Polyesters of Cholesterol
117
Figure 2. (A) DSC plot of PCHC (B) DSC plot of PCHDMA (C) TGA thermograms.
of 5% and 9%, respectively. Complete degradation of PCHDMA occurs at about 600◦C
and that of PCHC occurs at about 570◦C.
The phase transition temperatures from anisotropy to isotropy of the polyesters
PCHDMA and PCHC are observed under the hot stage optical microscope and pho-
tographed with the camera attached. Small amount of sample is heated under the microscope
for this purpose. The phase transitions shown by the DSC curves for these compounds cor-
relate with those observed under the hot stage microscope. The liquid crystalline phases
of the polyesters PCHDMA and PCHC under the hot stage optical microscope are shown
in Fig. 3. Both PCHDMA and PCHC do not exhibit colors other than green on increasing
the temperature, probably due to the temperature ranges of the chiral nematic phase being
narrow as observed by Pandey et al. in mesogen based on cholesterol derivatives [22].
Figure 3. Liquid crystalline phases of the compounds under Hot stage microscope.

118
P. Dutta et al.
Figure 4. (A) Arrhenius plots of the polymers showing semiconducting behavior, (B) log zꢀ vs. log
f, (C) log zꢀꢀ vs. log f, (D) log zꢀ vs. T, (E) log zꢀꢀ vs. T of PCHDMA.
3.5. Conductivity and Dielectric Studies
The variation of conductivity with increase in temperature of the polymers is investigated
within the temperature range of 35◦C–70◦C and a frequency range of 42 Hz to 1 MHz
at a step temperature of 5◦C. From the plots, it is observed that the conductivity of the
compounds increases nonlinearly with the increase in temperature. The polymers show
small rise in conductivity with the rise in temperature from 1.9 × 10−9 to 3.75 × 10−9
Scm−1 for PCHDMA and 1.08 × 10−9 to 3.93 × 10−9 Scm−1 for PCHC at a temperature
range of 35–70◦C.
3.5.1. Effect of Temperature and Frequency on Impedance Values of PCHDMA and PCHC.
Impedance measurement is also performed on the polymers as a variance of frequency and
that of temperature as shown in Fig. 4. Figure 4B and 4C show the variance of log Z with
log f at different temperatures (40◦C, 50◦C, 60◦C, 70◦C) of PCHDMA and it is observed

Liquid Crystalline Polyesters of Cholesterol
119
Figure 5. (A) εꢀ vs. log f, (B) εꢀꢀ vs. log f, (C) εꢀ vs. T, and (D) εꢀꢀ vs. T.
that impedance values decrease with the increase in frequency. Figure 3D and 3E show
the variance of real and imaginary parts of log Z with different temperatures at different
frequency ranges (100 Hz, 1 kHz, 10 kHz, 100 kHz and 1 MHz) for PCHDMA and it
is seen that impedance values decrease sharply with the increase in frequency. There is
about 4910 and 5100 fold decrease in the impedance values of PCHDMA and PCHC on
increasing the frequencies from 40 Hz to 1 MHz at 30◦C. This shows that there is a huge
rise in conductivity of the compounds with rise in frequency.
3.5.2. Effect of Temperature and Frequency on Dielectric Constant of PCHDMA and
PCHC. The effect of temperature on Dielectric constant of the polymer PCHDMA is
observed by plotting dielectric constant at different temperatures (40◦C, 50◦C, 60◦C, 70◦C)
against log of different frequency values (100 Hz, 1 kHz, 10 kHz, 100 kHz, and 1 MHz)
as shown in Fig. 5A and 5B. The frequency is changed at a particular temperature in equal
intervals and the values of Cp (static capacitance in parallel equivalent circuit mode), D
(tanδ), and G (conductance) are observed. The real part, εꢀ, and the imaginary part, εꢀꢀ of
permittivity are calculated from the equations:
εꢀ = (Cpd)/(εoA)
(1)
and
εꢀꢀ = εꢀ tan δ
(2)

120
P. Dutta et al.
Figure 6. I–V studies of the polymers at frequency ranges (A)10 kHz (B)100 kHz, and (C)1MHz.
Where d is the thickness of the sample; εo = 8.854 × 10−12 F/m and A (effective area)
= πr2 (m2)
Figure 5A shows the change of real part of permittivity with change in frequency
while in Fig. 5B the change of imaginary part of permittivity is observed with increase
in frequency. It is observed that the values of dielectric constant sharply decrease with
increasing frequencies from 100 Hz to 10 kHz while it again increase sharply with the
increase in frequency from 10 kHz to 1 MHz. The nature of all the curves in different
temperatures is similar in nature.
The effect of frequency on dielectric constant is observed by plotting dielectric constant
of the sample at different temperatures (40◦C, 50◦C, 60◦C, and 70◦C) as shown in Fig.
5C and 5D. It is clearly observed that with increase in temperature, the dielectric constant
increases for the compounds in different frequency ranges. This can be explained by the
fact that with increase in temperature, polarization increases which leads to the observed
increase in permittivity.
3.6. Amperomatric Analysis
The influence of applied voltage on the conductivity at different temperatures and frequency
ranges are studied by amperometric analysis. In Fig. 6A–6C, the current passing through
PCHDMA at different temperatures (40◦C, 50◦C, 60◦C, and 70◦C) is studied in frequency
ranges (A)10 kHz, (B)100 kHz, and in (C)1 MHz. The linear increase of conductivity

Liquid Crystalline Polyesters of Cholesterol
121
Figure 7. (A) Schematic representation of the cell circuit used to determine the transport number
(B) Polarized current Vs time plot of 1.PCHDMA and 2.PCHC.
with rise in temperature is verified by Ohm’s law (V = IR). When the applied potential is
increased from 0 V to 5 V, the current passing through PCHDMA is different in different
frequencies. It is clearly observed that with increase in frequency, the current passing
through the samples also increases. At a frequency of 1 MHz, the current passing though
the samples is in the range of mA. This again verifies that the compounds show much
better electrical properties at higher frequencies. The plots (as shown in Fig. 6) are found
to be continuous and no breakdown voltage is apparent for both the compounds within this
observed range.
3.7. Transport Number Determination by Wagner Polarization Method
The total ionic and electronic contribution to the conductivity is determined by standard
Wagner polarization technique, generally used to determine ionic transport number. Ac-
cordingly, on application of voltage to the cell, ionic migration occurs until steady state is
reached. At the steady state, the cell is polarized and any residual current flows because of
electron migration across the sample and interfaces. For ionic samples, the currents passing
through an ion-blocking electrode falls rapidly with time. A cell is prepared to observe
electronic conductivity, keeping the sample (in pellet form) in between two stainless steel
anvils. A step potential of about 1V is then applied and resulting potentiostatic current is
monitored as a function of time using a data loggers and software. The stainless steel anvils
acts as ion blocking electrodes for the above cell. The circuit diagram of the cell is shown
in Fig. 7A.
For the experimental polymers the current vs time plot, Fig. 7B shows no initial decrease
in total current with respect to time mostly observed in an ionic conduction. This observation
shows that the polymers are nonpolar/ nonionic in nature. The plots obtained from Wagner
polarization technique show that there is no ionic contribution for the conduction and the
small amount of conductivity showed by the polymers is purely electronic in nature.
4. Conclusion
In the present study, we have synthesized high molecular weight liquid crystalline polyesters
of Cholesterol by thermal polymerization technique. The phase transitions are studied under
a hot stage optical polarizing microscope which correlates with that shown in the DSC

122
P. Dutta et al.
thermograms. The compounds show good thermal stability up to about 300◦C as observed
from their thermogravimetric analysis. The electrical studies were also carried out which
includes ac conductivity, impedance studies, dielectric studies, current-voltage and transport
number measurements. The observed increase in conductivity with rise of temperature is due
to the semiconducting behavior of the materials and the conduction is purely electronic in
nature as is shown by the transport number measurements. There is about 5000 fold decrease
in impedance values with increase in frequency from 42 Hz to 5 MHz, which simultaneously
shows the huge increase in ac conductivity of the compounds with rise in frequency.
The dielectric properties also show significant variations with change in temperature and
frequency. The current-voltage studies show that the increase in temperature and frequencies
leads to significant rise in the current passing through the samples. Such cholesteric liquid
crystal polymers may be useful in various future applications as well as in the development
of different opto-electronic devices.
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