Interest in dye mixed liquid crystals and their practical application has increased recently for a number of reasons in the field of liquid crystalline material. Fast response time and high degree of ordering in dye mixed liquid crystals has been one of the reasons for their importance in practical application. In guest host displays the liquid crystalline material used are the solution of dye (guest) in pure liquid crystals (host). The presence of dye changes various properties of liquid crystals up to a great extent. When dye is dissolved in a nematic liquid crystals, molecules of dye possessing anisotropy tend to arrange in such a way that their long molecular axis align along the liquid crystals director. It is also well known that the presence of dye in nematic liquid crystals changes the range of nematic phase, which is caused due to either increase or decrease in nematic- isotropic transition temperature (Bauman and Haase, 1989). The properties, which can be varied with the help of doping dye in liquid crystals are dielectric permittivity, dielectric loss, optical transmittance, order parameter etc. A review of literature reveals that the effect of doping dichroic dyes in liquid crystals have been investigated by some groups for their use in electro optic display devices (Raina et al., 1999).
Dielectric and optical studies of liquid crystals and their mixture with mesogenic and non mesogenic compounds are important as they provide valuable information on molecular arrangement, molecular motion and intermolecular interaction especially in nematic and isotropic phase (Chen and Hwang, 2004). Therefore these properties of pure nematic liquid crystals have been studied by various groups (Scarmujja et al., 2001). But the dielectric measurement of dye doped nematic liquid crystals are less studied as most of the studies are concentrated around electro optic effect of the dye doped liquid crystals (Shussarenko and Francescangeli, 1997; White and Taylor, 1974). Therefore it has been considered important to make systematic and detailed study of nematic liquid crystal B08 and its mixture with Anthraquinone dye in low concentration. These investigations are expected to give information regarding interaction of dye molecule with nematic liquid crystal and their contribution to the occurrence of relaxation modes.
The phase transition temperature obtained using dielectric measurement technique has been supported by data obtained from optical transmittance study for pure as well as dye mixed liquid crystals.
MATERIALS AND METHODS
The material under investigation is a pure nematic liquid crystal B08 crystal
with the following structure.
|Chemical structure of B08
The other sample has been prepared by mixing Anthraquinone dye in pure nematic liquid crystal in concentration 2% wt/wt. The strong interaction between two different types of molecules allows the sample to maintain a good alignment (Gibbons et al., 1991) which has been confirmed by observing the sample holder under polarizing microscope.
The guest host sample and pure sample has been filled in a standard sandwich
type sample holder having coating of Indium Tin Oxide (ITO) on the glass substrate
and pretreated with polymide for planner type alignment. The molecular structures
for anthraquinone dye are given below.
|Chemical structure of anthraquinone dye
Two planar aligned and ITO coated cells of thickness 5 μ were used
for the dielectric measurements. One cell is filled with pure liquid crystal
and other is filled with dye doped liquid crystal. We have used an impedance/
Gain phase analyzer (HP-4194A) to determine the real and imaginary part of permittivity.
The values of Cp and Dp have been read on the screen of analyzer first without
sample and then after introducing the sample in the cell. These measurements
have been taken in the frequency range of 1 kHz to 5 MHZ for different temperatures.
In order to vary the temperature a microprocessor based heating device Julabo-F
24 has been used. The real and imaginary part of permittivity have been determined
by using the values of capacitance and dissipation of empty sample holder and
sample holder filled with sample as described in our earlier work (Singh et
al., 2005) by following equation (Manohar et al., 2006).
It directly gives the value of perpendicular component of dielectric permittivity ε⊥ as we are using planer aligned sample holders. The values of ε⊥ after the N→I transition temperature become equal to εi (dielectric permittivity in isotropic phase) and varies almost linearly with temperature in isotropic phase. By extrapolating this for nematic phase we can obtain the value of εavg or for the sample. Now εII can be calculated using the following relation (Bahadur, 1991)
Thermodynamical parameters have been determined by using the dielectric relaxation data with following equations
Where ω = 2τf where f is the frequency of the applied electric field.
Thermo dynamical parameter ΔF, ΔH and ΔS are related by the following equation
For optical transmittance measurements sample holder filled with sample
is placed on rotating stage between two crossed polarizers of polarizing microscope,
model CENSICO (7626). A detector has been fitted at one of the eyepieces of
microscope to monitor the change in intensity of light transmitted. The temperature
of the sample has been varied by using a refrigerated circulator of Julabo (Model
No. F-25) with accuracy of ±0.1° C. The variation of temperature
gives rise to change in phase of the sample. This allows the light of different
intensity to transmit through the sample and incident on detector. The optical
transmittance data has been normalized by considering 100% transmittance through
empty cell and 0% transmittance through sample holder filled with black ink.
RESULTS AND DISCUSSION
Figure 1 and 2 represent typical frequency dependences spectra of the real and imaginary part of the dielectric permittivity measured for pure liquid crystal and dye doped liquid crystal. The real part of dielectric permittivity is either constant or decreases as the frequency has been increased (Viciosa et al., 2002). Low values of ε at higher frequency suggest that the molecule rotate about their molecular axis (Arora and Agarwal, 1991) or some of its side group rotates and give rise to this kind of relaxation. This type of low frequency relaxation has also been reported by other workers for nematic liquid crystals (Biradar et al., 2000). The variation of dielectric permittivity with frequency shows almost similar behaviour for pure liquid crystal and dye doped liquid crystal but the values of permittivity are higher for dye mixed liquid crystal.
Maxima in loss curve has been observed at frequency 125 kHz for pure liquid crystal and observed at frequency 400 kHz for dye doped liquid crystal. The peak in the curve indicates dielectric relaxation and it shift towards higher frequency side as expected and also reported by other investigators (Bahadur, 1990). Each curve in Fig. 4 shows similar behaviour on both side of the peak and this type of behaviour has also been observed by many other researchers for nematic, smectic and cholesteryl liquid crystals (Thakur et al., 2003).
||Variation of dielectric permittivity with log of frequency
for pure liquid crystal and dye doped liquid crystal
||Variation of dielectric loss with log of frequency for pure
liquid crystal and dye doped liquid crystal
The nature of loss curve is similar for pure liquid crystal and dye doped liquid
crystal but it is evident from the Fig. 3 that the peak of
the curve obtained for pure liquid crystal has been significantly shifted from
the frequency of the peak obtained for dye doped liquid crystal. It suggests
that the relaxation frequency of the dye-doped liquid crystal is different from
that of pure sample and the variation also depends upon the type of dye used
and the compatibility of dye with liquid crystal host.
||Variation of dielectric permittivity with temperature for
pure liquid crystal and dye doped liquid crystal
The variation in the value of ε and εN due to addition of dye can be explained by considering the contribution of dipole moment of dye molecule in the host liquid crystal molecule. Each dye molecule has a preferential axis of orientation, which will get aligned along with the host liquid crystal molecule therefore resulting in the restriction on the freedom of movement. Thus the dipole moment of dye molecule contributes significantly to the actual value of ε and εN and also to the relaxation frequency value (Hohmutha and Schieweb, 1981).
Figure 3 and 4 shows the variation of ε
and εN as a function of temperature for pure liquid crystal and dye
doped liquid crystal. The values of real part of permittivity remain almost
constant in temperature range of 30 to 100°C and its value varies slightly
at 103°C shows crystalline to nematic transition of sample and a sharp change
in its value at 163°C shows that sample become isotropic at this temperature.
This type of behaviour has also been reported by other groups for other type
of liquid crystal sample (Agrawal et al., 1989). Similar type of behaviour
has also been observed for the curve of εN versus temperature at
varying frequencies shown in Fig. 5.
The transition scheme determine by dielectric study is as follows
The small difference in the experimental value from literature values of transition temperature can be attributed to the influence of boundary condition on phase transition temperature for sample.
||Variation of dielectric loss with temperature for pure liquid
crystal and dye doped liquid crystal
||Variation of ε11 and ε with temperature
for pure liquid crystal and dye doped liquid crystal
Enthalpy of activation energies was found to 1.11 kcal mol-1 deg
for pure liquid crystal and 1.25 kcal mol-1 deg-1 for
dye doped liquid crystal. For guest host sample energies are slightly higher
than the pure liquid crystal which implies an increase in the hindering potential
barrier in guest host sample.
|| Cole-Cole plot for pure liquid crystal and dye doped liquid
crystal at 110 and 125°C
In other words guest host sample requires a higher energy to overcome the
hindering potential barrier and to realize a rotational motion (Haase and Wrobel,
2003; Agrawal et al., 1989).
Dielectric anisotropy has been calculated from the data of dielectric permittivity corresponding to the various applied frequencies. The variation of ε11 and ε⊥ with temperature are shown in Fig. 5. Pure liquid crystal and dye doped liquid crystal shows almost similar behaviour. For Pure liquid crystal the values of ε⊥ remains constant in crystalline phase and then increases slowly up to a temperature until sample becomes isotropic. After that the values remain constant with further increase of temperature but the values of ε11 remains constant in crystalline phase and then decreases slowly up to the point sample becomes isotropic and after this the values remain constant. Similar type of behaviour has been observed in the case of dye doped liquid crystal.
The present data can also be represented by the symmetrical distribution, which
has been indicated by Cole-Cole plot. Representative curve at 110, 125°C
for pure liquid crystal and dye doped liquid crystal are shown in Fig.
6. For the qualitative analysis of the dielectric spectra the Cole -Cole
equation has been used
ε*(ω) = ε' (ω)-iε''(ω)
Here ε∞ is the high frequency limit of the permittivity; Δε is dielectric strength and τ is mean relaxation time. The exponent ∝ (0<∝<1) represent the distribution of relaxation times and goes to zero when single relaxation is effective. Both the real and imaginary part of permittivity has been analyzed.
||Variation of percentage optical transmittance with temperature
for pure liquid crystal and dye doped liquid crystal
The fitting of the Cole-Cole equation to the experimental spectrum has been performed and it has been found that the centre of Cole-Cole plot lies below the real axis indicating distribution of relaxation time (Manohar et al., 2000).
The optical transmittance data as a function of temperature has been plotted for pure liquid crystal and dye doped liquid crystal. Figure 7 shows the variation of percentage optical transmittance with temperature.
The samples show following phase sequence as determined by optical transmittance measurement
|Pure liquid crystal
Dye doped liquid crystal
The phase transition temperature measured for the dye doped liquid crystal is not same as in case of pure liquid crystal. It can be concluded that, when dye has been mixed in pure nematic sample the value of transition temperature decreases in comparison to pure sample. But the breath of nematic isotropic phase has been found to increase slightly in comparison to pure sample. This type of behaviour has also been reported by our group (Demus et al., 1978) and other workers (Shussarenko and Francescangeli, 1997) for cholesteric as well as nematic samples. Singh et al. (1990) have also observed similar behaviour and concluded that the breadth of the transition depends upon the purity of substance and it decreases with lower of purity.
||The measurement of dielectric parameters, optical transmittance
and dielectric anisotropy show the Iso-Nem and Nem-Cryst phase transitions.
||Transition temperature obtained from different techniques
i.e., optical transmittance, dielectric and birefringence measurements are
found to be in good agreement.
The support by Prof. B.K. Sadashiv of RRI, Bangalore in providing the sample is gratefully acknowledged.