PHASE TRANSITION STUDIES OF CHOLESTERYL PALMITATE, CHOLESTERYL DECANOATE AND THEIR MIXTURES USING DIELECTRIC, EPR AND DTA TECHNIQUES

The phase transition of two liquid crystalline materials, namely cholesteryl palmitate and cholesteryl decanoate and their mixtures (by weight)are studied using dielectric, Electron paramagnetic resonance (EPR) and differential thermal analysis (DTA) techniques. From the dielectric measurements, the phase transition temperatures were determined from the discontinuities in the curve between dielectric constant and temperatures for all the samples investigated. Further, since mixing of two liquid crystals modifies the transition of phases, the measurements were carried out on the mixtures of the two liquid crystals. Order parameter for each sample was calculated at varying temperatures by measuring the hyperfine splitting of three line EPR spectrums. The discontinuities in the curve between order parameter and the temperature again indicated the phase transition of the liquid crystals and their mixtures. A DTA study on these samples was also carried out in order to probe further their transitional behaviour. The DTA records of both the liquid crystals and their mixtures support the behaviour observed

The phase transition of two liquid crystalline materials, namely cholesteryl palmitate and cholesteryl decanoate and their mixtures (by weight)are studied using dielectric, Electron paramagnetic resonance (EPR) and differential thermal analysis (DTA) techniques. From the dielectric measurements, the phase transition temperatures were determined from the discontinuities in the curve between dielectric constant and temperatures for all the samples investigated. Further, since mixing of two liquid crystals modifies the transition of phases, the measurements were carried out on the mixtures of the two liquid crystals. Order parameter for each sample was calculated at varying temperatures by measuring the hyperfine splitting of three line EPR spectrums. The discontinuities in the curve between order parameter and the temperature again indicated the phase transition of the liquid crystals and their mixtures. A DTA study on these samples was also carried out in order to probe further their transitional behaviour. The DTA records of both the liquid crystals and their mixtures support the transition behaviour observed from the dielectric and EPR measurements.

Introduction:-
The study of phase transition of liquid crystalline materials has been considered to be one of the very interesting subjects for both experimental and theoretical investigations. Liquid crystals are highly anisotropic fluids that exist between the boundaries of the solid phase and the conventional, isotropic liquid phase. The phase is a result of long range orientational ordering among constituent molecules that occurswithin a certain range of temperature in melts and solutions of many organic compounds. The ordering is sufficient to impart some solid-likeproperties to the fluid but the forces of attraction usually are not strongenough to prevent flow. The term liquid crystal thus refers to the phasethat lies between the rigidly ordered solid phase where the mobility ofindividual molecule is restricted and the isotropic phase where molecularmobility and a general lack of molecular order exist.As a result, there isanisotropy in the mechanical, electrical and optical properties. Thedielectric and optical studies of liquid crystal are important as theyprovide information about molecular structure, intermolecular forcesand moleculardynamics [HillN.E. et al (1969)]. Structural changes arising from the transitionbetween the mesophases of liquid crystalline materials are generally studied using optical properties [Brown G.H. et al (1971), Nagappaet al (1991), Khandelwal [Coats D. et al (1972)]. Itis therefore important to rely on other techniques and the thermalmethods, for detecting the phase transition involving subtle changes inthe molecular order, have been found to be highly sensitive and have been widely used.
Among the various methods used to study the molecularordering and molecular motion of mesophases in liquid crystals, theEPR spin probe technique has proven to be very useful in elucidatingdynamic and structural characteristics of these ordered mesophase [Meiroviteh E. et al (1982)].In the initial applications of EPR to this field, the emphasis was on themore fluid nematicand smectic phases where molecular motion is rapidand ordering is often relatively low. A major effect is observed on spinrelaxation upon passing from the isotropic to the nematic phase is theonset of molecular ordering [Polnaszek G.E. et al (1979)]. The transition from the nematic toasmectic is also reflected mainly in changes in molecular orderingcharacteristics.The dielectric anisotropy in liquid crystals resulting from theangular correlation between the molecules not only throws light on themolecular structure but also on their ordering in the mesophase which ischaracterized by the order parameter [Chandrashekhar S.(1977) (1984)] on liquid crystals and it has been reported that phase transitions can be detected using temperature dependent dielectric constant data. Therefore thephase transitions are also studied, using dielectric data by measuringdielectric permittivity as a function of temperature andfrequency, whichis further supported by the data observed using differential thermalanalysis (DTA). The results of phase transition temperatures observedfrom the above three techniques dielectric, EPR and DTA are compared.The present papertherefore reports the phase transitionstudies of two liquid crystals, namely cholesteryl palmitate and cholesteryldecanoate and their mixtures by weight, ¼ ¾ (25% cholesterylpalmitate and 75% cholesteryl decanoate) and ½ ½ (50% cholesterylpalmitate and 50% cholesteryl decanoate).

Experimental Details
In order to conduct the dielectric studies, measurement ofdielectric permittivity of each sample were carried out in thetemperature range of 40-110°C in the frequency range of 1-10 kHzusing 1620-APGR capacitance measuring assembly.The cell (effective capacitance: 4.5 pf), filled with the sample,was first heated till the sample becomes isotropic liquid and then cooleddown to room temperature. The values of capacitance were thenmeasured at different temperatures while heating the sample atfrequencies 1, 2, 5 and 10 kHz and from these the dielectric permittivities (є') were evaluated.For evaluating the order parameter(s), the three line EPR spectraat varying temperature were taken for all the samples using E-line Centuryseries EPR spectrometer (X-band, Model 109). By measuring thehyperfine parameter <a>in non-isotropic phase, S was evaluated andwas plotted against temperature.Further the differential thermal analysis (DTA) were carriedout for the samples at a heating rate of 2°C per minute above roomtemperature up to 150°C, using DTA-50, Shimadzu Corporation, Japan. Figs(1-4) show the variation of dielectric constant withtemperature for respective samples at different frequencies. Fig5shows the EPR spectrum for cholesteryl palmitate at temperatures 58and 83°C. Fig 6 represents

Dielectric Measurement
The dielectric permittivity components (є') of the two liquidcrystalline materials viz. cholesteryl palmitate and cholesteryl decanoateand their two mixtures ¼ ¾ and ½ ½ obtained by dielectricstudies, were plotted as a function of temperature, as shown in Figs.(1-4), there is a marked variation in є' with temperature. It has 212 beenreported earlier that the magnitude of dielectric constant is very muchdependent on the orientation of the liquid crystal molecules, i.e. whetherthe long axis is parallel (є II ) or perpendicular (є T ) to the applied electricfield [Takezoe H. et al (1984)]. Any structural change which affects the molecular orientationwilltherefore necessarilyalter the dielectric constant (and hence thecapacitance of the cell) which is detected. It was for this reason thatno attempt was made to align the liquid crystals and run the risk ofconstraining it to a particular orientation. It has also been demonstratedearlier that a wide range of phase transitions can be detected usingtemperaturedependent dielectric constant and any phase transitioninvolving a structural rearrangement can in principle be detected.
The phase transition temperatures are easily observedbythe discontinuities in these curves (Figs 1-4). For cholesteryl palmitate,the curve showing variation of є' with temperature (Fig1) shows threediscontinuities at 65, 77.5 and 87°C which correspond to three successivephase transitions. In the case of cholesteryl decanoate, the discontinuitieswere observed at 60, 75 and 95°C. When these two liquid crystallinematerials were mixed by weight, the phase transition occurred atcomparatively lower temperatures. For ½ ½ the phase changeoccurredat 45, 70 and 85°C while in ¼ ¾ it occurred at 45, 67 and 85°C.
In the case of all the four samples when they are heated aboveroom temperature, the first two transition temperatures correspond to thephase change from solid to smectic and smectic to cholesteric while the third transition occurs from Cholesteric to isotropic. In the case ofthe two mixtures, it is observed that the transition takes place at lowertemperature than that of the pure liquid crystalline materials. Also forboth the mixtures, solid to smectic and cholesteric to isotropic transitionsappear at the same temperature. However, when the amount ofcholesteryldecanoate is increased in the mixture (as in ¼ ¾ )the smectic tocholesteric transition temperature further decreases. It hasbeen furtherobserved that in all the systems the value of dielectricconstant increaseswith decrease in temperature. This indicates considerable freedom ofdipole orientation. In the solid phase, є' has been found to decreasewith decreasing temperature. This indicates that the orientation of thedipoles is restricted as it approaches solid phase. The fact that the values of dielectric constant at 10 kHz are lessthan those at 1 kHz indicates that the materials under investigation showdielectric dispersion at these frequencies. However, complete dispersionis likely to be obtained if measurements are taken beyond 10 kHz. Thisbehaviour of the above liquid crystalline materials and their mixtures issimilar to those observed for other liquid crystals [Kressf H. et al (1991)

Electron Paramagnetic Resonance (EPR) Measurements
In the EPR spectrum, on passing from the amorphous isotropicphase to the nematic phase, the number of lines observed does notchange. Instead the spacing between the lines changes(Fig5).Therefore, a determination of the line spacing or their position providesinformation about the Order parameter.The order parameter is calculatedfrom the expression = < > − − Here 'a' is scalar hyperfine interaction and can be determined from themeasurements in the isotropic phase. Also<a> is a hyperfineparameterin non-isotropic phase and can be determined at varying temperature. 'A'is a constant for particular probe used.In the present work, nitroxide (N→0)is used as a paramagneticspin probe for whichA=30.8 G and a=14.7G. The order parameter(S)was evaluated for the samples at various temperatures from EPR spectra.The values of S are plotted as a function of temperature (Fig6). Thediscontinuities in the curve correspond to change of phase. Thereforefrom the curve phase transition temperatures were measured. Forcholesteryl palmitate the change of phase takes place at 65, 77 and 87°Cwhile in the case of cholesteryl decanoate only two transitiontemperatures76 and 95°C were detected. Similarly for ½ ½ and ¼ ¾ phasechange takesplace at 70, 85°C and 67, 85°C respectively. Thesetransition temperatures obtained using EPR spectrum are in excellentagreement with those obtained from dielectric measurement. However,in the case of above mixtures one of the phases observed using EPR ismissing.

Differential Thermal Analysis (DTA) Measurement
The DTA study on pure cholesteryl palmitate and cholesteryldecanoate has clearly identified the transition peaks at 65. 3

Conclusion:-
The phase transition temperatures observed for the twoliquid crystalline materials and their mixtures from three techniques viz.dielectric, EPR and DTA are given in Table-1. The transitiontemperatures observed from these techniques are in excellent agreementwith each other. However, the values of pure materials differ slightlyfrom literature values which might be because of some impurity in thesamples.