Abstract 

The conventional Twisted Nematic Liquid Crystal Display (TN-LCD's) 

uses Liquid Crystal materials with positive dielectric anisotropy. However, the 

greatest drawbacks of the TN-LCD's are their narrow viewing angle, low 

contrast and slow response times. In contrast, the recently introduced Vertically 

Aligned mode Liquid Crystal Displays (VA-LCD) combine the vertical 

alignment technique with nematic liquid crystals having negative dielectric 

anisotropy, resulting in wide viewing angle, excellent contrast and shorter 

response time. In VA mode, the name suggests, the liquid crystal molecules are 

normally aligned at right angles to the substrates, swinging through 90° to lie 

parallel with the substrates in the presence of an electric field. 

A particularly valuable class of materials satisfying the requirements of 

VA mode technology is the laterally fluorinated bi-phenyls and ter-phenyls. A 

(2, 3) difluoro substitution on the phenyl ring results in an effective dipole 

moment perpendicular to the principal molecular axis thus realizing negative 

dielectric anisotropy (~E<O) for the liquid crystalline molecule. Moreover, no 

single liquid crystalline compound can fulfill all the requirements of a display 

device; multicomponent mixtures with optimum values of material properties 

are required. The objective of this work is to prepare and characterize several 

multicomponent mixtures comprising of polar-fluorinated molecules suitable 

for VA mode application. The aim of this work is to produce materials with 

wide nematic temperature range, large negative dielectric anisotropy (~c), low 

rotational viscosity (y1), low values of bend elastic constant (K33) and low 

values of the optical birefringence (~) for their application in VA-LCD's. 

Several pure liquid crystalline materials with negative dielectric anisotropy 

have been selected as components of multi component mixtures and their 

physical properties have been characterized. Bicyclohexane compounds with 

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low values of the optical birefringence (~n) have also been characterized with a 

view to use them in mixtures for lowering the birefringence of the material. 

This work consists of careful formulation of multi component mixtures and the 

measurement of the dielectric anisotropy, optical birefringence, bend elastic 

constant and the rotational viscosity of these mixtures. The material properties 

of these mixtures are to be optimized with respect to improved driving voltages 

and high quality optical performance so that mixtures with the required 

specification may be identified for commercial application or patentability of 

the work. The following are the conclusions summarizing the work done in this 

thesis. 

A. Study of pure compounds: 

1. In this work an in-depth study of several liquid crystalline compounds 

(compound 1-15) from different experimental techniques has been undertaken. 

2. Compound 1 and compounds 3-9 with lateral fluorine substitution posses a 

higher value of birefringence (0.20-0.28 in the nematic range) while the phenyl 

bicyclohexane compounds 10 and 11 and the phenyl- cyclohexane compound 

12 exhibit a relatively lower birefringence values of around 0.08 - 0.11. 

Bicyclohexane compounds 14 and 15 show a very low birefringence of the 

order of0.03 and 0.08 respectively. 

3. It is observed that the dielectric anisotropy (~s) for compound 9 is around -2 

in the nematic phase. Compounds 1, 3, 6 and 8 however have a moderately low 

value of L\s of about -1. ~s value for compound 4, 5 and 7 are slightly higher 

than -1. Again, L\s for compounds 10 and 11 are around -2 at the nematic to Sm 

B phase transition and -6 within the Sm B phase. 

4. The K33 values vary from 4-16 pN at room temperature for all the laterally 

fluorinated pure compounds. The y1 values for compounds 1, 3-9 are around 

100-120 mPas in the nematic range which is considerably smaller than those 

obtained for phenyl bicyclohexane compounds 10 and 11 with values of 233 

mPas and 322 mPas respectively at the N-SmB phase transition. 

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5. The terphenyl compounds 1, 3-9 have improved values of the visco elastic 

coefficient (y1/K33) due to lower values of the rotational viscosity of these 

materials. Significantly higher Figure of Merit (FoM) (almost ten times or 

more) are observed for the laterally fluorinated compounds (1, 3-9) compared 

to bicyclohexane compounds with CH2-CH2 linkage group in the core 

(compounds 10 and 11). 

6. The physical properties of three laterally fluorinated liquid crystalline 

compounds 1, 10 and 11, have been thoroughly studied by different 

experimental techniques. Orientational Order Parameter (<P2>) values have 

been determined from the x-ray diffraction and refractive index measurements. 

Fairly good agreement is observed between the two experimental methods as 

well as the theoretically calculated values. 

B. Formulation and study of several multicomponent mixtures: 

Based on the physical properties of single compounds, seven multi 

component mixtures (mixtures A-G) ranging from three component (mixture A 

and mixture B) to fifteen component (mixture G) have been fommlated and 

their physical characterization has been done. 

1. In the tri component mixtures (mixtures A and B) birefringence results are 

close to the targeted values for VA mode display devices (around 0.12 and 0.13 

respectively) but ~E values are found too low(~ -1 at around 20°C). 

2. To meet the required specifications a five component mixture (mixture C) 

has been prepared whose birefringence values are almost satisfying targeted 

value (around 0.11) but again the ~E values are very low. 

3. A nine component mixture (mixture D) was formulated comprising of all 

laterally fluorinated bi- and tri-phenyl compounds, which showed a low 

viscosity (~ 48 mPas at around 20°C) and moderate birefringence ( ~ 0.18 at 

around 20°C) and higher values of dielectric anisotropy ( ~ -2.5 at around 20°C) 

with a broad nematic range. 

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4. By introducing two different dopants (laterally fluorinated tolanes) the nine 

component mixture have been modified into two ten component mixtures 

(mixtures E and F) where it was possible to tailor almost all of the physical 

properties (for a better effect on switching time, threshold voltage and the 

display contrast), only sacrificing the ~n values. 

5. The fifteen component mixture (mixture G) giVes a reasonable balance 

among all the required physical properties for a suitable VA display device. 

Mixtures D-F emerged to be the best mixture among all the mixtures studied. 

C. Study of the effect of the pretilted cell on different multicomponent 
mixture: 

The effects of 2° and 5° pretilt with respect to normal cell (cell with 

pretilt) have been thoroughly investigated for all of the multicomponent 

mixtures (mixture A-G). The voltage dependent transmittance curve for the 

mixture in different pretilted cells (0°, 2° and 5°) has also been compared. 

1. At = 20°C the response time decreases to 25% and 35% for mixture A for 

2° and 5° cells respectively compared to 0° pretilt. On the other hand, at the 

same temperature for B, the reductions in these values are 16% and 35% 

respectively. 

2. At T NI-T= 63°C (i.e. T = 20°C), the response time for the mixture decreases 

to 22% and 41% for 2° and 5° respectively, compared to zero pretilt. On the 

other hand, at the same temperature the Vth values are decreased by 5% and 9% 

respectively. 

3. It has been found that when 2° pretilted cells are used, the relaxation time for 

mixture D, E and Fare decreased by 6%, 14% and 11% respectively and for 5° 

pretilted cell the same values are reduced by 14%, 18% and 47% respectively 

for mixtures D, E and F. Similar behaviour is also found for Vth values of these 

mixtures. It is observed that the Vth values for mixture D, E and F is reduced by 

5%, 18% and 12% for 2° pretilted cell and for 5° pretilted cell the same values 

are reduced by 10%, 16% and 13% respectively. 

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4. The response time for the mixture decreases to 14% and 44% for 2° and 5° 

respectively, On the other hand, it is found that at the same temperature the Vth 

values are decreased by 6% and 11% respectively. 

D. Study of mesomorphic and structural properties of some liquid crystals 
possessing a bicyclohexane core: 

1. In this work the results of refractive indices and density measurements on 

seven alkenyl bicyclohexane compounds of which five compounds show only 

nematic phase while other two compounds possessing smectic B phase which 

precursor of nematic phase has been presented. The refractive indices as well as 

the density data have been used to determine the Orientational Order Parameter 

( <P2>) using the standard Vuks isotropic model and have also been compared 

with the theoretical mean field values. 

2. Optical Transmission (OT) method has also been employed to obtain a high 

resolution (accuracy~ 1 0"6
) measurement of the temperature dependences of the 

optical birefringence (iln), which provides a macroscopic measure of the 

anisotropy of the liquid crystalline phase, and can also, be considered as a 

measure of the orientational ordering. The optical birefringence data obtained 

from optical transmission method have been compared with the same as 

obtained from thin prism technique. 

3. Interestingly, the high resolution L'm data obtained from temperature 

scanning measurement of optical birefringence are quite successful in 

characterizing the transitional anomaly associated with the nematic-isotropic 

phase transition. For the investigated compounds, the values of the critical 

exponent P related to the limiting behavior of the nematic order parameter 

close to the N-I transitions, are found to be close to 0.25 and thus are in well 

accordance with the tricritical hypothesis and also excludes the possibility of 

any higher P values. 

4. Additionally, the dielectric permittivities parallel and perpendicular to the 

molecular long axis throughout their mesomorphic range of the seven pure 

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liquid crystalline compounds has also been measured. The structure property 

relationship of these compounds has also been discussed. 

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