70

CHAPTER 2

EXPERIMENTAL

In this chapter, the main experimental techniques employed will be

briefly introduced. At the beginning of the chapter the complementarities of

these techniques and their relevance to the study of LC systems will be

specifically addressed.

2.1 MATERIALS

Hexane, benzene, dichloromethane, chloroform, ethylacetate,

ethanol, methanol, tetrahydrofuran, acetone, N,N-dimethylformamide,

water and thionylchloride were purified by the reported

procedure (Perrin and Armarego 1988, Furniss et al 1994). Potassium

hydroxide, sodium hydroxide, potassium carbonate, hydrochloric

acid (35%), absolute ethanol were purchased from Merck,

India. 4-Hydroxybenzoic acid, 4-hydroxybenzaldehyde, 4-hydroxyacetanilide,

potassium iodide and triethylamine were purchased Spectrochem, India.

Palladium carbon (10%), potassium dichromate, resorcinol, benzyl chloride,

N,N-dicyclohexylcarbodiimide (DCC) and 4-(dimethylamino)pyridine

(DMAP) were purchased from Aldrich. All other reagents and chemicals were

used as received.

71

2.2 PURIFICATION OF SOLVENTS

2.2.1 Benzene

Benzene (500 mL) was shaken with about 15% of its volume of

concentrated sulphuric acid until free from thiophene, then washed with water

and 10% sodium carbonate solution, again with water and dried in fused

calcium chloride and distilled. The fraction boiling at 80 C was collected and

stored over metallic sodium wire (lit.b.p.80.1 C, Perrin and Armarego 1988).

2.2.2 Dichloromethane

Dichloromethane (100 mL) was shaken with portions of

concentrated sulphuric acid until the acid layer remains colorless and washed

with aqueous 5% sodium carbonate solution then with water. Pre-dried with

calcium chloride and distilled over phosphorus pentoxide. The fraction

boiling at 40 C was collected (lit.b.p.40 C, Perrin and Armarego 1988).

2.2.3 Chloroform

Chloroform (SRL) (500 mL) was shaken several times with half of

its volume of 10% aqueous sodium bicarbonate and followed by distilled

water; the chloroform layer was separated, dried over fused calcium chloride

for 48 h and distilled. The fraction boiling at 62 C was collected and

redistilled with P2O5 to get dry chloroform (lit.b.p.62 C, Furniss et al 1994).

2.2.4 Ethylacetate

Ethyl acetate (Spectrochem, India) (1L) was washed with aqueous

5% sodium carbonate then washed several times with sodium chloride and

dried with potassium carbonate. The fraction boiling at 77 C was collected.

(lit.b.p.77.1 C, Perrin and Armarego 1988).

72

2.2.5 Ethanol

Rectified spirit (1000 mL) was refluxed with calcium oxide for 6 h,

set aside overnight and distilled. The fraction distilling at 80 °C was collected

(lit.b.p.80 °C, Furniss et al 1994).

2.2.6 Methanol

Dried methanol was obtained by distilling the commercial

methanol (SRL) (1 L) which was refluxed over anhydrous calcium oxide. The

distilled methanol was treated with magnesium metal and re-distilled. The

fraction boiling at 65 C was collected (lit.b.p.65 C, Furniss et al 1994)

2.2.7 Acetone

Acetone (Merck) (1L) was refluxed with successive quantities of

potassium permanganate until the violet color persisted. It was then dried with

anhydrous potassium carbonate and distilled. The fraction boiling at 57 C

was collected (lit.b.p.57 C, Furniss et al 1994).

2.2.8 N,N'-Dimethylformamide (DMF)

To 500 mL of N,N'-Dimethylformamide (SRL), freshly roasted

copper sulphate (20 g) was added and stirred. This was left for 24 h till green

colored solution was obtained and filtered. The filtrate was then distilled

under reduced pressure and the fraction boiling at 75 C/12mm Hg, was

collected (lit. b.p.75-76 C/12 mm Hg, Furniss et al 1994).

2.2.9 Thionyl Chloride

Commercial thionyl chloride was first fractionated in an all glass

apparatus from quinoline to remove acid impurities (50 g thionyl chloride

73

from 10 mL of quinoline), the receiver is protected from the entry of moisture

by using a guard-tube, filled with anhydrous calcium chloride. The distillate

was then refractionated from boiled linseed oil the fraction boiling between

76 to 78 oC collected.

2.2.10 Water

Water (1 L) was distilled with 10 g of potassium permanganate and

sodium hydroxide. The distilled water was collected and then re-distilled to

get double distilled water (b.p.100 °C, Furniss et al 1994).

2.3 SYNTHESIS OF KEY INTERMEDIATES

2.3.1 Synthesis of 4-(10-Undecenoyloxy)biphenyl-4-carboxylic Acid

(1)

Figure 2.1 Synthesis of compound 1

10-Undecenoic acid (0.2117 mol) was dissolved in benzene (150

mL) with one drop of dimethylformamide, then thionyl chloride (75 mL;

0.6351 mol) added drop wise to the reaction mixture. The resultant mixture

was refluxed for 6 h with constant stirring. The benzene and excess thionyl

chloride were removed under vacuum to get acid chloride as colorless liquid

(yield 95%) (Petersen 1953). 10-Undecenoyl chloride (0.1mol) dissolved with

74

100 mL dry tetrahydrofuran (THF) and 4-hydroxybiphenyl-4-carboxylic acid

(0.1 mol) followed by dry triethylamine (0.12 mol) were added and stirred at

20 C for 12 h under nitrogen atmosphere. The precipitated triethylamine

hydrochloride was removed and product dissolved in THF and filtered. The

filtrate was evaporated under vacuum to get crude product then recrystallized

in ethanol to yield white crystals (yield 92 %).

2.3.2 Synthesis of 4-(Alkyloxy)benzoic Acids (2a-2g)

Figure 2.2 Synthesis of compound 2a

4-(Alkyloxy)benzoic acids (2a-2g) were synthesized by the

following method and as a representative synthetic procedure for the series,

the synthesis of compound 4-(hexyloxy)benzoic acid (2a) is as follows: . 4-

Hydroxybenzoic acid (6.4 g, 46 mmol) and potassium hydroxide (19 g,

138 mmol) were dissolved in ethanol (30 ml) and stirred for 1h at room

temperature. 1-Bromohexane (7.6 g, 46 mmol) was then added drop wise to

this solution followed by addition of potassium iodide (0.8 g, 0.0046 mmol)

in one portion. The solution refluxed for 24 h with constant stirring. The

reaction mixture was concentrated by distillation of ethanol. Then the mixture

was poured in ice water (500 mL) and neutralized with 10% hydrochloric acid

solution. Resultant precipitate was filtered and recrystallized from absolute

ethanol to get the desired product (yield 65%). A similar procedure was

75

adopted for preparation of other alkyl series such as heptyl, octyl, nonyl,

decyl, undecyl, and dodecyl.

2.3.3 Synthesis of 4-(4-n-Alkyloxybenzoyloxy)benzaldehydes (3a-3g)

Figure 2.3 Synthesis of compound 3a

4-(4-n-Alkyloxybenzoyloxy)benzaldehydes (3a-3g) were

synthesized by the following method and as a representative

synthetic procedure for the series, the synthesis of compound

4-(4-n-hexyloxybenzoyloxy)benzaldehyde (3a) is as follows: To a mixture

of 4-n-hexyloxybenzoic acid (5 g, 22 mmol)), 4-hydroxybenzaldehyde (1.8g

22 mmol), dicyclohexylcarbodiimide (DCC) (3g, 25mmol), and 5% w/w of

4-(N,N' dimethylamino)pyridine (DMAP) were dissolved in methylene

chloride (200 mL), and the resulting solution was stirred for 12 h at room

temperature under nitrogen atmosphere. Precipitated by product urea was

filtered from the reaction mixture and filtrate was concentrated by vacuum

evaporation. Crude product was purified by silica gel column chromatography

using chloroform as eluent. The product 4-(4-n-hexyloxybenzoyloxy)

benzaldehyde (5.8 g; 86 %) was obtained as a white powder. A similar

procedure was adopted for preparation of other alkyl series such as heptyl,

octyl, nonyl, decyl, undecyl, and dodecyl containing compounds.

COOHC6H13O CHOHO+

COOC6H13O CHO

DCC, DMAP/DCM

(3a)

76

2.3.4 Synthesis of 4-(4-n-Alkyloxybenzoyloxy)benzoic Acids (4a-4g)

Figure 2.4 Synthesis of compound 4a

4-(4-n-Alkyloxybenzoyloxy)benzoic acids (4a-4g) were

synthesized by the following method and as a representative synthetic

procedure for the series, the synthesis of compound 4-(4-n-

hexyloxybenzoyloxy) benzoic acid (4a) is as follows: 4-(4-n-

hexyloxybenzoyloxy)benzaldehyde (5 g, 15 mmol) was dissolved in acetone

(20 mL) and diluted with water to make the final volume of 100 mL.

Addition of Jones reagent [mixture of chromium oxide (26.72 g) with

concentrated sulfuric acid (23 mL)] was continued till red color persisted for

at least 1 min. Resultant mixture was stirred at room temperature for 30 min

to ensure the completion of oxidation. Excess oxidizing reagent was

quenched with 2-propanol. Reaction solution was then diluted with water and

repeated extraction with ether. Combined organic extracts were dried over

anhydrous sodium sulfate, filtered and concentrated in vacuum. Crude

product was purified by column chromatography (4.25 g; 77%). A similar

procedure was adopted for the preparation of other alkyl series such as heptyl,

octyl, nonyl, decyl, undecyl, and dodecyl containing compounds.

77

2.3.5 Synthesis of Resorcinolmonobenzylether (5)

Figure 2.5 Synthesis of compound 5

Resorcinol (11 g, 100 mmol) was dissolved in acetone (100 mL).

To this solution, powdered potassium carbonate (13.8 g, 300 mmol) and

potassium iodide (pinch) were added slowly. The mixture was refluxed and

benzyl chloride (11.6 mL, 100 mmol) added drop wise to the refluxing

mixture over a period of 30 min. The reaction was carried at for 48 h and

cooled the reaction to room temperature and potassium carbonate filtered and

washed with acetone, the collected filtrates concentrated by vacuum

distillation. Product thus obtained was purified by column chromatography

using chloroform and hexane (3:7) mixture as eluent to yield brown color

viscous liquid (12 g; 72%).

2.3.6 Synthesis of 4-((3-(Benzyloxy)phenoxy)carbonyl)phenyl-4-

(alkyloxy)benzoates (6a-6g)

Figure 2.6 Synthesis of compound 6a

78

4-((3-(Benzyloxy)phenoxy)carbonyl)phenyl-4-(alkyloxy)benzoates

(6a-6g) were synthesized by the following method and as a representative

synthetic procedure for the series, the synthesis of compound 4-((3-

(benzyloxy)phenoxy)carbonyl)phenyl-4-(hexyloxy)benzoate (6a) is as

follows: To a mixture of 4-(4-n-hexyloxybenzoyloxy)benzoic acid (3.42 g,

10 mmol), resorcinol monobenzylether (2 g, 10 mmol) DCC (2.5 g,

12 mmol), and a catalytic amount of DMAP in dry dichloromethane (50 ml)

were stirred for 12 h. The precipitated N, N -9-dicyclohexylurea was filtered,

washed with excess of dichloromethane and the filtrate concentrated in a

rotary evaporator. The residue was purified by silica gel column using

chloroform as eluent. The product obtained on removal of chloroform was

further purified by recrystallization using mixture of chloroform and hexane

(1:3) to get 4.5 g yield (85%). A similar procedure was adopted for

preparation of other alkyl series such as heptyl, octyl, nonyl, decyl, undecyl,

and dodecyl containing compounds.

2.3.7 Synthesis of 4-((3-Hydroxyphenoxy)carbonyl)phenyl-4-

(alkyloxy)benzoates (7a-7g)

Figure 2.7 Synthesis of compound 7a

79

4-((3-Hydroxyphenoxy)carbonyl)phenyl-4-(alkyloxy)benzoates

(7a-7g) were synthesized by the following method and as a representative

synthetic procedure for the series, the synthesis of compound 4-((3-

hydroxyphenoxy)carbonyl)phenyl 4-(hexyloxy)benzoate (7a) is as follows:

4-((3-(benzyloxy)phenoxy)carbonyl)phenyl-4-(hexyloxy)benzoate (4.2 g, 8.0

mmol) was dissolved in ethyl acetate (150 ml) containing a suspension of

palladium carbon(Pd/C) catalyst (10% Pd/C). The mixture was stirred for

24 h under hydrogen atmosphere, filtered and concentrated under vacuum.

Crude product thus obtained was purified by silica gel column

chromatography using ethyl acetate-hexane (1:3) as eluent to yield 88%. A

similar procedure was adopted for the preparation of other alkyl series such as

heptyl, octyl, nonyl, decyl, undecyl, and dodecyl containing compounds.

2.3.8 Synthesis of 3-(4-(4-(Alkyloxy)benzoyloxy)benzoyloxy)phenyl-

4-formylbenzoates (8a-8g)

Figure 2.8 Synthesis of compounds 8a

3-(4-(4-(Alkyloxy)benzoyloxy)benzoyloxy)phenyl-4-formylbenzoates

(8a-8g) were synthesized by the following method and as a representative

synthetic procedure for the series, the synthesis of compound 3-(4-(4-

(hexyloxy)benzoyloxy)benzoyloxy)phenyl-4-formylbenzoate (8a) is as

80

follows: A mixture of 4-((3-hydroxyphenoxy)carbonyl)phenyl-4-(hexyloxy)

benzoate (4.34 g, 10 mmol), and a 4-formylbenzoic acid (1.5 g, 10 mmol)

and a catalytic amount of 4-(N,N-dimethylamino)pyridine (DMAP) in dry

dichloromethane (25 mL) was stirred for 10 min. To this mixture,

N,N'- dicyclohexylcarbodiimide (DCC) (2.5 g, 12 mmol) was added and

stirred for about 12 h at room temperature. The precipitated

N,N'-dicyclohexyl urea was filtered off and washed with excess of

dichloromethane. The combined organic solution was washed with ice-cold

aqueous 5% sodium hydroxide solution (NaOH) (3 75 mL), 5% hydrochloric

acid (HCl) (3 75 mL) and finally with water (3 75 mL) and dried over

anhydrous sodium sulphate. The residue obtained on removal of solvent was

chromatographed on silica gel using chloroform as eluent to yield 4.9 g

(86.5 %). A similar procedure was adopted for the preparation of other alkyl

series such as heptyl, octyl, nonyl, decyl, undecyl, and dodecyl containing

compounds.

2.3.9 Synthesis of 4-(Alkyloxy)acetanilides (9a-9g)

Figure 2.9 Synthesis of compound 9e

4-(Alkyloxy)acetanilides (9a-9g) were synthesized by the following

method and as a representative synthetic procedure for the series, the

synthesis of compound 4-(hexyloxy)acetanilide (9e) is as follows: A mixture

of 4-hydroxyacetanilide (3 g, 20 mmol), anhydrous potassium carbonate

(K2CO3) (8.4 g, 60 mmol), 1- bromodecane (3.9 g, 19 mmol) and pinch of

potassium iodide (KI) in 200 mL of acetone were stirred at 70 °C for 48 h.

Then the reaction mixture was cooled to room temperature, filtered, washed

81

with excess of acetone. The solvent was removed under vacuum to give white

solid. The solid obtained was dissolved in diethyl ether and washed with

water (3 300 mL) to remove the unreacted 4-hydroxyacetanilide. The

organic layer was dried over anhydrous sodium sulphate, solvent removed

under vacuum and recrystallized from methylene chloride to give the 3.3 g of

bright-white crystals of 4-(decyloxy)acetanilide (Yield 60%) (Henderson et al

2001). Similar procedure was adopted for the preparation of hexyloxy,

heptyloxy, octyloxy nonyloxy, undecyloxy and dodecyloxy acetanilides.

2.3.10 Synthesis of 4-(Alkyloxy)anilines (10a-10g)

Figure 2.10 Synthesis of compounds 10e

4-(Alkyloxy)anilines (10a-10g) were synthesized by the following

method and as a representative synthetic procedure for the series, the

synthesis of compound 4-(decyloxy)aniline (10e) is as follows: The

compound 4-decyloxyacetanilide (5.82 g, 20 mmol) was dissolved in ethanol

(150 mL), 20 mL of concentrated HCl in ethanol (25 mL) added drop wise to

the mixture. The reaction mixture was heated to reflux for 12 h, cooled,

poured into ice-water mixture. The white solid thus obtained was extracted

with ether, washed with water (3 100 mL), brine solution (3 100 mL) and

dried over anhydrous sodium sulphate. The solvent was removed under rotary

evaporator to give 3.9 g of white crystal (Yield 81%). Similar procedure was

adopted for the preparation of hexyloxy, heptyloxy, octyloxy nonyloxy,

undecyloxy and dodecyloxy anilines.

82

2.4 SYNTHESIS OF TARGET COMPOUNDS

2.4.1 Synthesis of 1,3-Substituted Phenylenebis(4-(10-undecenoyl

oxy)-1,1'-biphenyl-4-carboxylate)s (Ia-Ie)

Figure 2.11 Synthesis of compounds Ia-Ie

A typical procedure for the synthesis of Ia-Ie is as follows:

4'-(10-undecenoyloxy)biphenyl-4-carboxylic acid (3.8 mmol), resorcinol (1.9

mmol) and dicyclohexylcarbodiimide (DCC) (8.7 mmol) were dissolved in

dry dichloromethane (25 mL), to this solution N,N'-(dimethylamino)pyridine

(DMAP; 5.0 mmol) added and then stirred at room temperature for 5 h. The

precipitate thus obtained was removed by filtration and the precipitate

dissolved in dichloromethane and filtered. The filtrate was washed with 5%

HCl (3 50 mL), saturated NaCl (3 50 mL), and followed by water. The

separated organic layer was dried over anhydrous sodium sulphate and

solvent removed under reduced pressure. The product was purified by column

chromatography using chloroform as eluent, and recrystallized from ethanol

O O

O O

O

O O

OR4

R2

DCC/DMAPDCM

8 8

HO

O

O

O

8

HO OH

R4

R2

+

Ia (R2=R4=H)

Ib (R2=H; R4=COOCH3)

Ic (R2=H; R4=Cl)

Id (R2=NO2; R4=H)

Ie (R2=NO2;R4=COOCH3)

83

to give white solid (yield 85 %). The above synthetic procedure was adopted

for all other homologoues.

2.4.2 Synthesis of 3-(4-(4-(Alkyloxy)benzoyloxy)benzoyloxy)phenyl

4'-(10-undecenoyloxy)biphenyl-4-carboxylates (IIa-IIg)

Figure 2.12 Synthesis of compounds IIa-IIg

A typical procedure for the synthesis of IIa-IIg is as follows: To a

mixture of 4'-(10-undecenoyloxy)biphenyl-4-carboxylic acid (10 mmol),

4-((3-hydroxyphenoxy)carbonyl)phenyl 4-(hexyloxy)benzoate (10 mmol)

DCC (15 mmol), and a catalytic amount of DMAP in dry dichloromethane

(100 mL) were stirred for 12 h under N2 atmosphere. Precipitated

N, N -9-dicyclohexylurea was filtered out, washed with excess of

dichloromethane and solvent removed in a rotary evaporator. The residue was

purified by silica gel column using chloroform as eluent. The product

obtained on removal of chloroform was further purified by column

chromatography using a mixture of chloroform and hexane (1:3) to get 80%

yield. The similar procedure was adopted for the preparation of other alkyl

84

series such as hexyl, heptyl, octyl, nonyl, decyl, undecyl, and dodecyl

containing compounds.

2.4.3 Synthesis of 3-(4-(4-(Alkyloxy)benzoyloxy)benzoyloxy)phenyl

4-((4-(alkyloxy)phenylimino)methyl)benzoates (IIIa-IIIg)

Figure 2.13 Synthesis of compounds IIIa-IIIg

A typical procedure for the synthesis of IIIa-IIIg is as follows:

A solution of 4-decyloxyaniline (1.43 g, 6.4 mmol) and 3-(4-(4-

(hexyloxy)benzoyloxy)benzoyloxy)phenyl-4-formylbenzoate (1.20 g,

3.2 mmol) in chloroform (100 mL) was heated under reflux for 4 h. The

reaction mixture was concentrated and recrystallized from ethanol to get

(1.78 g, 71%) yellow crystals.

Compound IIIa IIIb IIIc IIId IIIe IIIf IIIg

n 6 7 8 9 10 11 12

85

2.4.4 Synthesis of 3-(4-(4-(Alkyloxy)benzoyloxy)benzoyloxy)phenyl-

4-((4-(decyloxy)phenylimino)methyl)benzoates (IVa-IVg)

Figure 2.14 Synthesis of compounds IVa-IVg

A typical procedure for the synthesis of compounds IVa-IVg is

similar to the above discussed compounds IIIa-IIIg.

2.5 CHARACTERIZATION OF COMPOUNDS

In our studies, a combination of different experimental techniques

has been used to characterize the structural and phase behavior of liquid

crystalline materials. They include direct space techniques such as

spectroscopy (to ascertain the chemical structure), polarized optical

microscopy (POM) (to identification of mesophase) and X-ray diffraction

study (to conformation of mesophase), electro-optical study (to identify polar

property of the compound). Differential scanning calorimetry (DSC) was

employed to study the thermal stability and thermal transitions occurring in

liquid crystalline systems during heating and cooling ramps respectively.

Compound IVa IVb IVc IVd IVe IVf IVg

n 6 7 8 9 10 11 12

86

2.5.1 Fourier Transform-Infrared Spectroscopy

Fourier transform-infrared spectroscopy (FT-IR) is

multidisciplinary analytical tool yields information pertaining to the structural

details of a material. FT-IR involves the absorption of electromagnetic

radiation in the infrared region of the spectrum which results changes in the

vibrational energy of a molecule. It is a valuable and formidable tool in

identifying organic compounds has polar chemical bonds such as OH, NH,

CH, etc., with good charge separation. Since every functional group has

unique vibrational energy, the IR spectra can be seen as their fingerprints.

FT-IR spectrometer (Perkin Elmer PE 1600 FT-IR) was used to substantiate

the formation of products in this study. The spectra recorded for liquid

samples were made into a thin film in between two KBr windows. On the

other hand, solid samples were recorded by making KBr (Merck, IR Grade)

pellets. About 10 mg of the sample was ground with about 70 mg of spectral

grade KBr to form a mixture, which was then made into a pellet using a

hydraulic press. All the spectra were recorded in the range 4000-400 cm-1 at a

resolution of 4 cm-1 with a maximum of 100 scans. A background spectrum

was run before recording the spectra for each sample. The spectral calibration

of the instrument was made using a polystyrene film at regular intervals of

time.

2.5.2 Nuclear Magnetic Resonance Spectroscopy

Nuclear Magnetic Resonance (NMR) is a spectroscopic method is

even more important to the organic chemist than infrared spectroscopy. Many

nuclei may be studied by NMR techniques, but hydrogen and carbon are most

commonly investigated. Whereas infrared spectroscopy reveals the types of

functional groups present in a molecule, NMR gives information about the

number of magnetically distinct atoms of the type being studied.

87

High-resolution 1H and 13C-NMR spectra were recorded using

Brucker MSL 300P, 300 and 75.4 MHz NMR spectrometer. Deutriated

chloroform [Aldrich, CDCl3, 99.8% containing 0.03% v/v tetramethylsilane

(TMS)] was used as solvents for recording NMR spectra. The proton NMR

spectra were recorded using broadband inverse probe where the inner coil for

the protons and outer coil for ‘X’ nuclei. Solvent suppression was applied in

some cases where the solvent signal is very strong compared to the sample

signals. 13C spectra were recorded in the dual (13C/1H) probe where the inner

coil for 13C and the outer coil for protons.

2.5.3 Differential Scanning Calorimetry

Differential scanning calorimeter (DSC) has become a method of

choice for quantitative studies of thermal transition in polymers. Differential

scanning calorimetry was performed using the Perkin Elmer DSC-7 model

and Mettler Toledo STAR@ system thermal analysis unit attached to a DSC

module. The experiments were carried out in nitrogen atmosphere at a heating

rate of 10 C/min from ambient to 500 C with a nitrogen flow of 10 mL/min.

Generally, DSC measures the power released or absorbed by

materials during temperature treatments that can include dynamic (i.e.,

heating or cooling ramps) or isothermal segments. The measurement is

performed by comparing the temperature of the sample and that of the

reference materials. The instantaneous heat flux is computed from this

temperature difference using instrumental calibration constants. Standard

samples like pure indium or zinc with known transition enthalpies and

temperatures are used for the calibration.

The measuring cell of a calorimeter includes the sample and

reference materials enclosed in a single furnace. The DSC furnace is made of

silver and separated from the DSC sensor by a ceramic plate. The temperature

88

of each of the two containers (pans) is measured by thermocouples connected

in series and located around each of them.

The measure of the enthalpy variation can allow assigning a given

thermal event to a polymorphic crystal to crystal or to a mesophase to

mesophase transition in LC systems. This is based on the fact that the

enthalpy variation associated with crystal melting by far more important than

the one corresponding to the mesophase to mesophase or mesophase to

isotropic phase transitions. The assignment can become difficult when one

deals with ordered mesophases, sometimes called “soft crystals”, which

exhibit transition enthalpies comparable to the one corresponding to crystal

melting.

The DSC is a convenient tool to measure the temperatures and

transition enthalpies to determine the phase diagram of the system and to

study the kinetics of transitions as a function of heating/cooling rates or as a

function of time. Differential scanning calorimeter has become a method of

choice for quantitative studies of thermal transition in polymers.

2.5.4 Polarizing Optical Microscope

Polarizing Optical Microscope (POM) was carried out to find the

texture analysis and also to determine the phase transition with sensitivity of

0.1 C. Polarizing microscope studies were performed with a Euromex

polarizing microscope attached with a Linkem HFS 91 heating stage and a

TP-93 temperature programmer. Samples were placed in between two thin

glass cover slips and melted with heating and cooling at the rate of 2°C/min.

The photographs were taken from Nikon FM10 camera and printed on a

Konica 400 film. All the microphotographs were taken from the second

cooling stage of isotropic transition temperature up to melting temperature.

89

2.5.5 X-Ray Diffraction Measurements

X-Ray diffraction measurements were carried out to investigate the

texture of the mesophase. Generally powder samples and in a few cases

oriented samples were used to obtain diffraction patterns of the mesophases of

bent-core compounds. The powder samples held in sealed Lindemann

capillaries (diameter: 0.7 mm; wall thickness: 0.01 mm) were cooled from

isotropic state to the mesophase and irradiated. Oriented patterns were

obtained by slow cooling of a drop of sample on a glass plate from the

isotropic state.

The X-rays were generated by a 4 kW rotating anode generator

(Rigaku Ultrax 18). The beam was monochromated to obtain a radiation of

wavelength 1.54 Å (Cu-K radiation) by using a graphite crystal. A double slit

arrangement was used to collimate the beam, which subsequently interacted

with the sample in a sample holder. The temperature of the sample holder was

controlled by a computer to an accuracy of 0.1 ttern of

the mesophase was collected on a two-dimensional image plate detector. A

schematic representation of the X-ray set- up used is shown in Figure 2.15.

The layer spacing of the mesophase was calculated using Bragg’s Equation

(2.1),

(2.1)

where n = 1 (for first order reflection)

= 1.54 Å (wavelength of Cu-K radiation)

d = measured layer spacing

-1(R/D)

R = Radius of the diffraction pattern

D = Distance between the sample and the detector

90

Figure 2.15 Block diagram showing the X-ray diffraction experimental

set-up

The tilt angle of the molecules in the mesophase can be calculated

using the following Equation (2.2).

-1(d/l) (2.2)

where l = calculated molecular length.

2.5.6 Electro-Optical Investigations

To study the polar properties of the banana mesophases and to

measure the polarization value for the mesophase, the standard triangular-

wave method was used (Miyasato et al 1983). A block diagram of the

experimental set-up used is shown in Figure 2.16. The electro-optical

experiments were carried out using sample cells made up of transparent glass

plates, coated with indium tin oxide (ITO). In order to obtain a planar

alignment of the sample, the inner surfaces of these conducting glass plates

were coated with polyimide and unidirectionally rubbed. The thickness of the

cell was controlled by using Mylar spacers and the thickness was measured by

interferrometric technique. In some experiments, the commercially obtained

91

(EHC, Japan) cells were also used. The sample was filled in to the cell in the

isotropic phase and cooled slowly to get a good alignment of the sample. The

triangular wave of definite amplitude and frequency was produced by using a

wave from generator (Wavetek model 39) amplified hundred times using an

amplifier (Trek model 601-B). The output from the amplifier was divided into

two channels CH1 and CH2. The waveform Channel CH1 was directly

connected to the oscilloscope, which acted as a reference signal; CH2, the

output signal from the sample connected to the oscilloscope (Agilent 54621A)

via a 10 (or 1) k resistance. The resultant curve obtained on the oscilloscope

screen was a plot of switching current versus time. The polarization value was

calculated by integrating the area under the peaks obtained in the experiment

using the relation.

Figure 2.16 Block diagram of a circuit used for the measurement of

polarization