1. Chapter 2: Liquid Crystals States between crystalline and
isotropic liquid

2. Liquid Crystals,
Before discovery of LC, Lehmann designed a microscope that could be used to monitor phase transition process.

3. 1888 by Prof. Reinitzer, a botanist, University of Prague, Germany

4. Phase Transition first defined by
Georges Freidel in 1922

6. The ordering parameter
S=1/2<3cos2Q-1>
S=0, isotropic
S=1, Ordered
Nematic, S=

8. Smectic Liquid Crystals
Classification of
Smectic Liquid Crystals
A type: molecular alignment perpendicular to the surface of the layer, but lack of order within the layer.
B type: molecular alignment perpendicular to the surface of the layer, having order within the layer.
C type: having a tilted angle between molecular alignment and the surface of the layer.

9. Smectic B Liquid Crystals

10. Smectic C Liquid Crystals

11. Smectic A Liquid Crystals

12. More Detailed Classification of Smectic Phases

13. Nematic Liquid Crystals

14. Cholesteric Phase Liquid Crystals

15. Polymeric Liquid Crystal

16. For Display Propose Low Viscosity Fast Response Time
Advantages of Nematic Phase and Cholesteric Phase LC
For Display Propose
Low Viscosity
Fast Response Time

17. Discotic Liquid Crystals

19. Response to Electric and Magnetic Fields

20. External Electric Field and Dielectric Properties of LC molecules

21. Dielectric Constant
ke0L = C = q/V

22. Flow of ions in the presence of electric field
Internal Field Strength E = E0 – E’

23. Alignment of LC molecules in Electric Field
S = > S > 0

24. Dielectric Anisotropy and Permanent Dipole Moment

25. Dielectric Anisotropy and Induced Dipole Moment

27. Examples

28. Magnetic Susceptibility and Anisotropy

31. Light as Electromagnetic Wave
Plane Polarized light can be resolved into Ex and Ey

34. Birefringence

35. Extraordinary light travels in the crystal
Ordinary light travels in the crystal with the same speed v in all direction.
The refractive index n0=c/v in all direction are identical.
Extraordinary light travels in the crystal
with a speed v that varies with direction.
The refractive index n0=c/v also varies
with different direction

36. Generation of polarized light by crystal birefringence

37. Interaction of Electromagnetic Wave with LC Molecules

40. Circular Birefringence

43. Reflection of Circular Polarized Light

44. Devices for Liquid Crystal Display

45. Designs of LC cell
Electronic Drive
AM: active matrix; TFT: thin film transistor;
MIM: metal-insulator-metal

46. Alignment of LC molecules in a Display Device

48. Dynamic Scattering Mode LCD Device

49. Twisted Nematic (TN) Device 1971 by Schadt

50. Optical Response of a Twisted Nematic (TN) Device
Applied voltages and optical response

51. Super Twisted Nematic (STN) LC Device 1984 by Scheffer
By addition of appropriate amounts of chiral reagent
Twisted by o
N:Number of row for scanning
Vs: turn on voltage
Vns:turn off voltage

52. Sharp change in the voltage-transmittance curve

53. Electrically Controlled Birefringence (ECB) Device (DAP type)

55. Black and White
RF-STN Device

57. Optical response of Nematic LC in a Phase-Change
Guest-Host Type Device (by G. Heilmeier)

58. Phase Change (PC) in a Guest Host (GH) LC Device

59. In-Plane Switching (IPS) type LC Device

60. Polymer Dispersed Liquid Crystal (PDLC) Device

61. Polymeric Nematic LC Materials

62. Active Matrix LCD

63. Structure of a typical
LC Display

65. Hybrid Aligned Nematic (HAN) type
Fast response time,
Upto ms scale.

66. Full color reflective display

67. References
Liquid Crystals, P. J. Collings, Princeton
Introduction to liquid crystals, P. J. Collings and M. Hird, Taylor and Francis
Flat Panel Displays, J. A. Connor, RSC.

68. Structure of rigid rod like liquid crystal molecules
Core group: usually aromatic or alicyclic; to make the structure linear and rigid
Linker: maintaining the linearity and polarizability anisotropic.
Terminal Chain: usually aliphatic chain, linear but soft so that the melting point could be reduced. Without significant destroy the LC phase. Note that sometimes one terminal unit is replaced by a polar group to provide a more stable nematic phase.
Side group: to control the lateral interaction and thereore enhance the chance for nematic. Note that large side groups will weaken the lateral interaction

69. Common components for LC molecules
Core Group
Linker A, B
-(CH=N)-; -(N=N)-
-(N=NO)-; -(O-C=O)-
Terminal Group X, Y
Non-polar flexible groups
-R, -OR, -O2CR
Polar rigid group
-CN, -CO2H, -NO2, -F, -NCS
Side Branch
-F, -Cl, -CN, -CH3

70. Character of LC molecules
Rod like or Discotic
Empirical Length/Diameter parameter for LC phase  4 (Flory theory predicted critical L/D ratio = 6.4; Onsager theory predicted critical L/D ratio = 3.5)
Having polar or highly polarizable moiety
Large enough rigidity to maintain the rod or discotic like structure upon heating
Chemically stable.
Phase transition temperature is determined by DH and DS. At TCN or TNI, DGo = DHo –TDSo= 0. Therefore TCN= DHoCN/DSoCN and TNI= DHoNI/DSoNI

71. L D
L/D > 4 Ti > Tm (nematic)

72. When the length of the molecules increases, van der Waal’s interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.
In the above examples, the critical L/D is around 4. When L/D = 1, 2, or 3, no LC phase was observed.

73. 67 o
6-10 o
Flexible linker L D
Nematic phase could not be observed until L/D >4

74. 67 o
6-10 o
This type of linker group is more flexible. Entropy gain is more effective in isotropic liquid state. Therefore DSN-I is relatively large, leading to a low Ti. In the presence case, even for the LC molecules having the L/D upto 5.1, the Ti is only 254 oC

75. Other Options for the core group.

76. Thermal Stability: DT
TC-N
TN-I
Crystal
Nematic LC
Isotropic Liquid
Low TC-N; high TN-I
larger DT = TN-I - TC-N , higher the stability of the LC state
In general, shorter the LC molecule, lower the phase transition temperature it has.
For LC molecule contains more polarizable aromatic cores, or longer the body, Vander Waals interactions between LC molecules will increase. This will lead to higher thermal stability.

77. Nematogenic: structures that lead to nematic phase as the only LC phase
Smectogenic: smectic phase is the only mesophase exhibited
Calamitic: Both nematic and smectic phases would exhibited.

78. Smectic Phase
Smectic LC phase: Lamellar close packing structure are favored by a symmetrical molecular structure; Wholly aromatic core-alicyclic core each with two terminals alkyl/alkoxyl chains compatible with the core ten to pack well into a layer-like structures and generates smectic phase.
Long alkyl/alkoxyl chain would lead to strong lateral interactions that favors lamellar packing smectic phase formation.

79. Terminal groups for smectic phase
Salts from RCO2H/RNH2
Terminal groups contain -CO2R, -CH=CHCOR, -CONH2, -OCF3, -Ph, -NHCOCH3, -OCOCH3

80. Terminal group for nematic
Short chain

81. For Smectic Phase
NHCOCH3 > Br > Cl > F > NMe2 > RO > H > NO2 > OMe
For nematic Phase
NHCOCH3 > OMe > NO2 > RO > Br~ Cl > NMe2 > Me >F > H
-CN,-NO2 -MeO are nematogen: poor smectic/good nematic
-NHCOCH3, halogen, -NR2, good smectic/nematic

82. Nematic Phase.
Due to its fast response time, the nematic LC phase is technologically the most important of the many different types of LC phase
The smectic phases are lamellar in structure and more ordered than the nematic phase.
The smectic phases are favored by an symmetrical molecular structure.
Any breaking of the symmetry or where the core is long relative to the overall molecular length tends to destabilized the smectic formation and facilitate the nematic phase formation.

83. At least two rings are required to enable the generation of LC phase.
The nematic phase tends to be the phase exhibited when the conditions for the lamellar packing (smectic) cannot be met.
Molecular features for nematic phase: (a) breaking of the symmetry or (b) short terminal chain.

86. Stereochemistry of alicyclic systems
No LC phase

89. Heteroatom
effects
The heteroatoms enhances the polarity and higher melting point are seen. Nematic phase transition temperature is low than the melting point. The large sulfur atom further disrupts the nematic packing. The flexible sulfur containing ring gains more entropy from N to I and therefore lead to lower TNI.

90. MM2 space-filling models

91. The TCN and TNI orders: dicyclooctane > cyclohexane > phenyl

92. MM2 calculation
Linear
structure
Bent structure

93. Extending the number of the rings

94. Linking group:
Linking groups are used to extend the length and polarizability anisotropy of the molecular core in order to enhance the LC phase stability by more than any increase in melting point, producing wider LC phase ranges.
(A) Linking group should maintain the linearity of the molecule.

95. Odd number of CH2: Bent
Even number of CH2: Linear

96. (b) Linker groups that connect aromatic core units with the conjugation extended over the longer molecules. This could enhance the polarizability anisotropy.

98. Terminal Flexible Long Chain:
The function of the terminal flexible long chain is to suppress the melting point without serious destroying the LC phase.

102. Lateral Substitution
Lateral substitution is important in both nematic/smectic systems. However, because of the particular disruption to the lamellar packing, necessary for smectic phases, lateral substitution nearly always reduces smectic phase stability more than nematic phase stability except when the lateral substitutions lead to a strong dipole-dipole interaction.

106. Not quite linear for some substituents

108. Electronic effects arising from the lateral groups

109. Mixing of two Components may generate a LC phase

110. Mixture of two Components
A mixture of MBBA (60%) and EBBA (40%) would lead to LC at room temperature

111. Temperature Dependent Rotation of the Cholesteric Phase

114. Lyotropic Liquid Crystal Polymers
Fairly rigid rod like polymers; but soluble in certain solvents to form a LC phase

116. Examples
Poly(p-phenylenebenzobisthiazole) PBT
Soluble in PPA or H2SO2 and could be fabricated as high tensile strength polymeric wires