LC Applications Behzad Pourabbas Polymer Eng. Department Sahand University of Technology Tabriz-Iran

Overview: Order Parameter Anisotropic Properties Light, polarization and materials 2

ORDER PARAMETER “S”

 n The Order Parameter n perfect crystal isotropic fluid

Maier-Saupe Theory - Mean Field Approach Temperature Nematic Liquid Crystal Isotropic Fluid Order Parameter, S nn

The Order Parameter: How does it affects display performance ? The order parameter, S, is proportional to a number of important parameters which dictate display performance. ParameterNomenclature  Elastic ConstantK ii S 2 Birefringence  n S Dielectric Anisotropy  S Magnetic Anisotropy  S Viscosity Anisotropy  S Example: Does the threshold switching voltage for a TN increase or decrease as the operating temperature increases. Scales as the square root of S therefore lowers with increasing temperature proportional to

Response to Electric and Magnetic Fields

External Electric Field and Dielectric Properties of LC molecules

Anisotropy: Dielectric Constant E    E  positive negative all angles in the plane  to E are possible for the -  materials E

Anisotropy: Duel Frequency MLC-2048 (EM Industries), Duel Frequency Material Frequency (kHz) Dielectric Anisotropy (  ) low frequency,  >0 high frequency,  <0

Dielectric Constant   L = C = q/V Dielectric Constant

Dielectric Material? E Dielectric materials consist of polar molecules which are normally randomly oriented in the solid. They are not conductors. When a dielectric material is placed in an external electric field, the polar molecules rotate so they align with the field. This creates an excess of positive charges on one face of the dielectric and a corresponding excess of negative charges on the other face.

Dielectric Material is smaller in many materials than it would be in a vacuum for the same arrangement of charges. Eg. Parallel plates: EoEo ++++ Dielectric material This makes the potential difference smaller (V=Ed) between the parallel plates of the capacitor for the same charges on the plates and thus capacitance is larger, since Q=C/V. EiEi Net field: E=E o -E i

Dielectric Constant (“kappa”) = “dielectric constant” = (a pure number ≥ 1) So, (for parallel plates) Or Where C 0 is the capacitance without the dielectric. Hence, the capacitance of a filled capacitor is greater than an empty one by a factor

Dielectric Constants o C, 1kHz) *Mixture Application     BL038PDLCs MLC-6292TN AMLCDs ZLI-4792TN AMLCDs TL205AM PDLCs Fiber-Optics  material MaterialsDielectric Constant Vacuum Air Polystyrene2.56 Polyethylene2.30 Nylon3.5 Water78.54 *EM Materials PD: Polymer Dispersed AM: Active Matrix TN: Twisted Nematic

Flow of ions in the presence of electric field Internal Field Strength E = E 0 – E’

S = 0 1 > S > 0 Alignment of LC molecules in Electric Field

  Dielectric Anisotropy and Permanent Dipole Moment

Dielectric Constants: Temperature Dependence 4’-pentyl-4-cyanobiphenyl Temperature Dependence Average Dielectric Anistropy

Dielectric Anisotropy and Induced Dipole Moment

Magnetic Anisotropy: Diamagnetism Diamagnetism: induction of a magnetic moment in opposition to an applied magnetic field. LCs are diamagnetic due to the dispersed electron distribution associated with the electron structure. Delocalized charge makes the major contribution to diamagnetism. Ring currents associated with aromatic units give a large negative component to  for directions  to aromatic ring plane.  is usually positive since:

Magnetic Anisotropy: Diamagnetism Compound

Examples

Magnetic Susceptibility and Anisotropy

LIGHT, POLARIZATION AND MATERIALS 27

Optical polarization 28 for any wavevector, there are two field components light is a transverse wave: perpendicular to any wave may be written as a superposition of the two polarizations

Light as Electromagnetic Wave Plane Polarized light can be resolved into E x and E y

BIREFRENGENCE 32

Birefringence

O rdinary light travels in the crystal with the same speed v in all direction. The refractive index n 0 =c/v in all direction are identical. E xtraordinary light travels in the crystal with a speed v that varies with direction. The refractive index n 0 =c/v also varies with different direction

Interaction of Electromagnetic Wave with LC Molecules

Optical Anisotropy: Birefringence ordinary ray (n o, ordinary index of refraction) extraordinary ray (n e, extraordinary index of refraction)

Optical Anisotropy: Birefringence ordinary wave  extraordinary wave For propagation along the optic axis, both modes are n o optic axis

Birefringence (20 o 589 nm) EM Industry  n n e n o Application Mixture BL PDLC TL PDLC TL AM PDLC ZLI STN ZLI TN ZLI AM TN LCDs MLC AM TN LCDs ZLI AN TN LCDs MLC ECB  devices MLC IPS MLC IPS Fiber Optics ZLI  device

Birefringence: Temperature Dependence Average Index Temperature Dependence

CIRCULAR POLARIZATION OF LIGHT

Circular Birefringence

Categories of optical polarization 44 linear (plane) polarization coefficients differ only by real factor circular polarization coefficients differ only by factor elliptical polarization all other cases

Characterizing the optical polarization 45 wavevector insufficient to define electromagnetic wave we must additionally define the polarization vector e.g. linear polarization at angle

Reflection of Circular Polarized Light LCP RCP

Dynamic Scattering Mode LCD Device

Twisted Nematic (TN) Device 1971 by Schadt

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 V s : turn on voltage V ns: turn off voltage

Electrically Controlled Birefringence (ECB) Device (DAP type)

Polymer Dispersed Liquid Crystal (PDLC) Device

GENERAL STRUCTURE 55

A X Y Z Z’ Aromatic or saturated ring core X & Y are terminal groups A is linkage between ring systems Z and Z’ are lateral substituents CH 3 - (CH 2 ) 4 C N 4-pentyl-4’-cyanobiphenyl (5CB) General Structure

Mesogenic Core Linking Groups Ring Groups N N phenyl pyrimidine cyclohexane biphenyl terphenyl diphenylethane stilbene tolane schiffs base azobenzene azoxyben- zene phenylbenzoate (ester) phenylthio- benzoate Common Groups

Nomenclature Mesogenic Core phenyl benzyl benzene biphenyl terphenyl phenylcyclohexane (PCH) cyclohexane cyclohexyl Ring Numbering Scheme 3’2’ 1’ 6’5’ 4’

Terminal Groups (one terminal group is typically an alkyl chain) CH 3 CH 2 CH 3 CH 2 C*H CH 2 CH 3 straight chain branched chain (chiral) Attachment to mesogenic ring structure Direct - alkyl (butyl) Ether -O- alkoxy (butoxy)

CH 3 - CH 3 -CH 2 - CH 3 -(CH 2 ) 2 - CH 3 -(CH 2 ) 3 - CH 3 -(CH 2 ) 4 - CH 3 -(CH 2 ) 5 - CH 3 -(CH 2 ) 6 - CH 3 -(CH 2 ) 7 - methyl ethyl propyl butyl pentyl hexyl heptyl octyl CH 3 -O- CH 3 -CH 2 -O- CH 3 -(CH 2 ) 2 -O- CH 3 -(CH 2 ) 3 -O- CH 3 -(CH 2 ) 4 -O- CH 3 -(CH 2 ) 5 -O- CH 3 -(CH 2 ) 6 -O- CH 3 -(CH 2 ) 7 -O- methoxy ethoxy propoxy butoxy pentoxy hexoxy heptoxy octoxy Terminal Groups

Second Terminal Group and Lateral Substituents (Y & Z) H - Fflouro Clchloro Brbromo Iiodo CH 3 methyl CH 3 (CH 2 ) n alkyl CNcyano NH 2 amino N(CH 3 )dimethylamino NO 2 nitro phenyl cyclohexyl

Odd-Even Effect Clearing point versus alkyl chain length carbons in alkyl chain (n) clearing point CH 3 -(CH 2 ) n -OO-(CH 2 ) n -CH 3 C-O O

CH 3 -(CH 2 ) 4 C N CH 3 -(CH 2 ) 4 -O C N 4’-pentyl-4-cyanobiphenyl 4’-pentoxy-4-cyanobiphenyl Nomenclature Common molecules which exhibit a LC phase

Structure - Property N N CH 3 -(CH 2 ) 4 C N vary mesogenic core A AC-N ( o C)N-I( o C)  n 

Structure - Property CH 3 -(CH 2 ) 4 COO vary end group X XC-N ( o C)N-I ( o C) H F Br CN CH 3 C 6 H

Lateral Substituents (Z & Z’) A X Y Z Z’ Z and Z’ are lateral substituents Broadens the molecules Lowers nematic stability May introduce negative dielectric anisotropy

E Solid Liquid Crystal Isotropic Liquid Concentration (  2 ), % Why Liquid Crystal Mixtures Melt Temperature: Liquid Crystal-Solid ln  i =  H i (T eu -1 - T mi -1 )/R  H: enthalpies T eu : eutectic temperature T mi : melt temperature R: constant Nematic-Isotropic Temperature: T NI T NI =   i T NI i Temperature eutectic point