Liquid Crystal Materials

Lyotropics Thermotropics amphiphilic molecules, polar and non-polar parts form liquid crystal phases over certain concentration ranges when mixed with a solvent molecules consisting of a rigid core and flexible tail(s) form liquid crystal phases over certain temperature ranges hydrophilic polar head hydrophobic non-polar tail flexible tail rigid core Broad Classification

The Lyotropic Phases micelle reverse micelle cross section

CN Chemist’s View Physicist’s Engineer’s View Shape Anisotropy Length > Width The molecule above (5CB) is ~2 nm × 0.5 nm The Thermotropic Liquid Crystal Molecule

Geometrical Structures of Mesogenic Molecules Low Molecular Weight High Molecular Weight (polymers) ( ) n n disk-like rod-like most practical applications

n Temperature Crystal Nematic LC Isotropic The Liquid Crystal Phase

The Nematic Director n n The local average axis of the long molecular axis director

n Temperature Smectic C Smectic A Nematic n z  n Other Liquid Crystal Phases

left-handed right-handed mirror images non-superimposable H-C-C-C-C-C C N H HHHH H HH H H H-C-C-C-C-C C N H H H CH 3 H H HH H H non-chiral chiral (RH) The methyl group on the 2nd carbon atom on the alkyl chain of the molecules extends out of the plane of the paper and the hydro- gen atom extends into the plane of the paper. Therefore the 2nd carbon can be thought of as a right or left handed coordinate system Chirality

CN pitch P CN Ordinary Nematic Chiral Nematic director n The Chiral Nematic

The Chiral Doped Nematic You can create a cholesteric material by doping a conventional nematic with a chiral dopant. For dilute solutions Chiral DopantHTP (  m) -1 S IS indicates left twist sense For a 10% doping of S-811

The Chiral Smectic C: Ferroelectrics   Eye- dipole moment  fin - chiral ferroelectric LC has a dipole moment perp- endicular to its long axis, and is chiral.

The Chiral Smectic: TGB Twisted Grain Boundary (TGB) A twisted grain boundary smectic A phase (frustrated) - TGBA*

O C R C O C R O O C O R O C O R O C O R O C O R Discotic Liquid Crystal example: R=OCOC 11 H 23

Columnar, columns of molecules in hexagonal lattice Nematic discotic phase n Discotics Liquid Crystals n

Polymer Liquid Crystals Combining the properties of liquid crystals and polymers Main Chain Side Chain mesogenic moieties are connected head-to-tail mesogenic moieties attached as side chains on the polymer backbone rigid semi-flexible

Polymer Liquid Crystals forming nematic liquid crystal phases n main-chain side-chain

O C-O-(CH 2 ) n -O R2R2 C-O O Example of Side-Chain Polymer LCs -(-CH 2 -C-) X - R1R1 Too slow for display applications (switching times ~ s Useful for other applications such as: Optical filters Optical memory Alignment layers for low molecular weight LCs Non-linear optic devices (optical computing)

 n The Order Parameter n no order perfect order perfect crystal isotropic fluid

Interactions between individual molecules are represented by a potential of average force From Statistical Mechanics (Self Consistency) Maier-Saupe Theory - Mean Field Approach { V : minimum} when phase is ordered (  - P 2 (cos  )) { V : V =0} when phase is disordered (  ) factor for intermolecular strength (  )  =(kT) -1  n  

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

Landau-de Gennes Theory a =   (T-T*),  , b, c, T*, L are phenomenological constants G is a surface interaction strength Order Parameter, S Temperature Good near NI transition surface Predicts order near surface

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

Anisotropy: Dielectric Constant  Off-axis dipole moment, angle  with molecular axis N: number density h,f: reaction field, reaction cavity parameters S: order parameter  : anisotropy in polarizability  : molecular dipole moment k B : Boltzman constant T: Temperature For values of the angle  , the dipolar term is positive, and for values  , the dipolar term is negative, and may result in a materials with an overall - .

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