Скачать презентацию Liquid Crystal Materials Broad Classification Lyotropics Thermotropics Скачать презентацию Liquid Crystal Materials Broad Classification Lyotropics Thermotropics

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Liquid Crystal Materials Liquid Crystal Materials

Broad Classification Lyotropics Thermotropics amphiphilic molecules, polar and non-polar parts form liquid crystal phases Broad Classification 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. hydrophobic non-polar tail + - hydrophilic polar head flexible tail rigid core

The Lyotropic Phases micelle cross section reverse micelle cross section The Lyotropic Phases micelle cross section reverse micelle cross section

The Thermotropic Liquid Crystal Molecule Chemist’s View CN Physicist’s Engineer’s View • Shape Anisotropy The Thermotropic Liquid Crystal Molecule Chemist’s View CN Physicist’s Engineer’s View • Shape Anisotropy • Length > Width The molecule above (5 CB) is ~2 nm × 0. 5 nm

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

The Liquid Crystal Phase n Crystal Nematic LC Isotropic Temperature The Liquid Crystal Phase n Crystal Nematic LC Isotropic Temperature

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

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

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

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

The Chiral Doped Nematic You can create a cholesteric material by doping a conventional The Chiral Doped Nematic You can create a cholesteric material by doping a conventional nematic with a chiral dopant. For dilute solutions Chiral Dopant S-811 IS-4651 HTP (mm)-1 -14 -13. 6 - indicates left twist sense For a 10% doping of S-811

The Chiral Smectic C: Ferroelectrics q m Eye- dipole moment m fin - chiral The Chiral Smectic C: Ferroelectrics q m Eye- dipole moment m fin - chiral ferroelectric LC has a dipole moment perpendicular to its long axis, and is chiral.

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

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

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

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

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

Example of Side-Chain Polymer LCs R 1 -(-CH 2 -C-)XO C-O-(CH 2)n-O • • Example of Side-Chain Polymer LCs R 1 -(-CH 2 -C-)XO C-O-(CH 2)n-O • • • O C-O R 2 Too slow for display applications (switching times ~ 0. 5 -1 s Useful for other applications such as: Optical filters Optical memory Alignment layers for low molecular weight LCs Non-linear optic devices (optical computing)

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

Maier-Saupe Theory - Mean Field Approach Interactions between individual molecules are represented by a Maier-Saupe Theory - Mean Field Approach Interactions between individual molecules are represented by a potential of average force n q • {V: minimum} when phase is ordered ( -P 2(cosq)) • {V: V=0} when phase is disordered (

) • factor for intermolecular strength ( n) y f From Statistical Mechanics (Self Consistency) b=(k. T)-1

Maier-Saupe Theory - Mean Field Approach n Order Parameter, S 1. 0 Isotropic Fluid Maier-Saupe Theory - Mean Field Approach n Order Parameter, S 1. 0 Isotropic Fluid 0. 0 Nematic Liquid Crystal n -0. 6 Temperature

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

The Order Parameter: How does it affects display performance ? The order parameter, S, 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. Parameter Elastic Constant Birefringence Dielectric Anisotropy Magnetic Anisotropy Viscosity Anisotropy Nomenclature Kii Dn De Dc Dh proportional to S 2 S 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

Anisotropy: Dielectric Constant Off-axis dipole moment, angle b with molecular axis b N: h, Anisotropy: Dielectric Constant Off-axis dipole moment, angle b with molecular axis b N: h, f: S: Da: m: k. B: T: number density reaction field, reaction cavity parameters order parameter anisotropy in polarizability molecular dipole moment Boltzman constant Temperature For values of the angle b<54. 7 o, the dipolar term is positive, and for values b>54. 7 o, the dipolar term is negative, and may result in a materials with an overall -De.

Anisotropy: Dielectric Constant ++ +++ positive E e -- -- De = e - Anisotropy: Dielectric Constant ++ +++ positive E e -- -- De = e - e negative - + - + >0 E De = e - e e E <0 all angles in the plane to E are possible for the -De materials

Anisotropy: Duel Frequency low frequency, De>0 high frequency, De<0 MLC-2048 (EM Industries), Duel Frequency Anisotropy: Duel Frequency low frequency, De>0 high frequency, De<0 MLC-2048 (EM Industries), Duel Frequency Material Frequency (k. Hz) 0. 1 1. 0 10 50 Dielectric Anisotropy (De) 3. 28 3. 22 0. 72 -3. 0 100 -3. 4