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LC Applications Behzad Pourabbas Polymer Eng. Department Sahand University of Technology Tabriz-Iran [email protected] Overview: • Order Parameter • Anisotropic Properties • Light, polarization and materials 2 ORDER PARAMETER “S” The Order Parameter n 1 2 S P2 (cos q ) (3 cos q 1) 2 q n S P2 (cos q ) 1 perfect crystal S P2 (cos q ) 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 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 proportional to De Dc Dh S2 S S S S Example: Does the threshold switching voltage for a TN increase or decrease as the operating temperature increases. K S2 VTH S De S Scales as the square root of S therefore lowers with increasing temperature Response to Electric and Magnetic Fields External Electric Field and Dielectric Properties of LC molecules Anisotropy: Dielectric Constant ++ + ++ E e positive - --- De e e e >0 E negative De e e < 0 E 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 Material Frequency (kHz) 0.1 1.0 10 50 Dielectric Anisotropy (De) 3.28 3.22 0.72 -3.0 100 -3.4 Dielectric Constant Dielectric Constant ke0L = C = q/V Dielectric Material? • 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. E Dielectric Material E is smaller in many materials than it would be in a vacuum for the same arrangement of charges. Eg. Parallel plates: E k ke Eo Dielectric material Ei Eo + + + + Net field: E=Eo-Ei 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. Dielectric Constant (“kappa”) = “dielectric constant” k So, Or = (a pure number ≥ 1) C ke A (for parallel plates) d C kC 0 Where C0 is the capacitance without the dielectric. k Hence, the capacitance of a filled capacitor is greater than an empty one by a factor Dielectric Constants (@20oC, 1kHz) *Mixture Application De e e BL038 MLC-6292 ZLI-4792 TL205 18523 95-465 16.7 7.4 5.2 5 2.7 -4.2 21.7 11.1 8.3 9.1 7 3.6 5.3 3.7 3.1 4.1 4.3 7.8 PDLCs TN AMLCDs TN AMLCDs AM PDLCs Fiber-Optics -De material *EM Materials PD: Polymer Dispersed AM: Active Matrix TN: Twisted Nematic Materials Vacuum Air Polystyrene Polyethylene Nylon Water Dielectric Constant 1.0000 1.0005 2.56 2.30 3.5 78.54 Flow of ions in the presence of electric field Internal Field Strength E = E0 – E’ Alignment of LC molecules in Electric Field S=0 1>S>0 Dielectric Anisotropy and Permanent Dipole Moment m m Dielectric Constants: Temperature Dependence e 16 CH3-(CH2)4 De S(T ) 14 Dielectric Constant 4’-pentyl-4-cyanobiphenyl 12 Temperature Dependence Extrapolated from isotropic phase eis 10 De 1 2e e // 3 e 8 30 T-TNI (°C) De S(T ) Average Dielectric Anistropy 6 25 C N 35 1 De 2e e // 3 Dielectric Anisotropy and Induced Dipole Moment easily polarized + minduced is large r// e is large - + r Molecular axis - minduced is small e is small e dielectric constant along the direction perpendicular to the molecular axis e dielectric constant along the direction parallel to the molecular axis 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 c for directions to aromatic ring plane. Dc is usually positive since: Dc c ll c > 0 c ll > c Magnetic Anisotropy: Diamagnetism Dc /10 9 m3 kg 1 Compound C5H11 CN 1.51 C7H15 CN 1.37 CN 0.46 CN 0.42 C5H11 C7H15 CN C7H15 -0.38 Light is a high frequency electromagnetic wave and will only polarize electron cloud. In general, De = e e > 0 or e > e Positive De > 0 (10 to 20) Negative De < 0 (-1 to -2) For high electrical resistance materials, n is proportional to e1/2 Dn = n n > 0 in general Dn is a very important parameter for a LC device. Larger the Dn value, thinner the LC device and faster the response time Examples S C N C5H11 De = +33 C - N - I 76 98 O N C O C5H11 O C7H15 De = - 4.0 C - N - I 45 101 Magnetic Susceptibility and Anisotropy LIGHT, POLARIZATION AND MATERIALS 27 Optical polarization • light is a transverse wave: E perpendicular to k • for any wavevector, there are two field components • any wave may be written as a superposition of the two polarizations 28 Light as Electromagnetic Wave Plane Polarized light can be resolved into Ex and Ey BIREFRENGENCE 32 Birefringence 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 Interaction of Electromagnetic Wave with LC Molecules Propagation of the light is hindered by the molecule E field e// Induced dipole by electromagnetic wave Speed of the light is slowed down = C / e// Propagation of the light parallel to the molecular axis E field Induced dipole by electromagnetic wave e // Change of the speed is relatively small // = C// / e Optical Anisotropy: Birefringence ordinary ray (no, ordinary index of refraction) extraordinary ray (ne, extraordinary index of refraction) Optical Anisotropy: Birefringence n no ordinary wave extraordinary wave 1 cos2 q sin2 q 2 2 2 n no ne optic axis q For propagation along the optic axis, both modes are no Birefringence (20oC @ 589 nm) EM Industry Mixture BL038 TL213 TL205 ZLI 5400 ZLI 3771 ZLI 4792 MLC-6292 ZLI 6009 MLC-6608 95-465 MLC-6614 MLC-6601 18523 ZLI 2806 Dn 0.2720 0.2390 0.2175 0.1063 0.1045 0.0969 0.0903 0.0859 0.0830 0.0827 0.0770 0.0763 0.0490 0.0437 ne 1.7990 1.7660 1.7455 1.5918 1.5965 1.5763 1.5608 1.5555 1.5578 1.5584 ----------------1.5089 1.5183 no 1.5270 1.5270 1.5270 1.4855 1.4920 1.4794 1.4705 1.4696 1.4748 1.4752 ----------------1.4599 1.4746 Application PDLC PDLC AM PDLC STN TN AM TN LCDs AM TN LCDs AN TN LCDs ECB -De devices IPS IPS Fiber Optics -De device Birefringence: Temperature Dependence Average Index ne 1.8 Index of Refraction n 1.7 n 1 ne 2 2n02 3 2 2 1 2 2 ne 2n0 3 niso Extrapolated from isotropic phase 1.6 no Temperature Dependence 1.5 Dn S(T ) 1.4 50 40 30 T-TNI (°C) 20 10 0 CIRCULAR POLARIZATION OF LIGHT Circular Birefringence Categories of optical polarization • linear (plane) polarization • coefficients differ only by real factor • circular polarization • coefficients differ only by factor i • elliptical polarization • all other cases 44 Characterizing the optical polarization • wavevector insufficient to define electromagnetic wave • we must additionally define the polarization vector k x z • e.g. linear polarization at angle y 45 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 180-270 o N:Number of row for scanning Vs: turn on voltage Vns:turn off voltage Electrically Controlled Birefringence (ECB) Device (DAP type) Polymer Dispersed Liquid Crystal (PDLC) Device GENERAL STRUCTURE 55 General Structure Z’ Z Y A X • Aromatic or saturated ring core • X & Y are terminal groups • A is linkage between ring systems • Z and Z’ are lateral substituents CH3 - (CH2)4 4-pentyl-4’-cyanobiphenyl (5CB) C N Common Groups Mesogenic Core Ring Groups Linking Groups phenyl N pyrimidine N cyclohexane biphenyl terphenyl diphenylethane stilbene tolane schiffs base azobenzene azoxybenzene phenylbenzoate (ester) phenylthiobenzoate CH2 CH2 CH CH CH CH CH N N N O N N O CO O CS Nomenclature Mesogenic Core terphenyl biphenyl phenyl benzyl benzene phenylcyclohexane (PCH) 3’ 2’ 2 1’ 4’ 5’ 6’ cyclohexane cyclohexyl 3 1 4 6 5 Ring Numbering Scheme Terminal Groups (one terminal group is typically an alkyl chain) CH2 CH2 CH2 CH3 CH2 CH2 CH3 C*H straight chain branched chain (chiral) CH3 Attachment to mesogenic ring structure Direct alkyl (butyl) Ether -O- alkoxy (butoxy) Terminal Groups CH3- methyl CH3-O- methoxy CH3-CH2- ethyl CH3-CH2-O- ethoxy CH3-(CH2)2- propyl CH3-(CH2)2-O- propoxy CH3-(CH2)3- butyl CH3-(CH2)3-O- butoxy CH3-(CH2)4- pentyl CH3-(CH2)4-O- pentoxy CH3-(CH2)5- hexyl CH3-(CH2)5-O- hexoxy CH3-(CH2)6- heptyl CH3-(CH2)6-O- heptoxy CH3-(CH2)7- octyl CH3-(CH2)7-O- octoxy Second Terminal Group and Lateral Substituents (Y & Z) H F Cl Br I CH3 CH3(CH2)n CN NH2 N(CH3) NO2 flouro chloro bromo iodo methyl alkyl cyano amino dimethylamino nitro phenyl cyclohexyl Odd-Even Effect Clearing point versus alkyl chain length O CH3-(CH2)n-O C-O O-(CH2)n-CH3 18 clearing point 16 14 12 10 0 1 2 3 4 5 6 7 8 9 10 11 carbons in alkyl chain (n) Nomenclature Common molecules which exhibit a LC phase CH3-(CH2)4 C N 4’-pentyl-4-cyanobiphenyl CH3-(CH2)4-O 4’-pentoxy-4-cyanobiphenyl C N Structure - Property vary mesogenic core CH3-(CH2)4 A A C N C-N (oC) N-I(oC) Dn De 22.5 35 0.18 11.5 71 52 0.18 19.7 31 55 0.10 9.7 N N Structure - Property vary end group COO CH3-(CH2)4 X H F Br CN CH3 C6H5 C-N (oC) 87.5 92.0 115.5 111.0 106.0 155.0 X N-I (oC) 114.0 156.0 193.0 226.0 176.0 266.0 Lateral Substituents (Z & Z’) Z’ Z X •Z A and Z’ are lateral substituents • Broadens the molecules • Lowers nematic stability • May introduce negative dielectric anisotropy Y Why Liquid Crystal Mixtures Melt Temperature: Liquid Crystal-Solid Isotropic Liquid ln ci = DHi(Teu-1 - Tmi-1)/R Temperature Liquid Crystal DH: enthalpies Teu: eutectic temperature Tmi: melt temperature R: constant E Nematic-Isotropic Temperature: TNI eutectic point TNI = S ciTNIi Solid 0 50 Concentration (c2), % 100