Liquid Crystal and Liquid Crystal Polymer

Liquid Crystal polymer,types and its applications

1. L i q u i d C r y s t a l a n d L i q u i d C r y s t a l P o l y m e r s By, Saurav Chandra Sarma Int. Ph.D. 4 th Sem.

2. Liquid Crystal and Life Liquid crystals are also fundamentally important to life. DNA and cell membranes have liquid crystal phases. Our brains are around 70% liquid crystal, and liquid crystals are also found in muscles, the amazing iridescent colours of some insects, and also slug slime! Liquid crystals are beautiful and mysterious; I am fond of them for both reasons . - P.G. De Gennes

3. Liquid crystals (LCs)  are matter in a state that has properties between those of conventional liquid and those of solid crystal . For instance, an LC’s may flow like a liquid, but its molecules may be oriented in a crystal like way. There are many different type of LC phases, which can be distinguish by their different “optical properties” (such as birefringence. Which viewed under a microscope using a polarized light source, different liquid crystal phases will appear to have distinct textures. Introduction

4. Positional Order + Orientational Order = Crystal Phase Positional Order + No Orientational Order = Plastic Phase Varying Positional Order + Orientational Order = Liquid Crystal Phase No Positional Order + No Orientational Order = Isotropic Phase Liquid crystals are classified in terms of following criterion: (1) Translational order/ Positional Order (2) Bond orientational order (3) Correlation between smectic layers (4) With chirality? (5) Cubic structure?

5. Liquid Crystal-Is it a Solid or Liquid..??? The amount of energy required to cause the phase transition is called latent heat of the transition and is useful to measure of how different the two phases are. In the case of cholesteryl myristate , the latent heat of solid to liquid crystal is 65 calories/ gram,while the latent heat for liquid crystal to liquid transition is 7 calories/gram . The smallness the latent heat of liquid crystal to liquid phase transition is evidence that liquid crystal are more similar to liquids than they are to solids.

6. Mesophase : a phase lying between solid (crystal) and isotropic (liquid) states. Liquid crystals: fluid (l) but also show birefringence (c); have properties associated with both crystals and liquids . Thermotropic : liquid crystalline phase is formed when the pure compound is heated . Lyotropic : liquid crystalline phase forms when the molecules are mixed with a solvent ( solution ). Liquid Crystalline Phases

7. No translational order— Nematics The word “ Nematic " is derived from the Greek word for thread-like structure. It is the only liquid crystal phase with no long range translational order. It is the least ordered mesophase Preferred Orientation is denoted by the ‘Director’ n. This phase has a symmetrical axis C∞ along the director Point Group D∞ h . It has thread like structure when seen under polarizing microsope .

8. One-dimensional translational order— Smectic The word " Smectic " is derived from the Greek word for soap Liquid-like motion of the rods in each layer No correlation of the molecular positions from one layer to the next The layers can easily slide In the smectic A phase, molecules tend to be perpendicular to the smectic layers In the smectic C phase, the molecules in the layers are parallel and tilted in arrangement with respect to the normal of the layers by a tilt angle θ .

9. Chiral Liquid Crystal- Cholesteric Also known as “Chiral nematic ” Molecules have non-symmetrical carbon atoms and thus lose mirror symmetry Shows a helical structure. In general the helical pitch of cholesteric liquid crystals is of the order of visible light’s wavelength—about a few hundreds nm and so shows different color .

10. Lyotropic Liquid Crystal Lyotropic LCs are two-component systems where an amphiphile is dissolved in a solvent . Lyotropic mesophases are concentration and solvent dependent.

11. Thermotropic Liquid Crystal The transitions to the liquid crystalline state are induced thermally

12. Thermotropic Liquid Crystal The essential requirement for a molecule to be a thermotropic LC is a structure consisting of a central rigid core (often aromatic) and a flexible peripheral moiety ( generally aliphatic groups). This structural requirement leads to two general classes of LCs : Calamitic LCs: Calamitic or rod-like LCs are those mesomorphic compounds that possess an elongated shape. Divided into 2 groups: Nematic and Smectic 2. Discotic LCs:

13. Order Parameter To quantify just how much order is present in a material, an order parameter ( S) is defined . Theta is the angle between the director and the long axis of each molecule The brackets denote an average over all of the molecules in the sample . In an isotropic liquid, the average of the cosine terms is zero, and therefore the order parameter is equal to zero . For a perfect crystal, the order parameter evaluates to one Typical values for the order parameter of a liquid crystal range between 0.3 and 0.9, with the exact value a function of temperature , as a result of kinetic molecular motion. S=(1/2)<3Cos 2 q -1> Nematic LC

14. External influences on Liquid Crystals E xternal  perturbation can cause significant changes in the macroscopic properties of the liquid crystal system. The order of liquid crystals can be manipulated by mechanical, electric or magnetic forces. Electric and Magnetic field effect: Due to the effect of electric field permanent electric dipole results which aligns the director along the electric field. The effect of magnetic field is analogous to the electric field. Surface Preparations: It is possible, however, to force the director to point in a specific direction by introducing an outside agent to the system. For example, when a thin polymer coating (usually a polyimide) is rubbed in a single direction,on a glass substrate, with a cloth, it is observed that liquid crystal molecules in contact with that surface align with the rubbing direction.

15. Birefringence in Liquid Crystals When light enters a birefringent material, such as a nematic liquid crystal sample , the process is modeled in terms of the light being broken up into the fast ( called the ordinary ray) and slow (called the extraordinary ray) components. Because the two components travel at different velocities, the waves get out of phase. When the rays are recombined as they exit the birefringent material, the polarization state has changed because of this phase difference

16. Liquid Crystal Textures The term texture refers to the orientation of liquid crystal molecules in the vicinity of a surface. Each liquid crystal mesophase can form its own characteristic textures,which are useful in identification. We consider the nematic textures here. If mesogenic materials are confined between closely spaced plates with rubbed surfaces (as described above ) and oriented with rubbing directions parallel, the entire liquid crystal sample can be oriented in a planar texture, as shown in the following diagram

17. Defects Under the Microscope : The abrupt changes in brightness seen in the pictures signal a rapid change in director orientation in the vicinity of a line or point singularity known as a disclination . A disclination is a region where the director is undefined. The following is a diagram that shows the orientation of the director around a disclination . Defects in Liquid Crystal

18. Calamitic vs. Discotic Nematics

19. Experimental Identification of Liquid Crystals Differential Scanning Calorimetry (DSC ): I t provides valuable information like the exact transition temperatures and the enthalpies of the different phases Polarizing Microscope: When a liquid crystal material is placed on a microscope slide with a cover slip and the slide is heated and viewed using a polarizing microscope, textures characteristic of each type of liquid crystal can be seen.

20. Experimental Identification of Liquid Crystals X-ray Crystallography: This can be used to study the extent of translational or positional order , and thus infer the type of liquid crystal phase Extended X-ray absorption fine structure spectroscopy ( EXAFS ): EXAFS was used to investigate the local structure of the polar spines of metal ion soaps in the columnar liquid crystalline state

21. Applications of liquid crystals Display application of liquid crystals: The most common application of liquid crystal technology is liquid crystal displays (LCDs .) Thermal mapping and non-destructive testing Medicinal Uses: Cholesteric liquid crystal mixtures have also been suggested for measuring body skin temperature, to outlines tumours etc. Optical Imaging and Liquid Crystal Interactions with Nanostructure Liquid Crystal in Chromatography Liquid Crystal as Solvents in Spectroscopy

22. Characteristics: These are a class of aromatic polymer. Extremely unreactive and inert. Highly resistant to fire. Liquid crystallinity in polymers can be obtained : By dissolving in a solvent. ( Thermotropic ) By heating above melting transition point. ( Lyotropic ) Liquid Crystal Polymer (LCP)

23. Polymer Liquid Crystals

24. Advantage of Liquid Crystal Polymer (LCP)

25. Advantage of LCP High heat resistance Flame retardant Chemical resistance Dimensional stability Mold ability Heat aging resistance Adhesion Low viscosity Wieldable Low cost Disadvantage of LCP Form weak weld lines Highly anisotropic properties Drying required before processing High Z-axis thermal expansion coefficient

26. Soap Conducting foams Heat Sensitive cameras use liquid crystal screens that respond to heat. Applications

27. Kevlar, the most widely used body armor is made up of intertwined liquid crystal polymers. Applications