liquid crystal and liquid crystal polymer
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Liquid Crystal and Liquid Crystal PolymersBy,Saurav Chandra SarmaInt. Ph.D. 4th Sem.
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
Liquid crystals (LCs)arematter in a statethat has properties between those of conventionalliquidand those of solidcrystal. For instance, an LCs 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.IntroductionPositional 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 PhaseLiquid 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?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.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
No translational orderNematicsThe 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 Dh.
It has thread like structure when seen under polarizing microsope.
One-dimensional translational orderSmecticThe word "Smectic" is derived from the Greek word for soapLiquid-like motion of the rods in each layerNo correlation of the molecular positions from one layer to the nextThe layers can easily slideIn the smectic A phase, molecules tend to be perpendicular to the smectic layersIn 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 .
Chiral Liquid Crystal- Cholesteric
Also known as Chiral nematicMolecules have non-symmetrical carbon atoms and thus lose mirror symmetryShows a helical structure.In general the helical pitch of cholesteric liquid crystals is of the order of visible lights wavelengthabout a few hundreds nm and so shows different color.
Lyotropic Liquid Crystal
Lyotropic LCs are two-component systems where an amphiphile is dissolved in asolvent.Lyotropic mesophases are concentration and solvent dependent.Thermotropic Liquid CrystalThe transitions to the liquid crystalline state are induced thermally
Thermotropic Liquid CrystalThe 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:
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.
Nematic LCExternal influences on Liquid CrystalsExternalperturbationcan 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. 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
Liquid crystals are found to be birefringent, due to theiranisotropicnature. That is, they demonstrate double refraction (having two indices of refraction). Light polarized parallel to the director has a different index of refraction (that is to say it travels at a different velocity) than light polarized perpendicular to the director. In the following diagram, the blue lines represent the director field and the arrows show the polarization vector.
Thus, 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.
The birefringence of a material is characterized by the difference,Dn, in the indices of refraction for the ordinary and extraordinary rays. To be a little more quantitative, since the index of refraction of a material is defined as the ratio of the speed of light in a vacuum to that in the material, we have for this case, ne= c/V| |and no= c/V^for the velocities of a wave travelling perpendicular to the director and polarized parallel and perpendicular to the director, so that the maximum value for the birefringence,Dn = ne no. We wont deal here with the general case of a wave travelling in an arbitrary direction relative to the director in a liquid crystal sample, except to note thatDn varies from zero to the maximum value, depending on the direction of travel. The condition ne> nodescribes a positive uniaxial material, so that nematic liquid crystals are in this category. For typical nematic liquid crystals, nois approximately 1.5 and the maximum difference,Dn, may range between 0.05 and 0.5.The length of the sample is another important parameter because the phase shift accumulates as long as the light propagates in the birefringent material. Any polarization state can be produced with the right combination of the birefringence and length parameters.It is convenient here to introduce the concept of optical path in media since for the above two wave components travelling with different speeds in a birefringent material, the difference in optical paths will lead to a change in the polarization state of the wave as it progresses through the medium. We define the optical path for a wave travelling a distance L in a crystal as nL so that the optical path difference for the two wave components mentioned above will be L (ne no) = LDn. The resultant phase difference between the two components (the amount by which the slow, extraordinary component lags behind the fast, ordinary one) is just 2pLDn/lv wherelv is the wavelength in vacuum.The following simulation demonstrates the optical properties of a birefringent material. A linearly polarized light wave enters a crystal whose extraordinary (slow) index of refraction can be controlled by the user. The length of the sample can also be varied, and the outgoing polarization state is shown. The concept of optical path difference and its influence on polarization state can also be explored here. This leads to a discussion of optical