Liquid Crystal and Liquid Crystal Polymer
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Transcript of Liquid Crystal and Liquid Crystal Polymer
Liquid Crystal and
Liquid Crystal Polymers
By,
Saurav Chandra Sarma
Int. Ph.D. 4th Sem.
Liquid Crystal and Life
Liquid crystals are also fundamentallyimportant to life. DNA and cellmembranes have liquid crystal phases.Our brains are around 70% liquidcrystal, and liquid crystals are alsofound in muscles, the amazingiridescent colours of some insects, andalso slug slime!
Liquid crystals are beautiful and mysterious; I am fond of them for both reasons. - P.G. De Gennes
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
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?
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
Liquid Crystals
Thermotropic Lyotropic
High molecular
(molar) mass
[ polymers]
Low molecular
(molar) mass
Main-chain
polymers
Side-chain
polymers
Rod-like or
lath-like
molecules
Calamitic
Disc-like
molecules
Discotic
Single or multicomponent
systems
Homo- or co-polymers
Figure 9.1 The liquid crystal family tree.
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.
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.
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’swavelength—about a few hundreds nm andso 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 Crystal
The transitions to the liquid crystalline state are induced thermally
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:
1. Calamitic LCs: Calamitic or rod-like LCs arethose mesomorphic compounds that possess anelongated shape.
Divided into 2 groups:Nematic and Smectic
2. Discotic LCs:
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 longaxis of each molecule
The brackets denote an average over all of themolecules in the sample.
In an isotropic liquid, the average of the cosine termsis zero, and therefore the order parameter is equal tozero.
For a perfect crystal, the order parameter evaluatesto one
Typical values for the order parameter of a liquidcrystal range between 0.3 and 0.9, with the exactvalue a function of temperature, as a result of kineticmolecular motion.
S=(1/2)<3Cos2q -1>
Nematic LC
External influences on Liquid CrystalsExternal perturbation can cause significant changes in the macroscopic properties ofthe liquid crystal system. The order of liquid crystals can be manipulated bymechanical, electric or magnetic forces.
Electric and Magnetic field effect:Due to the effect of electric field permanent electricdipole results which aligns the director along theelectric field.The effect of magnetic field is analogous to the electricfield.
Surface Preparations: It is possible, however, to force the director to point in aspecific direction by introducing an outside agent to the system.
For example, when a thin polymer coating (usually a polyimide) is rubbed ina single direction,on a glass substrate, with a cloth, it is observed that liquid crystalmolecules 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 crystalsample, the process is modeled in terms of the light being broken up into thefast (called the ordinary ray) and slow (called the extraordinary ray)components. Because the two components travel at different velocities, thewaves get out of phase. When the rays are recombined as they exit thebirefringent material, the polarization state has changed because of this phasedifference
Liquid Crystal Textures
The term texture refers to the orientation of liquid crystal molecules in the vicinityof a surface. Each liquid crystal mesophase can form its own characteristictextures,which are useful in identification. We consider the nematic textures here.If mesogenic materials are confined between closely spaced plates with rubbedsurfaces (as described above) and oriented with rubbing directions parallel, theentire liquid crystal sample can be oriented in a planar texture, as shown in thefollowing diagram
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 adisclination. A disclination is a region where the director is undefined. Thefollowing is a diagram that shows the orientation of the director around adisclination.
Defects in Liquid Crystal
Calamitic vs. Discotic Nematics
Experimental Identification of Liquid Crystals
Differential Scanning Calorimetry (DSC): It
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.
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 crystalphase
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
Applications of liquid crystals
Display application of liquidcrystals: The most commonapplication of liquid crystaltechnology is liquid crystal displays(LCDs.)
Thermal mapping and non-destructive testing
Medicinal Uses: Cholesteric liquidcrystal mixtures have also beensuggested for measuring body skintemperature, to outlines tumoursetc.
Optical Imaging and Liquid CrystalInteractions with Nanostructure
Liquid Crystal in Chromatography
Liquid Crystal as Solvents in Spectroscopy
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)
Polymer Liquid Crystals
Advantage of Liquid Crystal Polymer (LCP)
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
• Soap
• Conducting foams
• Heat Sensitive cameras use liquid crystal screens that respond to heat.
Applications
• Kevlar, the most widely used body armor is made up of intertwined liquid crystal polymers.
Applications