Untitled Flashcards Set

Liquid Crystals (LCs): exhibit flow of liquids but maintain long-range order characteristic of crystals. They are an intermediate state between liquids and solid crystals formed by mesogens

Mesogens are: rigid rod-like molecules with “disorder-inducing” flexible tails that form mesophases phases

LCs are classified into two types: thermotropic and lyotropic

Lyotropic: formation of mesophases is assisted by solvent

Thermotropic mesogens do not require solvents to form mesophases

Thermotropic LCs: formed by aniosotropic molecules with some disordering moieties (alkyl chanins) so that upon packing they display both fluidity and retain long-range ordering in a given temperature range. Classified into calamitic (rod-like) and discotic (disc-like) by the mesogen shape/anisotropy

Nematic: characterized by no translational order but has long-range orientational order denoted by the director

Cholesteric (Chiral) LCs: formed by chiral molecules, the director twists/undergoes helical rotation along the helix. very large optical rotatory power and iridescent colour due to selective reflection

Calamitic mesogens of Nematic LC: have rigidity and rod-like anisotropy, polarizability increases with the anisotropy of molecules and conjugation length. polar substances increase polarity. Shorter alkyl chains introduce translational disorder, longer alkyl substituents induce formation of smectic phases

Smectic LC; have layering. SmA has director perpendicular to layers

SmC: has director tilted relative to layers

SmB: features hexagonal packing within layers

Calamitic Mesogens of Smectic LC: longer alkyl chains enhance layered phases, SmC is observed for tilted molecules with the side polar groups. 5CB and 8CB mixtures commonly used for LC displays. compounds can arrange from smectic to nematic phase simultaneously

SmC* is: chiral SmC, the director changes orientation around the cone generated by the tilt angle. unwinding of the helix it becomes ferrorelectric

Ferroelectricity: is a spontaneous electric polarization of materials that can be reversed by external stimuli (electric field)

Blue Phases: are formed by highly chiral mesogens that pack into double twist cylinders

Discotic LCs Columnar Phases: three levels of classification. Phase 1 is phase type so nematic of columnar. Phase 2 is packing within phases so disordered, ordered, tilted. Phase 3 is column arrangement so hexagonal, rectangular, or oblique.

Discotic Mesogens (disc-like): are aromatics with symmetric C3, C4, or C6 substituents, polar substituents used to increase polarity to tune donor-acceptor properties

Metallotropic LC: molecules that incorporate metal ions. lemellar, cubic, and hexagonal ohases has been demonstrated. Promising for magnetic, electronic, and optical applications.

Polymer LCs: polymers that can form LC phases due to the incorporation of the structural anisotropy or mesogens. PLC can by lyotropic or thermotropic. Lyotropic PLC can be rigid-rod like polymers (DNA) assembling into ordered mesophases in solution. Thermotropic PLC with incorporated mesogens can be classified into main chain PLC and side chain PLC.

All LC phases have long-range orientational order. Smectics and columnar phases have translational order, NOT nematics.

crystals have: constant peaks, liquids and nematics decays exponentially, smectics slowly decrease.

LCs are: anisotropic due to mesogen anisotropy that is retained in LC packing. Anisotropic response to stimuli is important for applications. Mesogens have dipole moment and interact with electromagenetic field.

Optical Anisotropy: nematic, smectic, and columnar LC phases are optically anisotropic. different refractive indices are observed for different directions. Birefringence defines optical anisotropiy and is used in polarized optical light microscopy, it is responsivle for milky/scattering LC appearance.

Mecahnical alignment: have two orientations relative to a flat substrate, homeotropic (perpendicular director) and homogeneous (parallel director)

Polarized Light microscopy (PLM): uses polarized light to characterize anisotropic/birefringent materials. PL can be used from monochromatic (lasers) or white light sources.

LCs are: anisotropic (& birefringent) and display diverse structural organization in different phases that can be distinguished using polarized light microscopy

Disclinations are: line defects in regular structural organization (point defects are 0D, line defects are 1D, wall defects are 2D). strength and sign of disclinations correspond to the number of full lines that appear or disappear. Disclinations have characteristic appearances in PLM depending on their type and angle or orientation

Disclinations in nematic phases: produce a schlieren texture. thicker nematic films show characteristic dark lines

PLM of SmA: Non-covered films of SmA can display terraces due to the layering. Fan-like texture is characteriztic of SmA mesophases confined between two glasses. flower texture arise from quasi-isotropic orientation of the focal conics that nucleate from the center toward the boundary of the droplet

PLM of SmC: SmC mesophases prepped from SmA phases develops characteric regions of schlieren texture coexisting with the fan textrure

PLM of SmB: SmB mesophases identified by mosaic or broken fan textures in non-planar orientations

Cholesteric mesophases: display fingerprint textures. the line pattern originates from helical structures where the axes of the helices lie in the substrate plane. Discotic LC in Colh display dendritic textures (snowflakes is hexagonally packed columns)

X-ray diffraction pattersn display spots and arcs, distacne of spot from center gives info on oredering periodicity and angular position is related to the orientation of molecules and layers.

Nematcis: display short-range smectic-like orientation.

Smectics: characterized by bragg spots related to their periodicity.

SmA: has bragg refelctions normal to layers,

SmC: display diff oreintations with diverse patterns. N and SmA director is vertical. SmC has distinct diffraction patterns

All LC molecules: have long-range order, anisotropic molecules align alone the same direction that defines the director. director is along long axes of calamitic mesogens and is perpendicular to the disc plane of discotic. orientational order parameter (P2) for isotropic phases is 0 and 2 for perfect crystals. P2 = <3/2 cos² B - 1/2>

nematic to smectic A: is simplest transition that involves translational order (layering), transition is either first order of continuous. cooling of nematic phase close to transition, pre-transitional fluctuations of smectic order appear called cybotactic clusters that are anisotrpoic and grow approaching the transition.

Phases transition with increasing temperature is: Crystal, Smectic A, Nematic, Isotropic

elastic deformation of LC is three types: splay, twist, bend

LC Displays: anisotropy and mobility of mesogens in LC phases make possible changes in orientation by electric of magnetic fields.

Fredericks transition involves field-induced reorientation from the orientation imposed by the substrate. Can occur in three modes by deformation of twist, bend, and splay. Common LC orientation is planar with deformation in twist

Twisted Nematic LCDs: commercial displayed in watches and calculators, operate on fredericks transitions, needs: transparent conductive coatings(graphene) to apply an electric field; substracte that anchors LCs in planar orientations (polyimide films); and two polarizers.

TN LCDs have director twisted and LC rotates the polarization to allow light to come through when OFF. Applied voltage causes director to orients parallel to the field so that the light can’t pass when ON (dark)

Ferroelectric LCDs: very fast switching time (0.05ms-0.1ms) and wide viewing angles. ahve unwinded helix of SmC* phase and require small cell gaps

Polymer-dispersed LC displays: LC droplets dispersed in polymer matrix (acrylates). INtensity of light transmitted modulated by application of electric field that changes the orientaiton of LC molecules. Prepared by encapsulation and phase separation.

Thin Film Transistor LCDs: active matrix, each pixel is addressed by a transistor taht enables sharpness, resolution and multiplexing. Made from semiconductor materials like silicon

LCD and OLED displays: LCD is large display screens while OLED is expensive phone screens. Display performance based on six metrics: response time, contrast ratio, colour gamut, lifetime, power efficiency, and panel flexibility. Motion picture response time affects image blue of moving picture, ambient contrast ratio (ACR) determines perceived image contrast under ambient lighting conditions. LCD have better MPRT and ACR values than OLEDs

LCD: liquid crystals do not emit light, backilighting required to illuminate display pannels (white light led). LC layer sandwiched between two crossed polarizers. Common LCD designs: twisted nematic, vertical alignment, in-plane switching, fringe-field switching

LCD TN: LC twists continuously introducing polarization rotation effect, applied voltage it starts to unwind and polarization decreases and transmittance is decreased. Overall mode has high transmittance and low operation. Wirstwatches, laptop computers

LCD VA: multi-domain VA used to solve viewing angle problem with TN. LCs are aligned in vertical direction, applied voltage and directors are tilted and incident light transmits through crossed polarizers. Used in large TVs, desktop computers

LCD IPS: LC directors homogeneously aligned, electric field applied in lateral direction, applied voltage, strong electric fields reorient the LC directors. useful for touch pannels bcuz of no ripple effect, limitation is peak transmittance bcuz of dead zones. Used in desktop computes, pads

LCD FFS: high transmittance, wide viewing anlge, weak colour shift, built-in storage capacitance, and robustness to touch pressure. Similar to IPS but pixel and electrode separated by thin passivation layer, electrode width and gap much smaller so dead zone reduced. Used in smartphones, pads, notebook computers

OLED: organic stacks between anode and cathode, electrons and holes are injected from electrode to organic layers for recombination and light emission

Biological Soft Matter:

most biological objects classified as soft matter, main are macromolecules, proteins, lipid bilayerm nucleic acids, vesicles and membranes

polymers, colloids, amphiphiles, and self-assembly are key areas for physiochemical understanding of biological and biochemical systems

Cell membrances: structural and functional cell element, containment and selective transport of ions and molecules, membranes are flexible and cholesterol adds ridigidty, embedded proteins are amphiphilic, example is of membrane is phospholipid bilayer

Lipid rafts: phase-separated domain in lipid membrane, rich in cholesterol and sphingolipids, tightly packed and more ordered, participate in endo and exocytosis and vesicular trafficking

Myelins: membrane instabilities, multilamellar tubes composed of nested cylindrical bilayer membranes, produced spontaneously when water brought into contact with concentrated surfactant that forms lamellae phases, coiling of single or double helix formations

DNAs/RNAs: forms double-helical structures held together by strong H-bonding. helices has major and minor grooves that are important for protein binding and interactions with cations. DNA/RNA are biopolymers, ridi-rod polymers, polyelectrolytes, self-assembled structures. persistence length quantifies stiffness of a polymer chain, polymer segment shorter than P behaves as rigid rods. DNA is a rigid-rod polymer. DNA is anionic and can form LC phases and condensed by PEI (cationic)

Polysaccharides: natural polymers built of monosaccharide units liked by glycosidic bonds. Glucose+Glucose=Cellulose (cotton, hemp, flax, wood), Starch is amylose, amilopectin and glycogen

Supramolecular assemblies: Tobacco Mosaic Virus is rod like virus with supramolecular self-assembly, first isolated virus that was model for rigid-rod nano-size objects. Icosahedral Viruses: 3 or 4 diff protein building blocks, has high symmetry and high stability, constructed from 20 identical near-tetrahedral units