Polymer Liquid Crystals

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Polymer Liquid Crystals. Behzad Pourabbas Polymer Eng. Dep. Sahand University of Technology Tabriz, Iran pourabas@sut.ac.ir www.sut.ac.ir. Crystals which are liquid or liquids with properties of crystals. Liquid Crystals States between crystalline and isotropic liquid. Overlook. - PowerPoint PPT Presentation

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Polymer Liquid Crystals

Polymer Liquid CrystalsBehzad PourabbasPolymer Eng. Dep.Sahand University of TechnologyTabriz, Iranpourabas@sut.ac.irwww.sut.ac.ir

7/21/20111Crystals which are liquid or liquids with properties of crystals7/21/20112Liquid CrystalsStates between crystalline and isotropic liquid

7/21/20113OverlookHistoryTerms and definitionsMesomorphism Basic Properties

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Discovery of liquid crystals Liquid crystals form from organic compounds and is thought of as the phase of matter between the solid and liquid state of a crystal. This phenomena was discovered in 1888 by Austrian chemist Frederich Reinitzer.

7/21/20115Liquid Crystals, 1805-1922. Before discovery of LC, Lehmann designed a microscope that could be used to monitor phase transition process.

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1888 by Prof. Reinitzer, a botanist, University of Prague, Germany7/21/20117

Phase Transition first defined by Georges Freidel in 19227/21/20118

7/21/20119Terms and definitions7/21/201110The term in EncyclopediasLiquid crystal (1891): n. A liquid in which the molecules are oriented parallel to each other resulting in birefringence and interference patterns visible in polarizing light.

Collins PJ (1997) Introduction to liquid crystals: chemistry and physics, vol 1.Taylor and Francis, New York.7/21/201111By orientation we mean:

7/21/201112The terms in EncyclopediasLiquidcrystal polymer (LC polymer, liquid crystalline polymer, and mesomorphic polymer).A polymer capable of forming regions of highly ordered structure (mesophase) while in the liquid (melt or solution) phase. The degree of order is somewhat less than that of a regular solid crystal. Four types have been identified: rodlike, including aromatic polyamides, esters, azomethines, and benzobisoxazoles; helical, mostly natural materials such as polypeptides; sidechain (comb polymers); and block copolymers with alternating rigid and flexible units. These polymers are described as nematic, in which the mesogens (ordered regions) show no positional order, only longrange order; Cholesteric or chiral, a modified nematic phase in which the orientation direction changes from layer to layer in a helical pattern; and smectic, in which the mesogens have both longrange order and 1 or 2dimensional positional order.

Collins PJ (1997) Introduction to liquid crystals: chemistry and physics, vol 1. Taylor and Francis, New York.

7/21/201113The term in EncyclopediasLiquidcrystal polymer (LC polymer, liquid crystalline polymer, and mesomorphic polymer).Liquidcrystal polymers are difficult to get into the molten condition because the solid crystals generally decompose before melting. The most commercially successful ones to date are those processed in solution, e.g., poly(pphenylene terephthalamide) (Kevlar). LC polymers are also classified as lyotropic and thermotropic. Lyotropic ones show their liquidcrystalline character only in solution, while thermotropic ones can show it in the melt without the presence of a solvent.

Collins PJ (1997) Introduction to liquid crystals: chemistry and physics, vol 1. Taylor and Francis, New York.7/21/201114General properties7/21/201115Main PropertiesRigid and Flexible sequences.Orderness Blend PLCS with EPS (Engineering Polymers) in such proportions that the good properties of PLCS show up, while at the same time there is in each case enough of an EP to keep the costs at bay. (Compatibility problem)

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7/21/201119Types of OrdernessThree types of ordering positional, orientational, and conformational ordering;

7/21/201120what PLCs arePositional Disordering: When methane melts, various relative positions of its quasi-spherical molecules become possiblesince positional disordering occurs.

7/21/201121what PLCs areOrientational Disordering:When we move to the next homolog in the n-alkane series, ethane, its melting is accompanied also by positional disordering; intermolecular distances become less uniform. However, melting of ethane involves at the same time orientational disordering since two molecules can now be perpendicular to each other, or parallel as they largely were before, or anything in between.

7/21/201122what PLCs areConformational ordering;A longer paraffin hydrocarbon molecule such as n-decane also undergoes positional and orientational disordering, but conformational disordering dominates here: the segments acquire freedom to execute rotations about single bonds.

7/21/201123LCs and types of ordernessliquid crystals exhibit positional disordering;plastic crystals show orientational disordering; andcondis crystals exhibit conformational disordering.

7/21/201124Orderness and the conditions

Thermotropic : PLC phases which appear in certain temperature intervals,Lyotropic: LC properties are induced by the presence of a solvent,Barotropic :liquid crystallinity induced by pressure elevation

7/21/20112526 Liquid Crystal Polymers (rigid and semiflexible molecules, with orientational ordering)Isotropic

Nematic

Smectic-A

directorlayers7/21/2011MOLECULAR STRUCTURESClassification of PLCsClassification of PLCs on the basis of molecular structures

7/21/201127Molecular StructureProperty ConnectionProperties of PLCs depend strongly on the molecular structures of the chains.Consider simple or one-row combs, subclass O. Transition from a LC state such as nematic into isotropic liquid, accomplished by a temperature increase, results in lowering the viscosity as usual and as expected.However, for longitudinal polymers, class , similar isotropization results in a viscosity increase: the rigid LC sequences were aligned in the LC state, but in the isotropic state all directions are equiprobable, and the flow is more difficult.

7/21/201128LC PHASES AND THERMOPHYSICAL PROPERTIES

Kinds of LC PhasesNematic, cholesteric, and a variety of smectic phases.These three names have been proposed by Friedel in 1922 who imagined that such phases should existlong before his concepts were confirmed by diffractometric experiments.In all these phases the entire molecules (in MLCs) or the LC sequences in the chains (in PLCs) are oriented approximatelybut not quiteperpendicularly to a preferred axis in space called director. The degree of alignment is characterized by the order parameter (also called the anisotropy factor) defined in 1946 by Hermans [36] as:

7/21/201129LC PHASES AND THERMOPHYSICAL PROPERTIES

Kinds of LC Phases

where is the angle between the molecular axis and the director, and the braces denote an average for the material (or a layer). We see from the Eq. that in a completely isotropic system s = 0 while a System perfectly aligned along the director would have s = 1.

7/21/201130Order Parameter S = The order parameter in a liquid crystal range from 0.3 to 0.9

7/21/201131Electric field EffectsIf an electric field is applied to a liquid crystal the molecules will align in the same direction as the field.

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7/21/201133LC PHASES AND THERMOPHYSICAL PROPERTIES

Kinds of LC Phases

An example of a nematic (a), cholesteric (b)and smectic C (c) phase.The simplest among LC phases are nematic, in which the orientation along the director is the only kind of long-range order present;A cholesteric phase is formed by a pile of nematic phases with the director changing from one layer to another;Smectic phases have also layers, but each layer has at least one more element of long-range order in addition to the director.7/21/20113435 Liquid Crystal Polymers (rigid and semiflexible molecules, with orientational ordering)Isotropic

Nematic

Smectic-A

directorlayers7/21/2011Nematic

Kinds of LC Phases:

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Nematic Liquid Crystals7/21/201137Cholesteric phases

Kinds of LC Phases:

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Cholesteric Phase Liquid Crystals7/21/201139Twisted NematicThis is called the twisted nematic liquid crystal and the spacing between the planes change with temperature. The spacing is associated with the wavelengths of light.

7/21/201140smectic phases

Kinds of LC Phases:

7/21/201141LC PHASES AND THERMOPHYSICAL PROPERTIES

Kinds of Smectit LC PhasesSmectic A phase: the centers of molecules (in MLCs) or of LC sequences (in PLCs) lie on equidistant planes perpendicular to the director.In smectic B phases there are also such planes, but there is additionally a two-dimensional hexagonal lattice within each plane.There is no hexagonal structure in smectic C phases, while the director is tilted with respect to the plane normal.

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Classification of Smectic Liquid CrystalsA type: molecular alignment perpendicular to the surface of the layer, but lack of order within the layer.

B type: molecular alignment perpendicular to the surface of the layer, having order within the layer.

C type: having a tilted angle between molecular alignment and the surface of the layer.7/21/201143

Smectic A Liquid Crystals7/21/201144

Smectic B Liquid Crystals7/21/201145

Smectic C Liquid Crystals7/21/201146

Smectic A Liquid Crystals7/21/201147

Polymeric Liquid Crystal 7/21/201148Picture Gallery 7/21/201149

Nematic Phase

Molecules in this phase are long and rod-like in shape. They are free to move in space.

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Self-organized pattern of a nematic liquid crystal created by interface orientations at the liquid crystal-isotropic phase transition temperature 7/21/201151Smectic PhaseThis phase can be reached at lower temperatures than the nematic phase. Molecules align themselves in layers.(They are restricted to their plane.)More order and higher visc