Polymer Liquid Crystals Behzad Pourabbas Polymer Eng. Dep. Sahand University of Technology Tabriz, Iran pourabas@sut.ac.ir www.sut.ac.ir 11/15/2018

Crystals which are liquid or liquids with properties of crystals 11/15/2018

States between crystalline and Liquid Crystals States between crystalline and isotropic liquid 11/15/2018

Overlook History Terms and definitions Mesomorphism Basic Properties 11/15/2018

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. 11/15/2018

Liquid Crystals, 1805-1922. Before discovery of LC, Lehmann designed a microscope that could be used to monitor phase transition process. 11/15/2018

1888 by Prof. Reinitzer, a botanist, University of Prague, Germany 11/15/2018

Phase Transition first defined by Georges Freidel in 1922 11/15/2018

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Terms and definitions 11/15/2018

The term in Encyclopedias Liquid 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. 11/15/2018

The terms in Encyclopedias Liquid‐crystal 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: rod‐like, including aromatic polyamides, esters, azomethines, and benzobisoxazoles; helical, mostly natural materials such as polypeptides; side‐chain (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 long‐range 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 long‐range order and 1‐ or 2‐dimensional positional order. Collins PJ (1997) Introduction to liquid crystals: chemistry and physics, vol 1. Taylor and Francis, New York. 11/15/2018

The term in Encyclopedias Liquid‐crystal polymer (LC polymer, liquid crystalline polymer, and mesomorphic polymer). Liquid‐crystal 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(p‐phenylene terephthalamide) (Kevlar). LC polymers are also classified as lyotropic and thermotropic. Lyotropic ones show their liquid‐crystalline 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. 11/15/2018

General properties 11/15/2018

Main Properties Rigid 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) 11/15/2018

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Types of Orderness Three types of ordering positional, orientational, and conformational ordering; 11/15/2018

what PLCs are Positional Disordering: When methane melts, various relative positions of its quasi-spherical molecules become possible—since positional disordering occurs. 11/15/2018

what PLCs are Orientational 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. 11/15/2018

what PLCs are Conformational 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. 11/15/2018

LCs and types of orderness liquid crystals exhibit positional disordering; plastic crystals show orientational disordering; and condis crystals exhibit conformational disordering. 11/15/2018

Orderness 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 11/15/2018

Liquid Crystal Polymers (rigid and semiflexible molecules, with orientational ordering) Isotropic Nematic Smectic-A director layers 11/15/2018

MOLECULAR STRUCTURES Classification of PLCs Classification of PLCs on the basis of molecular structures 11/15/2018

Molecular Structure–Property Connection Properties of PLCs depend strongly on the molecular structures of the chains. Consider simple or one-row combs, subclass εO. 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 α, 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. 11/15/2018

LC PHASES AND THERMOPHYSICAL PROPERTIES Kinds of LC Phases Nematic, cholesteric, and a variety of smectic phases. These three names have been proposed by Friedel in 1922 who imagined that such phases should exist— long 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 approximately—but not quite—perpendicularly 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: 11/15/2018

LC PHASES AND THERMOPHYSICAL PROPERTIES Kinds of LC Phases where θ 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. 11/15/2018

Order Parameter S = ½<(3cos²θ-1)> The order parameter in a liquid crystal range from 0.3 to 0.9 11/15/2018

Electric field Effects If an electric field is applied to a liquid crystal the molecules will align in the same direction as the field. 11/15/2018

LC PHASES AND THERMOPHYSICAL PROPERTIES A cholesteric phase is formed by a pile of nematic phases with the director changing from one layer to another; Kinds of LC Phases Smectic phases have also layers, but each layer has at least one more element of long-range order in addition to the director. The simplest among LC phases are nematic, in which the orientation along the director is the only kind of long-range order present; An example of a nematic (a), cholesteric (b) and smectic C (c) phase. 11/15/2018

Kinds of LC Phases: Nematic 11/15/2018

Nematic Liquid Crystals 11/15/2018

Kinds of LC Phases: Cholesteric phases 11/15/2018

Cholesteric Phase Liquid Crystals 11/15/2018

Twisted Nematic This 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. 11/15/2018

Kinds of LC Phases: smectic phases 11/15/2018

LC PHASES AND THERMOPHYSICAL PROPERTIES Kinds of Smectit LC Phases Smectic 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. 11/15/2018

Classification of Smectic Liquid Crystals A 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. 11/15/2018

Smectic A Liquid Crystals 11/15/2018

Smectic B Liquid Crystals 11/15/2018

Smectic C Liquid Crystals 11/15/2018

Smectic A Liquid Crystals 11/15/2018

Polymeric Liquid Crystal 11/15/2018

Picture Gallery 11/15/2018

Nematic Phase Molecules in this phase are long and rod-like in shape. They are free to move in space. 11/15/2018

Self-organized pattern of a nematic liquid crystal created by interface orientations at the liquid crystal-isotropic phase transition temperature 11/15/2018

Smectic Phase This 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 viscosity 11/15/2018

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Subjects to be studied BLENDING AND RHEOLOGICAL PROPERTIES OPTICAL PROPERTIES Nonlinear Optical (NLO) Effects PLCs for NLO Applications ELECTRICAL AND MAGNETIC PROPERTIES Effects of Electric Fields Effects of Magnetic Fields and NMR Spectroscopy BLENDING AND RHEOLOGICAL PROPERTIES Rheology of Pure PLCs and of EP ‏ PLC Blends Properties of Blends 11/15/2018

Complementary slides, information about the polymer lc. 11/15/2018

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Uses of Liquid Crystals Flat screen television Wristwatches Laptop screens Digital clocks Thermometers Switchable windows 11/15/2018

References [1.]LIQUID CRYSTAL. The Columbia Encyclopedia, 6th ed. New York: Columbia University Press. <www.bartleby.com> 2001-04. [2.] Georgia Tech Investigates Liquid Crystal Polymer for NASA Applications. Georgia Institute of Technology. <http://www.physorg.com>. 2006. [3.] Gosta Ekspong. The Nobel Prize in Physics 1991.World Scientific Publishing Co., Singapore, 1997. [4.] Liquid Crystal Phases. <http://plc.cwru.edu> 2006. [5.] Robert F. Service, Science. Vol. 292. 5518, pp.825-826. 2001. [6.] De Gennes, P.G., Prost, J. The Physics of Liquid Crystals. 2nd ed. Oxford University Press, New York. 1993. [7.] A.M. Glazer. The Structures of Crystals. Adam Hilger, Bristol.1987. [8.] Sparks, H. Bio-weapons Liquid Crystal Sensor. < www.space.com >. 2000. [9.] Buka, A. Modern Topics in Liquid Crystals. River Edge, New Jersey.pp.33- 35.1993. [10.] Johnston, B. A Gallery of Liquid Crystal Photomicrgraphs. <www.microscopy- uk.org>Micscape Magazine.2003. [11.]LiquidCrystal.Wikepedia.<www.wikipedia.com > [12.] Lavrentovich, O. About Liquid Crystals. < www.lci.kent.edu >. [13.] Desimpel, C. The Basics About Liquid Crystals. < www.elis.ugent.be >. [14.] Allen,M.TheoreticalPhysics. <www2.warwick.ac.uk>. [15.] Shakhashiri, B.Chemical of the Week. <www. scifun.chem.wisc.edu>. 11/15/2018

The Main Reference 11/15/2018

Orderness everywhere!!! 11/15/2018