Polymer Liquid Crystals Behzad Pourabbas Polymer Eng Dep

Polymer Liquid Crystals Behzad Pourabbas Polymer Eng. Dep. Sahand University of Technology Tabriz, Iran pourabas@sut. ac. ir www. sut. ac. ir 9/25/2021 1

CRYSTALS WHICH ARE LIQUID OR LIQUIDS WITH PROPERTIES OF CRYSTALS 9/25/2021 2

Liquid Crystals States between crystalline and isotropic liquid 9/25/2021 3

Overlook �History �Terms and definitions �Mesomorphism �Basic Properties 9/25/2021 4

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. 9/25/2021 5

Liquid Crystals, 1805 -1922. Before discovery of LC, Lehmann designed a microscope that could be used to monitor phase transition process. 9/25/2021 6

1888 by Prof. Reinitzer, a botanist, University of Prague, Germany 9/25/2021 7

Phase Transition first defined by Georges Freidel in 1922 9/25/2021 8

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TERMS AND DEFINITIONS 9/25/2021 10

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. 9/25/2021 11

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. 9/25/2021 13

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. 9/25/2021 14

GENERAL PROPERTIES 9/25/2021 15

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) 9/25/2021 16

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Types of Orderness �Three types of ordering ◦ positional, ◦ orientational, and ◦ conformational ordering; 9/25/2021 20

what PLCs are �Positional Disordering: ◦ When methane melts, various relative positions of its quasi-spherical molecules become possible—since positional disordering occurs. 9/25/2021 21

what PLCs are �Orientational Disordering: ◦ When we move to the next homolog in the nalkane 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. 9/25/2021 22

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. 9/25/2021 23

LCs and types of orderness �liquid crystals exhibit positional disordering; �plastic crystals show orientational disordering; and �condis crystals exhibit conformational disordering. 9/25/2021 24

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 9/25/2021 25

Liquid Crystal Polymers (rigid and semiflexible molecules, with orientational ordering) • Isotropic • Nematic director • Smectic-A layers 9/25/2021 26

MOLECULAR STRUCTURES Classification of PLCs �Classification of PLCs on the basis of molecular structures 9/25/2021 27

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. 9/25/2021 28

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: 9/25/2021 29

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. 9/25/2021 30

Order Parameter S = ½<(3 cos²θ-1)> The order parameter in a liquid crystal range from 0. 3 to 0. 9 9/25/2021 31

Electric field Effects �If an electric field is applied to a liquid crystal the molecules will align in the same direction as the field. 9/25/2021 32

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 The simplest among LC phases are nematic, in An example of athe nematic (b) which the orientation along director(a), is cholesteric the smecticorder C (c) present; phase. only kind of and long-range 9/25/2021 Smectic phases have also layers, but each layer has at least one more element of long-range order in addition to the director. 34

Kinds of LC Phases: NEMATIC 9/25/2021 36

Nematic Liquid Crystals 9/25/2021 37

Kinds of LC Phases: CHOLESTERIC PHASES 9/25/2021 38

Cholesteric Phase Liquid Crystals 9/25/2021 39

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. 9/25/2021 40

Kinds of LC Phases: SMECTIC PHASES 9/25/2021 41

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 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. 9/25/2021 42

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. 9/25/2021 43

Smectic A Liquid Crystals 9/25/2021 44

Smectic B Liquid Crystals 9/25/2021 45

Smectic C Liquid Crystals 9/25/2021 46

Smectic A Liquid Crystals 9/25/2021 47

Polymeric Liquid Crystal 9/25/2021 48

PICTURE GALLERY 9/25/2021 49

Nematic Phase �Molecules in this phase are long and rod -like in shape. They are free to move in space. 9/25/2021 50

Self-organized pattern of a nematic liquid crystal created by interface orientations at the liquid crystal-isotropic phase transition temperature 9/25/2021 51

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 9/25/2021 52

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Subjects to be studied �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 9/25/2021 54

COMPLEMENTARY SLIDES, INFORMATION ABOUT THE POLYMER LC. 9/25/2021 55

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Uses of Liquid Crystals • • • Flat screen television Wristwatches Laptop screens Digital clocks Thermometers Switchable windows 9/25/2021 63

References • • • • [1. ]LIQUID CRYSTAL. The Columbia Encyclopedia, 6 th 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. 2 nd 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. ]Liquid. Crystal. 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. Theoretical. Physics. <www 2. warwick. ac. uk>. [15. ] Shakhashiri, B. Chemical of the Week. <www. scifun. chem. wisc. edu>. 9/25/2021 64

The Main Reference 9/25/2021 65

ORDERNESS EVERYWHERE!!! 9/25/2021 66