L8: The Liquid Crystalline State

Transcription

1 L8: The Liquid Crystalline State Jiun-Tai Chen ( 陳俊太 ) Web: 1 Outline History Introduction of Liquid Crystals Liquid Crystalline Mesophases Liquid Crystalline Polymers Liquid Crystal Classification Applications 2 國立交通大學應用化學系陳俊太老師 1

2 Outline History Introduction of Liquid Crystals Liquid Crystalline Mesophases Liquid Crystalline Polymers Liquid Crystal Classification Applications 3 States of Matter Common States Solid Liquid Gas Materials can exist in other states. 4 國立交通大學應用化學系陳俊太老師 2

3 Liquid Crystals First noted for organic molecules about 100 years ago, when Reinitzer observed a peculiar melting behavior of cholesterol ( 膽固醇 ) esters. He found that the crystals of the substances melted sharply to form an opaque melt instead of the usual clear melt. He concluded that some type of order still existed in the molten state. The opacity vanished at a higher temperature Clearing temperature. 5 Liquid Crystal Shortly thereafter, Otto Lehmann reported ammonium oleate and p-azoxyphenetole exhibited turbid( 混濁 ) states between the truly crystalline and the truly isotropic fluid state. ammonium oleate p-azoxyphenetole To describe the strange behavior of the new state, Lehmann introduced the term Flussige Kristalle, or liquid crystals. 6 國立交通大學應用化學系陳俊太老師 3

4 Otto Lehmann Microscope with heating stages and polarizing filters (1889) From a letter to Reinitzer: my new results confirm your [previously declared view, that the [substance] consists of very soft crystals It is absolutely homogeneous, and another liquid-as you assumed formerly-is not present.. It is of a high interest for the physicist that crystals exist which are of such a considerable softness that one could almost call them liquid. le-chiffre.net/ 7 Richard Williams In 1962, he discovered the domain structure produced in nematic liquid crystals by application of an electric field a structure that was subsequently named "Williams Domains." He was also a pioneer in the development of the Liquid Crystal Display (LCD), He co-authored the first paper suggesting their use in 1963 RCA Laboratories producing the first experimental displays in 國立交通大學應用化學系陳俊太老師 4

5 Outline History Introduction of Liquid Crystals Liquid Crystalline Mesophases Liquid Crystalline Polymers Liquid Crystal Classification Applications 9 Definition of a Liquid Crystal Liquid crystals, LCs, are substances that exhibit long-range order in one or two dimensions, but not all three. Both small molecules and polymers may exist in the liquid crystalline state, but generally special spatial structures are required. Compare to amorphous and crystalline materials: Amorphous substances are more or less entirely disordered, Crystalline materials are ordered in all three dimensions, The LCs lie in-between in properties. 10 國立交通大學應用化學系陳俊太老師 5

6 Properties of Liquid Crystals Liquid crystal can flow like a liquid, due to loss of positional order Liquid crystal is optically birefringent( 各向異性 ), due to its orientational order Transition from crystalline solids to liquid crystals caused by a change of temperature gives rise to THERMOTROPIC ( 熱致型 ) liquid crystals to form a liquid crystal phase at a certain temperature molecules that are elongated & have some degree of rigidity. 11 Crystal vs Liquid Crystal A crystal is a highly ordered structure which possesses longrange positional & orientational order For many substances these two types of order are destroyed simultaneously when the crystal melts to form a liquid For some substances, these orders are destroyed in stages. These are liquid crystals 國立交通大學應用化學系陳俊太老師 6

7 An Example A sink filled with water. Onto the water surface, toothpicks are slowly added without overlap. At first the toothpicks are random in order. As the surface becomes more concentrated, the toothpicks begin to align into small groups more or less side by side. Now there are tow phases present The dilute, disordered phase The LC phase Outline History Introduction of Liquid Crystals Liquid Crystalline Mesophases Liquid Crystalline Polymers Liquid Crystal Classification Applications 14 國立交通大學應用化學系陳俊太老師 7

8 Liquid Crystalline Mesophases Nematic ( 向列型 ) Smectic ( 層列型 ) Cholesteric ( 膽固醇型 ) Discotic ( 盤型, 碟型 ) 15 Nematic ( 向列型 ) Organized in one dimension only, with their chains lying parallel to each other at equilibrium. long-range orientational order but no positional order The preferred direction is know as director. from the Greek word nematos meaning thread 絲狀體 ) Example: 16 國立交通大學應用化學系陳俊太老師 8

9 Smectic ( 層列型 ) The smectic LCs are ordered in two dimensions. Molecules align themselves approximately parallel and tend to arrange in layers Not all positional order is destroyed when a crystal melts to form a smectic liquid crystal from the Greek word smectos meaning soap ) Example: smectic A smectic C 17 Cholesteric Mesophase The cholesteric mesophase is like a twodimensional twisted nematic mesophase. Example cholesteric Cholesteryl nonanoate 18 國立交通大學應用化學系陳俊太老師 9

10 Discotic Mesophase The discotic mesophase resembles stacks of dishes or coins. imgarcade.com/ Stacks of Dishes Stacks of Coins Example: discotic 19 First-order Transitions The various mesophasic transitions are firstorder transitions, because volume and heat capacity go through discontinuities when plotted against temperature or pressure. As the temperature is raised, the polymer first melts to a smectic structure, then to a nematic structure, and then to an isotropic melt. 20 國立交通大學應用化學系陳俊太老師 10

11 First-order Transitions 21 Outline History Introduction of Liquid Crystals Liquid Crystalline Mesophases Liquid Crystalline Polymers Liquid Crystal Classification Applications 22 國立交通大學應用化學系陳俊太老師 11

12 Liquid Crystalline Polymers The first polymeric liquid crystal discovered was polypeptide poly( -benzyl L-glutamate) in forms birefringent( 雙折射的 ), anisotropic( 各向異性 ) phases in organic solutions. The shape of these polypeptides is in the form of α- helices, regular intramolecular hydrogen bonding and association of the solvent with the substituents to stabilize the structure in the form of a rod. 23 Liquid Crystalline Polymers (1) The stiff backbone types Very high modulus fibers and high temperature plastics Mesogenic unit in the main chain Meso: 介 (2) The side chain types Useful for their action in magnetic and electrical fields. Mesogenic unit in the side chain 24 國立交通大學應用化學系陳俊太老師 12

13 The Stiff Backbone Type: Rod-Shaped Chemical Structures Example: poly(p-phenylene terephthalamide), PPD-T The amide groups are located in the trans positions on the phenylene rings There is minimal bond rotation out of the chain axis. 25 The Stiff Backbone Type: Rod-Shaped Chemical Structures Another example: copolymer of p-hydroxybenzoic acid, p,p - biphenol, and terephthalic acid The ester groups, together with the parabonded phenylene rings, form strictly rodlike structures. 26 國立交通大學應用化學系陳俊太老師 13

14 The Side Chain Types Another type of liquid crystalline polymers are those having rod-shaped side chains. Thus, the backbone may be a random coil, but the side chains are organized into one-or two-dimensional liquid crystals. Mesogenic unit in the side chain 27 Outline History Introduction of Liquid Crystals Liquid Crystalline Mesophases Liquid Crystalline Polymers Liquid Crystal Classification Applications 28 國立交通大學應用化學系陳俊太老師 14

15 Liquid Crystal Classification Thermotropic ( 熱致型 ) LCs Mesophase formation is temperature (T) dependent. Absence of Solvent Lyotropic ( 溶致型 ) LCs Mesophase formation is concentration dependent. In Solvent Mesogenic side group compositions A subclass of the thermotropic LCs Usually have random coil backbone chains, with rodshaped side groups. Usually processed in the melt state. 29 Thermotropic LC Chemical Structures Most of the thermotropic LC polymers are aromatic copolyesters, some being terpolymers. Some homopolymer aromatic polyesters generally have too high a melting temperature to form thermotropic mesophases without decomposition but copolymerization reduces their melting temperatures 30 國立交通大學應用化學系陳俊太老師 15

16 Techniques to Reduce the Melting Temperatures. 1. Copolymerization of several mesogenic monomers To produces random copolymers with depressed melting temperatures. 2. Use of monomers with bulky side groups To Prevents close packing in the LC mesophases Sometimes referred to as frustrated chain packing 3. Use of bent comonomers To interrupt the order in the system Incorporation of flexible spacers To decrease rigidity. This permits the development of bends or elbows ( 肘部 ) in the chain. 31 Bulky Side Groups 2. Use of monomers with bulky side groups To Prevents close packing in the LC mesophases, Sometimes referred to as frustrated chain packing 32 國立交通大學應用化學系陳俊太老師 16

17 Bent Comonomers 3. Use of bent comonomers To interrupt the order in the system. 33 Flexible Group 4. Incorporation of flexible spacers To decrease rigidity. This permits the development of bends or elbows ( 肘部 ) in the chain. 34 國立交通大學應用化學系陳俊太老師 17

18 Flexible Group Mesogenic unit in the main chain Mesogenic unit in the side chain 35 Lyotropic Liquid Crystals ( 溶致液晶 ) The lyotropic LC form ordered states in concentrated solutions. Frequently rod-shaped polymer chains are dissolved to concentrations of about 30% for processing purposes, particularly for fiber formation. The main reason for using solutions rather than melts the polymers in question decompose below their very high melting temperatures. 36 國立交通大學應用化學系陳俊太老師 18

19 Lyotropic LC polymers Most often the lyotropic LC polymers form nematic mesophases. Most of the polymers in this class are aromatic polyamides with aromatic ring structures. 37 Important Polyamides yielding LC mesophases. The basis for the fiber Kevlar Widely used for bullet-proof jackets, parachuttes ( 降落傘 ). 38 國立交通大學應用化學系陳俊太老師 19

20 Lyotropic Solutions of Polyheterocyclic Compounds 39 Side-Chain Liquid Crystalline Mesogenic side group compositions A subclass of the thermotropic LCs Usually have random coil backbone chains, with rodshaped side groups. Usually processed in the melt state. 40 國立交通大學應用化學系陳俊太老師 20

21 Side-Chain Liquid Crystalline The first attempts to form side-chain LC structures involved attachment of short rodshaped moieties directly to the main chain. These efforts were disappointing for two reasons. The glass transition of the polymers is always at much higher temperature than for the corresponding polymers without mesogenic side chains. During the polymerizaton process, the LC packing of the monomers was destroyed by steric requirements. 41 Side-Chain Liquid Crystalline If the mesogenic moieties are linked to the backbone through alkyl spacer group they should develop an LC structure similar to that attained before polymerization. The effects need to be considered Backbone type Spacer group Length of the free substituent at the end of the mesogenic moiety. 42 國立交通大學應用化學系陳俊太老師 21

22 Packing of LC Side Chains Single-layer Packing Two-layer Pacing Packing with overlapping alkyl tails Packing with partial overlapping of the mesogenic side chains. 1: Main chain 2: Spacer 3: Mesogenic group D: Repeat distance 43 Outline History Introduction of Liquid Crystals Liquid Crystalline Mesophases Liquid Crystalline Polymers Liquid Crystal Classification Applications 44 國立交通大學應用化學系陳俊太老師 22

23 Fiber Formation Rod-shaped polymer chains make usually strong, high modulus fibers. because it is easy to orient the chains in the fiber direction, without folding. Examples: Aromatic Polyamides soluble in high-strength sulfuric acid or in mixed solvents containing N,N-dimethylacetamide. This has been As the concentration of the polymer is increased, the viscosity first increases, then shows an abrupt decrease. interpreted as the formation of the nematic mesophase, having been exceeded 45 LCD Applications casio.jp store.oregonscientific.com 46 國立交通大學應用化學系陳俊太老師 23

24 Electric field Effects If an electric field is applied to a liquid crystal the molecules will align in the same direction as the field. 47 TN LCD Illustration of construction and operation of a single pixel of a twisted nematic (TN) LC-cell. No voltage applied = field-off state Light Pass Voltage applied = field-on state Light Blocked 48 wechat.fingerdaily.com/ 國立交通大學應用化學系陳俊太老師 24