Liquid Crystal - Liquid crystals (LCs) are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. (Fourth state of matter) Liquid Crystal Orientation of molecules in the mesophase 1) Nematic phase 2) Smetic phase A 3) Smetic phase C - For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. Liquid crystalline polymer (LCP) - Lyotopic LCP : form in solution above critical conc. - Thermotropic LCP : form in the melt below critical temp. 81 - rod-shaped organic molecules have no positional order, - but tend to point in the same direction - found at lower temperatures and more solid-like than the nematic - form well-defined layers thus positionally ordered along one direction - In the Smectic A phase, the molecules are oriented along the layer normal, - In the Smectic C phase, they are tilted away from the layer normal. 82 Liquid Crystal 4) Chiral nematic (Cholestreric) phase - a type of liquid crystal with a helical structure and which is therefore chiral. Liquid Crystal Polymers ex. Aromatic Polyamide (Lyotropic LCP) : Kevlar (Dupont) O O C C NH NH - They organize in layers with no positional ordering within layers, but a director axis which varies with layers. - The period of this variation (pitch, p) varies with temperature and it can also be affected by the boundary conditions when the chiral nematic liquid crystal is sandwiched between two substrate planes. Advantages of LCP : easy processing (low viscosity) and improved mechanical property (ordered alignment of polymer molecules) 83 84
Liquid Crystal Polymers 85 86 Vulcanization of rubber - The properties of rubber can be dramatically altered by crosslinking the polymer chains. - This process, carried out with sulfur, is known as Vulcanization - The key chemical modification is that sulfide bridges are created between adjacent chains. 87 - The sulfur interconnects the chains by reacting with the double bonds. - The crosslinking makes rubber non-sticky and improves its tensile strength. The material is no longer thermoplastic. - is crucial in making elastomers, as it prevents the chains from slipping past one another 88
Chemical cross-linking (curing) A. Definition and physical property change. a. Linking the polymer chains together through covalent bonds to form a network. b. Not to be soluble but swelling by any solvent. c. Not to be melted but decompose at any temperature. d. For ionic crosslinking, to be melted at high temperature. B. The method of chemical crosslinking. Effect of vulcanization on rubber molecules: (1) raw rubber, and (2) vulcanized (cross-linked) rubber. Variations of (2) include: (a) soft rubber, low degree of cross- linking; and (b) hard rubber, high degree of cross-linking 89 a. Simultaneous crosslinking during polymerization, using polyfunctional monomer. b. Stepwise crosslinking. First step : to make free polymer (linear or branched polymer). Second step : curing step to be made network from free polymer. 90 Chemical cross-linking Physical cross-linking A. Definition a. Not covalent crosslink but strong secondary force attraction between polymer chains. b. Physical crosslinked polymer can be recycled. B. Examples of physical crosslinking. a. Crystalline polymer : To act like crosslinked amorphous polymer. b. Hydrogen bond : Gelatin, an animal-derived protein. c. Block copolymer : A-B-A type. A : styrene, short-hard segment, microdomain. B : butadiene, long-soft segment, matrix. d. TPE : Thermoplastic elastomer. 91 92
Physical cross-linking : thermoplastic elastomers (TPE) Disadvantages of chemical cross-liniking - stable mechanically and thermally, so once formed are difficult to break - cannot be dissolved or molded - cannot be recycled <Solution> Thermally labile crosslinks : Break apart on heating & Reform on cooling 1) Hydrogen bond : ex. Gelatin Physical cross-linking : thermoplastic elastomers (TPE) 3) Reversible covalent crosslink RT, 7days in bulk film 80 o C, > 2h 2) Ionic crosslink Reversible polymer cross-linking via Diels-Alder cycloaddition reaction between furan and maleimide 93 94 Physical cross-linking : Physical cross-linking : Block copolymer 4) Crystallite 5) Block copolymer 95 Representation of aggregation in an ABA block thermoplastic elastomer 96
Physical cross-linking : Block copolymer A. Definition of polymer blends. a. Physical mixture of two or more different polymer or copolymer. b. No covalent bonds among the polymers. c. Polymer alloys : like metal alloys. d. Blending is much easier than developing new polymers. 97 98 B. Technology of polymer blendings. A) Miscible polyblends : Binary homogeneous blend Chemical Blends ex.interpenetrating Crosslinked polymer is swollen with different monomer, then monomer is polymer network (IPN) polymerized and crosslinked 99 100