PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Introduction to Materials Science, Chapter 14, Polymer StructuresUniversity of Virginia, Dept. of Materials Science and Engineering1Molecular weight (II)Alternative way to express average polymer chain size is degree of polymerization - the average number of mer units in a chain:mMnnnnumber-average: weight-average:•Melting / softening temperatures increase with molecular weight (up to ~ 100,000 g/mol)Room T, short chain polymers (~ 100 g/mol) are liquids or gases, intermediate length polymers (~ 1000 g/mol) are waxy solids, •solid polymers: ~ 104 - 107 g/molmMnwwis the mer molecular weightmIntroduction to Materials Science, Chapter 14, Polymer StructuresUniversity of Virginia, Dept. of Materials Science and Engineering2Molecular shapeThe angle between the singly bonded carbon atoms is ~109o – carbon atoms form a zigzag pattern in a polymer molecule.Random kinks and coils lead to entanglement, like in the spaghetti structure:While maintaining the 109o angle between bonds polymer chains can rotate around single C-C bonds (double and triple bonds are very rigid).Introduction to Materials Science, Chapter 14, Polymer StructuresUniversity of Virginia, Dept. of Materials Science and Engineering3Molecular shapeMolecular chains may thus bend, coil and kinkNeighboring chains may intertwine and entangleLarge elastic extensions of rubbers correspond to unraveling of these coiled chainsMechanical / thermal characteristics depend on the ability of chain segments to rotateIntroduction to Materials Science, Chapter 14, Polymer StructuresUniversity of Virginia, Dept. of Materials Science and Engineering4Molecular structureThe physical characteristics of polymer material depend not only on molecular weight and shape, but also on molecular structure:1 Linear polymers: Van der Waals bonding between chains. Examples: polyethylene, nylon. 2 Branched polymers: Chain packing efficiency is reduced compared to linear polymers - lower densityIntroduction to Materials Science, Chapter 14, Polymer StructuresUniversity of Virginia, Dept. of Materials Science and Engineering5Molecular structure3 Cross-linked polymers: Chains are connected by covalent bonds. Often achieved by adding atoms or molecules that form covalent links between chains. Many rubbers have this structure.4 Network polymers: 3D networks made from trifunctional mers. Examples: epoxies, phenol-formaldehydeIntroduction to Materials Science, Chapter 14, Polymer StructuresUniversity of Virginia, Dept. of Materials Science and Engineering6Isomerism (repeat)Hydrocarbon compounds with same composition may have different atomic arrangements. Physical properties may depend on isomeric state (e.g. boiling temperature of normal butane is -0.5 oC, of isobutane -12.3 oC)Butane  C4H10  IsobutaneTwo types of isomerism are possible: stereoisomerism and geometrical isomerismIntroduction to Materials Science, Chapter 14, Polymer StructuresUniversity of Virginia, Dept. of Materials Science and Engineering7StereoisomerismStereoisomerism: atoms are linked together in the same order, but can have different spatial arrangement1 Isotactic configuration: all side groups R are on the same side of the chain.2 Syndiotactic configuration: side groups R alternate sides of the chain.3 Atactic configuration: random orientations of groups R along the chain.Introduction to Materials Science, Chapter 14, Polymer StructuresUniversity of Virginia, Dept. of Materials Science and Engineering8Geometrical isomerismGeometrical isomerism: consider two carbon atoms bonded by a double bond in a chain. H atom or radical R bonded to these two atoms can be on the same side of the chain (cis structure) or on opposite sides of the chain (trans structure).Cis-polyisopreneTrans-polyisopreneIntroduction to Materials Science, Chapter 14, Polymer StructuresUniversity of Virginia, Dept. of Materials Science and Engineering9Summary: Size – Shape -StructureIntroduction to Materials Science, Chapter 14, Polymer StructuresUniversity of Virginia, Dept. of Materials Science and Engineering10Copolymers (composed of different mers)Copolymers: at least two different types of mers, can differ in the way the mers are arranged:Random copolymerAlternating copolymerBlock copolymerGraft copolymerSynthetic rubbers are copolymersIntroduction to Materials Science, Chapter 14, Polymer StructuresUniversity of Virginia, Dept. of Materials Science and Engineering11Polymer Crystallinity (I)Atomic arrangement in polymer crystals is more complex than in metals or ceramics Unit cells are typically large and complex Polymer molecules are often partially crystalline (semi-crystalline), with crystalline regions dispersed within amorphous material.PolyethyleneIntroduction to Materials Science, Chapter 14, Polymer StructuresUniversity of Virginia, Dept. of Materials Science and Engineering12Polymer Crystallinity (II)Degree of crystallinity is determined by:Rate of cooling during solidification: time is necessary for chains to move and align into a crystal structureMer complexity: crystallization less likely in complex structures, simple polymers, such as polyethylene, crystallize relatively easilyChain configuration: linear polymers can crystallize, branches inhibit crystallization, network polymers almost completely amorphous, cross-linked polymers can be crystalline or amorphousIsomerism: isotactic, syndiotactic polymers can crystallize - geometrical regularity allows chains to fit together, atactic difficult to crystallizeCopolymerism: crystallize if mer arrangements are more regular - alternating, block can crystallize more easily as compared to random and graftMore crystallinity: higher density, more strength, higher resistance to dissolution and softening by heatingIntroduction to Materials Science, Chapter 14, Polymer StructuresUniversity of Virginia, Dept. of Materials Science and Engineering13Polymer Crystallinity (III)Crystalline polymers are denser than amorphous polymers, so the degree of crystallinity can be obtained from