Introduction To Materials Science, Chapter 15, Polymers .

Introduction to Materials Science, Chapter 15, Polymer StructuresChapter Outline: Polymer Structures¾ Hydrocarbon and Polymer Molecules¾ Chemistry of Polymer Molecules¾ Molecular Weight and Shape¾ Molecular Structure and Configurations¾ Copolymers¾ Polymer CrystalsOptional reading: noneUniversity Tennessee, Dept. of Materials Science and Engineering1Introduction to Materials Science, Chapter 15, Polymer StructuresPolymers: Introduction¾ Polymer - a large molecule consisting of (at least five)repeated chemical units ( mers') joined together, likebeads on a string. Polymers usually contain many morethan five monomers, and some may contain hundreds orthousands of monomers in each chain.¾ Polymers may be natural, such as cellulose or DNA, orsynthetic, such as nylon or polyethylene.Silk fiber is produced bysilk worms in a cocoon,to protect the silkwormwhile it metamorphosesin a moth.Many of the most important current research problems involvepolymers. Living organisms are mainly composed ofpolymerized amino acids (proteins) nucleic acids (RNA andDNA), and other biopolymers. The most powerful computers our brains - are mostly just a complex polymer material soakingin salty water! We are just making first small steps towardsUniversityDept.of Materials Science and ems.21

Introduction to Materials Science, Chapter 15, Polymer StructuresHydrocarbon molecules (I)¾ Most polymers are organic, and formed fromhydrocarbon molecules¾ Each C atom has four e- that participate in bonds,each H atom has one bonding eExamples of saturated (all bonds are single ones)hydrocarbon molecules:Methane, CH4Ethane, C2H6Propane, C3H8University Tennessee, Dept. of Materials Science and Engineering3Introduction to Materials Science, Chapter 15, Polymer StructuresHydrocarbon molecules (II)Double and triple bonds can exist between C atoms(sharing of two or three electron pairs). These bondsare called unsaturated bonds.Unsaturatedmolecules are more reactiveH-C H-CEthylene, C2H4Acetylene, C2H2Isomers are molecules that contain the same atoms but in adifferent arrangement.An example is butane andisobutane:Butane C4H10 IsobutaneUniversity Tennessee, Dept. of Materials Science and Engineering42

Introduction to Materials Science, Chapter 15, Polymer StructuresHydrocarbon molecules (III)Many other organic groups can be involved inpolymer molecules. In table above, R represents aradical, an organic group of atoms that remain as aunit and maintain their identity during chemicalreactions(e.g.Tennessee,CH3, Dept.C2Hof5,MaterialsC6H5)Science and EngineeringUniversity5Introduction to Materials Science, Chapter 15, Polymer StructuresPolymer molecules¾ Polymer molecules are very large: macromolecules¾ Most polymers consist of long and flexible chains witha string of C atoms as a backbone.¾ Side-bonding of C atoms to H atoms or radicals¾ Double bonds are possible in both chain and side bonds¾ A repeat unit in a polymer chain (“unit cell”) is a mer¾ A single mer is called a monomerUniversity Tennessee, Dept. of Materials Science and Engineering63

Introduction to Materials Science, Chapter 15, Polymer StructuresChemistry of polymer molecules (I)¾ Ethylene (C2H4) is a gas at room temp and pressure¾ Ethylene transforms to polyethylene (solid) by formingactive mers through reactions with an initiator orcatalytic radical (R.)¾ (.) denotes unpaired electron (active site)Polymerization:1. Initiation reaction:2. Rapid propagation 1000 mer units in 1-10 ms:3. Termination: When two active chain ends meet eachother or active chain ends meet with initiator or otherspecies with single active bond:University Tennessee, Dept. of Materials Science and Engineering7Introduction to Materials Science, Chapter 15, Polymer StructuresChemistry of polymer molecules (II)Replace hydrogen atomsin polyethylene: makepolytetraflouroethylene(PTFE) – TeflonReplace every fourthhydrogen atom inpolyethylene with Clatom: polyvinylchlorideReplace every fourthhydrogen atom inpolyethylene with CH3methyl group:polypropyleneUniversity Tennessee, Dept. of Materials Science and Engineering84

Introduction to Materials Science, Chapter 15, Polymer StructuresChemistry of polymer molecules (III)¾ When all the mers are the same, the molecule is called ahomopolymer¾ When there is more than one type of mer present, themolecule is a copolymer¾ Mer units that have 2 active bonds to connect withother mers are called bifunctional¾ Mer units that have 3 active bonds to connect withother mers are called trifunctional. They form threedimensional molecular network dehyde(trifunctional)University Tennessee, Dept. of Materials Science and Engineering9Introduction to Materials Science, Chapter 15, Polymer StructuresMolecular weight (I)¾ Final molecular weight (chain length) is controlled byrelative rates of initiation, propagation, terminationsteps of polymerization¾ Formation of macromolecules during polymerizationresults in distribution of chain lengths and molecularweights¾ The average molecular weight can be obtained byaveraging the masses with the fraction of times theyappear (number-average molecular weight) or withthe mass fraction of the molecules (weight-averagemolecular weight).number-average:Mn xiMiweight-average:Mw w iMiMi is the mean molecular weight of range iwi is weight fraction of chains of length ixi is number fraction of chains of length iUniversity Tennessee, Dept. of Materials Science and Engineering105

Introduction to Materials Science, Chapter 15, Polymer StructuresMolecular weight (II)¾ Alternative way to express average polymer chain sizeis degree of polymerization - the average number ofmer units in a chain:number-average:weight-average:Mnn nmnw mMwmis the mer molecular weight Melting / softening temperatures increasemolecular weight (up to 100,000 g/mol) At room temperature, short chain polymers (molarweight 100 g/mol) are liquids or gases, intermediatelength polymers ( 1000 g/mol) are waxy solids, solidpolymers have molecular weights of 104 - 107 g/molUniversity Tennessee, Dept. of Materials Science and Engineeringwith11Introduction to Materials Science, Chapter 15, Polymer StructuresMolecular shape¾ The angle between the singly bonded carbon atoms is 109o – carbon atoms form a zigzag pattern in apolymer molecule.¾ Moreover, while maintainingthe 109o angle between bondspolymer chains can rotatearound single C-C bonds(double and triple bonds arevery rigid).¾ Random kinks and coilslead to entanglement, likein the spaghetti structure:University Tennessee, Dept. of Materials Science and Engineering126

Introduction to Materials Science, Chapter 15, Polymer StructuresMolecular shape¾ Molecular chains may thus bend, coil and kink¾ Neighboring chains may intertwine and entangle¾ Large elastic extensions of rubbers correspond tounraveling of these coiled chains¾ Mechanical / thermal characteristics depend on theability of chain segments to rotateUniversity Tennessee, Dept. of Materials Science and Engineering13Introduction to Materials Science, Chapter 15, Polymer StructuresMolecular structureThe physical characteristics of polymer material dependnot only on molecular weight and shape, but also onmolecular structure:1 Linear polymers: Van der Waals bonding betweenchains. Examples: polyethylene, nylon.2 Branched polymers: Chain packing efficiency isreduced compared to linear polymers - lower densityUniversity Tennessee, Dept. of Materials Science and Engineering147

Introduction to Materials Science, Chapter 15, Polymer StructuresMolecular structure3 Cross-linked polymers: Chains are connected bycovalent bonds. Often achieved by adding atoms ormolecules that form covalent links between chains.Many rubbers have this structure.4 Networkpolymers:trifunctional mers.formaldehyde3D networks made fromExamples: epoxies, phenol-University Tennessee, Dept. of Materials Science and Engineering15Introduction to Materials Science, Chapter 15, Polymer StructuresIsomerismIsomerism: Hydrocarbon compounds with samecomposition may have different atomic compositions.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 erism and geometrical isomerismUniversity Tennessee, Dept. of Materials Science and Engineering168

Introduction to Materials Science, Chapter 15, Polymer StructuresStereoisomerismStereoisomerism: atoms are linked together in thesame order, but can have different spatial arrangement1 Isotactic configuration: all side groups R are onthe same side of the chain.2 Syndiotactic configuration:alternate sides of the chain.sidegroupsR3 Atactic configuration: random orientations ofgroups R along the chain.University Tennessee, Dept. of Materials Science and Engineering17Introduction to Materials Science, Chapter 15, Polymer StructuresGeometrical isomerismGeometrical isomerism: consider two carbon atomsbonded by a double bond in a chain. H atom or radical Rbonded to these two atoms can be on the same side of thechain (cis structure) or on opposite sides of the chain(trans rsity Tennessee, Dept. of Materials Science and Engineering189

Introduction to Materials Science, Chapter 15, Polymer StructuresSummary: Size – Shape -StructureUniversity Tennessee, Dept. of Materials Science and Engineering19Introduction to Materials Science, Chapter 15, Polymer StructuresCopolymers (composed of different mers)Copolymers, polymers with at least two different types ofmers, can differ in the way the mers are arranged:Random copolymerAlternating copolymerBlock copolymerGraft copolymerSynthetic rubbers are copolymersUniversity Tennessee, Dept. of Materials Science and Engineering2010

Introduction to Materials Science, Chapter 15, Polymer StructuresPolymer Crystallinity (I)Atomic arrangementin polymer crystalsis more complexthan in metals orceramics (unit cellsare typically largeand complex).PolyethylenePolymer molecules are oftenpartiallycrystalline(semicrystalline), with crystallineregionsdispersedwithinamorphous material.University Tennessee, Dept. of Materials Science and Engineering21Introduction to Materials Science, Chapter 15, Polymer StructuresPolymer Crystallinity (II)Degree of crystallinity is determined by:¾ Rate of cooling during solidification: time isnecessary for chains to move and align into a crystalstructure¾ Mer complexity: crystallization less likely in complexstructures, simple polymers, such as polyethylene,crystallize relatively easily¾ Chain configuration: linear polymers crystallizerelatively easily, branches inhibit crystallization,network polymers almost completely amorphous, crosslinked polymers can be both crystalline and amorphous¾ Isomerism: isotactic, syndiotactic polymers crystallizerelatively easily - geometrical regularity allows chainsto fit together, atactic difficult to crystallize¾ Copolymerism: easier to crystallize if merarrangements are more regular - alternating, block cancrystallize more easily as compared to random and graftMore crystallinity: higher density, more strength, higherresistance to dissolution and softening by heatingUniversity Tennessee, Dept. of Materials Science and Engineering2211

Introduction to Materials Science, Chapter 15, Polymer StructuresPolymer Crystallinity (III)Crystalline polymers are denser than amorphous polymers,so the degree of crystallinity can be obtained from themeasurement of density:%crystallin ity ρc ( ρ s ρa ) 100ρs ( ρc ρa )ρc: Density of perfect crystalline polymerρa: Density of completely amorphous polymerρs: Density of partially crystalline polymer that weare analyzingUniversity Tennessee, Dept. of Materials Science and Engineering23Introduction to Materials Science, Chapter 15, Polymer StructuresPolymer CrystalsThin crystalline platelets grown from solution - chains foldback and forth: chain-folded modelPolyethyleneThe average chain length is much greater than thethickness of the crystalliteUniversity Tennessee, Dept. of Materials Science and Engineering2412