PolymersPOLYMER STRUCTURESPolymerChemistry and Structure of PolyethyleneAncient PolymersCelluloseRubberSlide 8Hydrocarbon MoleculesSaturated HydrocarbonsUnsaturated HydrocarbonsIsomerismAddition (Chain) PolymerizationCondensation (Step) PolymerizationPolymerizationSome Common Addition PolymersSome Condensation PolymersMOLECULAR WEIGHTMOLECULAR WEIGHT DISTRIBUTIONSlide 20Slide 21Degree of Polymerization, DPPolymer Chain LengthsPolymers – Molecular ShapeChain End-to-End Distance, rMolecular Structures for PolymersMolecular Structures- BranchedMolecular Structures – Cross-linked, NetworkThermoplastics and ThermosetsSlide 30Slide 31More Examples of ThermoplasticsSpecific Thermoplastic PropertiesThermoset dataSlide 35Slide 36Thermoplastic vs ThermosetCopolymersCrystallinity in PolymersCrystal StructuresSlide 41Polymer CrystallinityPlastic Recycling SymbolsPaper or Plastic?PolymersChapter 412• What are the general structural and chemical characteristics of polymer molecules?• What are some of the common polymeric materials, and how do they differ chemically?• How is the crystalline state in polymers different from that in metals and ceramics ?POLYMER STRUCTURES3 Polymer Poly mer many repeat unit (building blocks)C C C C C CHHHHHHHHHHHHPolyethylene (PE)ClCl ClC C C C C CHHHHHHHHHPoly(vinyl chloride) (PVC)HHHHH HPolypropylene (PP)C C C C C CCH3HHCH3CH3HrepeatunitrepeatunitrepeatunitCarbon chain backbone4Chemistry and Structure of Polyethylene•Polyethylene is a long-chain hydrocarbon. •Top figure shows repeat unit and chain structures. •Other figure shows zigzag backbone structure. Tetrahedral arrangement of C-H5Ancient Polymers•Naturally occurring polymers (those derived from plants and animals) have been used for centuries.–Wood – Rubber–Cotton – Wool–Leather – Silk•Oldest known uses–Rubber balls used by IncasCellulose•Cellulose is a highly abundant organic compound. Extensive hydrogen bonding between the chains causes native celluose to be roughly 70% crystalline. It also raises the melting point (>280°C) to above its combustion temperature. •Cellulose serves as the principal structural component of green plants and wood.•Cotton is one of the purest forms of cellulose and has been cultivated since ancient times. •Cotton also serves (along with treated wood pulp) as the source the industrial production of cellulose-derived materials which were the first "plastic" materials of commercial importance.Rubber•A variety of plants produce a sap consisting of a colloidal dispersion of cis-polyisoprene. This milky fluid is especially abundant in the rubber tree (Hevea); it drips when the bark is wounded. •After collection, the latex is coagulated to obtain the solid rubber. Natural rubber is thermoplastic, with a glass transition temperature of –70°C.•Raw natural rubber tends to be sticky when warm and brittle when cold, so it was little more than a novelty material when first introduced in Europe around 1770. •It did not become generally useful until the mid-nineteenth century when Charles Goodyear found that heating it with sulfur — a process he called vulcanization — could greatly improve its properties.cis-polyisoprene8The International Journal for the Science and Technology of PolymersHydrocarbon Molecules•Many organic materials are hydrocarbons (composed of hydrogen and carbon). •Most polymers are made up of H and C.•The bonds between the hydrocarbon molecules are covalent.•Each carbon atom has 4 electrons that may be covalently bonded, the hydrogen atom has 1 electron for bonding. •A single covalent bond exists when each of the 2 bonding atoms contributes one electron (ex: methane, CH4).910Saturated HydrocarbonsEach carbon has a single bond to 4 other atoms; the 4 valence electrons are bonded, the molecule is stable. Examples are seen in the table.The covalent bonds in each molecule are strong, but only weak hydrogen and van der Waals bonds exist between the molecules.Most of these hydrocarbons have relatively low melting and boiling points. However, boiling temperatures rise with increasing molecular weight.11Unsaturated Hydrocarbons•Double & triple bonds are somewhat unstable – involve sharing 2 or 3 pairs of electrons, respectively. They can also form new bonds–Double bond found in ethylene - C2H4–Triple bond found in acetylene - C2H2 C CHHHHC C HH12Isomerism•Two compounds with same chemical formula can have different structures (atomic arrangements). for example: C8H18•normal-octane•2,4-dimethylhexaneC C C C C C C CHHHHHHHHHHHHHHHHHH H3C CH2CH2CH2CH2CH2CH2CH3=H3C CHCH3CH2CHCH2CH3CH3H3C CH2CH3( )61313Addition (Chain) Polymerization– Initiation– Propagation– Termination1414Condensation (Step) Polymerization15Polymerization•Free radical polymerization: ethylene gas reacts with the initiator (catalyst). (“R.” is the unpaired electron)C CH HHHmonomer(ethylene)R+free radicalR C CHHHHinitiationR C CHHHHC CH HHH+R C CHHHHC CH HH HpropagationdimerMonomer refers to the small molecule from which a polymer is synthesized.Some Common Addition PolymersSome Condensation Polymers18MOLECULAR WEIGHT• Molecular weight, M: Mass of a mole of chains.Low Mhigh M•Polymers can have various lengths depending on the number of repeat units. •During the polymerization process not all chains in a polymer grow to the same length, so there is a distribution of molecular weights. There are several ways of defining an average molecular weight.•The molecular weight distribution in a polymer describes the relationship between the number of moles of each polymer species and the molar mass of that species.19xi = number fraction of chains in size range i MOLECULAR WEIGHT DISTRIBUTIONiiwiinMwMMxM wi = weight fraction of chains in size range i Mi = mean (middle) molecular weight of size range iMn = the number average molecular weight (mass)__202122Degree of Polymerization, DPDP = average number of repeat units per chain weightmolecular unit repeat where mmMDPn Ex. problem 4.1b, for PVC: m = 2(carbon) + 3(hydrogen) + 1(Clorine) (from front of book) = 2(12.011) + 3(1.008) + 1(35.45) = 62.496 g/mol DP = 21,150 / 62.496 = 338.42Polymer Chain Lengths•Many polymer properties are affected by the length of the polymer chains. For example, the melting temperature increases with increasing molecular weight.•At room temp, polymers with very short