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530.352 Materials Selection

530.352 Materials Selection. Lecture #7: Molecular structure of polymers Monday September 26 th , 2005. Polymers. Poly + mers more than compounds 2 (building blocks). Atomic packing in polymers :. H H H H H H H | | | | | | |

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530.352 Materials Selection

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  1. 530.352 Materials Selection Lecture #7: Molecular structure of polymersMonday September 26th, 2005

  2. Polymers Poly + mers more than compounds 2 (building blocks)

  3. Atomic packing in polymers : H H H H H H H | | | | | | | C = C+Energy -> - C - C - C - C - C - | | | | | | | H H H H H H H Ethylene+ E -> Polyethylene Energy required to break C=C bonds

  4. Atomic packing in polymers : H C6H5 H C6H HH H C6H | | | | | | | | C = C -> - C - C - C - C - C - C - | | | | | | | | H H H H H C6H H H Styrene -> Polystyrene In all polymers: Covalently bonded C and H form the backbone.

  5. Atomic packing in polymers : H Cl H Cl HH H Cl | | | | | | | | C = C -> - C - C - C - C - C - C - | | | | | | | | H H H H H Cl H H Vinyl Chloride -> Polyvinyl chloride (PVC)

  6. Polymerization • Polymers are large - long chained molecules • Degree of polymerization (# of mers)typically 2,000 - 5,000 • # controlled by the heat input and catalysts or reagents

  7. Types of polymers - I • Linear • Branched • Cross-linked

  8. Types of polymers - II • Thermoplastic • linear and branched • recyclable plastics • Thermoset • cross-linked • epoxy , eggs, etc • Elastomers • only slightly cross-linked

  9. Types of polymers - III • Amorphous • disordered or “glassy” • produced by faster cooling rate • less dense • has a ‘glass transition temperature’ Tg • Crystalline • quasi crystalline • promoted by slow cooling • has a ‘melting temperature’ Tm

  10. Packing the molecules : Epoxy resin Low density polyethylene

  11. Packing the molecules : Rubber (T < Tg ) Rubber (T > Tg )

  12. | | | - C - C - C - | | | Bonding • Primary bonds • covalent • within molecule • Secondary bonds • between molecules • Vander Waals • Hydrogen • Dipole • Ionic

  13. Viscoelastic behavior • Elasticity • instantaneous • Viscosity • time dependent

  14. Elasticity in polymers • distortion of bonds and bond angles • entropy changes The old ‘straight and narrow’ - lower degree of freedom ‘Drunk’s walk home’ - more degrees of freedom

  15. Viscosity in polymers • slippage of molecules • friction between chains • Resistance to coiling and uncoiling

  16. Effect of strain rate • Short times / low T • glassy / brittle behavior • Long times / high T • rubbery / ductile behavior glassy log(E) rubbery liquid Tg Tm time , T

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