Lecture33.pdf

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Department
Mechanical Engineering
Course
MECHENG 382
Professor
All Professors
Semester
Winter

Description
Lecture 33 - Polymers POLYMERS 5 • Polymers consist of long-chain C-C molecules (up to 10 atoms / chain) • Atoms within a long-chain molecule are covalently bonded • Chains consist of a repeated representative unit (“mer”) • Degree of polymerization = number of repeated units (⇒ molecular weight) • Properties of polymers depend on molecular weight & bonding between chains • Commercial polymers have a range of molecular weights ∴ No unique set of properties • Accurate data for design must be measured or obtained from supplier Unlike metals, no standards or well-defined properties that can be looked up Data-book values are approximate (even for modulus) • General properties: (i) ρ ≈ 1000 kg/m ; E ≈ 1-8 GPa (down to 1 MPa for an elastomer) (ii) Design often stiffness limited (because of low modulus) (iii) Properties sensitive to temperature (around room temperature) and time (iv) Polymers are very sensitive to u/v radiation and 3 (become stiff & brittle) • Three types of polymers determined by nature of bonding between chains (i) Covalent cross links (ii) Secondary bonding (Hydrogen or van der Waals) (iii) Chain entanglements Thermoplastics • No covalent cross-links between chains • Below glass transition temperature, Tg, there is secondary bonding between chains Hydrogen or van der Waals ME 382 - 14/iv/14 1 Lecture 33 - Polymers • Above T gecondary bonds melt leaving molecules free to slide over each other Chain entanglements responsible for activation energy Viscosity drops with increased temperature • Thermoplastics can be melted and reformed ∴ Possible to recycle thermoplastics (if separated and not contaminated) • Thermoplastics are often made by an addition reaction • Molecules add to existing chains (at ends or branches) ⇒ no joining of chains • Thermoplastics can be linear (such as High-Density Polyethylene (H.D.P.E.) “2”) • Or they can be branched (such as Low-Density Polyethylene (L.D.P.E.) “4”) Three types of packing (i) Aligned fibers - draw fibers from the melt Very stiff because of covalent bonding ME 382 - 14/iv/14 2 Lecture 33 - Polymers (ii) Crystalline - Chains fold Can also form plates radiating out of a sphere “spherulite” (iii) Amorphous - random tangle of chains Amorphous polymers are less dense than crystalline polymers Have a lower modulus, because lower density of bonds between chains More likely if branching, large side groups, or irregular side groups (atactic polymers) - because ordered packing is more difficult H.D.P.E. (milk bottles) is denser and stiffer than L.D.P.E. (plastic bags) • Common thermoplastics (with recycling codes): Polyethylene terephthalate (PET “1”) (CO-C H -CO-CH -CH -O) 6 4 2 2 n Polyethylene (HDPE-“2”, LDPE-“4”) (CH ) 2 n Polyvinylchloride (PVC - “3”) (CH -CHCl) 2 n Polypropylene (“5”) (CH -CHCH ) 2 3 n Polystyrene (“6”) (CH -CHC H ) 2 6 5 n Polytetrafluoroethylene (PTFE “Teflon”) (CF 2 n Plexiglass (PMMA) (CH -2(CH )(C3OCH )) 3 n Thermosets • Contain covalent cross-links to form a network of covalent bonds
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