5 Pages
Unlock Document

Mechanical Engineering
All Professors

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
More Less

Related notes for MECHENG 382

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.