POLYMERS (Materials 2)

Polymers

long chain of hydrocarbon molecules made of repeating units called monomers

• has high molecular weight (1000s of carbons)

Intramolecular bonds

strong covalent bonds that hold

• carbon-carbon

• carbon-hydrogen

Intermolecular

weaker forces that attract a polymer chain to another

• H-bonds hold chains multiple chains together

• because of dipoles

Amorphous

• chains are flexible

• hydrogen atoms can rotate along axis

• like drained/cooked spaghetti

  • covalent bonds inside each noodle

  • hydrogen bonds between sticky noodles

Mechanical Behavior & Plasticity

under tensile stress, polymer chains slip past another

• H-bonds break and form new positions

• covalent bonds stay intact

• causes plastic deformation

Increase Molecular Weight (strengthen polymers)

• increase MW = increase tensile strength

• longer chain = more H-bonds = more solid at room temp

  • longer chains have more contact with each other = stronger attraction between chains = material is stronger

Glass Transition Temperature (Tg)

below Tg, chains cannot move freely

• polymer is “frozen” and brittle

• colder than solid

Melting Temperature (Tm)

when heated enough, polymer chains can slip

• H-bonds break

• covalent bonds don’t break

• material will flow like a liquid

Thermoplastics

have Tm & Tg

• soften when heated

• harden when cooled

• easily recyclable

Plasticity

• ability to sustain large permanent deformations

  • breaking a plastic bag: can stretch and change shape without breaking

• large chains of polymers can uncoil without breaking primary bonds

Tensile Strength (strengthen polymers)

• change functional groups = change side attachments

• change tensile strength by changing structure & composition

• large functional groups = harder to slip = stronger & more brittle

  • polyethylene = small H groups = easily slip

  • polystyrene = large ring shaped = hard to slip

Density, Crystallinity, Ductility

affected by polymer’s branches

• branches can have different compositions

  • co-polymers

Elasticity / Elastomer / Rubber

elastomers are polymers that stretch and return to original shape

• cross-links = strong bonds that connect different polymer chains, helps return to original shape

• vulcanization = add sulfur to rubber to create cross links = make it stronger & more elastic

Thermosets

don’t melt and recycle as easily

• brittle & temperature-resistant

• heavily cross-linked

  • epoxy, melanine