Polymers
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where do polymers come from in terms of raw material?
crude oil
what are polymers?
polymers are materials with large macro-molecules, of which plastics is just one group - long chains of hydrocarbons with sometimes other atoms like oxygen and silicon
what are some general properties of polymers?
• good thermal/electrical insulation • low density • easy to manufacture and low cost • useful as adhesives (including composites) • good transparency • durable
what is the main difference between thermoplastics and thermosets?
thermoplastics - can be re-heated thermosets - cannot be re-heated
what is an example of an elastomer?
rubber (can undergo higher strain)
what are polysiloxanes?
alternating oxygen and silicon atoms
what is the bonding in polymers?
atoms are bounded by covalent bonds to form long and flexible chains, secondary bonds between chains (VdW)
what is the structure of polymers?
amorphous or partially crystalline materials
what is teflon?
a polymer made from a hydrocarbon chain with fluorine - used in frying pans
what are the repeated units called in a polymer?
mers
what is polymerization?
the chemical process that causes a large no. of monomers to combine to form the polymer
why do polymers have low stiffness and low melting points (generally)?
because although they have a strong covalent backbone, chains will often bond by weaker secondary attraction
what determines the mechanical properties of the material?
molecular structure and molecular weight
what are linear polymers?
• single chains • flexible • ‘mass of spaghetti’ • van der walls between molecules
what are branched polymers?
• side branches • packing is less efficient • lower density • less crystalline than linear polymers
what are cross-linked polymers?
• adjacent chains joined at various positions by covalent bonds, e.g rubbers • cross-links are often non reversible due to chemical reactions • when heavily cross linked, may be referred to as a network
order the molecular structures of polymer from lowest to highest strength?
• network • cross-linked • branched • linear
how can we tailor the properties of polymers?
using different ‘mer’ units
what is condensation polymerisation?
• elimination of small molecule like H2O or HCl etc. - reaction between an hydroxyl (-OH) and carboxylic acid (-COOH) to make an ester and water
what is free radical polymerisation?
• produces an addition reaction - the 3 steps are: initiation, propagation, termination
what are the three main classes of polymers?
• thermoplastics • thermosets • elastomers
what are some properties of thermoplastics?
• they soften on heating • linear/branches chains (minimal cross linking) • weak intermolecular bonding • greater ability to form (semi-) crystalline regions • ductile: long period of plastic deformation • e.g PP, PE, PS
what are some properties of thermosets?
• only degrade on heating • react and harden (e.g epoxy resin) • heavily cross-linked (10-50% of mers) • do not soften on heating once formed, difficult to recycle • brittle: minimal period of plastic deformation • e.g vulcanised rubber, epoxies, urea formaldehyde, polyester resin, polyesters
what are some properties of elastomers?
• linear with some cross-linking, which provides shape ‘memory’ / retention • useful for heat shrink, to insulate electrical cables and structural bearing pads in construction