Materials Science Polymers

Polymers and plastics Polymers are materials with large macro- molecules, of which plastics is just one group. Plastics are used widely in engineered products as they can provide:  Good thermal / electrical insulation  Low density  Easy to manufacture and low cost  Useful as adhesives (including composites)  Good transparency  Durable (?)

Polymers and plastics Low density Transparency Thermal properties

Polymers and plastics Where’s the plastic?

Polymers and plastics Large civil engineering projects…

Polymer molecules Organic in origin (C, H, O atoms > Low density) Gigantic in comparison with hydrocarbons (macro- molecules) Atoms bound by covalent bonds to form long and flexible chains, secondary bonds between chains (VdW) Amorphous or partially crystalline materials

Polymer molecules Composed of structural entities, ‘mer’ units that are repeated Polymer = ‘many parts’ Polymerisation is the chemical process that causes a large no. of monomers to combine to form the polymer.

Polymers – properties and molecules Although polymers consist of a strong covalent backbone, chains will often bond by weaker secondary attraction. Polymers are normally characterized by:  Low stiffness  Low melting points The polymer chains determine the mechanical properties of the material  Molecular structure  Molecular ‘weight’

Polymers – molecular structures Linear polymers – Single chains, Flexible, ‘Mass of spaghetti’, Van der Waals between molecules  Polyethylene (PE), PVC, Polystyrene. Branched polymers – side branches, packing is less efficient, lower density, less crystalline than linear. Direction of increasing strength

Polymers – molecular structures 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’ Direction of increasing strength

Polymers – molecular weight The molecular weight (M w ) is the mass of a mole (fixed number) of chains. Stiffness, tensile strength and even viscosity of the material are linked to molecular weight. Longer chains are more likely to entangle and can ‘anchor’ better. Critical length needed before strength increases (~100 mers but only ~40 for nylons).

Polymers – molecular crystallinity Polymers are generally considered amorphous, but can show a degree of crystallinity (%C) by the alignment of polymer chains. Tensile strength and stiffness increases, but ductility decreases with %C. Annealing causes crystalline regions to grow. Bulky side groups / irregular chain structure reduces %C.

Copolymers We have considered that monomer units are all the same: A + A + A + A → -A-A-A-A-… Copolymers are composed of two ‘mer’ units, e.g. : A + B + A + B → -A-B-A-B-… (like polyamides and polyesters) This can provide improved and tailored properties. When more than one monomer is used, an irregular chain structure will result. This can be helpful to modify the intermolecular bonds.

Copolymers – categories Random copolymer -A-B-B-B-A-B-A-B-A-A- Alternating copolymer -A-B-A-B-A-B-A-B-A-B- Block copolymer -B-B-B-B-B-A-A-A-A-A- Graft copolymer -A-A-A-A-A-A-A-A-A-A- -B-B-B- Synthetic rubbers such as styrene butadiene (SBR) are random copolymers used in tyres.

Classes of polymers The extent of cross-linking between chains will greatly influence the final material. Stronger attractive forces between chains leads to a stronger, less flexible polymer (e.g. nylon). If chains are able to slide past each other easily (VdW bonding), the polymer is likely to be flexible (e.g. PE). Polymers are primarily classified by this cross-linking and resulting properties:  Thermoplastics  Thermosets  Elastomers

Classes of polymers Thermoplastics – soften on heating  Linear / branched chains – minimal cross-linking  Weak intermolecular bonding, ‘melted’ by heating.  Greater ability to form (semi-) crystalline regions  Ductile – long period of plastic deformation  Polyethylene, polypropylene, polycarbonate, polystryene, etc.

Classes of polymers Thermosets – only degrade on heating  React and harden (e.g. epoxy)  Heavily cross-linked (10 to 50% of mers)  Do not soften on heating once formed; Difficult to recycle.  Brittle – minimal period of plastic deformation  Vulcanized rubber, epoxies, polyester resin, phenolic resin Elastomers  Linear with some cross-linking, which provides shape ‘memory’ / retention.  Useful for ‘heat shrink’, to insulate electrical cables.

Mechanical properties The mechanical properties of polymers can vary dramatically based on the molecular structure of the material as well as the temperature it is operating at. General observations can be made:  Stiffness 7 MPa to 4 GPa (Low)  Maximum tensile strength 100MPa (Low)  Elongations 100 – 1000% (High!)

Mechanical properties

Mechanical properties – brittle

Mechanical properties - ductile

Cold drawing Where a material begins to ‘neck’, it has been drawn down. Drawing stretches the polymer prior to use and aligns the chains in the drawing direction.  Increases elastic modulus (E) in the stretched dir.  Increases tensile strength (  TS ) in the stretched dir.  Decreases ductility (%EL) As with cold working metals, we can also anneal after drawing.  Decreases alignment and reverses effect.

Kevlar – drawn fibres Kevlar TM (DuPont, 1962) –para-aramid synthetic fibre "...5 times stronger than steel on an equal weight basis..." Some clever chemistry – but properties ultimately arise from mechanical drawing

Mechanical properties - elastomers Compare to responses of other polymers: -brittle response (aligned, cross linked & networked case) -plastic response (semi-crystalline case)

Temperature dependence Polymers display a spectrum of mechanical behaviour near room temperature (-20 to 200°C). The mechanical state depends on the glass transition temperature (T g )- at which point the secondary bonds begin to melt.  Polyethylene: T g ~ Ductile / low E  PMMA: T g ~ Brittle / high E

Temperature dependence Reducing temperature of PMMA, what happens to…  Stiffness?  Strength?  Ductility ?

Manufacturing with polymers Generally low processing temperatures, providing reduced costs. Key methods include… Injection moulding – thermoset / thermoplastic  Inject precursor / molten polymer into die by screw  Can form complex shapes, rapid manufacturing  Expensive to produce the mould ‘tool’

Manufacturing with polymers Extrusion – normally thermoplastic  Molten polymer fed by screw squeezed through an orifice  Samples of constant cross-section Blow moulding – thermoplastic  Use air to blow polymer into required shape  Useful for hollow objects (bottles) Compression moulding – normally thermoset  Heated, mixed precursor (to react)  Pressed into shape under pressure

Summary Polymers are organic, macro-molecules made of repeating mer- units Material properties controlled by degree of cross-linking (balance of covalent to VdW) Classes: Linear - Branched - X-Linked - Network Affected by molecular weight and crystallinity Copolymers extend control of these properties Key classes:  Thermosets (epoxies, polyesters)  Thermoplastics (PE, PS, PP) Mechanical properties generally quite poor and temperature dependant Very easy material to process, hence low cost