Designing new materials

Two ways to make polymers

A polymer is a very long molecule made by joining thousands of small molecules called monomers. In Year 8 you built simple polymer chains. There are two main ways the monomers can join.

Nylon-6,6 is made from two monomers, a diamine (with an –NH₂ group at each end) and a diacid (with a –COOH at each end). Each time an –NH₂ meets a –COOH they join in an amide link and release a water molecule. The same amide link holds amino acids together in proteins, which is why nylon was first developed as an artificial silk.

Structure decides properties

Polymers made from the same monomer can behave very differently depending on how their chains are arranged:

• Linear chains can pack closely together, so the forces between them are stronger. High-density polyethene (HDPE), used for milk bottles, is linear: stiff, strong and fairly opaque.
• Branched chains can't pack closely. Low-density polyethene (LDPE), used for cling wrap and squeeze bottles, is branched: softer, more flexible and less dense, and it melts at a lower temperature.
• Lightly cross-linked chains are joined by a few strong covalent bonds. Stretch them and they uncoil, let go and the cross-links pull them back. These are elastomers, such as vulcanised rubber, where sulfur atoms form the cross-links.
• Heavily cross-linked chains form a rigid three-dimensional network. They don't soften when heated, they char. These thermosets are used for kitchen benchtops, electrical fittings and the glue in composites.

Polymers whose chains are not joined by cross-links soften on heating and can be remoulded: they are thermoplastics. Only thermoplastics can be melted down and recycled.

Synthetic fibres

Most synthetic fibres are made by melt spinning. Polymer chips are melted and forced through a spinneret, a metal plate with hundreds of tiny holes, like a shower head. The filaments cool and harden in a stream of air. They are then drawn: stretched to several times their length between rollers turning at different speeds. Drawing lines the chains up along the fibre so more of them can attract each other, making the fibre much stronger.

• Nylon (1935, Wallace Carothers at DuPont) was the first fully synthetic fibre. It is strong, elastic and resists wear: stockings, ropes, carpet, fishing line.
• Polyester (PET) doesn't stretch or crease and dries quickly. The same polymer as drink bottles, it is used for clothing, tents, sails and polar fleece. Much fleece is now made from recycled bottles.
• Kevlar has rigid, flat chains that line up like uncooked spaghetti in a packet and hydrogen-bond to each other. It is so strong and heat-resistant that it is used in bulletproof vests and firefighters' clothing.
• Elastane (Lycra, Spandex) has stretchy and rigid sections in its chains, so it can stretch to five times its length and spring back.

Synthetic fabrics shed tiny fibres when they are washed, and these microfibres are now one of the most common forms of microplastic in rivers and the ocean. Washing full loads in cold water, using a filter on the washing machine outlet and buying fewer, longer-lasting clothes all help.

Composite materials

A composite combines two or more materials to get properties neither has alone. Most have strong fibres or particles held in a matrix that glues them together and spreads the load:

• Fibreglass: glass fibres in polyester or epoxy resin; boats, surfboards, wind turbine blades, swimming pools.
• Carbon-fibre reinforced polymer: very stiff and light but expensive; racing bikes, tennis racquets, aircraft wings, Formula 1 cars.
• Reinforced concrete: concrete is strong when squeezed but cracks when stretched; steel bars inside take the tension.
• Natural composites: wood (cellulose fibres in lignin) and bone (collagen fibres with calcium phosphate crystals).

Fibres are strongest along their length, so designers lay layers at different angles, such as 0°, 90° and ±45°, when a part has to resist forces from several directions. The downside of many composites is that the fibres and matrix are hard to separate, so they are difficult to recycle. Thousands of wind turbine blades reaching the end of their life are a growing waste problem, and chemists are developing resins that can be dissolved or melted to recover the fibres.

Spinifex resin: an ancient material, a new technology

Across the arid inland, Aboriginal peoples have long made a powerful glue from spinifex (Triodia) grass. Resin is collected by threshing the grass and heated over a fire until it can be worked like putty. It is then pressed around the join between a stone blade and a wooden handle, or used to seal coolamons and repair tools. Warmed again, it softens and can be reshaped: it behaves like a natural thermoplastic. Resin was sometimes mixed with fillers such as charcoal, ochre or sand, which makes it tougher, a natural composite. Grass tree (Xanthorrhoea) resin was used in a similar way in other regions.

Spinifex is now the focus of new materials research. Scientists at the University of Queensland, working in partnership with the Indjalandji-Dhidhanu people of the Camooweal region through the Dugalunji Aboriginal Corporation, found that spinifex fibres can be broken down into extremely fine nanocellulose that strengthens latex rubber, and researchers are studying spinifex resin for modern adhesives and coatings. The partnership was set up so that Traditional Owners share in decisions and in any benefits, an approach that is increasingly expected when research uses First Nations knowledge and plants from their Country.

Nanomaterials

Nanomaterials have structures between 1 and 100 nanometres across (a nanometre is a millionth of a millimetre). At this size, a material's huge surface area and the behaviour of its electrons give it new properties.

• Graphene is a single layer of carbon atoms, first isolated in 2004 using sticky tape. It is extremely strong, flexible and conducts electricity well.
• Zinc oxide and titanium dioxide nanoparticles in sunscreens block UV but look clear on the skin, because the particles are too small to scatter visible light.
• Nanoparticle catalysts give a huge surface area for very little expensive metal.

Because nanoparticles are so small, scientists study whether they can enter cells or build up in the environment before they are widely used.