Silicones: From Earth’s silicon to versatile polymer tools

Overview

In this episode, Tim Harrison explains how silicones differ from elemental silicon and carbon-based polymers, highlighting the unique silicon–oxygen backbone of silicone polymers and how their properties are tuned by chain length, branching, and cross linking. The discussion covers common applications in consumer goods, cosmetics, and medicine, as well as the smell associated with curing in certain sealants and the role of dimethicone in hair care.

• Silicone polymers feature alternating silicon and oxygen in their backbones with organic groups such as methyls attached to silicon.
• Viscosity and elasticity arise from polymer chain length and cross linking, producing oils, elastomers, and viscoelastic materials.
• Applications span shampoos, sealants, dry cleaning alternatives, contact lenses, breast implants, and other medical uses.
• Future directions include advanced nanostructures like POSS cages and injectable silicone seals for fetal surgery, using UV curing for temporary implants.

Silicones and Silicon: A quick orientation

The podcast begins by clarifying the difference between elemental silicon and silicones. Silicon is the second most abundant element in the earth, primarily found bonded to oxygen in silica and silicates. Silicones, by contrast, are synthetic polymers that contain silicon atoms in their backbone, linked to oxygen atoms in a repeating Si–O chain with organic groups such as methyl attached to silicon. The polymer chain in silicones is thus silicon–oxygen based, a contrast to the carbon-based backbones of polythene, PVC, and polystyrene.

Polymer structure and what controls silicone properties

In silicones the repeating unit features two organic groups bound to each tetravalent silicon atom. The properties of silicone polymers depend on three main factors: the length of the polymer chains, the nature of the groups attached to the chain, and cross linking between chains. Short silicone chains produce low viscosity silicone oils, whereas long chains yield tougher elastomers. Elastomers are rubber-like materials with intermediate chain lengths. The general category of silicones is described as viscoelastic, meaning they exhibit both viscous flow and elastic properties. The material can be rolled into a ball and still flow over time to take the shape of its container, illustrating this viscoelastic behavior.

Applications and processing

Silicones are found in many familiar products, spanning personal grooming products such as shampoos (dimethicone is a common name), bathroom sealants, waterproofing agents, and environmentally friendly dry cleaning solvents. The talk mentions that in some bathroom sealants ethanoate groups can release ethanoic acid fumes during polymerization, which gives the characteristic vinegar smell as curing occurs. The discussion also touches on silicone-based dry cleaning agents as alternatives to traditional solvents such as tetrachloroethene.

Medical uses and safety

Silicones have a broad array of medical applications due to their chemical inertness and low toxicity. They are used in breathable contact lenses, replacement corneas, medical tubing, and in some breast implants. Silicone implants consist of elastomer shells filled with viscous silicone gel, while saline implants use an elastomer shell with saline solution. The body’s tolerance to silicone-based materials is aided by their inertness relative to most biological systems.

Future directions and Bristol research

The Bristol School of Chemistry has explored advanced silicone materials and nanostructures. One area is polyhedral oligomeric silsesquoxane cages, or pos cages, nanocubes where each edge is the length of a silicon–oxygen bond. These cages are terminated with variable alkyl side groups, enabling designed nanostructures with potential applications in nanotechnology. Another line of research describes a liquid silicone that could be injected into a fetus to form a temporary seal over exposed spinal tissue in spina bifida cases. This seal would be cured with ultraviolet light, after which birth would allow the baby to continue with surgery. The potential advantage is to improve survival by deferring major surgery until the baby is larger while preserving useful properties for the temporary seal.

Closing thoughts

The discussion closes with expectations that silicone polymer chemistry will expand commercially in personal care products and wound-healing contexts, as well as continued exploration of innovative materials like pos cages for designed nanostructures.