Polydimethylsiloxanes

Polydimethylsiloxanes (PDMS) are a family of organosilicon polymers characterized by a siloxane backbone and two methyl substituents on each silicon atom, giving repeating units [-Si(CH3)2-O-]n. The chain length and end groups determine viscosity, elasticity, and molecular weight. PDMS is typically colorless, highly slippery, and exhibits low glass transition temperature (around -125°C), allowing it to behave as a liquid oil at room temperature or form elastic rubbers at higher molecular weights. It shows outstanding thermal stability, chemical inertness, and high gas permeability, as well as a low surface energy and good biocompatibility.

Synthesis: PDMS is generally prepared by polymerizing either chlorosilane precursors such as dimethyldichlorosilane with water to form silanols that condense into -Si-O- linkages, or by polymerizing cyclic siloxanes (e.g., hexamethylcyclotrisiloxane) through ring-opening polymerization. End groups and molecular weight are controlled by process conditions and catalysts. Crosslinked PDMS elastomers are produced by hydrosilylation of vinyl-terminated PDMS with silicon-hydrogen-containing crosslinkers.

Forms and properties: Linear PDMS oils and fluids are clear, non-volatile liquids with low surface tension; higher molecular weight variants form flexible elastomers or gums. They resist UV and most solvents, are relatively non-reactive, and often used as lubricants, release agents, or sealants. In microfluidics, PDMS is valued for transparency, gas permeability, and ease of molding.

Applications and safety: PDMS and its derivatives are used in cosmetics, medical devices, coatings, electronics potting, and sealants. They are generally considered low-toxicity and biologically inert, though cyclic siloxanes and certain processing additives raise environmental and regulatory considerations. PDMS is persistent in the environment and subject to regulatory scrutiny in some jurisdictions.