Polydimethylsiloxane

One of the most widely used silicon-based organic polymers is polydimethylsiloxane or PDMS. The formula for this polymer is shown on the right. Because it is optically clear and considered to be inert, non-toxic and non-flammable, it is used in medical devices including contact lenses, and in personal care products such as shampoo in which it makes hair shiny and slippery.

Northwestern University scientists have also found that compressing polymers such as PDMS releases free radicals which can be used to power chemical reactions in water.

To demonstrate this, the scientists took a Nike Air LeBron, similar to that shown on the right, and filled the air pockets in the polymer sole of the shoe with a solution of a compound that lights up in the presence of free radicals. After a person walked in the shoe for 30 minutes or more, enough radicals were created to generate a blue glow visible to the naked eye.

The scientists demonstrated that they can squeeze a polymer, such as what might be found in a shoe, tire or plastic bag, and get a mechanical-to-chemical energy conversion of up to 30%, about the same as the energy efficiency of a car engine. You could recharge a battery from the energy produced by walking or by driving a car!
Just imagine how much energy could be released by compacting the millions of plastic bags in rubbish tips around the world!

Reference:
H. Tarik Baytekin, Bilge Baytekin, Bartosz A. Grzybowski. Mechanoradicals Created in “Polymeric Sponges” Drive Reactions in Aqueous Media. Angewandte Chemie, 2012; DOI: 10.1002/ange.201108110

Further Reading:
Polymers and Polymerization
Covalent Bonding


Suggested Study Questions:

1. Polydimethylsiloxane can be prepared from dimethylchlorosilane, Si(CH₃)₂Cl₂, and water. Draw a possible structural formula for dimethylchlorosilane.
2. When dimethylchlorosilane reacts with water to form polydimethylsiloxane, HCl is formed as a by-product of the reaction. Write a chemical equation to represent this polymerization reaction.
3. Draw a structural formula for polydimethylsiloxane showing 3 repeating monomer units.
4. Polydimethylsiloxane can also be produced using a monomer in which acetate groups, COO, replace the chlorines in dimethylchlorosilane. Draw a possible structural formula for this monomer.
5. What is the expected by-product of the reaction between the monomers in question 4 and water to produce polydimethylsiloxane?
6. The food additive E900 is mixture of polydimethylsiloxane and silicon dioxide. It is used as an antifoaming and anticaking agent in many processed foods including chicken nuggets and french fries. What properties of polydimethylsiloxane might make it suitable for use in fried food?
7. Write the chemical formula for silicon dioxide.
8. Compare the structural formula of silicon dioxide and polydimethylsiloxane. How would you expect the three dimensional structure of silicon dioxide to differ from the three dimensional structure of polydimethylsiloxane?
9. Silicon dioxide is a hard substance while polydimethylsiloxane feels slippery. Explain this difference based on your understanding of the three dimensional structure of each solid.