Sticking Non-stick Surfaces Together

Polymers made up of non-polar, or only very slightly polar, functional groups are said to have low surface energy and poor adsorption which means that the surfaces are not "sticky".
Teflon (polytetrafluoroethylene or PTFE) is an example of a polymer with a very low surface energy, so low that it is used to provide non-stick coatings to things like pots and pans.
Silicones (polysiloxanes), with the general formula [R₂SiO]_n in which R is an organic group such as a methyl or ethyl group, also tend to have low surface energies. Because most materials do not adhere to, or stick to, silicones, silicones have become widely used to make flexible "rubber" molds.

So, how do you join together materials like these that are not "sticky"?

This is the question that scientists at Kiel University in Germany have been studying, and the solution they have devised is to use nano-scaled crystal linkers as internal staples. These staples are made of zinc oxide in which the crystals are shaped like tetrapods, that is, each staple has 4 legs. Zinc oxide crystals are sprinkled evenly onto a heated layer of teflon. Then a layer of silicone is poured on top. The material is then heated to 100^oC for less than an hour in order to join the materials firmly together.When the zinc oxide crystals are heated, the tetrapods pierce the teflon and silicone materials, sink into them and get anchored.
Peeling the teflon layer off the silicone layer held together by the tetrapod staples is about the same as peeling sticky tape off glass.

Reference:
X. Jin, J. Strueben, L. Heepe, A. Kovalev, Y.K. Mishra, R. Adelung, S.N. Gorb, A. Staubitz. Joining the un-joinable: Adhesion between low surface energy polymers using tetrapodal ZnO linkers. Advances Materials, 2012 DOI: 10.1002/adma201201780

Further Reading
Polymers and Polymerization
Functional Groups
Molecule Polarity

Suggested Study Questions:

1. Define the term polymer
2. Give two examples of polymers that are commonly used in households.
3. Define the term functional group and give three exaples.
4. Explain what is meant by a polar functional group and a non-polar functional group.
5. Give the structural formula for the monomer that can be used to form teflon.
6. Are the bonds in the monomer you have drawn in question 5 polar or non-polar bonds. Explain your answer.
7. Is the molecule that is the monomer in question 5 polar or non-polar. Explain your answer.
8. Given the general formula for silicones provided in the article, write the formula for:
   • polydimethylsiloxane
   • polydiethylsiloxane
9. Give a possible structural formula for the monomer used to produce each of the silicone polymers in question 8.

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.
   

Titanium Dioxide To Treat Water

Scientists have discovered that adding silicone to titanium dioxide increases its ability to degrade aerosol and water-borne viruses.

Titanium dioxide (titanium IV oxide) is a naturally occurring compound that is used as a pigment in white paint, in sunscreens, in food colouring (E171), and as a disinfectant, used to kill viruses and bacteria and decompose organics via photocatalysis.

Silicone sealant is used to join glass plates in aquaria, silicone grease and lubricants are widely used in the automotive industry, silicone is used in cookware, liquid silicone is used as a dry cleaning solvent, silicone in gel form is used in medicine, and silicones are found as ingredients in shampoos, hair conditioners and gels.

Silicones are polymers made up of silicon as well as carbon, hydrogen, oxygen and sometimes other elements. They have the general chemical formula [R₂SiO]_n, where R is an organic group. Silicone polymers have an inorganic silicon-oxygen backbone: -Si-O-Si-O-Si-O-Si- with organic side chains attached to the silicon atoms.

Silicones can be produced from the reaction between chlorosilanes and water:
nSi(CH₃)₂Cl₂ + nH₂O →[Si(CH₃)₂O]_n + 2nHCl

In places where they don't have water treatment plants, water is disinfected using the SODIS method in which water is exposed to sunlight for its heat and ultraviolet radiation, but it takes a very long time to make water safe to drink. Treating titanium dioxide with silicone before adding it to the water sample to be disinfected can drastically reduce the time taken to disinfect the water.

Reference
Huma R. Jafry, Michael V. Liga, Qilin Li, Andrew R. Barron. Simple Route to Enhanced Photocatalytic Activity of P25 Titanium Dioxide Nanoparticles by Silica Addition. Environmental Science & Technology, 2010; 101231104927031 DOI: 10.1021/es102749e

Further Reading
Pure Substances and Mixtures
Elements and Compounds
Percentage Composition
Yield
Gravimetric Analysis
Water Analysis

Study Questions

1. What is the formula for titanium dioxide?
2. Calculate the percentage of titanium in titanium dioxide.
3. Calculate the percentage of oxygen in titanium dioxide.
4. A 100g sample of crude titanium oxide ore was known to contain 70% titanium dioxide. Calculate the mass of titanium present in this ore sample.
5. One method for obtaining pure titanium dioxide is to reduce the crude ore with carbon, then oxidize it with chlorine to produce titanium tetrachloride. The titanium tetrachloride is then distilled, then oxidized in a pure oxygen flame or plasma to produce pure titanium dioxide and regenerate the chlorine. Draw a flow chart to represent this purification process.
   
6. What property of titanium dioxide makes it useful as a pigment in white paint?
7. What is the formula for chlorosilane?
8. Given the general silicone formula [R₂SiO]_{n, calculate the percentage of carbon present in each of the following silicones:}
   
• R = methyl groups
  
• R = ethyl groups
• R = propyl groups
• R = butyl groups
9. Draw a structure for a silicone in which R are methyl groups.