Polymeric materials are simple organic molecules consisting of many individual parts (monomers). A monomer is anything which can chemically react with itself to form a long, unbroken chain of molecules. Some organic polymers (also called plastics) can be thousands, even millions of monomer units long.
Polymers are interesting because chemists can finely-tune their structures through organic synthesis; structure determines function, and therefore the chemical and physical properties of the material rely entirely on the skill of the chemist to correctly design the monomer that possesses the best qualities. One hot area of research in recent years has been the design of polymers which react to their environment in some way, acting as a sensor or at least, reacting to a stimulus such as temperature change.
A recent success from the Massachusetts Institute of Technology (published in the very prestigious science journal Nature: Materials) outlined the details of a recent success in this area. A team of chemists designed two types of monomers for the polymerization process: one monomer was hydrophobic (meaning that it “dislikes” water, and repels water – polystyrene is a good example), and the other monomer was hydrophilic (meaning that it “likes” water, and mixes well with it). Both monomers were then turned into plastics via polymerization chemical reactions – similar molecules reacted with themselves, and turned into long repeating chains. The key to the success of this “responsive” polymer is that the polymer gel the chemists made was fabricated using successive alternating layers of these hydrophobic / hydrophilic polymers. The hydrophilic layers – a polymer based on pyridine, a nitrogen-containing molecule – carried positive charges. As a result, upon exposure to water, this hydrophilic polymer swells. In this case, it swells to more than ten times its original size.
This response is a result of the water molecules (which have pairs of electrons on them) desperately swarming around and surrounding the positive charges, trying to stabilize them. A secondary effect is that this solvation gives the polymer chains some freedom of movement, and all of those positive charges attempt to repel each other (like charges repel, opposite charges attract) and so the net result is expansion. Chemists find that most charges don’t like to exist “in the open”, and having these positive charges (hundreds of them – up and down the polymer chain) stabilized by interactions with the lone pairs of electrons on the water molecules and by chain-chain repulsion (increasing the unfavorable close distance between the positive charges) brings the entire polymer structure down to a more stable arrangement, something that chemists would call a lower energy state.
The trick with this polymeric material (which the MIT chemists demonstrated) is that if you take this swollen polymeric gel and then sprinkle salt on it, the positively charged sodium ions and the negatively charged chlorine atoms (from sodium chloride, the table salt that was added) have to themselves be stabilized. The water molecules that were surrounding and swarming around the positive charges of the hydrophilic polymer backbone have to then rush to stabilize the salt cations and anions, which means that the gel “collapses” back to its original size.
Polymeric materials whose properties such as size can be tuned by something as simple as the addition of water and salt demonstrate a remarkable understanding of organic chemistry, and since the hydrophilic polymer in question interacts with light in other, interesting ways, this simple method demonstrated by the MIT researchers offers a way of easily controlling functional materials. The applications for this method will come later; right now, chemists are still discovering and developing ways that – on a fundamental, molecular level – we can use very simple stimuli to effect the physical and chemical properties of an organic material.
Youngjong Kang, Joseph J. Walish, Taras Gorishnyy, and Edwin L. Thomas.
“Broad-wavelength-range chemically tunable block-copolymer photonic gels”.
Nature Publications Archive.