Coil-to-Helix Transition of Poly(ethylene oxide) in Solution

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Poly(ethylene oxide) (PEO) is a simple polymer with repeating units [- C - C - O -] soluble in organic and aqueous solvents. The carbon atoms are hydrophobic; the oxygen atoms are hydrophilic and participate in hydrogen bonding. In all solvents in which PEO has previously been studied, PEO forms a coil in solution. Neutron scattering studies of PEO in isobutyric acid show that PEO undergoes a coil-to-rod transition in a solution of isobutyric acid (IBA). The stiffening is seen to progress smoothly with the addition of IBA, from a coil in D2O to a rod in pure deuterated-IBA. In addition to a solvent driven transition, a reversible rod-to-coil transition was seen to occur as a function of temperature, between 55 and 60 oC. Polarimetry experiments show that the rod formed by the PEO in solution is actually a helix, the conformation that PEO has in the solid state. It is also shown that, through the use of chiral impurities and temperature, the direction of the helix can be affected, allowing polymer folding to be influenced on a molecular level.