Helical and Coil Conformations of Poly(ethylene glycol) in Isobutyric Acid and Water

We show that when poly(ethylene glycol) (PEG) is dissolved in isobutyric acid at temperatures below about 55 °C, the polymer molecule can form helices. Small-angle neutron scattering indicates that in pure isobutyric acid and in isobutyric acid-rich aqueous solutions the polymer chains form stiff rods that coexist with polymer coils when the polymer molecular weight is 2.38 × 104, 2.13 × 105, and 2.87 × 105 g/mol, but that at the lower molecular weight of 1.73 × 102, only the polymer rods form. The addition of chiral dopants causes a net optical rotation in the solution, indicating that the rods are actually helices. Above about 60 °C in deuterated isobutyric acid (and above about 70 °C in hydrogenated isobutyric acid), the helices convert to coils. In water, the PEG molecules form coils which persist over the entire temperature range studied (25−60 °C).