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 degrees 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 x 10(4), 2.13 x 10(5), and 2.87 x 10(5) g/mol, but that at the lower molecular weight of 1.73 x 10(2), 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 degrees C in deuterated isobutyric acid (and above about 70 degrees 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 degrees C).