Partitioning, fractionation, and conformations of star poly(ethylene glycol) in isobutyric acid and water

We investigate the partitioning, fractionation, and conformations of star poly(ethylene glycol) (PEG) in coexisting liquid phases of isobutyric acid and water. Star PEG does partition: 98% in the upper isobutyric acid phase, versus 80-90% for linear PEG. There is no significant fractionation of the star PEG for molecular masses less than or equal to 10 kg/mol, but fractionation may occur at higher molecular masses. Small angle neutron scattering shows that the arms of the star PEG molecules form coils in DO but form stiff rods in deuterated isobutyric acid. At higher average molecular masses (>4 kg/mol) and higher temperatures (60 degrees C, some arms are coils and some are rods in isobutyric acid. Polarimetry studies indicate that these rodlike arms are actually helical conformations. At a star molecular mass of 2 kg/mol. the helical arms persist above 70 degrees C, but at larger molecular masses, the helical arms revert to coils at temperatures around 75 degrees C. The addition of PEG to isobutyric acid and H2O increases the critical temperature of the solvent mixture, and the increase is less as the star branching increases.