A NEUTRON-SCATTERING STUDY OF POLY(ETHYLENE GLYCOL) IN ELECTROLYTE-SOLUTIONS

The solution microstructure of aqueous (D2O) poly(ethylene glycol) (PE G) solutions was determined as a function of polymer concentration and molecular weight in the presence of salts such as NaNO3, Na2CO3, and Na3PO4. Measurements were made by using small-angle neutron scattering (SANS). In addition, anion effects on biphase formation were investig ated by cloud point measurements in H2O and D2O. In the presence of Na NO3, PEG behaves like a random coil with no evidence of aggregation, w hereas the presence of either Na3PO4 or Na2CO3 leads to aggregate form ation. Furthermore, the aggregate size increased with increasing salt concentration until a discontinuity appeared at the point of biphase f ormation, where the polymers in the PEG-rich phase form an entangled m esh with loss of chain identity. In the monophasic regime, the aggrega tes were elongated with a radius of approximately 19 Angstrom and a le ngth which varied with salt type and concentration. Analysis of the ef fects of electrolytes on the cloud points of PEG in H2O and D2O sugges ts that its phase behavior in solution is entropy driven. Salt effects on cloud points follow the well-known Hofmeister series. Anions leadi ng to increased structuring of water lower the cloud point, as does th e substitution of D2O for H2O. The cloud point data correlate linearly with the change in water entropy upon the addition of electrolytes.