title: Conformational change of poly(ethylene glycol) near the critical point of isobutyric acid + water.
abstract: In solutions of isobutyric acid + water, poly(ethylene glycol) (PEG) can assume a helical conformation. [Alessi, M. L.; Norman, A. I.; Knowlton, S. E.; Ho, D. L.; Greer, S. C. Macromolecules 2005, 35, 9333-9340.] Here we report new measurements of the kinematic viscosity, nu, as a function of temperature for a solution of isobutyric acid + water at the critical composition, to which PEG (number average molecular weight = 1.01 x10(3) g/mol) was added at a concentration of 6.01 mg/mL. The data show that nu decreases near the critical point, with a maximum in nu at about 0.05 degrees C above the critical temperature, Tc. We interpret this change in nu in terms of a change in conformation of the polymer from helix to coil. This interpretation is supported by polarimetry measurements on the same mixture doped with (S)-(+)-1,2-propanediol, which indicates the loss of helicity near Tc and also a second helix-to-coil transition at about 60 degrees C.
authors.affiliation: Department of Chemical and Biomolecular Engineering, The University of Maryland College Park, College Park, Maryland 20742, USA.
authors.forename: Patricia
authors.initials: P
authors.lastname: Castellanos
authors.forename: Alexander I
authors.initials: AI
authors.lastname: Norman
authors.forename: Sandra C
authors.initials: SC
authors.lastname: Greer