Association properties of triblock copolymers in aqueous solution: Copolymers of ethylene oxide and 1,2-butylene oxide with long E-blocks

Four triblock copolymers, EmB20Em, with m in the range 58-260, were prepared by sequential anionic polymerization of 1,2-butylene oxide followed by ethylene oxide. E denotes an oxyethylene repeat unit and B an oxybutylene repeat unit. The copolymers were characterized by gel permeation chromatography (for distribution width) and C-13 NMR spectroscopy (for absolute molar mass and confirmation of triblock architecture). Static and dynamic light scattering were used to study the micellization and micelle properties of the copolymers in dilute aqueous solution, particularly the mass-average association number and thermodynamic and hydrodynamic radii. At a given temperature, the micelle association number decreased as the E-block length was increased while the radii increased. The gelation of relatively concentrated aqueous solutions of the copolymers was also investigated by a tube inversion method. The minimum concentration for gelation decreased in the range 9-13.3 wt % as the E-block length was increased. Comparison is made with recent, results for four diblock EmBn copolymers (n = 17-19) of similar overall chain lengths. The diblock copolymers form micelles with larger association numbers and radii, but the scaling exponents for the effects of the E-block length on association number and radius are similar for the two series of copolymers. Comparison is also made with theoretical predictions of scaling exponents. Given the cubic packing of effectively spherical micelles (bcc or fcc), the critical concentrations for gelation of the micellar solutions of the diblock copolymers could be satisfactorily predicted from the thermodynamic radii of the micelles (using the related thermodynamic expansion factor), but this was not possible for the micellar solutions of the triblock copolymers, suggestive of a more complex structure.