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 prepare d by sequential anionic polymerization of 1,2-butylene oxide followed by et hylene oxide. E denotes an oxyethylene repeat unit and B an oxybutylene rep eat unit. The copolymers were characterized by gel permeation chromatograph y (for distribution width) and C-13 NMR spectroscopy (for absolute molar ma ss and confirmation of triblock architecture). Static and dynamic light sca ttering were used to study the micellization and micelle properties of the copolymers in dilute aqueous solution, particularly the mass-average associ ation number and thermodynamic and hydrodynamic radii. At a given temperatu re, the micelle association number decreased as the E-block length was incr eased while the radii increased. The gelation of relatively concentrated aq ueous 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 c hain 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 therm odynamic radii of the micelles (using the related thermodynamic expansion f actor), but this was not possible for the micellar solutions of the tribloc k copolymers, suggestive of a more complex structure.