Aqueous solutions and gels of diblock copolymers of 1,2-butylene oxide andethylene oxide studied by light scattering and rheology

Copolymers B20E430, B20E510, and B20E610 (B = oxybutylene repeat unit, E = oxyethylene repeat unit, subscripts indicate chain length in repeat units) were synthesized and characterized by gel permeation chromatography (for di stribution width) and C-13 NMR spectroscopy (for absolute molar mass and co mposition). Dynamic and static light scattering were used to determine mice llar properties in dilute aqueous solution: e.g., micelle association numbe rs and radii. A tube-inversion method was used to define the mobile-immobil e (hard gel) phase boundary. For copolymer B20E610, immobile gels form at c oncentrations as low as 2.9 wt %. Rheological measurements of dynamic modul us and yield stress served to characterize the gel properties and to confir m the phase boundaries. The results are combined with those from previous w ork on related block copolymers to obtain scaling relationships for the dep endence of micellar and gel properties on E-block length, thus enabling pre diction of the requirements for and properties of very dilute aqueous gels. The validity of treating the micelles as hard spheres is discussed.