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

Copolymers B(20)E(430), B(20)E(510), and B(20)E(610) (B = oxybutylene repeat unit, E = oxyethylene repeat unit, subscripts indicate chain length in repeat units) were synthesized and characterized by gel permeation chromatography (for distribution width) and (13)C NMR spectroscopy (for absolute molar mass and composition). Dynamic and static light scattering were used to determine micellar properties in dilute aqueous solution: e.g., micelle association numbers and radii. A tube-inversion method was used to define the mobile-immobile (hard gel) phase boundary. For copolymer B(20)E(610), immobile gels form at concentrations as low as 2.9 wt %. Rheological measurements of dynamic modulus and yield stress served to characterize the gel properties and to confirm the phase boundaries. The results are combined with those from previous work on related block copolymers to obtain scaling relationships for the dependence of micellar and gel properties on E-block length, thus enabling prediction of the requirements for and properties of very dilute aqueous gels. The validity of treating the micelles as hard spheres is discussed.