TRIBLOCK COPOLYMERS IN AQUEOUS-SOLUTION STUDIED BY STATIC AND DYNAMIC LIGHT-SCATTERING AND OSCILLATORY SHEAR MEASUREMENTS - INFLUENCE OF RELATIVE BLOCK SIZES

This paper describes a comparison of low molecular weight, nonionic, triblock copolymers in aqueous solution. The samples were chosen to have a common block length (39 propylene oxide units) and with varying endblock lengths (6, 67, and 96 ethyleneoxide units, respectively) analogous to the previously studied copolymer with the same central block and 27 ethyleneoxide units. Relaxation time distributions obtained by Laplace inversion of the dynamic light scattering (DLS) correlation functions demonstrate complex states of aggregation in solution. Monomer, micelles, and larger aggregates coexist in proportions that depend sensitively on temperature and concentration. The monomers have hydrodynamic radii in the size range 15-30 angstrom, micelles 80-130 angstrom (in sequence of increasing ethylene oxide block length); the clusters are 800 angstrom and larger. At higher temperatures and concentrations the micelles close-pick to form a glass-clear gel and at even higher temperatures the gel ''dissolves'' again. The poly(ethylene oxide) chains in the micellar mantle interpenetrate extensively in the gel, yielding a dynamic correlation length of magnitude 8 angstrom at the highest concentration used (about 0.35 g/mL). Oscillatory shear measurements show that the gelation onset temperature and the thermal stability range of the gel increase with increasing poly(ethylene oxide) (PEO) length.