Rheology and structures of aqueous gels of diblock(oxyethylene–oxybutylene) copolymers with lengthy oxyethylene blocks

Abstract

Aqueous solutions of diblock copolymers E₉₆B₁₈, E₁₈₄B₁₈, E₃₁₅B₁₇ and E₃₉₈B₁₉ (E=oxyethylene unit, B=oxybutylene unit) were investigated by rheometry and small-angle X-ray scattering (SAXS). Storage (G′) and loss (G″) modulus and yield strength (σ_y) were used to define hard- and soft-gel phases in experiments which covered the concentration range 2–14 wt.% copolymer. Values of G′ correlated with those of yield strength, the ratio G′/σ_y being ca. 0.03. SAXS was used to explore hard-gel structures, and to confirm hard-gel/soft-gel boundaries. The sol/soft-gel boundary was identified as a percolation threshold. The effect of an increase in E-block length was to move the gel phases to lower concentrations without changing the pattern of their behaviour. In this respect, the critical conditions for hard-gel formation (c*, T*) served as parameters for a ‘universal ’ phase diagram.