Micelles and gels of oxyethylene–oxybutylene diblock copolymers in aqueous solution: The effect of oxyethylene-block length

Abstract

Block copolymer E₉₀B₁₀ (E=oxyethylene, B=oxybutylene) was synthesised and characterised by gel permeation chromatography and ¹³C NMR spectroscopy. Dynamic light scattering (DLS) and static light scattering (SLS) were used to characterise the micelles in solution (both in water and in aqueous 0.2 mol dm^-3 K₂SO₄), yielding the micellar association numbers, the hydrodynamic and thermodynamic radii, and related expansion factors. Micellar parameters were also obtained by small-angle neutron scattering (SANS) for solutions of a similar copolymer, E₈₆B₁₀, in water, i.e., the association number, the hard-sphere radius, the micelle volume fraction and the corresponding expansion factors. A comparison of the appropriate quantities showed good agreement between the two techniques. SANS gave additional information e.g., volume fraction profiles for the micelles and volume fraction of water in the micelle core. Moderately concentrated solutions of copolymer E₉₀B₁₀ were studied in the gel state by small-angle X-ray scattering (SAXS) in tandem with rheology (oscillatory shear). Values for the dynamic elastic modulus (G′) of the gels significantly exceeded 10⁴ Pa across the range of temperature (25–80°C) and frequency (0.1–100 rad s^-1) explored. The SAXS patterns for the orientated gels showed them to have a body-centred-cubic structure, as expected for packed, spherical micelles. This structure persisted over a wide concentration range, e.g., until crystallisation of the E blocks occurred at high concentration (70 wt.% copolymer). By combining the present and published results, a comparison was made of the micelle and gel properties of copolymers with the same B-block length but different E-block lengths, i.e., E₉₀B₁₀, E₄₀B₁₀ and E₁₈B₁₀. As would be expected, as the E-block length was increased, the micelle association numbers decreased while the micelle radii and expansion factors increased. The critical gel concentration (cgc) also decreased as the E-block length was increased. Moreover, the cgc correlated quantitatively with the thermodynamic expansion factors obtained by SLS and SANS from the exclusion properties of the micelles.