Effect of block architecture on the micellisation and gelation of block copolymers of ethylene oxide and 1,2-butylene oxide in aqueous solution

Abstract

Block copolymers E₁₀₆B₁₆, E₂₁₀B₁₆ and E₁₀₃B₁₅E₁₀₃ (E = oxyethylene, B = oxybutylene) were synthesised and characterised by gel permeation chromatography (for molar mass distribution) and ¹³C NMR spectroscopy (for number-average molar mass and composition). Surface tensiometry was used to determine critical micelle concentrations at several temperatures, and thereby values of the enthalpy of micellisation. In the case of the diblock copolymers the micellisation was athermal. Dynamic and static light scattering were used to characterise the micelles in solution, yielding micellar association number, hydrodynamic and thermodynamic radius, and the related thermodynamic expansion factor. The latter was used to correlate the dilute solution properties of the copolymer micelles with their critical gelation concentrations. The gels were studied by small-angle X-ray scattering (SAXS) in tandem with rheology (oscillatory shear). Values of the elastic dynamic modulus (G′) of the gels significantly exceeded 10⁴ Pa across the range of temperatures (25–75 °C) and frequency (0.1–100 rad s⁻¹) explored, allowing the gels to be characterised as ‘hard’. SAXS, whether from unoriented or shear-oriented gels, showed them to comprise structures with body-centred cubic (bcc, space group Im3¯m) symmetry. In particular, the patterns from the sheared gels were indexed to a highly twinned structure in which a [111] direction of the bcc lattice lay along the shear direction, and flow was mainly in the {211} plane for E₁₀₃B₁₅E₁₀₃ gels, but in the intersecting {110}, {211} and {321} planes for the diblock copolymer gels. For one copolymer, E₁₀₆B₁₆, micellar radii (calculated assuming sphericity) and association numbers in the gel were obtained. Compared with equilibrium (thermodynamic) values in dilute solution, the micellar radii were similar but the micellar association number in the gel was larger.