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

Block copolymers E106B16, E210B16 and E103B15E103 (E = oxyethylene, B = oxy butylene) were synthesised and characterised by gel permeation chromatograp hy (for molar mass distribution) and C-13 NMR spectroscopy (for number-aver age 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 th e micellisation was athermal. Dynamic and static light scattering were used to characterise the micelles in solution, yielding micellar association nu mber, hydrodynamic and thermodynamic radius, and the related thermodynamic expansion factor. The latter was used to correlate the dilute solution prop erties of the copolymer micelles with their critical gelation concentration s. The gels were studied by small-angle X-ray scattering (SAXS) in tandem w ith rheology (oscillatory shear). Values of the elastic dynamic modulus (G' ) of the gels significantly exceeded 10(4) Pa across the range of temperatu res (25-75 degrees C) and frequency (0.1-100 rad s(-1)) explored, allowing the get to be characterised as 'hard'. SAXS, whether from unoriented or she ar-oriented gels, showed them to comprise structures with body-centred cubi c (bcc, space group Im (3) over bar m) symmetry. In particular, the pattern s 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 fl ow was mainly in the {211} plane for E103B15E103 gels, but in the intersect ing {110}, {211} and {321} planes for the diblock copolymer gels. For one c opolymer, E106B16, micellar radii (calculated assuming sphericity) and asso ciation numbers in the gel were obtained. Compared with equilibrium (thermo dynamic) values in dilute solution, the micellar radii were similar but the micellar association number in the gel was larger.