Micelles and gels of oxyethylene-oxybutylene diblock copolymers in aqueoussolution: The effect of oxyethylene-block length

Block copolymer E90B10 (E = oxyethylene, B = oxybutylene) was synthesised a nd characterised by gel permeation chromatography and C-13 NMR spectroscopy . Dynamic light scattering (DLS) and static light scattering (SLS) were use d to characterise the micelles in solution (both in water and in aqueous 0. 2 mol dm(-3) K2SO4), yielding the micellar association numbers, the hydrody namic and thermodynamic radii, and related expansion factors. Micellar para meters were also obtained by small-angle neutron scattering (SANS) for solu tions of a similar copolymer, E86B10, in water, i.e., the association numbe r, the hard-sphere radius, the micelle volume fraction and the correspondin g 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 i n the micelle core. Moderately concentrated solutions of copolymer E90B10 w ere studied in the gel state by small-angle X-ray scattering (SAXS) in tand em with rheology (oscillatory shear). Values for the dynamic elastic modulu s (G') of the gels significantly exceeded 10(4) Pa across the range of temp erature (25-80 degrees C) and frequency (0.1-100 rad s(-1)) explored. The S AXS patterns for the orientated gels showed them to have a body-centred-cub ic structure, as expected for packed, spherical micelles. This structure pe rsisted over a wide concentration range, e.g., until crystallisation of the E blocks occurred at high concentration (greater than or equal to 70 wt.% copolymer). By combining the present and published results, a comparison wa s made of the micelle and gel properties of copolymers with the same B-bloc k 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 associ ation numbers decreased while the micelle radii and expansion factors incre ased. The critical gel concentration (cgc) also decreased as the E-block le ngth was increased. Moreover, the cgc correlated quantitatively with the th ermodynamic expansion factors obtained by SLS and SANS from the exclusion p roperties of the micelles.