L. Derici et al., Micelles and gels of oxyethylene-oxybutylene diblock copolymers in aqueoussolution: The effect of oxyethylene-block length, PCCP PHYS C, 1(11), 1999, pp. 2773-2785
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.