The phase behavior of gels of E40B10 in
0.2 mol dm-3 aqueous
K2SO4 was studied as a
function
of temperature and concentration. E40B10
is a diblock copolymer of poly(oxyethylene) (E) and
poly(oxybutylene) (B), where the subscripts denote the number of repeats.
The phase of the material was
characterized by both simultaneous rheology and small-angle X-ray
scattering, (SAXS). Depending on
polymer volume fraction in the range 23−38% a body-centered cubic
(bcc) structure or a face-centered
cubic (fcc) structure was observed at low temperature, and at high
temperature a hexagonally packed
rod structure was formed. The phase transitions were shown to be
characterized by discontinuous changes
in the values of the dynamic shear moduli. A bcc−fcc transition
was observed at high concentration, the
corresponding transition temperature increasing with increasing polymer
concentration. The effects of
reciprocating shear were shown to increase the degree of order,
manifested as a sharpening of the
diffraction peaks in the SAXS pattern. The dynamic moduli
decreased rapidly on the application of
oscillatory shear and recovered equally rapidly when the deformation
ceased. The decrease in moduli
was shown, via the SAXS patterns acquired simultaneously, to be
correlated to structural changes within
the gel.
The simultaneous time-resolved study of structure development and reaction kinetics during polymer processing is an experimental method that has great potential in developing a deeper understanding of the parameters that govern the formation of structure and therefore polymer properties. A combination of synchrotron radiation small-angle x-ray scattering and Fourier-transform infrared spectroscopy experiments have been performed on a series of model segmented block copolyurethanes. These studies confirm that the driving force for structure development in polyurethanes is the thermodynamics of phase separation rather than hydrogen bonding.
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