Small-Angle X-Ray Diffraction Study of Thermal Transition in Styrene-Butadiene Block Copolymers.
CINCINNATI UNIV OH DEPT OF MATERIALS SCIENCE AND METALLURGICAL ENGINEERING
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The small-angle X-ray scattering SAXS technique is used to study the transition in styrene-butadiene block copolymers from a microdomain structure to a disordered homogeneous phase. The transition, occurring on raising the temperature, is reproducible and thermally reversible. The SAXS intensity pattern, exhibiting a main interdomain peak and a few secondary peaks at room temperature, shows a gradual decrease in intensity over a temperature range of more than 100 deg, but the position and the overall shape of the main peak remain unchanged. Analysis of the SAXS data shows that, with increasing temperature, the invariant and the length of inhomogeneity decrease, but the domain boundary thickness remains approximately constant. It is concluded that the disappearance of the microdomains proceeds by gradual intermixing of components into the opposite phases, with accompanying changes in the size of the microdomains, much in the same manner as would occur with coexisting phases of a binary liquid system. Even when the microdomains have finally disappeared and the mixture has become thermodynamically homogeneous, the SAXS pattern of diminished intensity persists, reflecting the presence of dynamic density inhomogeneity, as suggested in the theories be de Gennes and by Leibler. The similarity of our SAXS results with those obtainable with a supercooled mixture undergoing a spinodal decomposition is pointed out and its thermodynamic basis discussed.
- Physical Chemistry