Ordering kinetics of body-centered-cubic morphology in diblock copolymer solutions at low temperatures

The ordering kinetics of a body-centered cubic (bcc) mesophase in squalane solutions of poly(styrene-b-ethylene-alt-propylene) (SEP) at low temperatures was studied using rheological and small-angle x-ray scattering methods. At temperatures more than 110°C below the order-to-disorder transition temperature (T _ODT). the evolution of the storage modulus, G′, and the absolute value of the complex modulus, |G*|, was found to be describable by the Avrami equation, assuming a "series" mechanical model for the combination of a soft initial phase and a growing hard phase. The ordering half time, t _0.5, of the SEP/squalane solutions decreased with temperature from 50 to 120°C and increased with concentration from 5 to 10 wt%, indicating the diffusion-controlled nature of the ordering process in these micellar systems at low temperatures. The temperature dependence of t _0.5 followed the Arrhenius relationship with the activation energy showing negligible concentration dependence. Furthermore, the elastic modulus of the bcc phase was found to increase almost linearly with temperature, which is qualitatively consistent with the assumption of entropic elasticity of the corona chains in the micellar bcc lattice.