Thermodynamic Stability and Anisotropic Fluctuations in the Cylinder-to-Sphere Transition of a Block Copolymer

Abstract

The stability and fluctuations of the cylinder (C) and sphere (S) morphologies in the vicinity of the C and S equilibrium have been examined for a poly(styrene-b-isoprene-b-styrene) triblock copolymer by rheology, small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM). The spinodal temperatures of the C and S phases, and , and the equilibrium (coexistence) temperature, TOOT, have been determined by rheological measurements during thermal cycles over a wide range of heating and cooling rates. In the limit of high heating and cooling rates, and were determined as 202 ± 2 and 183 ± 3 °C, respectively. From the transitions at low heating and cooling rates, TOOT was determined as 196 ± 1 °C. The metastability window of the S phase, | S − TOOT|, was wider than that of the C phase, | − TOOT|, due to the osmotic barrier for merging spheres for the S → C transition. Prior to the C → S transition, a rheological signature of the fluctuations in the C phase was observed over a temperature int...