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

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, T OOT, 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, T OOT was determined as 196 ± 1 °C. The metastability window of the S phase, | S − T OOT |, was wider than that of the C phase, | − T OOT |, 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 interval 20 °C below T OOT . The elastic shear modulus along the cylinder orientation,, starts to increase with temperature at about 176 °C. This region of /d T > 0 is attributed to the pretransitional fluctuations in the C phase. TEM on the annealed samples at different temperatures provided real space images of various steps during the transition. TEM images from the sample annealed at 183 °C displayed many grains of undulating cylinders and supported the correlation between the anisotropic fluctuations and the rheological signature of /d T > 0. The symmetry of the anisotropic fluctuations in the C phase was characterized as twinned body-centered-cubic by SAXS after annealing at 189 °C, where /d T > 0.