Thermodynamic stability and anisotropie 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-e-isoprene-6-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, T_s^C and T_s^S, 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, T_s^C and T_s^C 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, |T_s^C -Toorl, was wider than that of the C phase, |Tf -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 interval 20 °C below TOOT. The elastic shear modulus along the cylinder orientation, dG′_∥, starts to increase with temperature at about 176 °C. This region of dG′_∥/dT > 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 dG′_∥/dT > 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 dG′_∥/dT > 0.