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, 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(S) were determined as 202 +/- 2 and 183 +/- 3 degrees C, respectively. From the transitions at low heating and cooling rates, T-OOT was determined as 196 +/- 1 degrees C. The metastability window of the S phase, \T-s(S) - T-OOT\, was wider than that of the C phase, \T-s(S) - 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 degrees C below T-OOT The elastic shear modulus along the cylinder orientation, G(parallel to)', starts tb increase with temperature at about 176 degrees C. This region of dG(parallel to)'/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 degrees C displayed many grains of undulating cylinders and supported the correlation between the anisotropic fluctuations and the rheological signature of dG(parallel to)'/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 degrees C, where dG(parallel to)'/dT > 0.