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) morpholog ies in the vicinity of the C and S equilibrium have been examined for a pol y(styrene-b-isoprene-b-styrene) triblock copolymer by rheology, small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM). The s pinodal temperatures of the C and S phases, T-s(C) and T-s(S), and the equi librium (coexistence) temperature, TOOT, have been determined by rheologica l measurements during thermal cycles over a wide range of heating and cooli ng 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 t he transitions at low heating and cooling rates, T-OOT was determined as 19 6 +/- 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 t ransition, a rheological signature of the fluctuations in the C phase was o bserved over a temperature interval 20 degrees C below T-OOT The elastic sh ear modulus along the cylinder orientation, G(parallel to)', starts tb incr ease 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 a nnealed at 183 degrees C displayed many grains of undulating cylinders and supported the correlation between the anisotropic fluctuations and the rheo logical signature of dG(parallel to)'/dT > 0. The symmetry of the anisotrop ic fluctuations in the C phase was characterized as twinned body-centered-c ubic by SAXS after annealing at 189 degrees C, where dG(parallel to)'/dT > 0.