Kinetics of Hexagonal-Body-Centered Cubic Transition in a Triblock Copolymer in a Selective Solvent: Time-Resolved Small-Angle X-ray Scattering Measurements and Model Calculations

Time-resolved small-angle X-ray scattering (SAXS) was used to examine the kinetics of the transition from hexagonal (hex) cylinders to body-centered cubic (bcc) spheres at various temperatures in poly(styrene-bethylene-co-butylene-b-styrene) (SEBS) in mineral oil, a selective solvent for the middle ethylene-co-butylene (EB) block. Temperature-ramp SAXS and rheology measurements show the hex to bcc order-order transition (OOT) at ∼127 °C and order-disorder transition (ODT) at ∼180 °C. We also observed the metastability limit of hex in bcc with a spinodal temperature, Ts ∼ 150 °C. The OOT exhibits three stages and occurs via a nucleation and growth mechanism when the final temperature Tf < Ts. Spinodal decomposition in a continuous ordering system was seen when Ts < Tf < TODT. We observed that hex cylinders transform to disordered spheres via a transient bcc state. We develop a geometrical model of coupled anisotropic fluctuations and calculate the scattering which shows very good agreement with the SAXS data. The splitting of the primary peak into two peaks when the cylinder spacing and modulation wavelength are incommensurate predicted by the model is confirmed by analysis of the SAXS data.