  Time-resolved small angle x-ray scattering (SAXS) was used to examine the
kinetics of the transition from HEX cylinders to BCC spheres at various
temperatures in poly(styrene-b- ethylene-co-butylene-b-styrene) (SEBS) in
mineral oil, a selective solvent for the middle EB block. Temperature-ramp SAXS
and rheology measurements show the HEX to BCC order-order transition (OOT) at
~127 oC and order-disorder transition (ODT) at ~180 oC. We also observed the
metastability limit of HEX in BCC with a spinodal temperature, Ts ~ 150 oC. The
OOT exhibits 3 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.
