A poly(isoprene-b-ethyleneoxide) diblock undergoes multiple ordered state t
ransitions: from a crystalline lamellar (L-c), to a hexagonal (Hex) mesopha
se, to a bicontinuous cubic phase (Gyroid) before disordering (Dis). We hav
e studied the kinetics of the Hex-to-Gyroid, Hex-to-L-c and L-c-to-Gyroid t
ransitions using synchrotron SAXS and rheology. The Hex-to-Gyroid transform
ation proceeds via a nucleation and growth mechanism with a small mismatch
between the two phases, as anticipated by recent theoretical predictions (M
.W. Matsen, Phys. Rev. Lett., 80 (1998) 4470). We provide the first quantit
ative measure of the activation barrier involved in an order-to-order trans
ition and show the importance of nucleation and growth in these transformat
ions.