Nucleation and growth in order-to-order transitions of a block copolymer

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.