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

A poly(isoprene-b-ethyleneoxide) diblock undergoes multiple ordered state transitions: from a crystalline lamellar (L-c), to a hexagonal (Hex) mesophase, to a bicontinuous cubic phase (Gyroid) before disordering (Dis). We have studied the kinetics of the Hex-to-Gyroid, Hex-to-L-c and L-c-to-Gyroid transitions using synchrotron SAXS and rheology. The Hex-to-Gyroid transformation 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 quantitative measure of the activation barrier involved in an order-to-order transition and show the importance of nucleation and growth in these transformations.