Fluctuation-Induced First-Order Transition of an Isotropic System to a Periodic State

Small-angle neutron-scattering data, obtained from a near-symmetric isotropic diblock copolymer, demonstrate non-mean-field behavior over a wide temperature range (57^) above the weakly first-order microphase-separation transition. These results are quantitatively explained by a fluctuation theory based on the Hartree approximation employed by Brazovskii to describe such phase transitions.