Coexistence of ordered and disordered phases in a nearly symmetric diblock copolymer near an order-disorder transition point.

We investigated the phase behavior near the order-disorder transition (ODT) temperature in a nearly symmetric diblock copolymer, using ultra-small-angle x-ray scattering method. In a narrow temperature range very close to the ODT temperature, we observed the scattering profiles that can be interpreted as a linear combination of the scattering from the disordered state and that from the ordered lamellar state. These profiles were stable during the observation time (50 h), revealing that the two-phase coexistence occurs at thermal equilibrium within this temperature range.