Simulating the Fluctuation-Induced Suppression of the Order-Disorder Transition in an Asymmetric Diblock Copolymer Melt
Mean-field theory predicts the phase behaviour of block copolymers with a great deal of success, however it systematically overestimates the order-disorder temperature (ODT) due to its inaccurate treatment of fluctuations in the disordered phase. We use Monte Carlo simulation to simulate a fluctuating melt of asymmetric diblock copolymers, and systematically investigate the suppression of the ODT as we decrease the invariant degree of polymerization, N̄, from 10⁶ to 10⁵. Using thermodynamic integration, we compare the ordered BCC sphere phase free energy to that of the disordered phase to locate the ODT. In the low-temperature regime of the disordered phase, we observe and characterize a disordered liquid of micelles that is stabilized by the inclusion of fluctuations in our simulation. We find that decreasing N̄ causes a suppression of the ODT that is in good agreement with a theory where fluctuations are incorporated on a single-mode Hartree level.