Field-theoretic Monte Carlo simulation of the suppression of the order-disorder temperature in a fluctuating asymmetric diblock copolymer melt

We use Monte Carlo simulation to sample density field configurations weighted by a Landau-Brazovskii effective Hamiltonian and systematically investigate the fluctuation-induced suppression of the order-disorder temperature (ODT) in an asymmetric diblock copolymer melt as we decrease the invariant degree of polymerization from 10⁶ to 10³. Derivatives of the thermodynamic free energy correspond to Monte Carlo averages, and we use thermodynamic integration to compute the free energy itself. We compare the ordered BCC sphere phase free energy to that of the disordered phase to locate the ODT. Turning on a BCC-ordering field enables us to bypass the discontinuity at the ODT and to integrate continuously from order to disorder. We find that the BCC free energy is relatively insensitive to fluctuations. However, in the low-temperature regime of disorder, fluctuations stabilize a disordered liquid of micelles, which we characterize. To accurately locate the ODT, we take care to ensure that the disordered micelles are equilibrated.