Computer simulations of asymmetric block copolymers

We simulate dense diblock copolymer melts using the lattice bond-fluctuation method. Letting the lengths N-A and N-B of the A- and B-subchains vary (with N-A+N-B=N) we study the dependence of the static and dynamic properties on f = N-A/N. Changes in the A-B interaction parameter allow to mimic large temperature variations. Thus at low T we find, depending on f, lamellar, hexagonal or micellar structures, as evident from the appearance of Bragg-reflexes in the collective structure factor S(q); for high temperatures S(q) is well approximated by a generalized Leibler form. The single chain statics reveals non-mean-field behavior even well above the order-disorder transition (ODT). Near the ODT the copolymer chains are, as a whole, stretched whereas the blocks contract slightly; the maximal contraction occurs near the spinodal T-sp. We evaluate the mean repulsive energy felt by the monomers and its dependence on the monomer's position along the chain. From the variance of the repulsive energy we calculate c(upsilon), the specific heat per chain; c(upsilon) is continuous both near T-sp and near the ODT. Surprisingly, c(upsilon) scales with epsilon(2)Nf(1-f), where epsilon is the microscopic energy parameter of the simulations. As dynamical features we compute D, the diffusion coefficient of single chains and the rotational relaxation times tau of the end-to-end vector: D scales with epsilon f(1-f), whereas the tau-times show complex f-dependencies, facts which stress that the diffusional motion and the rotational relaxation behave differently. (C) 1997 American Institute of Physics.