COMPUTER-SIMULATIONS OF ASYMMETRIC BLOCK-COPOLYMERS

We simulate dense diblock copolymer melts using the lattice bond-fluct uation method. Letting the lengths N-A and N-B of the A- and B-subchai ns vary (with N-A+N-B=N) we study the dependence of the static and dyn amic 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, d epending on f, lamellar, hexagonal or micellar structures, as evident from the appearance of Bragg-reflexes in the collective structure fact or S(q); for high temperatures S(q) is well approximated by a generali zed Leibler form. The single chain statics reveals non-mean-field beha vior even well above the order-disorder transition (ODT). Near the ODT the copolymer chains are, as a whole, stretched whereas the blocks co ntract slightly; the maximal contraction occurs near the spinodal T-sp . We evaluate the mean repulsive energy felt by the monomers and its d ependence on the monomer's position along the chain. From the variance of the repulsive energy we calculate c(upsilon), the specific heat pe r chain; c(upsilon) is continuous both near T-sp and near the ODT. Sur prisingly, c(upsilon) scales with epsilon(2)Nf(1-f), where epsilon is the microscopic energy parameter of the simulations. As dynamical feat ures we compute D, the diffusion coefficient of single chains and the rotational relaxation times tau of the end-to-end vector: D scales wit h epsilon f(1-f), whereas the tau-times show complex f-dependencies, f acts which stress that the diffusional motion and the rotational relax ation behave differently. (C) 1997 American Institute of Physics.