ASYMMETRIC BLOCK-COPOLYMER MELTS NEAR THE MICROPHASE SEPARATION TRANSITION - A MONTE-CARLO SIMULATION

A three-dimensional lattice model of a diblock copolymer melt with com position f = 3/4 is studied by Monte Carlo methods, extending our prev ious work for the symmetric case (f = 1/2). The polymers are represent ed as self-avoiding walks, at a volume fraction phi = 0.8 of occupied sites, and chain lengths N = 24, 32, and 40 are used. Different monome rs at nearest-neighbor sites repel each other with an energy k(B)Tepsi lon. Due to the use of a finite lattice size L = 32, the microphase se paration transition to the hexagonal phase can only be located rather roughly (epsilon(c)N = 11 +/- 1), but accurate data are obtained in th e disordered phase at temperatures above the transition temperature. A s in the symmetric case, we find that the Leibler theory predicts the peak of the structure factor S(q) only for E --> 0, while the peak pos ition q decreases gradually as epsilon increases by a relative amount of up to ca. 25 % at the transition. Also the gyration radius of the chains increases gradually by several percent, although the gyration r adii of the individual blocks even decrease slightly: the distance R(A B) between the centers of gravity of the two blocks increases by ca. 1 6% up to the transition, indicating a gradual formation of asymmetric dumbell-shaped molecules. This gradual stretching of the coils sets in far above the transition, indicating a breakdown of the random phase approximation as in the symmetric case.