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

A three-dimensional lattice model of a diblock copolymer melt with composition f = 3/4 is studied by Monte Carlo methods, extending our previous work for the symmetric case (f = 1/2). The polymers are represented 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 monomers at nearest-neighbor sites repel each other with an energy k(B)Tepsilon. Due to the use of a finite lattice size L = 32, the microphase separation transition to the hexagonal phase can only be located rather roughly (epsilon(c)N = 11 +/- 1), but accurate data are obtained in the disordered phase at temperatures above the transition temperature. As 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 position 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 radii of the individual blocks even decrease slightly: the distance R(AB) between the centers of gravity of the two blocks increases by ca. 16% 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.