Lattice Monte Carlo investigations on copolymer systems, 2 - Triblock copolymers

Symmetric triblock copolymers in common good solvents, common theta-solvents and in selective solvents (repulsive interactions being operative between outer and inner blocks in most cases) are investigated by means of lattice Monte Carlo simulation and compared to their constituent homopolymers. While copolymers in a common good solvent follow widely the behaviour of purely athermal chains, copolymers with compact blocks, e.g. blocks in a (near-) theta-solvent, show some particularities. Block copolymers in a selective solvent have different mean square radii of gyration, [s(2)], depending on whether the solvent is a good one for the outer or inner blocks (ABA or BAB, respectively, with B denoting theta-solvent and A athermal solvent) although their mean square end-to-end distances, [h(2)], are equal. The same behaviour is found for the scaling exponents of [s(2)] and [h(2)], respectively, leading to surprisingly strong chain-length dependences of the ratio between [h(2)],, and [s(2)] and of a parameter delta (asphericity) describing the shape of the molecule. Surprisingly, the shape of ABA-copolymers becomes more globule-like with increasing chain-length in spite of repulsive interaction between the blocks (opposite to the expected behaviour). The chain dimensions of block copolymers deviate by 30% from the sum of those of homopolymers of the same length and in the same solvent (as the blocks) if there are compact (B-type) blocks in the chain and by 20% when the blocks are all in a good solvent (A-type blocks). The number of contacts per segment is not much affected by the structure of the molecules but depends mostly on solvent quality. Like the number of contacts per segment between adjacent blocks also that between the two end-blocks quickly tends to zero with increasing chain-length.