SIMULATION OF THE SELF-ASSEMBLY OF SYMMETRICAL TRIBLOCK COPOLYMERS IN DILUTE-SOLUTION

The simulation of the self-assembly of symmetric triblock copolymers, A(NA)-B(NB)-A(NA), has been performed on a cubic lattice. In all of these simulations the pairwise energy of interaction of A with B (E(AB)) is identical with the pairwise interaction of A with a void, or solvent (E(AS)), and all other pairwise interactions are zero. The ratio of the number of observations of single dispersed chains that form internal loops to the number that have no interaction between their terminal blocks is studied for its implications for the influence of loop formation on the self-assembly of the system. As N(A) --> 1 the loop formation approaches that predicted by Jacobson and Stockmayer, but the entropic penalty associated with loop formation decreases as N(A) increases. This result has strong implications for the structures formed upon self-assembly of A(NA)-B(NB)-A(NA). The type of aggregation observed when E(AB) = E(AS) > 0 depends on whether the conditions correspond to weak or strong segregation. At weak segregation, the change in volume fraction of A(NA)-B(NB)-A(NA) produces two transitions. The first transition produces a very loosely organized micelle-like structure, and the second transition, at higher concentration, produces an enormous aggregate that we interpret as signifying separation into a gel-like macrophase. At strong segregation the system assembles into a micelle with a well-organized classical core-shell structure. The transition to a gel-like macrophase is suppressed.