MONTE-CARLO STUDIES OF PHASE-TRANSITIONS IN POLYMER BLENDS AND BLOCK-COPOLYMER MELTS

The unmixing transition of both symmetrical polymer blends AB (i.e. chain lengths N-A = N-B = N) and asymmetrical ones (N-B/N-A = 2, 3) is studied by large-scale Monte Carlo simulations of the bond fluctuation model. Combination of semi-grand-canonical simulation techniques, <<histogram reweighting>> and finitesize scaling allows an accurate location of the coexistence curve in the critical region. The variation of the critical temperature with chain length (N) is studied and compared to theoretical predictions. For the symmetrical case, use of chain lengths up to N = 512 allows a rough estimation of crossover scaling functions for the crossover from Ising to mean-field exponents. The order-disorder transitions in melts of both symmetric (composition f= N-A/(N-A + N-B) = 1/2) and asymmetric (f = 3/4) block copolymers is studied for very short chains (16 less than or equal to N less than or equal to 60). The interplay between structure and chain configuration is emphasized. Qualitative evidence for <<dumbell formation>> of chains and vacancy enrichment in A-B-interfaces and near hard walls is presented.