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

The unmixing transition of both symmetrical polymer blends AB (i.e. ch ain lengths N-A = N-B = N) and asymmetrical ones (N-B/N-A = 2, 3) is s tudied by large-scale Monte Carlo simulations of the bond fluctuation model. Combination of semi-grand-canonical simulation techniques, <<hi stogram reweighting>> and finitesize scaling allows an accurate locati on of the coexistence curve in the critical region. The variation of t he critical temperature with chain length (N) is studied and compared to theoretical predictions. For the symmetrical case, use of chain len gths up to N = 512 allows a rough estimation of crossover scaling func tions 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 t o 60). The interplay between structure and chain configuration is emph asized. Qualitative evidence for <<dumbell formation>> of chains and v acancy enrichment in A-B-interfaces and near hard walls is presented.