NUCLEATION PHENOMENA IN POLYMERIC SYSTEMS

Materials formed from long flexible macromolecules differ from their small-molecule analogs, because corresponding collective length scales are distinctly larger and many dynamical phenomena are very much slower, in addition, the variation of chain length N yields a control parameter that leaves intermolecular forces invariant, but allows a stringent test of theories. These concepts are exemplified in a discussion of nucleation barriers for symmetrical polymer (A,B)-mixtures (chain lengths N-A = N-B = N) near the critical temperature T-c, and for symmetrical block copolymers near the (fluctuation-induced) first order transition between the disordered melt and the lamellar mesophase. While in the latter case for N --> infinity the transition becomes second-order and the order of magnitude of the nucleation barrier vanishes as N--1/3, for the polymer mixtures it increases as N-1/2 in the mean-field critical regime. Experiments and simulations, however, both show that very long chains are needed to fully reach this mean-field critical regime. For asymmetrical block copolymers {f = N-A/(N-A+N-B) not equal 1/2} the nucleation barrier scales as N-1/2 \f - 1/2\(5).