Division of Chemistry and Chemical Engineering
California Institute of Technology
Pasadena
USA
zgw@cheme.caltech.edu
It is commonly believed that the weak interaction strength and large molecular size in most homopolymer mixtures and block copolymers make these systems exhibit mean-field behavior. While this belief is true in the limit of infinitely long chains, finite chain length effects can lead to significant, often qualitative deviations from the mean-field predictions. This talk will discuss some phenomena associated with the finite chain length effects. First, using results from a self-consistent field calculation, I will show that the barrier to homogeneous nucleation in a binary polymer blend becomes of order kT well before the mean-field spinodal for polymers of moderate lengths typically used in experiment. Thus, for typical polymer lengths, the metastable region is much narrower and the crossover from nucleation to spinodal decomposition sets in much earlier than commonly believed. Next, I will discuss the formation of disordered micelles in highly asymmetric diblock cop! olymers from the perspective of nucleation. While mean-field calculations predict a direct transition from a featureless disordered state to periodically ordered microphases, namely, simultaneous aggregation and ordering, finite molecular weights lead to a two-stage transition: ordering preceded by aggregation at a well-defined temperature. Finally, I discuss the possibility of microstructural glass formation in block copolymer melts as a result of localized fluctuations in Fourier space. Using a thermodynamic replica approach combined with perturbation theory, I show that microstructural glass transition can compete with disorder-order transition and construct a phase diagram that includes the glass transition.