Phase behaviour of random copolymers and crosslinked homopolymer blends

Abstract

The phase behaviour of molten linear random block copolymers and of randomly crosslinked homopolymer blends is studied in the framework of statistical mechanics based on microscopic models.In melts of random copolymers the concatenation of building blocks of two incompatible species counteracts their tendency to demixing. Therefore, macroscopic phase separation competes with microphase separation, i.e. local separation. The phase behaviour is investigated using a coarse-grained model (macrophases only) and a Landau expansion of the free energy of the full microscopic model (macro- and microphases). This yields substantial deviations from results obtained using continuum approximations. Based on Landau theory, a scheme for fractionation of the melt according to the chain sequences (homo- or copolymeric) is set up and studied. It is shown that fractionation leads to the coexistence of microstructured and macroscopic phases and thus constitutes an alternative to the established mechanism for microphase separation.In blends of incompatible homopolymers, sufficiently strong crosslinking stabilises the homogeneous state and replaces macroscopic phase separation with microphase separation. The crosslinking is modelled by permanent bonds between randomly selected pairs of monomers. Hence, the model permits the description of phase separation and gelation without ad hoc assumptions. The mean-field phase diagram is studied by means of the Landau expansion of the free energy, and and the degree of stabilisation of the homogeneous state is calculated. The investigation of the scattering functions of the homogeneous gel shows, e.g., the preservation of the fluctuations present prior to gelation and indications of a near-by microphase separation in the fluctuations. The microphases themselves are studied with an ansatz for different regular and disordered morphologies, which are selected depending on the compressibility and the composition of the gel.