Phase behavior of compressible melts of multiblock polydisperse copolymers

Abstract

In terms of the previously proposed model, specific features of the phase behavior of Markovian polydisperse copolymers with allowance for their compressibility have been investigated via bifurcation analysis followed by continuation with respect to a parameter that characterizes the deviation of the temperature of the system from its value on the spinodal. These features above all include competition between microphase separation and macrophase separation under conditions when the local instability of the homogeneous state appearing at the spinodal corresponds to the macrophase separation only. Nevertheless, it was shown that depending on the structural parameters, the global instability characterized by a cloud-point hypersurface can result in either macrophase or microphase separation, with the microphase separation occurring in the vicinity of the critical point. In this case, the results are consistent with the conclusions of the Landau theory of phase transitions, whose applicability limits with respect to deviation from the critical point have been evaluated in this study. Outside the range of applicability of this theory, cloud-point curves that correspond to macrophase separation and microphase separation are very similar. These conclusions remain valid over a wide range of compressibility whose influence has been assessed for the first time. It has been found that the type of copolymers under consideration has a characteristic feature that was not noticed previously: Namely, the distribution of density in the nucleus of a new phase in this case will look like a spatially localized solitonlike profile.