Mesoscopic segregation in binary liquid solutions: application of a model for block copolymers to non-macromolecular systems

Abstract

In this work, an analogy between block copolymer phase behaviour and microdomain spacial organization in certain liquid organic/aqueous binary systems is discussed. The peculiar phase behaviour in block copolymers has been described by a segregation of the constituting blocks in domains of mesoscopic dimensions that do not lead to actual macroscopic phase separation, in contrast to what is observed in ordinary binary polymer mixtures. In the liquid non-macromolecular systems under investigation here, such events can be described as a self-association of the organic moiety leading to the formation of initially spherical microdomains that grow in size and progressively change their geometry into interpenetrating patterns, as one proceeds towards composition regions of low water content, until a collapse of the microdomains is reached, and a non-polar continuum is produced. At that point, a reversion in the microconfiguration of the binary mixture takes place. In this work, vibrational spectroscopic data for one of those systems, namely, tetramethylurea/water, were analysed in order to determine changes in structural/dynamic parameters throughout the whole range of solvent composition, with the aim of allowing inferences relative to organization patterns of the micro/meso-domains. Along with evidence from previous work, the results strongly suggest a close parallelism between phase behaviour in these non-macromolecular liquid systems and that observed for block copolymers.