title: Interfaces and fluctuations in confined polymeric liquid mixtures: from immiscible to near critical systems.
abstract: In this paper the structure of the interface between polymer films is discussed to elucidate fluctuations and confinement effects in fluid polymer mixtures. The neutron reflectivity technique has been employed to investigate the dependence of the structure of the interface on the degree of immiscibility of the polymers over a wide range, as criticality is approached, and to characterize it in terms of intrinsic width, as calculated by mean field theories, and capillary fluctuations. For more immiscible systems, as the degree of incompatibility between the polymers is decreased, the width of the interface increases slowly, and it is independent of the molecular weight of the polymers. Closer to the critical point the dependence on the degree of miscibility becomes stronger and the way in which the interfacial width diverges, as criticality is approached, is related to both chain length and Flory-Huggins interaction parameter (χ). The results have been compared to the predictions of mean field theories. Self-consistent field numerical calculations, with the additional contribution due to capillary waves, provide a good description of the width of the interface between two polymer bulk phases, in particular at higher and intermediate degrees of immiscibility-the product of the Flory-Huggins interaction parameter χ and the number N of monomers of the chain, χN. For more miscible systems a crossover is observed to a region where the square gradient theory in the weak segregation limit better approximates the experimental results. Moreover, the mechanisms by which confinement affects the interface have been investigated. To understand the relative importance of the long ranged van der Waals forces and short ranged 'truncation forces' in modifying thermally excited fluctuations at the polymer/polymer interface, the thickness dependence of the interfacial width has been studied for different degrees of miscibility, approaching criticality. The results show a gradual transition from a region where long ranged dispersion forces are dominant in influencing the capillary wave spectrum, for higher degrees of immiscibility, to a region where short ranged forces-connected to the presence of the walls-become more important and the dependence of the interfacial width on film thickness is stronger. The thickness dependence of the interfacial width was also studied for systems with different molecular weights in different conditions of miscibility, to investigate the effects of a confined geometry on polymers with different chain lengths as criticality is approached.
authors.affiliation: Département de Physique, Université Libre de Bruxelles, Boulevard du Triomphe CP223, 1050 Bruxelles, Belgium. msferraz@ulb.ac.be
authors.forename: Michele
authors.initials: M
authors.lastname: Sferrazza
authors.forename: Clara
authors.initials: C
authors.lastname: Carelli