Abstract
The structure of the interface between two polymer phases is of considerable interest from a fundamental point of view. Polymers represent in fact model systems to address problems of statistical mechanics of fluids, and to test mean field theories. Moreover, polymer/polymer interfaces play an important role in technology, because the structure of the interface between immiscible or quasi-immiscible polymers controls adhesion, wetting, mechanical and optical properties of materials. In this thesis the interface between thin polymer films was extensively investigated by using neutron reflectivity. This study was focused on a particular class of polymers, random copolymers of ethylene and ethylethylene. In these systems it is possible to tune the size of the energetic interaction between polymer pairs, and thus to control the miscibility, by changing the copolymer ratio. One of the aims of the experimental work was to determine for the first time the dependence of the structure of the interface on the degree of immiscibility of the polymers over a wide range, as criticality is approached, characterising the interface in terms of intrinsic width, as calculated by mean field theories, and capillary fluctuations. Far from the critical point, the width of the interface increases slowly as the degree of immiscibility is decreased, and it is independent of the molecular weight of the polymers. Closer to criticality, the dependence on the miscibility becomes stronger, and the way in which the interfacial width diverges depends on both the interaction parameter and the length of the polymer chains. The mechanism by which confinement affects the interface between two polymer phases was also investigated. The aim was to understand the relative importance of long-ranged Van der Waals forces and short-ranged "truncation" forces in modifying interfacial fluctuations. To achieve this experimentally the dependence of the interfacial width on the film thickness was studied for different conditions of miscibility. It was observed that, as criticality is approached, there is a gradual transition from a region where long-ranged forces dominate, at a higher degree of incompatibility, to a region approaching criticality where truncation forces are relevant. The contrast match technique for thin polymer films was also investigated.