Investigation of interdiffusion between compatible polymers under static and co-extrusion processing conditions

When two compatible polymer melts are brought into contact, a diffuse interphase is formed that governs the adhesion between them. The degree of interdiffusion is defined mainly by the processing conditions and the physico-chemical properties of the materials. Despite the great industrial relevance of co-extrusion in manufacturing multilayer structures, little research has dealt with interdiffusion under these processing conditions, where polymer-polymer interfaces are subject to shear load. In this work, procedures for investigating interdiffusion both under static and real co-extrusion flow conditions were developed. Using various grades of poly(methyl methacrylate) and styrene-co-acrylonitrile copolymer, we studied both theoretically and experimentally the effects of interfacial contact time, temperature, material compatibility, and interfacial shear stress on interdiffusion. Spectroscopic analysis of the interfaces using confocal Raman microscopy showed that the general relationships between interdiffusion, time (Fick's law) and temperature (Arrhenius relationship) are identical under both static and co-extrusion conditions. However, the interdiffusion rate is significantly higher under shear influence. Since the prediction of the interdiffusion process supports material selection and process design in multilayer manufacturing, a simple analytical model was developed to quantify the effects of aforementioned material properties and processing conditions on the apparent mutual interdiffusion coefficient.