Interfacial slip reduces polymer-polymer adhesion during coextrusion

Abstract

Slip occurs at the interfaces between immiscible polymer melts at high shear stress. We demonstrate that this reduces adhesion during coextrusion. A 20-layer polystyrene (PS)/poly(methyl methacrylate) (PMMA) alternating layer sample was coextruded and the adhesion at each internal interface was measured with the asymmetric dual cantilever beam crack propagation test. When the shear stress experienced by an interface is low, interfacial slip is negligible and interfacial adhesion is high, comparable to a laminated interface. When the shear stress exceeds a critical value, interfacial slip begins to develop and interfacial adhesion begins to decrease with shear stress. Above another critical stress, full slip has been developed at the interface and adhesion reaches a plateau value, which is about 1∕3 of the equilibrium value. The changes in adhesion versus shear stress follow a master curve for different flow rates. This supports the hypothesis that polymer chains at the interface are disentangled by the shear stress during coextrusion. It was also found that annealing restored adhesion on the reptation time scale indicating that entanglements were reestablished at the interface. Creating block copolymer by a coupling reaction at the interface during coextrusion increased adhesion to level even higher than the laminated interface.