Morphological, interfacial and rheological properties in multilayer polymers: A review

The global market of multilayer polymer (MLP) films accounted for $5.50 billion in 2017 and is expected to reach $9.58 billion by 2026. In this paper, a comprehensive review on morphology, interfacial characteristics, and rheological behavior, as three main effective interim parameters on the overall properties of MLPs, has been presented. It has been shown that the morphology of MLPs is strongly governed by layer thickness, type of polymers, annealing, and the conditions in secondary processes. By manipulating these parameters in MLPs, various morphologies such as random 3-D spherulites, on-edge crystals, and planar lamellas are observed, leading to changes in properties such as gas permeability. Moreover, different levels of interfacial adhesion in MLPs are obtained by altering the interfacial entanglement density, highlighting that the interfacial entanglement density is a defining factor for many properties such as peeling strength. Addition of blends or copolymers to improve compatibility, use of adhesives, introduction of micro- or nanoparticles to the interface, and annealing have been shown to be effective techniques to improve the interfacial entanglement density. Followingly, the review of the rheological behavior in MLPs revealed that slip at the interface of MLPs is dependent at interactions between constitutive polymers. Also, it has shown that MLPs can present hardening in extension that contributes to improved properties such as reduced dripping in fire. This review provides a summary of the current status of MLPs from a structure-process-property perspective while shedding light on existing knowledge gaps to be addressed by future research works.