Probing Molecular Structural Changes of Buried Interfaces between Polyethylene and Nylon in Polymer Thin Films after Stretching

Polymer thin films are widely used in both industries and everyday life. The performance of thin films is improved by using multiple functional layers through either lamination with adhesives or multilayer coextrusions. Further machine direction stretching to achieve oriented (MDO) films or both machine direction and cross-machine direction stretching to achieve biaxially oriented (BO) films can continually enhance their performance. Polymer multilayer thin films can degrade upon stretching for certain uses due to the interlayer adhesion loss. In this research, we investigated the buried nylon/maleic anhydride (MAH)-functionalized ethylene octene (EO) copolymer (MAHfEO) interface in a multilayer thin film made from a coextrusion blowing process and followed with and without machine direction (MD) stretching. Sum frequency generation (SFG) vibrational spectroscopy was applied to study the chemical groups related to adhesion at the buried interface. The SFG signal of the unstretched film indicates weaker polymer chain entanglement and more ordered functional groups at the nylon/MAHfEO interface compared to the previously studied EVOH/MAHfEO interface in EVOH films. The signal increase from the unstretched to the 3× stretched films shows that reduced entanglement leads to adhesion loss. The comparatively small increase in signals from the 3× to the 6× stretched films indicates that the influence of entanglement loss due to the stretching becomes less significant for adhesion loss, while the chemical bond formed by the reaction at the buried interface plays a role in determining adhesion. Analysis of SFG spectra collected from orthogonal sample directions showed that the azimuthal orientation of interface functional groups was slightly reoriented toward the stretching direction. The research further explores the relationship between the polymer chain structure and its macromolecular properties, providing insights for rational material design and performance improvements.