Structural Analysis of the Adhesive Interface between Polyethylene and Polyamide 6 Using Scanning Transmission X-ray Microscopy

Abstract

The structure at the interface of multilayer polymer films is critical for determining their adhesion performance. However, the direct characterization of chemically reactive polymer–polymer interfaces remains challenging. This study examined the adhesive interface between coextruded polyethylene (PE) and polyamide 6 (PA6) films, which included a maleic anhydride-modified polyethylene (mPE) interlayer, using scanning transmission X-ray microscopy (STXM) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The analysis of the STXM image stacks through singular value decomposition revealed the development of a mixed interfacial layer, with its thickness systematically increasing as the maleic anhydride concentration increased. The thickness of this interface was found to be strongly linked to the peel strength, highlighting the critical influence of the nanoscale interfacial structure on macroscopic adhesion performance. Moreover, spectral features at the interface were identified that could not be reproduced by a linear combination of bulk components. The results indicate that the imide-like reaction products were localized within the interfacial layer. These findings demonstrate that STXM is a powerful method for directly investigating chemically reactive polymer interfaces and offers insights into the design of high-performance multilayer films with controlled interfacial chemistry.