Influence of the Polymer Interphase Structure on the Interaction between Metal and Semicrystalline Thermoplastics

Abstract

It is demonstrated that a lamellar crystalline structure may form near the metal–polymer interface during the comolding of metal–thermoplastic joints. The influence of the crystalline morphology near the interface on the rupture behavior of the joint is studied experimentally. High‐resolution scanning electron microscopy (HR–SEM), atomic force microscopy (AFM), and optical microscopy are used to characterize the microstructure of the metal–thermoplastic interface. The results show that a lamellar crystalline structure at the interface promotes cohesive failure, i.e., the crack runs in the polymer. Additional experiments show that a transcrystalline polymer structure at the interface results in failure at the interface. Herein, two methodologies have been developed to characterize the formation of the transcrystalline polymer interphase structure based on X‐ray diffraction and polarized light hot stage microscopy. The results show the importance of the polymer interphase structure for metal–thermoplastic interactions. Understanding of the formation of the polymer interphase and its influence on the interfacial bonding strength are vital for thermoplastic applications such as fiber‐reinforced thermoplastic composites, tool surface design for processing of thermoplastics, and their composites as well as for metal–polymer hybrid joints.

Formation of Flat‐on Lamellar Crystals in Absence of Nanoconfinement

“…A detailed description of the separation procedure can be found in our previous paper. [ 9 ] Due to its brittleness, a slightly different procedure was followed for the silicon wafer. Here, a steel‐PEKK‐Si sandwich is first prepared.…”

“…(PEKK on stainless steel has been published in our previous paper and is not included here). [ 9 ] The first column shows the obtained results for PPS. Figure 4a shows the optical image of the PPS residual on the stainless steel surface.…”

“…The thickness of the PEKK layer is characterized as 20 nm as published in our earlier work. [ 9 ] The results in Figure 4 show that when semicrystalline thermoplastics are melted and crystallized in contact with a stainless steel surface, a polymer layer structure is formed at the interface with the substrate. This polymer layer structure has a uniform thickness which varies between different types of thermoplastics.…”

“…The bond strength between the polymer layer and the stainless steel substrate is stronger than the bond strength between the polymer layer and the bulk polymer. [ 9 ] Thus, fracture occurs at the interface between the polymer layer and the bulk polymer during the opening of the stainless steel‐thermoplastic joint, hence, leaving a polymer residue on the substrate. Next, two characterization approaches as detailed in part 3.2 and 3.3 are used to make plausible that these polymer residual layers are, in fact, flat‐on lamellar crystals.…”

“…In our previous paper, on the influence of the polymer interphase structure on the adhesion between a semicrystalline thermoplastic and a metal, we showed that a lamellar structure can be found at the polymer‐metal interphase. [ 9 ] The formation of the lamellar structure has a significant influence on the fracture behavior of the metal‐thermoplastic interface, which is crucial in, for example, the release of thermoplastics from mold surfaces. These results indicate that a lamellar crystalline structure may form without the aforementioned nanoscale confinement.…”