Mechanism of adhesion between protein-based hydrogels and plasma treated polypropylene backing

We studied the mechanism of adhesion between N₂ plasma treated polypropylene (PP/N₂) backing and a hybrid hydrogel (HG) produced by chemical crosslinking between poly(ethylene glycol) and soy albumin. The work of adhesion, measured by peel testing, was found to be 25 times higher for PP/N₂ compared to untreated PP (≈5.0 J/m² versus ≈0.2 J/m²). In order to understand the adhesion mechanism, we performed a detailed analysis of the surface chemical composition of PP and PP/N₂ using X-ray photoelectron spectroscopy (XPS), chemical derivatization and attenuated total reflectance infra-red (ATR-IR) measurements. The results confirm incorporation of different nitrogen- (amine, amide,…) and oxygen- (hydroxyl, carboxyl,…) containing chemical groups on the PP/N₂ surface. The derivatized functions were primary amine, hydroxyl, carboxyl and carbonyl groups. Chemical derivatization reactions validated the XPS results (except for carbonyl groups), and they clearly underlined the essential role of primary amine groups in the adhesion process. In fact, after derivatization of the amine functions, the work of adhesion was found to be 0.41 ± 0.12 J/m². Participation of amine groups in the formation of covalent bonds at the interface between PP/N₂ and HG was directly confirmed by ATR-IR measurements.