Optimized multilayer coating using layer-by-layer assembly method for excellent oxygen barrier of poly(lactic acid) based film

Layer-by-layer assembly (LbL) of trilayer coatings of polyelectrolytes and nanoclays (NC) were produced on poly(lactic acid) based (PLAb) films. The focus of this study was on the optimization of the wetting properties of the PLAb substrate, and the adhesion properties of the prepared coating. Coatings produced by an optimized method, by combining UVC-treatment of the substrate and adjusting surface tension of the coating solutions, were compared to coatings made by typically used water-based solutions. Even though LbL-coatings with a high adhesion strength to its substrate is of outmost importance for most applications, this is often not evaluated. In this study, the adhesion of the coatings on the PLAb substrate was assessed using the cross-cut test according to ISO 2409, in which the adhesion is classified on a scale between 0 and 5, where a lower number reflects better adhesion. The coating prepared by the optimized procedure was categorized as class 0, while the control film was classified as 4. These results reveal the importance of the optimization of both the surface free energy of the substrate, and the surface tension of the dipping solutions. The surface morphology and roughness properties of the optimized films were further investigated with scanning electron microscopy (SEM), confocal optical microscopy (COM) and atomic force microscopy (AFM), showing a uniform distribution of NC on the substrate with increasing number of trilayers. X-ray photoelectron spectroscopy (XPS) confirmed the homogenous surface composition of polyelectrolytes and NC. Finally, the oxygen transmission rate (OTR) decreased considerably (by 99.7%), from 97.2 cc/(m²·day) for pristine PLAb substrate to 0.3 cc/(m²·day) for the 20-trilayer coated substrate.