A comprehensive study of the orientation mechanisms of montmorillonite in a LLDPE polymer matrix using multi-nano-layer polymer co-extrusion: from orientation quantification to polymer-filler structure-property relationships

Abstract

The development of flexible photovoltaic technology with low production costs is crucial for expanding access to solar energy and promoting its widespread adoption. One of the objectives of the CUSTOM-ART project (under grant agreement no. 952982) is to develop ultra-barrier films to protect photovoltaic cells using common, affordable plastic materials. To achieve this, the strategy adopted is based on the use of multi-nano-layer (MNL) polymer film co-extrusion technology aimed at orienting 2D montmorillonite nano-fillers dispersed in a poly(ethylene) matrix to produce films with a high degree of tortuosity. Therefore, this study focuses on characterizing the orientation of montmorillonite by systematic texture X-ray diffraction analysis. The results indicate that the orientation of montmorillonite with the MNL co-extrusion technology has a limited effect when the poly(ethylene) is in the molten state. However, the orientation is significantly improved with a biaxial stretching post-treatment of the films, which is performed at a temperature below the melting point of poly(ethylene). This raises the question of the influence of the matrix and the relationship between montmorillonite and poly(ethylene) on montmorillonite orientation. Further investigation of the crystallization of the poly(ethylene) phase revealed that it occurs around montmorillonite nano-fillers, allowing preferential orientation of poly(ethylene) crystals without affecting the rate of poly(ethylene) crystallinity. Consequently, the way poly(ethylene) crystallizes affects the orientation of montmorillonite. However, despite these variations in orientation, no significant improvement in the water barrier properties of the films was observed. Although the film has a controlled nanostructure, only 35% (including both montmorillonite and poly(ethylene) contributions) of the film consists of crystals, which may limit their ability to influence water diffusion through the films.