Morphological reorganization and mechanical enhancement in multilayered polyethylene/polypropylene films by layer multiplication or mild annealing

• Published in: Journal of polymer science. Part B, Polymer physics Polymer physics, 2018-03, Vol.56 (6), p.520-531
• Main Authors: Mauri, Michele, Ponting, Michael, Causin, Valerio, Simonutti, Roberto, Pisciotti, Francesco
• Format: Article
• Language: English
• Subjects: Annealing; confinement; Conversion; Crystallites; Mechanical properties; Mesophase; mesophases; Modulus of elasticity; multilayer films; nanostructures; NMR; NMR spectroscopy; Nuclear magnetic resonance; Polyethylene; Polyethylenes; polymer processing; polymers; Polypropylene; polypropylene crystallinity; polypropylene mesophases; Thickness; X‐ray Diffraction
• ISSN: 0887-6266; 1099-0488
• DOI: 10.1002/polb.24561

ABSTRACT Shortly after processing, Polyethylene/Polypropylene (PE/PP) multilayer films demonstrate an increase in tensile modulus and other mechanical properties when the individual layer thickness is below 0.5 µm. Subsequent annealing at 60 °C for 16 h brings the properties of all other samples to similar values. WAXD characterization of the layered films identified a prevalence of mesophase in the thicker PP layers. In samples with increased layer numerosity or subjected to annealing, WAXD detected its conversion to α crystalline phase that correlates with improved mechanical properties. SSNMR and DSC detailed the defective nature of α iPP crystallites. Comonomers, detected by NMR in the commercial polymers used for the films, are the source of “tunable disorder” that dictates the formation of the PP mesophase and the low temperature of conversion to the mechanically stronger defective α phase. Soft intrafilm layer interfaces instead enable nucleation and localized polymer chain rearrangement even without annealing. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 520–531 Films produced with alternating thin layers of commercial polypropylene and polyethylene display optimized mechanical properties due to PP crystallization. High layer thickness instead promotes the formation of a mesophase where PP chains have a parallel alignment but display lateral disorder. This phase has lower mechanical properties, but it can be reorganized to crystal with mild heating or simply by the presence of soft confining interfaces.