Nanomechanical Behavior of High Gas Barrier Multilayer Thin Films

Abstract

Nanoindentation and nanoscratch experiments were performed on thin multilayer films manufactured using the layer-by-layer (LbL) assembly technique. These films are known to exhibit high gas barrier, but little is known about their durability, which is an important feature for various packaging applications (e.g., food and electronics). Films were prepared from bilayer and quadlayer sequences, with varying thickness and composition. In an effort to evaluate multilayer thin film surface and mechanical properties, and their resistance to failure and wear, a comprehensive range of experiments were conducted: low and high load indentation, low and high load scratch. Some of the thin films were found to have exceptional mechanical behavior and exhibit excellent scratch resistance. Specifically, nanobrick wall structures, comprising montmorillonite (MMT) clay and polyethylenimine (PEI) bilayers, are the most durable coatings. PEI/MMT films exhibit high hardness, large elastic modulus, high elastic recovery, low friction, low scratch depth, and a smooth surface. When combined with the low oxygen permeability and high optical transmission of these thin films, these excellent mechanical properties make them good candidates for hard coating surface-sensitive substrates, where polymers are required to sustain long-term surface aesthetics and quality.

Fast Self‐Healing of Polyelectrolyte Multilayer Nanocoating and Restoration of Super Oxygen Barrier

“…This polyelectrolyte multilayer nanocoating, fabricated by alternate deposition of polyethyleneimine (PEI) and PAA ( Figure S1, Supporting Information), has an exceptionally high elastic modulus and ultrahigh oxygen barrier. [45,46] As expected, the gas barrier vanishes after cracking due to strain, but it is completely restored after high humidity exposure (i.e., self-healing efficiency of 100%). This simple self-healing nanocoating is potentially useful for protection of food, pharmaceuticals, and electronics.…”

“…Reported here for the first time is a repeatable self‐healing, and restoration of compromised gas barrier, in a stretched multilayer polyelectrolyte nanocoating upon exposure them to high humidity. This polyelectrolyte multilayer nanocoating, fabricated by alternate deposition of polyethyleneimine (PEI) and PAA (Figure S1, Supporting Information), has an exceptionally high elastic modulus and ultrahigh oxygen barrier . As expected, the gas barrier vanishes after cracking due to strain, but it is completely restored after high humidity exposure (i.e., self‐healing efficiency of 100%).…”

“…This low OTR originates from the strong ionic interaction between the polyelectrolyte chains, which results in small free volume in the nanocoating. It has been reported that this PEI/PAA multilayer film has a modulus near 20 GPa in the dry state and a glass transition temperature near 100 °C, but its gas barrier is compromised at high humidity . It is believed that water molecules swell and plasticize the thin film, resulting in enlarged free volume that eases oxygen diffusion.…”

“…Multilayer films were left in water overnight in a concave meniscus to reach full swelling before testing. Nanoindentation measurements were performed using a previously described method . More than 30 indents were performed on each sample.…”

Polyelectrolyte Multilayer Nanocoating Dramatically Reduces Bacterial Adhesion to Polyester Fabric

“…11,12 This diversity of materials leads to thin films with a variety of properties that include energy generation, 13 structural color, 14,15 flame retardation, 11,16 and gas barrier. 17,18 These properties can be tailored by changing the pH, 19 solution concentration 20 , and deposition time 21 . A number of biomedical applications have been explored using these multilayer assemblies, including polyelectrolyte microcapsules that can be used for targeted drug delivery, 22 antimicrobial surfaces 23,24 and magnetic-field-sensitive surfaces 25 .…”