1. Introduction
Gas barrier films have a wide range of applications in food/drug preservation and electronic device encapsulation.1−5The protection of flexible displays, inflation/deflation of tires and football/basketball require deformation. So the demand for flexible and stretchable gas barrier films has greatly increased. Conventional alumina/zinc oxide coatings display excellent barrier properties but are easy to break or crack when the barrier films are stretched. Although pure polymer materials such as ethylene/vinyl alcohol copolymer (EVOH), polyvinyl alcohol (PVA), and polyvinylidene chloride (PVDC) can meet the requirement of general packaging application, they cannot be applied to high barrier fields.6−12 Besides, such polymer materials have limited application range due to their humidity sensitivity and poor mechanical strength. By hybridizing inorganic nanoplatelets (such as hydrotalcite and montmorillonite) with polymers, multilayer organic-inorganic films called ”nano-brick” solve the problem of poor gas barrier property of polymers and simultaneously improve the mechanical property.13−18
Layer-by-layer (LBL) assembly is an effective method to prepare gas barrier coatings through nanoscale control of film architecture.19−23 Electrostatic layer assembly technology is the earliest and most common method for constructing multilayer films.13, 24 This method is suitable for assembly between oppositely charged elements, such as polymer electrolytes, inorganic nanoparticles, two-dimensional (2D) nanoplatelets, etc. Although the electrostatically assembled films have good barrier properties, these nano-walls are usually very strong and cracks upon stretching. Stretchable gas barrier films must be crack-free because the cracks act as gas transmission highways. For example, a 10% strain resulted in cracking of PEI/montmorillonite nanocoating and thus an increase in gas permeability by an order of magnitude.25, 26 Hydrogen bonded multilayer films are generally less rigid than ionically bonded films.27−29The weaker hydrogen bonds lead to a looser film structure with a lighter crosslink density that produces greater strain without damage. However, extensibility generally increases at the expense of gas permeability because the barrier properties of LBL deposited films are dependent on density and density is related to hardness. One strategy to break through this limitation is to modify the hydrogen bonding system to increase barrier properties while maintaining its extensibility.
Hydrotalcite, referred to as layered double hydroxide (LDH), is an important 2D layered material.30−34 The surface of the LDH laminate contains a large number of hydroxyl groups, and the presence of trivalent metal ions in metal-oxygen hexahedra makes the laminate positively charged. Therefore, LDH can be incorporated into hydroxyl group-containing or negatively-charged polymer matrixvia hydrogen bond or electrostatic interaction, respectively. In our recent work, we used LDH nanoplatelets to construct high-barrier film materials through electrostatic force.5 However, the barrier properties of such materials will decrease during long-term use or under bending conditions, which restricts their application in the fields that require good flexibility and stretchability.
In this work, we prepared single-layer LDH nanosheets followed by surface modification with tannic acid (TA) (named as TA@LDH), which were alternately assembled with polyethylene oxide (PEO) to prepare hybrid films with stretchable gas barrier property. It is revealed that the combination of TA@LDH into PEO prolongs the diffusion path of the gas molecules, resulting in enhanced gas barrier properties. The ability to prevent cracking and preserve the gas barrier up to 120% elongation provides a tremendous opportunity for improving the barrier of elastomeric materials. After several times of stretching 100%, the film still has a high oxygen barrier property. The hydrogen bond results in more loose film structure, lighter crosslinking density, and larger strain without damage. Due to the abundant hydrogen bond sites on the LDH surface, stretching may lead to the transfer of hydrogen bond from one bonding site to another, rather than fracture. These highly elastomeric assemblies are potentially useful for light-weighting inflatable devices, tyres and wrapping.