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.