Relatively fast exposure times (5 s) to aqueous solutions were found to improve the gas barrier of clay-polymer thin films prepared using layer-by-layer (LbL) assembly. Contrary to the common belief about deposition time (i.e., the longer the better), oxygen transmission rates (OTRs) of these nano-brick-wall assemblies are improved by reducing exposure time (from 1 min to 5 s). Regardless of composition, LbL films fabricated using shorter deposition time are always thicker in the first few layers, which correspond to greater clay spacing and lower OTR. A quadlayer (QL) assembly consisting of three repeat units of branched polyethylenimine (PEI), poly(acrylic acid) (PAA), PEI and montmorillonite (MMT) clay is only 24 nm thick when deposited with 1 min exposure to each ingredient. Reducing the exposure time of polyelectrolytes to 5 s not only increases this film thickness to 55 nm but also reduces the oxygen transmission rate (OTR) to 0.05 cm3/(m2 day atm), which is 2 orders of magnitude lower than the same film made using 1 min exposures. A conceptual model is proposed to explain the differences in growth and barrier, which are linked to polyelectrolyte relaxation, desorption, and interdiffusion. The universality of these findings is further exemplified by depositing clays with varying aspect ratios. This ability to quickly deposit high-barrier nanocomposite thin films opens up a tremendous opportunity in terms of commercial-scale processing of LbL assemblies.
Unlike
ionically bonded or clay-loaded gas barrier thin films,
which easily crack when moderately stretched, hydrogen-bonded poly(acrylic
acid) (PAA)/poly(ethylene oxide) (PEO) multilayer thin films remain
crack-free. Even after 100% strain, these nanocoatings provide more
than a 5× reduction in oxygen transmission rate. This study shows
that the lowest modulus PAA/PEO thin film is obtained at pH 3, but
maintains a high barrier. A total of 20 PAA/PEO bilayers (367 nm thick)
on 1.58 mm rubber reduced the oxygen transmission rate by 1 order
of magnitude. Stretching from 25–100% caused plastic deformation
and reduced gas barrier, but the oxygen transmission rate remained
at least 5× lower than the uncoated rubber. The ability to prevent
cracking and preserve the gas barrier up to 100% strain provides a
tremendous opportunity for reducing weight and improving the barrier
of elastomeric materials.
Hydrogen bonded poly(acrylic acid) (PAA)/poly(ethylene oxide) (PEO) layer-by-layer assemblies are highly elastomeric, but more permeable than ionically bonded thin films. In order to expand the use of hydrogen-bonded assemblies to applications that require a better gas barrier, the effect of assembling pH on the oxygen permeability of PAA/PEO multilayer thin films was investigated. Altering the assembling pH leads to significant changes in phase morphology and bonding. The amount of intermolecular hydrogen bonding between PAA and PEO is found to increase with increasing pH due to reduction of COOH dimers between PAA chains. This improved bonding leads to smaller PEO domains and lower gas permeability. Further increasing the pH beyond 2.75 results in higher oxygen permeability due to partial deprotonation of PAA. By setting the assembling pH at 2.75, the negative impacts of COOH dimer formation and PAA ionization on intermolecular hydrogen bonding can be minimized, leading to a 50% reduction in the oxygen permeability of the PAA/PEO thin film. A 20 bilayer coating reduces the oxygen transmission rate of a 1.58 mm natural rubber substrate by 20 ×. These unique nanocoatings provide the opportunity to impart a gas barrier to elastomeric substrates without altering their mechanical behavior.
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