| dc.description.abstract | High oxygen barrier materials are becoming increasingly important for food
packaging, pressurized systems, and flexible electronics protection. The widely used
traditional aluminized plastics and inorganic gas barrier coatings (SiOx and AlxOy) exhibit
low transparency and/or flexibility. Layer-by-layer assembly (LbL) provides a costeffective
and environmental-friendly alternative, with lower oxygen permeability and
greater transparency. This dissertation focuses on improvements of the coating process
and development of new functionalities. Four studies are discussed in detail on this topic
and two future research directions are also introduced.
Layer-by-layer assembled multilayer nanocoatings have been shown to provide
excellent oxygen barrier to poly(ethylene terephthalate) (PET) film, which is commonly
used for encapsulation and packaging. Polymer-clay (polyethylenimine (PEI)/vermiculite
(VMT)) and all-polymer ((PEI)/poly(acrylic acid) (PAA)) multilayer systems are shown
to be equally beneficial as barrier coatings for polyolefin substrates (e.g., polyethylene and
polypropylene), which suffer from a high oxygen transmission rate. A 30 bilayer
PEI/VMT nanocoating reduces the OTR of biaxially oriented polypropylene (BOPP) by
more than 160X, rivaling most inorganic coatings. WVTR is simultaneously reduced by
43%.
In addition to VMT, montmorillonite (MMT) clay has been widely used to prepare
gas barrier nanocoatings. In an effort to produce high oxygen barrier with fewer deposition
steps, pH of the MMT aqueous suspension was reduced. In a PEI/PAA/PEI/MMT
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quadlayer system, the reduced pH of MMT causes the preceding PEI layer to be more
charged, which results in more clay deposited. A compromise between high
polyelectrolyte diffusion (high pH) and high clay deposition (low pH) was found at pH 6,
where the best oxygen barrier is obtained. A 179 μm thick PET substrate, coated with just
three quadlayers (44 nm thick), exhibits an undetectable oxygen transmission rate (< 0.005
cm3/(m2 day atm)).
In another attempt to achieve better oxygen barrier of the PEI/MMT bilayer (BL)
system, a “salty clay” strategy was carried out, where an “indifferent” electrolyte (NaCl)
was introduced into the MMT suspension. NaCl alters the ionic strength of the clay
suspension and subsequently changes the rheological behavior of clay platelets. As a
result, a multilayer film with a more tortuous path and better clay coverage (and alignment)
is produced. Lower OTR is achieved as a result of this structural engineering. An 8 BL
PEI/MMT film, assembled with a clay suspension containing 5 mM NaCl, exhibits an
order of magnitude reduction in OTR relative to its no-salt counterpart.
The ability to self-repair after damaging (e.g., cracking) and restore original
properties is highly desired for polymers and composites. The PEI/PAA nanocoating has
the ability to self-heal after being cracked by stretching. Cracks in the film lead to loss of
oxygen barrier. High humidity is employed as an external healing stimulus to repair these
cracks. The OTR of a PEI/PAA multilayer nanocoating remains below detection after 10
stretching-healing cycles, which shows this healing process to be highly robust. | en |
