While antioxidants are widely known
as natural components of healthy
food and drinks or as additives to commercial polymer materials to
prevent their degradation, recent years have seen increasing interest
in enhancing the antioxidant functionality of newly developed polymer
materials and coatings. This paper provides a critical overview and
comparative analysis of multiple ways of integrating antioxidants
within diverse polymer materials, including bulk films, electrospun
fibers, and self-assembled coatings. Polyphenolic antioxidant moieties
with varied molecular architecture are in the focus of this Review,
because of their abundance, nontoxic nature, and potent antioxidant
activity. Polymer materials with integrated polyphenolic functionality
offer opportunities and challenges that span from the fundamentals
to their applications. In addition to the traditional blending of
antioxidants with polymer materials, developments in surface grafting
and assembly via noncovalent interaction for controlling localization
versus migration of antioxidant molecules are discussed. The versatile
chemistry of polyphenolic antioxidants offers numerous possibilities
for programmed inclusion of these molecules in polymer materials using
not only van der Waals interactions or covalent tethering to polymers,
but also via their hydrogen-bonding assembly with neutral molecules.
An understanding and rational use of interactions of polyphenol moieties
with surrounding molecules can enable precise control of concentration
and retention versus delivery rate of antioxidants in polymer materials
that are critical in food packaging, biomedical, and environmental
applications.
We report on mechanically
strong, water-insoluble hydrogen-bonded nanofiber mats composed of
a hydrophilic polymer and a natural polyphenol that exhibit prolonged
antioxidant activity. The high performance of fibrous mats resulted
from the formation of a network of hydrogen bonds between a low-molecular-weight
polyphenol (tannic acid, TA) and a water-soluble polymer (polyvinylpyrrolidone,
PVP) and could be precisely controlled by the TA-to-PVP ratio. Dramatic
enhancement (5- to 10-fold) in tensile strength, toughness, and Young’s
moduli of the PVP/TA fiber mats (as compared to those of pristine
PVP fibers) was achieved at the maximum density of hydrogen bonds,
which occurred at ∼0.2–0.4 molar fractions of TA. The
formation of hydrogen bonds was confirmed by an increase in the glass-transition
temperature of the polymer after binding with TA. When exposed to
water, the fibers exhibited composition- and pH-dependent stabilities,
with the TA-enriched fibers fully preserving their integrity in acidic
and neutral media. Importantly, the fiber mats exhibited strong antioxidant
activity with dual (burst and prolonged) activity profiles, which
could be controlled via fiber composition, a feature useful for controlling
radical-scavenging rates in environmental and biological applications.
We explore synthetic polymeric antioxidants as powerful inhibitors of corrosion of metals. Antioxidant copolymers were designed to have polyphenol, gallol, or catechol moieties along with hydrophobic groups brought by hexyl methacrylamide units. The polymers were deposited on aluminum alloy 2024-T3 as submicrometer thick films of individual polymers or as 10 wt % additives to epoxy-based ∼100 μm thick coatings. Thin antioxidant polymer films provided significant corrosion protection of AA2024. Unlike poly(hexyl methacrylamide), which lacked polyphenol functional units, the antioxidant polymers were compatible with standard epoxy coatings and provided exceptional protection of surfaces in extreme corrosion conditions (0.6 M NaCl solution) for over 100 days. Anticorrosion performance of antioxidant-containing coatings was characterized using scanning electron microscopy, optical microscopy, and electrochemical impedance spectroscopy (EIS) and further evaluated with the equivalent electrical circuit analysis of the EIS data. Importantly, unlike small-molecule analogues, such as tannic acid, antioxidant copolymers were not leachable from epoxy coatings into water or ethanol. Taken together, these results show that antioxidant copolymers are efficient materials for protection of metals against corrosion.
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