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Abstract
<p class="first" id="d4670134e193">Nanocellulose based gas barrier materials have
become an increasingly important subject,
since it is a widespread environmentally friendly natural polymer. Previous studies
have shown that super-high gas barrier can be achieved with pure and hierarchical
nanocellulose films fabricated through simple suspension or layer-by-layer technique
either by itself or incorporating with other polymers or nanoparticles. Improved gas
barrier properties were observed for nanocellulose-reinforced composites, where nanocellulose
partially impermeable nanoparticles decreased gas permeability effectively. However,
for nanocellulose-based materials, the higher gas barrier performance is jeopardized
by water absorption and shape deformation under high humidity conditions which is
a challenge for maintaining properties in material applications. Thus, numerous investigations
have been done to solve the problem of water absorption in nanocellulose-based materials.
In this literature review, gas barrier properties of pure, layer-by-layer and composite
nanocellulose films are investigated. The possible theoretical gas barrier mechanisms
are described, and the prospects for nanocellulose-based materials are discussed.
</p>
One of the most ubiquitous and long-lasting recent changes to the surface of our planet is the accumulation and fragmentation of plastics. Within just a few decades since mass production of plastic products commenced in the 1950s, plastic debris has accumulated in terrestrial environments, in the open ocean, on shorelines of even the most remote islands and in the deep sea. Annual clean-up operations, costing millions of pounds sterling, are now organized in many countries and on every continent. Here we document global plastics production and the accumulation of plastic waste. While plastics typically constitute approximately 10 per cent of discarded waste, they represent a much greater proportion of the debris accumulating on shorelines. Mega- and macro-plastics have accumulated in the highest densities in the Northern Hemisphere, adjacent to urban centres, in enclosed seas and at water convergences (fronts). We report lower densities on remote island shores, on the continental shelf seabed and the lowest densities (but still a documented presence) in the deep sea and Southern Ocean. The longevity of plastic is estimated to be hundreds to thousands of years, but is likely to be far longer in deep sea and non-surface polar environments. Plastic debris poses considerable threat by choking and starving wildlife, distributing non-native and potentially harmful organisms, absorbing toxic chemicals and degrading to micro-plastics that may subsequently be ingested. Well-established annual surveys on coasts and at sea have shown that trends in mega- and macro-plastic accumulation rates are no longer uniformly increasing: rather stable, increasing and decreasing trends have all been reported. The average size of plastic particles in the environment seems to be decreasing, and the abundance and global distribution of micro-plastic fragments have increased over the last few decades. However, the environmental consequences of such microscopic debris are still poorly understood.
This critical review provides a processing-structure-property perspective on recent advances in cellulose nanoparticles and composites produced from them. It summarizes cellulose nanoparticles in terms of particle morphology, crystal structure, and properties. Also described are the self-assembly and rheological properties of cellulose nanoparticle suspensions. The methodology of composite processing and resulting properties are fully covered, with an emphasis on neat and high fraction cellulose composites. Additionally, advances in predictive modeling from molecular dynamic simulations of crystalline cellulose to the continuum modeling of composites made with such particles are reviewed (392 references).
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