Aspect Ratio Control of Layered Double Hydroxide Nanosheets and Their Application for High Oxygen Barrier Coating in Flexible Food Packaging

Hybrid organic-inorganic coating with enhanced oxygen- and UV-barrier performance: Polyelectrolyte complex based on sodium alginate, poly (vinyl alcohol), and reconstructed layered double hydroxide

Organic-inorganic hybrid materials with high oxygen- and UV-barrier properties were developed using a polyelectrolyte complex comprising sodium alginate (SA), poly (vinyl alcohol) (PVA), and reconstructed layered double hydroxide (RLDH). These materials were applied to poly (ethylene terephthalate) (PET) as a barrier coating layer at a harsh drying temperature of 120 °C, similar to environments for the industrial coating process. The RLDH nanoplatelets within the coating matrix restricted the polymer chain mobility, elevating the glass transition temperature to 105.222-159.114 °C. Below RLDH 0.2 %, the apparent coating density significantly increased to 0.93-0.94 g/cm3. The embedded RLDH gave a tortuosity within the matrix, as evidenced by an intensified (003) diffraction peak in the XRD analysis. These structural alterations contributed to high oxygen- and UV-barrier performance. Notably, the PET/SA1.0PVA0.5RLDH0.2 film exhibited an extremely low oxygen transmission rate of <0.005 cm3/m2·day, with effectively blocking UV-A (62.41 %), -B (92.45 %), and -C light (100 %). Moreover, the susceptibility of the coated film to water vapor was mitigated by laminating cast polypropylene, achieving a water vapor transmission rate of 1.17 g/m2·day. Overall, the packaging materials with advanced oxygen-, water vapor-, and UV-barrier properties show great potential for practical applications in various sectors, including food packaging and medical/electrical devices.

Separation of amino acids by selective sorption through reconstructive intercalation in calcined MgAl-layered double hydroxide

Mg-Al layered double hydroxide (MgAl-LDH) exhibits selectivity in the intercalation of amino acids (AAs). When the MgAl-LDH derived mixed metal oxide was treated with different mixtures of AAs, preferential sorption of one AA over the other(s) was observed as indicated by XRD analysis of the products and HPLC analysis of the interlayer AA contents in the products. The order of preference was aspartic acid, glutamic acid (acidic AAs) > glycine, alanine (neutral AAs) > hystidine, and arginine (basic AAs). Among the acidic AAs, aspartic acid was preferred over glutamic acid and among the basic AAs, histidine was preferred over arginine. LDH shows equal preference for glycine and alanine. The selectivity can be explained on the basis of the isoelectric pH (pI) of the AA. A similar selectivity order was obtained when the mixtures of AAs were treated with nitrate-intercalated LDH (direct anion exchange) although the net AA intercalated is much lower due to competition with carbonate derived from atmospheric CO₂. The high selectivity observed in some cases (such as aspartic acid and glycine) would result in the quantitative separation of the individual AAs from their mixture.

Aged layered double hydroxide nanosheet-polyvinyl alcohol dispersions for enhanced gas barrier coating performance

Whilst applying a coating layer to a polymer film is a routine approach to enhance the gas barrier properties of the film, it is counter-intuitive to consider that the gas barrier performance of the film would improve by ageing the coating dispersion for weeks before application. Herein, we report that the oxygen barrier performance of a 12 μm PET film coated with a dispersion of inorganic nanosheets in polyvinyl alcohol can be significantly enhanced by ageing this coating dispersion for up to 8 weeks before application. We found up to a 37-fold decrease in the oxygen transmission rate (OTR) of the PET coated film using aged dispersions of [Mg_0.66Al_0.33(OH)₂](NO₃)_0.33 layered double hydroxide nanosheets (Mg₂Al-LDH NS) in polyvinyl alcohol (PVA) compared to the film coated with an equivalent freshly prepared LDH/PVA dispersion. A limiting OTR value of 0.31 cc m^-2 day^-1 was achieved using the PET film coated with a 3 week aged LDH NS/PVA dispersion. X-ray diffraction experiments show that the degree of in plane alignment of LDH NS on the PET film surface increased significantly from 70.6 ± 0.6 to 86.7 ± 0.6 (%) (100% represents complete alignment of LDH NS platelets on the film surface) for the 4 week aged dispersion compared to the freshly prepared layer. We postulate that when the Mg₂Al-LDH NS are aged in PVA the coiled PVA aggregates start to unwrap and attach onto the Mg₂Al-LDH NS through hydrogen bonding and eventually form a hydrogen bonded ordered network that facilitates the alignment of nanosheet dispersions during the coating process. Our results suggest that the ageing of inorganic nanosheet dispersions in PVA or other potential hydrogen bonding adhesive systems could be a general approach to improve the alignment of the nanosheets on the polymer film surface once applied and thus improve their performance characteristics for barrier coating applications.

Finite Element Analysis of Gas Diffusion in Polymer Nanocomposite Systems Containing Rod-like Nanofillers

Polymer-based films with improved gas barrier properties are of great interest for a large range of applications, including packaging and coatings. The barrier effect is generally obtained via the addition of a sufficient amount of impermeable nanofillers within the polymer matrix. Due to their low environmental footprint, bio-based nanocomposites such as poly(lactic acid)-cellulose nanocrystal (PLA-CNC) nanocomposites seem to be an interesting alternative to synthetic-polymer-based nanocomposites. The morphology of such systems consists of the dispersion of impermeable rod-like fillers of finite length in a more permeable matrix. The aim of this work is to analyze, through finite element modeling (FEM), the diffusion behavior of 3D systems representative of PLA-CNC nanocomposites, allowing the determination of the nanocomposites' effective diffusivity. Parametric studies are carried out to evaluate the effects of various parameters, such as the filler volume fraction, aspect ratio, polydispersity, and agglomeration, on the improvement of the barrier properties. The role of the filler-matrix interfacial area (or interphase) is also investigated and is shown to be particularly critical to the overall barrier effect for highly diffusive interphases.

Cellulose nanocrystal-coated TEMPO-oxidized cellulose nanofiber films for high performance all-cellulose nanocomposites

High performance biopolymer films are of great interest as effective alternatives to non-biodegradable and petroleum-based polymer films. However, most natural biopolymer films possess weak mechanical and poor gas barrier properties, limiting their applicability. In this work, we developed all-cellulose nanocomposite films through a simple vacuum filtration process, using cellulose nanocrystals (CNCs) and 2,2,6,6-tetramethylpiperidine-1-oxy-oxidized cellulose nanofibers (TEMPO-CNFs). The TEMPO-CNFs were employed to construct a transparent, free-standing substrate matrix and the CNCs were used as a coating material to improve the mechanical and water vapor barrier properties of the final material. We have demonstrated that the top and bottom CNCs-coated TEMPO-CNF substrates (CNC/TEMPO-CNF/CNC) have excellent mechanical and good water vapor barrier properties. The resulting CNC/TEMPO-CNF/CNC films revealed a high tensile strength of 114 MPa and a low specific water vapor transmission rate (SWVTR) of 19 g∙mm/m²∙day. In addition, the CNC/TEMPO-CNF/CNC films were resistant to various solvents including water, ethanol, tetrahydrofuran (THF), and acetone. This type of high performance cellulose nanocomposite can be used as a renewable material for a broad range of potential applications.

Layered Double Hydroxides in Bioinspired Nanotechnology

Layered Double Hydroxides (LDHs) are a relevant class of inorganic lamellar nanomaterials that have attracted significant interest in life science-related applications, due to their highly controllable synthesis and high biocompatibility. Under a general point of view, this class of materials might have played an important role for the origin of life on planet Earth, given their ability to adsorb and concentrate life-relevant molecules in sea environments. It has been speculated that the organic–mineral interactions could have permitted to organize the adsorbed molecules, leading to an increase in their local concentration and finally to the emergence of life. Inspired by nature, material scientists, engineers and chemists have started to leverage the ability of LDHs to absorb and concentrate molecules and biomolecules within life-like compartments, allowing to realize highly-efficient bioinspired platforms, usable for bioanalysis, therapeutics, sensors and bioremediation. This review aims at summarizing the latest evolution of LDHs in this research field under an unprecedented perspective, finally providing possible challenges and directions for future research.