NaHec undergoes the rare phenomenon of repulsive osmotic delamination, which is a thermodynamically allowed process, resulting in gentle and quantitative delamination into a nematic liquid crystalline suspension. Previously in our group, synthetic sodium fluorohectorite (NaHec, inter oct tetO 10F 2) clay has been used to create nanocomposite barrier coatings with various polymer matrices.
Silicate platelets act as an impermeable barrier to diffusion, creating a tortuous path for gas molecules, including oxygen and water vapor. In separate reports, clay nanocomposites have also been used to address the poor barrier properties of degradable polymers for food packaging. For these PLA/clay nanocomposites, an increase in biodegradation rate and reduction of lag time compared to PLA has been observed and is usually attributed to exposure to terminal hydroxylated edge groups.
While this helps disaggregation during melt compounding, delamination into single nanosheets of maximized aspect ratio is difficult, if at all possible. Since natural clays are hydrophilic, modification of the clay surface via ion exchange with alkyl ammonium compounds is necessary to disperse clay into degradable, hydrophobic polyesters. The incorporation of clay filler for this purpose stands out for its practicality and effectiveness as such layered silicate nanocomposites can provide additional improvement in the mechanical, barrier, and thermal properties. Various attempts to accelerate PLA degradation include optimizing its microbial environment or by blending it with other more degradable or water-soluble polymers, which increases surface area but ultimately worsens mechanical properties. Nevertheless, PLA is a weak competitor to typical packaging materials in terms of gas barrier properties, which are necessary to prevent spoilage and extend the shelf-life of packaged food. Therefore, PLA offers a limited solution to the accidental release of plastic into our water systems whose plastic litter is disproportionally composed of packaging. While PLA meets ASTM standards for compost degradation, numerous reports have shown it to have poor degradation in soil, seawater, and freshwater. Poly(lactic acid) (PLA) is a frontrunner for this position due to several favorable properties, including availability and low cost, zero toxicity, high mechanical strength, ease of production, and ability to derive its raw material from renewable (bio-)resources. After all, mitigating potentially dangerous effects from micro- and nanoplastics in our ecosystem begins with material design and waste management. However, with 8 million tons of plastic waste entering the ocean each year, an increasing effort is being made to replace those plastics that would persist in the environment when improperly disposed with degradable alternatives. Single-use food packaging has become an essential part of our lifestyle, for not only consumer convenience and hygiene but also to prevent food waste as goods are transported globally.
Accelerated degradation is observed under controlled enzymatic conditions and in an environmentally relevant wastewater medium during CO 2 evolution testing. Additionally, the film shows surprisingly high resistance to swelling at elevated humidity, but once soaked in water, clay swelling is triggered, which fragments the film and drastically increases the surface area by 2500%. This unique self-assembled nanostructure combines the best of two worlds: The aspect ratio remains high and results in a 99.3% reduction in oxygen permeability. Upon drying, thermodynamics drive the suspension toward segregation into sublayers of PLA and partially restacked 18C6Hec in situ. Here a synthetic, large aspect ratio Na-Hectorite is used that may be utterly delaminated in an organic solvent and composited with PLA by modification with 18-crown-6 (18C6Hec), yielding a castable, homogeneous nematic suspension. Compositing with clay has been used to improve barrier properties according to tortuous path theory. Additionally, PLA suffers from very poor barrier properties, which could result in food spoilage. However, PLA has shown to not be degradable in some environmentally relevant conditions, including within the freshwater systems. Conventional biodegradable polymers such as poly(lactic acid) (PLA) are an attractive alternative to replace traditional nondegradable food packaging films which plague the environment.
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