The petroleum-based plastics are widely used for food packaging applications because of their low cost, easy processability, and tunable properties to meet the specific needs of food packaging. However, these polymers are non-biodegradable, leading to a substantial amount of plastic waste in both land and marine ecosystems. Therefore, it is crucial to find biodegradable polymers from renewable resources to achieve the sustainability goals set by the United Nations. In this context, poly(lactic acid) (PLA) biopolymer is a potential candidate due to its biodegradability, low toxicity, and eco-friendly behavior; it also has excellent mechanical properties, transparency, and economic viability in comparison to many other biopolymers. However, despite the aforementioned benefits, PLA has disadvantages that limit its use in food packaging applications. These include inherent brittleness, poor melt strength, and moderate gas barrier performance. The primary objective of this thesis was to improve these properties of PLA by producing nanocomposites with biopolymer, processing aids, and nano-size reinforcement, as well as modifying by processing to meet the requirements for food packaging applications.

In this work, PLA and PLA-poly(hydroxybutyrate) (PHB) blend nanocomposites with chitin nanocrystal (ChNC) were prepared via a liquid-assisted extrusion process. Glyceroltriacetate (GTA), triethyl citrate (TEC), and lactic acid oligomer (OLA) were used as plasticizers/compatibilizers, dispersing, and processing aids. The effect of the addition of PHB, chitin nanocrystals (ChNCs), and dispersing agents on the properties of PLA was studied. The effect of different processing techniques, such as iso-thermal crystallization as well as the melt-state and solid-state drawing on the properties of the PLA nanocomposites were also investigated. In addition, the influence of ChNCs and liquid assisted extrusion on the processing and properties of blown films were assessed.

The results of the first study demonstrated that the dispersion and distribution of ChNCs in the PLA matrix progressively improved with increasing TEC dispersing aid content, with the effect being most pronounced in the nanocomposite containing 15 wt% plasticizers. PLA with 15 wt% of TEC resulted in enhanced flexibility and toughness, but negatively influenced its mechanical and thermal properties; however, the incorporation of 1 wt% ChNCs minimizes these effects. In the second study, it was shown that the polymer chain orientation of PLA/ChNC nanocomposite achieved via a combination of melt state and solid-state drawing resulted in a material with excellent mechanical properties, including an increase in toughness of nearly 100-fold compared to that of unoriented nanocomposite film. The orientation of the nanocomposite also enhanced the material's crystallinity. In the third and fourth studies, it was found that the crystallinity of PLA was increased by using an isothermal crystallization process and the addition of 25 wt% PHB. The crystallinity was further enhanced by the addition of a very small amount of ChNC (1 wt%), which acted as a nucleation agent, resulting in a faster crystallization rate and enhanced crystallinity in both cases. The nanocomposites PLA/ChNC or PLAPHB/ChNC with ChNCs, higher crystallinity, and/or orientation created a more tortuous path for gas molecules resulting in significant improvements in the O2 and CO2 barrier performance. In the final study, it was demonstrated that the PLA-PHB/ChNCs nanocomposite produced by liquid-assisted extrusion exhibited a stable process during the film-blowing operation and exhibited smooth and homogenous surface film compared to the nanocomposite produced via conventional melt compounding. Moreover, the blown film exhibited comparable mechanical properties with petroleum polymers and also degraded within 45 days under standard composting conditions.

In conclusion, this thesis shows that the properties of PLA can be tailored through the composition of the blend and nanocomposite, or during the processing of the material to make it suitable for food packaging applications. It was also demonstrated that the processing technique in this study can be a step forward for the large-scale production of bionanocomposites.