Development of biodegradable nanostructured hydrophobic and superhydrophobic packaging films
Creators: name: Ghasemlou, Mehran
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 300604
subject: Food packaging, preservation and processing

description: Plastics pollution is recognized as one of the greatest current environmental challenges. The plastic-waste crisis is exacerbated when the plastics are inevitably broken down into smaller particles, littering both marine and terrestrial environments, with multifaceted impacts on human health. The environmental and healthcare footprints associated with conventional petroleum-based plastics have driven the implementation of bioplastics that are compostable and produced from renewable resources, and thus cause a smaller environmental impact. This PhD thesis not only provides a simple, scalable, and straightforward approach to developing multifunctional starch-based bioplastics with exceptional mechanical performance and waterproof capabilities, but also highlights their potential commercial developments by determining their underlying biodegradation behavior upon disposal in a soil environment. 

The first experimental chapter hypothesizes that the intermolecular hydrogen bonding between hydroxyl groups of starch and those of polyhydroxyurethanes (PHUs) predominantly contribute to improvements in the thermomechanical properties of their hybrid films. To accomplish this, a new class of green crystallizable PHUs was produced in-house through a facile, non-isocyanate, and catalyst-free route with step-growth polymerization between a cyclic carbonate and three different diamines (1,2-ethanediamine, 1,4-butanediamine, and 1,6-hexanediamine). The PHU monomers were then combined with starch to yield starch-PHU hybrid materials. The tunable mechanical properties of the starch-PHU hybrids indicated that the PHUs were moderately miscible with the starch matrix, and that this could be a probable reason for the observed improvement in the mechanical properties of the hybrids. With these encouraging outcomes, the work was expanded to the second experimental chapter to gain a better understanding on the strength and interfacial affinity between starch and PHUs. In this work, multiple advanced techniques on a molecular-level standpoint were used to probe the internal morphology and interfacial interactions between the starch and PHUs. The findings concluded that the hydroxyl groups of the starch backbone had good affinity in building a well-developed interface with functional groups of PHUs via the formation of noncovalent hydrogen bonds. However, it was demonstrated through AFM analysis that these two components became poorly interactive at the interface region, behaving like two immiscible phases when large concentrations of PHUs were reinforced into the starch matrix.

 The findings reported in the second experimental chapter set the stage for the next phase of the work: providing a new perspective on the unique merits of reinforcing nanoscale components in developing ternary nanohybrid comprised starch, PHUs, and cellulose nanocrystals (CNCs), with simultaneous improvements in both toughness and stiffness. CNCs are emerging as building blocks that can ideally serve as a valuable renewable resource for the production of high-performance materials. The synergistic effects between the starch-PHU and CNCs contributed to tailoring the microstructure and mechanical properties of their nanohybrids. The free-standing nanohybrid films showed no evidence of macroscopic phase separation and possessed remarkable optical transparency (&gt;80%) and an unprecedented tensile strength (up to 8.5 MPa). The multiple intramolecular hydrogen bonds at the interface between CNCs and starch-PHU, as confirmed by X-ray photoelectron spectroscopy (XPS) and Fourier transformation infrared and multinuclear NMR spectroscopy results concurrently facilitated energy dissipation and transferred the stress across the interfacial region. The formation of these highly stable associations between the individual components considerably improved the nanohybrid films’ thermal stability, giving a maximum degradation temperature of 335 °C. In spite of the undoubtedly high potential of these nanocomposites, their environmental aspects have equal importance to their industrial relevance, and thus should not be overlooked. 

The fourth experimental chapter aims to gain an insight into understanding the interactions and transformations of these nanocomposites in a natural soil and ambient environment. Overall, the soil microbiomes not only managed to effectively and quickly digest PHUs, but also led to pronounced degradation of their hybrids with starch (starch-PHU). The results discussed in this chapter revealed the exceptional biodegradability of starch-PHU hybrids, with mass losses of up to ~88% at a soil-burial time of only 120 days. While the presence of CNCs reduced the potential for the soil microbial community to degrade starch-PHU-CNC nanohybrids, microbially digested bioplastics with PHUs presented fewer stages of thermal degradation, and reduced the intensities of FTIR, NMR, and XPS signals compared to the original films, indicating improvement of the biodegradation mechanism. In view of a great demand for non-wettable bioplastics in packaging industries, the work on this theme continued to advance to the next phase, aiming to render hydrophobic characteristics to starch-PHU-CNC nanohybrids through a lotus-inspired strategy.

The fifth experimental chapter presents a facile and environmentally friendly biomimetic route to prepare mechanically robust superhydrophobic surfaces for deposition onto starch-PHU-CNC bioplastics with engineered multiscale hierarchical structures that mimic the surface architecture of lotus leaves. A direct soft-imprinting lithography onto starch-PHU-CNC films formed micro-scaled features resembling the pillar architecture of lotus leaves. Spin-coating was then used to assemble a thin layer of low surface-energy poly(dimethylsiloxane) (PDMS) over these microstructures. A further modification with hydrophobic functionalized silica nanoparticles contributed to the superhydrophobic properties. The fabricated hierarchical films with lotus-leaf-like micro/nanostructures had outstanding superhydrophobicity, with a water contact angle (WCA) of ∼150° and a sliding angle (SA) of &lt;10°. The resulting coatings resisted extensive knife-scratching and finger-rubbing, 20 cycles of sandpaper abrasion under 50 g loading, and tape-peeling for at least 10 repetitions without losing their superhydrophobic properties. Graphene oxide quantum dots (GOQDs) are new category of carbon-based nanomaterials that have gained recent interest due to their photoluminescence properties and biocompatibility features. Given their outstanding Young’s modulus, they offer an immense potential to be used as a toughening nanofiller.

The last experimental phase of the work presented in this thesis (Chapter 12) aimed to develop a one-pot and green route to dope silicon into a GOQD network. The Si-doped GOQD nanoconjugates had an average dimension of ~8 nm, with a graphite-structured core and amorphous carbon on their shells. We further used infrared nanoimaging based on scattering-type scanning near-field optical microscopy (s-SNOM) to reveal the spectral near-field response of the GOQD samples and to measure the nanoscale IR response of their networks. The conducted antibacterial tests on the Si-doped GOQD showed moderate antibacterial activity against E. coli and S. aureus bacterial cells. The results highlighted the potential usage of Si-doped GOQDs in different fields, especially biomedical applications. This chapter explored the development of a multifunctional nanoplatform through the co-assembly of silicon-doped GOQDs and PDMS onto starch-PHU-CNC nanohybrids using a facile spin-coating. Silicon resin PDMS was chosen as a coating host matrix because of its good mechanical ductility, which imparted flexibility to the substrate and served as an adhesive to tightly anchor Si-doped GOQDs to the surface. By co-assembling PDMS/Si-GOQD, followed by linking the resulting interconnected network to starch nanohybrids plastics, this work envisions the preparation of a new material capable of harnessing the unique merits of quantum-confined GOQDs into polymers, yielding concomitant improvements in thermal, mechanical, and morphological characteristics. The outcome of this chapter suggests the potential for exploring the fascinating possibilities of GOQD-functionalized polymers, which will help in increasing the scope of GOQDs for a wide range of applications such as bioimaging, drug delivery, gene delivery, sensing, and catalysis.

The systematic work outlined in this thesis thus lays the groundwork for future research, paving a feasible path for developing bionanocomposites with tunable optical, mechanical, and barrier properties. The findings of this thesis are expected to contribute to current knowledge on bioplastic packaging by fast-tracking the realization of the capabilities of starch-based bioplastics. This thesis can also serve as a toolkit to guide governmental agencies and local councils for the transition from synthetic plastics to a more renewable circular bioeconomy based on starch-based bioplastics.