Ready to proof -- Clare 2/7/22

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CLJ revised on 2/7

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The alarming volumes of plastic waste in landfills require sustainable alternatives. Biobased polymers are a promising replacement, but performance challenges limit their introduction in a competitive, cost-driven market. One approach to increase the performance and functionality of biobased polymers is through addition of nanoparticles. In this work we demonstrate that strategic assembly of nanoparticles can yield new and interesting properties to the biopolymer matrix. For example, nanoassembly can be used to design waterborne biobased polymer nanocomposites with superhydrophobicity or UV-blocking properties. Detailed characterization and computation modelling have shown that the morphology of biobased polymers plays an important role in determining the assembly structures and the ultimate performance.

Introduction

Increasing amounts of plastic waste (348 million tons in 2017)^{1, 2} and food waste in retail (up to 133 billion pounds and 161 billion USD),³ require green solutions immediately. Biobased polymers are attractive alternatives to petroleum polymer systems, but poor performance in terms of mechanical properties, thermal stability and water sensitivity limit their integration into petroleum-dominated industries. Nanoparticles have been widely used to enhance the performance of petroleum polymers. Interestingly, few studies address nanoparticle integration in biobased polymer systems.^{4, 5} Less fundamental research in biobased nanocomposites is likely a result of inherently variable sources of biobased polymers, which make the results less reproducible.^6-11 Nanoparticles have been demonstrated to impart properties including antibacterial activity,^12-14 corrosion resistance,^{15, 16} strength,^{17, 18} and hydrophobicity^19-21 to biobased polymers. It has also been shown that biobased polymers can be used to design complex nanoparticle assembly structures including clusters,^{22, 23} networks,^24-26 and films.^27-29

The aforementioned structures can be formed in-situ (via sol-gel chemistry or hydrothermal/solvothermal methods)^30-33 or by simply blending the nanoparticles and matrix.^34-36 These nanostructures can afford new and unique properties to the biobased polymer matrix, together lessening environmental impact and fulfilling the necessary performance metrics.   

In this report, we take advantage of the morphology of biobased polymers and design unique nanoassembly structures for producing high-performance biobased coatings. Two examples are demonstrated here: transparent UV-blocking coating and superhydrophobic coating. Starch and cellulose with hydroxyethyl modifications (HEC and HES) are used in the scope of this work. Though chemically identical, cellulose exhibits a rod-like morphology while starch is a coil. The conformation of the chain has monumental effects on the assembly of nanoparticles within the matrix, and the properties that result. The nanoassembly structures formed by silica (SiO₂) and zinc oxide (ZnO) nanoparticles impart superhydrophobicity and UV-blocking properties (respectively) to the biobased polymer matrix.  Therefore, polymer morphology can be used to develop highly performing green nanocomposites whose nanostructures grant them a competitive edge against traditional plastics.

Results

UV-Blocking Coating

The UV-blocking coating is composed of three elements: ZnO nanoparticle (30 nm, 0.8% wt.), biobased binder (HEC or HES) (4% wt.), and a chemical dispersant (Tween 20) (0.5% wt.). ZnO was chosen here for its unique band gap, which promotes UV-blocking in the UVA range.⁶

Using small nanoparticles helps produce a transparent UV-blocking film. It was found that the choice of polymeric binder drastically changes the UV-blocking capacity of the coating, with HEC largely outperforming HES (Figure 1) with impressive visible transparency.⁷

SEM revealed the presence of loosely branched particle assemblies with HEC, and small, dense aggregates with HES (Figure 2), suggesting that the functionality of the coating is highly dependent on nanoparticle aggregation pattern. This result is surprising considering the similarities between polymer molecular structures. HEC and HES are chemically identical, only differing by their intermolecular bonds. HES takes on a coiled structure due to its cis glycosidic linkage. HEC, on the other hand, is rod-like as a result of its trans bonds (Figure 2). The variation in polymeric structure clearly impacts on the assembly of nanoparticles within the matrix.

Superhydrophobic Coating

Silica dioxide nanoparticles form similar structures with HEC and HES at the same particle loading. However, when larger quantities of silica (3% wt.) are introduced in place of zinc oxide in the HEC and HES polymer systems, new capabilities were introduced to the coating system. Upon vaporized treatment of fluorinated silane on the dried coating surface, superhydrophobicity was achieved. Without nanoparticles, the silane treatment is shown to form small clusters on the polymer surface. These clusters appear larger in HEC than in HES. When nanoparticles are included in the treated coating, the cellulose polymer introduces multiscale roughness (with branched features), while the starch matrix only disperses the particles. The roughness post silane treatment with and without nanofiller is quantified via confocal and AFM techniques, respectively (Figure 3).      

The roughness and assembly features largely alter the water repellency of the composite, with HEC achieving a 160° contact angle, outperforming HES by 20°. This is a significant improvement compared to the polymer without nanofiller (Figure 4). The structures formed by HEC are more robust than the HES derivative, sustaining the water repellency even after being immersed in water for 1 day.

The network structure formed by HEC has also been demonstrated to enhance the adhesion of the coating to the substrate. Even after immersion, the silane-treated HEC-silica coating is robust to adhesion testing via the crosshatch adhesion test (Figure 5). The untreated HEC shows adhesive strength prior to immersion, but after immersion little coating remains to be tested. HES, on the other hand, demonstrates poor adhesion, even after silane treatment (Figure 5).

Performance Highlights

Polymer chain conformation was used to mediate unique assembly structures of nanoparticles. The rod-like conformation of HEC formed a porous branched network, and the coil-like conformation of HES formed a dispersion of aggregates. The network assembly yielded a number of favorable properties based on nanoparticle selection.

• ZnO nanoparticles with HEC

 - Highly efficient UV-blocking (95%) while maintaining 80% visible transparency

 - Ultrathin thickness of 200 nm

• SiO₂ nanoparticles with HEC

 - Superhydrophobicity due to multiscale roughness from the silane treatment and nanoassembly     pattern

 - Maintained performance post immersion in water

 - Strong adhesion post crosshatch test

Conclusion

Through this study, we demonstrate that biobased polymer morphology can be used to influence nanoparticle assembly structures. High-performing coatings can be fabricated with water-dispersible biobased polymers. The results bridge the assembly structures at the nanoscale, influenced by molecular conformation of biobased polymers, to the coating performance at the macroscopic level. Cellulose-derived biobased polymers HEC has created a unique network structure of nanoparticle assemblies, which has proven to be beneficial to enhance the functionality and performance of biobased coatings. Superhydrophobicity and adhesive strength are demonstrated with silica nanoparticles and UV blocking with zinc oxide nanoparticles. In addition, these structures are generalizable to different types of nanoparticles, which may inspire more innovative coating designs in future. Through this study we unveil new opportunities in economical and sustainable development of high-performance biobased materials. The potential applications may impact broad fields such as medical, civil, automotive and aerospace.