Cellulosic materials, which come from the cell walls of plants, have emerged as attractive, sustainable replacements for traditional plastics. However, the moisture sensitivity of cellulose and its incompatibility with many soft hydrophobic polymers are challenges to their widespread application. From a materials design perspective, gaining the benefit of both hydrophilic cellulose and hydrophobic polymers at the same time without any chemical treatment of raw materials is mystifying. It will be of desirable to have interface with a third component, having favorable interactions with both cellulose and soft polymers such as polycaprolactone. To address these perplexing challenges, we suggest a nature-inspired solution exploiting the inherent properties of cellulose nanofibrils and another natural polymer, lignin. The conversion of crude lignin into colloidal lignin nanoparticles overcomes the hurdles commonly posed by its complex structural heterogeneity. In a new study, by bioproduct researchers from Aalto University in Finland led by professor Monika Österberg demonstrated that lignin nanoparticles with their well-defined morphology and active surface sites can interact with both cellulose, in this case cellulose nanofibrils, and polycaprolactone and act as a compatibilizer between hydrophilic cellulose and hydrophobic polycaprolactone. Although it looks complex, the solution is simple. The research work is now published in Advanced Materials Interfaces.
The research team first, dissolved polycaprolactone dissolved in an organic solvent is mixed with the lignin nanoparticles in water. The lignin particles assembles at the oil water interface and stabilize the emulsion. Emulsions stabilizes with solid particles are called Pickering emulsions. This emulsion is then mixed with aqueous cellulose nanofibrils suspension prior to film formation. This Pickering emulsion strategy creates an even dispersion of a polymer within the cellulose network, increasing the wet strength and water resistance of the composite, meanwhile retaining all the positive characteristics of the cellulose fibers or fibrils. The outcomes are excellent: the developed composite has a higher strength than pure CNF nanopaper or pure polymer in both dry and wet conditions, even after fully immersing it in water for a day.
The research team demonstrated that the Pickering emulsion stabilized with lignin nanoparticles approach can provide a convenient platform to design nanocomposite systems incorporating hydrophobic polymer and hydrophilic cellulose nanofibrils, overcoming the high interfacial tension between hydrophilic cellulose nanofibrils and hydrophobic polymers with the aid of amphiphilic colloidal lignin nanoparticles. This strategy allows the even dispersion of polycaprolactone onto the cellulose nanofibrils network inducing waterproof characteristics to the cellulose nanofibrils -based nanocomposite films and enabling synergistic effects of polycaprolactone and cellulose nanofibrils, resulting in composites with higher tensile strength and toughness than pure cellulose nanofibrils nanopapers in both dry and wet conditions. To elucidate the mechanism of colloidal lignin nanoparticles as the interfacial mediator between cellulose nanofibrils and polycaprolactone, the authors characterized the surface and bulk structures of nanocomposite films. Furthermore, they show that the inclusion of colloidal lignin nanoparticles induces additional functionality, such as UV shielding and antioxidant properties to the developed free-standing films, making them interesting for packaging applications. Overall, the new study offers a facile route to design multifunctional cellulose-based nanocomposites using only biodegradable polymers and lignocellulosic nanoparticles by controlling the interfacial interactions.
The authors achieved a wet strength up to 87 MPa for the composite, the highest obtained wet strength for cellulosic composites developed without any direct covalent surface modifications or synthetic additives. Furthermore, the new strategy added additional functionality, such as UV shielding and antioxidant properties to the developed composites, making them interesting for packaging applications.
In a nutshell. The Bioproduct Chemistry team at Aalto University have designed a sustainable method to produce strong and flexible cellulosic films that incredibly maintain their strength even when wet. The material is made through an innovative combination of wood-based and biodegradable polymers without any chemical modification, harnessing the maximum benefit of each component. For experts in the field, this approach opens new possibilities to eliminate the need for cellulose chemical modification to impart new functionalities, promoting the sustainable use of natural resources from the forest. Furthermore, the new study offers a generic foundation for combining hydrophilic cellulose with varied hydrophobic soft polymers to design multifunctional cellulose-based composites using only biodegradable polymers and lignocellulosic materials, taking a big step toward fully sustainable use of natural resources. As a follow up, the researchers are now exploring a broad framework to identify the sustainability of this early-stage technology in environmental and economic aspects by integrating techno-economic and life cycle assessments.
Erfan Kimiaei, Muhammad Farooq, Rafael Grande, Kristoffer Meinander, Monika Österberg. Lignin Nanoparticles as an Interfacial Modulator in Tough and Multi-Resistant Cellulose–Polycaprolactone Nanocomposites Based on a Pickering Emulsions Strategy. Advanced Materials Interfaces (2022). https://doi.org/10.1002/admi.202200988