Durable Green Plastic from Cellulose
Green alternatives to plastics are often flimsy and not as durable as conventional plastics. Cellulose, the most abundant biopolymer, has previously been turned into a soft plastic, but now researchers have devised a way to make it into a durable and rigid plastic [1]. The new technique tweaks the structure of cellulose films made of nanoscale crystals in a way that allows the material to bend without breaking, a key property of plastics. The researchers believe that their technique could allow a wide range of nanomaterials—those with structure on the nanoscale—to be converted into eco-friendly plastics.
The word “plastic” refers to a material but also to its key property: the ability to be permanently deformed without breaking or snapping back. Making a polymer material into a plastic usually involves altering the polymers—either via heat or plasticizing agents—to make them slide over each other. The process is highly energy intensive and generates significant greenhouse gas emissions. In 2021, Kai Zhang of the University of Göttingen in Germany and his colleagues developed a substitute with a lower-energy footprint [2]. They modified the cellulose molecule and turned it into a so-called hydroplastic polymer—a material that becomes plastic when exposed to water at room temperature. The chemical modification allows water molecules access to the polymer network of cellulose, where they break intermolecular hydrogen bonds, allowing the polymer chains to move.
However, this process doesn’t work for hard plastics. For example, cellulose nanocrystals—crystalline rods with nanometer-scale diameters—have incredibly dense hydrogen networks that give the crystals high rigidity and durability. But this dense network also makes them impregnable to water molecules. Now Zhang and his colleagues have found a hydroplastification approach that works with cellulose nanocrystals. The crystals are assembled into a film, and the nanocrystals move within the film—there is no need for individual polymers to slide past one another. The 2- to 50-nanometer-wide spaces between nanocrystals are called mesopores; humidity can affect their sizes and, in turn, affect the mobility of the nanocrystals.
The researchers isolated cellulose from the shells of sea squirts using several different chemical procedures and cast them into thin films. They then ran a series of experiments where films were subjected to vapor training—a protocol of alternating 30-minute periods of 30% and 80% relative humidity, during which films were held in various conditions. For one film type, applying a stretching force, or stress, of 1.5 megapascals (MPa) continuously for about a day during vapor training led to a slight shortening of the film after the stress was released. But if the stress was 5 MPa, the same process led to a lengthening.
To explain these results, Zhang and his colleagues measured the effects of humidity on stiffness and on the amount of water condensation within the mesopores. They also recorded electron-microscope images. They found that the capillary pressure that sucks water into the mesopores also serves as a strong shrinking force on the films, forcing the nanocrystals to become denser and reducing the average mesopore size. If an external stretching stress is large enough, above about 3 MPa in the experiments, it can overcome the shrinking force and cause the film to lengthen. Whether a film shortens or lengthens, it undergoes plastic deformation rather than fracture, which was the team’s goal. The researchers also demonstrated the films’ ability to be permanently deformed in various shapes by wrapping them around cylindrical and cubic forms during vapor training. These films could then be reformed into new shapes with further vapor training, extending the life cycle of the plastic material.
The researchers observed that the length and surface chemistry of the nanocrystals affected the ease of plasticization. Longer nanocrystals have larger mesopores, which allows them to be more mobile. In contrast, the presence of surface charges—which appeared in some of the nanocrystal preparations—causes repulsion among the nanocrystals, making them harder to plasticize.
The researchers plan to fabricate larger samples to test the material’s suitability for practical applications, such as for packaging. Zhang says that there are also basic science questions to address. “We still do not fully understand how single water molecules bind to the surface of the crystal, impact the stress and strain, and turn the material into plastic,” he says.
“It is a clever way of reorienting a crystalline material,” says Lucian Lucia, a green-materials scientist at North Carolina State University. Harnessing the capillary pressure and mesopores in this way could also be important for applications such as nonlinear optics, he adds.
–Sachin Rawat
Sachin Rawat is a freelance science writer based in Bangalore, India.
References
- S. Liu et al., “Hydroplasticity of nanocrystal films induced by mesopore change,” Phys. Rev. X 16, 021026 (2026).
- J. Wang et al., “Hydroplastic polymers as eco-friendly hydrosetting plastics,” Nat. Sustain. 4, 877 (2021).