A team from McMaster University has created polymer functionalized, pH-sensitive cellulose nanocrystals (CNCs) that could have applications in fields as disparate as biocomposites and wastewater treatment.
Cellulose from wood pulp has both amorphous and crystalline regions. When properly digested with acid, the crystalline regions form nanocrystals about 100-200 nanometres long and five to ten nanometres wide. These tough little particles could impart their strength to polymer composites if mixed in during the manufacturing process. Unfortunately, because CNCs are hydrophilic, they are often repelled by the hydrophobic polymers and form clumps instead of well-dispersed mixtures.
One solution is to attach bulky polymers to the CNC surface, which both prevents them from sticking together and makes them more hydrophobic. Polymers can be grafted onto regular cellulose by treating it with ceric ammonium nitrate, which turns part of the cellulose backbone into hydroxyl (OH) radicals. These radicals then serve as the initiators for polymer chain growth. “The process is known to be pretty easy to do industrially,” says Emily Cranston, a professor of chemical engineering at McMaster University. “But until now everyone thought that it only works on amorphous cellulose.” In a paper recently published in Biomacromolecules, Cranston and her team demonstrated for the first time that it is indeed possible to use ceric ammonium nitrate as the initiator to grow polymer chains on the surface of CNCs, improving their compatibility in composites.
But there’s another twist: the polymer they chose to add, poly (vinylpyridine), is sensitive to pH, a property that was passed on to the CNCs. Below pH 5, they remain dissolved in water and appear clear. Above pH 5, they come crashing out as a cloudy precipitate. This ‘switchable solubility’ could be useful in removing pharmaceutical compounds from wastewater, something current treatment methods can’t do. If the CNCs could be made to bind to such compounds, a simple rise in pH is all it would take to pull the entire complex out of solution. Cranston says the team is already working on this, as well as ways to rinse off and re-suspend the CNCs. “If we can do that, we’d have a recyclable system, which would be really nice,” she says.