Cellulose in ionic liquid solution as a potential synthetic polymer substitute

Cellulose is a naturally-occurring polymer that, when dissolved, can be fashioned into useful forms such as fibers, films, gels or beads. It can also be combined with other substances to produce composites. The most widely used industrial solvent for cellulose is cupriethylenediamine hydroxide, or Cuen. However, this strongly alkaline solvent tends to depolymerize cellulose, leading to inaccurate measurements.

Utomo et. al report measurements of cellulose’s molecular weight using ionic liquids (ILs) as the solvent. These are promising “green” solvents since they can be recycled and reused many times. Adding water or other non-solvents to the cellulose/IL solution causes cellulose to crystallize, allowing the removal of the non-solvent and recycling of the IL.

The authors determined the molecular weight of cellulose from viscosity measurements. Once the viscosity is known, a previously reported empirical relation is used to calculate the molecular weight of cellulose.

While ILs are promising alternative solvents for cellulose, previous experiments showed a puzzling temperature dependence of the intrinsic viscosity for cellulose/IL solutions. In this work, the investigators showed that the temperature dependence occurred only for certain rheological systems used to measure the viscosity. They attributed this effect to adsorption of the cellulose at the boundary between the solution and air, which creates a viscoelastic interface.

The interfacial and bulk contributions to the total viscosity are additive and can be experimentally decoupled.

“By using different rheometer geometries – parallel plate, concentric cylinder, and double wall ring – we were able to decouple the interfacial contribution from the total rheological response,” said author Daniele Parisi. “Furthermore, by knowing the bulk contribution and rescaling the interfacial response, it is possible to obtain the expected rheological response for any geometry.”

Source: “Determination of intrinsic viscosity of native cellulose solutions in ionic liquids,” by Nyalaliska W. Utomo, Behzad Nazari, Daniele Parisi, and Ralph H. Colby, Journal of Rheology (2020) The article can be accessed at https://doi.org/10.1122/8.0000015 .