SusChEM: Rheology of Cellulose and other Biopolymers in Ionic Liquids

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AbstractNon-Technical: Cellulose is the most abundant renewable polymeric natural resource on our planet. For many years cellulose has been chemically modified to make it water-soluble or at least water-dispersible, enabling solution processing of cellulose. However, the modifications are expensive and inhibit the crystallization of cellulose that imparts final mechanical properties. Ionic liquids are nonvolatile solvents and recently, certain ionic liquids have been found to dissolve native cellulose. There is a large inductrial research effort aimed at fiber spinning and other solution processing operations on unmodified cellulose with companies developing large patent portfolios on cellulose/ionic liquid solutions. In support of such technologies, a fundamental study of the flow properties of cellulose solutions in ionic liquids is planned. Since there are several unusual aspects of cellulose/ionic liquid rheology, including a seemingly large effect of trace amounts of water, this research may be transformative and enabling. This study will provide a basis for understanding the rheology of native cellulose, enabling it to be processed in ionic liquid solutions. This green processing scheme allows cellulose to be processed without chemical modification, which is already known to produce fibers with superior mechanical properties, and ionic liquids are nonvolatile and routinely water-extracted from the cellulose to be 99% recovered without use of any volatile organic solvents. Consequently, ionic liquid solutions may provide a viable pathway to high modulus polymer products from renewable resources that are non-petroleum based.Technical: At a given concentration, ionic liquid solutions have higher viscosity than aqueous solutions and are slightly more elastic (longer relaxation time); both are advantageous for stability of coating and fiber spinning operations. The planned rheology experiments will thoroughly characterize the viscoelastic response of cellulose in three ionic liquids (one that appears to be a theta-solvent and another that preliminary data suggests to be a good solvent) over wide ranges of temperature and concentration. While the concentration dependences of viscosity, relaxation time and terminal modulus of dry cellulose/ionic liquid solutions in linear response have the expected scalings, the linear viscoelasticity is quite sensitive to small quantities of water. Additionally, unexpected results are observed in stronger shear flows relevant to coating, fiber spinning and other solution processing; the shear viscosity is significantly larger than the linear viscosity from oscillatory shear, whereas conventional polymer solutions either show the two to be identical or find the shear viscosity is lower from chain alignment in shear. This will be explored in detail using X-ray scattering in shear flow to detect alignment of cellulose chains. The PI and his team will also begin to explore the solution rheology of other polysaccharides (chitin/chitosan, the second-most abundant polymeric natural resource on the planet) in ionic liquids.