(769c) Ionic Liquids: Green Solvents for Dry Native Cellulose and Chitosan

Cellulose and chitosan (the deacetylated form of chitin) are the most abundant renewable polymeric natural resources on our planet. Native cellulose and chitosan can dissolve in certain ionic liquids. Having nearly zero vapour pressure and being almost completely recyclable make these liquids strong alternatives for the traditional solvents with quite large environmental impacts and derivative effects on the polymers. It is crucial to gain better understanding of the physical state of cellulose and chitosan in ionic liquids. We report solution rheology of six native cellulose samples of different molecular weight in two different ionic liquids. Based on the concentration dependences of viscosity and longest relaxation time, 1-ethyl-3-methylimidazolium acetate (EMImAc) and 1-butyl-3-methyl imidazolium chloride (BMImCl]) are θ-solvents for cellulose. The strange failure of the Cox-Merz rule (shear viscosity larger than linear complex vsicosity in the shear-thinning region) for cellulose solutions in ionic liquids suggests that cellulose is not a simple flexible polymer in solution. This was hypothesized to be related to intra-chain hydrogen bonding at shear rates that align and stretch the longest chains. Trace amounts of water compete for hydrogen bonds and can impart a yield stress to the solution rheology, which grows in magnitude as more water is incorporated. However, simply heating the solution to 80°C drives off the water and reverts the solutions to viscoselastic liquids with no yield stress. Rheology of five different chitosan samples (with deacetylation ratio ~75% and of different molecular weight) was also studied in EMImAc. Unlike those of dry cellulose, chitosan solutions contained non-crystalline aggregates, identified using linear viscoelastic tests. This was related to the chemical crosslinks present in the samples due to reaction between amine and hydroxyethyl groups on the chitosan backbone while being extracted from crustacean shells at elevated temperatures in strong base. The chitosan solutions were much less sensitive than the cellulose ones to the presence of water in the system. Upon adding water (<3 wt.%) to the system. the solutions of dry cellulose transition from having terminal behaviour to weak gels, whereas the chitosan solutions already show non-terminal characteristic and do not tend to change at small water content (<20 wt.%).