A pinboard by
Behzad Nazari

Postdoctoral Associate , Massachusetts Institute of Technology


Trying to find green ways to process green materials (i.e. minimizing environmental impacts)

Cellulose and chitosan (the deacetylated form of chitin) are the most abundant renewable polymeric natural resources on our planet. Despite our long history with cellulose and chitin (both discovered over 170 years ago), while the underground carbon source keeps shrinking and concerns toward their environmental impact continue to grow, we now live in a cellulose-chitin renaissance, reconsidering new applications and processes for these two green polymeric materials. This puts an emphasize on the importance of research on finding new solvents for these materials since both polymers only get processed via wet-spinning and not in molten form. 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 (η>η* 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 adequate for polymer conformational ordering. 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 in the IL 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 manufactured at elevated temperatures.