(607i) Interfacial Phenomena in Rheology of Cellulose/Ionic Liquid Solutions

It is crucial to fully understand the physical state of cellulose, as the most abundant bio-polymer on Earth, in ionic liquids (ILs). Attributable to low volatility and nearly complete solvent recovery, cellulose/IL solutions are considered green. If cellulose/IL systems are to facilitate the bio-polymer use in real-world products and operations, such as fibers, films, aerogels, etc, characterizing how cellulose behaves at the air-liquid interface is important. By means of small amplitude oscillatory shear measurements, using different geometries such as cone partitioned plate, parallel plates, and double-wall ring, it is demonstrated that the total rheological response of the cellulose/IL solution is simply the sum of bulk solution and interfacial contributions, and we show that those contributions can be separated. The contribution from the interface is particularly important for dilute solutions. The intrinsic viscosity [𝜂] in ethyl methyl imidazolium acetate (EMImAc) is measured using a gravity-driven glass capillary viscometer (with virtually zero interfacial effects) and found to be independent of temperature in the range 30–80℃ (−𝑑ln[𝜂]/𝑑𝑇≅0.001/℃). The measurements from concentric cylinders and cone-plate geometries showed −𝑑ln[𝜂]/𝑑𝑇≅0.009 and 0.021/℃, respectively. This was in line with the difference in the interfacial length scale 𝐾 for each geometry, determining the relative importance of bulk and surface contributions (𝐾 ~ 1.5, 3.5, and ∞ for cone-plate, concentric cylinders, and gravity-driven glass capillary geometries, respectively). It is demonstrated that cellulose adsorbs at the air/solution interface in different ionic liquids, to create a viscoelastic liquid interfacial layer of higher concentration. Adsorption at the air/solution interface gives an extra contribution to the measured torque in various rotational rheometer geometries, which simply adds to the torque from the pure bulk solution. Higher temperatures cause the cellulose chains to desorb from the interface because adsorption has an entropic cost and consequently ensuring a more homogeneous concentration throughout the solution at 80℃.