Rheological Properties and Molecular Structure of Tunicate Cellulose in LiCl/1,3-Dimethyl-2-imidazolidinone

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Abstract
Solution properties and molecular structure of tunicate cellulose (TC), an animal cellulose from Halocynthia roretzi, were investigated in terms of rheological and dilute solution properties. The solvent used is 8 wt % LiCl/1,3-dimethyl-2-imidazolidinone (DMI). A solution of dissolving pulp (DP), derived from plant, was also used for comparison. The weight-average molecular weight, Mw, and the limiting viscosity number, [η], of the TC were evaluated to be 413 × 106 and 2645 mL/g, respectively. The TC solution showed the same concentration dependence of GN (GN = 5.49 × 106φw2.14 Pa; φw: weight fraction of cellulose in solution; GN: plateau modulus) as the DP solution and, moreover, also as the solution of cotton linter (CC) in 8 wt % LiCl/N,N-dimethylacetamide (DMAc). This exponent of 2.14 indicates that network structure by entanglements was formed in these solutions. According to the theory of Fetters et al., moreover, such identity means that all of these celluloses have the identical chain structure though their biological origins are far different. On the other hand, the φw-dependence of η0 − ηs (η0 = zero shear rate viscosity of solution; ηs = solvent viscosity) was different between the TC and the DP solution in the semidilute regime: the TC solution exhibited η0 − ηs ∝ φw7.5 and the DP solution η0 − ηs ∝ φw4. According to the theory of Doi−Edwards, this exponent of 4 (the DP solution) indicates that the DP behaves as flexible polymers in the solution. In contrast, the dependence for the TC solution seems unexplainable on the basis of molecular theories. This difference probably signifies the difference in the relaxation process or mechanism in entanglement systems.