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Effect of molecular structure on the viscoelastic properties of cellulose acetate in a ternary system Hsieh, Chia-wen Carmen

Abstract

A series of ternary systems composed of cellulose acetate (CA), N,N-dimethylacetamide (DMA), and water were prepared by varying the mixing temperature, order of component addition, and polymer substitution pattern with increasing water content. The viscoelastic properties of the resulting ternary systems were measured using steady state and dynamic rheology. The CA/DMA/H₂O mixture formed physical gels at 17.5 and 19 wt% nonsolvent concentrations after heating to 50 and 70/90°C respectively. Gel formation was characterized by the loss of a Newtonian plateau in the steady state as well as the transition of the elastic (G') modulus becoming greater than the viscous (G") modulus in the dynamic state. The molecular structure of the polymer influenced the viscoelastic properties of the resulting gel. Commercially available CA was found to be partially acetylated at the C2, C3, and C6 positions and contained a total degree of substitution (DS) of 2.47. As CA cluster size in solution decreases with increasing temperature, viscosity measurements showed higher viscosity for samples heated at 50°C, where the loss of the linear stress-strain relationship occurred at 17.5 wt% water. In the dynamic state, higher heating temperature produced higher elastic moduli with a longer linear viscoelastic region (LVR), indicative of a stable system. Changing the sequence of polymer addition by adding CA to a DMA/H₂O solution resulted in lower overall viscoelastic moduli as compared to adding water to a CA/DMA solution. CA that was regioselectively synthesized to a DS of 2.4 showed different viscoelastic behaviour than the commercial CA. This polymer was completely acetylated at C2 and C3 and partially acetylated at position 6. The system underwent phase separation induced gelation at much lower nonsolvent content. Stress sweep experiments confirmed a shorter LVR and higher G' than commercial CA. Increasing the DS of the regioselective polymer to 2.8 led to a longer LVR and higher G' than all other polymers at the same nonsolvent content. The enhanced steady shear viscosity and dynamic viscoelastic properties were a result of the intensification of hydrogen bonding and hydrophobic interactions between the polymer, solvent, and nonsolvent.

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