Cellulose gelation in aqueous hydroxide solutions by CO2(g): Fact and theory

Abstract

Understanding how the solvent structure affects the stability of the dissolved state and the following precipitation is important for designing future dissolution-coagulation systems for cellulose processing. In this study, two morpholinium hydroxides with different alkyl chain lengths, namely N,N-dimethylmorpholinium hydroxide (NDMMOH(aq)) and N‑butyl‑N-methyl morpholinium hydroxide (BMMorOH(aq)), were studied and compared with the previously thoroughly investigated cellulose solvent benzyltriemethylammonium hydroxide (Triton B(aq)) which is well-known for its superior ability to stabilize the dissolved state. Cellulose solutions in each solvent were characterized by NMR, flow and frequency sweeps, while cellulose coagulation by CO2(g) was followed by in situ FTIR, pH and temperature measurements. The coagulated systems were characterized by CP/MAS 13C NMR, flow, and frequency sweep measurements. Complementary molecular dynamic (MD) simulations were performed to gain a deeper insight into the observed gelation behavior. The intrinsic viscosity indicated more extended cellulose chains in the solvents with more hydrophobic moieties (Triton B and BMMorOH). Even though the course of coagulation did not show any significant differences during monitoring, both the properties of the obtained gels and the MD simulations indicated differences in formation and properties of the coagulated materials that could be related both to the choice of solvent and coagulant.

Keywords

• Cellulose
• Gelation
• Quaternary ammonium hydroxide
• CO2(g), MD simulations