Abstract
Cellulose can be regenerated from cellulose-ionic liquid (IL) solutions by immersion in water or alcohols. These compounds are potent non-solvents due to their proton-donating ability in hydrogen bonds to IL anions. Although they share this fundamental way of reducing IL solvent quality, coagulation in water is distinctly different from coagulation in alcohols with regard to the microstructures formed and the mechanisms that generate the microstructures. In this study, the possibility of mass-transport effects on microstructures was investigated. The mass-transport of all components: non-solvent (EtOH, 2PrOH), IL ([C2mim][OAc]), and a co-solvent (DMSO), during coagulation was studied. The data was compared to previous data with water as the non-solvent. Results showed that diffusion is essentially limited to a continuous non-solvent-rich phase that is formed during phase separation in all non-solvents. There were also significant differences between non-solvents. For instance, [C2mim][OAc] diffusion coefficients were 6–9 times smaller in 2PrOH than in water, and there were apparent effects from cellulose concentration in 2PrOH that were not observed in water. The differences stem from the interactions between solvent, non-solvents, and cellulose, which can be both mutual and competitive. Weaker [C2mim][OAc]-non-solvent interactions with alcohols give more persistent [C2mim][OAc]-cellulose interactions than with water as the non-solvent, which has consequences for mass-transport.Graphic abstract
Highlights
Cellulose extracted from trees or other plants can be reshaped into foams, films, or fibers using dissolution– precipitation processes
In this study we found that the apparent diffusion coefficients of the non-solvents, DNSs, during the coagulation of [C2mim][OAc]-cellulose solutions are proportional to (& 1/6th of) the self-diffusivity of each non-solvent
This indicates that, during coagulation in alcohol, mass transport is limited to a continuous non-solvent phase that is separated from cellulose during coagulation, similar to coagulation in water
Summary
Cellulose extracted from trees or other plants can be reshaped into foams, films, or fibers using dissolution– precipitation processes. In the second article (Hedlund et al 2017), the mass-transport of non-solvent, co-solvent, and [C2mim][OAc] during the coagulation of a solution membrane in water, and coagulation rates, were studied. Water with both N-methylmorpholin-N-oxide, NMMO (Biganska and Navard 2005; Gavillon and Budtova 2007), and in ILs (Hauru et al 2016; Sescousse et al 2011) None of these measured the mass transport of the non-solvent, which is the active compound that induces coagulation. The methodology for mass transport and coagulation rate measurements from the previous paper (Hedlund et al 2017) is reapplied with two alcohols, EtOH and 2PrOH, as the non-solvents in order to compare them to water. We enquire whether the different microstructures found for cellulose materials coagulated in water or in 2PrOH (Hedlund et al 2019) can be attributed to differences in mass-transport, as suggested by the recurring concept of hard and soft coagulation, see e.g. (Fink et al 2001)
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