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New Insights on the Role of Urea on the Dissolution and Thermally-Induced Gelation of Cellulose in Aqueous Alkali

1
CQC, Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal
2
Faculty of Sciences and Technology (MEDITBIO), University of Algarve, Campus de Gambelas, Ed. 8, 8005-139 Faro, Portugal
3
Fibre Science and Communication Network (FSCN), Mid Sweden University, SE-851 70 Sundsvall, Sweden
4
Division of Physical Chemistry, Department of Chemistry, Center for Chemistry and Chemical Engineering, Lund University, SE-221 00 Lund, Sweden
5
Department of Chemical Engineering and the Russell Berrie Nanotechnology Institute (RBNI), Technion-Israel Institute of Technology, Haifa 3200003, Israel
*
Author to whom correspondence should be addressed.
Received: 17 October 2018 / Revised: 29 November 2018 / Accepted: 4 December 2018 / Published: 11 December 2018
(This article belongs to the Special Issue Polysaccharide Hydrogels)
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Abstract

The gelation of cellulose in alkali solutions is quite relevant, but still a poorly understood process. Moreover, the role of certain additives, such as urea, is not consensual among the community. Therefore, in this work, an unusual set of characterization methods for cellulose solutions, such as cryo-transmission electronic microscopy (cryo-TEM), polarization transfer solid-state nuclear magnetic resonance (PTssNMR) and diffusion wave spectroscopy (DWS) were employed to study the role of urea on the dissolution and gelation processes of cellulose in aqueous alkali. Cryo-TEM reveals that the addition of urea generally reduces the presence of undissolved cellulose fibrils in solution. These results are consistent with PTssNMR data, which show the reduction and in some cases the absence of crystalline portions of cellulose in solution, suggesting a pronounced positive effect of the urea on the dissolution efficiency of cellulose. Both conventional mechanical macrorheology and microrheology (DWS) indicate a significant delay of gelation induced by urea, being absent until ca. 60 °C for a system containing 5 wt % cellulose, while a system without urea gels at a lower temperature. For higher cellulose concentrations, the samples containing urea form gels even at room temperature. It is argued that since urea facilitates cellulose dissolution, the high entanglement of the cellulose chains in solution (above the critical concentration, C*) results in a strong three-dimensional network. View Full-Text
Keywords: cellulose; gelation; urea; NaOH; microrheology; cryo-transmission electronic microscopy; polarization transfer solid-state NMR; hydrophobic interactions cellulose; gelation; urea; NaOH; microrheology; cryo-transmission electronic microscopy; polarization transfer solid-state NMR; hydrophobic interactions
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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MDPI and ACS Style

Alves, L.; Medronho, B.; Filipe, A.; E. Antunes, F.; Lindman, B.; Topgaard, D.; Davidovich, I.; Talmon, Y. New Insights on the Role of Urea on the Dissolution and Thermally-Induced Gelation of Cellulose in Aqueous Alkali. Gels 2018, 4, 87.

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