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

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.

Keywords: NaOH; cellulose; cryo-transmission electronic microscopy; gelation; hydrophobic interactions; microrheology; polarization transfer solid-state NMR; urea.

Figure 1

Cryo-transmission electronic microscopy (cryo-TEM) images of 0.5 wt % MCC dissolved in (a) 8 wt % NaOH(aq.) solution and (b) in 8 wt % NaOH(aq.)/12 wt % urea system. Scale bars correspond to 100 nm.

Figure 2

Polarization transfer solid-state nuclear magnetic resonance (PTssNMR) spectra for 10 wt % MCC dissolved in 8 wt % NaOH(aq.), with (bottom) and without (top) urea. The cross-polarization (CP, blue line) and the insensitive nuclei enhanced by the polarization transfer signal (INEPT, red line) spectra were acquired at 25 °C. The vertical lines represent the chemical shifts of native dry microcrystalline cellulose (MCC; blue dotted lines) [2] and dissolved cellulose [33] (red dotted lines).

Figure 3

Viscoelastic parameters determined by mechanical rheometry. G′ (full symbols) and G″ (empty symbols) were obtained for samples with (squares) and without (circles) urea after a temperature sweep from 25 to 60 °C (heating rate of 1 °C/min) at a constant shear stress of 10 Pa and frequency of 0.1 Hz. (a) 5 wt % MCC and (b) 10 wt % MCC dissolved in 8 wt % NaOH(aq.)/12 wt % urea.

Figure 4

(a) Intensity correlation function (ICF) obtained with increasing temperature for the solution of 5 wt % MCC in NaOH(aq.), and (b) the corresponding H (t = 1 s; grey circles) and G′ (ω = 100 rad/s; black squares) as a function of temperature. The arrow on the left indicates how the ICF plateaus (and the H parameter) evolve with temperature rise, while the arrow on the right indicates the inflection point (Tg), where both the H parameter and G′ start increasing exponentially.

Figure 5

Variation of the H parameter with temperature for the 5 wt % (grey squares) and 10 wt % (black circles) MCC samples dissolved in NaOH(aq.). The arrows indicate the Tg.

Figure 6

Variation of the H parameter with temperature for the 5 wt % (black squares) and 10 wt % (grey circles) MCC samples dissolved in 8 wt % NaOH(aq.)/12 wt % urea.