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Cellulose is a typical hygroscopic material of major importance in nature and industry. As vapor is absorbed in the amorphous matrix of cellulose, the material swells which gives rise to unusual coupled behaviors between the fluid and the porous solid. In this work, we investigate by NMR/MRI how the drying and wetting behavior of cellulose fiber stacks depends on its degree of compression, and we propose a tentative interpretation of the observed response based on poromechanics.
Cellulose samples, either dry or wet (i.e., equilibrated close to 0% or 100% RH, respectively), are compressed to various level of pressure (from 1 atm to a few MPa). While the wet samples are dried and the dry samples are wetted, their bound water content is followed by NMR/MRI. The kinetics of drying can be very well reproduced by a simple diffusion equation, and how the diffusion coefficient depends on the degree of compression can be well explained by the combined effect of vapor and bound water diffusion, the relative contribution of which evolve as the pore volume available for vapor diffusion decreases with compression. The kinetics of wetting however, appears incompatible with this diffusion model: for highly compressed samples, wetting appears much slower than expected and the material never reach the water content expected at saturation, reaching less that 70% of the expected content after waiting several weeks (whereas the drying usually reaches equilibration after a few days at most). Interestingly, for small compression, the wetting kinetics appears consistent with the drying kinetics, suggesting that the observed anomaly originates from the pressure applied on the material. A possible explanation is that compression affects the adsorption isotherm, in the same way adsorption makes the material swell when it is free of stress. To explore this idea, we set up a non-linear poromechanical model of the cellulose fiber stacks, that satisfies i) the free swelling response, ii) the simple compression response, and iii) the Maxwell relations of the grand potential (to guarantee thermodynamic validity). The factor capturing the effect of compression is inspired from the theoretical derivation of a Langmuir model extended to adsorption, assuming, as is the case in amorphous cellulose, that when adsorption takes place, an adsorbed water molecule increases the volume of its adsorption site of about the size of the water molecule. This model is able to capture quantitatively the decrease in amount adsorbed observed during the wetting tests of compressed cellulose fibers stacks. This work highlights the critical importance of internal stresses on the hygroscopic behavior of bio-based materials, and proposes a fundamental explanation for it.
| References | Zou, Y., Maillet, B., Coussot, P., & Brochard, L. (2025). Sorption hysteresis in nanoporous swelling adsorbents. Physical Review Research, 7(1), 013205. https://doi.org/10.1103/PhysRevResearch.7.013205 |
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| Country | France |
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