(346h) Water Sorption and Diffusion in Nanofibrillated Cellulose Films and Composites

Nanocelluose is an innovative nano-sized material, which is attracting significant interest in view of its possible applications in various fields, such as, among the others, gas barrier materials or gas separation membranes. Indeed, nanocellulose films and nanocomposites are strongly barrier towards gases in dry conditions, while the permeabilities increase up to 2 or more orders of magnitude in humid environments, with significantly high selectivities toward CO₂. Water vapor can thus tune the nanocellulose properties from a barrier materials to a very promising membrane and the study of water sorption in these materials is crucial to completely understand the potentialities and current limitation of such new material for this kind of applications.

This work investigates the solubility and transport of water vapor in different types of Nanofibrillated Cellulose (NFC) as well as in polyvinylamine (PVAm)-NFC nanocomposites, by means of direct sorption and diffusion experiments in different experimental conditions.

In particular, NFC obtained by different cellulose pretreatments (carboxymethylation or enzymatic) prior to mechanical processing, were tested spanning over wide ranges of water activity and temperatures, between 16 to 65°C, while PVAm-NFC composite are investigated at various NFC loadings at 35°C.

Interestingly, the water diffusivity increased both with temperature and water activity, approaching a maximum value, independent on the operative condition considered for the tests. In addition, non-Fickian behaviors are observed especially at higher temperatures and R.H. inspected, clearly indicating the existence of different processes controlling the kinetics water sorption in the NFC film.

Similar behavior was also observed for in the case of nanocomposites, which also presented analogous water diffusivity, even if they showed significantly higher water uptake (larger than 50 % wt.) with respect to the pure NFC films that never exceed 10%wt.

The experimental data are then analyzed by appropriate modeling descriptions, seeking for the clear understanding of the process of moisture diffusion in nanocellulose based materails. Different approaches have been considered for solubility, accounting for both absorption and adsorption processes, and diffusivity, covering single or multiple Fickian diffusion, diffusion relaxation models, as well as the so-called parallel exponential kinetics model (PEK), often employed to represent the water sorption in cellulose fiber.