Viscoelastic properties of poly (vinyl alcohol) hydrogels with cellulose nanocrystals fabricated through sodium chloride addition: rheological evidence of double network formation

Abstract This report investigates viscoelastic and mechanical behavior of hybrid polyvinyl alcohol (PVA)-cellulose nanofibers (CNC) hydrogel aggregated with sodium chloride (NaCl) salt. In almost all samples, wide distribution of pores with roughly equal average pore size was observed, denoting that the average porosity is independent from CNC concentration. To assess dispersion quality of CNCs in CNC-PVA/salt hybrid network, transmission electron microscopy (TEM) was employed. Small and large amplitude oscillatory shear (SAOS and LAOS) measurements were also performed on CNC-loaded PVA-salt hydrogels to further characterize their microstructure via monitoring viscoelastic parameters. At CNC loadings near the percolation threshold (15 g/L), a sudden jump in storage modulus versus frequency curve was observed, attributing to polymer-CNC 3D network formation. Interestingly, appearance of a more dramatic jump in the value of the rheological parameters at higher CNC concentrations (30 g/L) indicates the establishment of a secondary network structure upon formation of a direct contact between individual CNC particles and/or clusters. The viscoelastic behavior of the hydrogels was further investigated via LAOS approach based on Lissajous-Bowditch plots and sequence of physical process method. In addition, through a wide range of rheological characterizations, we demonstrated successful fabrication of a healable hydrogel able to regain original strength after disruption of the structure at sufficiently large deformations. Afterward, to explore the interactions between CNC-CNC and PVA-CNC, the Foglar-Tucker model, designed to account for modeling the orientation of CNC particles within the matrix, was fitted on stress-overshoot experiments. The present study on PVA-CNC hydrogels opens avenues for further developing advanced materials with tunable microstructures for tissue engineering and regenerative medicine applications.