Thick Polyvinyl Alcohol Films Reinforced with Cellulose Nanocrystals for Coating Applications

The strain-induced elastic buckling instability for mechanical measurements (SIEBIMM) method was used to quantify Young’s modulus of polyvinyl alcohol (PVOH) coatings reinforced with cellulose nanocrystals (CNCs). The effect of processing parameters, namely, deposition method and drying temperature, and changes in relative humidity were assessed. The coatings were relatively thick (0.6–17.6 μm) compared to freestanding PVOH/CNC nanocomposite films tested previously, and the ability to robustly measure the modulus of substrate-supported coatings without specialized equipment or the need for “model systems” offered new material insights. In solvent cast coatings, the addition of 10 wt % CNCs into PVOH increased Young’s modulus from 1.6 ± 0.1 GPa (neat PVOH) to 21 ± 2 GPa. The 13-fold increase in modulus is attributed to three factors: the rigid CNCs forming a percolated network, the high compatibility between CNCs and PVOH, and the CNCs nucleating PVOH crystallitesalmost doubling the matrix degree of crystallinity. Bar coating further increased Young’s modulus of the coating by 3.5-fold (compared to solvent casting), likely due to alignment of CNCs and PVOH crystallites. Higher drying temperatures and lower relative humidity also improved the CNC/PVOH coating mechanical performance; annealing increased the matrix degree of crystallinity and the modulus (ca. 6 times more for CNC/PVOH coatings than for neat PVOH), and the modulus increased by 0.4 GPa per unit decrease in relative humidity (vs 0.1 GPa for neat PVOH). Overall, CNCs can be used to tailor coating moduli over a wide range and impart higher sensitivity to processing parameters; furthermore, SIEBIMM is a facile tool to quantify mechanical properties of nanocomposite coatings prepared in different ways, such that their performance and potential applications may be fully assessed.