Oriented polyvinyl alcohol films using short cellulose nanofibrils as a reinforcement

Short cellulose nanofibrils (SCNF) were investigated as a new kind of reinforcement for polyvinyl alcohol (PVA) films. SCNF were mechanically isolated from hardwood pulp after enzymatic pretreatment. Various concentrations of SCNF were added to an aqueous PVA solution, which were cast into composite films and then hot drawn with various draw ratios at 200C. SCNF were effective in improving PVA film tensile properties (i.e., ultimate strength and elastic modulus), which depended on SCNF loading, PVA crystal orientation, and the draw ratio. For example, the ultimate strength and modulus of a composite film with a SCNF weight ratio of 3% and a draw ratio of 7.5 were nearly 46 and 61% higher than that of the neat PVA. The PVA crystal orientation increased when small amounts of SCNF were added but decreased as the SCNF content increased above about 6%, likely due to SCNF percolation resulting in network formation that inhibited alignment. Stress-induced crystallization during hot drawing increased the crystallinity of PVA in both the PVA and its composites. Cryogenic and tensile-fractured SEM images of PVA/SCNF composite films showed a uniform SCNF dispersion in the PVA matrix. Young's moduli of drawn composite films were predicted surprisingly well by the rule of mixtures except at intermediate levels of SCNF where the increased orientation of the matrix was not considered in the model.