Controlled shrinkage of cellulose nanofibril films to enhance mechanical and barrier properties

Abstract

Standalone cellulose nanofibril (CNF) films have a natural tendency to shrink upon drying from wet conditions due to capillary drying stresses. This shrinkage happens in both the radial direction, and the vertical direction. In this study, we prepared two types of CNF films- one in a restrained condition that did not allow shrinkage in the radial direction but enabled it in the vertical direction and another with 11 % radial shrinkage but limited vertical shrinkage. The radial shrinkage led to a more porous structure than the vertical shrinkage, which brought about poorer oxygen/moisture barrier performance. However, the density and oxygen permeability of the films converged to a similar value upon a simple thermocompression process. Radial shrinkage resulted in 140 % and 90 % higher strain at break and toughness in films with a significant sacrifice in strength and modulus. Scanning electron microscopy revealed that radial shrinkage formed wavy layers in the core structure leaving more free space, whereas vertical shrinkage formed flatter layers. Radial shrinkage is likely to produce a thicker individual layer in the core structure of CNF films than vertical shrinkage. The insight from this study will help tune the mechanical and barrier performance of CNF films and their composites.