Cellulose nanofibrils (CNFs) are considered as one of the promising alternatives to current petroleum-based packaging materials attributed to their ability to form dense self-assembled structures imparting high mechanical and gas barrier properties. The shrinkage phenomenon is one of the most noticeable characteristics of any CNF-containing material throughout the drying process. This shrinking can occur in the horizontal as well as vertical directions.  Interestingly, we found that wet films dried on a cellulose-based substrate resulted in a greater vertical shrinkage (z-shrinkage), whereas a substrate-free drying of the same film led to more horizontal shrinkage (xy-shrinkage). The mechanical and barrier properties of films obtained from these two alternative shrinkage directions were found to be significantly different. We anticipated the core structure of CNF film as multilayered, with each layer consisting of discrete segments (can be considered as tectonic plates) rather than a single continuous layer. These discrete pieces can converge and overlap each other more during xy-shrinkage leading to a higher thickness compared to z-direction shrinkage. We can also apply a simple thermal compression technique on dried films to further reduce the thickness of horizontally shrunk films potentially leading to improved mechanical and barrier properties compared to their counterparts. This study will reveal the core structure of CNF films and establish the structure-property relationship between shrinkage direction and mechanical as well as gas barrier properties.

Horizontal vs vertical shrinkage in cellulose nanofibril films

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