Swelling and Free-Volume Characteristics of TEMPO-Oxidized Cellulose Nanofibril Films

Cellulose nanofibrils (CNFs) are becoming increasingly ubiquitous in diverse technologies requiring sustainable nanoscale species to form or modify films. The objective of the present study is to investigate the swelling behavior and accompanying free volume of self-standing TEMPO-oxidized (TO) CNF films in the presence of water vapor. For this purpose, we have performed time-resolved swelling experiments on films, prepared according to different experimental protocols, at 90% relative humidity (RH) and ambient temperature. Corresponding free-volume characteristics are elucidated by positron annihilation lifetime spectroscopy (PALS) conducted at ambient temperature and several RH levels. Increasing the drying temperature of the films (from ambient to 50 °C) is observed to promote an increase in film density, which serves to reduce bulk swelling. These elevated drying temperatures likewise cause the free-volume pore size measured by PALS to decrease, while the corresponding total free-volume fraction remai...