Engineering nanocellulose superabsorbent structure by controlling the drying rate

Hypothesis: The absorption performance and structure of superabsorbents prepared from carboxylated nanocellulose are strongly influenced by the rate of water removal. Their structure can be engineered by changing the drying profile. Experiments: TEMPO-oxidised nanocellulose superabsorbents were prepared using five different drying techniques, each providing a distinct drying rate. The absorption capacity of deionised water was measured as a function of time and the swelling kinetics was determined, modelled and related to the superabsorbent structure. Superabsorbent phytotoxicity was assessed through seed emergence tests. Findings: The absorption performance of nanocellulose superabsorbents is controlled by the drying rate. In most cases, drying the nanocellulose superabsorbents via evaporation increases the absorption capacity compared to freeze-dried superabsorbents. The best nanocellulose superabsorbent was the air-dried, absorbing around 230 g water/g dry fibre. The high absorption capacity of the evaporative dried superabsorbents is due to their high pore area which increases the interaction between water molecules and fibres. This leads to a stronger physical entrapment of water by capillary forces. Seed germination studies demonstrated that oven-dried 50°C superabsorbent increased germination by 40%. Carboxylated nanocellulose superabsorbents emerge as high-performance renewable materials which can be used extensively in many applications, including agriculture.