Abstract
A novel mechanism of heat-triggered gelation for oxidised cellulose nanofibrils (OCNF) is reported. We demonstrate that a synergistic approach combining rheology, small-angle X-ray scattering (SAXS) and saturation transfer difference NMR (STD NMR) experiments enables a detailed characterisation of gelation at different length scales. OCNF dispersions experience an increase in solid-like behaviour upon heating as evidenced by rheological studies, associated with enhanced interfibrillar interactions measured using SAXS. Interactions result in an increased fibrillar overlap and increased population of confined water molecules monitored by STD NMR. In comparison, cationic cellulose nanofibrils (produced by reaction of cellulose with trimethylglycidylammonium chloride) were found to be heat-unresponsive.
Highlights
There is an enormous interest in cellulose based hydrogels as inexpensive and biodegradable gels, for their industrial and sustainable applications
In this work we describe a novel mechanism of temperature triggered gelation for oxidised nanocellulose hydrogels, along with a thorough study of the process, from the rheological macroscopic behaviour, through nanoscale fibril aggregation, down to the molecular level of the water dynamics within the oxidised cellulose nanofibrils (OCNF) network
saturation transfer difference NMR (STD NMR) experiments detected an increase in the population of confined water with temperature, this behaviour not observed for cationic cellulose nanofibrils (CCNF), suggesting a correlation between water confinement and gel formation
Summary
There is an enormous interest in cellulose based hydrogels as inexpensive and biodegradable gels, for their industrial and sustainable applications. Studies of the structure and dynamics of water by NMR have so far largely relied on the determination of T1 and T2 relaxation times.[19,20] while the former can be misinterpreted due to its symmetric behaviour at short and long correlation times (i.e. high T1 times might indicate either very fast or very slow dynamics), the latter is strongly affected by the kinetics of chemical exchange, hindering the real impact of molecular motion on T2 relaxation On these grounds, we have instead used an NMR approach based on the saturation transfer difference (STD) experiment on D2O hydrogel samples which overcomes the drawbacks of NMR relaxation measurements. Coupling STD-NMR with rheology measurements and small angle x-ray scattering to determine network structures, provides detailed structural insights on the gelation mechanisms upon temperature, both at the mesoscale level and at the molecular level
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