Hydrogels are smart materials, which in recent years received plenty of attention due to their excellent properties, such as response to stimuli, durability, capability of adsorption and relatively high storage modulus. Nowadays, great emphasis is put on biodegradable hydrogels with wide aspect of applications. Such examples are nanocellulose hydrogels, which are made from cellulose nanofibrils or nanocrystals, and can be used in various fields such as sensors, drug delivery systems and additives in food and health industry. Two of the most important fields for hydrogel applications are medicine and pharmacy.
Nanofillers such as carbon nanotubes and graphene are used to improve hydrogel properties, so they can be used in specific applications. Rheological properties of hydrogels are measured by rheometers, which are used to measure rheological parameters such as viscosity, storage and loss moduli.
In my master’s thesis hydrogels made of TEMPO modified nanocellulose are investigated. Three different graphene nanoparticles were added to TEMPO CNF suspensions; graphene, graphene modified with amino groups and graphene modified with carboxyl groups. Firstly, I investigated the effect of aging of TEMPO CNF suspensions with specific additive and the influence of the amount of specific additive on rheological properties. The results showed higher viscosity and storage modulus of TEMPO CNF hydrogels over time. The reason for such behaviour is the microstructural change in suspensions, the arranging of nanocellulose chains over time and the creation of hydrogen and van der Waals bonds inside the hydrogel.
Results from rheological measurements did not show major differences when comparing the effect of concentration of specific graphene on rheological behaviour of hydrogels. Later on, I was able to explain these results with conductometric titration. The amount of amino and carboxyl groups was low, there was between 0,042 – 0,045 mmol/g of -COOH groups in graphene modified with carboxyl groups and 0,979 mmol/g of -NH_2 groups in graphene modified with amino groups. I was able to obtain an increase in viscosity with higher amount of each graphene, which was the consequence of increase of dispersed phase in hydrogels.
In addition, I researched the effect of pH value on TEMPO CNF hydrogels. Rheological results showed the increase in viscosity, elastic and storage moduli in acidic environment pH = 3. The reason for such behaviour could be water elimination from hydrogels as well as weaker repulsion between polymer chains. Nanocellulose chains also started to agglomerate due to the van der Waals interactions. Rheological behaviour of hydrogels at pH = 11 was similar to hydrogels with pH = 7. Furthermore, I concluded that higher pH does not affect the rheological behaviour of TEMPO CNF hydrogels.
In my research, I managed to determine the effect of aging, the effect of modified graphene nanoplatelets addition, and the effect of pH value on the rheological properties of TEMPO CNF hydrogels. I hope my work will help with further research of TEMPO CNF composite hydrogels and the use of these smart materials in various new applications.
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