The first part of my master's thesis investigates crosslinking syntheses of commercial carboxymethylcellulose sodium salt using different crosslinking reagents based on natural dicarboxylic acids of different chain length (citric acid, succinic acid, malic acid) and 2,5-furandicarboxylic acid as molecules from biomass. The crosslinking reactions were carried out under different conditions (reaction time and drying temperature as well as different molar ratios between the cellulose derivative and the crosslinking reagent). Infrared spectroscopy and thermogravimetric analysis were carried out to determine the best crosslinking conditions, and cellulose derivative crosslinking reactions were transferred to a larger scale. It was found that properties of hydrogels depended on drying time and concentration of the crosslinking reagent. The hydrogel with the highest water absorption rate is obtained when 10 wt % of the crosslinking reagent is used. In this case, the water absorption rate was 54%, which is approximately 10% more than that of the commercial hydrogel with a water absorption rate of 45%.
In the second part of my master's thesis, crosslinking reactions directly on native cellulose fibers were studied. Primarily, acid chlorides of aliphatic carboxylic acids with different aliphatic chain lengths (succinyl-, adipoyl-, suberoyl- and sebacoyl-dichloride) were used to carry out the crosslinking reactions. In this way, the influence of the distance between individual cross-linked cellulose chains on the degree of water absorption was investigated. This is also directly related to the size of 3D spaces to which water molecules bind. The reaction time of cellulose crosslinking, the crosslinking temperature, and the ratio between cellulose and the crosslinking reagent were optimized. In addition, crosslinking reactions were also performed directly with dicarboxylic acids and anhydrides. Reactions and properties of the hydrogels synthesized using different crosslinking reagents were compared. Various analytical techniques were used to determine the optimal crosslinking reagent or optimal crosslinking conditions, and the crosslinking reaction of cellulose fibers was transferred to a larger scale. The results showed that the properties of cellulose-based hydrogels have the greatest influence on the mass ratio between cellulose and the crosslinking reagent used. With lower amounts of crosslinking reagents and consequently a lower degree of crosslinking, absorption properties, compared to those of cellulose, improve. The best absorption properties were achieved when cellulose was crosslinked with a lower amount of the crosslinking reagent and then additionally modified with succinic anhydride. In this case, the proportion of bound water was about 44%, based on the weight of the dry hydrogel, which is comparable to the properties of a commercial hydrogel. The best absorption properties are obtained when cellulose is modified with succinic acid in a medium comprising N,N-dimethylacetamide and lithium chloride. The hydrogel has a water absorption rate of 70%.
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