The Synthetic Strategies for Unique Properties in Cellulose Nanocrystal Materials

Abstract

Cellulose is renewable, biodegradable and widely available natural biopolymer which upon acid hydrolysis yields highly crystalline rod-like rigid hydrophilic particles having nanoscale dimensions. The acid hydrolysis of cellulose fibers is a heterogeneous acid diffusion process wherein acid penetrates the less ordered amorphous regions and causes the cleavage of glycosidic bonds while leaving the highly organized crystalline regions undamaged. The penetration and the glycosidic bond breakage are known to depend on the hydrolysis conditions, the acid type, hydrolysis temperature, and acid concentration. Acid hydrolysis is typically done using either hydrochloric acid or sulfuric acid. The effect of reaction conditions to the production of cellulose nanocrystals (CNCs) was investigated in the present research. It was demonstrated that the use of ultrasonic energy during the acidic hydrolysis (HBr) elevates the yields of cellulose nanocrystals at lower reaction temperatures (80°C). At higher reaction temperatures (100°C), the ultrasonication was not seen to improve the yields of CNCs. The optimum conditions for the hydrolysis reaction, at given experimental set up, were found to be 2.5 M HBr, 3 hours at 100°C applying the ultrasonic energy in the course of the reaction. A novel quantitative 31P NMR methodology has been developed to determine the amount of reactive hydroxyl groups in cellulose. It was then used for the monitoring the amount of accessible hydroxyl groups in relation with the mechanical treatment and moisture content of cellulose. Cellulose nanocrystals were modified starting either from the reducing end aldehyde or the surface hydroxyl groups. TEMPO-mediated oxidation was applied to selectively oxidize the primary hydroxyl groups on the surface of cellulose nanocrystals. The produced carboxylic sites were used for the grafting reactions with various amine bearing compounds using coupling agents such as N-hydroxysuccinimide (NHS) and 1-Ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC). Moreover, suitable CNC precursors for the 1,3-dipolar cycloaddition reaction (“Click†-reaction) were developed. As a result, the cross linking of cellulose nanocrystals accompanied with the gel formation was demonstrated. Moreover, the “Click†-chemistry was successfully used for the creation of fluorescence cellulose nanocrystals.