Effects Of Polymorphs On Dissolution Of Cellulose And Preparation Of Novel Hydrogel In NaOH/Urea Aqueous Solution

The efficient utilization of renewable lignocellulose biomass is becoming increasingly important due to the depletion of petroleum resource as well as global warning caused by greenhouse gas emissions. Cellulose, the most abundant biopolymer on Earth, has many advantageous properties such as good mechanical strength and environmentally friendly and is therefore a desirable material source in a future biomaterial-based and sustainable economy. Research group developed a new type of solvent mixture for cellulose, that is, aqueous NaOH/urea solution, at low temperature, in which cellulose can be dissolved rapidly to obtain a transparent cellulose solution. Previous researches have shown that the solubility of cellulose in NaOH/urea aqueous solution depended on the temperature of solvent, molecular weight of cellulose, crystallinity of cellulose, stirring time and stirring rate. Four different crystalline allomorphs have been identified for cellulose:â… , â…¡, â…¢, and â…£. However, the study on the effect of crystal type in dissolution of cellulose is quite scarce and dispersed. In this work, the effects of polymorphs of cellulose on the dissolution in NaOH/urea aqueous solution were demonstrated. The cellulose samples were characterized by XRD, GPC, SEM and 13C CP/MAS NMR spectroscopy. Additionally, the properties of cellulose solution were analyzed by solubility test, viscosity measurements,1H NMR spectroscopy and DSC.1. The results indicated that, compared with the degree of polymerization (DP) and crystallinity index of cellulose, the cellulose crystal type had greater impact on cellulose dissolution. Specially, the solubility of the cellulose â… , â…¡, and â…¢ was progressively decreased, probably due to different hydrogen bond network and packing energy in it.2. The ability of cellulose â… , â…¡, â…¢, and â…£ to form hydrogen bonds with sodium hydroxides was nearly the same, while gradually increased when cellulose preparations were associated with urea. Moreover, all the cellulose solutions showed good thermal stability.3. A novel hydrogel was prepared successfully from graphene oxide (GO) and cellulose in the NaOH/urea aqueous system by using epichlorohydrin (ECH) as cross-linker and the obtained hydrogel has good potential to adsorb metal ions. GO nanosheets containing carboxyl and hydroxyl groups were introduced into the surface of the cellulose hydrogel with retention of the gel structure and its nanoporous property. Due to the introduction of GO, the GO/cellulose composite hydrogels exhibited good compressive strength. Adsorption capacity of Cu2+ significantly increases with an increase in the GO/cellulose ratio and GO/cellulose hydrogel showed high adsorption rates. The calculated adsorption capacities at equilibrium(qecal) for GO/cellulose hydrogel (GO:cellulose= 20:100 in weight) was up to 94.34 mg g-1, which was much higher than that of the pristine cellulose hydrogels. Furthermore, GO/cellulose hydrogel exhibited high efficient regeneration, metal ion recovery efficiency and high adsorption capacity for Zn2+, Fe3+, and Pb2+.