Mechanochemical radical formation in cellulose ball milling and production of cellulose-metal nanoparticles composites
Embargo Lift Date: 2020-12-25
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Cellulose is the most abundant biopolymer in nature, which contains linear chains of repeating D-glucose molecules connected by 𝛽-1,4-glycosidic linkages. Over the past decades, due to growing interest in sustainability and green chemistry, cellulosic materials have received much attention. Since cellulose is highly abundant, light weight, strong, biodegradable, and nonabrasive, composite materials including cellulose can be environmental friendly, biocompatible, low cost, low weight, and multifunctional. Cellulose composites including metal nanoparticles (cellulose-metal NPs composites) find application in many fields due to the combined properties of both metal NPs and cellulose matrices. However, current production methods of cellulose-metal NPs composites have generally multistep, long, and non-environment friendly procedures due to their requirement for hazardous chemicals to reduce the metal ion precursor and stabilize the metal NPs that form. This thesis work focuses on the investigation of cellulose mechanoradicals formation produced by ball milling qualitatively and quantitatively with the changes occurred in cellulose samples after mechanical treatment, and using formed cellulose mechanoradicals as reducing agent to reduce metal cations in cellulose matrices to obtain cellulose-metal NPs composites. Firstly, formation of cellulose mechanoradicals -free radicals that are formed by the homolytic breaking of the bonds in cellulose chains under mechanical input (ball milling)- was analyzed qualitatively and quantitatively. Qualitative analysis of cellulose mechanoradicals by ESR spectroscopy showed that, formed radicals could be generally peroxyl and alkoxyl types. The numbers of mechanoradicals formed by milling cellulose samples (cotton and microcrystalline cellulose) in wet and dry conditions were detected by using DPPH solutions with UV-Vis spectroscopy. It was shown that, dry grinding method led to the higher number of mechanoradicals formation. Changes in cellulose samples occurred after milling probed by using SEM, XRD, and FTIR-ATR analyses were found to follow the mechanoradical formation. Due to more efficient grinding in dry conditions, with increasing milling time, the progressive decrease in fiber size leading more accessible regions with smaller particle size of cellulose samples, and forming mostly amorphous cellulose samples were observed. FTIR-ATR analyses of ground cellulose samples, especially dry ground samples, showed the breaking of intra and intermolecular hydrogen bonds and 𝛽-1,4-glycosidic linkages. Lastly, cellulose-metal NPs composites were produced by using cellulose mechanoradicals as reducing agent for the first time. The composites were characterized by SEM, EDX, XRD, and XPS analyses. Au, Ag, Pt, and Pd NPs in their metallic forms, and Cu NPs in its metallic form Cu0 or in 1+ oxidation state as Cu2O, and Co in 2+ oxidation state as CoO NPs are successfully produced in cellulose matrices. This mechanochemical method can be proposed as a new and green method for cellulose-metal NPs composites production. Consequently, our findings can contribute to the area of mechanochemistry and composite materials.