Dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) investigations of surface functionalized cellulose materials

This thesis focusses on the application of solid state nuclear magnetic resonance (NMR) combined with Dynamic Nuclear Polarization(DNP) to probe molecule structures on the surface of cellulose materials. As DNP-enhanced NMR improve the sensitivity by several orders of magnitude and thus allows the measurement of nuclei such as 13C, 15N in natural isotope abundance, it is a powerful tool for the study of chemical structures of heterogeneous catalysts, and functionalized cellulose materials, and may offer a pathway to investigate the cellulose surface chemical structure, from nano-cellulose to paper material. Three examples are employed to show how DNP can be efficiently used for structure determination of functionalized cellulose materials and paper hybrid materials. As first example, the material of imidazole physically doped on nano crystalline cellulose (Cell-Im), an excellent solid-state proton conductor for fuel cell, was investigated by 13C and 15N CPMAS NMR and DNP. Interactions and dynamics of hydrogen related with imidazole molecules on the cellulose surface have been revealed in detail. First, the local structure of imidazole on the surface of cellulose was studied by using 13C CPMAS spectroscopy of Cell-Im at 105 K (DNP) and Im at room temperature (NMR). The rigid and amorphous imidazole were revealed by 15N CPMAS NMR experiments of Cell-Im-15N, Cell-Im and Im at 105 K (DNP). Moreover, variable temperature 15N CPMAS NMR and 1H-15N HETCOR of Cell-Im-15N indicated the dynamic of protons in Cell-Im. They were applied to monitor proton fast exchange processes and provide the exchange rate and activation energy distribution for the processes in the material. 1H-15N HETCOR experiments at variable temperature were performed to study the interactions among imidazole, water and cellulose. Imidazole was stabilized on the cellulose surface by two types of hydrogen bonding with Cellulose-OH and water molecule. According to the obtained NMR data, a possible proton transfer mechanism in Cell-Im material was proposed, which may induce the highly efficient proton conductivity in this material. As second example, DNP-enhanced NMR revealed the structure of rhodamine derivatives immobilized on crystalline nano cellulose (Cell-RhB). Such compound is sensitive to external stimuli such as pHvalue, heating and UV light. The presence of RhB in Cell-RhB was confirmed by using AFM and FT-IR. The carboxyl binding sites were revealed by DNP-enhanced 13C CPMAS NMR. Switching color experiments of Cell-RhB were performed and the optical color change refers to a ring opening and closing process of the rhodamine spiroamide groups which was analyzed in detail by 15N DNP NMR. The 15N NMR shift obtained from DNP experiments indicated a temporary bond through an electrostatic interaction formed within the confined environment on the cellulose surface during heat treatment. As third example, solid state NMR techniques were applied to study the structure of benzophenonecontaining copolymer (P(DMAA-co-MABP)) modified paper materials. To identify the covalent bonding between copolymer and cellulose, the structure of cellulose, copolymer and modified paper were studied by conventional 13C CPMAS NMR. Moreover, the covalent bonding within copolymer was confirmed by the DNP-enhanced 15N CPMAS.