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THERMAL AND MECHANICAL PROPERTIES OF POLYMER COMPOSITES REINFORCED BY SULFURIC ACID-HYDROLYZED AND TEMPO-OXIDIZED NANOCELLULOSE: A COMPARATIVE STUDY

The main drawback of cellulose nanocrystals (CNCs) obtained by conventional sulphuric acid hydrolysis is their low thermal stability in consequence of pyrolysis catalyzed by sulfo-groups on the CNC surface. Replacement of surface sulfo-groups by carboxyl groups as a result of oxidation allows the thermal stability of CNCs to be enhanced significantly. Although a great number of studies have reported properties of polymer nanocomposites reinforced by CNCs, thermal properties of the composites compared to the neat polymers are discrepant and still poorly understood. In this work, CNCs were produced from microcrystalline cellulose by sulfuric acid hydrolysis and (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) oxidation. The CNC composites with water-soluble polymers – polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone and polyacrylamide – were obtained. The composites were characterized by various methods, i.e. transmission electron and scanning electron microscopies, energy-dispersive X-ray analysis, Fourier-transform infrared spectroscopy, X-ray diffraction and thermogravimetric analyses, differential scanning calorimetry, and tensile testing. A side-by-side comparison between the thermal and mechanical properties of the polymer composites reinforced by sulfuric acid-hydrolyzed and TEMPO-oxidized nanocellulose was conducted. Analysis of the thermal properties of CNC shows that the surface sulfonate groups replacement with carboxyl groups leads to significant increase of initial temperature of thermal degradation and temperature of the maximum decomposition rate of the CNC. However, the thermal behavior of the composites is much more complicated, and such thermal properties are discussed in detailed. The tensile properties analysis of the composites demonstrates that an addition of TEMPO-oxidized nanocellulose does not improve significantly the tensile strength and Young’s modulus as compared with sulfuric acid-hydrolyzed one.