Surface modification of cellulose nanomaterials with amine functionalized fluorinated ionic liquids for hydrophobicity and high thermal stability

A highly hydrophobic fluorinated ionic liquid (IL), 3-aminopropyl-tributylphosphonium bis(trifluoromethylsolfonyl)imide ([aP4443][NTf2]), was synthesized, and applied for the surface modification of cellulose nanomaterials (CNMs) by reductive amination. The modified CNMs were fully characterized for their chemical structure, morphology, thermal stability, and surface hydrophobicity. Results obtained from Nuclear Magnetic Resonance spectroscopy (1H, 13C, 19F and 31P), Fourier Transform Infrared spectroscopy, X-ray Photoelectron Spectroscopy, and X-ray diffraction confirmed the successful grafting of [aP4443][NTf2] onto the surface of CNMs up to a degree of surface functionalization of 2.5 %. Transmission Electron Microscopy analysis confirmed the dimensions of the CNMs were retained after modification but with significant aggregation for modified cellulose nanocrystals (CNCs). Thermal Gravimetric Analysis demonstrated significant increases in the degradation temperatures of modified CNCs from ∼252 °C to ∼310 °C. Modified cellulose nanofibers (CNFs) did not show any increase in thermal stability. The modified CNM suspensions showed reduced affinity for water and the formation of aggregates in aqueous media. Furthermore, a water contact angle test demonstrated enhanced hydrophobicity for modified CNMs. This modification approach holds potential for the use of the [aP4443][NTf2] IL for functional materials to achieve novel hydrophobic CNMs suitable for aqueous processing with thermoplastics, for fabrication of thermally stable composite materials, and for polymer gel electrolytes for batteries.