Facile cellulose nanofibrils amidation using a ‘one-pot’ approach

Abstract

We report a ‘one-pot’ synthetic approach for TEMPO (2,2,6,6-tetramethyl-piperidin-1-oxyl) oxidized cellulose nanofibrils (TOCN) surface amidation by coupling up to 70% of superficial carboxylic units in TOCN with long alkyl chain primary amines (dodecylamine and octadecylamine) using TBTU [O-(1H-benzotriazol-1-yl)-N,N,N’,N’-tetramethyluronium tetrafluoroborate] uronium salt as coupling agent. Raw TOCNs were produced from commercial microcrystalline cellulose and have an elongated highly tangled morphology with nanometric diameters and lengths up to several micrometers with a crystallinity of 62%. TOCN´s degree of oxidation was 1.25 mmol –COOH groups/g cellulose. The TOCN-amidated products named TOCN-AMDC12 and TOCN-AMDC18 depending upon to the primary amine used for the synthesis (dodecylamine –C12– for the former and octadecylamine –C18– for the latter), were characterized using field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy with attenuated total reflectance and X-ray diffraction. After amidation the hydrated gel-like TOCNs become a cream-colored gel insoluble in water and soluble in toluene. IR data indicates formation of an amide bond after the reaction, and FESEM images suggest a slight increase in fibril width as a result of the long-chain amide derivatives grafted onto the TOCN’s surface. Thermal stability and hydrophobicity of the products were determined using thermogravimetrical analysis and contact angle (CA) measurements. The TOCN-amidated products showed improved thermal properties with maximum decomposition temperatures of 347 and 358 °C, for TOCN-AMDC12 and TOCN-AMDC18 respectively, when compared to the raw TOCN (T_d 340 °C); and high hydrophobicity with CA of 61° and 67°, respectively. Finally, the proposed method for TOCN amidation has advantages over current amidation approaches in that it is performed in aqueous media, does not require heating and occurs rapidly (2 h).