Surface-hydrophobized TEMPO-nanocellulose/rubber composite films prepared in heterogeneous and homogeneous systems

A surface-carboxylated nanocellulose was prepared from wood cellulose by catalytic oxidation with 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO). The fibrous TEMPO-oxidized cellulose with sodium carboxylate groups (TOC-Na) was surface-hydrophobized by counterion exchange with tetra-n-butylammonium [TOC-N(n-Bu)₄]. This fibrous TOC-N(n-Bu)₄ was mechanically disintegrated in water and N,N-dimethylformamide (DMF) to prepare dispersions of TEMPO-oxidized cellulose nanofibrils (TOCNs) with tetra-n-butylammonium counterions, i.e., TOCN-N(n-Bu)₄/water and TOCN-N(n-Bu)₄/DMF. TOCN-N(n-Bu)₄/rubber composite films were prepared by mixing TOCN-N(n-Bu)₄ and hydrogenated acrylonitrile–butadiene rubber (H-NBR), used as a polymer matrix, in heterogeneous and homogeneous systems with water and DMF, respectively, followed by casting and drying. The TOCN-N(n-Bu)₄/H-NBR composite films prepared in the heterogeneous and homogeneous systems both had a high Young’s modulus of ~ 45 MPa and low coefficients of thermal expansion of ~ 20 ppm/K at a TOCN/H-NBR ratio of 5/100 (w/w). In contrast, the tensile strengths and strain-to-failure values of the composite films prepared using the two systems clearly differed. These different properties are probably caused by differences between the TOCN distributions in the H-NBR matrix and between the H-NBR matrix structures in the two systems. The composite films prepared in the homogeneous system with DMF as the medium are likely to have a more homogeneous distribution of TOCN elements in a homogeneous H-NBR polymer matrix, resulting in a higher tensile strength and work-of-fracture at TOCN/H-NBR = 5/100 (w/w) compared with those of the films prepared in the heterogeneous system with water.