Manipulating the colloidal properties of (non-)sulfated cellulose nanocrystals via stepwise surface cyanoethylation/carboxylation

When creating new materials, surface modification is an effective means of controlling the characteristics and the colloidal properties of cellulose nanocrystals (CNC), thereby increasing its compatibility with composite matrices. In this paper, the CNC surface is modified in aqueous media by cyanoethyl groups, followed by the hydrolysis of grafted nitrile groups. Particles having a surface composition close to that of native cellulose (N-CNC) and particles with sulfated surfaces (S-CNC) are used. The influence of the functional composition of CNC, as well as that of temperature and the ratio of reagents on the modified particles is investigated. It is shown that under identical synthesis conditions, a higher degree of cyanoethylation can be achieved for N-CNC. Particles modified with only nitrile groups and those with a polyfunctional composition modified with amide and carboxyl groups are obtained. Atomic force microscopy (AFM) and X-Ray diffraction (XRD) data indicate the preservation of both the morphology and structure of CNC post modification. The modified CNC particles are characterized by elemental analysis, synchronous thermal analysis (STA) and Fourier-transform infrared (FTIR) spectroscopy. The thermal stability of the resulting particles is close to that of the starting materials but higher for N-CNC and its derivatives. The acid-base properties of the CNC surface are established using a combination of methods (photometric measurements), namely potentiometric titration, dynamic light scattering, and microelectrophoresis. It is shown that the aggregative stability of modified particles along with the activity and number of acid-base centers on their surface increases sharply after the hydrolysis of nitrile groups.