Hydrophobization of Cellulose Nanocrystals for Aqueous Colloidal Suspensions and Gels

Abstract

Surface hydrophobization of cellulose nanomaterials has been used in the development of nanofiller-reinforced polymer composites and formulations based on Pickering emulsions. Despite the well-known effect of hydrophobic domains on self-assembly or association of water-soluble polymer amphiphiles, very few studies have addressed the behavior of hydrophobized cellulose nanomaterials in aqueous media. In this study, we investigate the properties of hydrophobized cellulose nanocrystals (CNCs) and their self-assembly and amphiphilic properties in suspensions and gels. CNCs of different hydrophobicity were synthesized from sulfated CNCs by coupling primary alkylamines of different alkyl chain lengths (6, 8, and 12 carbon atoms). The synthetic route permitted the retention of surface charge, ensuring good colloidal stability of hydrophobized CNCs in aqueous suspensions. We compare surface properties (surface charge, ζ potential), hydrophobicity (water contact angle, microenvironment probing using pyrene fluorescence emission), and surface activity (tensiometry) of different hydrophobized CNCs and hydrophilic CNCs. Association of hydrophobized CNCs driven by hydrophobic effects is confirmed by X-ray scattering (SAXS) and autofluorescent spectroscopy experiments. As a result of CNC association, CNC suspensions/gels can be produced with a wide range of rheological properties depending on the hydrophobic/hydrophilic balance. In particular, sol–gel transitions for hydrophobized CNCs occur at lower concentrations than hydrophilic CNCs, and more robust gels are formed by hydrophobized CNCs. Our work illustrates that amphiphilic CNCs can complement associative polymers as modifiers of rheological properties of water-based systems.

Employing photoluminescence to rapidly follow aggregation and dispersion of cellulose nanofibrils

“…Under excitation at 405 nm, the emission spectra for CNMs, such as OCNFs (Figure 1a) and CNCs, 35 appear to have two dominant emission wavebands centred at 468 and 504 nm. The excitation/emission shift observed for the first band is the same as that reported by Gong et al (emission maxima at 427 nm under excitation at 365 nm), 29 for native cellulose.…”

“…This theory was previously used to accurately predict the concentration at which the chiral nematic phase of a CNC suspension forms and the CAC of hydrophobized CNCs, 35 both of which could be reasonably assumed to be inflexible spherocylinders due to their highly crystalline structure. 1,2 Comparatively, OCNFs have higher aspect ratios and consist of multiple crystalline and disordered domains, resulting in flexible chains that impact the concentration at which interactions occur and their ability to pack.…”

“…2,25,26 The autofluorescence of poly-and oligo-saccharides, from various sources, has previously been widely reported. 6,[27][28][29][30][31][32][33][34][35] This autofluorescence is generally attributed to contamination of the sample with aromatic compounds, i.e. lignin, or proteins, dependent on the source.…”

“…We confirmed that the intensity ratio between the two observed bands, centred at 468 and 504 nm, and attributed to intra-and inter-particle molecular forces respectively, was concentration dependent, enabling a prediction of the critical aggregation concentration (CAC). 35 Here we investigate the effects of concentration and protonation on the autofluorescence of oxidised CNFs (OCNFs). We demonstrate that the ratio between the two previously reported bands are impacted by both phenomena and support our previous assumptions.…”

Nanocellulose: From Fundamentals to Advanced Applications

“…Furthermore, the use of non-polar media causes a poor dispersion of CNCs due to their ability to generate aggregates owing to the presence of polar chemical groups and the high surface energy of these nanoparticles (Dufresne, 2019;Younas et al, 2019). Hence, surface modifications via covalent binding, surfactants, ionic interactions, reductive amination, physical adsorption, and molecule/polymer grafting have been demonstrated as efficient approaches for enhancing the compatibility between nanocomposite components, thereby improving the dispersion and interaction between them (Thakur et al, 2013;Abitbol et al, 2016;Younas et al, 2019;Nigmatullin et al, 2020). Moreover, the processing temperature of CNCs and some engineering plastics such as polyethylene and polypropylene is another shortcoming, which requires the development of high-performance natural fiber-reinforced composites (Gopi et al, 2019).…”