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.
Cellulose nanocrystals (CNCs) that bind to each other through associative hydrophobic interactions have been synthesized by modifying sulfated CNCs (sCNCs) with hydrophobic moieties. These octyl-CNCs form gels at significantly lower concentrations than parent sCNCs, producing extremely strong hydrogels. Unlike sCNCs, these octyl-CNCs do not form ordered liquid crystalline phases indicating a random association into a robust network driven by hydrophobic interactions. Furthermore, involvement of the octyl-CNCs into multicomponent supramolecular assembly was demonstrated in combination with starch. AFM studies confirm favorable interactions between starch and octyl-CNCs, which is thought to be the source of the dramatic increase in gel strength.
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