Abstract
Solvent-induced physical hydrogels of TEMPO-oxidized cellulose nanofibrils (OCNFs) were obtained from aqueous/alcoholic dispersions of fibrils in lower alcohols, namely, methanol, ethanol, 1-propanol and 2-propanol. The sol-gel transition occurs above a critical alcohol concentration of ca. 30 wt% for all alcohols tested. The rheological properties of the hydrogels depend on the nature of the alcohol: for ethanol, 1-propanol and 2-propanol the magnitude of the shear storage modulus follows the alcohol hydrophilicity, whilst methanol produces the weakest gels in the group. Above a second critical concentration, ca. 60 wt% alcohol, phase separation is observed as the gels undergo syneresis. Analysis of small-angle X-ray scattering data shows that the OCNFs may be modelled as rigid rods. In the presence of lower alcohols, attractive interactions between nanofibrils are present and, above the alcohol concentration leading to gelation, an increase of the OCNF cross-section is observed, suggesting alcohol induced aggregation of nanofibrils.
Highlights
Self-assembly is a spontaneous natural process involving the formation of large complex structures from existing disordered building blocks,[1,2] which may be directed by the intrinsic properties of the building blocks, e.g., shape and size,[3,4,5] as well as specific or nonspecific attractive and repulsive interactions.[6]
We studied the influence of low molecular weight alcohols on the self-assembly of oxidized cellulose nanofibrils (OCNFs) dispersions
The four alcohols tested resulted in similar rheological behaviour regarding the sol–gel transition, the figures presented focus on ethanol/water systems, with data pertaining to other alcohols contained in the Electronic supplementary information (ESI).‡ Two main behaviours can be readily observed with the increase of ethanol content: the storage modulus (G0) increases with the alcohol content and becomes less frequency dependent, indicated by the shallower slope of the curves, as illustrated for ethanol in Fig. 1A and for the other alcohols in the ESI,‡ Fig. S1
Summary
Self-assembly is a spontaneous natural process involving the formation of large complex structures from existing disordered building blocks,[1,2] which may be directed by the intrinsic properties of the building blocks, e.g., shape and size,[3,4,5] as well as specific or nonspecific attractive and repulsive interactions.[6]. Cellulose nanofibrils have lengths of a few hundred nanometres and cross-sections of up to tens of nanometres, leading to particles with very large aspect ratios.[15,16,17] The ease with which cellulose-based particles can be surface modified offers additional advantages, as particles can be tailored for specific applications.[18,19] TEMPO-mediated oxidation[20] is a chemoselective oxidation of the glucosyl C6 primary hydroxyl groups by NaOCl mediated by (2,2,6,6-tetramethyl-piperidin-1-yl)oxyl (TEMPO)/NaBr in water. These oxidized cellulose nanofibrils (OCNFs) are anionic particles and form stable dispersions of individualized nanofibrils in water.[20,21,22] OCNFs can selfassemble in an aqueous environment due to the influence of concentration,[23] OCNF aspect ratio,[24] co-solutes, such as surfactants,[16] salts,[9,25,26] or block copolymers[27] as well as pH.[28,29,30] This flexibility makes OCNFs a good choice as building blocks in self-assembled hydrogels
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