Soft and tough cellulose nanocrystals interlocked with polyacrylate-bearing cyanobiphenyl ionogels through a double network strategy.

Abstract

There are several examples of double network gels prepared from cellulose nanocrystals (CNC) conjugated with water-soluble polymers. However, soft but tough gels composed of mostly CNCs have been challenging to design due to aggregation or precipitation of CNCs at higher concentrations in solvated and gelled systems. We report a general strategy to introduce covalent and non-covalent interactions within cellulose nanocrystals (CNCs) to develop tough, processable, liquid crystalline (LC) gels. A liquid crystalline 12-methylene spacer cyanobiphenyl (CB12OH), is grafted onto acrylic acid via Steglich esterification and subsequently copolymerized with acrylic acid via free radical polymerization to afford an amphiphilic liquid crystalline polymer (LCP), PAACB12-r-PAA. The free carboxyl (COOH) groups from polyacrylic acid (PAA) in the copolymer are randomly interlocked with the hydroxyl (OH) groups of CNC through covalent crosslinking and host-guest supramolecular hydrogen bonding in DMSO and 1-butyl-3-methylimidazolium acetate ionic liquid. The dominant cyanobiphenyl-cyanobiphenyl (CB12-CB12) interactions yield hierarchically structured, tough, free-standing liquid crystalline nanocomposite ionogels. Rheological studies of the interlocked free-standing ionogels display a typical crosslinked gel behavior with the storage moduli (G') ranging 10³-10⁴ Pa across the entire angular frequency range. Mechanical properties are tailored by varying the concentration of CNCs in the nanocomposite gel. This facile design provides a general strategy to prepare soft and tough liquid crystalline gels from renewable resources.