Solvent-free liquid crystals from combination of cellulose nanocrystals with surfactants

Abstract

Biobased and renewable rod-like cellulose nanocrystals (CNCs) classically show cholesteric liquid crystalline (LC) behavior in aqueous media. This helically twisting assembly allows tunable light reflection and structural color by altering the chiral pitch of CNCs. On the other hand, it would be desirable also to achieve CNC-based nematic uniaxially aligned LCs and also plasticized deformable assemblies, thus overcoming the volatile liquid solvent phase. This work addresses the latter problem by introducing CNC-surfactant adducts via combination of polyethyleneglycol alkyl-3-sulfopropyl diether potassium salt surfactants (referred hereafter as AES) with CNCs. The compositions were prepared by mixing CNC aqueous dispersions and AES aqueous solutions at prescribed CNC:AES weight ratios ranging from 1:1 wt:wt up to 1:20 wt:wt, i.e., in excess of AES. Next, water was removed by lyophilization. The samples were characterized by means of polarized optical microscopy (POM), small angle X-ray scattering (SAXS), and UV-Vis spectrophotometry. Pure AES shows self-assembly in bulk: at low temperatures -20 °C and 0 °C it crystallizes, probably promoted by the inherent residual water. At 25 °C and 50 °C, it self-assembles to an oblique 2D liquid crystalline state, whereas at higher temperatures at 100 °C it forms a disordered liquid. CNC:AES 1:1 wt:wt forms macroscopically aggregated structures where the CNC packs in uncontrolled domains. However, at higher AES weight fractions, homogeneous structures appear based on POM. They also allow increased birefringence upon shearing where CNC:AES 1:5 wt:wt shows the highest birefringence. The lower birefringence in CNC:AES 1:10 and 1:20 wt:wt may arise from the reduced CNC steric interactions due to their larger distance. SAXS analysis of CNC:AES 1:5 and 1:20 wt:wt showed that the AES domain involve a related structure as pure AES, albeit less ordered as they are confined between the CNCs. The CNCs did not show clear SAXS peaks probably because of their excessive size distributions. Still, they showed clear q^(−2) scaling, at the length scale relevant for CNC, where q is the magnitude of the scattering vector, thus potentially manifesting the CNC/AES interfaces. The obtained specimens were used to produce free-standing, aligned films of different thicknesses via free-radical photopolymerization. In conclusion, this work shows promising observations to allow CNC-nanocomposites plasticized by the nonvolatile surfactant AES, which allows shear alignment of CNCs.