Lyotropic liquid crystal behavior of carboxylated cellulose nanocrystals

In this work, the lyotropic liquid crystal (LC) behavior of carboxylated cellulose nanocrystals (CNs) has been studied for cotton cellulose prepared by TEMPO-mediated oxidation of cellulose nanocrystals (TOCNs). It is found that suspensions of TOCNs at a critical concentration of 4.1 wt% form anisotropic phase with clear optical birefringence. Furthermore, fingerprint texture is clearly observed when the concentration is 9.0 wt% under a polarizing optical microscope. Fingerprint texture is characteristic of chiral nematic liquid crystal (N*-LC) phase. The band-spacing corresponds to half the helical pitch, and thus, the helical pitch of N*-LCs (9.0 wt%) is about 6.0 μm. The degree of oxidation (DO) and zeta potential of TOCNs in suspension are 0.15 and −59.5 mV, respectively. Relatively larger aspect ratio and higher surface charges density of TOCNs make the critical concentration of LC formation decreased and the helical pitch increased compared to that of the LC prepared by H₂SO₄ hydrolysis system. The interparticle electrostatic repulsion between CNs with different surface charges influence the helical pitch of N*-LCs.