Figure 5
Polarizing microscopy textural traces of CNC helix formation in the liquid crystalline (wet) state (a) and in solid CNC samples obtained by drying (b–d). Scale bars correspond to 50 μm. (a) Characteristic regularly spaced helix lines in an aqueous suspension with 5 wt% CNC derived by sulfuric acid hydrolysis of wood pulp, observed in transmission. The distance between two lines is half the pitch (p/2), as illustrated in the schematic; hence, the equilibrated helix of this sample has p≈13 μm. (b–d) Typical dried film of CNC (same source as above, starting concentration 4.8 wt%) with nonuniform helix orientations and a strong spread in p, observed in reflection. In (b), the film is illuminated with linear polarized light and observed through a crossed polarizer (analyzer). In (c) and (d), it is illuminated with unpolarized light and observed through a λ/4 phase plate followed by the analyzer, together constituting an analyzer for left- (c) and right-handed (d) circular polarization, respectively. The inset in (d) shows the iridescence from a macroscopic, dried CNC film on a circular glass substrate (25 mm in diameter) against a black background, viewed at a slight angle.
