Formation of Chiral Nematic Films from Cellulose Nanocrystal Suspensions Is a Two-Stage Process

The evaporation of aqueous suspensions of cellulose nanocrystals (CNCs) gives iridescent chiral nematic films with reflection colors at visible wavelengths. A key problem is controlling the chiral nematic pitch, P, and hence the reflection colors of CNC films. By adding D-(+)-glucose to the suspension, we show that the change in P during evaporation occurs in two distinct stages. The first stage is the decrease in P as the concentration of CNC in the chiral nematic suspension increases due to evaporation; the addition of glucose causes a decrease in P at this stage. In a second stage, a concentration of CNC is reached where the formation of ordered gels and glasses prevents further major changes in P. The addition of glucose lowers the CNC concentration at which this occurs, leading to an increase in P and hence an overall shift to the red end of the spectrum in the final film.