Unexpected microphase transitions in flow towards nematic order of cellulose nanocrystals

Abstract

Organization of nanoparticles is essential in order to control their light-matter interactions. We present cellulose nanocrystal suspension organization in flow towards a unidirectional state. Visualization of evolving polarization patterns of the cellulose nanocrystal suspensions is combined with steady and oscillatory shear rheology. Elucidation of the chiral nematic mesophase in a continuous process towards unidirectional order enables control of alignment in a suspension precursor for structural films and reveals thus far in situ unrevealed transition states that were not detectable by rheology alone. The coupled analytics enabled the suspensions of interest to be divided into rheological gels and rheological liquid crystal fluids with detailed information on the microtransition phases. Both populations experienced submicron organization and reached macro-scale homogeneity with unidirectional ordering in continued shear. We quantify the time, shear rate, and recovery time after shear to design an optimizing formation process for controlled wet structures as precursors for dry products. Roland Kádár and Tiina Nypelö have contributed equally to this work.

In Situ Visualization of the Structural Evolution and Alignment of Lyotropic Liquid Crystals in Confined Flow

“…[ 33 ] This is an indication that the rheological material response in both phases is dominated by the interaction between the self‐assembled structures. [ 34 ] The moduli from strain sweep tests are presented in Figure 2b, for an angular frequency of 1.0 rad s −1 . From a quantitative point of view, the dynamic moduli in the linear viscoelastic region (≈<0.1%) correspond to the frequency sweep data in Figure 2a, confirming again the gel‐like material response.…”

Shear-Coated Linear Birefringent and Chiral Cellulose Nanocrystal Films Prepared from Non-Sonicated Suspensions with Different Storage Time

“…In contradistinction, the larger viscosity of the non-sonicated suspensions in the biphasic regime investigated here locks the alignment of CNC. Microphase transitions in flow towards the nematic order of CNC suspensions showing shear thinning behavior have been reported [ 36 ]. However, shear-thinning behavior of CNC suspensions does not ensure alignment of nanocrystals, but a breakup of CNC aggregates might appear [ 37 ].…”

Experimental Studies of Flow-Induced Birefringence in Complex Fluids

“…Recently, FIB measurement has also been used to analyze cellulose nanocrystals (CNCs), and CNC birefringence patterns using the crossed-Nicol method have been reported [11][12][13][14] . Kádár et al 14) reported birefringence patterns as being sensitive to flow-induced structural changes, thus FIB measurement is considered to be an essential tool for examining the flow-induced structure of CNC suspensions. Tanaka et al 10) developed a highly sensitive apparatus for oscillatory FIB measurement and succeeded in measuring a low-bire-fringent suspension of cellulose nanofibers.…”