Effect of Anisotropy of Cellulose Nanocrystal Suspensions on Stratification, Domain Structure Formation, and Structural Colors.

Konrad W Klockars,Maryam Borghei,Anurodh Tripathi,Blaise L Tardy,Luiz G Greca,Orlando J Rojas

doi:10.1021/acs.biomac.8b00497

Konrad W Klockars, Maryam Borghei + Show 4 more

Open Access

https://doi.org/10.1021/acs.biomac.8b00497

Copy DOI

Abstract

Outstanding optical and mechanical properties can be obtained from hierarchical assemblies of nanoparticles. Herein, the formation of helically ordered, chiral nematic films obtained from aqueous suspensions of cellulose nanocrystals (CNCs) were studied as a function of the initial suspension state. Specifically, nanoparticle organization and the structural colors displayed by the resultant dry films were investigated as a function of the anisotropic volume fraction (AVF), which depended on the initial CNC concentration and equilibration time. The development of structural color and the extent of macroscopic stratification were studied by optical and scanning electron microscopy as well as UV–vis spectroscopy. Overall, suspensions above the critical threshold required for formation of liquid crystals resulted in CNC films assembled with longer ranged order, more homogeneous pitches along the cross sections, and narrower specific absorption bands. This effect was more pronounced for the suspensions that were closer to equilibrium prior to drying. Thus, we show that high AVF and more extensive phase separation in CNC suspensions resulted in large, long-range ordered chiral nematic domains in dried films. Additionally, the average CNC aspect ratio and size distribution in the two separated phases were measured and correlated to the formation of structured domains in the dried assemblies.

Highlights

Cellulose nanocrystals (CNCs) offer a significant opportunity to form hierarchically structured materials mimicking the most robust materials observed in nature.[1,2] cellulose nanocrystals (CNCs) are generally obtained by hydrolysis of plant or marine biomass,[3] resulting in highly crystalline and rodlike assemblies with dimensions ranging from 3 to 70 nm in width and 35 to 3000 nm in length.[3]
The helical order in dry films formed from aqueous suspensions of cellulose nanocrystals (CNCs) was determined as a function of the anisotropic volume fraction (AVF) of the suspensions
The AVF increased as a function of concentration and comprised CNCs of a larger aspect ratio, which phase-separated at a lower concentration compared to that required for smaller CNCs

Summary

INTRODUCTION

Cellulose nanocrystals (CNCs) offer a significant opportunity to form hierarchically structured materials mimicking the most robust materials observed in nature.[1,2] CNCs are generally obtained by hydrolysis of plant or marine biomass,[3] resulting in highly crystalline and rodlike assemblies with dimensions ranging from 3 to 70 nm in width and 35 to 3000 nm in length.[3]. We evaluated CNC segregation by size and size polydispersity as a function of the state of equilibration of the liquid crystalline CNC suspension, i.e., the role of AVF on the long-range order and optical properties of CNC films. For this evaluation, our experiments included CNC films produced from suspensions with identical total solid content but varied initial concentrations. The resultant films exhibited a more distinct gradient of pitches along the cross section, resulting in narrower specific absorption bands, as evidenced by UV−vis These effects were significantly less pronounced for films formed from suspensions below the critical concentration. The results bear meaning to the formation of structured materials from liquid crystalline suspensions formed from other mesogens

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES