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See detailSelf-Organizing Cellulose Nanorods: From the fundamental physical chemistry of self-assembly to the preparation of functional films
Honorato Rios, Camila UL

Doctoral thesis (2019)

Cellulose nanocrystals (CNCs), nanorods isolated by acid hydrolysis from cellulose sources, be- long to a selective type of functional biomaterials. The intriguing ability of these nanoparticles to self ... [more ▼]

Cellulose nanocrystals (CNCs), nanorods isolated by acid hydrolysis from cellulose sources, be- long to a selective type of functional biomaterials. The intriguing ability of these nanoparticles to self-organize and develop a chiral nematic liquid crystal phase when suspended in aqueous suspensions, is increasing interest regardless of the diverse range of research fields. Unfortu- nately (or fortunately, for this thesis), pristine CNCs are always disperse, with great variations in rod length within a single sample. Of particular interest is the fractionation of CNC rods by separation of the coexisting phases: isotropic phase from the liquid crystalline (LC) part. Since the aspect ratio is considered to be the critical parameter that dictates the particle fraction at which cholesteric-isotropic phase separation starts, it is expected that the high aspect ratio rods will separate from low aspect ratio rods, and this is indeed what I found in this thesis. By a systematic repetition of separation of phases, I could reach a quality of separation of long from short rods that is remarkable. The fractionation procedure was then improved by varying the equilibrium phase volume fraction at which the phases were separated, reducing with this new procedure the multiple separations from five cycles to only one. The onset of liquid crystallinity was drastically reduced in the long rod fraction and the decrease in the threshold for complete liquid crystallinity was even stronger. The mass fraction threshold at which gelation of the CNC suspension is triggered is not at all affected by the fractionation. Since gelation is a percolation phenomenon, the expectation was that also the onset of gelation would move to lower mass fractions, but this remained at about the same value. Together with the shift to lower mass fractions of the cholesteric liquid crystal phase formation we have thus opened access to a whole new range of the equilibrium phase diagram, where the full sample is cholesteric yet not gelled. I demonstrate that the critical parameter for inducing gelation is in fact not the fraction of CNC, but the concentration of counterions in the solution. This suggests that the gelation is more complex than direct percolation between individual CNC rods, and instead is related to loss of colloidal stability due to reduced electrostatic screening. I also show that the behavior of key parameters, such as the period of the helical modulation, so-called pitch, that is characteristic of the cholesteric phase, is very different in the range of phase coexistence compared to the range of complete liquid crystallinity. In addition, I found that the dependence of the pitch on CNC mass fraction has less to do with the size of the nanorods but rather than with the variation of effective volume fraction as a result of more rods in the suspension or higher counterion concentration. I corroborate this hypothesis by adding different amounts of salt to CNC suspensions of varying mass fraction such that the ion concentration is held constant, thereby tuning the pitch to the same value throughout the suspensions. In films prepared by drying CNC suspensions, the pitch can go down to a few hundred nanome- ters, resulting in circularly polarized colorful Bragg re ection of visible light. By working with the long-rod fraction we can absolutely obtain a highly-ordered monodomain structure that results in uniform color of films, with only one circular polarization re ected, as should be the case. While the study is carried out on CNCs, the implications go far beyond this particular nanoma- terial, revealing new challenges and opportunities in general liquid crystal and colloid physics, as well as in strategic research where fractionation and the drying of initially disperse populations of nanorods is desirable. [less ▲]

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See detailFractionation of cellulose nanocrystals: enhancing liquid crystal ordering without promoting gelation
Honorato Rios, Camila UL; Lehr, Claudius Moritz UL; Schütz, Christina UL et al

in NPG asia materials (2018)

Colloids of electrically charged nanorods can spontaneously develop a fluid yet ordered liquid crystal phase, but this ordering competes with a tendency to form a gel of percolating rods. The threshold ... [more ▼]

Colloids of electrically charged nanorods can spontaneously develop a fluid yet ordered liquid crystal phase, but this ordering competes with a tendency to form a gel of percolating rods. The threshold for ordering is reduced by increasing the rod aspect ratio, but the percolation threshold is also reduced with this change; hence, prediction of the outcome is nontrivial. Here, we show that by establishing the phase behavior of suspensions of cellulose nanocrystals(CNCs) fractionated according to length, an increased aspect ratio can strongly favor liquid crystallinity without necessarily influencing gelation. Gelation is instead triggered by increasing the counterion concentration until theCNCs lose colloidal stability, triggering linear aggregation, which promotes percolation regardless of the original rod aspect ratio. Our results shine new light on the competition between liquid crystal formation and gelation in nanoparticle suspensions and provide a path for enhanced control of CNC self-organization for applications in photonic crystal paper or advanced composites. [less ▲]

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See detailEnhancing Self-Assembly in Cellulose Nanocrystal Suspensions Using High-Permittivity Solvents
Bruckner, Johanna; Kuhnhold, Anja UL; Honorato Rios, Camila UL et al

in Langmuir (2016)

Helical liquid crystal self-assembly in suspensions of cellulose nanocrystals (CNCs), bioderived nanorods exhibiting excellent mechanical and optical properties, opens attractive routes to sustainable ... [more ▼]

Helical liquid crystal self-assembly in suspensions of cellulose nanocrystals (CNCs), bioderived nanorods exhibiting excellent mechanical and optical properties, opens attractive routes to sustainable production of advanced functional materials. For convenience, in most studies until now, the CNCs were suspended in water, leaving a knowledge gap concerning the influence of the solvent. Using a novel approach for aggregation-free solvent exchange in CNC suspensions, here we show that protic solvents with a high dielectric permittivity εr significantly speed up self-assembly (from days to hours) at high CNC mass fraction and reduce the concentration dependence of the helix period (variation reducing from more than 30 μm to less than 1 μm). Moreover, our computer simulations indicate that the degree of order at constant CNC content increases with increasing εr, leading to a shorter pitch and a reduced threshold for liquid crystallinity. In low-εr solvents, the onset of long-range orientational order is coupled to kinetic arrest, preventing the formation of a helical superstructure. Our results show that the choice of solvent is a powerful parameter for tuning the behavior of CNC suspensions, enhancing our ability to control the self-assembly and thereby harvesting valuable novel cellulose-based materials. [less ▲]

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See detailEquilibrium Liquid Crystal Phase Diagrams and Detection of Kinetic Arrest in Cellulose Nanocrystal Suspensions
Honorato Rios, Camila UL; Kuhnhold, Anja UL; Bruckner, Johanna et al

in Frontiers in Materials (2016), 3

The cholesteric liquid crystal self-assembly of water-suspended cellulose nanocrystal (CNC) into a helical arrangement was observed already more than 20 years ago, and the phenomenon was used to produce ... [more ▼]

The cholesteric liquid crystal self-assembly of water-suspended cellulose nanocrystal (CNC) into a helical arrangement was observed already more than 20 years ago, and the phenomenon was used to produce iridescent solid films by evaporating the solvent or via sol–gel processing. Yet, it remains challenging to produce optically uniform films and to control the pitch reproducibly, reflecting the complexity of the three-stage drying process that is followed in preparing the films. An equilibrium liquid crystal phase formation stage is followed by a non-equilibrium kinetic arrest, which in turn is followed by structural collapse as the remaining solvent is evaporated. Here, we focus on the first of these stages, combining a set of systematic rheology and polarizing optics experiments with computer simulations to establish a detailed phase diagram of aqueous CNC suspensions with two different values of the surface charge, up to the concentration where kinetic arrest sets in. We also study the effect of varying ionic strength of the solvent. Within the cholesteric phase regime, we measure the equilibrium helical pitch as a function of the same parameters. We report a hitherto unnoticed change in character of the isotropic–cholesteric transition at increasing ionic strength, with a continuous weakening of the first-order character up to the point where phase coexistence is difficult to detect macroscopically due to substantial critical fluctuations. [less ▲]

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