Enantioselective disruption of cellulose nanocrystal self-assembly into chiral nematic phases in D-alanine solutions.

The chiral environment of enantiomerically pure D-alanine solutions is observed to disrupt and modify the entropy-driven assembly of cellulose nanocrystals (CNCs) into a chiral nematic mesophase. The effect is specific to D-alanine and cannot be attributed to the adsorption of alanine molecules (neither D- nor L-alanine) onto the CNC particles.

Charge Regulation of Poly(acrylic acid) in Solutions of Non-Charged Polymer and Colloids.

Weak polyelectrolytes (WPEs) are responsive materials used as active charge regulators in a variety of applications, including controlled release and drug delivery in crowded bio-related and synthetic environments. In these environments, high concentrations of solvated molecules, nanostructures, and molecular assemblies are ubiquitous. Here, we investigated the effect of high concentrations of non-adsorbing, short chains of poly(vinyl alcohol), PVA, and colloids dispersed by the very same polymers on charge regulation (CR) of poly(acrylic acid), PAA. PVA does not interact with PAA (throughout the full pH range) and thus can be used to examine the role of non-specific (entropic) interactions in polymer-rich environments. Titration experiments of PAA (mainly 100 kDa in dilute solutions, no added salt) were carried out in high concentrations of PVA (13-23 kDa, 5-15 wt%) and dispersions of carbon black (CB) decorated by the same PVA (CB-PVA, 0.2-1 wt%). The calculated equilibrium constant (and pKa) was up-shifted in PVA solutions by up to ~0.9 units and down-shifted in CB-PVA dispersions by ~0.4 units. Thus, while solvated PVA chains increase the charging of the PAA chains, as compared to PAA in water, CB-PVA particles reduce PAA charging. To investigate the origins of the effect, we analyzed the mixtures using small-angle X-ray scattering (SAXS) and cryo-TEM imaging. The scattering experiments revealed re-organization of the PAA chains in the presence of the solvated PVA but not in the CB-PVA dispersions. These observations clearly indicate that the acid-base equilibrium and the degree of ionization of PAA in crowded liquid environments is affected by the concentration, size, and geometry of seemingly non-interacting additives, probably due to depletion and excluded volume interactions. Thus, entropic effects that do not depend on specific interactions should be taken into consideration when designing functional materials in complex fluid environments.

Effects of Non-Ionic Micelles on the Acid-Base Equilibria of a Weak Polyelectrolyte.

Weak polyelectrolytes (WPEs) are widely used as pH-responsive materials, pH modulators and charge regulators in biomedical and technological applications that involve multi-component fluid environments. In these complex fluids, coupling between (often weak) interactions induced by micelles, nanoparticles and molecular aggregates modify the pKa as compared to that measured in single component solutions. Here we investigated the effect of coupling between hydrogen bonding and excluded volume interactions on the titration curves and pKa of polyacrylic acid (PAA) in solutions comprising PEO-based micelles (Pluronics and Brij-S20) of different size and volume fraction. Titration experiments of dilute, salt-free solutions of PAA (5 kDa, 30 kDa and 100 kDa) at low degree of polymer ionization (α < 0.25) drive spatial re-organization of the system, reduce the degree of ionization and consequentially increase the pKa by up to ~0.7 units. These findings indicate that the actual degree of ionization of WPEs measured in complex fluids is significantly lower (at a given pH) than that measured in single-component solutions.

Surfactant-Mediated Co-Existence of Single-Walled Carbon Nanotube Networks and Cellulose Nanocrystal Mesophases.

Hybrids comprising cellulose nanocrystals (CNCs) and percolated networks of single-walled carbon nanotubes (SWNTs) may serve for the casting of hybrid materials with improved optical, mechanical, electrical, and thermal properties. However, CNC-dispersed SWNTs are depleted from the chiral nematic (N*) phase and enrich the isotropic phase. Herein, we report that SWNTs dispersed by non-ionic surfactant or triblock copolymers are incorporated within the surfactant-mediated CNC mesophases. Small-angle X-ray measurements indicate that the nanostructure of the hybrid phases is only slightly modified by the presence of the surfactants, and the chiral nature of the N* phase is preserved. Cryo-TEM and Raman spectroscopy show that SWNTs networks with typical mesh size from hundreds of nanometers to microns are distributed equally between the two phases. We suggest that the adsorption of the surfactants or polymers mediates the interfacial interaction between the CNCs and SWNTs, enhancing the formation of co-existing meso-structures in the hybrid phases.

Nano-to-meso structure of cellulose nanocrystal phases in ethylene-glycol-water mixtures.

The self-assembly and phase behavior of cellulose nanocrystals (CNCs) in binary liquid mixtures of ethylene-glycol (EG):water was investigated. Our findings indicate that a small fraction of water delays the onset of colloidal jammed states previously reported in water-free organic solvents. Here the full phase diagram of CNCs evolves, including the chiral nematic phase (N*), characterized by long-range orientational order and non-isotropic macroscopic properties. Furthermore, the effect of the solvent-mixture composition on the properties of the CNC mesophases is found to be scale-dependent: the micron-size pitch of the N* phase decreases as the dielectric constant (εr) of the solvent mixture is reduced (higher EG content). Yet the nanometric inter-particle spacing of the CNC rods (measured using SAXS and cryo-TEM) is almost independent on the EG content. Also, unlike theoretical predictions, the transition to the biphasic regime is not sensitive to εr of the solvent mixtures and takes place at a higher CNC volume fraction than in aqueous suspensions. These observations may be rationalized by hypothesizing that vicinal water, adsorbed at the CNC surface, prevents kinetic arrest, and dictates the local dielectric constant and thus the effective diameter of the rods (via the Debye length), while εr of the liquid-mixture dominates the pitch length (micron scale) and the optical properties. These findings indicate that the water content of EG:water mixtures may be used for engineering colloidal inks where delayed kinetic arrest and jamming of the CNCs enable printing and casting of tunable, optically-active thin films and coatings.

The role of polymer-solvent interactions in polyvinyl-alcohol dispersions of multi-wall carbon nanotubes: from coagulant to dispersant.

Dispersion of carbon nanotubes in solutions of polyvinyl-alcohol is required for solution casting of composite materials with improved interfacial adhesion where chains adsorbed on the nanotubes serve in the dual role of dispersant and compatible "connector" to the polyvinyl-alcohol matrix. Yet polyvinyl-alcohol is known to induce coagulation of nanotubes in aqueous solutions and thus far, it has not been used for dispersing pristine nanotubes. Here, we report that non-fully hydrolyzed (80-90%) polyvinyl-alcohol can be used for the preparation of stable, surfactant-free, dispersions of multi-wall carbon nanotubes in ethanol-water mixtures (of at least 50 vol% ethanol). Cryo-TEM imaging and rheological measurements of stable, long-lived dispersions reveal the formation of random networks of suspended tubes, with an averaged mesh size of ∼500 nm, indicating that the individual tubes do not aggregate or coagulate. We hypothesize that the polyvinyl-acetate sequences found in non-fully hydrolyzed polymers swell in the presence of ethanol, leading to the formation of a long-ranged steric (entropic) repulsion among polymer-decorated nanotubes. The unexpected role of the polyvinyl-acetate sequences along with a detailed dispersion mechanism are described.

Two-year follow-up of bone mineral density changes in the knee after meniscal allograft transplantation: Results of an explorative study.

BACKGROUND: The potential chondroprotective effect of meniscal allograft transplantation (MAT) is unclear. Subchondral bone mineral density (BMD) and subchondral bone remodeling play important roles in osteoarthritis development. Evaluation of subchondral BMD after MAT might give more insight into the potential chondroprotective effect. The purpose of this study was to determine early BMD changes in the knee after MAT. METHODS: Twenty-six consecutive patients underwent MAT during 2010-2013. The BMD was measured using dual-energy x-ray absorptiometry (DXA) scan preoperatively, and six months, one and two years postoperatively. Bone mineral density was measured in six regions of interest (ROIs) in the tibia and femur (medial, central, lateral) in both treated and healthy contralateral knees. RESULTS: The BMD levels of MAT knees did not significantly change during two years of follow-up in almost all ROIs. Bone mineral density was significant higher in nearly all ROIs in MAT knees at almost all follow-ups compared to healthy contralateral knees. In the healthy contralateral knees, BMD slightly, but not statistically, decreased in the first postoperative year, where it normalized to baseline values at two-year follow-up. The BMD levels in all ROIs did not significantly differ between the patients with or without chondropathy at baseline and two-year follow-up. CONCLUSION: Based on the findings, MAT did not show a significant influence on BMD in the first two postoperative years. Longer follow-up is necessary to prove the potential chondroprotective effect of MAT using BMD measurements.