Nonclassical Crystal Growth of Supramolecular Polymers in Aqueous Medium.

A mechanistic understanding of the principles governing the hierarchical organization of supramolecular polymers offers a paradigm for tailoring synthetic molecular architectures at the nano to micrometric scales. Herein, the unconventional crystal growth mechanism of a supramolecular polymer of superbenzene(coronene)-diphenylalanine conjugate (Cr-FFOEt ) is demonstrated. 3D electron diffraction (3D ED), a technique underexplored in supramolecular chemistry, is effectively utilized to gain a molecular-level understanding of the gradual growth of the initially formed poorly crystalline hairy, fibril-like supramolecular polymers into the ribbon-like crystallites. The further evolution of these nanosized flat ribbons into microcrystals by oriented attachment and lateral fusion is probed by time-resolved microscopy and electron diffraction. The gradual morphological and structural changes reveal the nonclassical crystal growth pathway, where the balance of strong and weak intermolecular interactions led to a structure beyond the nanoscale. The role of distinct π-stacking and H-bonding interactions that drive the nonclassical crystallization process of Cr-FFOEt supramolecular polymers is analyzed in comparison to analogous molecules, Py-FFOEt and Cr-FF forming helical and twisted fibers, respectively. Furthermore, the Cr-FFOEt crystals formed through nonclassical crystallization are found to improve the functional properties.

Edge-On (Cellulose II) and Face-On (Cellulose I) Adsorption of Cellulose Nanocrystals at the Oil-Water Interface: A Combined Entropic and Enthalpic Process.

Nanocelluloses can be used to stabilize oil-water surfaces, forming so-called Pickering emulsions. In this work, we compare the organization of native and mercerized cellulose nanocrystals (CNC-I and CNC-II) adsorbed on the surface of hexadecane droplets dispersed in water at different CNC concentrations. Both types of CNCs have an elongated particle morphology and form a layer strongly adsorbed at the interface. However, while the layer thickness formed with CNC-I is independent of the concentration at 7 nm, CNC-II forms a layer ranging from 9 to 14 nm thick with increasing concentration, as determined using small-angle neutron scattering with contrast-matched experiments. Molecular dynamics (MD) simulations showed a preferred interacting crystallographic plane for both crystalline allomorphs that exposes the CH groups (100 and 010) and is therefore considered hydrophobic. Furthermore, this study suggests that whatever the allomorph, the migration of CNCs to the oil-water interface is spontaneous and irreversible and is driven by both enthalpic and entropic processes.

Quantifying the Contribution of the Dispersion Interaction and Hydrogen Bonding to the Anisotropic Elastic Properties of Chitin and Chitosan.

The elastic tensors of chitin and chitosan allomorphs were calculated using density functional theory (DFT) with and without the dispersion correction and compared with experimental values. The longitudinal Young's moduli were 114.9 or 126.9 GPa for α-chitin depending on the hydrogen bond pattern: 129.0 GPa for ß-chitin and 191.5 GPa for chitosan. Furthermore, the moduli were found to vary between 17.0 and 52.8 GPa in the transverse directions and between 2.2 and 15.2 GPa in shear. Switching off the dispersion correction led to a decrease in modulus by up to 63%, depending on the direction. The transverse Young's moduli of α-chitin strongly depended on the hydroxylmethyl group conformation coupled with the dispersion correction, suggesting a synergy between hydrogen bonding and dispersion interactions. The calculated longitudinal Young's moduli were, in general, higher than experimental values obtained in static conditions, and the Poisson's ratios were lower than experimental values obtained in static conditions.

Small Angle Neutron Scattering Shows Nanoscale PMMA Distribution in Transparent Wood Biocomposites.

Transparent wood biocomposites based on PMMA combine high optical transmittance with excellent mechanical properties. One hypothesis is that despite poor miscibility the polymer is distributed at the nanoscale inside the cell wall. Small-angle neutron scattering (SANS) experiments are performed to test this hypothesis, using biocomposites based on deuterated PMMA and "contrast-matched" PMMA. The wood cell wall nanostructure soaked in heavy water is quantified in terms of the correlation distance d between the center of elementary cellulose fibrils. For wood/deuterated PMMA, this distance d is very similar as for wood/heavy water (correlation peaks at q ≈ 0.1 Å-1). The peak disappears when contrast-matched PMMA is used, indeed proving nanoscale polymer distribution in the cell wall. The specific processing method used for transparent wood explains the nanocomposite nature of the wood cell wall and can serve as a nanotechnology for cell wall impregnation of polymers in large wood biocomposite structures.

Bottom-up Construction of Xylan Nanocrystals in Dimethyl Sulfoxide.

A new type of polysaccharide (hemicellulose) nanocrystal, bearing the shape of an anisotropic nanoflake, emerged from a dimethyl sulfoxide (DMSO) dispersion of wood-based xylan through heat-induced crystallization. The dimensions of these xylan nanocrystals were controlled by the crystallization conditions. Sharp signals in solid-state NMR indicated a well-ordered crystal structure. The unit cell is constituted of two asymmetric xylose residues, and DMSO molecules resided in a host-guest type of arrangement with more than one local environment. This corroborates with the identical 1H NMR relaxation time between DMSO and xylan, indicative of intimate mixing of the two at the tens of nanometer length scale. X-ray and electron diffraction indicated a 2-fold helical helix along the chain in a monoclinic unit cell with an antiparallel arrangement, with chains placed on the 2-fold helix axes: at the corner and at the center. The 2-fold helical structure is unique for xylan for which only a 3-fold helical form has been reported. The DMSO molecules participated in the crystallization, and they were shown to be vital in stabilizing the crystalline structure. The manipulation of temperature, concentration, and incubation time of the xylan/DMSO dispersion provided pathways for the crystallization to form size-adjustable nanocrystals. As 20-30% of biomass consists of hemicelluloses, this work will serve as a starting point to understand the controlled assembly of hemicelluloses to discover their full application potential.

Cellulose crystals plastify by localized shear.

Cellulose microfibrils are the principal structural building blocks of wood and plants. Their crystalline domains provide outstanding mechanical properties. Cellulose microfibrils have thus a remarkable potential as eco-friendly fibrous reinforcements for structural engineered materials. However, the elastoplastic properties of cellulose crystals remain poorly understood. Here, we use atomistic simulations to determine the plastic shear resistance of cellulose crystals and analyze the underpinning atomic deformation mechanisms. In particular, we demonstrate how the complex and adaptable atomic structure of crystalline cellulose controls its anisotropic elastoplastic behavior. For perfect crystals, we show that shear occurs through localized bands along with noticeable dilatancy. Depending on the shear direction, not only noncovalent interactions between cellulose chains but also local deformations, translations, and rotations of the cellulose macromolecules contribute to the response of the crystal. We also reveal the marked effect of crystalline defects like dislocations, which decrease both the yield strength and the dilatancy, in a way analogous to that of metallic crystals.

Supramolecular Assembly and Chirality of Synthetic Carbohydrate Materials.

Hierarchical carbohydrate architectures serve multiple roles in nature. Hardly any correlations between the carbohydrate chemical structures and the material properties are available due to the lack of standards and suitable analytic techniques. Therefore, designer carbohydrate materials remain highly unexplored, as compared to peptides and nucleic acids. A synthetic D-glucose disaccharide, DD, was chosen as a model to explore carbohydrate materials. Microcrystal electron diffraction (MicroED), optimized for oligosaccharides, revealed that DD assembled into highly crystalline left-handed helical fibers. The supramolecular architecture was correlated to the local crystal organization, allowing for the design of the enantiomeric right-handed fibers, based on the L-glucose disaccharide, LL, or flat lamellae, based on the racemic mixture. Tunable morphologies and mechanical properties suggest the potential of carbohydrate materials for nanotechnology applications.

Periodate Oxidation Followed by NaBH4 Reduction Converts Microfibrillated Cellulose into Sterically Stabilized Neutral Cellulose Nanocrystal Suspensions.

The periodate oxidation of microfibrillated cellulose followed by a reduction treatment was implemented to produce a new type of sterically stabilized cellulosic nanocrystals, which were characterized at the molecular and colloidal length scales. Solid-state NMR data showed that these treatments led to objects consisting of native cellulose and flexible polyols resulting from the oxidation and subsequent reduction of cellulose. A consistent set of data from dynamic light scattering, turbidimetry, transmission electron microscopy, and small-angle X-ray scattering experiments further showed that stable neutral elongated nanoparticles composed of a crystalline cellulosic core surrounded by a shell of dangling polyol chains were produced. The dimensions of these biosourced nanocrystals could be controlled by the degree of oxidation of the parent dialdehyde cellulose sample. The purely steric origin of the colloidal stability of these nanoparticles is a strong asset for their use under conditions where electrostatics no longer provides colloidal stability.

Molecular interactions in nanocellulose assembly.

The contribution of hydrogen bonds and the London dispersion force in the cohesion of cellulose is discussed in the light of the structure, spectroscopic data, empirical molecular-modelling parameters and thermodynamics data of analogue molecules. The hydrogen bond of cellulose is mainly electrostatic, and the stabilization energy in cellulose for each hydrogen bond is estimated to be between 17 and 30 kJ mol-1 On average, hydroxyl groups of cellulose form hydrogen bonds comparable to those of other simple alcohols. The London dispersion interaction may be estimated from empirical attraction terms in molecular modelling by simple integration over all components. Although this interaction extends to relatively large distances in colloidal systems, the short-range interaction is dominant for the cohesion of cellulose and is equivalent to a compression of 3 GPa. Trends of heat of vaporization of alkyl alcohols and alkanes suggests a stabilization by such hydroxyl group hydrogen bonding to be of the order of 24 kJ mol-1, whereas the London dispersion force contributes about 0.41 kJ mol-1 Da-1 The simple arithmetic sum of the energy is consistent with the experimental enthalpy of sublimation of small sugars, where the main part of the cohesive energy comes from hydrogen bonds. For cellulose, because of the reduced number of hydroxyl groups, the London dispersion force provides the main contribution to intermolecular cohesion.This article is part of a discussion meeting issue 'New horizons for cellulose nanotechnology'.

X-ray crystal structure of anhydrous chitosan at atomic resolution.

We determined the crystal structure of anhydrous chitosan at atomic resolution, using X-ray fiber diffraction data extending to 1.17 Å resolution. The unit cell [a = 8.129(7) Å, b = 8.347(6) Å, c = 10.311(7) Å, space group P21 21 21 ] of anhydrous chitosan contains two chains having one glucosamine residue in the asymmetric unit with the primary hydroxyl group in the gt conformation, that could be directly located in the Fourier omit map. The molecular arrangement of chitosan is very similar to the corner chains of cellulose II implying similar intermolecular hydrogen bonding between O6 and the amine nitrogen atom, and an intramolecular bifurcated hydrogen bond from O3 to O5 and O6. In addition to the classical hydrogen bonds, all the aliphatic hydrogens were involved in one or two weak hydrogen bonds, mostly helping to stabilize cohesion between antiparallel chains. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 361-368, 2016.

Fast and Robust Nanocellulose Width Estimation Using Turbidimetry.

The dimensions of nanocelluloses are important factors in controlling their material properties. The present study reports a fast and robust method for estimating the widths of individual nanocellulose particles based on the turbidities of their water dispersions. Seven types of nanocellulose, including short and rigid cellulose nanocrystals and long and flexible cellulose nanofibers, are prepared via different processes. Their widths are calculated from the respective turbidity plots of their water dispersions, based on the theory of light scattering by thin and long particles. The turbidity-derived widths of the seven nanocelluloses range from 2 to 10 nm, and show good correlations with the thicknesses of nanocellulose particles spread on flat mica surfaces determined using atomic force microscopy.

Linear, non-linear and plastic bending deformation of cellulose nanocrystals.

The deformation behaviour of cellulose nanocrystals under bending loads was investigated by using atomistic molecular dynamics (MD) simulations and finite element analysis (FEA), and compared with electron micrographs of ultrasonicated microfibrils. The linear elastic, non-linear elastic, and plastic deformation regions were observed with increasing bending displacements. In the linear elastic region, the deformation behaviour was highly anisotropic with respect to the bending direction. This was due to the difference in shear modulus, and the deformation could be approximated by standard continuum mechanics using the corresponding elastic tensors. Above the linear elastic region, the shear deformation became a dominant factor as the amplitude of shear strain drastically increased. Plastic deformation limit was observed at the bending angle above about 60°, independent of the bending direction. The morphology of the atomistic model of plastically deformed cellulose crystals showed a considerable similarity to the kinked cellulose microfibrils observed by transmission electron microscopy. Our observations highlight the importance of shear during deformation of cellulose crystals and provide an understanding of basic deformations occurring during the processing of cellulose materials.

X-ray texture analysis indicates downward spinning of chitin microfibrils in tubeworm tube.

The direction of ß-chitin deposition in the tube of tubeworm Lamellibrachia satsuma was investigated by texture analysis using X-ray diffraction. The ß-chitin crystallite in the tube has planar orientation with the (110) plane perpendicular to the surface, and the c-axis is aligned parallel to the tube. The monoclinic unit cell of ß-chitin allowed determination of the sense of c-axis from the orientation of (010) and (100) planes. This means that the reducing end of ß-chitin is pointing up in the tube. This orientation can be ascribed to possible secretion mechanisms of the ß-chitin microfibrils, i.e. the chitin-synthesizing enzyme complex travels unidirectionally from top to bottom when the worm body contracts in the tube.

Thermodynamics of the swelling work of wood and non-ionic polysaccharides: A revisit.

Polysaccharides, even non-ionic ones, swell in water with potentially huge pressure, which can be sources of intended actuation or a cause of structural damage. The swelling pressure has been investigated since the 19th century, and thermodynamic considerations developed at the beginning of the 20th century. Such treatment is revisited with currently available data showing the swelling to be mostly enthalpy driven. The molecular origin of the heat of swelling is discussed, considering the specificity of polysaccharides or biopolymers with relatively stiff chain conformation that contradicts the compact packing of enthalpy stabilization. Part of the heat of swelling, and thus the potential work of swelling, would originate from the elastic energy stored in the rigid structure. This vision can be tested in the conception of actuation or control of swelling.

Processing strategy for reduced energy demand of nanostructured CNF/clay composites with tailored interfaces.

Nacre-mimicking nanocomposites based on colloidal cellulose nanofibrils (CNFs) and clay nanoparticles show excellent mechanical properties, yet processing typically involves preparation of two colloids followed by a mixing step, which is time- and energy-consuming. In this study, a facile preparation method using low energy kitchen blenders is reported in which CNF disintegration, clay exfoliation and mixing carried out in one step. Compared to composites made from the conventional method, the energy demand is reduced by about 97 %; the composites also show higher strength and work to fracture. Colloidal stability, CNF/clay nanostructure, and CNF/clay orientation are well characterized. The results suggest favorable effects from hemicellulose-rich, negatively charged pulp fibers and corresponding CNFs. CNF disintegration and colloidal stability are facilitated with substantial CNF/clay interfacial interaction. The results show a more sustainable and industrially relevant processing concept for strong CNF/clay nanocomposites.

Structural Anisotropy Governs the Kink Formation in Cellulose Nanocrystals.

Understanding the defect structure is fundamental to correlating the structure and properties of materials. However, little is known about the defects of soft matter at the nanoscale beyond their external morphology. We report here on the molecular-level structural details of kink defects of cellulose nanocrystals (CNCs) based on a combination of experimental and theoretical methods. Low-dose scanning nanobeam electron diffraction analysis allowed for correlation of the local crystallographic information and nanoscale morphology and revealed that the structural anisotropy governed the kink formation of CNCs. We identified two bending modes along different crystallographic directions with distinct disordered structures at kink points. The drying strongly affected the external morphology of the kinks, resulting in underestimating the kink population in the standard dry observation conditions. These detailed defect analyses improve our understanding of the structural heterogeneity of nanocelluloses and contribute to the future exploitation of soft matter defects.

Residual Strain and Nanostructural Effects during Drying of Nanocellulose/Clay Nanosheet Hybrids: Synchrotron X-ray Scattering Results.

Cellulose nanofibrils (CNF) with 2D silicate nanoplatelet reinforcement readily form multifunctional composites by vacuum-assisted self-assembly from hydrocolloidal mixtures. The final nanostructure is formed during drying. The crystalline nature of CNF and montmorillonite (MTM) made it possible to use synchrotron X-ray scattering (WAXS, SAXS) to monitor structural development during drying from water and from ethanol. Nanostructural changes in the CNF and MTM crystals were investigated. Changes in the out-of-plane orientation of CNF and MTM were determined. Residual drying strains previously predicted from theory were confirmed in both cellulose and MTM platelets due to capillary forces. The formation of tactoid platelet stacks could be followed. We propose that after filtration, the constituent nanoparticles in the swollen, solid gel already have a "fixed" location, although self-assembly and ordering processes take place during drying.

The thermodynamics of enhanced dope stability of cellulose solution in NaOH solution by urea.

The addition of urea in pre-cooled alkali aqueous solution is known to improve the dope stability of cellulose solution. However, its thermodynamic mechanism at a molecular level is not fully understood yet. By using molecular dynamics simulation of an aqueous NaOH/urea/cellulose system using an empirical force field, we found that urea was concentrated in the first solvation shell of the cellulose chain stabilized mainly by dispersion interaction. When adding a glucan chain into the solution, the total solvent entropy reduction is smaller if urea is present. Each urea molecule expelled an average of 2.3 water molecules away from the cellulose surface, releasing water entropy that over-compensates the entropy loss of urea and thus maximizing the total entropy. Scaling the Lennard-Jones parameter and atomistic partial charge of urea revealed that direct urea/cellulose interaction was also driven by dispersion energy. The mixing of urea solution and cellulose solution in the presence or absence of NaOH are both exothermic even after correcting for the contribution from dilution.

Direct determination of the hydrogen bonding arrangement in anhydrous ß-chitin by neutron fiber diffraction.

The hydrogen bonding arrangement in anhydrous ß-chitin, a homopolymer of N-acetylglucosamine, was directly determined by neutron fiber diffraction. Data were collected from a sample prepared from the bathophilous tubeworm Lamellibrachia satsuma in which all labile hydrogen atoms had been replaced by deuterium. Initial positions of deuterium atoms on hydroxyl and acetamide groups were directly located in Fourier maps synthesized using phases calculated from the X-ray structure and amplitudes measured from the neutron data. The hydrogen bond arrangement in the refined structure is in general agreement with predictions based on the X-ray structure: O3 donates a hydrogen bond to the O5 ring oxygen atom of a neighboring residue in the same chain; N2 and O6 donate hydrogen bonds to the same carbonyl oxygen O7 of an adjacent chain. The intramolecular O3···O5 hydrogen bond has the most energetically favorable geometry with a hydrogen to acceptor distance of 1.77 Å and a hydrogen bond angle of 171°.

Reorientation of cellulose nanowhiskers in agarose hydrogels under tensile loading.

Agarose hydrogels filled with cellulose nanowhiskers were strained in uniaxial stretching under different humidity conditions. The orientation of the cellulose whiskers was examined before and after testing with an X-ray laboratory source and monitored in situ during loading by synchrotron X-ray diffraction. The aim of this approach was to determine the process parameters for reorienting the cellulose nanowhiskers toward a preferential direction. Results show that a controlled drying of the hydrogel is essential to establish interactions between the matrix and the cellulose nanowhiskers which allow for a stress transfer during stretching and thereby promote their alignment. Rewetting of the sample after reorientation of the cellulose nanowhiskers circumvents a critical increase of stress. This improves the extensibility of the hydrogel and is accompanied by a further moderate alignment of the cellulose nanowhiskers. Following this protocol, cellulose nanowhiskers with an initial random distribution can be reoriented toward a preferential direction, creating anisotropic nanocomposites.