Effect of Moisture Content on Ion Diffusion and Glass Transition Temperature in Wood Cell Walls.

Understanding and controlling the diffusion of ions and chemicals within the secondary plant cell walls are pivotal in various applications of biomasses. Recent studies have shown that inorganic ion diffusion through secondary cell walls is controlled by a moisture-induced glass transition in amorphous polysaccharides, including amorphous cellulose and hemicelluloses. Understanding the diffusion of ions in these structures has been the subject of numerous recent experiments; however, a deep understanding of the underlying mechanisms of interactions between ion atoms and water/hemicellulose molecules is still lacking. This study uses molecular dynamics simulations to elucidate the diffusion mechanisms of potassium and chloride ions in the cell walls under varying moisture content. The results reveal that a higher moisture content leads to the formation of solvent layers around the ions and reduces the charge interaction between the functional groups of wood polymers and ions. Hence, a higher moisture content results in an improved diffusion rate of ions within the domain. The simulation results also show that higher moisture content lowers the glass transition temperature, promoting diffusion of ions in the system. In contrast, increases in the ion concentration increase the glass transition temperature of the system and degrade the diffusion of ions in the system.

Diffusion in intact secondary cell wall models of plants at different equilibrium moisture content.

Secondary plant cell walls are composed of carbohydrate and lignin polymers, and collectively represent a significant renewable resource. Leveraging these resources depends in part on a mechanistic understanding for diffusive processes within plant cell walls. Common wood protection treatments and biomass conversion processes to create biorefinery feedstocks feature ion or solvent diffusion within the cell wall. X-ray fluorescence microscopy experiments have determined that ionic diffusion rates are dependent on cell wall hydration as well as the ionic species through non-linear relationships. In this work, we use classical molecular dynamics simulations to map the diffusion behavior of different plant cell wall components (cellulose, hemicellulose, lignin), ions (Na+, K+, Cu2+, Cl-) and water within a model for an intact plant cell wall at various hydration states (3-30 wt% water). From these simulations, we analyze the contacts between different plant cell wall components with each other and their interaction with the ions. Generally, diffusion increases with increasing hydration, with lignin and hemicellulose components increasing diffusion by an order of magnitude over the tested hydration range. Ion diffusion depends on charge. Positively charged cations preferentially interact with hemicellulose components, which include negatively charged carboxylates. As a result, positive ions diffuse more slowly than negatively charged ions. Measured diffusion coefficients are largely observed to best fit piecewise linear trends, with an inflection point between 10 and 15% hydration. These observations shed light onto the molecular mechanisms for diffusive processes within secondary plant cell walls at atomic resolution.

Crystal misorientation correlates with hardness in tooth enamels.

The multi-scale hierarchical structure of tooth enamel enables it to withstand a lifetime of damage without catastrophic failure. While many previous studies have investigated structure-function relationships in enamel, the effects of crystal misorientation on mechanical performance have not been assessed. To address this issue, in the present study, we review previously published polarization-dependent imaging contrast (PIC) maps of mouse and human enamel, and parrotfish enameloid, in which crystal orientations were measured and displayed in every 60-nm-pixel. By combining those previous results with the PIC maps of sheep enamel presented here we discovered that, in all enamel(oid)s, adjacent crystals are slightly misoriented, with misorientation angles in the 0°-30° range, and mean 2°-8°. Within this limited range, misorientation is positively correlated with literature hardness values, demonstrating an important structure-property relation, not previously identified. At greater misorientation angles 8°30°, this correlation is expected to reverse direction, but data from different non-enamel systems, with more diverse crystal misorientations, are required to determine if and where this occurs. STATEMENT OF SIGNIFICANCE: We identify a structure-function relationship in tooth enamels from different species: crystal misorientation correlates with hardness, contributing to the remarkable mechanical properties of enamel in diverse animals.

Measurement of moisture-dependent ion diffusion constants in wood cell wall layers using time-lapse micro X-ray fluorescence microscopy.

Our future bioeconomy depends on increased utilization of renewable lignocellulosic biomass. Controlling the diffusion of chemicals, such as inorganic ions, within secondary plant cell walls is central to many biomass applications. However, insufficient understanding of intra-cell-wall diffusion within secondary plant cell walls is hindering the advancement of many lignocellulosic biomass applications. In this work, X-ray fluorescence microscopy was used to measure diffusion constants of K+, Cu2+, and Cl- diffusing through loblolly pine (Pinus taeda) cell wall layers under 70%, 75%, or 80% relative humidity (RH). Results revealed that diffusion constants increased with RH, the larger Cu2+ diffused more slowly than the K+, and the Cl- diffusion constant was the same as that for the counter cation, indicating cations and anions diffused together to maintain charge neutrality. Comparison with electrical conductivity measurements showed that conductivity is being controlled by ion mobility over these RH. The results further support that intra-cell-wall diffusion of inorganic ions is a Fickian diffusion process occurring through rubbery amorphous polysaccharides, which contradicts previous assertions that intra-cell-wall diffusion is an aqueous process occurring through water pathways. Researchers can now utilize polymer science approaches to engineer the molecular architecture of lignocellulosic biomass to optimize properties for specific end uses.

Mechanism for Diffusion through Secondary Cell Walls in Lignocellulosic Biomass.

The future bioeconomy depends on the increased utilization of renewable lignocellulosic resources from trees and other bioenergy crops. However, considering that the diffusion of ions, chemicals, and enzymes into secondary cell walls is critical to effectively utilize lignocellulosic biomass, the unidentified mechanisms underpinning this diffusion have hindered progress. Here, nanomechanical spectroscopy was used to measure changes in moisture-dependent relaxations of amorphous polysaccharides inside loblolly pine ( Pinus taeda) cell wall layers. The comparison with recent ion mobility measurements made in similar cell wall layers revealed that the mineral ion diffusion occurs via interconnecting nanoscale pathways of rubbery amorphous polysaccharides. This result contradicts previous assertions of cell wall transport being an aqueous process occurring through simple interconnecting water pathways. Because polymer diffusion and aqueous transport via water channels are such different phenomena, the identification of the diffusion mechanism in this manuscript opens up a new paradigm in lignocellulosic research. The utilization of lignocellulosic resources is expected to be accelerated because the extensive polymer science literature can now be used to design the molecular architecture of lignocellulosic biomass to optimize diffusion properties for specific uses, including biorefinery feedstocks, advanced materials, and wood-based construction materials.

Synchrotron-based X-ray fluorescence microscopy enables multiscale spatial visualization of ions involved in fungal lignocellulose deconstruction.

The role of ions in the fungal decay process of lignocellulose biomaterials, and more broadly fungal metabolism, has implications for diverse research disciplines ranging from plant pathology and forest ecology, to carbon sequestration. Despite the importance of ions in fungal decay mechanisms, the spatial distribution and quantification of ions in lignocellulosic cell walls and fungal hyphae during decay is not known. Here we employ synchrotron-based X-ray fluorescence microscopy (XFM) to map and quantify physiologically relevant ions, such as K, Ca, Mn, Fe, and Zn, in wood being decayed by the model brown rot fungus Serpula lacrymans. Two-dimensional XFM maps were obtained to study the ion spatial distributions from mm to submicron length scales in wood, fungal hyphae with the dried extracellular matrix (ECM) from the fungus, and Ca oxalate crystals. Three-dimensional ion volume reconstructions were also acquired of wood cell walls and hyphae with ECM. Results show that the fungus actively transports some ions, such as Fe, into the wood and controls the distribution of ions at both the bulk wood and cell wall length scales. These measurements provide new insights into the movement of ions during decay and illustrate how synchrotron-based XFM is uniquely suited study these ions.

Size Dependence of Nanoscale Wear of Silicon Carbide.

Nanoscale, single-asperity wear of single-crystal silicon carbide (sc-SiC) and nanocrystalline silicon carbide (nc-SiC) is investigated using single-crystal diamond nanoindenter tips and nanocrystalline diamond atomic force microscopy (AFM) tips under dry conditions, and the wear behavior is compared to that of single-crystal silicon with both thin and thick native oxide layers. We discovered a transition in the relative wear resistance of the SiC samples compared to that of Si as a function of contact size. With larger nanoindenter tips (tip radius ≈ 370 nm), the wear resistances of both sc-SiC and nc-SiC are higher than that of Si. This result is expected from the Archard's equation because SiC is harder than Si. However, with the smaller AFM tips (tip radius ≈ 20 nm), the wear resistances of sc-SiC and nc-SiC are lower than that of Si, despite the fact that the contact pressures are comparable to those applied with the nanoindenter tips, and the plastic zones are well-developed in both sets of wear experiments. We attribute the decrease in the relative wear resistance of SiC compared to that of Si to a transition from a wear regime dominated by the materials' resistance to plastic deformation (i.e., hardness) to a regime dominated by the materials' resistance to interfacial shear. This conclusion is supported by our AFM studies of wearless friction, which reveal that the interfacial shear strength of SiC is higher than that of Si. The contributions of surface roughness and surface chemistry to differences in interfacial shear strength are also discussed.

Directed plant cell-wall accumulation of iron: embedding co-catalyst for efficient biomass conversion.

BACKGROUND: Plant lignocellulosic biomass is an abundant, renewable feedstock for the production of biobased fuels and chemicals. Previously, we showed that iron can act as a co-catalyst to improve the deconstruction of lignocellulosic biomass. However, directly adding iron catalysts into biomass prior to pretreatment is diffusion limited, and increases the cost of biorefinery operations. Recently, we developed a new strategy for expressing iron-storage protein ferritin intracellularly to accumulate iron as a catalyst for the downstream deconstruction of lignocellulosic biomass. In this study, we extend this approach by fusing the heterologous ferritin gene with a signal peptide for secretion into Arabidopsis cell walls (referred to here as FerEX). RESULTS: The transgenic Arabidopsis plants. FerEX. accumulated iron under both normal and iron-fertilized growth conditions; under the latter (iron-fertilized) condition, FerEX transgenic plants showed an increase in plant height and dry weight by 12 and 18 %, respectively, compared with the empty vector control plants. The SDS- and native-PAGE separation of cell-wall protein extracts followed by Western blot analyses confirmed the extracellular expression of ferritin in FerEX plants. Meanwhile, Perls' Prussian blue staining and X-ray fluorescence microscopy (XFM) maps revealed iron depositions in both the secondary and compound middle lamellae cell-wall layers, as well as in some of the corner compound middle lamella in FerEX. Remarkably, their harvested biomasses showed enhanced pretreatability and digestibility, releasing, respectively, 21 % more glucose and 34 % more xylose than the empty vector control plants. These values are significantly higher than those of our recently obtained ferritin intracellularly expressed plants. CONCLUSIONS: This study demonstrated that extracellular expression of ferritin in Arabidopsis can produce plants with increased growth and iron accumulation, and reduced thermal and enzymatic recalcitrance. The results are attributed to the intimate colocation of the iron co-catalyst and the cellulose and hemicellulose within the plant cell-wall region, supporting the genetic modification strategy for incorporating conversion catalysts into energy crops prior to harvesting or processing at the biorefinery.

Synchrotron-based X-ray fluorescence microscopy in conjunction with nanoindentation to study molecular-scale interactions of phenol-formaldehyde in wood cell walls.

Understanding and controlling molecular-scale interactions between adhesives and wood polymers are critical to accelerate the development of improved adhesives for advanced wood-based materials. The submicrometer resolution of synchrotron-based X-ray fluorescence microscopy (XFM) was found capable of mapping and quantifying infiltration of Br-labeled phenol-formaldehyde (BrPF) into wood cell walls. Cell wall infiltration of five BrPF adhesives with different average molecular weights (MWs) was mapped. Nanoindentation on the same cell walls was performed to assess the effects of BrPF infiltration on cell wall hygromechanical properties. For the same amount of weight uptake, lower MW BrPF adhesives were found to be more effective at decreasing moisture-induced mechanical softening. This greater effectiveness of lower MW phenolic adhesives likely resulted from their ability to more intimately associate with water sorption sites in the wood polymers. Evidence also suggests that a BrPF interpenetrating polymer network (IPN) formed within the wood polymers, which might also decrease moisture sorption by mechanically restraining wood polymers during swelling.

Thermomechanical properties of lignin-based electrospun nanofibers and films reinforced with cellulose nanocrystals: a dynamic mechanical and nanoindentation study.

We produced defect-free electrospun fibers from aqueous dispersions of lignin, poly(vinyl alcohol) (PVA), and cellulose nanocrystals (CNCs), which were used as reinforcing nanoparticles. The thermomechanical performance of the lignin-based electrospun fibers and the spin-coated thin films was improved when they were embedded with CNCs. Isochronal dynamic mechanical analysis (DMA) was used to assess the viscoelastic properties of the lignin:PVA electrospun fiber mats loaded with CNCs. DMA revealed that α relaxation processes became less prominent with an increased lignin content, an effect that correlated with the loss tangent (tan δ = E″/E') and α peak (Tg) that shifted to higher temperatures. This can be ascribed to the restraint of the segmental motion of PVA in the amorphous regions caused by strong intermolecular interactions. The reinforcing effect and high humidity stability attained by addition of CNCs (5, 10, or 15 wt %) in the multicomponent fiber mats were revealed. Nanoindentation was performed to assess the elastic modulus and hardness of as-prepared and cross-section surfaces of spin-coated lignin:PVA (75:25) films loaded with CNC. The properties of the two surfaces differed, and only the trend in cross-section elastic modulus correlated with DMA results. After addition of 5 wt % CNCs, both the DMA and nanoindentation elastic modulus remained constant, while after addition of 15 wt % CNCs, both increased substantially. An indentation size effect was observed in the nanoindentation hardness, and the results provided insight into the effect of addition of CNCs on the microphysical processes controlling the yield behavior in the composites.

Spontaneous phase transformation and exfoliation of rectangular single-crystal zinc hydroxy dodecylsulfate nanomembranes.

Free-standing two-dimensional (2D) nanostructures, exemplified by graphene and semiconductor nanomembranes, exhibit exotic electrical and mechanical properties and have great potential in electronic applications where devices need to be flexible or conformal to nonplanar surfaces. Based on our previous development of a substrate-free synthesis of large-area, free-standing zinc hydroxy dodecylsulfate (ZHDS) hexagonal nanomembranes, herein, we report a spontaneous phase transformation of ZHDS nanomembranes under extended reaction time. The hexagonal ZHDS sheets transformed into rectangular single crystal nanomembranes with sizes of hundreds of micrometers. They contain long-range-ordered zinc vacancies that can be fitted into an orthorhombic superlattice. A surplus of dodecylsulfate ions and a deficit of Zn(2+) diffusion near the water surface are believed to be the factors that drive the phase transformation. The phase transformation starts with the formation of zinc vacancies at the topmost layer of the hexagonal hillock, and propagates along the spiral growth path of the initial hexagonal sheets, which bears a great resemblance to the classic "periodic slip process". Mechanical property characterization of ZHDS nanomembranes by nanoindentation shows they behave much like structural polymers mechanically due to the incorporation of surfactant molecules. We also developed a one-step exfoliation and dehydration method that converts ZHDS nanomembranes to ZnO nanosheets using n-butylamine. This work provides a further understanding of the growth and stability of ZnO-based nanomembranes, as well as advisory insight for the further development on solution-based synthesis of free-standing, single-crystalline 2D nanostructures.

Interfacial properties of lignin-based electrospun nanofibers and films reinforced with cellulose nanocrystals.

Sub-100 nm resolution local thermal analysis, X-ray photoelectron spectroscopy (XPS), and water contact angle (WCA) measurements were used to relate surface polymer distribution with the composition of electrospun fiber mats and spin coated films obtained from aqueous dispersions of lignin, polyvinyl alcohol (PVA), and cellulose nanocrystal (CNC). Defect-free lignin/PVA fibers were produced with radii which were observed to increase with lignin concentration and with the addition of CNCs. XPS and WCA results indicate a nonlinear relationship between the surface and the bulk compositions. A threshold around 50 wt % bulk composition was identified in which extensive partitioning of PVA and lignin components occurred on the surface below and above this value. In 75:25 wt % lignin/PVA solvent cast films, phase separated domains were observed. Using nanoscale thermal analyses, the continuous phase was determined to be lignin-rich and the discontinuous phase had a lignin/PVA dispersion. Importantly, the size of the phase separated domains was reduced by the addition of CNCs. When electrospun fiber surfaces were lignin-rich, the addition of CNCs affected their surfaces. In contrast, no surface effects were observed with the addition of CNCs in PVA-rich fibers. Overall, we highlight the importance of molecular interactions and phase separation on the surface properties of fibers from lignin as an abundant raw material for the fabrication of new functional materials.

Lignin-based electrospun nanofibers reinforced with cellulose nanocrystals.

Lignin-based fibers were produced by electrospinning aqueous dispersions of lignin, poly(vinyl alcohol) (PVA), and cellulose nanocrystals (CNCs). Defect-free nanofibers with up to 90 wt % lignin and 15% CNCs were achieved. The properties of the aqueous dispersions, including viscosity, electrical conductivity, and surface tension, were examined and correlated to the electrospinnability and resulting morphology of the composite fibers. A ternary lignin-PVA-water phase diagram was constructed as a tool to rationalize the effect of mixing ratios on the dispersion electrospinability and morphology of the resulting fibers. The influence of reinforcing CNCs on the thermal properties of the multicomponent fibers was investigated by using thermal gravimetric analysis and differential scanning calorimetry. The thermal stability of the system was observed to increase owing to a strong interaction of the lignin-PVA matrix with the dispersed CNCs, mainly via hydrogen bonding, as observed in Fourier transform infrared spectroscopy experiments.