The effects of hydrostatic pressure and indentation strain on nanoindentation elastic modulus ( E_s ) and hardness ( H ) were investigated in the five polymers poly(methyl methacrylate) (PMMA), polycarbonate (PC), polystyrene (PS), poly(ether ether ketone) (PEEK), and low density polyethylene (LDPE) using four different pyramidal probes with equivalent cone angle ( θ ) = 47.7° (cube corner), 35.0°, 19.7° (Berkovich), or 10.0°. Both E_s and H generally increased with θ , with the strongest effects in the glassy polymers. Consistent with a hydrostatic pressure effect, the measured E_s were 10–50 E_s versus H was extrapolated to H=0 assuming a linear relationship. These extrapolated values were within 15 (σ_H )–representative strain ( ϵ_r ) curves were also calculated and compared to literature uniaxial compression stress–strain curves. σ_H compared best to the literature for data for a Berkovich probe at an ϵ_r = 0.07. The empirical Tabor–Marsh–Johnson correlation used to calculate σ_H does not work as well for these polymers at either higher or lower ϵ_r .

Positron annihilation lifetime spectroscopy (PALS) was employed to study the water-dependent free volume characteristics of wood cell walls in earlywood and latewood of loblolly pine (Pinus taeda). Measurements were conducted across relative humidity levels (1–80

Moisture- and orientation-dependent mechanical properties of the S2 secondary cell wall layer are needed to better understand wood mechanical properties and advance wood utilization. In this work, nanoindentation was used to assess the orientation-dependent elastic moduli and Meyer hardness of the loblolly pine (Pinus taeda) S2 layer under environmental conditions ranging from 0% to 94% relative humidity (RH). The elastic moduli were fit to a theoretical transverse isotropic elasticity model to calculate the longitudinal elastic modulus, transverse elastic modulus, axial shear modulus, and transverse shear modulus for the S2 layer at 0%, 33%, 75%, and 94% RH and 26 °C. The longitudinal elastic modulus was consistently higher than the transverse elastic modulus because of the orientation of the stiff cellulose microfibrils in the S2 layer. The axial shear modulus was consistently higher than the transverse shear modulus. The Meyer hardness had a much smaller orientation dependence than the elastic properties. Moisture generally plasticized the S2 layer. Over the range of RH tested, the longitudinal elastic modulus decreased by 30%, the transverse elastic modulus and transverse shear modulus decreased by 83%, the axial shear modulus did not have an observable trend with RH, and the hardness decreased by 68% to 82% with the hardness in the longitudinal direction softening less than in the transverse direction.

Volatile products from thermal decomposition of lignocellulosic biomass have been well characterized, but the solid- and liquid-phase reactions during the early stages of decomposition are largely unknown. Here the initial solid-phase biomass thermal deconstruction reactions were analyzed in situ and with high particle heating rates, delineating how these processes occur. A variety of instrumentation was used to quantify the extent and relative rates of deconstruction, demonstrating that biopolymers resist the thermally energetic conditions to differing degrees, even when ensconced in biomass cell walls. Hemicellulose and the more frangible lignin components decompose and volatilize more readily than cellulose, which temporarily enriches biomass with cellulose. These chemical changes manifest in larger cell wall structural and mechanical property transformations. In all, this investigation concludes that these solid-phase reactions strongly influence the production rates of volatile species and will require additional study before these processes can be modeled precisely to improve yields of desired product.

Previous research found that some organosilicon treatments proved effective in stabilizing waterlogged wood dimensions during drying. The present research aimed to determine the mechanism of wood stabilization by these chemicals to understand their mode of action. The study used chemically (ChP) and biologically degraded (BP) model Scots pine wood treated with Methyltrimethoxysilane (MTMS), (3-Mercaptopropyl) trimethoxysilane (MPTMS), or 1,3-Bis(diethylamino)-3-propoxypropanol)-1,1,3,3-tetramethyldisiloxane (DEAPTMDS). Synchrotron-based X-ray fluorescence microscopy (XFM) was used to investigate the penetration of organosilicons into the wood cellular structure and cell walls, and nanoindentation was used to study the mechanical properties of the treated wood cell walls. All treatments resulted in high volumetric anti-shrink efficiency (ASEV) values of 74–82

To compare hardness versus strain rate data from different kinds of indentation creep test can be challenging. It is often difficult to determine whether measured differences in material response along different loading paths, such as constant load creep or load relaxation, are real or merely arise as artifacts from the analysis. We argue that the difficulty lies in how indentation strain rate is defined. For traditional definitions of strain rate, such as ε̇_h , which measures the rate of penetration, or ε̇_A , which measures the growth of indent area, material response might seem path-dependent even when it is not. We introduce a new definition of plastic strain rate, ε̇_irr , which is based on irreversible work and is well-posed in the sense that it gives the same results from different loading paths when deformation is path-independent. This property frees experimenters to isolate and explore deformation mechanisms that respond differently along different loading paths, which is useful for revealing the influences of time and strain rate on evolution of structure. ε̇_irr is important for exploring path-dependence in high-hardness/modulus (≳ 0.02) materials like ceramics and polymers. For low-hardness-modulus materials other measures of strain rate suffice.

Berkovich nanoindentation was used to characterize viscoelastic and viscoplastic deformations in the semicrystalline polymers, poly(ether ether ketone) (PEEK) and low density polyethylene (LDPE). The quasistatic experiments generated viscoelastic moduli over four decades in frequency and hardness over more than four decades of indentation strain rate. These semicrystalline polymer results augment analogous results previously obtained from the amorphous polymers, poly(methyl methacrylate), polycarbonate, and polystyrene. Although nanoindentation viscoelastic moduli were all systematically higher than those from conventional measurements, viscoelastic moduli from nanoindentation and conventional measurements followed the same trends with frequency, which indicated that both types of measurements are sensitive to the same microphysical processes giving rise to the viscoelasticity. Differences in the path dependence of hardness were observed between polymers and attributed to differences in polymer glass transition temperatures. Flow stress versus strain rate data were also calculated from hardness versus indentation strain rate data and compared to literature values. Flow stress versus strain rate obtained from both nanoindentation and conventional uniaxial experiments agreed very closely. Collectively, the comparisons between nanoindentation and conventional measurements over wide timescales further confirmed that viscoelastic and viscoplastic deformations characterized by Berkovich nanoindentation can be related back to conventional measurements and have similar mechanistic interpretations.

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.

Research on new conservation treatments for historical wood requires considerable amounts of appropriate wood material, which is hard to acquire. Thus, we produced biologically and chemically degraded model wood that could be used as a representative material in future research on consolidating agents. Using chemical composition determinations, we found that fungal decay targeted mainly polysaccharides, while alkaline treatment mostly reduced hemicelluloses and lignin content. X-ray and neutron scattering showed that all decayed samples had increased disorder in microfibril alignment and larger elementary fibril cross-sections, and alkaline-treated samples had much larger elementary fibril spacing compared to those decayed by fungi. These nanoscale and chemical differences correlate with physical property changes. For example, decreased cellulose crystallinity and increased disorder of the microfibrils in degraded cell walls likely contribute to the lower elastic moduli measured for these cell walls. The obtained data improves understanding of how degradation alters wood structures and properties across length scales and will be valuable for future studies focusing on archeological wood. Moreover, it leads to the conclusion that it is more appropriate to develop treatments that consider not only spatial variability and degree of wood degradation but also the corresponding molecular and nanoscale changes in the cell walls.

To further enhance the performance of wood products, improved tools are needed to study in situ cellular scale phenomena like mechanical deformations and moisture swelling. Micro-X-ray computed tomography (μXCT) using brilliant synchrotron light sources now has the spatial and temporal resolution for real-time visualization of phenomena in three-dimensional cellular structures. However, the tradeoff for speed includes the loss of intensity contrast between different types of materials within the imaged structure, such as cell wall and air in wood. This loss of contrast prevents traditional histogram-based segmentation methods from being used effectively. A new convolutional neural network (CNN) approach was therefore developed to segment fast μXCT images of wood into cell wall and air volumes. The fast μXCT and segmentation were demonstrated in the study of moisture swelling in loblolly pine (Pinus taeda) earlywood and latewood cellular structures conditioned at 0%, 33%, 75%, and 95% relative humidity (RH). The CNN segmentation results had a mean intersection over union (IoU) metric accuracy of 96%. Initial analysis of the swelling in the latewood revealed cell walls swelled about 25% when conditioned from 0% to 95% RH. Additionally, the widths of ray cell lumina in the transverse plane of latewood could be observed to increase at higher RH. The segmentation method presented here will facilitate future quantitative analyses in in situ μXCT studies of wood and other similar cellular materials.

Quasistatic nanoindentation is a proven tool that provides information on the micromechanical behavior of wood cell walls. However, quasistatic tests cannot probe the time-dependent mechanical behavior shown by wood. Nanoindentation dynamic mechanical analysis (nanoDMA) can mesaure the viscoelastic properties of wood cell walls. This research aimed to study the quasistatic and viscoelastic properties of individual radiata pine wood (Pinus radiata D. Don) cell wall layers. To minimize variability and retrieve both properties at the same locations, a load function composed of a multiload-quasistatic function followed by dynamic reference frequency segments was developed. Nanoindentations were then performed on the S2 layer and compound corner middle lamella (CCML) of unembedded latewood cells. Because the S2 layer is anisotropic, both transverse and longitudinal-tangential wood planes were studied. In the transverse plane, the average results of the quasistatic elastic moduli (Es) for the S2 layer and CCML were 15.7 GPa and 4.6 GPa, respectively. In the longitudinal-tangential plane, the E_s was 3.9 GPa. In the transverse section, the hardness (H) of the S2 layer and CCML were 331 MPa and 277 MPa, respectively, and in the longitudinal-tangential section H was 244 MPa. To acquire the viscoelastic properties, measurements were made over more than three decades of frequency. An increase of the storage modulus (E'), and a reduction of the loss modulus (E'') and loss factor (tan⁡ δ ) as frequency increased were found in both wood orientations. The quasi-static and dynamic indentations equivalent at 0.1 Hz showed similar values for Es and E’. This study contributes to our knowledge of wood cell wall micromechanical properties.

Quasistatic nanoindentation is a proven tool that provides information on the micromechanical behavior of wood cell walls. However, quasistatic tests cannot probe the time-dependent mechanical behavior shown by wood. Nanoindentation dynamic mechanical analysis (nanoDMA) can measure the viscoelastic properties of wood cell walls. This research aimed to study the quasistatic and viscoelastic properties of individual radiata pine wood (Pinus radiata D. Don) cell wall layers. To minimize variability and retrieve both properties at the same locations, a load function composed of a multiload-quasistatic function followed by dynamic reference frequency segments was developed. Nanoindentations were then performed on the S2 layer and compound corner middle lamella (CCML) of unembedded latewood cells. Because the S2 layer is anisotropic, both transverse and longitudinal–tangential wood planes were studied. In the transverse plane, the average results of the quasistatic elastic moduli Es for the S2 layer and CCML were 15.7 GPa and 4.6 GPa, respectively. In the longitudinal–tangential plane, the Es was 3.9 GPa. In the transverse section, the hardness H of the S2 layer and CCML were 331 MPa and 277 MPa, respectively, and in the longitudinal–tangential section H was 244 MPa. To acquire the viscoelastic properties, measurements were made over more than three decades of frequency. An increase of the storage modulus E′, and a reduction of the loss modulus E″ and loss factor tanδ as frequency increased were found in both wood orientations. The quasi-static and dynamic indentations equivalent at 0.1 Hz showed similar values for Es and E′. This study contributes to our knowledge of wood cell wall micromechanical properties.

Nanoindentation (NI) is capable to investigate mechanical properties on a small scale and is also suitable to examine cross-sections of co-extruded or laminated multilayer films with thicknesses in the $\mu$m-regime. The standard Oliver-Pharr (O&P) NI method [1] is typically employed to measure the elastic modulus and hardness. However, this standard method assumes a homogeneous and semi-infinite sample that is rigidly supported. NI on thin, multilayer film cross-sections violate these assumptions because nanoindentations are always near free edges and heterophase interfaces. The structural compliance method was developed by Jakes et al. to correct NI results for edge effects and specimen-scale flexing [2]. The method is based on the discovery, that the effect of both edges and specimen-scale flexing is to introduce a structural compliance (Cs) into the measurement. Applied to a multilayer, the Cs showed a strong position dependence, i.e., the effect was larger near the edges and layer interfaces. The Cs correction had little effect on the hardness values; however, the influence on measured elastic modulus was significant. There, the corrected modulus values tended to be higher than the uncorrected ones in the stiff layers. After the Cs correction, the position dependence of the elastic modulus within a given layer was not observed within experimental uncertainties.

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 wallat 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 10-15% hydration. These observations shed light onto the molecular mechanisms for diffusive processes within secondary plant cell walls at atomic resolution.

Waterlogged wood conservation is a complex and challenging task. Detailed knowledge about the interactions between the applied chemicals and wood is necessary to ensure the effective and safe conservation of wooden artifacts. The present research aims to determine the mechanism of dimensional stabilization of archeological wood by organosilicon compounds using the combination of synchrotron-based X-ray fluorescence microscopy (XFM) and nanoindentation. Archeological oak wood was treated with methyltrimethoxysilane, (3-mercaptopropyl)trimethoxysilane, or 1,3-bis-[(diethylamino)-3-(propoxy)propan-2-ol]-1,1,3,3-tetramethyldisiloxane, which in previous studies were found to be more effective than other organosilicons in stabilizing wood dimensions. The XFM and nanoindentation results showed that all three organosilicons infiltrated wood cell walls and enhanced their mechanical properties. The XFM also showed that part of the chemicals filled some void spaces like cell lumina. Based on the results obtained here and in our previous research, it is determined that the mechanism of archeological wood dimensional stabilization by organosilicon treatment is complex and likely involves both filling cell lumina and infiltration into cell walls where organosilicons interact with wood polymers.

In this study, we investigated the impact on surface topography and micro-mechanical properties of polyvinyl alcohol (PVA) thin films when loaded with hardwood-derived biocarbon particles (BCP). The PVA/BCP composites were prepared with the conventional film casting method, after varying BCP concentrations of 6, 8, 10, 12, and 20 wt% were added to the PVA solution. Atomic force microscopy (AFM) investigations and nanoindentation tests were performed. The average roughness of the thin films increased with the increase in BCP content. The alternation between dark and light patterns observed in the AFM images showed an irregular surface topography with alternating high peaks and deep valleys. The skewness and kurtosis parameters showed that the different dispersion degrees of the BCP within the PVA matrix influenced the composites' surface roughness. The micro-mechanical properties of the thin film composites depended on the BCP type and concentration. Films reinforced with red oak-derived BCP had higher hardness and Young's modulus compared to films reinforced with willow SV1 and yellow-poplar BCP, which was attributed to the high carbon content and low ash content of red oak BCP. We argue that these results may be extrapolated to other types of BCP-reinforced thin films and can significantly contribute to enabling more efficient methods and protocols for reinforcing polymers with hardwood biocarbon.

Understanding and controlling water in wood is critical to both improving forest products moisture durability and developing new sustainable forest products-based technologies. Due to the inherent complexity of wood structure and chemistry, our knowledge on the wood-water interactions needed for increased durability is still lacking. Research at macro levels has provided some insight, but an improved understanding of the wood-water interactions at the subcellular level (from 1 to 100 nm) has been missing. Advanced neutron scattering techniques are ideally suited to probe changes in wood nanostructure because they have increased contrast due to their isotope sensitivity and allow for in situ humidity control. Time-resolved scattering techniques offer a unique opportunity to provide new insights on this subject, yet they are still underutilized in forest products research. In this talk, I will provide a brief overview on the opportunities and challenges for using time-resolved scattering to study moisture-induced structural and dynamics changes in unmodified and chemically modified wood. SANS studies capable of measuring the moisture-induced swelling of wood nanostructure from 1 to 100nm will be discussed. Additionally, QENS studies meant to probe these interactions spatially from 0.3 to 3 nm and temporally in the range of 3 to 400 ps will also be highlighted. These findings are expected to inform the development of new protection treatments that can improve the decay resistance and overall durability of forest products used outdoors.

A correction is needed to minimize errors in contact area caused by surface tilt in nanoindentation. Surface tilt decreases the contact area calculated using standard analyses that assume the tested surface is not tilted and thus results in overestimation of hardness and elastic modulus. Both the direction of tilt with respect to the pyramid face and the angle of the pyramidal probe are important when characterizing error caused by surface tilt. Here, a geometric model was used to create contour plots from which the area correction factor can be directly determined using only the ratios of side lengths measured from an image of the triangular nanoindentation impression. Contour plots for Berkovich, cube corner, and two nonstandard pyramidal probe geometries are given. The efficacy of the method was demonstrated in the correction of Berkovich nanoindentation on facets in freshly prepared poly(methyl methacrylate) with surface tilts as high as 6°.

Elastic modulus and hardness are commonly measured using nanoindentation. Calculating these properties from measured force–displacement curves typically requires knowledge of the contact depth of the indenter into the specimen. However, surface detection methods in many nanoindentation experiments can lead to an error in the contact depth measurement and, subsequently, the measured properties. Here, the contributions of elastic and plastic deformations to surface detection errors in nanoindentation experiments are examined through experiments and modeling. The model is used to quantify errors in elastic modulus and hardness measurements due to elastic–plastic deformation during surface detection as a function of the specimen properties, indenter geometry, preload, and contact depth. Nanoindentation measurements on polystyrene, an aluminum alloy, and fused silica specimens with a Berkovich indenter are used to illustrate the effects of surface detection error and are compared to the model. The experiments and model both demonstrate that surface detection error can lead to measurement of apparent depth-dependent properties in homogenous materials.

Brown rot fungi utilize iron as part of a chelator-mediated Fenton (CMF) reaction during wood biodegradation. Research suggests these fungi reduce Fe3+ to promote oxygen radical generation resulting in depolymerization of the wood cell wall. High levels of Mn are also found in wood decayed by brown rot fungi. However, little is known about the oxidation states of Fe and Mn during the decay process. X-ray absorption near edge spectroscopy (XANES) can be used to examine metal oxidation states and coordination chemistry. XANES experiments were conducted on wood decayed by Gloeophyllum trabeum over 2–8 weeks with results showing that Mn2+ and Fe3+ predominated for metal oxidation states. However, Fe2+ was present at sites of greater fungal growth In certain cases, the μXANES measurements showed that the fraction of Fe2+ in the wood samples was as high as 50%. Localized areas of reduced iron corresponded with areas of greater fungal hyphal mass which is in agreement with how brown rot fungi decay wood via the CMF reaction. The limited change in oxidation state of Mn observed in wood with active fungal activity suggests that the role of manganese in CMF biodegradation chemistry should be further explored.