Joseph E. Jakes

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.

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.

Analysis of indentation creep

Finite element analysis is used to simulate cone indentation creep in materials across a wide range of hardness, strain rate sensitivity, and work-hardening exponent. Modeling reveals that the commonly held assumption of the hardness strain rate sensitivity ( m H ) equaling the flow stress strain rate sensitivity ( m σ ) is violated except in low hardness/modulus materials. Another commonly held assumption is that for self-similar indenters the indent area increases in proportion to the (depth)2 during creep. This assumption is also violated. Both violations are readily explained by noting that the proportionality “constants” relating (i) hardness to flow stress and (ii) area to (depth)2 are, in reality, functions of hardness/modulus ratio, which changes during creep. Experiments on silicon, fused silica, bulk metallic glass, and poly methyl methacrylate verify the breakdown of the area-(depth)2 relation, consistent with the theory. A method is provided for estimating area from depth during creep.

The effect of water uptake on the mechanical properties of low-k organosilicate glass

Water uptake in porous low-k dielectrics has become a significant challenge for both back-end-of-line integration and circuit reliability. The influence of absorbed water on the mechanical properties of plasma-enhanced chemical-vapor-deposited organosilicate glasses (SiCOH) was investigated with nanoindentation. The roles of physisorbed (α-bonded) and chemisorbed (β-bonded) water were examined separately through annealing at different temperatures. Nanoindentation measurements were performed on dehydrated organosilicate glass during exposure to varying humidity conditions. The elastic modulus and hardness for as-deposited SiCOH are intimately linked to the nature and concentration of the absorbed water in the dielectric. Under mild-annealing conditions, the water-related film mechanical property changes were shown to be reversible. The mechanical properties of UV-cured SiCOH were also shown to depend on absorbed water, but to a lesser extent because UV curing depopulates the hydrophilic chemical groups in...

The edge effect in nanoindentation

Until recently, obtaining unambiguous data from a nanoindentation measurement placed near a free edge has not been possible because the discontinuity associated with the edge introduces artifacts into the measurement. The primary consequence of a free edge is to introduce a structural compliance, C s, into the measurement. Like the machine compliance, C m, C s is independent of the size of the indent and it adds to the measured unloading compliance; but unlike C m, C s is a function of position. Accounting for C s in nanoindentation analyses removes the artifacts in nanoindentation measurements associated with the edge, allowing researchers to more accurately probe material properties near an edge. Expressions were obtained for the effect of the free edge on hardness and modulus measured using the Oliver–Pharr method. The theory was tested on specimens including fused silica, poly(methyl methacrylate), and a layered silicon-on-insulator specimen.

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.

Threshold for ion movements in wood cell walls below fiber saturation observed by X-ray fluorescence microscopy (XFM)

Abstract Diffusion of chemicals and ions through the wood cell wall plays an important role in wood damage mechanisms. In the present work, free diffusion of ions through wood secondary walls and middle lamellae has been investigated as a function of moisture content (MC) and anatomical direction. Various ions (K, Cl, Zn, Cu) were injected into selected regions of 2 μm thick wood sections with a microinjector and then the ion distribution was mapped by means of X-ray fluorescence microscopy with submicron spatial resolution. The MC of the wood was controlled in situ by means of climatic chamber with controlled relative humidity (RH). For all ions investigated, there was a threshold RH below which the concentration profiles did not change. The threshold RH depended upon ionic species, cell wall layer, and wood anatomical orientation. Above the threshold RH, differences in mobility among ions were observed and the mobility depended upon anatomical direction and cell wall layer. These observations support a recently proposed percolation model of electrical conduction in wood. The results contribute to understanding the mechanisms of fungal decay and fastener corrosion that occur below the fiber saturation point.

Effects of plasma and vacuum-ultraviolet exposure on the mechanical properties of low-k porous organosilicate glass

The effects of plasma exposure and vacuum-ultraviolet (VUV) irradiation on the mechanical properties of low-k porous organosilicate glass (SiCOH) dielectric films were investigated. Nanoindentation measurements were made on SiCOH films before and after exposure to an electron-cyclotron-resonance plasma or a monochromatic synchrotron VUV beam, to determine the changes of film hardness, elastic modulus, and crack threshold due to these exposures. This permits the effects of ion bombardment and photon bombardment to be analyzed separately. The role of energetic ions was examined with a variety of inert plasma-exposure conditions. The role of VUV photons was analyzed as a function of synchrotron photon energy. It was found that both energetic ions and VUV photons with energies larger than the bond energy of the Si-O bond cause a significant increase in film hardness along with a smaller increase in elastic modulus and crack threshold. Differential Fourier transform infrared spectra and x-ray photoemission spe...

Nanomechanical study of amorphous and polycrystalline ALD HfO2 thin films

Thin films of hafnium oxide (HfO2) were deposited by atomic layer deposition (ALD). The structural properties of the deposited films were characterised by transmission electron microscopy (TEM) and X-ray diffraction (XRD). We investigated the effect of phase transformations induced by thermal treatments on the mechanical properties of ALD HfO2 using nanoindentation. The elastic modulus of the amorphous low temperature deposited ALD HfO2 films was measured to be 370 ± 20 GPa. Subsequent to crystallisation by annealing in a rapid thermal annealing (RTA) chamber, the elastic modulus dropped to 240 ± 20 GPa. Similarly, the Meyer hardness decreased from a value of 18 ± 1 GPa for amorphous HfO2 to 15 ± 1 GPa following the transition temperature from amorphous to polycrystalline HfO2.

The role of chemical transport in the brown-rot decay resistance of modified wood

Chemical modification of wood increases decay resistance but the exact mechanisms remain poorly understood. Recently, Ringman and coauthors examined established theories addressing why modified wood has increased decay resistance and concluded that the most probable cause of inhibition and/or delay of initiation of brown-rot decay is lowering the equilibrium moisture content. In another recent study, Jakes and coauthors examined moisture-induced wood damage mechanisms, including decay and fastener corrosion, and observed that these mechanisms require chemical transport through wood cell walls. They proposed that chemical transport within wood cell walls is controlled by a moisture-induced glass transition in interconnected networks of hemicelluloses and amorphous cellulose. This paper shows how these models jointly suggest mechanisms by which wood modifications can inhibit brown-rot. Alternative mechanisms are also discussed. These models can be used to understand and further improve the performance of wood modification systems.