This paper aims at providing a methodological framework for investigating wood polymers using atomistic modeling, namely, molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations. Atomistic simulations are used to mimic water adsorption and desorption in amorphous polymers, make observations on swelling, mechanical softening, and on hysteresis. This hygromechanical behavior, as observed in particular from the breaking and reforming of hydrogen bonds, is related to the behavior of more complex polymeric composites. Wood is a hierarchical material, where the origin of wood-moisture relationships lies at the nanoporous material scale. As water molecules are adsorbed into the hydrophilic matrix in the cell walls, the induced fluid–solid interaction forces result in swelling of these cell walls. The interaction of the composite polymeric material, that is the layer S2 of the wood cell wall, with water is known to rearrange its internal material structure, which makes it moisture sensitive, influencing its physical properties. In-depth studies of the coupled effects of water sorption on hygric and mechanical properties of different polymeric components can be performed with atomistic modeling. The paper covers the main components of knowledge and good practice for such simulations.
Heat and moisture are known to have important mechanical effects on polymers such as hygric swelling, thermal expansion, and mechanical weakening. A common approach when investigating such effects is to assume the effects of heat and moisture to be similarthe so-called time−temperature−moisture superposition. Through molecular dynamics simulation, this study evaluates the extent of the similarity of the effects of moisture and heat on the hygric swelling, thermal expansion, and mechanical weakening of a biopolymer: an uncondensed type of lignin, one of the most abundant polymers in the plant regime. We introduce as a microscopic metric the local stiffness (T/ ⟨u 2 ⟩, temperature divided by the amplitude of segmental motion ⟨u 2 ⟩), to analyze the mechanisms of mechanical effects of heat and moisture. The local stiffness of polymer skeleton and the overall stiffness of the composite material are shown to be strongly correlated, with a Pearson correlation coefficient of 0.96. Under the assumptions of harmonic vibration and isotropy, an explicit equation relating bulk moduli and the local stiffness can be derived, and the theoretically predicted moduli are in good agreement with measurement. The thermal expansion and weakening are shown to be related to each other and both dependent on the local stiffness. The analysis of the potential energy further points out that heating weakens both primary and secondary bonds of the polymer skeleton, while hydration only affects the secondary bonds. This major difference is thought to be the origin of the different impacts of heat and moisture on biopolymer mechanics, offering a different view of the time−temperature−moisture superposition principle.
-In this paper two dimensional stagnation point of unsteady nanofluid flow over a stretching/shrinking sheet embedded in a porous medium is analyzed numerically and effects of porosity, heat generation and volume fraction on Nusselt number, skin friction coefficient and convergence time and velocity and temperature distribution are investigated in detail. Also different behaviors of stretching and shrinking sheets are observed. Results are reported for three different volume fraction values (0 − 0.1 − 0.2). It is found that velocity step change increases skin friction coefficient while it has opposite effect on Nusselt number. Stretching sheet showed higher variation in skin friction coefficient value with time while shrinking sheet presented higher variation of Nusselt number.
Prevailing absorbents like wood-derived porous scaffolds or polymeric aerogels are normally featured with hierarchical porous structures. In former molecular simulation studies, sorption, deformation, and coupled sorption-deformation have been studied for single-scale materials, but scarcely for materials where micropores (<2 nm) and mesopores (2−50 nm) coexist. The present work, dealing with a mesoscopic slit pore between two slabs of microporous amorphous cellulose (AC), aims at modeling sorption-deformation interplay in hierarchical porous cellulosic structures inspired by polymeric modern adsorbents. Specifically, the atomic system is modeled by a hybrid workflow combining molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations. The results clarify the multiple sorption/deformation mechanisms in porous materials with different slit-pore sizes, including water filling in micropores, surface covering at the solid−air interface, and subsequent capillary condensation in mesopores. In particular, before the onset of capillary condensation, the sorption behavior of the AC matrix in the hybrid system is almost the same as that of bulk AC, in which sorption and deformation enhance each other through sorptioninduced swelling and additional sorption in the newly created voids. Upon capillary condensation, the interaction between the micropores and the mesopore emerges. Water molecules in the mesopore exert a negative hydrostatic pressure perpendicular to the slab surface on the matrices, resulting in an increase in porosity and water content, a decrease in distance between the centers of mass (COMs) of the slabs, and thus a thinning of the slit pore. As described by Bangham's Law, the surface area of the rough slitpore slab increases proportionally to the surface energy variation during surface covering. For a system composed of a compliant polymer like AC, however, the surface area enlargement does not result in an in-plane swelling as expected but instead in an in-plane shrinkage along with an increase in local roughness or irregularity (an accordion effect).
The biogenic synthesis of silver nanoparticles has recently attracted more attention to counter microbial resistance, which has been one of the medical concerns in the last decade. This research expresses the biogenic synthesis of silver nanoparticles utilizing Ferula assafoetida aqueous extract (Fer@AgNP) as a reducing and capping agent. The total parts of the plant were extracted from an aqueous solution (FerEX) and characterized using GC/MS apparatus. The Fer@AgNP and chemically synthesized silver nanoparticles (AgNPs) were characterized using UV-vis, Fourier transform infrared (FTIR) spectroscopies, field emission-scanning transmission electron microscopy, powder X-ray diffraction analysis, and energy-dispersive X-ray spectroscopy. The impacts of nanoparticles and FerEX were evaluated against four pathogenic bacterial strains, including Staphylococcus aureus, Escherichia coli, Salmonella typhi, and Enterococcus faecalis, using the microdilution method. The biocompatibility of compounds was also evaluated on human cell line L-929 using MTT and human blood cells using the hemolytic assay. The major compounds found in FerEX were sulfur-containing compounds such as butyl disulfides (45.36%) and monoterpenes such as α-pinene (25.66%), β-pinene (16.31%), and ocimene (7.26%). The characterizations of materials confirmed the hexagonal structure of AgNPs. The sizes of cAgNP and Fer@AgNP were about 42.7 nm and 22.5 nm. The antimicrobial activity of Fer@AgNP was considerably developed and reached MIC values ranging from 10 to 50 μg/mL compared to AgNP, which showed MIC values ranging from 50 to 100 μg/mL. The biocompatibility assessment showed that the Fer@AgNP was improved compared to AgNP and had a minimal toxic impact on the normal fibroblast cell line. The Fer@AgNP also indicated outstanding compatibility with human RBCs. The results illustrated that biosynthesized Fer@AgNPs have improved antimicrobial efficacy against Gram-negative and Gram-positive pathogenic bacteria with promising biocompatibility and can be used as potential antibacterial agents.
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