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A multiscale modeling approach is proposed to account for the interfacial load transfer characteristics of epoxy nanocomposites. The localized stress evolutions in nanocomposites are examined with ...

Y2O3 coating on Al2O3 substrate was prepared by atmospheric plasma spray (APS). Computational fluid dynamics (CFD) was carried out to predict the state of in-flight Y2O3 particles at different powder feeding rates. Microstructure and mechanical properties were found to be affected by the spray distance and powder feeding rate. In this study, the hardness was calculated using a field emission-scanning electron microscope (FE-SEM) because the indentation in the coating is too small to measure using a hardness test machine. The formation of pores causes a decrease in the mechanical property, and the pore length of over 10 μm substantially decreases the hardness. Meanwhile, the solidification behavior is affected by the maximum temperature of the in-flight particles. Based on computational fluid dynamics (CFD) analysis, the maximum temperature of the in-flight particles was found to decrease with increase of the powder feeding rate at the same spray distance. At the powder feeding rate of 60 g/min, a lower adhesion strength was confirmed than that at feeding rate of 30 g/min because splats were insufficiently spread due to the lower maximum temperature of the in-flight particles. The roughness and height of the coating surface were evaluated by confocal microscopy and atomic force microscopy(AFM) analyses. The roughness is the resultant of accumulated splats and the accumulation mechanism of splats is affected by the state of the in-flight particles. Furthermore, there were nano-scale differences of height on the splat surface, on which the nucleation looks like ‘rugged bark’ during solidification of splats when the in-flight particles impact the substrate.

The mechanical characterization of the interphase zone has attracted considerable attention in the fields of mechanical engineering and material design. Especially, the constitutive modeling for the description of the nonlinear mechanical behaviors of the interphase zone within the finite strain is essential for the material design of the polymer nanocomposites (e.g., the thermo-mechanical design, the fracture toughness design, and the fatigue life design). In this study, we propose an efficient multiscale homogenization modeling approach to describe the hyperelastic behavior of the polymer nanocomposites. This is the first attempt to achieve the hyperelastic constitutive modeling of the interphase zone by the multiscale framework, which is based on the full-atomistic molecular dynamics simulations and the multiscale homogenization analysis. The role of interfacial interactions between the nanoparticle and the polymer matrix on the nonlinear mechanical behaviors of polymer nanocomposites is characterized within the finite strain range by the proposed multiscale framework. To overcome the numerical inefficiencies induced by the excessively large number of iterations, the proper orthogonal decomposition method is merged into the proposed multiscale framework. The equivalent continuum models of the polymer nanocomposites are verified by the full atomistic molecular dynamics simulations.

Cross-linked liquid crystalline polymers (CLCPs) incorporated with photochromic azobenzene moieties can exhibit large and reversible deformations under the exposure of actinic lights. To practicall...

Cracks are typically associated with the failure of materials. However, cracks can also be used to create periodic patterns on the surfaces of materials, as observed in the skin of crocodiles and elephants. In synthetic materials, surface patterns are critical to micro- and nanoscale fabrication processes. Here, a strategy is presented that enables freely programmable patterns of cracks on the surface of a polymer and then uses these cracks to pattern other materials. Cracks form during deposition of a thin film metal on a liquid crystal polymer network (LCN) and follow the spatially patterned molecular order of the polymer. These patterned sub-micrometer scale cracks have an order parameter of 0.98 ± 0.02 and form readily over centimeter-scale areas on the flexible substrates. The patterning of the LCN enables cracks that turn corners, spiral azimuthally, or radiate from a point. Conductive inks can be filled into these oriented cracks, resulting in flexible, anisotropic, and transparent conductors. This materials-based processing approach to patterning cracks enables unprecedented control of the orientation, length, width, and depth of the cracks without costly lithography methods. This approach promises new architectures of electronics, sensors, fluidics, optics, and other devices with micro- and nanoscale features.

We propose a scale-bridging methodology to link the microscopic photoreaction of an azobenzene-containing liquid crystalline polymer (LCP) and the macroscopic interfacial and elastic properties of carbon nanotube (CNT)-reinforced photo-responsive nanocomposites. The photo-isomerization of the azobenzene moieties is described by implementing a photo-switching potential that represents the light-excited energy transition path. The relevant time evolution of the molecular shape and the concurrent changes in the interfacial morphology are observed using molecular dynamics (MD) simulations. Finally, the effective elastic properties of the photo-responsive polymer (PRP) nanocomposite with respect to the isomerization ratio are numerically derived using the micromechanics-based homogenization method. It is verified that the size of the CNT and the photo-deformation of the azobenzene molecules influence the intermolecular interactions and the nematic phase of the LCP at the interfacial region. The continuum-scale finite element (FE) model, which reflects the microscopic information, clearly predicts the reinforcing effect of the CNT filler on the elastic properties of the composite and their variation under photo-actuation. We expect our results to shed light on designing the photomechanical energy conversion efficiency of nano-sized soft actuators composed of CNT-reinforced composites.

We propose an efficient simulation-based methodology to characterize the quasi-static (experimental low strain rate) yield stress of an amorphous thermoset polymer, which has generally been considered a limitation of molecular dynamics (MD) simulations owing to the extremely short time steps involved. In an effort to overcome this limitation, the temperature-accelerated method – in which temperature is treated as being equivalent to time in deformation kinetics – is employed to explore the experimental strain rate conditions. The mechanical tensile behavior of a highly crosslinked polymer is then investigated with MD simulations by considering different strain rates and temperatures below the glass transition temperature. The derived yield stress represents the time- and temperature-dependent characteristics, showing that the yield stress decreases with increasing temperature and decreasing strain rate. Changeable vertical and horizontal shift factors are introduced for the first time to reflect nonlinear characteristics of the yield stress across a broad range of strain rates and to quantify the correlation between increasing temperatures and decreasing strain rates. With the proposed method, the Eyring plot, which describes the rate effect on yield from quasi-static to high-rate conditions, is predicted from MD simulations, and agrees well with macroscopic experimental results. From the constructed Eyring plot, the experimentally validated quasi-static stress-strain response is also estimated by using linear elastic model and Ludwick's hardening model. The proposed method provides new avenues for the design of glassy polymers using only fully atomistic MD simulations, thus overcoming the existing temporal scale limitations.

The nature of the inelastic-deformation characteristics of highly-crosslinked epoxy polymers has been understood at the microscopic level and in consideration of the structural network-topology differences. The structural differences that arise from different types of curing agents (aliphatic and aromatic) have been estimated using the compressive loading–unloading responses in terms of the energy, stress, and geometric characteristics. The energy and stress distributions at 300 K revealed that the nonbonded interactions of the polymer chains and the local dihedral-angle behaviors are key internal-potential components that accommodate the applied levels of the deformation energy and stress. In particular, a residual dihedral-angle stress was observed in the monomers of aromatic curing agents after the unloading, while the aliphatic-cured system displayed a spring-like elastic response. The plastic response of the aromatic-cured epoxy is attributed to the plastic folding of a local dihedral angle that is owing to the mobility discrepancy of a benzene ring and the flexible chain segments that are linked to the benzene ring. From the energy perspective, plastic dihedral-angle transitions were observed in the 1-K deformation simulations. The plastic-folding behaviors of the dihedral angles are evident near the yield point, which is coincident with the molecular-kink behaviors of the classical yielding theory.

We describe an experimental and computational study of increased electrical double layer capacitance (EDLC) changes and improved harvesting performances in multi-walled carbon nanotube (MWCNT) yarn-based energy harvesters using smaller-diameter MWCNTs. Twisted MWCNT yarn-based energy harvesters convert tensile mechanical energy into electrical energy by utilizing changes in the EDLC on the surface of MWCNTs. The correlation of the variance in EDLC and open circuit voltage (OCV) of the coiled MWCNT yarn was investigated under the mechanical stretch applied to the twistron energy harvester, and both OCV and capacitance increased as the MWCNT diameter decreased. We performed molecular dynamics (MD) simulations to elucidate the mechanisms of the increasing EDLC and OCV of twistron energy harvesters, which were verified by experiment. The computational results elucidate the relationship between the decrease in MWCNT diameters and the increasing number density of interstitial spaces between multi-walled MWCNTs. The decrease in the total surface area of the interstitial spaces is an important factor in the capacitance change of the twistron energy harvester under longitudinal stretching. Such novel findings are a foundation for the development of a twistron harvester with significantly improved energy harvesting performance.

We propose a new model for predicting the trans-cis-trans reorientation (TCTR)-based photodeformation of the azobenzene-doped liquid-crystal polymer (azo-LCP). The model was validated through the results of a bi-directional photobending experiment performed at various temperatures and mole ratios of the azobenzene monomer within the azo-LCP. Through both numerical and experimental investigations, we found that the photobending curvature of the azo-LCP shows a maximum point at certain temperatures, but only shows a proportional relationship with the azobenzene mole ratio within the azo-LCP. We confirmed that this tendency is caused by the change in the polymeric constraint with the temperature and the low light propagation in azo-LCP.

Azobenzene-containing crosslinked liquid crystalline polymer (CLCP) exhibits macroscopic deformation through irradiation with light, and recently, carbon nanotubes (CNTs) were inserted to improve the efficiency of the photo-mechanical energy conversion. We conducted molecular dynamics simulations to study the effects of the arrangement of the single-walled CNT (SWCNT) and photo-isomerization of the photochromic moieties on the opto-mechanical properties of the composite. We characterized the composites in terms of the interfacial shear strength (IFSS), photostrain, and elastic modulus according to the isomerization ratio. The SWCNT-CLCP composite enables 19–29% larger photostrain and 20–41% higher modulus compared to those of neat azo-PRP. As the angle between the CNT axis and the nematic director increases, mechanical reinforcing effect decreases. Nevertheless, the applicable photo-shrinkage increases because more space for polymer diffusion is made by the nematic defects in the interphase. The isomerization of azobenzene weakens the interfacial adhesion due to the decrease in the π-π interaction. However, changes in the elastic stiffness of the composite can be modulated by adjusting the arrangement between the SWCNT and matrix. These results provide insight into the mechanism of changes in the interfacial and opto-mechanical properties of the CNT-PRP composite caused by photo-isomerization and the mechanical design of a photo-responsive actuator.

A multiscale modeling approach is proposed to characterize the interfacial behavior and the interphase properties of epoxy nanocomposites. The interfacial characteristics between the filler and matrix are investigated using molecular dynamics (MD) and molecular mechanics (MM) simulations. With increasing crosslink conversions, the interfacial adhesion between the filler and matrix is reduced which is attributed to the changes of inherent non-bond interaction characteristics at the interface, resulting in retarded reinforcing effect on the stiffness and thermal stability of epoxy nanocomposites. Moreover, to understand the structural change in the interphase region of nanocomposites with crosslinking, the radial density profile, the local crosslinks distribution, and the free volume at the filler surface are further examined. The results of structural features consistently demonstrate that the structural conformation of the interphase is substantially influenced by the reduction of interfacial communication with increasing crosslink conversion. In order to take into account the variations of interfacial compliance and the thermomechanical property of the interphase region, the effective interphase concept is implemented. Further, the micromechanics-based multi-inclusion model provides a reasonable prediction for the thermomechanical property of composites using the effective interphase concept.

Synthesizing surface-grafted Au nanoparticles (grafted-AuNPs) for incorporation into a liquid crystalline elastomer (LCE) matrix yields high contents of nanoparticles that are chemically compatible and soluble in the matrix. To investigate the change in the internal microstructure by inserting grafted-AuNPs and the mechanical role of the interfaces in the LCE/Au nanocomposites, all-atom molecular dynamics simulations were conducted. The results suggest that the insertion of the grafted-AuNP disrupts the LCE mesogen arrangement, especially in the interfacial area. Additionally, the strain energy density distributed to each molecular component revealed that the grafted unit on the side of the LCE matrix forms a highly entangled network with a semi-rigid microstructure and enables high load transfer efficiency. By contrast, a ductile grafted layer formed on the side of the AuNP surface acts as a cushion, which allows the AuNP to exist in the unloading state. Furthermore, the curvature of the inserted AuNPs is crucial in changing the mechanical and structural properties of the LCE matrix phase.

The oxide growth mechanism on Al (100), (110) and (111) and the corresponding oxide film properties were examined using all-atom molecular dynamics simulations. The growth kinetics of the aluminum oxide film at a constant dose rate of oxygen were analyzed. Al (110), with a structurally unstable surface and a high surface energy, showed the highest oxide growth rate, oxygen adsorption rate, and adsorption energy. However, as the crystallinity of the surface structure collapsed as oxidation progressed, the reaction energy of Al (110) with atomic oxygen converged to the same value as that of Al (100) and (111). Instead, a high anisotropic residual stress remained in the oxide layer on Al (110) with the aid of high surface energy and anisotropic surface structure. The results suggest that although the chemical bonding features of the as-prepared oxide layers were similar, the intermediate process of oxide film formation and the resultant mechanical properties were highly dependent on surface crystallinity. The oxidation kinetics model presented in this work showed consistency with other reported ab initio calculations. Moreover, it also successfully reproduced the experimental fact that the formation speed of oxygen islands on Al (100) is more delayed than that on Al (111).

Photoresist (PR) nanopatterning using extreme-ultraviolet-lithography (EUVL) drives significant advances in integrated-circuit downsizing; however, sub-10 nm nodes severely suffer from collapse in the rinsing process (structural rigidity ↓, capillary force ↑ in denser line patterns). To mitigate collapse, we propose a new type of photoresist, hybrid material, with delicate polymer microstructure across the exposure boundary; a blend of linear chains for exposed domain and crosslinked network for masked area. This hybrid system is synthesized in a computational model through exposure-bake-curing process that generates a steep chemical gradient at the exposure boundary of polymer and triggers selective crosslinking reaction on the protected functional groups. The crosslinked structure is formed exclusively for the protection group-rich unexposed region; thus, the chemical joints tightly anchor the masked chains in aqueous solution, leading to nanoscale smoothing on the interfacial surface. Moreover, chemical linkage on the residual chains contributes to force delivery on the polymer matrix under macroscopic strain. Among the candidate materials, the hybrid resist incorporating a bicyclic crosslinker exhibits the best load transfer efficiency (E 101.4% ↑) and uniform interfacial surface (roughness 26.4% ↓), which significantly alleviates mechanical deformation and extends the critical aspect ratio of the pattern over the neat system.

In a nematic solid, wherein liquid crystal molecules are incorporated into polymeric chains, the chromophore phase is projected onto the polymer conformation, changing the stress-free configuration metric. Stimulated actuation cannot be separated from the structure itself, since the mesoscopic polymer properties dictate the degree and type of shape change. In this research, we focused on self-deforming device programming, inspired by recent optical techniques, to pattern nontrivial alignment textures and induce exotic strain fields on specimens. A finite-element framework incorporating a light-thermo-order coupled constitutive relation and geometric nonlinearities was utilized to compute mechanical deformations for given external stimuli. The distortion of planar strips into various exotic 3D shapes was simulated, and disclination-defect-like liquid crystal texture topographies with different defect strengths produced various many-poled shapes upon irradiation, as observed experimentally. The effects of the boundary conditions and geometric nonlinearities were also examined, exemplifying the need for a comprehensive finite-element-based framework. The same method was applied to textures naturally emerging due to static distortion, and the effects of the prescribed inhomogeneities on the overall deformations, which is the basis of inverse design, were observed. Furthermore, we analyzed the local Poisson-effect-induced instability resulting from inscribing a hedgehog disclination texture onto a solid; the onset of buckling-like deformations was observed energetically, and the relations between this onset and other physical properties were elucidated to enable microstate design while maintaining structural stability. These results will facilitate the development and comprehension of the mechanisms of remotely light-controlled self-assembly and propulsion systems that may soon be realized.

Oriented polystyrene (PS) has attracted attention due to its capability of sequential self-folding behaviors by infrared (IR) light heating. It has been seen that temporal changes of the shape and properties of PS are affected by its pre-defined molecular structure. In this study, full atomistic molecular dynamics (MD) simulations are carried out to study the effect of initially applied strain on the thermoelastic properties and shape recovery behaviors of the oriented PS. The shape memory creation procedure (SMCP) is described by long-time simulation and initial strains between 15% and 100% are applied by in-plane tensile loadings. As the extent of pre-stretching increases, the anisotropy of elastic modulus and linear coefficient of thermal expansion (CTE) increase primarily due to the molecular alignment. The shape fixity ratio increases by up to 24% and the recovery ratio decreases by up to 32% by increasing the initial strain. In particular, the shape recovery ratio is mainly affected by the orientational order parameter of the oriented PS. The rate of the recoveries of the thermomechanical properties and shape reduce for a more pre-stretched PS. The retardation of conformational transformation comes from a reduction in the sizes of free volume elements. The results demonstrate a microstructure-property relationship which is important in designing the self-folding behaviors of oriented PS.

We investigated the optical and thermal actuation behavior of densely cross-linked photoresponsive polymer (PRP) and polymer nanocomposites containing gold nanoparticles (PRP/Au) using all-atom molecular dynamics (MD) simulations. The modeled molecular structures contain a large number of photoreactive mesogens with linear orientation. Flexible side chains are interconnected through covalent bonds under periodic boundary conditions. A switchable dihedral potential was applied on a diazene moiety to describe the photochemical trans-to-cis isomerization. To quantify the photoinduced molecular reorientation and its effect on the macroscopic actuation of the neat PRP and PRP/Au materials, we characterized the photostrain and other material properties including elastic stiffness and thermal stability according to the photoisomerization ratio of the reactive groups. We particularly examined the effect of nanoparticle size on the photothermal actuation by varying the diameter of the nanofiller (10-20 Å) under the same volume fraction of 1.62%. The results indicated that the insertion of the gold nanoparticles enlarges the photostrain of the material while enhancing its mechanical stiffness and thermal stability. When the diameter of the nanoparticle reaches a size similar to or smaller than the length of the mesogen, the interfacial energy between the nanofiller and the surrounding polymer matrix does not significantly affect the alignment of the mesogens, but rather the adsorption energy at the interface generates a stable interphase layer. Hence, these improvements were more effective as the size of the gold nanoparticle decreased. The present findings suggest a wider analysis of the nanofiller-reinforced PRP composites and could be a guide for the mechanical design of the PRP actuator system.

In this study, a multiscale model fully representing mechanical deformation is developed for the identification of geometrical and mechanical properties of the interfacial layer in single-walled carbon nanotube (SWNT)-epoxy nanocomposite. The mechanical properties of nanocomposite reinforced with SWNTs are derived using all-atom molecular dynamics (MD) simulations for different diameter nanotubes under constant composition conditions. A nanotube size effect on axial stiffness along the nanotube alignment direction is clearly observed, whereas the transverse axial and shear stiffness components are less than the corresponding values of neat polymer. Through the analysis of deformation energy and its distribution inside the nanocomposite unit cell, the presence of an inner soft and slippery polymer layer at the vicinity of the nanocarbon surface is revealed. Taking account of this unusual reinforcing effect using a finite element (FE) model, we implicitly solve for the size and mechanical properties of the effective interphase, which has an equivalent deformation energy around the nanotube, as well as the global elastic stiffness of the nanocomposite that is equivalent to the corresponding value from MD simulations. The equivalent continuum model thus properly predicts the local stress distribution at the adsorbed polymer-SWNT interface as well as the overall mechanical properties of nanocomposite, along with their inherent nanotube size effect.

Embedding nanoparticles in a responsive polymer matrix is a formidable way to fabricate hybrid materials with predesigned properties and prospective applications in actuators, mechanically tunable optical elements, and electroclinic films. However, achieving chemical compatibility between nanoparticles and organic matter is not trivial and often results in disordered structures. Herein, it is shown that using nanoparticles as exclusive cross-linkers in the preparation of liquid-crystalline polymers can yield long-range-ordered liquid-crystalline elastomers with high loadings of well-dispersed nanoparticles, as confirmed by small-angle XRD measurements. Moreover, the strategy of incorporating NPs as cross-linking units does not result in disruption of mechanical properties of the polymer, and this phenomenon was explained by the means of all-atom molecular dynamics simulations. Such materials can exhibit switchable behavior under thermal stimulus with stability spanning over multiple heating/cooling cycles. The presented strategy has proven to be a promising approach for the preparation of new types of hybrid liquid-crystalline elastomers that can be of value for future photonic applications.

Concentrated light-absorption on specific areas of polystyrene (PS) sheet induces self-folding behaviour. Such localized light-absorption control is easily realized by black-coloured line pattern printing. As the temperature in the line patterns of PS sheet increases differently due to the transparencies in each line pattern, localized thermal contraction generates folding deformation of the PS sheet. The light-activated folding technique is caused by the shape memory effect of PS sheet. The shape memory creation procedure (SMCP) is described by using molecular dynamic (MD) simulation, and the constitutive model of PS sheet is identified. This study employs the shell/cohesive line element for the folding deformation of PS sheet, and utilizes the constitutive model obtained from the MD simulation. Based on the continuum-model analysis of the PS sheet folding deformation activated by light, various self-folding structures are designed and manufactured.

A liquid crystal network whose chromophores are functionalized by photochromic dye exhibits light-induced mechanical behaviour. As a result, the micro-scaled thermotropic traits of the network and the macroscopic phase behaviour are both influenced as light alternates the shape of the dyes. In this paper, we present an analysis of this photomechanical behaviour based on the proposed multiscale framework, which incorporates the molecular details of microstate evolution into a continuum-based understanding. The effects of trans-to-cis photoisomerization driven by actinic light irradiation are first examined using molecular dynamics simulations and are compared against the predictions of the classical dilution model; this reveals certain characteristics of mesogenic interaction upon isomerization, followed by changes in the polymeric structure. We then upscale the thermotropic phase-related information with the aid of a nonlinear finite element analysis; macroscopic deflection with respect to the wide ranges of temperature and actinic light intensity are thereby examined, which reveals that the classical model underestimates the true deformation. This work therefore provides measures for analysing photomechanics in general by bridging the gap between the micro- and macro-scales.