Bulk Mechanical Properties Research Articles

Prediction of bulk mechanical properties of PVC foam based on microscopic model:Part I-Microstructure characterization and generation algorithm

The mechanical properties of porous materials greatly depend on their microstructures, so the generation of realistic microstructures is the premise for a sound numerical simulation and analytical prediction. This paper presented the microstructure generation of three kinds of transversely isotropic closed-cell polyvinyl chloride (PVC) foams with different densities based on the geometric characteristics obtained from the X-ray computed tomography (CT) technology. Advancing front method was used to densely pack a set of spheres whose volume distribution is proportion to the cell volume distribution and the microstructure was formed by Laguerre tessellation. Then the geometric characteristics of the numerical models were compared with those from the CT. The good consistence proves that the proposed method is accurate and efficient to produce the representative microstructure of the PVC foams. Finally, a method was proposed to optimize the microstructure of small size numerical specimens, by which the small specimens could give similar geometric characteristics to those of the large specimens. • Systematic investigation of geometric characteristics of PVC foams by micro-CT scan. • Wall thickness and volume difference between spheres and corresponding polyhedrons considered at the beginning of the modelling process. • The same radius distribution of packing spheres for foams of different densities can be assumed if the wall thickness has been considered. • Method to reduce the size of representative volume element and thus the computational expense.

Hard ultralight systems by thermal spray deposition of WC-CoCr onto AZ31 magnesium alloy

Magnesium alloys in general, and AZ31 in particular, have huge potential as materials for energy saving and emissions reduction in the transportation of people and goods, because of their low density (about 1.7 g∙cm −3 ) coupled to good bulk mechanical properties. The main weakness is represented by the alloy surface, prone to corrosion and rather soft. Coatings are therefore mandatory, especially in harsh environments. To obtain AZ31 materials with exceptional surface properties High Velocity Oxygen-Fuel (HVOF) thermal spray was exploited to deposit a WC-CoCr coating with very high surface hardness. Because of the heat sensitivity of magnesium alloys, the influence of torch-substrate kinematics and substrate surface preparation on the surface temperature during deposition, the metallurgical characteristics of AZ31, and the residual stress state of the coated samples were investigated. The overall quality of the coatings was also evaluated by abrasion and electrochemical corrosion resistance tests. Proper adjustment of the torch/substrate standoff distance and torch patterning strategy reduced the surface temperature during deposition by around 100 °C, as measured with an IR pyrometer. This also resulted in a decreased magnitude of the average compressive stress in the coating. At the same time, these adjustments did not impair measurably the performances of the coatings, which, regardless of the deposition conditions, had approximately 1200 HV 0.3 hardness, 2–4 μA∙cm −2 corrosion current density in 3.5 wt./vol% NaCl aqueous solution, and 7–8 × 10 −4 mm 3 ∙(N∙m) −1 specific wear rate under dry particles' abrasion conditions. Cyclic impact tests also showed that the coatings adhered equally well to grit-blasted or machined surfaces. On the other hand, transverse cracks developed across the coatings during impact tests because of the much higher elastic and plastic deformability of the AZ31 substrate. The avoidance of grit-blasting and the lowered deposition temperatures were also important to prevent anomalous grain growth in the AZ31 substrate. • Stand-off distance and torch/substrate kinematics controlled the deposition temperature. • The compressive stress level in the coating depended on the deposition temperature. • High deposition temperature caused anomalous grain growth in grit-blasted AZ31. • HVOF-sprayed WC-CoCr particles penetrated deep into the AZ31 substrate. • Thus, grit-blasting was not needed to achieve sound coating/substrate adhesion.

Detergent-Free Decellularization Preserves the Mechanical and Biological Integrity of Murine Tendon.

Tissue decellularization has demonstrated widespread applications across numerous organ systems for tissue engineering and regenerative medicine applications. Decellularized tissues are expected to retain structural and/or compositional features of the natural extracellular matrix (ECM), enabling investigation of biochemical factors and cell-ECM interactions that drive tissue homeostasis, healing, and disease. However, the dense collagenous tendon matrix has limited the efficacy of traditional decellularization strategies without the aid of harsh chemical detergents and/or physical agitation that disrupt tissue integrity and denature proteins involved in regulating cell behavior. In this study, we adapted and established the advantages of a detergent-free decellularization method that relies on latrunculin B actin destabilization, alternating hypertonic-hypotonic salt and water incubations, nuclease-assisted elimination of cellular material, and protease inhibitor supplementation under aseptic conditions. Our method maintained the collagen molecular structure (i.e., minimal extent of denaturation), while adequately removing cells and preserving bulk mechanical properties. Furthermore, we demonstrated that decellularized tendon ECM-derived coatings isolated from different mouse strains, injury states (i.e., naive and acutely injured/"provisional"), and anatomical sites harness distinct biochemical cues and robustly maintain tendon cell viability in vitro. Together, our work provides a simple and scalable decellularization method to facilitate mechanistic studies that will expand our fundamental understanding of tendon ECM and cell biology. Impact statement In this study, we present a decellularization method for tendon that does not rely on any detergent or physical processing techniques. We assessed the impact of detergent-free decellularization using tissue, cellular, and molecular level analyses and validated the preservation of gross fiber architecture, collagen molecular structure, and extracellular matrix (ECM)-associated biological cues that are essential for studying physiological cell-ECM interactions. Finally, we demonstrated the applicability of this method for healthy and injured tendon environments, across mouse strains, and for different types of tendons, illustrating the utility of this approach for isolating the contributions of biochemical cues within unique tendon ECM microenvironments.

Effects of plasma-induced grafting modification on the adhesive strength and mechanical properties of fiber posts.

This study aimed to assess the effects of plasma grafting modification on the micro-push-out adhesive strength and mechanical properties of fiber posts and to assess the stability of these treatment effects. Glass-fiber posts were divided into four groups based on the treatment methods used, as follows: (1) Group NT: no treatment; (2) Group PT: Helium (He) plasma treatment; (3) Group PIG: He-plasma-induced post-irradiation grafting; and (4) Group SIG: He-plasma-induced syn-irradiation grafting. The treated fiber posts were bonded using self-adhesive resin cement exposure to air for 0, 1, 6 or 12 hours separately after surface treatment. Micro-push-out adhesive strength, flexural modulus, and flexural strength were measured. Plasma treatment, post-irradiation grafting, and syn-irradiation grafting improved adhesive strength at the 0-hours level. However, the improved adhesive strength disappeared in group PT after exposure for one or more hours. In group PIG, the adhesive strength after 1-hour exposure was 20.5% lower than that of 0-hour exposure (adhesive immediately after treatment), and no statistically significant differences in adhesive strength were observed between the 1, 6, and 12-hour exposure. In group SIG, no statistically significant differences in adhesive strength were observed among the 0, 1, and 6-hour exposure. Although the adhesive strength was 23% lower at the 12-hour exposure than that of 0-hour exposure in group SIG, the adhesive strength of fiber posts received syn-irradiation grafting still presented the best adhesive strength compared with the other treatment methods. The three-point flexural modulus and strength remained unaffected by the treatment methods used. Plasma-induced syn-irradiation grafting provided the ideal improvement and stability in adhesive strength in fiber posts. In addition, plasma-induced grafting modification successfully overcame the surface aging effect caused by plasma treatment alone without affecting the bulk mechanical properties of fiber posts.

Recycling of Concrete Demolition Waste Powder as a Sustainable Material in Portland Cement Pastes Modified with Nano-silica

In recent years, researchers dedicated themselves to explore the possibility of introducingconcrete waste powder (CWP) into Portland cement as a sustainable material for Portland cement to reduce as possible the environmental pollution with this concrete demolition wastes. To optimize the effect of CWP on cement-based materials, this paper usesnano-silica (NS) to improve the hydration and mechanical properties of cement-based materials with CWP. Results indicated that after adding NS into CWP cement pastes, the setting time of cement pastes was significantly reduced, while the early rate of hydration and hydration heat increased. Besides, the mechanical strengths of the cement pastes increased as CWP replaced at the expense of the cement only up till 15 weights %, and then decreased. The addition of nanosilica (NS) can compensate the mechanical strength loss caused by CWP as supplementary cementing materials. The higher ratios of CWP than 15 weight % increased the pore volume or porosity. In contrast, NS in CWP blended cement significantly decreased the porosity, and increased the proportion of harmless pores. Hence, NS reduced the porosity, which in turn improved and enhanced the bulk density and mechanical properties. The optimum amount of NS is 2.5 weight % which resulted in the best results. The heat of hydration of the different cement batches in the two groups adversely affected with the incorporation of CWP, but little improved with NS. The obtained results were confirmed by the ultrasonic pulse velocity (USPV) test, where the cement batch PP15 (Group I) and PS2.5 (Group II) achieved the highest conformance.

Prediction of bulk mechanical properties of PVC foam based on microscopic model：Part II-Material characterization and analytical formulae

The bulk mechanical properties of a foam are strongly related with the modulus of elasticity and yield strength of the base material and the geometric features of the foam microstructure. This paper proposed a method to predict the bulk mechanical properties of transversely isotropic closed-cell polyvinyl chloride (PVC) foams based on the numerical analysis of microscopic models. Firstly, the elastic modulus of the base material was determined through nanoindentation and the macro compressive stress-strain curves in three orthogonal directions were measured for the Divinycell H100 foam. Then, the influence of variation of the scaling factor, which transformed an isotropic microstructure to a transversely isotropic one, on the mechanical properties were investigated numerically. It is found that the ratio of the two moduli of elasticity in the rise direction and in the transverse direction linearly depends on the scaling factor, so does the ratio of the yield strengths. Next, the relations of the bulk mechanical properties of the transversely isotropic foam in the two directions with the relative density were formulated. Finally, this method was validated by being successfully applied to foams of the same base material but different densities.

Polymeric multimaterials by photochemical patterning of crystallinity.

An organized combination of stiff and elastic domains within a single material can synergistically tailor bulk mechanical properties. However, synthetic methods to achieve such sophisticated architectures remain elusive. We report a rapid, facile, and environmentally benign method to pattern strong and stiff semicrystalline phases within soft and elastic matrices using stereo-controlled ring-opening metathesis polymerization of an industrial monomer, cis-cyclooctene. Dual polymerization catalysis dictates polyolefin backbone chemistry, which enables patterning of compositionally uniform materials with seamless stiff and elastic interfaces. Visible light-induced activation of a metathesis catalyst results in the formation of semicrystalline trans polyoctenamer rubber, outcompeting the formation of cis polyoctenamer rubber, which occurs at room temperature. This bottom-up approach provides a method for manufacturing polymeric materials with promising applications in soft optoelectronics and robotics.

Superior mechanical properties by exploiting size-effects and multiscale interactions in hierarchically architected foams

Protective applications in extreme environments demand thermally stable materials with superior modulus, strength, and specific energy absorption (SEA) at lightweight. However, these properties typically have a trade-off. Hierarchically architected materials--such as the architected vertically aligned carbon nanotube (VACNT) foams--offer the potential to overcome these trade-offs to achieve synergistic enhancement in mechanical properties. Here, we adopt a full-factorial design of experiments (DOE) approach to optimize multitier design parameters to achieve synergistic enhancement in SEA, strength, and modulus at lightweight in VACNT foams with mesoscale cylindrical architecture. We exploit the size effects from geometrically-confined synthesis and the highly interactive morphology of CNTs to enable higher-order design parameter interactions that intriguingly break the diameter-to-thickness (D/t)-dependent scaling laws found in common tubular architected materials. We show that exploiting complementary hierarchical mechanisms in architected material design can lead to unprecedented synergistic enhancement of mechanical properties and performance desirable for extreme protective applications.

Supershear surface waves reveal prestress and anisotropy of soft materials

Surface waves play important roles in many fundamental and applied areas from seismic detection to material characterizations. Supershear surface waves with propagation speeds greater than bulk shear waves have recently been reported, but their properties are not well understood. Here we describe theoretical and experimental results on supershear surface waves in rubbery materials. We find that supershear surface waves can be supported in viscoelastic materials with no restriction on the shear quality factor. Interestingly, the effect of prestress on the speed of the supershear surface wave is opposite to that of the Rayleigh surface wave. Furthermore, anisotropy of material affects the supershear wave much more strongly than the Rayleigh surface wave. We offer heuristic interpretation as well as theoretical verification of our experimental observations. Our work points to the potential applications of supershear waves for characterizing the bulk mechanical properties of soft solid from the free surface.

Impact of pearl-necklace-like skeleton on pore sizes and mechanical properties of porous materials: A theoretical view

The structural and mechanical properties of open-porous cellular materials are often described in terms of simple beam-based models. A common assumption in these models is that the pore walls have a constant cross section, which may be in agreement for a vast majority of such materials. However, for many of those materials that are characterized by a pearl-necklace-like network, this assumption seems too idealized. Aerogels are perfect examples of such materials. In this paper, we investigate the effect of such pore walls having a string of pearls-like morphology on the properties of such open-porous materials. First, the pore size is mathematically modeled. Three scenarios are described, where the pore sizes are calculated for cells in 2D, 3D, and 3D with overlapping particles. The dependency of the skeletal features on the resulting pore size is investigated. In the second part, pore walls with 3D overlapping spheres are modeled and subjected to axial stretching, bending, and buckling. The effect of the particle sizes and the amount of overlap between the particles on the mechanical features is simulated and illustrated. The results are also compared with models that assume a constant cross section of pore-walls. It can be observed that neglecting the corrugations arising from the pearl-necklace-like morphology in open-porous cellular materials can result in serious miscalculations of their mechanical behavior. The goal of this paper is not to quantify the bulk mechanical properties of the materials by accounting for the pearl-necklace-like morphology but rather to demonstrate the significant deviations that may arise when not accounted for.

Determination of peak ordering in the CrCoNi medium-entropy alloy via nanoindentation

Local chemical ordering (LCO) in the CrCoNi medium-entropy alloy was investigated by transmission electron microscopy (TEM) after different annealing treatments and their corresponding mechanical properties by bulk tensile tests and nanoindentation. A cold-rolled alloy was annealed at 1000°C for 0.5 h followed by ice water quenching and then aged at a number of different temperatures (600°C, 700°C, 800°C, 900°C, and 1000°C) under vacuum for 240 h to generate different degrees of chemical ordering. A splat-quenched sample rapidly cooled from the liquid phase was also examined. While bulk mechanical properties did not vary among samples with equivalent grain sizes, nanoindentation tests revealed notable differences. As indicated by the load at first pop-in using a Berkovich tip or the indentation yield strength via continuous stiffness measurements using a 10 μm spherical tip, the nanoindentation tests revealed that the stress for onset of plasticity during indentation varied with heat treatment and peaked in the 900°C aged sample. Energy-filtered TEM characterization indicated the presence of ordering in all specimens, with a higher degree of LCO in the aged samples relative to the splat-quenched and 1000°C-quenched samples. The evolution of LCO during aging was determined to occur on the time scale similar to those of bulk diffusion. The difference in nanoindentation strength was attributed to the difference in dislocation nucleation barriers imposed by different degrees of LCO.

Peptide Sequence Determines Structural Sensitivity to Supramolecular Polymerization Pathways and Bioactivity.

Pathways in supramolecular polymerization traverse different regions of the system's energy landscape, affecting not only their architectures and internal structure but also their functions. We report here on the effects of pathway selection on polymerization for two isomeric peptide amphiphile monomers with amino acid sequences AAEE and AEAE. We subjected the monomers to five different pathways that varied in the order they were exposed to electrostatic screening by electrolytes and thermal annealing. We found that introducing electrostatic screening of E residues before annealing led to crystalline packing of AAEE monomers. Electrostatic screening decreased intermolecular repulsion among AAEE monomers thus promoting internal order within the supramolecular polymers, while subsequent annealing brought them closer to thermodynamic equilibrium with enhanced β-sheet secondary structure. In contrast, supramolecular polymerization of AEAE monomers was less pathway dependent, which we attribute to side-chain dimerization. Regardless of the pathway, the internal structure of AEAE nanostructures had limited internal order and moderate β-sheet structure. These supramolecular polymers generated hydrogels with lower porosity and greater bulk mechanical strength than those formed by the more cohesive AAEE polymers. The combination of dynamic, less ordered internal structure and bulk strength of AEAE networks promoted strong cell-material interactions in adherent epithelial-like cells, evidenced by increased cytoskeletal remodeling and cell spreading. The highly ordered AAEE nanostructures formed porous hydrogels with inferior bulk mechanical properties and weaker cell-material interactions. We conclude that pathway sensitivity in supramolecular synthesis, and therefore structure and function, is highly dependent on the nature of dominant interactions driving polymerization.

Evaluation of lateritic soils of Mbé for use as compressed earth bricks (CEB)

This work was carried out on the lateritic soils of Mbé in the Adamawa region of Cameroon. For this study, twenty (20) soil samples were taken from four (04) sites (Kabaa, Ndom, Mbé Norwegian camp, and Nyéssé). To assess their suitability for the production of compressed earth bricks, physical parameters were obtained by geotechnical tests to determine particle size distribution, water content, atterbergs limits, specific gravity, methylene blue value, maximum dry density (MDD), and optimum moisture content (OMC). Chemical and mineralogical properties were obtained by X-ray fluorescence and X-ray diffraction, respectively, followed by the determination of technological parameters (water absorption and mechanical properties) on compressed earth brick specimens. The results show that the studied lateritic soils contain mainly sand (52.8–90.3 wt%) and clay particles (8.4–42.1 wt%), gravel particles are slightly represented (1.1–16.3 wt%). The plasticity index of the studied materials is average (5–38%), about their methylene blue values (1–5), these soils correspond to clayey-silty soils. The materials studied are classified by the USCS as clayey sands/silty clays; according to the Highway Research Board (HRB), they are classified as A-7-5 or A-7-6 clayey soils with group index of 1 for Kabaa, 7 for Ndom, 9 for Mbé Norwegian Camp, and 12 for Nyéssé. These soils are rich in silica (SiO2, 62.6–78.1 wt%), followed by aluminum (Al2O3, 11.8–18.2 wt%) and iron oxides (Fe2O3, 3.2–8.1 wt%); other oxides are in lower proportions (<1 wt%). The mineralogical content consists mainly of clay minerals such as kaolinite, illite, smectite (montmorillonite), and non-clay minerals such as quartz, muscovite, biotite, gibbsite, and hematite. The bulk density and mechanical properties of the specimens are within the standard NC 102-115 (2007) of compressed earth bricks (CEB), which recommends minimum compressive strengths of 2 Mpa for unstabilized CEB and 4 Mpa for stabilized CEB. Water absorption of CEB and physical parameters of soils have a significant impact on the mechanical behavior of CEB. According to the test results, Mbe lateritic soils are suitable for engineering applications in the production of stabilized compressed earth bricks.

Controlling mixed-mode fatigue crack growth using deep reinforcement learning

Mechanical discontinuity embedded in a material determines the bulk mechanical, physical, and chemical properties. Under external forces, mechanical discontinuity undergo spatiotemporal propagation; thereby altering various properties of the material. This paper is a proof-of-concept development and deployment of a reinforcement learning framework, based on deep deterministic policy gradient, to precisely control both the direction and rate of the fatigue crack growth. The ability to control mechanical discontinuity in essence determines the key material properties. The desired control is relatively hard to achieve considering the large, continuous state and action spaces along with the exponential relationship between crack growth and stress cycle. The reinforcement-learning scheme is capable of learning an optimal and computational tractable control strategy. In the proposed approach, the reinforcement learning framework is integrated into an OpenAI-Gym-based environment that implements the mechanistic equations governing the fatigue crack growth. The learning agent does not explicitly know about the underlying physics, nonetheless, the learning agent can infer the control strategy by continuously interacting the numerical environment. The paper formulates an adaptive reward function involving reward shaping that can be generalized to similar control problems to improve the training efficiency. The reinforcement learning framework can successfully control the fatigue crack growth in a material despite the complexity of the propagation/growth pathway determined by multiple goal points. The paper provides the mathematical/physical basis of the reward function and the effect of neural network size and architecture and the state and action space that boosts the training speed while preserving the stability of the RL agents for the desired control problem. • Reinforcement learning successfully controls direction and rate of crack growth. • Robust control is achieved by adaptively changing stress angle and stress frequency. • Optimal neural network size, reward function, and state/action space are critical. • Reward shaping is crucial for a fast and robust convergence of the control policy. • Reward function follows a power-law distribution with step-wise additional reward.

Biomechanical changes in myopic sclera correlate with underlying changes in microstructure

Myopia alters the microstructural and biomechanical properties of the posterior sclera, which is characterized as a layered structure with potentially different inter-layer collagen fibril characteristics. Scanning acoustic microscopy (SAM) has been used to investigate how the micron-scale bulk mechanical properties of the posterior sclera are affected by myopia. Other investigators have employed second harmonic generation (SHG) imaging to characterize the collagen microstructure of tissues. In the present study, SAM and SHG imaging were used to investigate the existence of biomechanically-distinct scleral layers and identify relationships between mechanical properties and tissue microstructure in myopic guinea pig (GP) eyes. Diffusers were worn over the right eyes of six, 1-week-old GPs for one week to induce unilateral form-deprivation myopia. GPs were euthanized, enucleated, and eyes were cryosectioned. Twelve-micron-thick adjacent vertical cryosections were scanned with SAM or SHG. SAM maps of bulk modulus, mass density, and acoustic attenuation were estimated. A fiber-extraction algorithm applied to SHG images estimated collagen fiber length, width, straightness, alignment, and number density. Results revealed that the posterior sclera may exhibit biomechanically distinct layers that are affected differently in myopia. Specifically, a layered structure was observed in the mechanical-parameter maps of control eyes that was less apparent in myopic eyes. Collagen fibers in myopic eyes had smaller diameters and were more aligned. Myopia-associated biomechanical changes were most significant in the outermost and innermost scleral layers. SAM-measured mechanical parameters were correlated with collagen fiber microstructure, particularly fiber length, alignment, and number density, which may imply the biomechanical parameters estimated from SAM measurements are related to tissue microstructure. Interestingly, some changes were greatest in more-peripheral regions, suggesting interventions to strengthen the sclera may be effective away from the optic nerve and efficacy may be achieved best when intervention is applied to the outermost layer.

Constructive adaptation of 3D-printable polymers in response to typically destructive aquatic environments.

In response to environmental stressors, biological systems exhibit extraordinary adaptive capacity by turning destructive environmental stressors into constructive factors; however, the traditional engineering materials weaken and fail. Take the response of polymers to an aquatic environment as an example: Water molecules typically compromise the mechanical properties of the polymer network in the bulk and on the interface through swelling and lubrication, respectively. Here, we report a class of 3D-printable synthetic polymers that constructively strengthen their bulk and interfacial mechanical properties in response to the aquatic environment. The mechanism relies on a water-assisted additional cross-linking reaction in the polymer matrix and on the interface. As such, the typically destructive water can constructively enhance the polymer's bulk mechanical properties such as stiffness, tensile strength, and fracture toughness by factors of 746% to 790%, and the interfacial bonding by a factor of 1,000%. We show that the invented polymers can be used for soft robotics that self-strengthen matrix and self-heal cracks after training in water and water-healable packaging materials for flexible electronics. This work opens the door for the design of synthetic materials to imitate the constructive adaptation of biological systems in response to environmental stressors, for applications such as artificial muscles, soft robotics, and flexible electronics.

Novel backbone modified polyetheretherketone (PEEK) grades for powder bed fusion with enhanced elongation at break

This study presents a new family of backbone modified polyetheretherketone (PEEK) grades in the powder bed fusion (PBF) process, coded as PAEK1 and PAEK2, with the same chemical structure but different molecular weights. By incorporating polyetherbiphenyletherketone (PEDEK) comonomer, the optimal powder bed temperature of the new grades could be reduced to 290 °C, approximately 40 °C lower than that of PEEK. The underlying crystallisation kinetics in the PBF process was investigated by replicating the in-process temperature profiles using a flash differential scanning calorimetry (DSC) machine. This method enabled monitoring of the development of crystallisation during the process and at different deposition thicknesses. The results confirmed that the PAEK2 grade with a higher molecular weight is slower crystallising. A significant part of the crystallisation of PAEK2 was found taking place in the later stage of the printing process, i.e. the free cooling stage. This leads to a better particle coalescence and enhanced elongation, a feature previously difficult to obtain in the PBF process. The PBF PAEK2 tensile bars achieved a significant increase in mechanical performance with an outstanding 13% in elongation, reaching its yielding point. The data supports that PAEK2 is the first high temperature polymer grade in the PBF process to match its bulk mechanical properties while maintaining a high level of crystallinity in the printed parts. • PAEK2 is the first PAEK grade recording high elongation with high crystallinity in PBF. • PEEK/PEDEK copolymer has a lower melting temperature with a high Tg. • The high molecular weight of the PAEK2 grade leads to a slow crystallising speed. • The cooling temperature profiles of the EOS P800 system was replicated with flash DSC. • This study provides insight into designing dedicated polymers for the PBF process.

Nano- to macro-scale control of 3D printed materials via polymerization induced microphase separation

Although 3D printing allows the macroscopic structure of objects to be easily controlled, controlling the nanostructure of 3D printed materials has rarely been reported. Herein, we report an efficient and versatile process for fabricating 3D printed materials with controlled nanoscale structural features. This approach uses resins containing macromolecular chain transfer agents (macroCTAs) which microphase separate during the photoinduced 3D printing process to form nanostructured materials. By varying the chain length of the macroCTA, we demonstrate a high level of control over the microphase separation behavior, resulting in materials with controllable nanoscale sizes and morphologies. Importantly, the bulk mechanical properties of 3D printed objects are correlated with their morphologies; transitioning from discrete globular to interpenetrating domains results in a marked improvement in mechanical performance, which is ascribed to the increased interfacial interaction between soft and hard domains. Overall, the findings of this work enable the simplified production of materials with tightly controllable nanostructures for broad potential applications.

Study on Bulk Texture and Mechanical Properties of As-Extruded Wide Mg-Al-Zn Alloy Sheets with Different Al Addition.

The wide Mg alloy sheets produced by hot extrusion usually can easily form an inhomogeneous texture, resulting in anisotropic mechanical properties and poor formability. However, few studies have been carried out on the bulk texture investigation at different areas of as-extruded Mg alloy sheets, especially the Mg alloys with different alloying elements. In this work, the effect of Al on the bulk texture and mechanical properties at different areas for three wide Mg-Al-Zn alloy sheets with different Al contents (Mg-3Al-0.5Zn, Mg-8Al-0.5Zn and Mg-9Al-0.5Zn) are mainly investigated by neutron diffraction. The results showed that a strong and uneven basal texture was formed in the Mg-3Al-0.5Zn sheet. Meanwhile, the intensity of the basal texture was significantly weakened due to the numerous fine precipitates of Mg17Al12 particles, with the Al content increasing, which hinder the grain growth during extrusion, while fine recrystallized grains have a more random orientation. The enhanced tensile properties in Mg-8Al-0.5Zn and Mg-9Al-0.5Zn alloy sheets are ascribed to the cooperation effect of a refined microstructure, precipitates and weakened basal texture. Among the three Mg alloy sheets, the Mg-8Al-0.5Zn alloy sheet has a yield strength of about 270 MPa, an ultimate tensile strength of about 330 MPa and ultimate elongation of about 16% in the extrusion direction, which possesses the most excellent comprehensive mechanical properties.

3D-Printed Electroactive Hydrogel Architectures with Sub-100µm Resolution Promote Myoblast Viability.

3D-printed hydrogel scaffolds functionalized with conductive polymers have demonstrated significant potential in regenerative applications for their structural tunability, physiochemical compatibility, and electroactivity. Controllably generating conductive hydrogels with fine features, however, has proven challenging. Here, micro-continuous liquid interface production (μCLIP) method is utilized to 3D print poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogels. With a unique in-situ polymerization approach, a sulfonated monomer is first incorporated into the hydrogel matrix and subsequently polymerized into a conjugated polyelectrolyte, poly(4-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-2-ylmethoxy)-butane-1 sulfonic acid sodium salt (PEDOT-S). Rod structures are fabricated at different crosslinking levels to investigate PEDOT-S incorporation and its effect on bulk hydrogel electronic and mechanical properties. After demonstrating that PEDOT-S does not significantly compromise the structures of the bulk material, pHEMA scaffolds are fabricated via μCLIP with features smaller than 100µm. Scaffold characterization confirms PEDOT-S incorporation bolstered conductivity while lowering overall modulus. Finally, C2C12 myoblasts are seeded on PEDOT-pHEMA structures to verify cytocompatibility and the potential of this material in future regenerative applications. PEDOT-pHEMA scaffolds promote increased cell viability relative to their non-conductive counterparts and differentially influence cell organization. Taken together, this study presents a promising new approach for fabricating complex conductive hydrogel structures for regenerative applications.