We report a tough, semicrystalline, ternary thiol–ene polymer system containing linear dithiols, cross-linking trithiols, and spiroacetal alkene units in the main chain backbone that is synthesized ...by “click” ultraviolet photopolymerization in a one-step, solvent-free process. We varied the cross-link density to tune crystallinity and microstructure; in turn, thermomechanical properties such as yield strength, glass transition temperature, failure strain, and stress–strain behavior could be modified and controlled. Thiol–enes containing 7.5 and 10 thiol mol % cross-linker resulted in networks that balanced crystallinity, elasticity, and cross-linking to maximize toughness. These materials demonstrate how the presence of spiro units throughout a polymer’s backbone creates semicrystalline networks of substantial toughness from traditionally weak chemistries such as thiol–enes. This system can be synthesized in a neat, one-step photopolymerization process; as such, it illustrates the power of spirochemistry in designing photopolymers with tunable, robust thermomechanical properties.
We performed atomistic simulations to study the effect of free surfaces on the structure and elastic properties of gold nanowires aligned in the
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0
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and
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1
1〉
crystallographic directions. ...Computationally, we formed a nanowire by assembling gold atoms into a long wire with free sides by putting them in their bulk fcc lattice positions. We then performed a static relaxation on the assemblage. The tensile surface stresses on the sides of the wire cause the wire to contract along the length with respect to the original fcc lattice, and we characterize this deformation in terms of an equilibrium strain versus the cross-sectional area. While the surface stress causes wires of both orientations and all sizes to increasingly contract with decreasing cross-sectional area, when the cross-sectional area of a
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nanowire is less than
1.83
nm×1.83
nm
, the wire undergoes a phase transformation from fcc to bct, and the equilibrium strain increases by an order of magnitude. We then applied a uniform uniaxial strain incrementally to 1.2% to the relaxed nanowires in a molecular statics framework. From the simulation results we computed the effective axial Young's modulus and Poisson's ratios of the nanowire as a function of cross-sectional area. We used two approaches to compute the effective elastic moduli, one based on a definition in terms of the strain derivative of the total energy and another in terms of the virial stress often used in atomistic simulations. Both give quantitatively similar results, showing an increase in Young's modulus with a decrease of cross-sectional area in the nanowires that do not undergo a phase transformation. Those that undergo a phase transformation experience an increase of about a factor of three of Young's modulus. The Poisson's ratio of the
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0
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wires that do not undergo a phase transformation show little change with the cross-sectional area. Those wires that undergo a phase transformation experience an increase of about 10% in Poisson's ratio. The
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1
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wires show, with a decrease of cross-sectional area, an increase in one of Poisson's ratios and small change in the other.
The structural evolution in NiTi shape memory alloys subjected to pseudoelastic cycling is examined in the present study. Single crystals with 100 and 111 orientations were subjected to repeated ...compressive cycles and then studied by transmission electron microscopy (TEM). TEM observations were made at cycle numbers 1, 2, 5, 10, and 20 since the majority of degradation occurs during these initial cycle numbers. Under compression, single crystals with 111 orientations degraded much faster than crystals with 100 orientations. Under tension, single crystals with 100 orientations fractured in the elastic region, and crystals with 111 orientations showed considerable degradation as a function of cycling. Intermittent TEM observations on single crystals oriented along the 111 direction showed an increase in dislocation density on multiple active slip systems as a function of cycling. Single crystals oriented along the 100 orientation show a less dramatic increase in dislocation density as a function of cycling. TEM observations have revealed that dislocation structures formed near martensite plates have a similar periodicity as internal twin modes within the martensite. This observation implies that, although the interface between the martensite and parent phase is a low-energy boundary, the local disruptions due to internal twins create preferential nucleation sites for the formation of lattice defects.
Powder bed fusion (PBF), including selective laser melting and electron beam melting, fabricates complex, porous, osseointegrative implants for widespread clinical use. Fatigue testing is imperative ...for predicting long-term strength and durability of rough and surface porous implants while bone remodels around and grows into the implant. This study analyzes different materials (Ti6Al4V and Co28Cr6Mo) with varying topographies including as-printed surface roughness and the addition of a surface porous layer common to implants. The results are compared to wrought and PBF controls that are polished and machined. Moreover, different PBF techniques for titanium result in different as-printed surface roughness (∼0.07–17 μm) and microstructure. The fatigue data demonstrates that the surface finish impact was stronger in Ti6Al4V versus CoCr and SLM Ti6Al4V HIP + surface porous gyroid samples didn't perform worse than the roughest solid sample without surface porosity (EBM Ti6Al4V). With the same mechanical surface finishes, the SLM and wrought Ti6Al4V samples display similar fatigue resistance (800 and 850 MPa respectively), while EBM samples remain inferior (350 MPa). This study provides a foundation to compare fatigue resistance across materials and surface topographies through different fabrication techniques to optimize the lifespan of orthopedic implants while incorporating rough as printed surfaces and added surface porosity, both of which are essential for osseointegration.
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•This study systematically tested the effect of material, manufacturing technique, and surface topography (surface finish and porosity) on fatigue life.•Distinct microstructures were found amongst the samples generated with different manufacturing techniques.•A critical surface roughness of ∼0.2 μm was found in which further reducing the surface roughness was relatively ineffective in increasing fatigue strength.•Increasing gyroid thickness onto the surface of a SLM Ti6Al4V HIP sample had limited additional impact on decreasing fatigue strength.
We present results from a systematic study linking material microstructure to monotonic and fatigue properties of NiTi shape memory alloys. We consider Ni-rich materials that are either (1) hot ...rolled or (2) hot rolled and cold drawn. In addition to the two material processing routes, heat treatments are used to systematically alter material microstructure giving rise to a broad range of thermal, monotonic and cyclic properties. The strength and hardness of the austenite and martensite phases initially increase with mild heat treatment (300
°C), and subsequently decrease with increased aging temperature above 300
°C. This trend is consistent with transmission electron microscopy observed precipitation hardening in the hot-rolled material and precipitation hardening plus recovery and recrystallization in the cold-drawn materials. The low-cycle pseudoelastic fatigue properties of the NiTi materials generally improve with increasing material strength, although comparison across the two product forms demonstrates that higher measured flow strength does not assure superior resistance to pseudoelastic cyclic degradation. Fatigue crack growth rates in the hot-rolled material are relatively independent of heat treatment and demonstrate similar fatigue crack growth rates to other NiTi product forms; however, the cold-drawn material demonstrates fatigue threshold values some 5 times smaller than the hot-rolled material. The difference in the fatigue performance of hot-rolled and cold-drawn NiTi bars is attributed to significant residual stresses in the cold-drawn material, which amplify fatigue susceptibility despite superior measured monotonic properties.
Abstract The cell response to an implant is regulated by the implant’s surface properties including topography and chemistry, but less is known about how the mechanical properties affect cell ...behavior. The objective of this study was to evaluate how the surface stiffness and chemistry of acrylate-based copolymer networks affect the in vitro response of human MG63 pre-osteoblast cells. Networks comprised of poly(ethylene glycol) dimethacrylate (PEGDMA; Mn ∼750) and diethylene glycol dimethacrylate (DEGDMA) were photopolymerized at different concentrations to produce three compositions with moduli ranging from 850 to 60 MPa. To further decouple chemistry and stiffness, three networks comprised of 2-hydroxyethyl methacrylate (2HEMA) and PEGDMA or DEGDMA were also designed that exhibited a range of moduli similar to the PEGDMA–DEGDMA networks. MG63 cells were cultured on each surface and tissue culture polystyrene (TCPS), and the effect of copolymer composition on cell number, osteogenic markers (alkaline phosphatase specific activity and osteocalcin), and local growth factor production (OPG, TGF-β1, and VEGF-A) were assessed. Cells exhibited a more differentiated phenotype on the PEGDMA–DEGDMA copolymers compared to the 2HEMA–PEGDMA copolymers. On the PEGDMA–DEGDMA system, cells exhibited a more differentiated phenotype on the stiffest surface indicated by elevated osteocalcin compared with TCPS. Conversely, cells on 2HEMA–PEGDMA copolymers became more differentiated on the less stiff 2HEMA surface. Growth factors were regulated in a differential manner. These results indicate that copolymer chemistry is the primary regulator of osteoblast differentiation, and the effect of stiffness is secondary to the surface chemistry.
Shape memory polymers (SMPs) have the capacity to recover large strains when pre-deformed at an elevated temperature, cooled to a lower temperature, and reheated. The thermomechanical behavior of ...SMPs can be tailored by modifying the molecular structure of the polymer, or by using the polymer as a matrix for multiphase composites. Here we study the thermomechanics of a SMP polymer and its composites made by adding nano-scale SiC reinforcements. Our experimental study focuses on the thermomechanical behavior in three-point flexure. The results show that the SMP nanocomposites have a higher elastic modulus and are capable of generating higher recovery forces as compared to the SMP resin. When pre-deformed at a temperature well above the glass transition temperature,
T
g, the stress–strain response at the pre-deformation temperature governs the relationship between the recovery stress/strain and the corresponding constraining strain/stress. When pre-deformed at a temperature below
T
g, the recoverable stress/strain is not governed by the stress–strain response at the pre-deformation temperature. Rather, a peak recovery stress, which is less than the pre-deformation stress, appears at a temperature near
T
g. Ramifications of the results on future research efforts and emerging applications of SMPs and their composites are discussed.
Nickel–titanium (NiTi) shape memory alloys undergo relatively large recoverable inelastic deformations via a stress-induced martensitic phase transformation. Although stress-induced phase ...transformations in shape memory alloys are well characterized and utilized at micrometer to meter length scales, significant opportunity exists to understand and exploit martensitic transformations at nanometer scales. Displacive stress-induced martensitic phase transformations may constitute an ideal nanometer-scale actuator, as evident in certain biological systems, such as the T4 bacteriophage. The present work uses nanoindentation to study the fundamentals of stress-induced martensitic phase transformations in NiTi shape memory alloys. The experimental results presented are the first to show evidence of discrete forward and reverse stress-induced thermoelastic martensitic transformations in nanometer-scaled volumes of material. Shape recovery due to indentation, followed by subsequent heating, is demonstrated for indent depths in the sub-10
nm range. The indentation results reveal that stress-induced martensitic phase transformations nucleate at relatively low stresses at nanometer scales, suggesting a fundamental departure from traditional size scale effects observed in metals deforming by dislocation plasticity. It is also shown that the local material structure can be utilized to modify transformation behavior at nanometer scales, yielding an insight into the nature of stress-induced martensitic phase transformations at small scales and providing an opportunity for the design of nanometer-sized NiTi actuators.
The ability to control the surface topography of orthopedic implant materials is desired to improve osseointegration but is often at the expense of mechanical performance in load bearing ...environments. Here we investigate the effects of surface modifications, roughness and porosity, on the mechanical properties of a set of polymers with diverse chemistry and structure. Both roughness and surface porosity resulted in samples with lower strength, failure strain and fatigue life due to stress concentrations at the surface; however, the decrease in ductility and fatigue strength were greater than the decrease in monotonic strength. The fatigue properties of the injection molded polymers did not correlate with yield strength as would be traditionally observed in metals. Rather, the fatigue properties and the capacity to maintain properties with the introduction of surface porosity correlated with the fracture toughness of the polymers. Polymer structure impacted the materials relative capacity to maintain monotonic and cyclic properties in the face of surface texture and porosity. Generally, amorphous polymers with large ratios of upper to lower yield points demonstrated a more significant drop in ductility and fatigue strength with the introduction of porosity compared to crystalline polymers with smaller ratios in their upper to lower yield strength. The latter materials have more effective dissipation mechanisms to minimize the impact of surface porosity on both monotonic and cyclic damage.
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•Surface roughness and porosity significantly decrease the toughness of polymers.•Greatest effect of surface defects is significant decrease in ductility.•Fracture toughness correlates with and predicts notched polymer properties.•Crystalline polymers are more resistant to defects due to two-phase structure.•Shape of the stress–strain curve important in determining the sensitivity to notches.
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