Softening Transition Research Articles

Molecular dynamics simulation of phase transformation and wear behavior of Ni35Al30Co35 high temperature shape memory alloy

Ni–Al–Co alloys as one of the ferromagnetic high-temperature shape memory alloys (HTSMA) are attractive in aerospace and defense applications, due to their high phase transition temperature range (400–700K), high recoverable strain and good machinability. However, the underlying physics for the phase transformation behavior of β single-phase and β+γ two-phase Ni–Al–Co ternary alloys remain unclear. In this work, molecular dynamics simulation of phase transition, pseudoelasticity (PE) and wear behavior of Ni35Al30Co35 alloy were analyzed using the EAM potential. It shows that Ni35Al30Co35 alloy has a high martensite start temperature of 825K and an austenite start temperature of 631K, respectively. After five thermal cycles, martensite and austenite start and finish temperatures are stable around 500K and 700K. The initial structure of Ni35Al30Co35 alloy at 1000K is identified as BCC (β phase) and FCC (γ phase) structure, transforming to L10 structure through martensitic transformation. Furthermore, PE of Ni35Al30Co35 alloy exhibits about a 9.1% recoverable strain under <001> oriented uniaxial tension tests and about a 6.6% recoverable strain under <001> oriented uniaxial compression tests at a temperature of 1200K, showing the potential used in extreme environment. Moreover, spherical indenters with a radius of 15 Å were used to investigate the wear behavior of Ni35Al30Co35 alloy under different temperature. It was found that the PE effect affects the frictional behavior during nano-scratching. Combining the effect of high temperature softening and phase transition PE, the wear rate increases nearly by 5.3% when the temperature rises from 300K to 1000K. Furthermore, the friction force curve obtained is consistent with the friction force curve obtained by simulation at 300K.

Peptide-polyurea hybrids: a platform for tunable, thermally-stable, and injectable hydrogels.

Drawing inspiration from natural systems, such as the highly segmented structures found in silk fibroin, is an important strategy when designing strong, yet dynamic biomaterials. Polymer-peptide hybrids aim to incorporate the benefits of hierarchical polypeptide structures into synthetic platforms that are promising materials for hydrogel systems due to aspects such as their biocompatibility and structural tunability. In this work, we demonstrated the utility of poly(ethylene glycol) (PEG) peptide-polyurea hybrids as self-assembled hydrogels. Specifically, poly(ε-carbobenzyloxy-L-lysine)-b-PEG-b-poly(ε-carbobenzyloxy-L-lysine) and poly(β-benzyl-L-aspartate)-b-PEG-b-poly(β-benzyl-L-aspartate) triblock copolymers were used as the soft segments in linear peptide-polyurea (PPU) hybrids. We systematically examined the effect of peptide secondary structure and peptide segment length on hydrogelation, microstructure, and rheological properties of our PPU hydrogels. Polymers containing α-helical secondary structures resulted in rapid gelation upon the addition of water, as driven by hierarchical assembly of the peptide segments. Peptide segment length dictated gel strength and resistance to deformation via complex relationships. Simulated injection experiments demonstrated that PPU hydrogels recover their original gel network within 10 s of cessation of high shear. Finally, we showed that PPU hydrogels remain solid-like within the range of 10 to 80 °C; however, a unique softening transition occurs at temperatures corresponding to slight melting of secondary structures. Overall, this bioinspired PPU hybrid platform provides opportunities to design synthetic, bioinspired polymers for hydrogels with tunable microstructure and mechanics for a wide range of thermal and injection-based applications.

Temperature controlled decohesion regimes of an elastic chain adhering to a fixed substrate by softening and breakable bonds

We study the rate-independent decohesion process for a chain linked to a substrate through a series of breakable elements with a softening mechanism. Such an assumption describes the realistic case when connecting links can undergo softening transitions before breaking. For instance, this is a diffuse mechanism observed both in fracture of soft materials and biological adhesion. The analysis of this model is developed in the framework of equilibrium statistical mechanics. In order to describe mechanically induced detachment of the chain from the substrate both in the cases of hard devices (prescribed extension) or soft devices (applied force), we consider both Helmholtz and Gibbs ensembles. In any case, the model can be exactly solved and is characterized by a phase transition at a given critical temperature, corresponding to the complete detachment of the chain even without mechanical actions. Interestingly, according to the ‘size’ of the softened region, we observe two different regimes. In one case (fragile regime) during the decohesion the measure of the softened region is negligible, whereas in the other case (ductile regime) we obtain a finite measure of the softened region that is constant, giving a temperature dependent analytic measure of the process zone.

An Injectable Meta-Biomaterial: From Design and Simulation to In Vivo Shaping and Tissue Induction.

A novel type of injectable biomaterial with an elastic softening transition is described. The material enables invivo shaping, followed by induction of 3D stable vascularized tissue. The synthesis of the injectable meta-biomaterial is instructed by extensive numerical simulation as a suspension of irregularly fragmented, highly porous sponge-like microgels. The irregular particle shape dramatically enhances yield strain for invivo stability against deformation. Porosity of the particles, along with friction between internal surfaces, provides the elastic softening transition. This emergent metamaterial property enables the material to reversibly change stiffness during deformation, allowing native tissue properties to be matched over a wide range of deformation amplitudes. After subcutaneous injection in mice, predetermined shapes can be sculpted manually. The 3D shape is maintained during excellent host tissue integration, with induction of vascular connective tissue that persists to the end of one-year follow-up. The geometrical design is compatible with many hydrogel materials, including cell-adhesion motives for cell transplantation. The injectable meta-biomaterial therefore provides new perspectives in soft tissue engineering and regenerative medicine.

Swelling of Tendomer Gels

We propose a new type of polymer gels made of tendomers as elastic elements. Tendomers are a pair of rotaxanes with a specific nonlinear response and an almost step-like softening transition by increasing their extension. We consider the swelling equilibrium of tendomer gels in good solvents based on a Flory–Huggins type of approach. Using the exact solution for the single tendomer force–extension relation, we calculate the swelling equilibrium of the gels numerically. For the limiting cases of the linear and nonlinear elastic regimes of the tendomer strands, we derive analytic solutions for the equilibrium degree of swelling as a function of the system parameters. We show that the resulting tendomer networks can swell much more than conventional gels and that the preparation conditions can be tuned such that the gels swell predominantly in a linear or a nonlinear response regime. Tendomer gels are also a prototype of a super-soft and practically entanglement-free gel since at preparation state, the tendomers consist mainly of long elastically inactive tails that act as a reservoir of chain segments upon swelling or deformation. This results in a c*-like state at swelling equilibrium for appropriate preparation and swelling conditions. Besides swelling in a good solvent, we also consider the swelling equilibrium of charged tendomer gels. Specific preparation conditions are taken into account in terms of a prestrain coefficient of the network. Monte-Carlo simulations and numerical calculations are performed to test the theoretical predictions. Finally, we discuss a possible route for the synthesis of tendomer gels.

Reversible Self-Healing for Preserving Optical Transparency and Repairing Mechanical Damage in Composites.

This research concentrates on the healing of optical properties, roughness, contact angle hysteresis, and shallow scratches in polymer/nanoparticle composites. A series of ternary composite blends [epoxy/halloysite nanotubes (HNTs)/cellulose acetate butyrate (CAB)] with various CAB concentrations were fabricated and subjected to a series of mechanical damages. The optimized concentration of a nanoparticle is 1.0 vol %, and the CAB concentration is 3.0 vol % based on the mechanical reinforcement and wear resistance. Nanoscale scratching, microlevel falling-sand test, and macrolevel Taber abrasions were utilized to damage the surfaces. The induced damage (roughness and surface scratch up to hundreds of nanometers in depth) healed upon heating. At any temperatures above the softening transition of the semi-interpenetrating network structure of the polymer composites, CAB migrates into the microcracks, and the essential mechanical parameters (modulus, strength, strain to failure) are recovered; in our particular epoxy/HNTs/CAB system, optical transparency is also recovered efficiently. CAB also moves to the macroscopic air/specimen interface and favorably modifies the surface properties, reducing the roll-off angles of water droplets from ∼90° to ∼20°. Through an appropriate choice of CAB additives with different molecular weights, the healing temperature can be tailored.

Strengthening to softening transition in lath martensite

Size induced strengthening, i.e. increase in strength with decreasing sample size, has been reported for a variety of single crystal body centered cubic (bcc) materials. However, studies on bcc lath martensite are very limited, in part due to its complex microstructure and the difficulty of small-scale experimentation. In this work, we demonstrate for the first time a size induced strengthening followed by a softening in bcc lath martensite pillars by reducing pillar diameter from 8 µm to 250 nm. These martensitic pillars are tested under uniaxial compression conducted in-situ inside of a scanning electron microscope. High-resolution microstructural characterization by transmission electron microscopy is used for post-mortem examination of the compressed pillars in order to study the deformation mechanisms governing their plasticity. We find that the size-induced strengthening during compression can be rationalized in terms of the competition between dislocation slip and boundary sliding, while softening at the finest scales is attributed to the boundary sliding mechanisms. Our work provides direct observations of block/lath boundaries behavior during compression as well as new insights into the deformation mechanisms in lath martensite.

Opportunities for materials science: From molecules to neural networks

Abstract

Ultrathin Shell Layers Dramatically Influence Polymer Nanoparticle Surface Mobility.

Advances in nanoparticle synthesis, self-assembly, and surface coating or patterning have enabled a diverse array of applications ranging from photonic and phononic crystal fabrication to drug delivery vehicles. One of the key obstacles restricting its potential is structural and thermal stability. The presence of a glass transition can facilitate deformation within nanoparticles, thus resulting in a significant alteration in structure and performance. Recently, we detected a glassy-state transition within individual polystyrene nanoparticles and related its origin to the presence of a surface layer with enhanced dynamics compared to the bulk. The presence of this mobile layer could have a dramatic impact on the thermal stability of polymer nanoparticles. Here, we demonstrate how the addition of a shell layer, as thin as a single polymer chain, atop the nanoparticles could completely eliminate any evidence of enhanced mobility at the surface of polystyrene nanoparticles. The ultrathin polymer shell layers were placed atop the nanoparticles via two approaches: (i) covalent bonding or (ii) electrostatic interactions. The temperature dependence of the particle vibrational spectrum, as recorded by Brillouin light scattering, was used to probe the surface mobility of nanoparticles with and without a shell layer. Beyond suppression of the surface mobility, the presence of the ultrathin polymer shell layers impacted the nanoparticle glass transition temperature and shear modulus, albeit to a lesser extent. The implication of this work is that the core–shell architecture allows for tailoring of the nanoparticle elasticity, surface softening, and glass transition temperature.

INTERPRETATION OF THE TANδ PEAK HEIGHT FOR PARTICLE-FILLED RUBBER AND POLYMER NANOCOMPOSITES WITH RELEVANCE TO TIRE TREAD PERFORMANCE BALANCE

ABSTRACT The aim of this research is to clarify the meaning of the peak height for the viscoelastic loss tangent (tanδ) in the glass transition region of particle-filled rubber, polymer nanocomposites, and polymer systems in general. Filler, oil, and curative loadings were systematically varied in a model styrene-butadiene rubber formulation with carbon black as the reinforcing filler. The dynamic mechanical responses of these compounds enabled a detailed study of the glass-to-rubber softening transition, which is known to play an important role with respect to the balance of traction, handling, and rolling resistance characteristics of tire treads. From the temperature-dependent viscoelastic results that were acquired at fixed frequency and small dynamic strain, it was demonstrated that a higher peak value for tanδ was correlated with a lower dynamic modulus in the rubbery state. Adjusting filler volume fraction was found to be an effective way of changing the rubbery modulus and hence the tanδ peak height. It was furthermore verified that such a correlation is a universal material-independent viscoelastic effect by mathematically producing a similar trend by varying the rubbery modulus parameter in the Havriliak–Negami viscoelastic model. This investigation also showed why glass transition temperature should be determined from the position of the loss modulus peak and not the tanδ peak. Cure behavior, tensile stress–strain properties, and extent of filler networking (Payne effect) for these rubber compounds will additionally be discussed.

RE2O3-alkaline earth-aluminosilicate fiber glasses: Melt properties, crystallization, and the network structures

Glasses and/or glass fibers of a new glass system, RE2O3 (RE = Sc,Y, La)-MgO-CaO-Al2O3-SiO2, were studied in detail, covering rare earth (RE) effects on high temperature viscosity, liquidus temperature and crystalline phases, softening and glass transition temperature, and speciation reactions of aluminum (AlOx, x = 4, 5) and silicate network (Qn, n = 4, 3, 2) using 27Al and 29Si MAS NMR and Raman spectroscopic methods. Rare earth oxides (RE2O3) were shown to have a dual functionality, i.e., decreasing melt viscosity like a network modifier depolymerizing glass network, but increasing viscosity at low temperature, i.e., glass softening temperature (Tsoft) and transition temperature (Tg), strengthening the glass network functioning like Al2O3. Liquidus temperature (TLiq) was found to be sensitive to the type of RE2O3 modifications in terms of their ionic field strength (IFS); Sc3+ of the highest IFS increased, but La3+ of the lowest IFS decreased TLiq. Effect of Y3+ (intermediate IFS) on TLiq resembles Sc3+ only at higher concentration. The overall results suggest that RE ions of different IFS have different “true” solubility limits in the host glass matrix, above which RE2Si2O7 crystallizes. All of the glasses with and without RE2O3 contained predominately four-fold coordinated aluminum, AlO4; yet replacement of CaO by RE2O3 increased AlO5 according to 27Al MQMAS NMR spectroscopic measurements. Raman spectroscopic results suggested that depolymerization of the silicate network.

Production of vitreous materials from mineral coal bottom ash to minimize the pollution resulting from the waste generated by the thermoelectrical industry

Mineral coal bottom ash exerts a great impact on the environment due to the presence of heavy metals in its composition and the lack of an adequate area for disposal. Vitreous materials were synthesized from bottom ash to be employed as a by-product. The bottom ash was subjected to an X-ray fluorescence (XRF) analysis to evaluate the oxide composition present in the material. To study the effect of bottom ash in the attainment of glass, a simplex lattice design for experiments with blends was employed. The elements considered in the design were: bottom ash; sodium carbonate (Na2CO3) and calcium oxide (CaO), both used as melting agents; magnesium oxide (MgO), which was used as a stabilizer for the vitreous network. For the characterization of the glasses, X-ray diffraction (XRD), differential scanning calorimetry (DSC) and Fourier transform infrared spectrometry (FTIR) were carried out. Ten different formulations were tested. The results indicated that two out of the ten formulations formed a crystalline phase, which is undesirable for a vitreous material. In the statistical analyses, the Pareto Diagram and the response surface showed that the glass transition and softening temperatures were strongly influenced by the level of calcium oxide and magnesium oxide, as well as that of bottom ash, resulting in an increase in the softening and glass transition temperatures.

Flexibility of DNA/PNA, DNA/LNA, DNA/RNA hybrids measured with a nanoscale transducer

We report a new application for a previously introduced enzyme-DNA supramolecular construct, as a method to assess bending stiffness of short (∼10 nm long) synthetic nucleic acids. We obtain the first measurements of bending elasticity for DNA/PNA and DNA/LNA hybrids, and also assess the DNA/RNA hybrid. In order of increasing bending stiffness, we find: DNA/RNA, DNA/DNA, DNA/LNA, DNA/PNA. The experiments also probe the nonlinear elasticity regime of the coupled enzyme-nucleic acid molecules. We find the measurements consistent with the existence of a softening transition in the mechanics of the enzyme. This previously proposed nonlinearity maybe a general property of the folded state of proteins.

Effects of binders on the properties of bio-char pellets

Bio-char, produced from biomass pyrolysis, can be pelletized to improve its undesirable characteristics, such as low bulk and energy densities, poor transportation and storage properties, and troublesome to co-fire with coal. In this study, rice husk char was compressed into pellets with four kinds of binders (lignin, starch, calcium hydroxide and sodium hydroxide). The compressive process, mechanical strength, basic fuel properties, and combustion characteristics were investigated to elucidate the effect of binders on the properties of bio-char pellets. Results showed that starch pellets had good hydrophobicity, but low volume density and poor mechanical strength. The softening and morphological transition of lignin during compression may account for the high elastic modulus and good bonding of lignin pellets. The low saturated moisture content of Ca(OH)2 pellets is mainly ascribed to its hydration. NaOH pellets showed the highest compressive strength among all pellets, and also exhibited the highest moisture uptake that may worsen the handling and storage treatment of bio-char. Compared with raw bio-char, bio-char pellets had a lower ignition temperature, wider temperature interval, and higher oxidation activity according to the thermogravimetric analysis. The lignin and Ca(OH)2 pellets showed lower compression energy consumption and moisture uptake, enhanced mechanical strength and promoted combustion performance, which demonstrated more desirable properties for utilization as biofuels.

Thermomechanical transitions of egg-ceramide monolayers.

Ceramides have unique biophysical properties. Their high melting temperature and their ability to form lateral domains have converted ceramides into the paradigm of rigid lipids. Here, using shear surface rheology of egg-ceramide Langmuir monolayers, a solid to fluid transition was evidenced as a vanishing shear rigidity at lower temperatures than the lipid melting temperature. Such a mechanical transition, which depends on the lipid lateral pressure, was found in a broad range temperature (40-50 °C). The solid to fluid transition was correlated to a LC to LC+LE phase transition, as confirmed by BAM experiments. Interestingly, together with the softening transition, a supercooling process compatible with a glassy behavior was found upon freezing. A new phase scenario is then depicted that broadens the mechanical behavior of natural ceramides. The phase diversity of ceramides might have important implications in their physiological roles.

Elastic Softening and Phase Transition Characteristics in YbPtGe

YbPtGe with the orthorhombic ɛ-TiNiSi-type structure is reported as a heavy-fermion ferromagnet with TC = 5.4K. Recent research suggested that its magnetic anisotropy can be ascribed to the crystal electric field (CEF) effect which acts on the eight-fold multiplet of Yb3+ ions. In order to clarify the CEF effect and to investigate the magnetic phase transition in this compound, ultrasonic measurements of YbPtGe and related theoretical calculation have been performed. The transverse elastic modulus C66 exhibits a softening characteristic with a peak at 180K while the longitudinal modulus C33 shows a continuous hardening in the whole temperature range. According to the theoretical strain-susceptibility fitting of the transverse elastic modulus C66 by using the CEF parameters reported by Katoh et al., we argue the softening at higher temperatures (>50K) to be a result of quadrupole interaction between the CEF ground and excited Kramers doublets. With further decreasing the temperature below TC, C66 mode exhibits a step-like softening which is quite different from that of the C33 mode and the possible mechanism is discussed.

Links between energy dissipation and wear mechanisms in solid epoxy/epoxy sliding contact

This paper covers wear and energy dissipation of solid epoxy induced by the alternative rubbing between two samples of identical thermosetting polymer. Varying normal load, sliding velocity and sliding distance, the authors were able to define and discuss wear and friction laws and associated energy dissipation. Moreover, traces of several wear mechanisms were distinguished on the worn surfaces and associated with applied conditions. Observed under higher velocity, polymer softening and local state transition were explained by surface temperature estimate and confirmed by infra-red spectroscopy measurements. To conclude this study, all observed phenomena are classified into two wear scenarios according to sliding velocity.

High Molecular-Weight Thermoplastic Polymerization of Kraft Lignin Macromers with Diisocyanate

A high molecular-weight thermoplastic lignin-based polymer was successfully synthesized by adjusting the degree of polymerization while inducing linear growth of lignin macromers via methylene diphenyldiisocyanate. The thermoplastic lignin-urethane polymer was desirably achieved in a narrow range of reaction conditions of 2.5 to 3.5 h at 80 oC in this study, and the molecular weight of the resulting lignin-based polyurethanes (LigPU) reached as high as 912,000 g/mole, which is far above any reported values of lignin-based polymer derivatives. The thermal stability of LigPU was greatly improved by the urethane polymerization, giving the initial degradation temperature (T2%) at 204 °C, which should be compared with T2% = 104 °C of the pristine lignin. This was due to the fact that the OH groups in the lignin macromers, having low bond-dissociation energy, were replaced by the urethane bonds. In dielectric analysis, the synthesized LigPU exhibited a softening transition at 175 °C corresponding to a combinatorial dual process of the dry Tg,dry of the lignin macromers and the softening of methylenediphenyl urethane chains. This work clearly demonstrated that a high molecular weight of thermoplastic LigPU could be desirably synthesized, broadening the lignin application for value added and eco-friendly products through common melt processes of polymer blend or composites.

Critical bending torque of DNA is a materials parameter independent of local base sequence

Short double-stranded DNA molecules exhibit a softening transition under large bending which is quantitatively described by a critical bending torque τ_{c} at which the molecule develops a kink. Through equilibrium measurements of the elastic energy of short (∼10 nm), highly stressed DNA molecules with a nick at the center we determine τ_{c} for different sequences around the nick. We find that τ_{c} is a robust materials parameter essentially independent of sequence. The measurements also show that, at least for nicked DNA, the local structure at the origin of the softening transition is not a single-stranded "bubble."

Phase-Transition Induced Elastic Softening and Band Gap Transition in Semiconducting PbS at High Pressure

We have investigated the crystal structure and phase stability, elastic incompressibility, and electronic properties of PbS based on high-pressure neutron diffraction, in-situ electrical resistance measurements, and first-principles calculations. The refinements show that the orthorhombic phase is structurally isotypic with indium iodide (InI) adopting a Cmcm structure (B33). The cubic-to-orthorhombic transition occurs at ∼2.1(1) GPa with a 3.8% volume collapse and a positive Clausius-Clapeyron slope. Phase-transition induced elastic softening is also observed, which is presumably attributed to the enhanced metallic bonding in the B33 phase. On the basis of band structure simulations, the cubic and orthorhombic phases are typical of direct and indirect semiconductors with band gaps of 0.47(1) and 1.04(1) eV, respectively, which supports electrical resistivity measurements of an abrupt jump at the structural transition. On the basis of the resolved structure for B33, the phase transition paths for B1→B33→B2 involve translation of a trigonal prism in B1 and motion of the next-nearest neighbor Pb atom into {SPb7} coordination and subsequent lattice distortion in the B33 phase.