Temperature-dependent Phase Transition Behavior Research Articles

Trace sulfur-induced embrittlement of ultrahigh-purity copper

Sulfur (S), as an impurity element in copper and copper-based alloys, usually leads to intergranular embrittlement even at ppm concentrations. However, the intrinsic mechanisms of the embrittlement remain largely unclear. In this study, we systematically investigate the influence of trace amounts of S (53 and 300 wt ppm) on the ductility of an ultrahigh-purity copper (99.99999%) in the range of room temperature to 900 °C. Tensile results show an excellent plastic deformation capacity at any test temperature for the ultrahigh-purity copper, while a remarkable reduction in ductility for both S-containing alloys, particularly at 300–750 °C. A microanalysis reveals formation of micro- and nanoscale Cu–S precipitates, mainly distributed at grain boundaries, acting as the nucleation sites for small cavities leading to the crack initiation. This is responsible for the ductility loss in the S-containing alloys in the whole temperature range. The distinct embrittlement behavior between 300 and 750 °C is independent on the crystallographic orientation relationship between the Cu–S precipitates and Cu matrix, but is dominated by the temperature-dependent phase transition behavior of the Cu–S precipitates. Such a structural transformation with the volume expansion accelerates the nucleation and propagation of the microcracks, causing the intergranular embrittlement at intermediate temperatures. Our findings provide a comprehensive experimental understanding on the fundamental mechanisms of impurity-induced the embrittlement in metallic materials.

Exploring Ba(Ti, Sn)O3: An experimental and theoretical study of structural, ferroelectric, electronic, and optical properties

Ba(Sn, Ti)O3 (BSnTO) is an excellent functional material that can be utilized for its high electrocaloric effect, high pyroelectric effect, and good photovoltaic characteristics. The current report is based on a combined experimental and theoretical study of ferroelectric Sn-doped BaTiO3 ceramic. The sample was successfully synthesized via the solid-state sintering method. The phase purity was verified by the XRD measurements and subsequent Rietveld refinement. The sample showed good ferroelectric and dielectric properties at room temperature. Specifically, the temperature-dependent phase transition behavior was studied with dielectric measurements. The band structure, the density of states, and optical properties were calculated using the CASTEP code. Various optical constant as a function of energy was calculated and the underlying physics is explored.

Acetylation of Cyclodextrin-Threaded Polyrotaxanes Yields Temperature-Responsive Phase Transition and Coacervate Formation Properties.

Stimuli-responsive materials have received considerable attention for their application in biomedical fields. Herein, the temperature-dependent phase transition behavior of acetylated polyrotaxanes (Ac-PRXs) consisting of acetylated cyclodextrins threaded onto a linear axle polymer in aqueous solution is investigated. The aqueous solutions of Ac-PRXs exhibit temperature-dependent transmittance changes when the degree of acetylation is 30-40%. Upon increasing the temperature, Ac-PRXs are dehydrated and acetyl groups are subsequently associated through hydrophobic interactions. Interestingly, the Ac-PRXs form coacervate droplets with a diameter of ≈2-3 µm above their cloud points. These temperature-responsivities of Ac-PRXs are observed for other PRXs with different axle polymer molecular weights and compositions. Consequently, the acetylation of PRXs is an attractive chemical modification for yielding temperature-responsivities, and Ac-PRXs can be applied as temperature-responsive supramolecular materials.

On the competition between interface energy and temperature in phase transition phenomena

Phase nucleation and propagation phenomena can be characterized by a cooperative behavior regulated by non local interactions between the multistable domains and with the loading device. Cooperativity is often macroscopically witnessed by a stress-peak, distinguishing the nucleation from the propagation stress, and by a larger size of the first nucleated domain. When low dimensional scales are considered, both in nanostructures or single molecule behaviors, the interfacial energy can compete with entropic effects, leading to the experimental observation of a temperature dependent phase transition strategy. We propose a fully analytical model, in the framework of equilibrium Statistical Mechanics, measuring such energetic competition and temperature dependent behavior, that well reproduces important experimental evidences. The effectiveness of the model is successfully tested in predicting the temperature dependent phase transition behavior of shape memory nanowires.

In Situ Phase Transition of Elastin-Like Polypeptide Chains Regulates Thermoresponsive Properties of Elastomeric Protein-Based Hydrogels.

Engineering protein-based hydrogels that can change their physical and mechanical properties in response to environmental stimuli have attracted considerable interest due to their promising applications in biomedical engineering. Among environmental stimuli, temperature is of particular interest. Most thermally responsive protein hydrogels are constructed from thermally responsive elastin-like polypeptides (ELPs), which exhibit a lower critical solution temperature (LCST) transition, or nonstructured elastomeric proteins fused with ELPs. Here we report the engineering of thermally responsive elastomeric protein-based hydrogels by fusing ELPs to elastomeric proteins made of tandemly arranged folded globular proteins. By fusing ELP sequence (VPGVG)n to an elastomeric protein (GR)4, which is made of small globular protein GB1 (G) and random coil sequence resilin (R), we engineered a series of protein block copolymers, Vn-(GR)4. The fusion proteins Vn-(GR)4 exhibit temperature-responsive behaviors in aqueous solution that are different from that of Vn-ELPs, as they did not exhibit the macroscopic phase transitions in the turbidity test. Instead, V48-(GR)4 and V72-(GR)4 form micelles at temperatures higher than the transition temperature of V48 and V72 at the same concentration. Using the well-developed ruthenium-mediated photochemical cross-linking method, Vn-(GR)4 polymers can be cross-linked into hydrogels, in which Vn-ELP serve as side chains of the hydrogel network. These hydrogels exhibited thermoresponsive properties due to the temperature dependent phase transition behaviors of the incorporated Vn-ELPs blocks. At elevated temperatures, the Vn-ELPs side chains in the hydrogel network underwent aggregation, leading to secondary physical cross-linking. The aggregation of the Vn-ELPs resulted in higher Young's modulus and reduced swelling ratio. Furthermore, the amplitude of such property changes can be tuned by side chain length and composition. These results demonstrate that in situ phase behaviors of ELP side chains can regulate thermoresponsiveness of protein-based hydrogels. We anticipate that this method can be applied to other elastomeric proteins for potential biomedical applications.

Photothermal-modulated drug delivery and magnetic relaxation based on collagen/poly(γ-glutamic acid) hydrogel.

Injectable and stimuli-responsive hydrogels have attracted attention in molecular imaging and drug delivery because encapsulated diagnostic or therapeutic components in the hydrogel can be used to image or change the microenvironment of the injection site by controlling various stimuli such as enzymes, temperature, pH, and photonic energy. In this study, we developed a novel injectable and photoresponsive composite hydrogel composed of anticancer drugs, imaging contrast agents, bio-derived collagen, and multifaceted anionic polypeptide, poly (γ-glutamic acid) (γ-PGA). By the introduction of γ-PGA, the intrinsic temperature-dependent phase transition behavior of collagen was modified to a low viscous sol state at room temperature and nonflowing gel state around body temperature. The modified temperature-dependent phase transition behavior of collagen/γ-PGA hydrogels was also evaluated after loading of near-infrared (NIR) fluorophore, indocyanine green (ICG), which could transform absorbed NIR photonic energy into thermal energy. By taking advantage of the abundant carboxylate groups in γ-PGA, cationic-charged doxorubicin (Dox) and hydrophobic MnFe2O4 magnetic nanoparticles were also incorporated successfully into the collagen/γ-PGA hydrogels. By illumination of NIR light on the collagen/γ-PGA/Dox/ICG/MnFe2O4 hydrogels, the release kinetics of Dox and magnetic relaxation of MnFe2O4 nanoparticles could be modulated. The experimental results suggest that the novel injectable and NIR-responsive collagen/γ-PGA hydrogels developed in this study can be used as a theranostic platform after loading of various molecular imaging probes and therapeutic components.

Effects of Hydrophobic Modifications and Phase Transitions of Polyvinylamine Hydrogel Films on Reversible CO2 Capture Behavior: Comparison between Copolymer Films and Blend Films for Temperature‐Responsive CO2 Absorption

The separation of CO2 from large emission sources is essential to both mitigate the greenhouse effect, as well as generate carbon‐based energy. However, energy consumption of conventional CO2 separation processes, which using aqueous amine solution as absorbent, is too large. It is has been previously reported that hydrogel films that are consisting of temperature‐responsive amine‐containing polymers can be energy efficient CO2 absorbent—the films can reversibly capture and release large amount of CO2 via temperature‐induced phase transition of hydrogels. However, the study is limited to the films consisting of gel particles of polyacrylamides. In this study, a series of hydrogel films consisting of a mass‐produced amine‐containing linear polymer, polyvinyl amine (PVAm), are prepared, and the efficiencies of their reversible CO2 capture are tested. The effects of hydrophobic modifications and the temperature dependent phase transition behaviors of the films on the reversible CO2 capture efficiency are studied in detail. The function of hydrogel films containing modified PVAm (copolymers), as well as blend films of nonmodified PVAm and 100% modified PVAm, are compared for the first time. The results reveal that the reversible CO2 capture efficiency of polyamine films can be improved just by blending with temperature‐responsive polymers. image

A thermoresponsive amphiphilic dendron — Synthesis, characterization, and self-assembled micelles for controlled drug release

A new third-generation thermoresponsive amphiphilic dendron consisting of a hydrophobic poly(benzyl ether) dendritic core and hydrophilic oligo(ethylene glycol) peripheries was synthesized by an efficient convergent approach. Its structure was confirmed by 1H NMR, 13C NMR, IR, GPC, MALDI-TOF MS, and elemental analysis. Turbidity and dynamic light scattering (DLS) measurements demonstrated that the dendron showed a reversible temperature-dependent phase-transition behavior in aqueous solution and its lower critical solution temperature (LCST) was lower than that of the corresponding second-generation dendron, indicating the dependence of LCSTs on the generation of dendrons. Fluorescent spectroscopy and TEM studies revealed that the dendron would self-assemble into nanospherical micelles with a very low critical micelle concentration (CMC) in water. The core-shell structure of the micelles was proved by 1H NMR analyses of the micelles in D2O. The drug-loading capacity of the dendron micelles is about 29 wt % for podophyllotoxin (POD) used as a model drug, and in vitro release tests showed a desired thermoresponsive drug-release behavior. These results indicate that the dendron is promising as stimuli-responsive material for biomedical applications.

Phase transition behavior for ZrW 2−xMo xO 8 compositions at elevated temperatures

A systematical study on cubic ZrW 2− x Mo x O 8 ( x=0.73, 0.53, 0.33, 0.11) solid solutions reveals that their temperature-dependent phase transition behaviors are related to the Mo fraction x. A phase diagram of cubic ZrW 2− x Mo x O 8 solid solutions has been drawn over a wide temperature range (298–1473 K) on the basis of the temperature-dependent phase transition behaviors observed.

Phase Transitions in Thin Films of a Diblock Copolymer Composed of a Linear Polymer Block and a Brush Polymer Block with Mesogenic Oligothiophenyl Bristles

In this study, we quantitatively investigated the temperature-dependent phase transition behaviors in thin films of an interesting semirod−coil diblock copolymer, poly(styrene)-b-poly(oligothiophene side chain modified isoprene) (PS-b-POTI), and the resulting morphological structures using synchrotron grazing incidence X-ray scattering combined with differential scanning calorimetry. These analyses provided detailed information about the morphologies of the phase-separated domains and the molecular structures formed in each domain. At room temperature, the diblock copolymer molecules in thin films were found to form an alternately stacked structure comprised of amorphous PS and smectic-A POTI microdomains whose stacking direction is parallel to the film plane. On heating, the films underwent a phase transformation to a hexagonal cylinder structure with cylindrical POTI domains in the PS matrix oriented normal to the film plane. This phase conversion is induced by the transformation of the POTI domains fro...

Order−Order and Order−Disorder Transitions in Thin Films of an Amphiphilic Liquid Crystalline Diblock Copolymer

In this study, we quantitatively investigated the temperature-dependent phase transition behaviors of thin films of an interesting amphiphilic diblock copolymer, poly(ethylene oxide)-b-poly(11-[4-(4-butylphenylazo)phenoxy]undecyl methacrylate) (p(EO)-b-p(MAAZ)) and the resulting morphological structures by using synchrotron grazing incidence X-ray scattering (GIXS) and differential scanning calorimetry. The quantitative GIXS analysis showed that the diblock copolymer in the homogeneous, isotropic melt state undergoes phase-separation near 190 degrees C and then forms a body-centered cubic (BCC) structure of spherical p(EO) domains in the p(MAAZ) matrix, at which point the p(EO) domains and the p(MAAZ) matrix are both in amorphous, liquid states. The BCC structure of spherical p(EO) domains is converted to a hexagonal cylinder structure near 120 degrees C, which is induced by the transformation of the isotropic phase of the p(MAAZ) matrix to the smectic A phase, which is composed of a laterally ordered structure of p(MAAZ) blocks with fully extended side groups. The resulting hexagonal cylinder structure is very stable below 120 degrees C. This microscopic hexagonal cylinder structure is retained as the smectic A phase of the p(MAAZ) matrix undergoes further transitions to smectic C near 104 degrees C and to a smectic X phase near 76 degrees C, while the amorphous, liquid phase of the p(EO) cylinders undergoes crystallization near -15 degrees C. These complicated temperature-dependent disorder-order and order-order phase transitions in the films were found to take place reversibly during the heating run. A face-centered orthorhombic structure of p(EO) domains was also found during the heating run and is an intermediate structure in the hexagonal cylinder structure to BCC structure transformation. We use these structural analysis results to propose molecular structure models at various temperatures for thin films of the diblock polymer.

Size effect and evidence of a size-driven phase transition in Bi 4Ti 3O 12 nanocrystals

Bi 4Ti 3O 12 nanocrystals (nc-BTO) were prepared by chemical coprecipitation technique. The crystal structure of the nanocrystals covering mean grain sizes from 16 to 48 nm was determined by X-ray diffraction and refined by the Rietveld method. Differential thermal analysis was employed to investigate the temperature dependent phase transition behavior in nc-BTO. With decreasing grain size the tetragonal–orthorhombic phase transition temperature decreases and ferroelectricity weakens. A size-driven phase transition was found at a critical size d c=44 nm for nc-BTO. The high-temperature tetragonal phase stabilizes at room temperature when d< d c. The origin of the structural anomaly is attributed to higher strain energy induced by lattice strain and to higher inter-phase surface energy difference in nc-BTO system.

Synthesis and Properties of Ionically Modified Polymers with LCST Behavior

Ionically modified LCST polymers were synthesized by free radical copolymerization of N-isopropylacrylamide (NIPAM) with various cationic, anionic, and amphoteric comonomers, resulting in products with comparable molecular masses and charge concentrations. The temperature-dependent phase transition behavior of the copolymers was investigated in water, aqueous sodium chloride solutions, and mixtures of water with dimethylformamide (DMF) using light transmission measurements, static light scattering, viscometry, and sedimentation experiments in an analytical ultracentrifuge. The LCST of the cationic modified copolymers remains unchanged compared with the homopolymer, but it is increased as a result of an amphoteric or anionic modification. Phase transitions are accompanied by conformational changes followed by aggregation processes. These changes are enhanced in the presence of salt. The phase transition of poly-NIPAM-co-MADAMBQ ((methacryloyloxy)ethyl)dimethylbenzylammonium chloride) in 0.5 M NaCl was studied in detail investigating the temperature dependence of molecular parameters as intrinsic viscosity, particle mass, radius of gyration, second virial coefficient, and sedimentation coefficient.