Tunable Transition Research Articles

Biomass-Derived Degradable Polymers via Alternating Ring-Opening Metathesis Polymerization of Exo-Oxanorbornenes and Cyclic Enol Ethers.

Degradable polymers made via ring-opening metathesis polymerization (ROMP) hold tremendous promise as eco-friendly materials. However, most of the ROMP monomers are derived from petroleum resources, which are typically considered less sustainable compared to biomass. Herein, we present a synthetic strategy to degradable polymers by harnessing alternating ROMP of biomass-based cyclic olefin monomers including exo-oxanorbornenes and cyclic enol ethers. A library of well-defined poly(enol ether)s with modular structures, tunable glass transition temperatures, and controlled molecular weights was achieved, demonstrating the versatility of this approach. Most importantly, the resulting copolymers exhibit high degrees of alternation, rendering their backbones fully degradable under acidic conditions.

Reentrant canonical spin-glass dynamics and tunable field-induced transitions in (GeMn)Co2O4 Kagomé lattice

We report on the reentrant canonical semi spin-glass characteristics and controllable field-induced transitions in distorted Kagomé symmetry of (GeMn)Co2O4. This B-site spinel exhibits complicated, yet interesting magnetic behaviour in which the longitudinal ferrimagnetic (FiM) order sets in below the Néel temperature T FN ∼ 77 K due to uneven moments of divalent Co (↑ 5.33 μ B) and tetravalent Mn (↓ 3.87 μ B) which coexists with transverse spin-glass state below 72.85 K. Such complicated magnetic behaviour is suggested to result from the competing anisotropic superexchange interactions (J AB/k B ∼ 4.3 K, J AA/k B ∼ −6.2 K and J BB/k B ∼ −3.3 K) between the cations, which is extracted following the Néel’s expression for the two-sublattice model of FiM. Dynamical susceptibility (χ ac (f, T)) and relaxation of thermoremanent magnetization, M TRM (t) data have been analysed by means of the empirical scaling-laws such as Vogel–Fulcher law and Power law of critical slowing down. Both of which reveal the reentrant spin-glass like character which evolves through a number of intermediate metastable states. The magnitude of Mydosh parameter (Ω ∼ 0.002), critical exponent zυ = (6.7 ± 0.07), spin relaxation time τ 0 = (2.33 ± 0.1) × 10−18 s, activation energy E a/k B = (69.8 ± 0.95) K and interparticle interaction strength (T 0 = 71.6 K) provide the experimental evidences for canonical spin-glass state below the spin freezing temperature T F = 72.85 K. The field dependence of T F obtained from χ ac (T) follows the irreversibility in terms of de Almeida–Thouless mean-field instability in which the magnitude of crossover scaling exponent Φ turns out to be ∼2.9 for the (Ge0.8Mn0.2)Co2O4. Isothermal magnetization plots reveal two field-induced transitions across 9.52 kOe (H SF1) and 45.6 kOe (H SF2) associated with the FiM domains and spin-flip transition, respectively. Analysis of the inverse paramagnetic susceptibility after subtracting the temperature independent diamagnetic term (=−3 × 10−3 emu mol−1 Oe−1) results in the effective magnetic moment = 7.654 μ B/f.u. This agrees well with the theoretically obtained = 7.58 μ B/f.u. resulting the cation distribution in support of the Hund’s ground state spin configuration and of Mn4+ and Co2+, respectively. The H–T phase diagram has been established by analysing all the parameters (T F(H), T FN(H), H SF1(T) and H SF2(T)) extracted from various magnetization measurements. This diagram enables clear differentiation among the different phases of the (GeMn)Co2O4 and also illustrates the demarcation between short-range and long-range ordered regions.

Cellulose Nanofiber-Alginate Biotemplated Cobalt Composite Multifunctional Aerogels for Energy Storage Electrodes.

Tunable porous composite materials to control metal and metal oxide functionalization, conductivity, pore structure, electrolyte mass transport, mechanical strength, specific surface area, and magneto-responsiveness are critical for a broad range of energy storage, catalysis, and sensing applications. Biotemplated transition metal composite aerogels present a materials approach to address this need. To demonstrate a solution-based synthesis method to develop cobalt and cobalt oxide aerogels for high surface area multifunctional energy storage electrodes, carboxymethyl cellulose nanofibers (CNF) and alginate biopolymers were mixed to form hydrogels to serve as biotemplates for cobalt nanoparticle formation via the chemical reduction of cobalt salt solutions. The CNF-alginate mixture forms a physically entangled, interpenetrating hydrogel, combining the properties of both biopolymers for monolith shape and pore size control and abundant carboxyl groups that bind metal ions to facilitate biotemplating. The CNF-alginate hydrogels were equilibrated in CaCl2 and CoCl2 salt solutions for hydrogel ionic crosslinking and the prepositioning of transition metal ions, respectively. The salt equilibrated hydrogels were chemically reduced with NaBH4, rinsed, solvent exchanged in ethanol, and supercritically dried with CO2 to form aerogels with a specific surface area of 228 m2/g. The resulting aerogels were pyrolyzed in N2 gas and thermally annealed in air to form Co and Co3O4 porous composite electrodes, respectively. The multifunctional composite aerogel's mechanical, magnetic, and electrochemical functionality was characterized. The coercivity and specific magnetic saturation of the pyrolyzed aerogels were 312 Oe and 114 emu/gCo, respectively. The elastic moduli of the supercritically dried, pyrolyzed, and thermally oxidized aerogels were 0.58, 1.1, and 14.3 MPa, respectively. The electrochemical testing of the pyrolyzed and thermally oxidized aerogels in 1 M KOH resulted in specific capacitances of 650 F/g and 349 F/g, respectively. The rapidly synthesized, low-cost, hydrogel-based synthesis for tunable transition metal multifunctional composite aerogels is envisioned for a wide range of porous metal electrodes to address energy storage, catalysis, and sensing applications.

High-quality VO2 films synthesized on polymer substrates using room-temperature pulsed laser deposition and annealing

With the growing trend towards portable and adaptable devices in various fields, the demand for flexible electronics and coatings utilizing phase transition materials like VO2 as a functional layer is rising. However, depositing high-quality crystalline VO2 films directly onto polymer substrates remains challenging due to the thermosensitivity of such materials being barely compatible with usual VO2 deposition methods. In this study, we present a simple method for directly growing VO2 on Kapton substrates. VOx films were first deposited onto the Kapton substrate at room temperature using pulsed laser deposition and subsequently annealed in an oxygen atmosphere at 390 °C. The electrical and optical properties of the VO2/Kapton samples with different film thicknesses were investigated. The 200 nm-thick vanadium dioxide films exhibit the best resistivity contrast with 3.1 orders of magnitude (OOM), while the 50 nm-thick films show the best transmittance change of 54 % at 2500 nm. The switching performances are comparable or even superior to those achieved previously using more complicated synthesis processes such as film transfer, introduction of buffer layers, and ink injection. Additionally, by coupling W elemental doping and thin film strain, we successfully tuned the VO2 transition temperature to room temperature with a dopant concentration of only 1.1 at.% while maintaining a high electrical contrast (2 OOM). The impressive electrical and optical transition performance of the VO2/Kapton samples, combined with their tunable transition temperature, makes them promising candidates for flexible electronic devices and thermochromic smart coatings.

Amine-Functionalized Polybutadiene Synthesis by Tunable Postpolymerization Hydroaminoalkylation.

Early transition metal-catalyzed hydroaminoalkylation is a powerful single-step method to selectively add amines to polybutadienes, offering an efficient strategy to access amine-functionalized polyolefins. Aryl and alkyl secondary amines were used with a tantalum catalyst to functionalize both 28 wt% (PBD13) and 70 wt% (PBD50) 1,2-polybutadiene polymers. The degree of amination was controlled by modifying amine and catalyst loading in both small- and multigram-scale reactions. The vinyl groups of 1,2-polybutadiene were aminated with ease, and unexpectedly the hydroaminoalkylation of challenging internal alkenes of the 1,4-polybutadiene unit was observed. This unanticipated reactivity was proposed to be due to a directing group effect. This hypothesis was supported with small-molecule model substrates, which also showed directed internal alkene amination. Increasing degrees of amination resulted in materials with dramatically higher and tunable glass transition temperature (Tg) values, due to the dynamic cross-linking accessible to hydrogen-bonding, amine-containing materials. Primary amine-functionalized polybutadiene was also prepared, demonstrating that a broad new class of amine-containing polyolefins can be accessed by postpolymerization hydroaminoalkylation.

Leveraging of both positive and negative magnetocaloric effects in ZnFe2O4 layers

The novel magnetocaloric thin film material with a tunable magnetic transition is the cornerstone of magnetic refrigeration, which plays a crucial role in cooling on-chip devices. Zn-ferrite (ZnFe2O4) occupies a significant place due to its interesting grain size-controlled magnetism. In this study, we investigate the magnetocaloric properties of Zn-ferrite layers consisting of a mixture of superparamagnetic (SPM), ferrimagnetic (FiM), and bulk-type antiferromagnetic (AFM) grains. Due to the co-existence of different magnetic grains, a crossover from the conventional magnetocaloric effect (MCE) to the inverse magnetocaloric effect (IMCE) is observed. The magnetic entropy change (-ΔSM) exhibits high-temperature positive values around the Curie temperature (TC) and low-temperature negative values around either the Néel temperature (TN) or blocking temperature (TB). The maximum value of -ΔSM (0.10 J/kg K) is almost double in AFM-dominant samples compared to SPM-dominant samples, while maintaining a relative cooling power (RCP) of 11.57–14.82 J/Kg under applied fields of 0 – 2T. FiM-dominant samples, with a wide working temperature window around room temperature, are suitable for large RCP applications. Thus, the discovery of new IMCE materials like single layers of Zn-ferrite is equally important in the search for suitable conventional MCE materials for magnetic refrigeration devices, which can exhibit multiple -ΔSM and RCP peaks in low and high temperatures.

Reducing phase transition temperature of vanadium dioxide by ascorbic acid

The phase transition of vanadium dioxide brings huge change in its optical property, which is well used in thermochromic window, fixed-temperature heat sensor, laser protection and other fields. Tunable phase transition temperature is one key for its wide applications. In this paper we verified a new simple method to reduce phase transition temperature. A coordination effect of ascorbic acid on VO2 powder reduces its phase transition temperature to about 32 °C. This simple method offers a new efficient option to deal with VO2, which may dramatically promote the applications of VO2.

Inducing an LCST in hydrophilic polysaccharides via engineered macromolecular hydrophobicity

Thermoresponsive polysaccharide-based materials with tunable transition temperatures regulating phase-separated microdomains offer substantial opportunities in tissue engineering and biomedical applications. To develop novel synthetic thermoresponsive polysaccharides, we employed versatile chemical routes to attach hydrophobic adducts to the backbone of hydrophilic dextran and gradually increased the hydrophobicity of the dextran chains to engineer phase separation. Conjugating methacrylate moieties to the dextran backbone yielded a continuous increase in macromolecular hydrophobicity that induced a reversible phase transition whose lower critical solution temperature can be modulated via variations in polysaccharide concentration, molecular weight, degree of methacrylation, ionic strength, surfactant, urea and Hofmeister salts. The phase separation is driven by increased hydrophobic interactions of methacrylate residues, where the addition of surfactant and urea disassociates hydrophobic interactions and eliminates phase transition. Morphological characterization of phase-separated dextran solutions via scanning electron and flow imaging microscopy revealed the formation of microdomains upon phase transition. These novel thermoresponsive dextrans exhibited promising cytocompatibility in cell culture where the phase transition exerted negligible effects on the attachment, spreading and proliferation of human dermal fibroblasts. Leveraging the conjugated methacrylate groups, we employed photo-initiated radical polymerization to generate phase-separated hydrogels with distinct microdomains. Our bottom-up approach to engineering macromolecular hydrophobicity of conventional hydrophilic, non-phase separating dextrans to induce robust phase transition and generate thermoresponsive phase-separated biomaterials will find applications in mechanobiology, tissue repair and regenerative medicine.

Spectral Variability in Fixed Windows using Fractional Fourier Transform: Application in Power Spectral Density Estimation

In statistical signal processing, power spectral density estimation is a frequency domain analysis in which power contents of a signal are measured with respect to frequency components of that signal. The power estimation of a signal can be carried out more precisely by using a window with a narrower 3-dB bandwidth and higher side-lobe attenuation. Theoretically, these two spectral parameters show trade-off in variable windows and remain constant in fixed windows. In this work, spectral behavior of fixed windows has been elaborated using Fractional Fourier Transform (FRFT) keeping their inherent time domain behavior intact. The FRFT is an extension of conventional Fourier transform with an additional variable parameter, known as rotation angle, which makes it more flexible and useful in various signal processing applications viz. power estimation and designing of tunable transition band FIR filters. In this article, variability in 3-dB bandwidth and sidelobe attenuation of fixed windows has been achieved by exploiting the available flexibility in FRFT and obtained variability has been applied in the estimation of signal power. Simulation results demonstrate that both of these two spectral parameters are improved and hence, trade-off problem between resolution and spectral leakage in the power spectral density estimation is overcome upto an extent.

Multifunctional deoxybenzoins: Preparation of low heat release polymer networks by orthogonal crosslinking

Multifunctional epoxide monomers present opportunities to design cross-linked networks with conveniently tunable thermal and mechanical properties. This paper describes the synthesis of a novel, multifunctional deoxybenzoin monomer containing both epoxide and allyl groups, and its use in the preparation of epoxide networks. The orthogonal reactivity of these functional groups, operating via epoxide-amine and thiol-ene chemistry, facilitated the formation of networks with tunable glass transition temperatures and crosslink densities. Thermogravimetric analysis (TGA) and microscale combustion calorimetry (MCC) characterization revealed these networks to possess impressively low heat release properties (∼50% lower than conventional bisphenol A (BPA)-based networks) and very high char residues that reached ∼60%. Overall, introducing functional deoxybenzoin epoxide monomers into these types of network architectures imparts a method for combining low heat release structures with the advantageously tunable physical and mechanical properties of polymer networks.

Tunable Interband Transitions in Twisted h-BN/Graphene Heterostructures.

In twisted h-BN/graphene heterostructures, the complex electronic properties of the fast-traveling electron gas in graphene are usually considered to be fully revealed. However, the randomly twisted heterostructures may also have unexpected transition behaviors, which may influence the device performance. Here, we study the twist-angle-dependent coupling effects of h-BN/graphene heterostructures using monochromatic electron energy loss spectroscopy. We find that the moiré potentials alter the band structure of graphene, resulting in a redshift of the intralayer transition at the M point, which becomes more pronounced up to 0.22eV with increasing twist angle. Furthermore, the twisting of the Brillouin zone of h-BN relative to the graphene M point leads to tunable vertical transition energies in the range of 5.1-5.6eV. Our findings indicate that twist-coupling effects of van der Waals heterostructures should be carefully considered in device fabrications, and the continuously tunable interband transitions through the twist angle can serve as a new degree of freedom to design optoelectrical devices.

Ultra-Tough Waterborne Polyurethane-Based Graft-Copolymerized Piezoresistive Composite Designed for Rehabilitation Training Monitoring Pressure Sensors.

Effective training is crucial for patients who need rehabilitation for achieving optimal recovery and reducing complications. Herein, a wireless rehabilitation training monitoring band with a highly sensitive pressure sensor is proposed and designed. It utilizes polyaniline@waterborne polyurethane (PANI@WPU) as a piezoresistive composite material, which is prepared via the in situ grafting polymerization of PANI on the WPU surface. WPU is designed and synthesized with tunable glass transition temperatures ranging from -60 to 0 °C. Dipentaerythritol (Di-PE) and ureidopyrimidinone (UPy) groups are introduced, endowing the material with good tensile strength (14.2MPa), toughness (62 MJ-1 m-3 ), and great elasticity (low permanent deformation: 2%). Di-PE and UPy enhance the mechanical properties of WPU by increasing the cross-linking density and crystallinity. Combining the toughness of WPU and the high-density microstructure derived by hot embossing technology, the pressure sensor exhibits high sensitivity (168.1 kPa-1 ), fast response time (32ms), and excellent stability (10 000 cycles with 3.5% decay). In addition, the rehabilitation training monitoring band is equipped with a wireless Bluetooth module, which can be easily applied to monitor the rehabilitation training effect of patients using an applet. Therefore, this work has the potential to significantly broaden the application of WPU-based pressure sensors for rehabilitation monitoring.

A photocured bio-based shape memory thermoplastics for reversible wet adhesion

Development of reversible wet or underwater adhesives remains a grand challenge, because weakened intermolecular interactions by water molecules or/and low effective contact area cause poor interface to the wet surfaces, which significantly decreases adhesive strength. Herein, a new photocured, bio-based shape memory polymer (SMP) that shows both chemical and structural wet adhesion to various types of surfaces is developed. The SMP is polymerized from three monomers to form linear polymer chains dangled with hydrophobic side chains. The hydrogen acceptor and donor groups in the chains form hydrogen bonding with the surfaces, which is protected by the hydrophobic chains in the interface. The SMP shows tunable phase transition temperature (Tg) of 17–38 °C. In a rubbery state above Tg, the adhesive forms conformable contact with the targeted surfaces. Below Tg, a transition to a glassy state locks the conformed shapes to largely increase the effective contact area. As a result, the adhesive exhibits long-term underwater adhesion of >15 days with the best adhesion strength of ∼0.9 MPa. Its applications in leak repair, underwater on-skin sensors were demonstrated. This new, general strategy would pave avenues to designing bio-based, long-lasting, and reversible adhesives from renewable feedstocks for widespread applications.

Second-order topological insulators and tunable topological phase transitions in honeycomb ferromagnets

Second-order topological insulators and tunable topological phase transitions in honeycomb ferromagnets

Hygroscopic Tunable Multishape Memory Effect in Cellulosic Macromolecular Networks with a Supramolecular Mesophase

Tunable multishape memory polymers offer intriguing opportunities for memorizing multiple temporary shapes with tunable transition temperatures from one material composition. However, such multishape memory effects have been exclusively correlated with the thermomechanical behaviors of polymers, significantly limiting their applications in heat-sensitive scenarios. Here we report a nonthermal tunable multishape memory effect in covalently cross-linked cellulosic macromolecular networks, which spontaneously organize into supramolecular mesophases by water evaporation induced self-assembly. The supramolecular mesophase endows the network with a broad, reversible hygromechanical response combined with a unique moisture memory effect at ambient temperature, enabling diverse multishape memory behaviors (dual-, triple-, and quadruple-shape memory) under highly tunable and independent control of relative humidity (RH) alone. Significantly, such a hygroscopic tunable multishape memory effect readily extends the implications of shape memory polymers beyond the conventional thermomechanical regimes with potential advantages for biomedical applications.

Spatio-temporal programming of lyotropic phase transition in nanoporous microfluidic confinements

HypothesisThe nanoporous polydimethylsiloxane (PDMS) surfaces of a rectangular microfluidic channel, selectively uptakes water molecules, concentrating the solute molecules in an aqueous phase, that could drive phase transitions. Factors such as surface wettability, channel geometry, the surface-to-volume ratio, and surface topography of the confinements could play a key role in tuning the phase transitions spatio-temporally. ExperimentsUsing a lyotropic chromonic liquid crystal as model biological material, confined within nanoporous microfluidic environments, we study molecular assembly driven by nanoporous substrates. By combining timelapse polarized imaging, quantitative image processing, and a simple mathematical model, we analyze the phase transitions and construct a master diagram capturing the role of surface wettability, channel geometry and embedded topography on programmable lyotropic phase transitions. FindingsIntrinsic PDMS nanoporosity and confinement cross-section, together with the imposed wettability regulate the rate of the N-M phase transition; whereas the microfluidic geometry and embedded topography enable phase transition at targeted locations. We harness the emergent long-range order during N-M transition to actuate elasto-advective transport of embedded micro-cargo, demonstrating particle manipulation concepts governed by tunable phase transitions. Our results present a programmable physical route to material assembly in microfluidic environment, and offer a new paradigm for assembling genetic components, biological cargo, and minimal synthetic cells.

Tunable optical topological transition of Cherenkov radiation

Chasing efficient methods to generate and manipulate Cherenkov radiation (CR) is always the lofty target of many scientific researchers. In this work, by combining the optical topological transition (OTT) of graphene hyperbolic metamaterials (GHMs), we study the properties and clarify the mechanisms of the OTT of CR for the first time. Specifically, we indicate two representative examples to reveal the potential of the OTT in manipulating CR properties and further applications, which correspond to the ultrafast photothermal and nonlocal properties of graphene individually. These results and conclusions pave the way for the photonic devices and applications of CR.

Anomalous magnetic entropy changes of bulk and powder Mn0.5Fe0.5-xNi1+xSi0.94Al0.06 intermetallic system

The multi-component MnTX (T = Ni, Co; X = p-block elements) intermetallic system have attracted intensive attention due to their tunable magneto-structural phase transition and associated giant magnetocaloric effect. Here, we report an experimental study on the magnetic, magnetocaloric, and electrical properties of alloys that belong to the MnTX family- Mn0.5Fe0.5-xNi1+xSi0.94Al0.06. For all values of x (0≤ x ≤0.10), the materials exhibited a first-order magneto-structural phase transition from a low-temperature ferromagnetic orthorhombic phase to a high-temperature paramagnetic hexagonal phase. The transition was accompanied by large magnetic entropy changes of up to –22 and −57 J kg−1K−1 at T = 322 K for field changes of 2 T and 5 T, respectively. Most noticeably, a sharp jump in electrical resistivity was observed at T = 320 K for x = 0.1, which confirmed that the first-order phase transition preserved the mechanical stability of the material. The alloy with x = 0.1 was further crushed into powder and the magnetic properties were compared to its bulk counterpart. The enhanced mechanical stability and nearly zero magnetic hysteresis loss along with the fact that the system is constituted of cheap, non-toxic elements make this system worthy of consideration for near-room temperature magnetic refrigeration applications.

Intersubband Transitions in Lead Halide Perovskite-Based Quantum Wells for Mid-Infrared Detectors.

Due to their excellent optical and electrical properties as well as versatile growth and fabrication processes, lead halide perovskites have been widely considered as promising candidates for green energy and applications related to optoelectronics. Here, we investigate their potential applications at infrared wavelengths by modeling the intersubband transitions in perovskite-based quantum well systems. Both single-well and double-well structures are studied, and their energy levels as well as the corresponding wave functions and intersubband transition energies are calculated by solving the one-dimensional Schrödinger equations. Via adjustment of the quantum well and barrier thicknesses, the intersubband transition energies can be tuned to cover a broad infrared wavelength range. We also find that the lead halide perovskite-based quantum wells possess high absorption coefficients. The widely tunable transition energies and high absorption coefficients of the perovskite-based quantum well systems, combined with their unique material and electrical properties, may enable an alternative material system for infrared photodetector applications.

Tunable topological phase transition in the telecommunication wavelength

Recent progress in the Valley Hall insulator has demonstrated a nontrivial topology property due to the distinct valley index in 2D semiconductor systems. In this work, we propose a highly tunable topological phase transition based on valley photonic crystals. The topological phase transition is realized by the inversion symmetry broken due to the refractive index change of structures consisting of optical phase change material (OPCM) with thermal excitation of different sites in a honeycomb lattice structure. Besides, simulations of light propagation at sharp corners and pseudo-spin photon coupling are conducted to quantitatively examine the topological protection. Compared with other electro-optical materials based on reconfigurable topological photonics, a wider bandwidth and greater tunability of both central bandgap frequency and topological phase transition can happen in the proposed scheme. Our platform has great potential in practical applications in lasing, light sensing, and high-contrast tunable optical filters.