Thermal Stiffening Research Articles

3D printable and mechanically tunable hydrogels achieved through hydrophobic and ionic interactions.

Thermal stiffening materials are commonly applied in the aerospace and automotive industries, among others, since their dimensional stabilities and stiffness characteristics improve at high temperatures. In this study, temperature-triggered modulus-tunable hydrogels were prepared by combining Pluronic F-127 with charged polymers. Pluronic F-127, a triblock copolymer micelle, provided three-dimensional printing capabilities of fine resolution with high viscosity, while hydrophobic and ionic interactions among polymer networks provided thermal stiffening. The hydrogel ink's printability was demonstrated by successfully creating complex 3D structures. A calcium ion carrying a hydrophobic propionate and carboxylate group in polymer chains was used to form additional physical crosslinking at high temperature, ultimately leading to the thermal stiffening effect without volume change. The thermal stiffening behavior was found to be fully reversible and repeatable. Finally, to demonstrate the versatility of this work, graphene oxide was added to produce a light-controllable modulus based on its photothermal properties.

Abnormal In-Plane Thermomechanical Behavior of Two-Dimensional Hybrid Organic-Inorganic Perovskites.

The implementation of two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) in semiconductor device applications will have to accommodate the co-existence of strain and temperature stressors and requires a thorough understanding of the thermomechanical behavior of 2D HOIPs. This will mitigate thermomechanical stability issues and improve the durability of the devices, especially when one considers the high susceptibility of 2D HOIPs to temperature due to their soft nature. Here, we employ atomic force microscopy (AFM) stretching of suspended membranes to measure the temperature dependence of the in-plane Young's modulus (E∥) of model Ruddlesden-Popper 2D HOIPs with a general formula of (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (here, n = 1, 3, or 5). We find that E∥ values of these 2D HOIPs exhibit a prominent non-monotonic dependence on temperature, particularly an abnormal thermal stiffening behavior (nearly 40% change in E∥) starting around the order-disorder transition temperature of the butylammonium spacer molecules, which is significantly different from the thermomechanical behavior expected from their 3D counterpart (CH3NH3PbI3) or other low-dimensional material systems. Further raising the temperature eventually reverses the trend to thermal softening. The magnitude of the thermally induced change in E∥ is also much higher in 2D HOIPs than in their 3D analogs. Our results can shed light on the structural origin of the thermomechanical behavior and provide needed guidance to design 2D HOIPs with desired thermomechanical properties to meet the application needs.

Entropy‐Mediated Polymer–Cluster Interactions Enable Dramatic Thermal Stiffening Hydrogels for Mechanoadaptive Smart Fabrics

Thermal stiffening materials that are naturally soft but adaptively self-strengthen upon heat are intriguing for load-bearing and self-protection applications at elevated temperatures. However, to simultaneously achieve high modulus change amplitude and high mechanical strength at the stiffened state remains challenging. Herein, entropy-mediated polymer-mineral cluster interactions are exploited to afford thermal stiffening hydrogels with a record-high storage modulus enhancement of 13 000 times covering a super wide regime from 1.3 kPa to 17 MPa. Such a dramatic thermal stiffening effect is ascribed to the transition from liquid-liquid to solid-liquid phase separations, and at the molecular level, driven by enhanced polymer-cluster interactions. The hydrogel is further processed into sheath-core fibers and smart fabrics, which demonstrate self-strengthening and self-powered sensing properties by co-weaving another liquid metal fiber as both the joule heater and triboelectric layer.

Entropy‐Mediated Polymer–Cluster Interactions Enable Dramatic Thermal Stiffening Hydrogels for Mechanoadaptive Smart Fabrics

AbstractThermal stiffening materials that are naturally soft but adaptively self‐strengthen upon heat are intriguing for load‐bearing and self‐protection applications at elevated temperatures. However, to simultaneously achieve high modulus change amplitude and high mechanical strength at the stiffened state remains challenging. Herein, entropy‐mediated polymer–mineral cluster interactions are exploited to afford thermal stiffening hydrogels with a record‐high storage modulus enhancement of 13 000 times covering a super wide regime from 1.3 kPa to 17 MPa. Such a dramatic thermal stiffening effect is ascribed to the transition from liquid‐liquid to solid–liquid phase separations, and at the molecular level, driven by enhanced polymer–cluster interactions. The hydrogel is further processed into sheath–core fibers and smart fabrics, which demonstrate self‐strengthening and self‐powered sensing properties by co‐weaving another liquid metal fiber as both the joule heater and triboelectric layer.

Seismic Performance Assessment of a Retrofitted Bridge with Natural Rubber Isolators in Cold Weather Environments Using Fragility Surfaces

Rubber-based seismic isolation has been demonstrated to be one of the most effective measures to protect structural elements from damage during earthquakes and a viable option to retrofit existing structures with poor seismic detailing. The main constituent of these isolation units is rubber, a material that is subject to stiffening when exposed to low air temperatures. In the case of isolated highway bridges, thermal stiffening might reduce the efficiency of isolators, transferring higher forces to the substructure. Assessment of the seismic response of retrofitted structures using rubber isolators in cold regions is thus necessary. Accordingly, in this study, the effect of low temperatures on the seismic performance of a highway bridge retrofitted with natural rubber (NR) isolators is quantified using a probabilistic framework based on fragility surfaces. From the component- and system-level surfaces, it is revealed that the effects of cold temperatures on highway bridges retrofitted with elastomeric isolators may be negligible, depending on the configuration of lateral restraining structures. However, when isolators are able to perform their function without impediment, their thermal stiffening might be significantly detrimental to the bridge’s substructure, mainly affecting bent columns.

Determination of the atomic cohesive energy and Debye temperature from thermal stiffening of the dielectric constant of semiconductors

From the perspective of bond-order-length-strength correlation, we have formulated the thermally stiffened dielectric constant and refractive index for semiconductors. It is clarified that: (i) The dielectric constant and refractive index of semiconductors turn from non-linear to linear thermal stiffening at one third of its Debye temperature; (ⅱ) The Debye temperature determines the width of the non-linear part and the reciprocal atomic cohesive energy determines the slope of linear part at high temperatures of the dielectric constant and refractive index curves.

Temperature dependence of electron-phonon interactions in vanadium

First-principles calculations were used to study the Fermi surface of body-centered cubic vanadium at elevated temperatures. Supercell calculations accounted for effects of thermal atom displacements on band energies, and band unfolding was used to project the spectral weight of the electron states into the Brillouin zone of a standard bcc unit cell. An electronic topological transition (ETT, or Lifshitz transition) occurred near the $\mathrm{\ensuremath{\Gamma}}$ point with increasing temperature, but the large thermal smearings from the atomic disorder and the Fermi-Dirac distribution reduced the effect of this ETT on the electron-phonon interactions. The phonon dispersions showed thermal stiffening of their Kohn anomalies near the $\mathrm{\ensuremath{\Gamma}}$ point and of the longitudinal N phonon mode. In general the effects of the ETT were overcome by the thermal smearing of the Fermi surface that reduces the spanning vector densities for anomalous phonon modes.

Optimizing filler network formation in poly(hexahydrotriazine) for realizing high thermal conductivity and low oxygen permeation

Shape, size, and orientation of fillers within polymer matrix are important factors for realizing various composite functionalities. Here, we describe an industrially scalable approach to preparing composites bearing highly aligned model hexagonal boron nitride filler (hBN) by simple melt-pressing. This outcome is achieved by using a malleable but thermal stiffening polymer matrix. The matrix maintains or even increases its stiffness during processing at elevated temperatures, producing the composites with highly aligned hBN and consequently a high thermal conductivity (28 W/mK). Furthermore, the composite bearing aligned hBN exhibits a 62% reduction in oxygen permeation with only 2.7 vol% of hBN. Since the matrix can be chemically depolymerized with an aid of acid, it is also possible to recover the hBN from the composite without physical/chemical denaturation of the filler, thus the recovered filler can be re-used in the future.

Role of adsorbed chain rigidity in reinforcement of polymer nanocomposites

ABSTRACTThe thermomechanical behavior of polymer nanocomposites is mostly governed by interfacial properties which rely on particle–polymer interactions, particle loading, and dispersion state. We recently showed that poly(methyl methacrylate) (PMMA) adsorbed nanoparticles in poly(ethylene oxide) (PEO) matrices displayed an unusual thermal stiffening response. The molecular origin of this unique stiffening behavior resulted from the enhanced PEO mobility within glassy PMMA chains adsorbed on nanoparticles. In addition, dynamic asymmetry and chemical heterogeneities existing in the interfacial layers around particles were shown to improve the reinforcement of composites as a result of good interchain mixing. Here, the role of chain rigidity in this interfacially controlled reinforcement in PEO composites is investigated. We show that particles adsorbed with less rigid polymers improve the mechanical properties of composites. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2019,57, 9–14

Thermal stiffening of hydrophobic association hydrogels

Strong and tough hydrogels, important for several materials and bio-applications, typically consist of nanocomposites and double networks. There is the need to create hydrogels that exhibit tunable mechanical properties on demand without compromising other functional properties. This work demonstrates the rapid thermal stiffening of a hydrophobic association hydrogel while maintaining its optical quality and chemical composition. Up to 100-fold increase in modulus of the methacrylate-based hydrogel was achieved by an increase in temperature from 25 °C to 50 °C. Based on various characterizations, we proposed that the thermal stiffening is related to the polymer conformational changes and the ensuing increase in inter-chain hydrophobic associations at the expense of intra-chain associations. The temperature above which thermal stiffening occurs can be tuned with the polymer content in the hydrogel. Furthermore, hydrogels containing a lower mole fraction of the hydrophobic groups exhibit unusual “gel-sol-gel” transitions with temperature increase.

Thermal stiffening of clamped elastic ribbons

We use molecular dynamics to study the vibrations of a thermally fluctuating two-dimensional elastic membrane clamped at both ends. We directly extract the eigenmodes from resonant peaks in the frequency domain of the time-dependent height and measure the dependence of the corresponding eigenfrequencies on the microscopic bending rigidity of the membrane, taking care also of the subtle role of thermal contraction in generating a tension when the projected area is fixed. At finite temperatures we show that the effective (macroscopic) bending rigidity tends to a constant as the bare bending rigidity vanishes, consistent with theoretical arguments that the large-scale bending rigidity of the membrane arises from a strong thermal renormalization of the microscopic bending rigidity. Experimental realizations include covalently-bonded two-dimensional atomically thin membranes such as graphene and molybdenum disulfide or soft matter systems such as the spectrin skeleton of red blood cells or diblock copolymers.

Phonons and elasticity of cementite through the Curie temperature

Phonon partial densities of states (pDOS) of ^(57)Fe_3 C were measured from cryogenic temperatures through the Curie transition at 460 K using nuclear resonant inelastic x-ray scattering. The cementite pDOS reveal that low-energy acoustic phonons shift to higher energies (stiffen) with temperature before the magnetic transition. This unexpected stiffening suggests strongly nonharmonic vibrational behavior that impacts the thermodynamics and elastic properties of cementite. Density functional theory calculations reproduced the anomalous stiffening observed experimentally in cementite by accounting for phonon-phonon interactions at finite temperatures. The calculations show that the low-energy acoustic phonon branches with polarizations along the [010] direction are largely responsible for the anomalous thermal stiffening. The effect was further localized to the motions of the Fe_(II) site within the orthorhombic structure, which participates disproportionately in the anomalous phonon stiffening.

Assessment of low temperature exposure for design and evaluation of elastomeric bridge bearings and seismic isolators in Canada

This paper presents key data and relevant analyses assessing low temperature exposure for design and evaluation of elastomeric bridge bearings and isolators in Canada. A large database of temperature records is processed to investigate the potential for instantaneous thermal stiffening and crystallization. The results mainly show that: (i) temperature conditions at some locations lead to a significant potential for instantaneous stiffening in terms of intensity and frequency; (ii) in warmer locations, thermal stiffening is highly improbable; (iii) the number of consecutive days below a given low temperature is very variable geographically; and (iv) the crystallization testing criteria prescribed in CSA S6-14 can be too conservative. The results are also illustrated using contour maps. Important differences in temperature variation trends between eastern and western Canada are highlighted. The proposed methodology and obtained results constitute efficient tools to determine site-specific temperature conditions for enhanced performance-based design and evaluation of bridges in Canada.

Rigid–elastic-thermal coupling dynamics and its application

Spacecraft dynamics belongs to rigid–elastic coupling dynamics in general. If thermal stress and thermal effect are further considered, it will cause the more complex rigid–elastic-thermal coupling dynamics. For the aforementioned kind of coupling dynamics, it is difficult to establish the basic equations directly. It has proved that a suitable way to study complex coupling problems is by work–energy theorem and the law of conversation of energy. Using the energy method, Hamilton principle of rigid–elastic-thermal coupling dynamics is established. The stationary value conditions of Hamilton principle of rigid–elastic-thermal coupling dynamics are derived. The physical meaning of the stationary value conditions is indicated. As an application of rigid–elastic-thermal coupling dynamics, the actual example of spacecraft is given to reveal that there existed thermal stiffening or softening problems in rigid–elastic-thermal coupling dynamics, just like dynamic hardening or softening problems in spacecraft dynamics.

First‐Principles Study of Sc1−xTixF3 (x ≤ 0.375): Negative Thermal Expansion, Phase Transition, and Compressibility

We present the first‐principles investigation of (x ≤ 0.375). Controllable thermal expansion of is achieved by different Ti contents. The negative thermal expansion (NTE) behavior is weakened gradually with increasing Ti content, which is consistent with experimental measurements. The Jahn–Teller effect plays an important role in the cubic‐to‐rhombohedral phase transition, which stems from the enhanced energy stability when the 3d orbitals of cation split into triply degenerate and sets. The unusual thermal stiffening of is found, which is similar to that of and but contrary to other NTE materials.

Structural Relationship between Negative Thermal Expansion and Quartic Anharmonicity of CubicScF3

Cubic scandium trifluoride (ScF3) has a large negative thermal expansion over a wide range of temperatures. Inelastic neutron scattering experiments were performed to study the temperature dependence of the lattice dynamics of ScF3 from 7 to 750 K. The measured phonon densities of states show a large anharmonic contribution with a thermal stiffening of modes around 25 meV. Phonon calculations with first-principles methods identified the individual modes in the densities of states, and frozen phonon calculations showed that some of the modes with motions of F atoms transverse to their bond direction behave as quantum quartic oscillators. The quartic potential originates from harmonic interatomic forces in the DO9 structure of ScF3, and accounts for phonon stiffening with the temperature and a significant part of the negative thermal expansion.

Effect of initial thermal structure on the collapse and explosion of iron stellar cores

We investigate the adiabatic collapse of 1.7 and 1.5 M/sub sun/ iron cores using the LLPR equation of state and Fermi gas electron capture rates, and assuming complete neutrino trapping. For a variety of initial configurtions, infall deleptonization leaves a homologous core of only 1 M/sub circle-solid/; the large overlay mass that the shock must penetrate and dissociate then prevents significant ejection of mass and kinetic energy. If all electron capture is artificially suppressed, we do obtain ejection of 0.1 M/sub circle-solid/, with 5 x 10/sup 50/ and 2 x 10/sup 52/ ergs kinetic energy for the 1.7 and 1.5 M/sub circle-solid/ initial cores, respectively. In stars of these masses, neutrino processes are not responsible for supernova explosion but instead kill the otherwise efficient thermal stiffening mechanism. The initial iron core configuration needed for a supernovva explosion must be cooler, and therefore lighter and more isentropic, than those cores heretofore considdered. Such a cooler pre-supernova configuration can evolve if hydrostatic electron capture leads to greater neutrino cooling before the contraction becomes dynamic.

Thermomechanical behavior of poly(carborane–siloxane)s containing CB5H5C cages

AbstractThe thermomechanical spectra of two new carborane–siloxane polymers containing five‐boron carborane cages in the backbones are reported and discussed. The polymers are the homopolymer, HO[Si(CH3)2CB5H5CSi(CH3)2O]nH, and the random copolymer with 20 mole‐% of the ten‐boron meta‐carborane analogue, [Si(CH3)2CB10H10CSi(CH3)2O]. The mechanical spectra (∼1 cps) were determined from −180° → +625° → −180°C (ΔT/Δl = 3.6°C/min for T > 25°C and 2°C/min for T < 25°C) using the semimicro thermomechanical technique, torsional braid analysis. In nitrogen, both polymers displayed secondary transitions at −140°C. The glass transition (Tg) for the homopolymer was −60°C and for the copolymer was −52°C. The homopolymer had a melting point of +70°C. The copolymer was amorphous. The high‐temperature stability in nitrogen of both polymers appeared to be identical; thermal stiffening commenced at 400°C, continued to 625°C, and resulted in materials that were typical of highly crosslinked resins. In air, the homopolymer began to stiffen catastrophically near 270°C, while the copolymer began to stiffen similarly nearly 50°C higher. The intrinsic elastomeric nature together with the thermomechanical results prompted further study of the copolymer. Thermomechanical cycling studies in nitrogen and air are reported for the copolymer. Some correlating TGA and DTA are also discussed.