A commercial bio-epoxy resin was investigated by means of DSC experiments. Dynamic ramps of 1, 2.5, 5 and 10 °C/min were used to attain the total heat of curing reaction (ΔHtotal) and glass ...transition temperatures of the unreacted monomers (Tgo) and the fully cured sample (Tgoo), estimated to be 398.3 J/gr, −46.4 °C and 63.9 °C, respectively. Isothermal measurements at 50, 65, 70 and 80 °C were performed in order to construct Di Benedetto equation, which resulted in the values of −44.1 °C and 61.2 °C for Tgo and Tgoo. Isothermal predictions, based solely on dynamic DSC data, were further conducted. Model-free kinetic analysis revealed a slight decrease in the effective activation energy Eα, which found to vary with conversion in the range of 30–60 kJ/mol. Utilizing Kamal and Sourour (KS) with diffusion reaction model under dynamic conditions, Ea values of 49 kJ/mol and 56 kJ/mol were reported for catalytic and autocatalytic mechanism, while a strong compliance was accomplished between experiment and simulation. Vyazovkin isoconversional method accurately predicted isothermal curing for temperatures exceeding 65 °C, could not however capture curing rate hindrance below Tgoo. Eventually, KS with diffusion model was incorporated in the production of Time Temperature Transformation (TTT) plot. The present comprehensive kinetic analysis, aims for divulging the optimum curing temperature window for the investigated epoxy system, leading the way to materials with advanced properties.
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•Epoxy curing accurately monitored through DSC instrumentation.•Model-free and model-fitting methods as valuable tools for curing kinetics study.•Isothermal curing predictions based solely on dynamic ramps.•Tracking glass transition temperature evolution.•Construction of TTT diagram for optimal curing conditions.
Effectively tailoring the temperature coefficient of resistance (TCR) is critical for multifunctional carbon nanotube (CNT) polymer composites with sensing capability. By developing a new multiscale ...percolation network model, this work reveals theoretically that the zero-TCR could be achieved by adjusting competing contributions from thermally assisted tunnelling transport at CNT junctions and thermal expansion of matrices. On the other hand, the negative temperature coefficient of nanocomposites above glass transition temperature could be greatly enhanced because the transport mechanism at the CNT junctions experienced a transition from tunnelling to hopping. Both tube-tube and/or tube-matrix interactions at conjunction and the structural distortion of nanotubes are considered in the newly proposed model. To validate the model, CNT/polymer nanocomposites with nearly constant resistance values (zero-TCR) below the glass transition temperature and a high TCR (98% resistance change ratio) resulting from the glass transition of the polymer matrix are successfully developed. The study also suggests that the desired parameters to achieve the zero-TCR property and the potential resistance change ratio could be improved by the glass transition in nanocomposites. This could be beneficial for the development of high quality sensing materials.
Weyl semimetal Td‐MoTe2 has recently attracted much attention due to its intriguing electronic properties and potential applications in spintronics. Here, Fe‐intercalated Td‐FexMoTe2 single crystals ...(0 < x < 0.15 ) are grown successfully. The electrical and thermoelectric transport results consistently demonstrate that the phase transition temperature TS is gradually suppressed with increasing x. Theoretical calculation suggests that the increased energy of the Td phase, enhanced transition barrier, and more occupied bands in 1T′ phase is responsible for the suppression in TS. In addition, a ρα–lnT behavior induced by Kondo effect is observed with x ≥ 0.08, due to the coupling between conduction carriers and the local magnetic moments of intercalated Fe atoms. For Td‐Fe0.15MoTe2, a spin‐glass transition occurs at ≈10 K. The calculated band structure of Td‐Fe0.25MoTe2 shows that two flat bands exist near the Fermi level, which are mainly contributed by the dyz and dx2−y2${{\rm{d}}_{{x^2} - {y^2}}}$ orbitals of the Fe atoms. Finally, the electronic phase diagram of Td‐FexMoTe2 is established for the first time. This work provides a new route to control the structural instability and explore exotic electronic states for transition‐metal dichalcogenides.
Fe‐intercalated Td‐FexMoTe2 single crystals (0 ≤ x ≤ 0.15) are grown successfully. The Fe intercalation significantly tunes the structural instability and brings about exotic electronic properties for the Weyl semimetal MoTe2, including the first observed Kondo effect and spin‐glass transition in its topologically nontrivial Td phase at low temperature.
The glass-transition temperature (Tg) of polymer thin films can be strongly influenced by the combined effects of the supporting solid substrate and the free surface. The relative importance of these ...two effects, which often compete with each other, depends on the strength of the substrate–film interactions. Utilizing an atomistically informed coarse-grained model for poly(methyl methacrylate) (PMMA), here we uncover the relationship between the substrate–film interfacial energy and the spatial distribution of Tg across thin films. We find that above a critical interfacial energy, the linear dependence of film Tg on the interfacial energy breaks down and film Tg attains an asymptotic value. Analyses on the spatial variation of Tg across the thin film reveal that the short-range interface near the cohesive surface generates a long-range interphase that leads a spatially uniform appreciation of Tg throughout the film, unlike weakly cohesive surfaces that show sharp gradients along the depth of film. These findings explain recent experiments and reveal a versatile approach for tuning film Tg via engineered substrate-film interactions.
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•Genetic algorithm achieves new polymer designs with high bandgap and high glass transition temperature.•Machine learning prediction models assist rapid evaluation of fitness ...function.•Chemical fragments leading high performance of polymers are highlighted.
Data driven or machine learning (ML) based methods have been recently used in materials science to provide quick material property predictions. Although powerful and robust, these predictive models are still limited in terms of their applicability towards the design of materials with target property or performance objectives. Here, we employ a nature-mimicking optimization method, the genetic algorithm, in tandem with ML-based predictive models to design polymers that meet practically useful, but extreme, property criteria (i.e., glass transition temperature, Tg>500 K and bandgap, Eg>6 eV). Analogous to nature, the characteristic properties of a polymer are assumed to be determined by the constituting types and sequence of chemical building blocks (or fragments) in the monomer unit. Evolution of polymers by natural operations of crossover, mutation, and selection over 100 generations leads to creation of 132 new (as compared to 4 already known cases) and chemically unique polymers with high Tg and Eg. Chemical guidelines on what fragments make up polymers with extreme thermal and electrical performance metrics have been selected and revealed by the algorithm. The approach presented here is general and can be extended to design polymers with different property objectives.
Broadening of glass transition and even two glass transitions in polymer ultrathin films were repeatedly reported in experiments. The glass transition and dynamic gradient in substrate-supported ...polymer films are studied by performing molecular dynamics simulations in this work. Two glass transitions are observed in ultrathin films with sufficiently strong attraction of the substrate. Due to the strong attraction of the substrate and spatial confinement of the ultrathin film, the adsorbed chains adopt a flattened conformation with slow dynamics, resulting in a pronounced dynamic gradient across the ultrathin film. The dynamics of chains near the free surface are suppressed by the adsorbed chain as the tail of a few of adsorbed chains is longer than the film thickness, therefore two glass transitions occur at higher temperatures than the bulk glass transition temperature. However, only a single glass transition is observed when the substrate attraction weakens or the film thickness increases.
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•Substrate's strong attraction yields large dynamic gradient in polymer thin film.•Glass transition temperature (Tg) increases with film thickness decreasing.•Two Tgs appear in ultrathin polymer film under strong attraction of substrate.
Vitrimers are a new type of polymers with promising applications in innovative materials. Epoxidized soybean oil (ESO) is an ideal candidate for vitrimer preparation owing to its abundant epoxy ...groups. However, preparing plant oil-based vitrimers with high glass transition temperatures (Tg) and strength remains challenging. A novel and fully bio-based vitrimer with a Tg above room temperature was synthesized from ESO and a rosin derivative-fumaropimaric acid (FPA) and exhibited excellent self-healing, shape memory, and reprocessing due to the presence of dynamic covalent bond exchange. The fully bio-based ESO-FPA vitrimer exhibited a Tg of 65 °C and a tensile strength of 16 MPa, which resulted from the rigid structure and tricarboxylic groups of FPA. The effect of transesterification reactions on the network structure was confirmed through stress relaxation. Our work expands the applications of commercial ESO in vitrimer materials and provides a novel approach to prepare bio-based vitrimers with desirable properties.
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•A bio-based epoxy vitrimer was fabricated by curing epoxy soybean oil (ESO) with fumaropimaric acid (FPA).•The introduction of FPA can effectively improve the glass transition temperature of ESO-based vitrimer above room temperature.•FPA is an efficient curing agent for increasing the mechanical properties of ESO-based vitrimer.•The vitrimer exhibited self-healing, shape memory and reprocessing due to the dynamic covalent bond exchange.
Nowadays, the glass transition kinetics is most commonly described in terms of the Tool–Narayanaswamy–Moynihan (TNM) model. Evaluation of one of the most prominent features of the structural ...relaxation motions—the relaxation nonlinearity—was traditionally done using the peak‐shift (PS) method. This paper introduces, for the first time, extensive testing of the PS method by means of theoretical simulations for all practically observable types of structural relaxation behavior and all types of glassy/amorphous materials (tested range of the glass transition temperatures: −55 to 1000°C; tested range of the relaxation activation energies: 300–1300 kJ⋅mol−1). For the majority of types of structural relaxation behavior, the PS method tends to slightly overestimate the value of the TNM nonlinearity parameter x (by ∼ 0.05–0.10). In the specific cases of the highly linear behavior (↑ x) combined with a broad distribution of relaxation times, the PS method systematically vastly underestimates the value of x. A new, improved temperature program was proposed for the PS method, eliminating the major intrinsic drawback of the originally proposed version of the PS methodology. In addition, based on the comparison between the theoretically simulated and real‐life experimental data, a new approach (based on the characteristic PS dependence shape) was introduced to estimate the width of the relaxation times distribution.
Homo-poly(ionic liquid) (HomoPIL) microsphere particles of poly2-(methacryloyloxy)ethyltrimethylammoniumbis(trifluoromethanesulfonyl)imide (PMTMATFSI) and ...poly2-(methacryloyloxy)ethyltrimethylammoniumhexafluorophosphate (PMTMAPF6), as well as their corresponding copolymer microsphere particles obtained through copolymerization with hexyl methacrylate (HMA), were synthesized as electrorheological (ER) materials. The morphology and structure of these synthesized particles were characterized using scanning electron microscopy (SEM), 1H nuclear magnetic resonance spectroscopy (1H NMR), Fourier-transform infrared spectroscopy (FT-IR), and small-angle X-ray scattering spectra (SAXS). Electrorheological measurements were conducted for these polyelectrolyte-based suspensions at room temperature, revealing that the HMA composition has a different effect on the ER effect of the two poly(ionic liquid)s. Differential scanning calorimetry (DSC) and temperature-modulated dielectric analysis (TMDA) were employed to investigate the effect of HMA composition on the glass transition temperature and interfacial polarization of these polyelectrolytes, suggesting that the incorporation of HMA affect the ER effect of PILs via changing the density of charge carriers and the counterion mobility. This study presents a potential approach for enhancing the electrorheological properties of poly(ionic liquid)s.
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•Copolymer microspheres of poly(ionic liquid) incorporating with HMA were synthesized as ER particles.•The incorporation of HMA improves the ER effect of TFSI-typed linear PILs, but reduces it of PF6-typed ones.•The flexible composition affects the ER effect of PILs via changing the mobility and concentration of counterions.
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