Showing papers on "Glass transition published in 2018"

A Self-Healable High Glass Transition Temperature Bioepoxy Material Based on Vitrimer Chemistry

Abstract: The design of high glass transition temperature (Tg) thermoset materials with considerable reparability is a challenge. In this study, a novel biobased triepoxy (TEP) is synthesized and cured with an anhydride monomer in the presence of zinc catalyst. The cured TEP exhibits a high Tg (187 °C) and comparable strength and modulus to the cured bisphenol A epoxy. By adopting the vitrimer chemistry, the cross-linked polymer materials are imparted significant stress relaxation and reparability via dynamic transesterification. It is noted that the reparability is closely related to the repairing temperature, external force, catalyst content, and the magnitude of rubbery modulus of the sample. The width of the crack from the cured TEP can be efficiently repaired within 10 min. This work introduces the first high-Tg biobased epoxy material with excellent reparability and provides a valuable method for the design of high-Tg self-healing materials suitable for high service temperature.

A nonconjugated radical polymer glass with high electrical conductivity

Abstract: Solid-state conducting polymers usually have highly conjugated macromolecular backbones and require intentional doping in order to achieve high electrical conductivities. Conversely, single-component, charge-neutral macromolecules could be synthetically simpler and have improved processibility and ambient stability. We show that poly(4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl), a nonconjugated radical polymer with a subambient glass transition temperature, underwent rapid solid-state charge transfer reactions and had an electrical conductivity of up to 28 siemens per meter over channel lengths up to 0.6 micrometers. The charge transport through the radical polymer film was enabled with thermal annealing at 80°C, which allowed for the formation of a percolating network of open-shell sites in electronic communication with one another. The electrical conductivity was not enhanced by intentional doping, and thin films of this material showed high optical transparency.

Silicon oxycarbide glasses and glass-ceramics: “All-Rounder” materials for advanced structural and functional applications

Abstract: Silicon oxycarbides can be considered as being carbon‐containing silicates consisting of glass networks in which oxygen and carbon share bonds with silicon. The carbon‐for‐oxygen substitution in silicate glass networks has been shown to induce significant changes in the network connectivity and consequently strong improvements in the properties of the silicate glass network. For instance, SiOC glasses exhibit Young's moduli, hardness values, glass transition, and crystallization temperatures which are superior to those of vitreous silica. Moreover, the silicon oxycarbide glass network exhibits unique structural features such as reduced mass fractal dimension and nano‐heterogeneity, which significantly affect and/or dictate its properties and behavior. In the present Review, a consideration of the current state of the art concerning the synthesis, processing, and various structural and functional properties of silicon‐oxycarbide‐based glasses and glass‐ceramics is done. Thus, the synthesis of silicon oxycarbides starting from macromolecular precursors such as polysiloxanes or alkoxysilanes‐based sol‐gel systems as well as current advances related to their processing will be critically reviewed. In addition, various structural and functional properties of silicon oxycarbides are presented. Specific emphasis will be put on the intimate correlation between the molecular architecture of the precursors and the structural features and properties of the resulting silicon oxycarbides.

High-Temperature and High-Energy-Density Dipolar Glass Polymers Based on Sulfonylated Poly(2,6-dimethyl-1,4-phenylene oxide).

Abstract: A new class of high-temperature dipolar polymers based on sulfonylated poly(2,6-dimethyl-1,4-phenylene oxide) (SO2 -PPO) was synthesized by post-polymer functionalization. Owing to the efficient rotation of highly polar methylsulfonyl side groups below the glass transition temperature (Tg ≈220 °C), the dipolar polarization of these SO2 -PPOs was enhanced, and thus the dielectric constant was high. Consequently, the discharge energy density reached up to 22 J cm-3 . Owing to its high Tg , the SO2 -PPO25 sample also exhibited a low dielectric loss. For example, the dissipation factor (tan δ) was 0.003, and the discharge efficiency at 800 MV m-1 was 92 %. Therefore, these dipolar glass polymers are promising for high-temperature, high-energy-density, and low-loss electrical energy storage applications.

Mode-coupling theory of the glass transition: A primer

Abstract: Understanding the physics of glass formation remains one of the major unsolved challenges of condensed matter science. As a material solidifies into a glass, it exhibits a spectacular slowdown of the dynamics upon cooling or compression, but at the same time undergoes only minute structural changes. Among the numerous theories put forward to rationalize this complex behavior, Mode-Coupling Theory (MCT) stands out as a unique framework that provides a fully first-principles-based description of glass phenomenology. This review outlines the key physical ingredients of MCT, its predictions, successes, and failures, as well as recent improvements of the theory. We also discuss the extension and application of MCT to the emerging field of non-equilibrium active soft matter.

Enhanced thermal conductivity and ideal dielectric properties of epoxy composites containing polymer modified hexagonal boron nitride

Abstract: Surface modification of chemically-inert hexagonal boron nitride (h-BN) to reduce its interfacial thermal resistance remains problematic, thereby hindering its application in thermal conductive composites. Here, poly(glycidyl methacrylate) (PGMA) chains were grafted onto the surface of h-BN by using a simple free radical polymerization. The prepared PGMA grafted h-BN (h-BN-PGMA) was incorporated into epoxy (EP) to enhance the thermal conductivity of EP composites. Adding 3, 9 or 15 vol% of h-BN-PGMA into EP leads to 60%, 203% or 505% increase s in thermal conductivity, respectively. Meanwhile, the surface modification of h-BN is benefit to enhance the compatibility between the fillers and EP matrix, which reduces the apparent viscosity of composite materials. Furthermore, compared with EP/h-BN, EP/h-BN-PGMA composites with the same filler-loading exhibit higher storage modulus and glass transition temperature. Additionally, the dielectric constant of the composites hardly depends on the testing frequency while the dielectric loss maintained at a very low level.

A facile access to stiff epoxy vitrimers with excellent mechanical properties via siloxane equilibration

Abstract: A new high performance stiff epoxy vitrimer based on siloxane equilibration has been fabricated in a simple way in which a polysiloxane oligomer containing aminopropyl side groups and catalytic potassium silanolate end groups was synthesized and used as a curing agent. The viscoelastic properties of the cured epoxy can be controlled by varying the concentration of potassium silanolate groups and the vitrimer possessing a complete stress relaxation behavior can be prepared, with relaxation times ranging from 376.8 s at 90 °C to 40.2 s at 170 °C. Due to the dynamic siloxane equilibration of polysiloxane catalyzed by potassium silanolate groups, this epoxy vitrimer exhibits excellent self-healing and recycling abilities. It can self-repair with a high healing efficiency, and be recycled at 130 °C within 40 min, retaining its original mechanical and thermal properties after at least four recycling cycles. Furthermore, such a material exhibits simultaneously a high service temperature (a glass transition temperature of 83 °C and an initial degradation temperature of 358 °C) and a strong (a stress at break of 46.6 MPa) and stiff (Young's modulus of 2.2 GPa) nature.

Conductive bio-polymer electrolyte iota-carrageenan with ammonium nitrate for application in electrochemical devices

Abstract: Bio-polymer electrolyte iota-carrageenan (Ι-carrageenan) with ammonium nitrate (NH4NO3) has been prepared by solution-casting technique. The increase in amorphous nature of the bio-polymer electrolyte due to the addition of salt has been confirmed by X-ray diffraction analysis. From the XRD spectra, the average particle size has been calculated using the Debye–Scherrer formula and the value is 0.98 nm. The complex formation between the polymer and the salt has been confirmed by Fourier transform infra-red spectroscopy. The glass transition temperature of the bio-polymer Ι-carrageenan with NH4NO3 electrolytes has been found using differential scanning calorimetric technique. From the AC impedance spectroscopic analysis, the ionic conductivity value has been found to be 1.46 × 10− 3 S/cm at ambient temperature for the composition of 1.0 g Ι-carrageenan: 0.4 wt% NH4NO3. The temperature dependent conductivity of the polymer electrolyte obeys an Arrhenius relationship. The dielectric behavior has been analyzed using dielectric permittivity (e*) and the relaxation frequency (τ) has been calculated from the loss tangent spectra (tan δ). The modulus spectra indicate non-Debye nature of the material. Ionic transference number has been found to be 0.95 for the polymer 1.0 g Ι-carrageenan: 0.4 wt% NH4NO3. The result reveals that the conducting species are predominantly due to ions. Electrochemical stability window of 2.46 V has been measured by using linear sweep voltammetry for the highest ionic conducting membrane. A primary proton battery has been constructed with the highest conductivity sample and open circuit voltage has been found to be 1.04 V. Fuel cell has been constructed with the highest proton conductivity polymer 1.0 g Ι-carrageenan: 0.4 wt% NH4NO3 and the open circuit voltage found to be 442 mV.

Revealing Hidden Structural Order Controlling Both Fast and Slow Glassy Dynamics in Supercooled Liquids

Abstract: A new theoretical analysis reveals hidden structural order driving the dynamics of glass-forming liquids, which indicates that the long-debated glass transition is thermodynamic in nature.

Mechanical glass transition revealed by the fracture toughness of metallic glasses

Abstract: The fracture toughness of glassy materials remains poorly understood. In large part, this is due to the disordered, intrinsically non-equilibrium nature of the glass structure, which challenges its theoretical description and experimental determination. We show that the notch fracture toughness of metallic glasses exhibits an abrupt toughening transition as a function of a well-controlled fictive temperature (Tf), which characterizes the average glass structure. The ordinary temperature, which has been previously associated with a ductile-to-brittle transition, is shown to play a secondary role. The observed transition is interpreted to result from a competition between the Tf-dependent plastic relaxation rate and an applied strain rate. Consequently, a similar toughening transition as a function of strain rate is predicted and demonstrated experimentally. The observed mechanical toughening transition bears strong similarities to the ordinary glass transition and explains the previously reported large scatter in fracture toughness data and ductile-to-brittle transitions. Understanding the fracture toughness of metallic glasses remains challenging. Here, the authors show that a fictive temperature controls an abrupt mechanical toughening transition in metallic glasses, and can explain the scatter in previously reported fracture toughness data.

Highly-Toughened Polylactide- (PLA-) Based Ternary Blends with Significantly Enhanced Glass Transition and Melt Strength: Tailoring the Interfacial Interactions, Phase Morphology, and Performance

Abstract: The inherent shortcomings of polylactide (PLA) including brittleness, low glass transition temperature, and melt strength during processing were addressed through a facile melt blending of PLA with polybutadiene-g-poly(styrene-co-acrylonitrile) (PB-g-SAN) core–shell impact modifier and poly(methyl methacrylate) (PMMA). Highly tough PLA-based ternary blends with drastically enhanced glass transition temperature (≈ 21 °C) and melt strength were successfully prepared. The effect of PMMA content (ranging from 0 to 30 wt %) on the phase miscibility, morphology, mechanical properties, thermal behavior, rheological properties, and toughening mechanisms of PLA/PB-g-SAN/PMMA blends with 30% PB-g-SAN was systematically investigated. It was found that PMMA can effectively tune the interfacial interactions, phase morphology and performance of incompatible PLA/PB-g-SAN blend owing to its partial miscibility with PLA matrix and miscibility with SAN shell of PB-g-SAN, as evidenced by DMTA analysis. Increase in PMMA cont...

Light-induced mechanical response in crosslinked liquid-crystalline polymers with photoswitchable glass transition temperatures

Abstract: Energy conversion of light into mechanical work is of fundamental interest in applications. In particular, diligent molecular design on nanoscale, in order to achieve efficient photomechanical effects on macroscopic scale, has become one of the most interesting study topics. Here, by incorporating a “photomelting” azobenzene monomer crosslinked into liquid crystalline (LC) networks, we generate photoresponsive polymer films that exhibit reversible photoswitchable glass transition temperatures (Tg) at room temperature (~20 °C) and photomechanical actuations under the stimulus of UV/visible light. The trans-to-cis isomerization of azo chromophores results in a change in Tg of the crosslinked LC polymers. The Tg of the polymer network is higher than room temperature in the trans-form and lower than room temperature in the cis-form. We demonstrate the photoswitchable Tg contribute to the photomechanical bending and a new mechanism for photomechanical bending that attributes the process to an inhomogeneous change in Tg of the film is proposed.

Glass-Transition Temperature of the β-Relaxation as the Major Predictive Parameter for Recrystallization of Neat Amorphous Drugs.

Abstract: Recrystallization of amorphous drugs is currently limiting the simple approach to improve solubility and bioavailability of poorly water-soluble drugs by amorphization of a crystalline form of the drug. In view of this, molecular mobility, α-relaxation and β-relaxation processes with the associated transition temperatures Tgα and Tgβ, was investigated using dynamic mechanical analysis (DMA). The correlation between the transition temperatures and the onset of recrystallization for nine amorphous drugs, stored under dry conditions at a temperature of 296 K, was determined. From the results obtained, Tgα does not correlate with the onset of recrystallization under the experimental storage conditions. However, a clear correlation between Tgβ and the onset of recrystallization was observed. It is shown that at storage temperature below Tgβ, amorphous nifedipine retains its amorphous form. On the basis of the correlation, an empirical correlation is proposed for predicting the onset of recrystallization for dr...

Molecular Origin of the Glass Transition in Polyelectrolyte Assemblies

Abstract: Water plays a central role in the assembly and the dynamics of charged systems such as proteins, enzymes, DNA, and surfactants. Yet it remains a challenge to resolve how water affects relaxation at a molecular level, particularly for assemblies of oppositely charged macromolecules. Here, the molecular origin of water’s influence on the glass transition is quantified for several charged macromolecular systems. It is revealed that the glass transition temperature (Tg) is controlled by the number of water molecules surrounding an oppositely charged polyelectrolyte–polyelectrolyte intrinsic ion pair as 1/Tg ∼ ln(nH2O/nintrinsic ion pair). This relationship is found to be “general”, as it holds for two completely different types of charged systems (pH- and salt-sensitive) and for both polyelectrolyte complexes and polyelectrolyte multilayers, which are made by different paths. This suggests that water facilitates the relaxation of charged assemblies by reducing attractions between oppositely charged intrinsic ...

A characteristic energy scale in glasses.

Abstract: Intrinsically generated structural disorder endows glassy materials with a broad distribution of various microscopic quantities - such as relaxation times and activation energies - without an obvious characteristic scale. At the same time, macroscopic glassy responses - such as Newtonian (linear) viscosity and nonlinear plastic deformation - are widely interpreted in terms of a characteristic energy scale, e.g., an effective temperature-dependent activation energy in Arrhenius relations. Nevertheless, despite its fundamental importance, such a characteristic energy scale has not been robustly identified. Inspired by the accumulated evidence regarding the crucial role played by disorder- and frustration-induced soft quasilocalized excitations in determining the properties and dynamics of glasses, we propose that the bulk average of the glass response to a localized force dipole defines such a characteristic energy scale. We show that this characteristic glassy energy scale features remarkable properties: (i) It increases dramatically in underlying inherent structures of equilibrium supercooled states approaching the glass transition temperature Tg, significantly surpassing the corresponding increase in the macroscopic shear modulus, dismissing the common view that structural variations in supercooled liquids upon vitrification are minute. (ii) Its variation with annealing and system size is very similar in magnitude and form to that of the energy of the softest non-phononic vibrational mode, thus establishing a nontrivial relation between a rare glassy fluctuation and a bulk average response. (iii) It exhibits striking dependence on spatial dimensionality and system size due to the long-ranged fields associated with quasilocalization, which are speculated to be related to peculiarities of the glass transition in two dimensions. In addition, we identify a truly static growing lengthscale associated with the characteristic glassy energy scale and discuss possible connections between the increase of this energy scale and the slowing down of dynamics near the glass transition temperature. Open questions and future directions are discussed.

Glass transition temperature of functionalized graphene epoxy composites using molecular dynamics simulation

Abstract: Molecular Dynamics (MD) simulations were carried out to investigate the effect of functionalization of graphene on glass transition temperature (Tg) of epoxy composites were built using LY556 epoxy...

Glass Forming Acceptor Alloys for Highly Efficient and Thermally Stable Ternary Organic Solar Cells

Abstract: The stability of donor:acceptor (D:A) semiconductor blends plays a key role in the development of solution-processed organic solar cells. One essential condition for both high-yield production and a long lifetime is excellent thermal stability. Recently, A1:A2 acceptor mixtures have received considerable attention and alloys of two miscible acceptors are singled out as a powerful tool for the design of efficient and durable organic solar cells. This progress report introduces a thermodynamic rationale for the superior thermal stability and reproducibility that is observed for some ternary blends. The increase in entropy upon mixing of several acceptors reduces the tendency for phase separation as well as crystallization, which facilitates the controlled formation of a fine blend nanostructure. Further, when combined with a high glass transition temperature many ternary blends can be readily quenched into a glassy state. Recent progress with regard to the thermal stability and efficiency of D:A1:A2 ternary blends is summarized in the light of the thermodynamic and kinetic arguments discussed in this article. Both, fullerene and fullerene-free acceptor alloys now yield solar cell efficiencies in excess of 10%, which indicates that ternary blends are a promising avenue that is poised to considerably enhance the prospect of organic photovoltaics.

Ultrastable metallic glasses formed on cold substrates.

Abstract: Vitrification from physical vapor deposition is known to be an efficient way for tuning the kinetic and thermodynamic stability of glasses and significantly improve their properties. There is a general consensus that preparing stable glasses requires the use of high substrate temperatures close to the glass transition one, Tg. Here, we challenge this empirical rule by showing the formation of Zr-based ultrastable metallic glasses (MGs) at room temperature, i.e., with a substrate temperature of only 0.43Tg. By carefully controlling the deposition rate, we can improve the stability of the obtained glasses to higher values. In contrast to conventional quenched glasses, the ultrastable MGs exhibit a large increase of Tg of ∼60 K, stronger resistance against crystallization, and more homogeneous structure with less order at longer distances. Our study circumvents the limitation of substrate temperature for developing ultrastable glasses, and provides deeper insight into glasses stability and their surface dynamics.

Effect of temperature, crystallinity and molecular chain orientation on the thermal conductivity of polymers: a case study of PLLA

Abstract: The crystallinity of semicrystalline polymers and molecular orientation of polymer have long been considered to be significant influencing factors on the thermal conductivity of polymer materials, but more clear-cut understanding on their impact on the thermal conductivity is still needed. In this work, poly-l-lactide (PLLA), whose crystallinity and orientation can be adjusted in a wide range, is selected to discuss the effect of degree of crystallinity and orientation on the thermal conductivity of PLLA. Meanwhile, the influence of temperature on the thermal conductivity is also discussed. PLLA compression-molded samples were heat-treated at 120 °C to tune the crystallinity of the samples, while the degrees of orientation were tuned by stretching the amorphous PLLA bars at 60 °C to different strains. It is found that environmental temperature of application affects the thermal conductivity obviously and the glass transition temperature of polymers shows a strong influence on the thermal conductivity of PLLA. Below Tg, the thermal conductivity of PLLA with different crystallinity increases with temperature and when the temperature is higher than Tg, the thermal conductivity of PLLA with different crystallinity decreases remarkably. It is also demonstrated that the thermal conductivity of PLLA increases with the increase in crystallinity, and the tensile strain linearly increases the thermal conductivity in the direction of molecular orientation and decreases the thermal conductivity in the perpendicular direction, which are in agreement with other semicrystalline polymers that has been reported.