Showing papers on "Glass transition published in 2022"

A hyperbranched P/N/B-containing oligomer as multifunctional flame retardant for epoxy resins

Abstract: Flame-retardant epoxy resins (EPs) with superior optical, mechanical and dielectric properties are highly desired in high-tech industries. In this work, a multifunctional hyperbranched additive (BDHDP) was synthesized for EPs. Our results showed that BDHDP catalyzed the curing of epoxy resin because of its tertiary amine and hydroxyl groups. At a low addition level (<3.0 wt%), BDHDP increased the glass-transition temperature and maintained the optical transmittance of epoxy thermoset. Meanwhile, BDHDP improved the mechanical strength and toughness, and reduced the dielectric constant and loss of EP because of the rigid phosphaphenanthrene groups and intra-molecular cavities. Moreover, BDHDP reduced the heat release and smoke generation during the EP combustion. Adding 1.5 wt% of BDHDP led to a UL-94 V-0 rating, and reduced the total smoke production by 16.4%. Hence, this study offers an effective method to create transparent EP thermosets with outstanding mechanical, dielectric and fire-retardant properties via incorporating a P/N/B-containing hyperbranched oligomer.

Metal–Organic Network-Forming Glasses

Abstract: The crystal-liquid-glass phase transition of coordination polymers (CPs) and metal-organic frameworks (MOFs) offers attractive opportunities as a new class of amorphous materials. Unlike conventional glasses, coordination chemistry allows the utilization of rational design concepts to fine-tune the desired properties. Although the glassy state has been rare in CPs/MOFs, it exhibits diverse advantages complementary to their crystalline counterparts, including improved mass transport, optical properties, mechanical properties, and the ability to form grain-boundary-free monoliths. This Review discusses the current achievements in improving the understanding of anomalous phase transitions in CPs/MOFs. We elaborate on the criteria for classifying CP/MOF glasses and comprehensively discuss the three common strategies employed to obtain a glassy state. We include all CP/MOF glass research progress since its inception, discuss the current challenges, and express our perspective on future research directions.

Imide-DOPO derivative endows epoxy resin with excellent flame retardancy and fluorescence without losing glass transition temperature

Abstract: The aggregation and plasticizing of most 9,10-dihydro-9-oxa-10-phosphaphenanthrene −10-oxide (DOPO) derived additives often result in deteriorating the glass transition temperature and mechanical properties of epoxy resins (EP). To address this issue, an imide-DOPO derivative (BMP) with bulky aromatic structure was successfully synthesized. Benefitting from the space hindering and intermolecular interaction (π-π stacking) of BMP, it does not lead to the loss of glass transition temperature, and only a minor damage of tensile and impact strengths is caused. Additionally, with only 0.78 wt% loading of phosphorus (10 wt% BMP), the EP/BMP10% achieves UL-94 V-0 and high LOI value of 35%. Simultaneously, both the heat release rate and total smoke production are markedly reduced. More excitingly, BMP exhibits aggregation-induced emission enhancement of fluorescence in the N, N-dimethylformamide/water mixed solution as well as epoxy resin. This work provides a new insight in simultaneously imparting flame retardancy and fluorescence to epoxy resins without losing glass transition temperature.

Redesigned Hybrid Nylons with Optical Clarity and Chemical Recyclability.

Abstract: Aliphatic polyamides, or nylons, are typically highly crystalline and thermally robust polymers used in high-performance applications. Nylon 6, a high-ceiling-temperature (HCT) polyamide from ε-caprolactam, lacks expedient chemical recyclability, while low-ceiling temperature (LCT) nylon 4 from pyrrolidone exhibits complete chemical recyclability, but it is thermally unstable and not melt-processable. Here, we introduce a hybrid nylon, nylon 4/6, based on a bicyclic lactam composed of both HCT ε-caprolactam and LCT pyrrolidone motifs in a hybridized offspring structure. Hybrid nylon 4/6 overcomes trade-offs in (de)polymerizability and performance properties of the parent nylons, exhibiting both excellent polymerization and facile depolymerization characteristics. This stereoregular polyamide forms nanocrystalline domains, allowing optical clarity and high thermal stability, however, without displaying a melting transition before decomposition. Of a series of statistical copolymers comprising nylon 4/6 and nylon 4, a 50/50 copolymer achieves the greatest synergy in both reactivity and polymer properties of each homopolymer, offering an amorphous nylon with favorable properties, including optical clarity, a high glass transition temperature, melt processability, and full chemical recyclability.

Exploring the physical properties of PVA/PEG polymeric material upon doping with nano gadolinium oxide

Abstract: Nano gadolinium oxide doped polyvinyl alcohol/polyethylene glycol blend (PVA/PEG) was prepared by sol–gel and casting methods. Rietveld refinement method was used for structural and microstructural investigation. The X-ray diffraction (XRD), scanning electron microscope (SEM) and Fourier transform infrared (FTIR) techniques were applied to study the formation of nano Gd2O3 doped PVA/PEG-blend. Differential scanning calorimetry technique (DSC) was utilized to investigate the glass transition, the melting and degradation temperatures of undoped and doped PVA/PEG blends. The melting temperature was increased slightly while the degradation temperature was reduced upon doping with nano Gd2O3. The behavior of the absorbance and other different optical parameters were studied in detail using UV-spectrophotometer. A variation in the optical bandgap of the PVA/PEG blend was observed while the values of refractive index and extinction coefficient were enhanced as the amount of Gd2O3 increased in the blend. The influence of nano Gd2O3 doping on the different dispersion parameters was examined using Wemple and DiDomenico approximation model. The nature of electronic transitions of different samples was examined by comparing the optical band gap obtained from Tauc’s relation and dielectric loss parameter.

Engineering and evaluation of thermostable IsPETase variants for PET degradation.

Abstract: Polyethylene terephthalate (PET) is a mass-produced petroleum-based synthetic polymer. Enzymatic PET degradation using, for example, Ideonella sakaiensis PETase (IsPETase) can be a more environmentally friendly and energy-saving alternative to the chemical recycling of PET. However, IsPETase is a mesophilic enzyme with an optimal reaction temperature lower than the glass transition temperature (Tg) of PET, where the amorphous polymers can be readily accessed for enzymatic breakdown. In this study, we used error-prone PCR to generate a mutant library based on a thermostable triple mutant (TM) of IsPETase. The library was screened against the commercially available polyester-polyurethane Impranil DLN W 50 for more thermostable IsPETase variants, yielding four variants with higher melting points. The most promising IsPETaseTMK95N/F201I variant had a 5.0°C higher melting point than IsPETaseTM. Although this variant showed a slightly lower activity on PET at lower incubation temperatures, its increased thermostability makes it a more active PET hydrolase at higher reaction temperatures up to 60°C. Several other variants were compared and combined with selected previously published IsPETase mutants in terms of thermostability and hydrolytic activity against PET nanoparticles and amorphous PET films. Our findings indicate that thermostability is one of the most important characteristics of an effective PET hydrolase.

Highly disordered VO2 films: appearance of electronic glass transition and potential for device-level overheat protection

Abstract: In this work, the phase transition of a highly disordered amorphous VO2 film is studied. It is found that the electronic transport behavior follows the Arrhenius or Vogel–Tammann–Fulcher model, showing different thermal sensitivity under different thicknesses. Based on it, the concept of a prototype device with overheating protection capability is demonstrated. The results, reflecting the relationship between phase transition and structural disorder, open up a unique pathway to understand the metal insulator transition in strongly correlated electronic systems and to its functionality in electronic devices.

Microscopic origin of excess wings in relaxation spectra of supercooled liquids

Abstract: Glass formation is encountered in diverse materials. Experiments have revealed that dynamic relaxation spectra of supercooled liquids generically become asymmetric near the glass transition temperature, $T_g$, where an extended power law emerges at high frequencies. The microscopic origin of this "wing" remains unknown, and was so far inaccessible to simulations. Here, we develop a novel computational approach and study the equilibrium dynamics of model supercooled liquids near $T_g$. We demonstrate the emergence of a power law wing in numerical spectra, which originates from relaxation at rare, localised regions over broadly-distributed timescales. We rationalise the asymmetric shape of relaxation spectra by constructing an empirical model associating heterogeneous activated dynamics with dynamic facilitation, which are the two minimal physical ingredients revealed by our simulations. Our work offers a glimpse of the molecular motion responsible for glass formation at relevant experimental conditions.

Ultrastable glasses: new perspectives for an old problem

Abstract: Abstract Ultrastable glasses (mostly prepared from the vapor phase under optimized deposition conditions) represent a unique class of materials with low enthalpies and high kinetic stabilities. These highly stable and dense glasses show unique physicochemical properties, such as high thermal stability, improved mechanical properties or anomalous transitions into the supercooled liquid, offering unprecedented opportunities to understand many aspects of the glassy state. Their improved properties with respect to liquid-cooled glasses also open new prospects to their use in applications where liquid-cooled glasses failed or where not considered as usable materials. In this review article we summarize the state of the art of vapor-deposited (and other) ultrastable glasses with a focus on the mechanism of equilibration, the transformation to the liquid state and the low temperature properties. The review contains information on organic, metallic, polymeric and chalcogenide glasses and an updated list with relevant properties of all materials known today to form a stable glass.

Synergistic Stimulation of Metal-Organic Frameworks for Stable Super-cooled Liquid and Quenched Glass.

Abstract: Metal-organic framework (MOF) glasses are a fascinating new class of materials, yet their prosperity has been impeded by the scarcity of known examples and limited vitrification methods. In the work described in this report, we applied synergistic stimuli of vapor hydration and thermal dehydration to introduce structural disorders in interpenetrated dia-net MOF, which facilitate the formation of stable super-cooled liquid and quenched glass. The material after stimulus has a glass transition temperature (Tg) of 560 K, far below the decomposition temperature of 695 K. When heated, the perturbed MOF enters a super-cooled liquid phase that is stable for a long period of time (>104 s), across a broad temperature range (26 K), and has a large fragility index of 83. Quenching the super-cooled liquid gives rise to porous MOF glass with maintained framework connectivity, confirmed by EXAFS and PDF analysis. This method provides a fundamentally new route to obtain glassy materials from MOFs that cannot be melted without causing decomposition.

High-entropy induced a glass-to-glass transition in a metallic glass

Abstract: Abstract Glass-to-glass transitions are useful for us to understand the glass nature, but it remains difficult to tune the metallic glass into significantly different glass states. Here, we have demonstrated that the high-entropy can enhance the degree of disorder in an equiatomic high-entropy metallic glass NbNiZrTiCo and elevate it to a high-energy glass state. An unusual glass-to-glass phase transition is discovered during heating with an enormous heat release even larger than that of the following crystallization at higher temperatures. Dramatic atomic rearrangement with a short- and medium-range ordering is revealed by in-situ synchrotron X-ray diffraction analyses. This glass-to-glass transition leads to a significant improvement in the modulus, hardness, and thermal stability, all of which could promote their applications. Based on the proposed high-entropy effect, two high-entropy metallic glasses are developed and they show similar glass-to-glass transitions. These findings uncover a high-entropy effect in metallic glasses and create a pathway for tuning the glass states and properties.

Self-healing and polymer welding of soft and stiff epoxy thermosets via silanolates

Abstract: Abstract Incorporating dynamic bonds into polymers enables static thermosets to be transformed into active materials, possessing the reprocessability of thermoplastics while maintaining the bulk properties of fully crosslinked networks. This new class of materials, termed covalent adaptable networks (CANs), has helped bridge the gap between traditional thermosets and thermoplastics. Here, epoxy-based adaptable networks were synthesized by combining a diepoxide monomer with an oligosiloxane prepolymer containing aminopropyl groups, which crosslink irreversibly, and silanolate end-groups, which participate in dynamic bonding. Two separate diepoxide crosslinkers were used to give a range of soft to stiff materials with a Young’s modulus varying from 12 MPa to 2.2 GPa. This study documents how the thermal and mechanical properties (e.g., glass transition temperature and modulus) are affected by compositional changes in these silanolate networks. Dynamic bonding also results in self-healing properties, offering the ability to repair structural polymers and composites. When combined with tunable mechanical properties, self-healing capabilities make these materials well-suited to be sustainable alternatives for many traditional thermosets. For example, we demonstrated the ability to weld a stiff epoxy thermoset to a dissimilar soft material, a feature traditional epoxies do not permit. Graphical abstract

Study of microplastics with semicrystalline and amorphous structure identification by TGA and DSC.

Abstract: The potential of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) to identify, distinguish among the most relevant microplastics, and quantification, has been studied, including semicrystalline polymers (polypropylene (PP), polyamide (PA), high density polyethylene (HDPE), low density polyethylene (LDPE) and polyethylene terephthalate (PET)) and amorphous polymers (polystyrene (PS) and Polyvinyl Chloride (PVC)). The identification has been carried out by analyzing the decomposition, melting and glass transition temperatures. Both amorphous polymers (PVC and PS) can be properly identified by DSC using the glass transition temperature. Semicrystalline polymers, PA, PET, PP, HDPE and LDPE can be identified by using the melting temperature without the interference of other polymers, neither semicrystalline nor amorphous. The main limitation of the evaluated techniques is to distinguish between the semicrystalline polymer LDPE and the amorphous PS, as LDPE melting temperatures overlap with PS glass transition temperature, and this parameter would be useful only if one of these two polymers is present in the sample to be analyzed, since interference with other polymers (PA, PP, PET, LDPE, PVC and PS) do not exist. If both LDPE and PS are present in the sample, the best but not ideal option would be to analyze the decomposition temperature curves, since overlapping is weak in this case. • Amorphous polymers can be identified by their glass transition temperature. • Quantification is possible Integrating differentiate glass transition temperature. • Differential Scanning Calorimetry is a suitable technic to identify microplastics.

Dielectric Properties of Fluorinated Aromatic Polyimide Films with Rigid Polymer Backbones

Abstract: Fluorinated aromatic polyimide (FAPI) films with rigid polymer backbones have been prepared by chemical imidization approach. The polyimide films exhibited excellent mechanical properties including elastic modulus of up to 8.4 GPa and tensile strength of up to 326.7 MPa, and outstanding thermal stability including glass transition temperature (Tg) of 346.3–351.6 °C and thermal decomposition temperature in air (Td5) of 544.1–612.3 °C, as well as high colorless transmittance of >81.2% at 500 nm. Moreover, the polyimide films showed stable dielectric constant and low dielectric loss at 10–60 GHz, attributed to the close packing of rigid polymer backbones that limited the deflection of the dipole in the electric field. Molecular dynamics simulation was also established to describe the relationship of molecular structure and dielectric loss.