Catalyst-Free α-Acetyl Cinnamate/Acetoacetate Exchange to Enable High Creep-Resistant Vitrimers.

Vitrimers represent an emerging class of polymeric materials that combine the desirable characteristics of both thermoplastics and thermosets achieved through the design of dynamic covalent bonds within the polymer networks. However, these materials are prone to creep due to the inherent instability of dynamic covalent bonds. Consequently, there are pressing demands for the development of robust and stable dynamic covalent chemistries. Here, we report a catalyst-free α-acetyl cinnamate/acetoacetate (α-AC/A) exchange reaction to develop vitrimers with remarkable creep resistance. Small-molecule model studies revealed that the α-AC/A exchange occurred at temperatures above 140 °C in bulk, whereas at 120 °C, this reaction was absent. For demonstration in the case of polymers, copolymers derived from common vinyl monomers were crosslinked with terephthalaldehyde to produce α-AC/A vitrimers with tunable thermal and mechanical performance. All resulting α-AC/A vitrimers exhibited high stability, especially in terms of creep resistance at 120 °C, while retaining commendable reprocessability when subjected to high temperatures. This work showcases the α-AC/A exchange reaction as a novel and robust dynamic covalent chemistry capable of imparting both reprocessability and high stability to cross-linked networks.

Enhanced age-hardening response and creep resistance of an Al-0.5Mn-0.3Si (at.%) alloy by Sn inoculation.

Precipitation-strengthening at ambient and high temperatures is examined in Al-0.5Mn-0.3Si (at.%) alloys with and without 0.02 at.% Sn micro-additions. Isochronal aging experiments reveal that Sn inoculation results in a pronounced age-hardening response: a hardening increment of 125 MPa is achieved at peak-aging (475 °C), which is five times greater than that of a Sn-free alloy. Scanning electron microscopy and synchrotron x-ray diffraction analyses demonstrate that, while the structure of the α-Al(Mn,Fe)Si precipitates formed in the peak-aged alloys is identical, their mean radius is smaller (R ~ 25 vs. 100-500 nm) and their number density is greater (~1021 vs. ~1019-20 m -3) in the Sn-modified alloy. Atom-probe tomography analyses reveal that the enhanced dispersion of the α-precipitates is related primarily to the formation of Sn-rich nanoprecipitates at intermediate temperatures, which act as nucleation sites for Mn-Si-rich nanoprecipitates. High-resolution transmission electron microscopy analyses demonstrate that these Mn-Si-rich nanoprecipitates exhibit icosahedral quasicrystal ordering (I-phase), which transform into the cubic-approximant α-phase upon peak aging. Significant Sn segregation at the semi-coherent interfaces of the α-precipitates in the peak-aged Sn-modified alloy is observed via APT, which promotes homogeneous nucleation of the I/α-precipitates at aging temperatures > 400 °C. At 300 °C, creep threshold stresses are observed in both alloys in the peak-aged state, which increases from ~30 MPa in the Sn-free alloy to ~52 MPa in the Sn-modified alloy. This boost in creep resistance is consistent with the enhanced aging response (higher Orowan stress).

Suppressing Creep and Promoting Fast Reprocessing of Vitrimers with Reversibly Trapped Amines.

We report a straightforward chemical strategy to tackle current challenges of irreversible deformation in low Tg vitrimers at operating temperature. In particular, vinylogous urethane (VU) vitrimers were prepared where reactive free amines, necessary for material flow, were temporarily shielded inside the network backbone, by adding a small amount of dibasic ester to the curing mixture. The amines could be released as reactive chain ends from the resulting dicarboxamide bonds via thermally reversible cyclisation to an imide moiety. Indeed, (re)generation of the required nucleophilic amines as network defects ensured reprocessing and rapid material flow at higher temperature, where exchange dynamics are (re)activated. As a result, VU vitrimers were obtained with limited creep at service temperature, yet with good reprocessability at elevated temperatures. Thus, by exerting strong control on the molecular level over the availability of exchangeable functional groups, a remarkable improvement of VU properties was obtained.

A Highly Dynamic Covalent Polymer Network without Creep: Mission Impossible?

Dynamic covalent polymer networks provide an interesting solution to the challenging recyclability of thermosets and elastomers. One of the remaining design constraints, however, is balancing thermal reprocessability in the form of material flow with dimensional stability during use. As a result, many chemistries are being investigated in order to improve bond reactivity control and material robustness. This Minireview highlights a number of promising concepts, with a particular emphasis on disconnecting chemical reactivity in low and high temperature regimes to obtain creep resistant, yet highly dynamic polymer networks. In addition, we will highlight the impact of sharp reactivity changes when applying extrapolation-based approaches during rheological analysis. As a result, we are confident that abandoning the myth of "permanent" reactivity will aid in the development of sustainable polymeric materials that can truly combine the benefits of thermoplastic and thermoset behaviour.

Enhanced oxidation resistance of SiC/SiC minicomposites via slurry infiltration of oxide layers.

SiC based composite materials commonly have protective silica surface in air. Under humid environments at high temperatures, like occur in jet engines, the silica surface layer reacts with water molecules to form volatile silicon hydroxide (Si(OH)4) and the protection is reduced which cause jet engine degradation. An alternative approach to protect SiC based composites would be to infiltrate the SiC matrix via slurry with an oxide material that is resistant to the high-temperature and humid environment. As proof of concept, aqueous based mullite particle slurries were infiltrated by pressurized flow and by capillarity of the wetting slurry on the external surface of the porous SiC matrix of single-fiber-tow SiC/SiC minicomposites. Minicomposites were precracked at room temperature during tensile tests then tested in tensile creep in air at 1200 °C to study the degree of protection that the infiltrated mullite provided at high temperatures. Next, fracture surfaces were examined using SEM.

Assembly of Branched Colloidal Nanocrystals in Polymer Films Leads to Enhanced Viscous Deformation Resistance.

Progress in the integration of nanocrystals with polymers has enabled the creation of materials for applications ranging from photovoltaics to biosensing. However, controlling the nanocrystal segregation and aggregation in the polymer phase remains a challenging task, especially because nanocrystals tend to form amorphous clusters inside the polymer matrix. Here, we present the ability of octapod-shaped particles to overcome their strong entropy-driven tendency to aggregate disorderly and form instead centipede-like linear arrays that are randomly oriented and fully embedded in polystyrene films upon controlled solvent evaporation. This behavior cannot be entirely described by short-range van der Waals interactions between the octapods in the polymer solution. An important role here is played by the increment of the viscosity of the medium during the evaporation of the solvent, which prevents disaggregation of the chains once they are formed. We show that increasing the octapod loading in the blends does not impact the length of the linear arrays beyond a critical length, while it favors instead chain demixing to form self-segregated regions of parallel interlocked chains. Our experiments evidence that softening of the polymer matrix by ex situ heating of the films induces a tail-to-tail coupling of the preformed chains and leads to the formation of longer linear structures of octapods, up to 2 µm long. The presence of 1D arrays of octapods in free-standing polystyrene films improves the creep response by a remarkable 37%, owing to an octapod pinning effect of the polymer matrix.

Effect of Sb Content on the Microstructure and Mechanical Properties of Eutectic SnPb Solder.

SnPb solder was widely used in electronic packaging for aerospace devices due to its high reliability. However, its creep resistance is poor and can be improved by adding alloying elements. The effects of Sb content on the microstructure, tensile, and creep properties of eutectic SnPb solder were investigated. Sb addition effectively improved the mechanical properties of the SnPb solder. When Sb content exceeds 1.7 wt.%, SbSn intermetallic compounds (IMCs) occurred. And increasing the Sb content increased the tensile strength. Furthermore, Sb addition decreased the steady-state creep rate and increased the stress exponent n, suggesting that the creep resistance had been enhanced, which may be attributed to the hindrance of dislocation movement by SbSn IMCs, as well as the reduction in phase boundaries, which consequently reduced grain boundary sliding.

Improvement in Cohesive Properties of Adhesion Systems Using Movable Cross-Linked Materials with Stress Relaxation Properties.

A strong, tough, and stable adhesion system used in various environments must be developed. A long-lasting adhesion system should effectively perform in the following five aspects: adhesion strength, toughness, energy dissipation property, self-restoration property, and creep resistance property. However, these properties are difficult to balance using conventional adhesives. Here, a new topological adhesion system using single-movable cross-network (SC) materials [SC(DMAAm) Adh] was designed. 3-(Trimethoxysilyl) propyl acrylate was used as the anchor, N,N-dimethyl acrylamide (DMAAm) was used as the main chain monomer, and γ-cyclodextrin (γ-CD) units acted as movable cross-links. The movable cross-links provided SC(DMAAm) Adh with energy dissipation properties, thereby improving its toughness. The γ-CD units also acted as bulky stoppers that provided a high adhesion strength and self-restoration properties. Moreover, the combination of the movable cross-links and bulky stoppers provided creep resistance to SC(DMAAm) Adh. The performance of the adhesion systems under different mobilities of the polymer chains was examined by adjusting the water content. In proper water-containing states, all mechanical properties of SC(DMAAm) Adh were better than those of the adhesion systems using homopolymers [P(DMAAm) Adh] and polymers with covalent cross-linking points [CP(DMAAm) Adh].

Dynamic Ablative Networks: Shapeable Heat-Shielding Materials.

Thermoset materials sacrifice recyclability and reshapeability for increased chemical and mechanical robustness because of an immobilized, cross-linked polymeric matrix. The robust material properties of thermosets make them well-suited for applications such as heat-shielding materials (HSMs) or ablatives where excellent thermal stability, good mechanical strength, and high charring ability are paramount. Many of these material properties are characteristic of covalent adaptable networks (CANs), where the static connectivity of thermosets has been replaced with dynamic cross-links. This dynamic connectivity allows network mobility while retaining cross-link connectivity to permit damage repair and reshaping that are traditionally inaccessible for thermoset materials. Herein, we report the synthesis of hybrid inorganic-organic enaminone vitrimers that contain an exceptionally high weight percent of polyhedral oligomeric silsesquioxane (POSS)-derivatives. Polycondensation of ß-ketoester-containing POSS with various diamine cross-linkers led to materials with facile tunability, shapeability, predictable glass transition temperatures, good thermal stability, and high residual char mass following thermal degradation. Furthermore, the char materials show notable retention of their preordained shape following decomposition, suggesting their future utility in the design of HSMs with complex detailing.

Effect of Mortise and Tenon Structure on the Properties of Wood Flour Polyvinyl Chloride-Laminated Veneer Lumber Co-Extruded Composites.

Core-shell composites with strong weather resistance, mechanical strength and creep resistance can be prepared using co-extrusion technology. Considering the weak bonding strength between core-shell interfaces, this study started from the concept of a mortise and tenon combination; three types of conical, rectangular and trapezoidal mortise and tenon joints were prepared, and their bending properties, long-term creep properties, interfacial bonding properties, and dimensional stability properties were tested. Results showed that the mortise and tenon structure could form a mechanical interlock between the outer-shell-layer polyvinyl chloride (PVC) wood-plastic composite (WPVC) and the inner-core-layer laminated veneer lumber (LVL), which could effectively improve the interface bonding property between the two layers. Among them, the trapezoidal mortise and tenon structure had the largest interface bonding force compared with the tapered and rectangular mortise and tenon structure, where the interface bonding strength reached 1.01 MPa. Excellent interface bonding can effectively transfer and disperse stress, so the trapezoidal mortise and tenon structure had the best bending properties and creep resistance, with a bending strength of 59.54 MPa and a bending modulus of 5.56 GPa. In the long-term creep test, the deformation was also the smallest at about 0.2%, and its bending properties, creep resistance and interface bonding performance were also the best. The bending strength was 59.54 MPa and the bending modulus was 5.56 GPa; in the long-term creep test, the strain curve was the lowest, about 0.2%. In addition, the mortise and tenon structure could disperse the stress of the inner shell LVL after water absorption and expansion, thus significantly improving the dimensional stability of the co-extruded composite after water absorption.

Evaluation on the Performance of Hydraulic Bitumen Binders under High and Low Temperatures for Pumped Storage Power Station Projects.

The high and low-temperature performance of five hydraulic bitumen binders was evaluated using the dynamic shear rheometer (DSR) test, infrared spectrum test and direct tensile (DT) test. These hydraulic bitumen binders were respectively applied for several pumped storage power stations (PSPS) projects that were constructed or under construction. In order to relate the bitumen performance to the mixture performance, the slope flow test, three-point bending test and thermal stress restrained specimen test were carried out on hydraulic asphalt mixtures. The test results indicated the DSR rheological master curves can well distinguish the difference of each bitumen binder as well as the effect of polymer modification. Phase angle master curves, black diagrams and infrared spectra all indicated that several penetration-grade hydraulic bitumen binders were not virgin bitumen binders but were modified with relatively lower SBS polymer content when compared with traditional SBS-modified bitumen. When selecting the commonly used Karamay SG70 hydraulic bitumen as a reference, the normal SBS-modified bitumen was superior to other bitumen in terms of low- and high-temperature performance. Several slightly SBS-modified bitumen binders did not always show consistent results, which indicated that slightly modified bitumen may not really have the desired performance as expected. Therefore, SBS-modified bitumen will be more promising when dealing with extremely low or high temperatures. Bitumen performance was well compared with the mixture performance by using the bitumen creep, relaxation and tensile failure strain corresponding to the asphalt concrete slope flow, the maximum bending strain and the failure temperature, respectively. Compared with the traditional penetration, softening point and ductility test, it indicated that the DSR rheological test, creep test, direct tensile test and stress relaxation test can be used as more powerful tools for the characterization and optimization of hydraulic bitumen binders.

Enhanced Elevated-Temperature Strength and Creep Resistance of Dispersion-Strengthened Al-Mg-Si-Mn AA6082 Alloys through Modified Processing Route.

In the present work, we investigated the possibility of introducing fine and densely distributed α-Al(MnFe)Si dispersoids into the microstructure of extruded Al-Mg-Si-Mn AA6082 alloys containing 0.5 and 1 wt % Mn through tailoring the processing route as well as their effects on room- and elevated-temperature strength and creep resistance. The results show that the fine dispersoids formed during low-temperature homogenization experienced less coarsening when subsequently extruded at 350 °C than when subjected to a more typical high-temperature extrusion at 500 °C. After aging, a significant strengthening effect was produced by ß″ precipitates in all conditions studied. Fine dispersoids offered complimentary strengthening, further enhancing the room-temperature compressive yield strength by up to 72-77 MPa (≈28%) relative to the alloy with coarse dispersoids. During thermal exposure at 300 °C for 100 h, ß″ precipitates transformed into undesirable ß-Mg2Si, while thermally stable dispersoids provided the predominant elevated-temperature strengthening effect. Compared to the base case with coarse dispersoids, fine and densely distributed dispersoids with the new processing route more than doubled the yield strength at 300 °C. In addition, finer dispersoids obtained by extrusion at 350 °C improved the yield strength at 300 °C by 17% compared to that at 500 °C. The creep resistance at 300 °C was greatly improved by an order of magnitude from the coarse dispersoid condition to one containing fine and densely distributed dispersoids, highlighting the high efficacy of the new processing route in enhancing the elevated-temperature properties of extruded Al-Mg-Si-Mn alloys.

The Effect of Service on Microstructure and Mechanical Properties of HR3C Heat-Resistant Austenitic Stainless Steel.

The physical metallurgical tests were performed on the test samples made of HR3C steel, taken from a section of a pipeline in the as-received condition and after approximately 26,000 h of service at 550 °C. In the as-received condition, the test material had austenitic microstructure with numerous large primary Z-phase precipitates inside the grains. The service of the test steel mainly contributed to the precipitation processes inside the grains and at the grain boundaries. After service, the following precipitates were identified in the microstructure of the test steel: Z-phase (NbCrN) and M23C6 carbides. The Z-phase precipitates were observed inside the grains, whereas M23C6 carbides - at the boundaries where they formed the so-called continuous grid. The service of the test steel contributed to the growth of the strength properties, determined both at room and elevated temperature (550, 600 °C), compared to the as-received condition. Moreover, the creep properties of HR3C steel after service were higher than those of the material in the as-received condition. The increase in the strength properties and creep resistance was connected with the growth of strengthening of the test steel by the precipitation of Z-phase and M23C6 carbides.

Fabrication of Photo-Crosslinkable Poly(Trimethylene Carbonate)/Polycaprolactone Nanofibrous Scaffolds for Tendon Regeneration.

BACKGROUND: The treatment of tendon injuries remains a challenging problem in clinical due to their slow and insufficient natural healing process. Scaffold-based tissue engineering provides a promising strategy to facilitate tendon healing and regeneration. However, many tissue engineering scaffolds have failed due to their poor and unstable mechanical properties. To address this, we fabricated nanofibrous polycaprolactone/methacrylated poly(trimethylene carbonate) (PCL/PTMC-MA) composite scaffolds via electrospinning. MATERIALS AND METHODS: PTMC-MA was characterized by nuclear magnetic resonance. Fiber morphology of composite scaffolds was evaluated using scanning electron microscopy. The monotonic tensile test was performed for determining the mechanical properties of composite scaffolds. Cell viability and collagen deposition were assessed via PrestoBlue assay and enzyme-linked immunosorbent assay, respectively. RESULTS: These PCL/PTMC-MA composite scaffolds had an increase in mechanical properties as PTMC-MA content increase. After photo-crosslinking, they showed further enhanced mechanical properties including creep resistance, which was superior to pure PCL scaffolds. It is worth noting that photo-crosslinked PCL/PTMC-MA (1:3) composite scaffolds had a Young's modulus of 31.13 ± 1.30 MPa and Max stress at break of 23.80 ± 3.44 MPa that were comparable with the mechanical properties of native tendon (Young's modulus 20-1200 MPa, max stress at break 5-100 MPa). In addition, biological experiments demonstrated that PCL/PTMC-MA composite scaffolds were biocompatible for cell adhesion, proliferation, and differentiation.

The Effect of Withdrawal Rate on Crystal Structure Perfection, Microstructure and Creep Resistance of Single Crystal Castings Made of CMSX-4 Nickel-Based Superalloy.

This study focuses on the evaluation of the crystal structure perfection in the single crystal made of CMSX-4 nickel superalloy and its effect on creep resistance. Single crystal castings were manufactured by directional solidification process at the withdrawal rate of 1, 3, 5 and 7 mm/min. Light (LM) and electron (SEM, TEM) microscopy, X-ray diffraction and Mossbauer spectroscopy were used for evaluation of the microstructure and crystal structure perfection. Castings were also subjected to creep tests. The best creep resistance was obtained for the casting manufactured at the withdrawal rate of 3 mm/min, characterized by the highest crystal structure perfection compared to the other castings examined.

Evolution of Elevated-Temperature Strength and Creep Resistance during Multi-Step Heat Treatments in Al-Mn-Mg Alloy.

The present work has systematically investigated the evolution of dispersoids and elevated-temperature properties including strength and creep resistance during various multi-step heat treatments in Al-Mn-Mg 3004 alloys. Results show that only the α-Al(MnFe)Si dispersoid is observed in the studied temperature range (up to 625 °C), and that it coarsens with increasing temperature to 500 °C, but dissolves at 625 °C. The evolution of elevated-temperature strength and creep resistance is greatly related to the temperature of each step during the multi-step heat treatments. Generally, lower temperature at the first-step heat treatment leads to higher properties, while the properties decrease with increasing temperature of last-step heat treatment. Suitable models have been introduced to explain the evolution of strength and the creep threshold stress at elevated-temperatures during the various heat treatments.

Drastically Enhancing Moduli of Graphene-Coated Carbon Nanotube Aerogels via Densification while Retaining Temperature-Invariant Superelasticity and Ultrahigh Efficiency.

Lightweight open-cell foams that are simultaneously superelastic, possess exceptionally high Young's moduli (Y), exhibit ultrahigh efficiency, and resist fatigue as well as creep are particularly desirable as structural frameworks. Unfortunately, many of these features are orthogonal in foams of metals, ceramics, and polymers, particularly under large temperature variations. In contrast, foams of carbon allotropes including carbon nanotubes and graphene developed over the past few years exhibit these desired properties but have low Y due to low density, ρ = 0.5-10 mg/mL. Densification of these foams enhances Y although below expectation and also dramatically degrades other properties because of drastic changes in microstructure. We have recently developed size- and shape-tunable graphene-coated single-walled carbon nanotube (SWCNT) aerogels that display superelasticity at least up to a compressive strain (ε) = 80%, fatigue and creep resistance, and ultrahigh efficiency over -100-500 °C. Unfortunately, Y of these aerogels is only ∼0.75 MPa due to low ρ ≈ 14 mg/mL, limiting their competitiveness as structural foams. We report fabrication of similar aerogels but with ρ spanning more than an order of magnitude from 16-400 mg/mL through controlled isostatic compression in the presence of a polymer coating circumventing any microstructural changes in stark contrast to other foams of carbon allotropes. The compressive stress (σ) versus ε measurements show that the densification of aerogels from ρ ≈ 16 to 400 mg/mL dramatically enhances Y from 0.9 to 400 MPa while maintaining superelasticity at least up to ε = 10% even at the highest ρ. The storage (E') and loss (E″) moduli measured in the linear regime show ultralow loss coefficient, tan δ = E″/E' ≈ 0.02, that remains constant over three decades of frequencies (0.628-628 rad/s), suggesting unusually high frequency-invariant efficiency. Furthermore, these aerogels retain exceptional fatigue resistance for 106 loading-unloading cycles to ε = 2% and creep resistance for at least 30 min under σ = 0.02 MPa with ρ = 16 mg/mL and σ = 2.5 MPa with higher ρ = 400 mg/mL. Lastly, these robust mechanical properties are stable over a broad temperature range of -100-500 °C, motivating their use as highly efficient structural components in environments with extreme temperature variations.