Increase In Crosslink Density Research Articles

Investigations on thermomechanical and structure properties of pure and wet polyimine networks by molecular dynamics simulations

Polyimine polymer is a kind of covalent adaptable networks with excellent reprocessing and self-healing properties. It has attracted wide attentions in recent years because it can release stress by reversible bond exchange reactions without using catalysts. Since polyimines containing -NH- groups are strongly hydrophilic, the influence of water content on their thermomechanical and structure properties is significant. In this study, we establish atomic models involving diamine and dialdehyde monomers combined with triamine cross-linkers to investigate the thermomechanical and structure properties of polyimine networks. Furthermore, different amounts of water molecules are added into networks to explore the interaction between polyimines and water molecules in aqueous polyimine systems. It is concluded that the increase of cross-linking density and conversion degree will improve the thermomechanical properties of polyimines. The water molecules in the aqueous polyimine systems will reduce glass transition temperature and enhance plasticity. Moreover, the diffusion coefficient is found to positively correlate with water content and temperature. Further researches on the structure of polyimines shows that the radius of gyration of polyimines and the radial distribution function of water molecules around the polyimines are also sensitive to temperature and water content. This investigation provides comprehensive information on different factors affecting the thermomechanical properties of pure and wet polyimine networks and contributes toward predicting the evolution of properties of polyimines under different circumstances.

Temperature-directed fluorescent switchable nanoparticles based on P3OT–PNIPAM nanogel composite

Poly(N-isopropylacrylamide) (PNIPAM) is a unique stimuli-responsive material that exhibits a lower critical solution temperature (LCST). Owing to this characteristic temperature-dependent behavior, PNIPAM has found extensive utilization as an active material in various applications, including sensors, drug delivery, and cellular imaging. Herein, we demonstrate temperature-directed fluorescent switchable nanoparticles based on poly(3-octylthiophene-2,5-diyl) (P3OT) nanoaggregate-embedded PNIPAM nanogel composites (POPNs) featuring different crosslinker contents. The amount of P3OT loading in the nanogel composites can be gradually controlled by varying the crosslinking density of the PNIPAM matrix; this may be attributable to the efficient entrapment of P3OT nanoaggregates in case of a dense polymeric network with the increase in crosslinking density. POPNs exhibit dramatic temperature-dependent fluorescence enhancement (by a factor of 2.11). This is based on the environmental changes affecting fluorescent P3OT chains at temperatures below and above the LCST of the PNIPAM matrix. Based on this temperature-directed fluorescent switching capability, POPN could find potential applications in various fields, including biomedical imaging and sensors.

Vanillin and organosilicon functionalized graphene oxide modified ester resin composite coatings with excellent anti-corrosion properties

Epoxy vinyl ester resin is widely used in the field of corrosion protection because of its excellent corrosion resistance and mechanical properties. However, due to the ease of hydrolysis of the ester group in the ester resin, the barrier ability of the coating is reduced, which will accelerate metal corrosion. The existing literature shows that the use of nano-sheet materials such as graphene oxide (GO) can effectively enhance the barrier properties of the coating. In order to solve the poor compatibility and electrical corrosion phenomenon of GO, in this work, renewable vanillin and silicone with excellent anti-corrosive properties were grafted to GO (APGO) by insertion reaction and D-A reaction. Subsequently, a series of new APGO-based epoxy vinyl ester resin composites with excellent mechanical properties and anti-corrosion properties were prepared by thermal crosslinking polymerization. The results showed that when the additive amount of APGO was 0.1 wt%, the comprehensive performance of the composite was the best: the conductivity (1.9 × 10−13–5.6 × 10−13 S/cm) of the composite was far lower than the critical electrical insulation value (10−9 S/cm), the breaking strength was improved by 38.0 %, the contact angle was increased by 65.7 %, and its anti-corrosion efficiency was up to 99.03 %. After 70 days of immersion in 3.5 wt% salt water, the impedance modulus of the composite at low frequency (Zf = 0.01 Hz) remained 2.23 × 1010 Ω cm2, which was 2 orders of magnitude higher than that of pure epoxy vinyl ester resin (2.43 × 108 Ω cm2). The above results were not only related to the maze effect formed by APGO, but also related to the increased crosslinking density of the system. This work provided a novel strategy for preparing GO-based composites with excellent comprehensive properties.

Computationally developed acrylated epoxidized methyl ester based pressure-sensitive adhesives

Healthcare sector is increasingly demanding pressure-sensitive adhesives (PSAs) to be bio-sourced and biodegradable to replace petro-based PSAs, which are neither renewable nor sustainable. This investigation employs MD simulations to develop bio-based PSAs by replacing low Tg monomers such as 2-ethylhexyl acrylate partially and fully with acrylated epoxidized methyl ester (AEME). The simulation procedures used in this study were verified by comparing specific properties with published literature. The glass transition temperatures of all developed systems show insignificant differences and are well below application as desirable for developing PSA products. Also, these values closely match Fox equation estimates and are in good accord with experimental values. Surface characteristics like surface energy and interfacial adhesion with Al substrate have been predicted. A ∼79 % increment in surface energy with AEME content ensures good substrate wettability and thermodynamic work of adhesion. The interaction energy with Al substrate has been increased by ∼365 % with addition of AEME reflecting improved adhesion strength and enhanced tackiness of the PSA copolymer. AEME provides polar groups like acrylate and hydroxyl groups, which helps improve adhesion strength. The polar groups associated with AEME and increased crosslinking density enhanced mechanical properties of developed PSA systems. This computational research lays the path for developing innovative bio-based PSAs used in adhesive bandages and transdermal patches.

Ageing behaviour and molecular/network structure evolution of EPDM/carbon black composites under compression and in thermal-oxidative environments

EPDM/carbon black (CB) composites were first prepared using sulfur as vulcanizing system, and the mechanical properties of EPDM/CB composites gradually increased with increasing CB content. Compressive stress-thermo oxidative ageing was performed in terms of three compressive stress levels and temperature conditions. During ageing process, two typical reactions of chains scission and crosslinking were deduced. On the one hand, degradation reaction of EPDM molecules occurred and unstable free radicals were produced under thermo/stress effect, which were oxidized to generate oxygenated compounds, leading to the increase of dangling chains ratio and the breakage of CB network with two-phase separation. On the other hand, the active free radicals were further consumed to initiate secondary crosslinking reaction, resulting in increase of crosslinking density and restriction of chains mobility. During the whole ageing process, the crosslinking reaction was dominant over chain scission reaction, resulting in the increase of crosslinking density and poor uniformity of network structure. Consequently, the surface/fractured surface morphologies of aged EPDM showed coarse features with obvious CB aggregates and the rupture of continuous CB network due to interfacial debonding, eventually leading to the remarkable deterioration of mechanical properties and decline of sealing resilience.

Investigating the Effect of Curing Activators on the Cure Kinetics of Acrylonitrile–Butadiene Rubber Filled with Graphene Oxide and Reduced Graphene Oxides Nanocomposites

For the first time, acrylonitrile–butadiene rubber (NBR)–graphene oxide (GO) and reduced graphene oxide (rGO) composites were prepared without cure activators: zinc oxide/stearic acid (ZnO/SA) and studied. The vulcanization characteristics of the compounds were systematically studied at 160–190°C, with the aid of rheometer and differential scanning calorimetry (DSC) techniques. NBR revealed rapid curing time (t90) with greater cure rate index compared with NBR–GO/rGO composites for the rheometer measurement. This results were in correspondence with the activation energies Ea (kJ/mol) calculated by Ozawa and Kissinger models of vulcanization kinetics. NBR–rGO obtained reduced t90 and Ea (kJ/mol) than NBR–GO, perhaps due to lower oxygenated groups: epoxide (–C–O–C–), carboxyl (–O–C=O), and hydroxyl (–OH) present. Although, the composites delayed in curing, they significantly recorded high tensile properties with high reinforcing factors than NBR. The order of increasing mechanical properties: NBR < NBR–rGO < NBR–GO followed the same order of increasing crosslinking density. In terms of tensile strength, NBR–GO-1 obtained 62.5% and 18.2% increment than NBR and NBR–rGO-1, respectively. The findings from this study indicate that the absence of ZnO/SA in rubber compounds may slow down curing of rubber–GO/rGO composites and lower networks compared with those containing activators ZnO/SA. However, optimization of ZnO/SA and with desired functional groups on graphene and derivative graphene sheets (GDS) including other proposed factors may enhance the curing speed of rubber–GDS based systems, without compromising their mechanical integrity for advanced applications.

A novel approach in wood waste utilization for manufacturing of catalyst-free polyurethane-wood composites (PU-WC)

In recent decades, due to the increase in environmental awareness and noticeable environmental degradation, the area of wood waste management has attracted increasing attention. The purpose of this study is to develop a new type of highly filled polyurethane wood-composite (PU-WC) by the utilization of large amount of wood wastes without addition of a catalyst. Although wood-plastic composites (WPCs) are widely known, there is still a lack of knowledge about WPCs with a polyurethane matrix. Obtained results showed that composites with a wood content of up to 80% show mechanical properties to commonly used MDF boards. This may be caused proper adhesion between polyurethane matrix and the wooden filler which was confirmed by scanning electron microscopy. The flexural strength of the manufactured composites varied between 19.25 and 25.11 MPa, while the flexural modulus varied between 966 and 1255 MPa. Dynamic mechanical analysis (DMA) of the composites showed a shift of Tgβ (from −70.3 to −52.3 °C) and Tgα (from 94.9 to 117.8 °C) to higher temperatures with increasing filler amount. The observed shift could be interpreted as a reduction of polymer chain mobility and an increase in cross-linking density of composites with a higher amount of wood. This is caused by chemical reactions between isocyanates and reactive hydroxyl groups on the wood surface. Thermogravimetric analysis has shown that PU-WC degrades in one step with Tmax at around 360 °C and T2% significantly reduce with greater addition of wood. Water absorption tests that water absorption strongly depends on wood content and varies between 13 and 80%. Moreover, cyclic water absorption tests showed no considerable difference between the water adsorption of samples after each cycle. Our work suggests that PU-WC can be used successfully as a potential substitute for wood or different types of wood-plastic composites (WPC).

Preparation of Cellulose Hydrogels and Hydrogel Nanocomposites Reinforced by Crystalline Cellulose Nanofibers (CNFs) as a Water Reservoir for Agriculture Use

An effective strategy for the management of available water and fertilizers is imperative to improve agricultural production and food quality, including releasing them sustainably to the soil and plants. Polymeric hydrogels with three-dimensional (3D) networks can absorb ample water and soluble fertilizers without network dissolution. Subsequently, the absorbed water and fertilizers can be released to arid and semi-arid agricultural land to furnish enough moisture and nutrients for plant growth. In this study, carboxymethyl cellulose sodium salt (NaCMC)- and hydroxyethyl cellulose (HEC)-based hydrogels have been synthesized using citric acid (CA) as a crosslinker. The Fourier transform infrared (FTIR) spectroscopy study revealed the formation of hydrogels with porous structures depicted by field emission scanning electron microscopy images. The prepared hydrogels have shown a remarkable swelling ratio of ∼1070% for the hydrogel prepared using NaCMC/HEC: 2/1 and ∼840% for NaCMC/HEC: 3/1 and a reduction in the swelling ratio with increasing crosslinking density. Moreover, crystalline (shown by X-ray diffraction) cellulose nanofibers (CNFs: ∼600 nm), prepared by acid (H2SO4) hydrolysis of cotton fibers, have been used to prepare hydrogel nanocomposites. The evaluation of hydrogel nanocomposites’ mechanical properties (universal testing matching test) depicted an improved tensile strength (16.27 MPa using 0.7% CNFs) compared to that of the hydrogel without CNFs. The soil burial test for biodegradation study of prepared hydrogels revealed soil degradation of hydrogel, confirmed by FTIR analysis and visual appearance. The current study provides comprehensive data establishing the potential of hydrogels for agriculture applications.

Entanglement in Smart Hydrogels: Fast Response Time, Anti‐Freezing and Anti‐Drying

AbstractThe common techniques to improve hydrogel's mechanical properties include increasing crosslinking density and forming crosslinked double‐network hydrogel, which may cause some hydrogels to lose their smart functionalities. Inspired by entanglement‐induced strengthening, a simple approach to introducing hydroxypropyl cellulose (HPC) fibers entangled with different smart hydrogel matrix systems are reported. Different from the conventional methods which hinder the movement of the polymer network, through entanglement with HPC fibers, the composite hydrogel shows both improved Young's modulus and toughness and more importantly improved smart functionalities including response speed, anti‐drying, and anti‐freezing capabilities and cycle stability. This strategy provides a new design rule to fabricate durable and strengthened smart hydrogels which can be used in smart windows, sensors, and soft robots.

Chemical Modification of Human Decellularized Extracellular Matrix for Incorporation into Phototunable Hybrid-Hydrogel Models of Tissue Fibrosis.

Tissue fibrosis remains a serious health condition with high morbidity and mortality rates. There is a critical need to engineer model systems that better recapitulate the spatial and temporal changes in the fibrotic extracellular microenvironment and enable study of the cellular and molecular alterations that occur during pathogenesis. Here, we present a process for chemically modifying human decellularized extracellular matrix (dECM) and incorporating it into a dynamically tunable hybrid-hydrogel system containing a poly(ethylene glycol)-α methacrylate (PEGαMA) backbone. Following modification and characterization, an off-stoichiometry thiol-ene Michael addition reaction resulted in hybrid-hydrogels with mechanical properties that could be tuned to recapitulate many healthy tissue types. Next, photoinitiated, free-radical homopolymerization of excess α-methacrylates increased crosslinking density and hybrid-hydrogel elastic modulus to mimic a fibrotic microenvironment. The incorporation of dECM into the PEGαMA hydrogel decreased the elastic modulus and, relative to fully synthetic hydrogels, increased the swelling ratio, the average molecular weight between crosslinks, and the mesh size of hybrid-hydrogel networks. These changes were proportional to the amount of dECM incorporated into the network. Dynamic stiffening increased the elastic modulus and decreased the swelling ratio, average molecular weight between crosslinks, and the mesh size of hybrid-hydrogels, as expected. Stiffening also activated human fibroblasts, as measured by increases in average cellular aspect ratio (1.59 ± 0.02 to 2.98 ± 0.20) and expression of α-smooth muscle actin (αSMA). Fibroblasts expressing αSMA increased from 25.8 to 49.1% upon dynamic stiffening, demonstrating that hybrid-hydrogels containing human dECM support investigation of dynamic mechanosensing. These results improve our understanding of the biomolecular networks formed within hybrid-hydrogels: this fully human phototunable hybrid-hydrogel system will enable researchers to control and decouple the biochemical changes that occur during fibrotic pathogenesis from the resulting increases in stiffness to study the dynamic cell-matrix interactions that perpetuate fibrotic diseases.

Improvement in alkali‐resistance of basalt fiber‐reinforced polymer by Ti3C2TX (MXene) modified matrix

AbstractFiber‐reinforced polymer (FRP) is a kind of potential construction material due to its low density and strong mechanical properties, but its weak alkali stability impedes its widespread use. MXene nanosheets' two‐dimensional (2D) property makes them an attractive contender as a nanofiller for polymer‐based composites. Here, a basalt fiber‐reinforced polymer (BFRP) laminate with a matrix modified with MXene nanosheets was made using a vacuum‐assisted resin‐transfer molding (VaRTM) process. The flexural strength (530.1 MPa) and flexural modulus (26.7 GPa) of the M0.5/BFRP composite were superior to those of the BFRP laminate, increasing by 39.4% and 4.6%, respectively. According to DMA examination, the MXene/BFRP composite exhibited increased crosslinking density, which could improve stability as well as aid in energy dissipation. As anticipated, the BFRP composites enhanced with MXene nanosheets demonstrated exceptional long‐term stability in a hostile alkali environment. After being submerged in alkali solution at 40°C for 60 days, the M0.5/BFRP's flexural strength reached 398.0 MPa, exceeding that of pristine BFRP that had not been submerged in alkali solution. This study gives an insight into the effect of MXene Ti3C2TX nanosheets modified resin on the durability of BFRP in strong alkali environment.

Thermal and Mechanical Characterization of Epoxy/Polyimide Blends via Postcuring Process

In this study, the effects of polyimide (PI) content and postcuring on thermal and mechanical properties in PI and epoxy (EP) blending systems were investigated. EP/PI (EPI) blending reduced the crosslinking density and improved the flexural and impact strength due to ductility. On the other hand, in the postcuring of EPI, the thermal resistance improved due to the increased crosslinking density and the flexural strength increased by up to 57.89% due to the enhanced stiffness, but the impact strength decreased by up to 59.54%. EPI blending induced the improvement in the mechanical properties of EP, and the postcuring process of EPI was shown to be an effective method to improve heat resistance. It was confirmed that EPI blending induces improvement in the mechanical properties of EP, and the postcuring process of EPI is an effective method for improving heat resistance.

Performance Enhancement of Kaolin/Chitosan Composite-Based Membranes by Cross-Linking with Sodium Tripolyphosphate: Preparation and Characterization.

A new family of environmentally friendly and low-cost membranes based on readily available mineral and polymeric materials has been developed from cast suspensions of kaolin and chitosan using aqueous phase separation and polyethylene glycol as a pore-forming agent. The as-fabricated membranes were further cross-linked with sodium tripolyphosphate (STPP) in order to strengthen the properties of the obtained samples. The functional groups determined by FTIR and EDX confirmed that the reaction occurred. A detailed study of the effects of cross-linking time on the physicochemical, surface and permeation properties showed that a 30-minute reaction enabled the composite membrane to be stable in acidic media (up to pH 2) and increased the mechanical strength twofold compared to the non-cross-linked membrane. A similar morphology to that generally observed in polymeric membranes was obtained, with a sponge-like surface overlaying a finger-like through structure. The top layer and cross-section thicknesses of the membranes increased during STPP post-treatment, while the pore size decreased from 160 to 15 nm. At the same time, the molecular weight cut-off and permeance decreased due to the increase in cross-linking density. These results observed in a series of kaolin/chitosan composite membranes showed that STPP reaction can provide control over the separation capability range, from microfiltration to ultrafiltration.

Co‐catalytic system design for deriving reactive diluent to construct robust benzoxazine resin with microphase‐separated structure

AbstractExploitation a facile and effective strategy for preparing benzoxazine resins with outstanding processability and robust mechanical properties still exists a challenge. In this work, a co‐catalytic concept was proposed to fabricate high‐performance benzoxazine resin by employing curing agent to derive reactive diluent to construct microphase‐separated system. Given on the design, the reactive diluent dissolved solid benzoxazine into a low‐viscosity solution, after which the curing agent presented a catalytic effect on diluent and benzoxazine to optimize the curing crosslinking reactions. As a result, it not only increased crosslinking density for a stable mechanical network, but also induced a microphase‐separated structure to effectively dissipate energy for a strong yet tough resin system, as proved by Fourier transform infrared, differential scanning calorimetry, and dynamic mechanical analyses measurements. Moreover, the as‐prepared modified benzoxazine resin showed great potential to fabricated composites with glass fiber, in which the values of tensile strength, elongation at break, and toughness presented an improvement of 212.1%, 106.8%, and 75.0% compared to composite with pristine PH‐ddm and glass fiber, respectively. These results suggested an effective path to improve the processability and overall mechanical properties of benzoxazine resins, opening a new strategy to develop high‐performance benzoxazine materials.

The study of composition-properties relationships for composite hydrogels based on poly(methacrylic acid) and high concentrations of MFI zeolite

High concentrations of hydrophobic MFI zeolite (26.0–51.3 wt%) were successfully embedded in poly(methacrylic acid) (PMAA) network in simple, one-pot synthesis of PMAA-MFI composite hydrogels. The effects of zeolite concentration regarding pure PMAA xerogel, for all of the investigated properties (primary structural parameters, morphological properties, water swelling ability, thermal degradation mechanism and stability, and mechanical properties) were determined and discussed. The PMAA-MFI composite hydrogels had remarkably higher density than PMAA xerogel (1660/1420 kg m−3), higher crosslinking density (29/1.0 × 10−4), improved mechanics in the dry and swollen state (up to 4.9 times higher storage modulus) and lower swelling degree (42/420 g g−1), average molar mass between the network crosslinks (3.6/110 × 104 g mol−1), and distance between the macromolecular chains (64/710 nm). Despite high zeolite concentration, the composites kept porous hydrogel structure with uniformly dispersed zeolite particles. FTIR spectra disclosed the existence of functional groups of PMAA network and MFI zeolite that interacted mutually and led to increased crosslinking density. The effect of zeolite concentration on the thermal degradation mechanism was established. The composition-properties correlations were determined and described by the adequate linear or power law equations revealing the additive nature of the composites and the fractal structure of the networks. These correlations enable the achievement of desired properties by simple adjustment of the initial composition, i.e. zeolite concentration. Therefore, this work presents the base for further development of tailor-made PMAA-MFI composite hydrogels aimed for specific application.

Smart Pellets for Controlled Delivery of 5-Fluorouracil.

This work aimed to develop a new one-pot and readily scaled-up formulation capable of retaining 5-fluorouracil and prolonging its release to obtain a site-specific medication delivery for the potential treatment of colorectal cancer. Six polymer-based formulations were successfully produced using a thermal bulk polymerization method and loaded with 5-fluorouracil, which is a chemotherapeutic agent used in the treatment of colorectal carcinoma. The pellets produced were characterized by measuring the glass transition temperature, tensile strength, Young's modulus, and tensile elongation at break. Studies on in vitro swelling and release were carried out in phosphate-buffered saline to evaluate the behaviour of the developed system. The Young's modulus, glass transition temperature, and tensile strength all increased significantly as the crosslinker concentration increased, but the fracture strain value reduced significantly. The in vitro swelling profile of the produced formulations was significantly reduced by increasing crosslinking density. Less than 27% cumulative drug release was achieved for all formulations after 5 h of starting the release study. The highest cumulative drug release reached after 24 h was 69%. The developed drug delivery system demonstrated the ability to delay the release of 5-fluorouracil in upper gastrointestinal tract-mimicking conditions, while permitting its release in a controlled way afterward, which makes it promising for the potential delivery of 5-fluorouracil to the colon.

Evaluation of the interactions of typical wood extracts on the bonding performance of soybean-based adhesives

The substitution of environmentally friendly, renewable soybean-based adhesives (SBAs) for formaldehyde-based synthetic resins to produce formaldehyde-free wood composites is of great significance for resource recycling and environmental protection. However, SBA-bonded wood composites often exhibit great variations in bonding properties, due to the diverse interactions of the extractive components in wood. In this study, the extracts of four typical commercial fast-growing timbers were isolated, and their effects on the structures and properties of the SBAs were systematically investigated using FTIR, NMR, TGA, contact angle, buffer capacity, sol–gel test, and plywood evaluation. The analytical data indicated that some of the wood extracts could cause important effects on the bonding properties of the SBAs by impacting the PAE cross-linking capacity and unfolding degree of the globular soybean proteins. Fast-growing hardwood extracts from poplar and eucalyptus timbers containing active groups resulted in increased cross-linking density and improved water resistance of the SBAs. Meanwhile, fast-growing softwood extracts from Chinese fir and Scotch pine containing inert hydrophobic components had slight impacts on PAE cross-linking, but resulted in the agglomeration of unfolded soybean proteins. This formed a weak boundary layer on the wood surface, causing a significant decrease in the water resistance of the SBAs by >35%. These findings presented available analytic approaches of wood extract–SBA interactions and useful guidance for the proper applications of SBAs in the commercial production of wood composites.

Physicochemical, mechanical and electrical properties of epoxy-based syntactic foam under thermal and hydrothermal aging conditions

In this study, epoxy resin containing hollow polymeric microspheres as an insulating material was synthesized and subjected to thermal and hydrothermal aging. Scanning electron microscopy (SEM) and three-dimensional computed tomography (3D CT) were used to examine the internal structures of the specimens as they aged. Thermogravimetric analysis (TGA) and Fourier transform infrared spectra (FT-IR) helped to elucidate the physical and chemical changes associated with aging. Tensile, flexural, dynamic mechanical analysis (DMA), and other mechanical tests were used to examine the mechanical properties of the syntactic foams (SFs) as they aged. Electrical tests such as the leakage current and breakdown strength were used to measure how effectively the SF materials maintained their insulating characteristics with aging. By comparing the aged and unaged specimens, the aging mechanism of the SF was elucidated. The findings demonstrated that the epoxy resin matrix composite foam packed with hollow polymeric microspheres had little effect on thermal aging and exhibited good hydrothermal aging resistance. The microstructures of the specimens subjected to thermal aging barely changed; however, hydrothermal aging resulted in dramatic changes. Notably, the comprehensive properties of the SF increased during thermal aging and hydrothermal aging, possibly due to an increase in secondary crosslinking density after a certain period of aging. The thermal and hydrothermal aging specimens had maximum tensile strengths of 14 and 13 MPa, respectively. The tensile strength dropped as aging time increased to 11.6 and 7 MPa, respectively. Furthermore, while the breakdown strengths of the hydrothermal samples continued to fall, the breakdown strengths of the thermal specimens initially increased and subsequently decreased, with a maximum value of 40.76 kV/mm. This was possibly related to the fact that the crosslinking reaction dominated the thermal aging process, whereas hydrothermal aging had numerous implications on the characteristics of the SF materials. This study showed that the SF filled with hollow polymeric microspheres had the potential for long-term durability in hot and humid environments, supporting its application as an insulation material in composite insulation cores. • The aging-time related micromorphology, physicochemical, mechanical and electrical performance of the syntactic foam are investigated by accelerated thermal aging and hydrothermal aging tests. • Both thermal and hydrothermal aging diminish the characteristics of specimens, while hydrothermal aging exhibits a more serious threat to the overall performance. • The trend of water absorption of epoxy resin-based syntactic foam shows non-Fick law. • The results of three-dimensional computed tomography reveal that thermal and hydrothermal aging increase the number and length of pores.

A Study on the Preparation and Cavitation Erosion Mechanism of Polyether Polyurethane Coating.

Polyurethane elastomers are anticipated to be applied in the field of cavitation erosion (CE) resistance, but their protection and damage mechanisms are not clear, which greatly restricts their further development. In this article, five polyether polyurethanes (PUx) with different crosslinking densities were prepared. Their mechanical properties, thermal properties, water absorption, surface morphology and chemical structure before and after CE tests were compared with ESEM, OM, TG-DSC, FTIR and XPS in detail. The results showed that with an increase in crosslinking density, the tensile strength of PUx increased first and then decreased, elongation at break and water absorption reduced gradually and thermal decomposition temperature and adhesion strength increased steadily. During the CE process, cavitation load aggravated the degree of microphase separation and made brittle hard segments concentrate on the coating surface; meanwhile, cavitation heat accelerated hydrolysis, pyrolysis, oxidation and the fracture of molecular chains. As a result, the mechano-thermal coupling intensified the formation and propagation of fatigue cracks, which should be the fundamental reason for the CE damage of polyurethane elastomer. PU0.4 exhibited the best CE resistance among the five coatings thanks to its good comprehensive properties and may find potential applications on the surface of hydraulic components.

Tensile performance and viscoelastic properties of rubber nanocomposites filled with silica nanoparticles: A molecular dynamics simulation study

Understanding the relationship between the macro-properties and microstructure of rubber and rubber nanocomposites (RNCs) is crucial for the design and optimization of relevant rubber composite products. Herein, we explore the mechanical and viscoelastic properties of rubber and RNCs filled with silica nanoparticles by using atomistic molecular dynamics simulation. It is found that increasing cross-linking density can greatly enhance the tensile performance of pure rubber, and reduce the permanent deformation and hysteresis loss. The addition of silica nanoparticles causes significant changes in the static distribution, movement, and bond orientation, and the Tg of RNCs increases by more than 6% when the volume fraction of NPs reaches 10%. At fixed volume fraction of NPs, decreasing the NP size can further enhance the mechanical properties of RNCs, and the maximum tensile stress can increase by 50%. In addition, cross-linking in the RNCs can improve the viscoelastic properties, and more highly crosslinked RNCs present larger storage modulus and loss modulus, and smaller loss angle. This work gives an in-depth understanding of the effect of microstructure on the mechanical properties of RNCs.