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Due to the good extracellular matrix biomimetic properties, silk fibroin (SF) hydrogels have been extensively concerned and studied in the fields of tissue engineering scaffold and 3D cell culture. The most widely used ones among them are mild chemical crosslinked SF hydrogels. However, as the inevitable movement and self-assembly of hydrophobic segments of silk fibroin in such hydrogels, the conformation of silk fibroin transforms from random coil to the β-sheet conformation with lower energy level, which will dynamically change the basic characteristics (such as the stiffness and pore size) of the hydrogels. Consequently, unknown and non-negligible impacts on the biomedical application of the hydrogel were induced. Based on this perspective, the SF hydrogel, which was crosslinked by horseradish peroxidase in a mild manner, was chosen as model materials. The strategy of changing related crosslinking density was used to successfully regulate the rate of protein conformational transition in SF hydrogels. At the same time, the effect of protein conformational transition rate in such an SF hydrogel on the proliferation of cells adhering to its surface was investigated. Results showed that the rate of protein conformational transition in corresponding hydrogels was inversely related to the crosslinking density. The lower the crosslinking density was, the faster the conformational transition was. Meanwhile, the proliferation of mouse embryonic fibroblasts (NIH3T3) cells on the hydrogel surface was generally positively correlated with the rate of protein conformational transition in corresponding hydrogels. However, as it also existed the effects of the accompanied initial crosslinking densities, the final trend was that the total number of cells on hydrogels with medium conformational transition rate was relatively low, and that on hydrogels with the fastest conformational transition rate was relatively high. [ABSTRACT FROM AUTHOR]

Stimuli‐responsive hydrogels are of great interest in many biomedical applications, from drug delivery to tissue engineering. In this work, thermo‐responsive poly(N‐isopropylacrylamide) (PNIPAM) hydrogels are prepared by adding different amounts of crosslinking agent (N,N′‐methylenebisacrylamide, BIS) in a two‐step‐freezing polymerization method, which is simple and potentially able to reduce fabrication times and costs. The hydrogel swelling behavior and the temporal evolution of the volume variation in response to temperature changes are investigated. Results demonstrate how the thermo‐responsive behavior of these materials is related to the content of the BIS crosslinking agent, which is helpful to guide the synthesis of dynamic hydrogels with tunable functional properties. [ABSTRACT FROM AUTHOR]

In order to analysis the effect of temperature on thermal oxidative aging behavior of latex sponge. The morphology and structural changes of latex sponge after aging directly affect its product use value, and its aging behavior is closely related to temperature. This study is not limited to mechanical properties. By measuring the changes of whiteness index, yellowness index, crosslink density, and indentation hardness of latex sponge at 70 ℃ and 120 ℃, the thermal oxidative aging behavior of materials under different temperature conditions was analyzed. The experimental results show that the peak of the yellowness index is reached at 86 hours at 120 ℃, the pore structure disappears and residue powder is formed on the surface. The crosslinking density is initially increased first and subsequently decreased, and the indentation hardness is sharply increased in a short time. Under the condition of 70 ℃, a slow decrease in the whiteness index of the material is noted. The other indexes display a slow upward trend, and the complete pore structure is retained. It is concluded that the change procedure and final shape of the latex sponge morphology and structure during the aging process under the condition of 120 ℃ are significantly different from those under the relatively low temperature condition. The product value is rapidly lost due to thermal oxidative aging behavior, and the thermal oxygen aging resistance of 80NR latex sponge needs improvement. [ABSTRACT FROM AUTHOR]

The thermo-oxidative aging of rubber products is inevitable during their use and leads to product failure and can even endanger safety. Oxygen is an important factor that cannot be ignored during the thermo-oxidative aging process. Thus, the gas barrier property of rubber products is of significant concern. In this work, a strategy of crystallizing rubber in unfilled rubber composites was designed by firstly constructing a dual synergistic mechanism of crosslinking density and crystallization on the oxygen barrier properties. As a crystallizable polymer, trans-butadiene-co-isoprene rubber (TBIR) shows dendritic fibril crystals or spherulites in natural rubber (NR)/TBIR vulcanizates. Meanwhile, the vulcanizates containing TBIR have a higher crosslinking density than NR vulcanizates. These TBIR-rich crystals and high-crosslinking-density structures are distributed in vulcanizates like continuous islands. Contrary to what has been reported in the literature, the decrease in oxygen permeability of NR/TBIR is not only due to the high crosslinking density and free volume of the polymer matrix, but more importantly, the spherulites of TBIR play a role in blocking and prolonging the oxygen diffusion path during the diffusion of oxygen in the polymer composites. We propose that the compatible crystalline polymer can replace the lamellar filler, play the role of the oxygen barrier in rubber composites, reduce the diffusion and dissolution of oxygen, and achieve the effect of improving the thermo-oxidative aging property of the rubber composite. Future research will follow the morphology evolution of TBIR crystals, their crosslinking structure and density, and interactions between TBIR and NR on the oxygen barrier and thermo-oxidative aging property. [ABSTRACT FROM AUTHOR]

Polyurethane (PU) materials with high strength, toughness, anti‐wear properties, and high heat resistance are in high demand for dynamic sealing applications. In this study, a series of high‐performance crosslinking polyurethanes (CPUs) were prepared by pre‐polymerization of polycarbonate diol (PCDL) with toluene‐2,4‐diisocyanate (TDI), followed by crosslinking with trimethylolpropane (TMP). The molar ratio (R) of PCDL/ TDI and the degree of cross‐linking were found to significantly affect the microphase separation. The results indicate that as the R‐value increases, the transition temperature of CPUs increases from 6.0°C to 82.4°C, and the tensile strength increases from 21.1 ± 2.5 MPa to 46.0 ± 3.6 MPa. Moreover, the CPUs exhibit excellent heat resistance (with a heat resistance index of ~150°C) and hydrolysis resistance, attributed to the high crystallinity of PCDL and the synergistic effect of chemical crosslinking. Additionally, the CPUs demonstrate low friction coefficient (COF = 0.01–0.062), exceptional anti‐wear properties (Akron wear, 0.11–0.07 cm3/1.61 km), and outstanding fatigue resistance. These desirable characteristics make CPUs suitable for use as dynamic sealing or lubricating materials in harsh conditions, thereby extending the service life of seals and preventing leakage and other related issues. [ABSTRACT FROM AUTHOR]

Copyright of Acta Materiae Compositae Sinica is the property of Acta Materiea Compositae Sinica Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Benzoxazine resin shows great potential as matrix for high performance composites, possessing good processabilty, dielectric property, mechanical property, and thermal properties, but its further application is limited by low crosslinking density and poor curing activity, especially for mono‐cyclic benzoxazines. In this work, melamine, a tertiary amine with a stable triazine structure, was introduced into the aldehyde‐containing mono‐cyclic benzoxazine, in order to achieve higher crosslinking density, higher curing activity, and better thermal properties. The chemical structure of prepolymers was verified by Fourier transform infrared spectroscopy (FTIR), 1H nuclear magnetic resonance (1H NMR) spectra, the curing behavior was investigated by differential scanning calorimetry, the thermal properties of polymers were studied by thermogravimetric analysis (TGA) and thermal mechanical analysis (TMA). It was found that the introduction of trifunctional melamine (MA) could lower curing temperature and promote the curing degree. Additionally, a proper amount of MA could increase the crosslinking density and thermal properties of the resultant polybenzoxazine. For example, poly(BZ‐3MA) showed the highest thermal stability with the temperature at 5% weight loss (Td5) of 415°C, carbon residue (Yc) of 67.7%, 23°C, and 2.9% higher than those of poly(BZ). Moreover, the glass transition temperature of poly(BZ‐5MA) reached 204.06°C, 36.51°C higher than that of poly(BZ). [ABSTRACT FROM AUTHOR]

Although there have been some studies on the preparation and characteristics of nano‐Fe3O4‐filled polyurethane (PU) composites, research focusing on the quantitative influence of the degree of microphase separation (DPS) and the crosslinking density (CLD) on dynamic viscoelastic properties is rare. In this paper, PU composites are synthesized by incorporating nano‐Fe3O4 as fillers. The dynamic mechanical properties under alternating loads are investigated using dynamic mechanical analysis. The influences of the DPS and the CLD on the dynamic hysteresis loss are analyzed. The experimental results show that the DPS and CLD of PU nanocomposites increase and then decrease with the increase of nano‐Fe3O4 addition and reached a maximum at the addition amount of 1.0 wt%. The quantitative relationships between DPS, CLD and the dynamic hysteresis loss of PU nanocomposites at different filler ratios are obtained. With a 1.0 wt% nano‐Fe3O4 content, the strain energy density (SED) and hysteresis energy density (HED) are increased by 416% and 354%, respectively. The hysteretic loss H indexes of all composites are kept lower than that of pure PU. These characteristics make nano‐Fe3O4‐filled PU a promising material for reducing the rolling resistance and increasing the service life of fuel‐saving tire materials. [ABSTRACT FROM AUTHOR]

Cars are the most important mode of transportation and play a significant role in the development of modern communities. Tyres, one of the main components of cars, hold strategic importance. Therefore, research aimed at improving the quality and increasing the production speed of tyres is an integral part of the strategic policy on this subject. In this study, the effects of pressing time in curing press (time of curing with pressure) on the structural properties of radial tyre tread were investigated. According to the one-factor response surface methodology, 10 samples that had different pressing times during their curing press were produced. The swelling test was used to study the cured compound structure. In this way, essential parameters such as diffusion coefficient and crosslinking density were calculated. The results showed that the vulcanised samples in free pressure conditions had the lowest amounts of crosslinking density. Also, these samples had the highest intrinsic diffusion coefficient. It is also observed that the intrinsic diffusion coefficient and crosslinking density have a relationship with pressing time. Mechanical tests were also performed to determine the dependence of the mechanical properties of the tyre tread on the duration of pressing time. As an obvious result, the cured samples in zero-pressure conditions had the lowest tensile strength, tear strength, hardness and the highest amount of resilience percentage. However, the abrasion parameter had the most negligible dependence on the pressing time. The results showed the dependence of the swelling and mechanical properties of the tread tyre compound on the pressing time. So, according to the optimal targeting for different variables and results, the optimal time of applying pressure during curing for the tyre tread compound has been obtained. As an important result, in the curing process, the pressurising can be stopped after about 14 min while the curing continues only with heat. [ABSTRACT FROM AUTHOR]

The relationship between crosslinking density and storage modulus of chloroprene rubber (CR) / ethylene-propylene-diene rubber (EPDM) blend was studied and the fitting model was established. The relationship between the crosslinking density, storage modulus and stress relaxation properties of each component in the blended rubber was also investigated. The results showed that the applicable temperature range of the model was 35-80 ℃ and the crosslinking density of the two phases were linearly related to the storage modulus. When m(CR)/m(EPDM) was 70/30, the predicted value of the storage modulus model of CR/EPDM blend was close to the measured value, which proved that the model had high reliability. The effect of CR phase on the total storage modulus of CR/EPDM blend was greater. The stress relaxation property of the blend had a linear relationship with the crosslinking density and storage modulus of each component.

With the growing volume of waste rubber products, the pressures on the environment and resources become increasingly serious. The reclamation of waste rubber products is one of the practical methods to meet this challenge. Therefore, the application of reclaimed rubber (RR) attracts researchers' attention. In this work, different contents of natural rubber (NR) matrix are replaced by RR and the reinforcing behavior of the RR phase is discussed. The results show that the crosslinking density, Payne effect and 100% modulus increase with increase in RR content in the NR/RR blends when the RR content is not more than 60 phr, and then drop when the RR content gets to 80 phr. It is inferred that both the NR and the RR constitute co‐continuous phases when the RR content is 60 phr, and the RR phase is the only continuous phase when its content reaches 80 phr, which can be confirmed by SEM observation. Besides, the curing rate is enhanced with increase of the RR content. It is also found that structures with high crosslinking density appear when the RR is incorporated into the NR matrix. Finally, it is proposed that carbon black in the RR, a structure with high crosslinking density and a continuous rubber matrix are important factors influencing the reinforcement behaviour in RR filled rubber composites. © 2023 Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

This study investigated the relationship between the parameters of the DLP manufacturing process and the structure of photopolymerizable acrylic resins. Four different process parameters were established to produce different thin-walled acrylic sample series: exposure time, layer thickness, area offset, and number of transition layers. The structure and the surface of the obtained samples were examined with the use of the FTIR–ATR method and an optical microscope, respectively. It was proved that extension of the exposure time increases the density of crosslinking and sample thickness. A decreasing crosslinking density due to rising layer thickness is observed. The area offset affects only the dimensions of the sample, predictably reducing the dimensions of the sample as the compensation increases. The absence of transition layers proved unfavorable in many respects, both structurally and geometrically. [ABSTRACT FROM AUTHOR]

The aging behavior and degradation mechanism of fluoroelastomers (FKM) after exposure to thermal air, commercial lubricating oil, and the corresponding base oil at elevated temperatures were investigated. The FKMs immersed in oil and exposed to air are denoted by FKM‐O and FKM‐A, respectively. The crosslinking densities of the samples were determined by equilibrium swelling and dynamic mechanical analyses, and the results were different. Compared to the performances of FKM‐A, noticeable changes in the mechanical and swelling performances of FKM‐O were observed, attributed to the severe deterioration induced by both the lubricating oil and the high‐temperature. The FTIR results revealed that the dehydrofluorination induced by the oil caused the fracture of the CF bonds, and the degradation of the oil itself exerted negative impacts on the properties of the FKM. The findings of this study will facilitate the development of FKMs with degradation‐resistant properties for various applications. [ABSTRACT FROM AUTHOR]

A way to improve high-temperature mechanical behavior and reduce conductivity of direct-current cable insulation material is introduced by using low-crosslinked linear low density polyethylene (LLDPE). LLDPE and low density polyethylene (LDPE) with different dicumyl peroxide (DCP) content are analyzed. The mechanical properties of LLDPE and LDPE with DCP are assessed by hot-set test, dynamic mechanical analysis (DMA) and tensile test. The DC conductivity and DC breakdown strength of these materials are also tested. The results show that crosslinked LLDPE behaves better than LDPE does in elongation in the range from 0.7 wt.% to 2.0 wt.% DCP addition, and corssslinked LLDPF meets the requirements of HVDC cable insulation for crosslinked polyethylene (XLPE) elongation with 0.7 wt.% DCP addition. In this condition, LLDPE with 0.7 wt.% DCP addition is chosen to be a substitute for traditional XLPE for the purpose of minimizing by-products. DMA and tensile test results verify that LLDPE with 0.7 wt.% DCP addition has higher Young's modulus and elongation at break than traditional XLPE does. DC conductivity and DC breakdown experiments show that LLDPE with 0.7 wt.% DCP addition has low conductivity, good temperature stability of conductivity and DC breakdown strength at the temperature range from 30 ℃ to 90 ℃. As the crosslinking by-products in the actual cable are difficult to be removed completely and unevenly distributed, it seriously affects the long-term safe operation of the cable. The low crosslinked LLDPE is recommended as the replacement of the traditional XLPE for the insulation material in HVDC cable insulation system, so as to reduce DC conductivity and enhance the temperature stability of conductivity while meeting high temperature mechanical properties.

In a multi-component system, the interfacial interactions between the various components trigger the behavior of the composite under external loading and thus determine the bulk properties of the material. In the present work, composite laminates made from TiO2 modified epoxy reinforced with glass fiber are subjected to thermal loading and the effort is made to analyze the interfacial interactions through the experimental values of storage modulus, loss modulus and tan delta. The loading of TiO2 nanoparticles in the epoxy resin is varied to also understand the effect of nanoparticle addition on the interfacial interactions through DMA. The addition of TiO2 nanoparticles helped to increase the storage and loss modulus by improving the rigidity of the polymeric chain. To understand the effect of varying the loading of TiO2 nanoparticles and to gauge the optimum wt.% addition of such nanoparticle, Tan delta, the degree of entanglement and the coefficient of the effectiveness of the nanofiller is found and the results indicate that the laminates successfully withstand the applied thermal load when the TiO2 loading is kept at 2 wt.%. This optimum nanoparticle loading wt.% is further validated through the adhesion factor which is found with the help of Tan delta values. [ABSTRACT FROM AUTHOR]

The aging behavior and degradation mechanism of nitrile rubber (NBR) under free and compression states were investigated in simulated air and hydraulic oil environments at various elevated temperatures. The variations in the crosslinking density, volatile components and the chemical structure of NBR during aging were studied using equilibrium swelling testing and dynamic mechanical analysis (DMA), pyrolysis‐gas chromatography–mass spectrometry and attenuated total reflection‐Fourier transform infrared spectroscopy. The migration of additives in different media and the crosslinking density of NBR aged in different conditions were compared. The tensile strength, DMA, and thermogravimetric analysis results were consistent with the changes in the crosslinking density of NBR and the migration of additives. Four effects of the simulated hydraulic system on the thermal aging of NBR were examined. Both the compression stress and the oil medium slowed the aging of NBR. The results can provide new insights into the use of NBR products in lubricating oil environments and NBR lifespan prediction in hydraulic working conditions. [ABSTRACT FROM AUTHOR]

Two kinds of latex sponge (80 NR and 0 NR) with different natural latex contents were exposed to 100 ℃ by accelerated hot air aging method. Their thermal-oxidative aging behavior was investigated by using yellowness index, whiteness index, crosslinking density and indentation hardness, and the hot air aging rule of the materials was discussed. The results showed that the yellowness index of 80 NR was always higher than that of 0 NR, while the whiteness index was always lower than the latter. The crosslinking density of 80 NR was always bigger than that of 0 NR, so the latex sponge of 80 NR was more prone to molecular chain breakage and crosslinking reaction. The indentation hardness of 80 NR was affected obviously by aging time, and would increase sharply in a short time. The indentation hardness of 0 NR was linearly correlated with aging time and had a small increase. The aging resistance of 0 NR was better than that of 80 NR.

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Copyright of China Rubber Industry is the property of Editorial Office of China Rubber Industry and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This study investigated the relationship between the parameters of the DLP manufacturing process and the structure of photopolymerizable acrylic resins. Four different process parameters were established to produce different thin-walled acrylic sample series: exposure time, layer thickness, area offset, and number of transition layers. The structure and the surface of the obtained samples were examined with the use of the FTIR–ATR method and an optical microscope, respectively. It was proved that extension of the exposure time increases the density of crosslinking and sample thickness. A decreasing crosslinking density due to rising layer thickness is observed. The area offset affects only the dimensions of the sample, predictably reducing the dimensions of the sample as the compensation increases. The absence of transition layers proved unfavorable in many respects, both structurally and geometrically.

The thermo-oxidative aging of rubber products is inevitable during their use and leads to product failure and can even endanger safety. Oxygen is an important factor that cannot be ignored during the thermo-oxidative aging process. Thus, the gas barrier property of rubber products is of significant concern. In this work, a strategy of crystallizing rubber in unfilled rubber composites was designed by firstly constructing a dual synergistic mechanism of crosslinking density and crystallization on the oxygen barrier properties. As a crystallizable polymer, trans-butadiene-co-isoprene rubber (TBIR) shows dendritic fibril crystals or spherulites in natural rubber (NR)/TBIR vulcanizates. Meanwhile, the vulcanizates containing TBIR have a higher crosslinking density than NR vulcanizates. These TBIR-rich crystals and high-crosslinking-density structures are distributed in vulcanizates like continuous islands. Contrary to what has been reported in the literature, the decrease in oxygen permeability of NR/TBIR is not only due to the high crosslinking density and free volume of the polymer matrix, but more importantly, the spherulites of TBIR play a role in blocking and prolonging the oxygen diffusion path during the diffusion of oxygen in the polymer composites. We propose that the compatible crystalline polymer can replace the lamellar filler, play the role of the oxygen barrier in rubber composites, reduce the diffusion and dissolution of oxygen, and achieve the effect of improving the thermo-oxidative aging property of the rubber composite. Future research will follow the morphology evolution of TBIR crystals, their crosslinking structure and density, and interactions between TBIR and NR on the oxygen barrier and thermo-oxidative aging property.

Crosslinking hydrogel membranes were prepared by esterification reaction with different ratios of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA). The purpose of the study was to research the effect of the crosslinking agent PAA on the hydrogel properties of the membranes used for soilless cultivation. Increasing of PAA ratio resulted in a significant increase in the crosslinking density (ρx) and a decrease in the average molecular weight of the polymer chain between the crosslinks (M¯c) and the mesh size (ξ) of the hydrogel membranes. In addition, the ion permeability coefficient decreased when the PAA ratio increased. Increasing the PAA ratio significantly affected water uptake value, tensile strength, and elongation of the hydrogel membranes. A PVA:PAA ratio of 9:1 was suitable for plant cultivation. The cultivated plants germinated well and generated many small fine roots on the upper surface of the hydrogel membrane. These small fine roots adhered well to the hydrogel membrane to absorb the water and feed nutrient ions. This finding showed that the hydrogel membranes have the potential of being used for soilless cultivation in controlled experimental conditions. [ABSTRACT FROM AUTHOR]

Copyright of Polymer Materials Science & Engineering is the property of Sichuan University, Polymer Research Institute and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of China Rubber Industry is the property of Editorial Office of China Rubber Industry and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of China Rubber Industry is the property of Editorial Office of China Rubber Industry and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In this work, a post-crosslinking method is used to construct crosslinked epoxy-polyimine networks (C-PIs) based on the imine bond. Due to the dynamic nature of the imine bond, bond exchange reactions (BERs) and rearrangements would take place in polyimine segments under heat or water external stimuli. Molecular dynamic (MD) simulations are performed to investigate imine polycondensation and post-crosslinking procedures. The results indicate that C-PIs exhibit different glass transition temperatures (Tgs) and crosslinking densities by tuning the ratio of epoxy and imine groups, and the Tg values of C-PIs increase linearly with the crosslinking density. In addition, the interface welding behaviour of different C-PIs is investigated, and our results show that BERs can improve the diffusion rate and the final penetration depth. By tracking the microstructures and examining the mechanical properties of samples varying with healing time, it is revealed that the C-PI with lower crosslinking density is easy to self-heal. Furthermore, MD results also imply that the internal stress of the C-PI can be eliminated under the coupling effect of chain relaxation and BERs. This study proves the reliability of a new crosslinking method and provides some useful guidance for designing new materials. [ABSTRACT FROM AUTHOR]

Copyright of China Rubber Industry is the property of Editorial Office of China Rubber Industry and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Shape-memory polymers (SMPs) show great potential in various emerging applications, such as artificial muscles, soft actuators, and biomedical devices, owing to their unique shape recovery-induced contraction force. However, the factors influencing this force remain unclear. Herein, we designed a simple polymer blending system using a series of tetra-branched poly(ε-caprolactone)-based SMPs with long and short branch-chain lengths that demonstrate decreased crystallinity and increased crosslinking density gradients. The resultant polymer blends possessed mechanical properties manipulable across a wide range in accordance with the crystallinity gradient, such as stretchability (50.5–1419.5%) and toughness (0.62–130.4 MJ m−3), while maintaining excellent shape-memory properties. The experimental results show that crosslinking density affected the shape recovery force, which correlates to the SMPs' energy storage capacity. Such a polymer blending system could provide new insights on how crystallinity and crosslinking density affect macroscopic thermal and mechanical properties as well as the shape recovery force of SMP networks, improving design capability for future applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • The effect of hydrogel structure parameters on uranium sorption kinetics is studied. • Nano-structured hydrogels with interconnected micropores facilitate mass transfer. • Freeze-assisted soaking endows hydrogel with strong shrinkage resistance in seawater. • The adsorption equilibrium time can be shortened to 11 days in natural seawater. Synthetic hydrogels, which are 3D networks composed of crosslinked hydrophilic polymer chains, have been created using various crosslinked ways for uranium capture due to full exposure of chelate functional groups. However, the conventional honeycomb-like hydrogels exhibit non-connected micropore structure, which locks abundant water molecules and hence is unfavorable for mass transfer. Herein, we demonstrate a simple synthesis of the nano-structured hydrogel with activated nanogels as nano-crosslinkers (denoted as PGP hydrogel), and probe into the relationship between hydrogel structural parameters and uranium adsorption kinetics. It is found that the PGP hydrogel with a low crosslinking density of ∼0.02 mol/m3 possesses interconnected micropores, thereby facilitating diffusion of uranyl ions (UO 2 2+) in 3D polymeric networks. Furthermore, a freeze-assisted soaking strategy is proposed to endow PGP hydrogel (denoted as target hydrogel) with strong shrinkage resistance in natural seawater, resulting in a ∼40 % increase in water permeance to reach 369 Lm−2h−1MPa−1. Correspondingly, the adsorption equilibrium time is shortened to ∼11 days. Although the uranium uptake capacity of PGP hydrogels with and without freeze-assisted soaking treatment remain basically unchanged (∼9.73 mg-U/g-Ads), a high uranium adsorption rate of 0.88 mg/g/d for the target hydrogel surpasses most currently available polymer adsorbents. The presented strategy is generalizable to a broader range of applications involving the adsorption of economically valuable ions in seawater or brine lake. [ABSTRACT FROM AUTHOR]

Shape memory polymers (SMPs) have received a lot of attention lately due to their potential use as actuators in various applications, including robotics and biomedical device. SMPs are a better choice than other smart materials due to their remarkable ability to recover from significant deformations and their ability to be stimulated remotely. Here, we investigate the mechanical and recovery behaviors of SMPs composed of acrylate-based monomers and crosslinkers with varying crosslink densities, using full-atomistic molecular dynamics (MD) simulations. Physical parameters, such as density and glass transition temperature, are determined for systems with different crosslinking densities. Tensile loading is applied to characterize the mechanical characteristics and shape memory behaviors. Furthermore, the shape-memory behavior of the system with highest crosslinking density is investigated to understand the effects of recovery temperature and deformation temperature on shape recovery. Our findings shed light on the critical structural elements that determine shape recovery ability, giving insights for advanced SMP material design and development. This study adds to our fundamental understanding of SMP behavior and has ramifications for a wide range of technological applications. [Display omitted] • A cross-linking technique is introduced to create a series of systems with different crosslink densities. • Large-scale simulations involving 447,552 atoms are conducted. • Crosslinking density affects material properties, including Young's modulus and glass transition temperature. • Effects of recovery and deformation temperatures on shape recovery in the thermo-mechanical loading cycle are studied. [ABSTRACT FROM AUTHOR]

Copper chloride (CuCl2) is used in sulfur‐cured natural rubber (NR) to regulate disulfide exchange reaction in self‐healing process while factors influencing the healing capability remain unclear. Herein CuCl2 and other additives including zinc oxide and sulfur are used to mediate self‐healing efficiency of NR vulcanizates. For revealing the role of proteins and phospholipids in NR, the mediated self‐healing capability is compared with sulfur‐cured isoprene rubber (IR) vulcanizates. The results reveal that CuCl2 interferes rheological responses and vulcanization dynamics in both rubbers, and it causes intermolecular disassociation in NR gums. As self‐healing capability is closely related to crosslinking density, formulation is adjusted to balance healing efficiency and tensile strength. Healing efficiency of around 70% and the highest tensile strength of about 6 MPa are achieved at crosslinking density about 70 mol/m3 when vulcanizates are healed at 140°C for 30 min. [ABSTRACT FROM AUTHOR]

Hydrogels belong to the group of polymers with a three-dimensional crosslinked structure, and their crosslinking density strongly affects their physicochemical properties. Here, we verified the impact of both the average molecular weight of crosslinking agents used during the photopolymerization of hydrogels and that of their content on selected properties of these materials. First, PVP-based hydrogels modified with Aloe vera juice and L-ascorbic acid were prepared using UV radiation. Next, their surface morphology was characterized via optical scanning electron microscopy, whereas their chemical structure was investigated by FT-IR spectroscopy. Moreover, we verified the tendency of the hydrogels to degrade in selected physiological liquids, as well as their tensile strength, percentage of elongation, and swelling capability. We found that the more crosslinking agent in the hydrogel matrix, the higher its tensile strength and the less elongation. The hydrogels showed the highest stability during incubation in SBF and 2% hemoglobin solution. A sharp decrease in the pH of distilled water observed during the incubation of the hydrogels was probably due to the release of Aloe vera juice from the hydrogel matrices. This was additionally confirmed by the decrease in the intensity of the absorption band derived from the polysaccharides included in this additive and by the decrease in the swelling ratio after 48 h. Importantly, all hydrogels demonstrated swelling properties, and it was proven that the higher content of the crosslinking agent in hydrogels, the lower their swelling ability. [ABSTRACT FROM AUTHOR]

Hydrogels belong to the group of polymers that are more and more often considered as innovative dressing materials. It is important to develop materials showing the most advantageous properties from the application viewpoint wherein in the case of hydrogels, the type and the amount of the crosslinking agent strongly affect their properties. In this work, PVP-based hydrogels containing Aloe vera juice and L-ascorbic acid were obtained via UV-induced polymerization. Next, their surface morphology (via both optical, digital and scanning electron microscope), sorption capacity, tensile strength, and elongation were characterized. Their structure was analyzed via FT-IR spectroscopy wherein their impact on the simulated body liquids was verified via regular pH and temperature measurements of these liquids during hydrogels' incubation. It was demonstrated that as the amount of the crosslinker increased, the polymer structure was more wrinkled. Next, hydrogels showed relatively smooth and only slightly rough surface, which was probably due to the fact that the modifiers filled also the outer pores of the materials. Hydrogels demonstrated buffering properties in all incubation media, wherein during the incubation the release of Aloe vera juice probably took place as evidenced by the decrease in the pH of the incubation media and the disappearance of the absorption band deriving from the polysaccharides included in the composition of this additive. Next, it was proved that as the amount of the crosslinker increased, hydrogels' crosslinking density increased and thus their swelling ratio decreased. Hydrogels obtained using a crosslinking agent with higher average molecular weight showed higher swelling ability than the materials synthesized using crosslinker with lower average molecular weight. Moreover, as the amount of the crosslinking agent increased, the tensile strength of hydrogels as well as their percentage elongation also increased. [ABSTRACT FROM AUTHOR]

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The growing concerns and impending regulations on the usage of monomeric isocyanates in the production of polyurethane resins have led to the introduction of alternative non-isocyanate polyurethane (NIPU) systems. Although two-component NIPU coatings based on the reaction of cyclic carbonates and aliphatic amines have emerged as a promising option, they are still associated with two significant drawbacks: lower crosslinking densities due to reduced functionality of the oligomers and low ambient-temperature reactivity. This study reports the utilization of a hybrid approach to address these drawbacks. Amine-functional NIPU oligomers (NI-PUPA) were synthesized by the reaction of cycloaliphatic amine functional compounds and multi-functional cyclic carbonates in an excess amine molar ratio. After mixing the NI-PUPAs with (3-glycidyloxypropyl) trimethoxysilane (GPTMS), a dual-curable coating could be achieved by ambient curing of amines and epoxies and moisture curing of alkoxy silanes. A comparative experimental design was implemented to evaluate the effect of an additional curing mechanism. The results revealed that the additional moisture curing led to faster ambient curing, faster development of properties, enhanced flexibility even at higher crosslinking densities, and better corrosion resistance. Such advancement could facilitate the future implementation of NIPUs in high-performance ambient-curing coating applications. [ABSTRACT FROM AUTHOR]

In order to predict the storage life of a certain type of HTPB (hydroyl-terminated polybutadiene) coating at 25 °C and analyze the influence of pre-strain on the storage life, the accelerated aging tests of HTPB coating at 40 °C, 50 °C, 60 °C, 70 °C with the pre-strain of 0%, 3%, 6%, 9%, respectively were carried out. The variation regularity of the change of crosslinking density was analyzed and the aging model of HTPB coating under pre-strained thermally-accelerated aging was proposed. The storage life of HTPB coating at 25 °C was estimated by using the Berthelot equation as the end point of the aging life with a 30% decrease in maximum elongation. The results showed that the change of crosslinking density of HTPB coating increased with the increase of aging temperature and aging time, and decreased with the increase of pre-strain. Under 0% pre-strain, the relationship between the change of crosslinking density of HTPB coating and the aging time can be described by the logarithmic model with the confidence probability greater than 99%.The stress relaxation phenomenon existed under 3%, 6% and 9% pre-strained aging. The aging model considering chemical aging and pre-strain was established with the confidence probability greater than 90%. The storage life of HTPB coating was 15.2935 years at 25 °C under 0% pre-strain, which was reduced by 13.9007%, 75.6949% and 89.7859% under 3%, 6% and 9% pre-strain, respectively. The existence of pre-strain has a serious impact on the storage life of HTPB coating, therefore, the pre-strain should be avoided as much as possible during the actual storage.

This paper investigates the influence of the time of enzymatic polymerization of glycerol and divinyl adipate on the chain microstructure and the thermal and linear viscoelastic properties (LVE) of the poly(glycerol adipate) prepolymer (pPGA). The immobilized Candida antarctica lipase B (CALB) was used as a regioselective catalyst in the polytransesterification reaction. A series of pPGA prepolymers were synthesized at different duration times of 8, 24, 48 and 72 h and the degree of polymerization (DP) and degree of branching (DB) were determined based on a quantitative NMR analysis. Due to lack in the literature of the described relationship between the properties of the pPGA prepolymers and the PGA elastomers, the main goal of the research was to correlate the microstructure of the prepolymers with linear viscoelastic properties of the elastomers obtained by thermal crosslinking of the respective prepolymers. The respective pPGA prepolymers were cured at 110 °C for 5 days to obtain PGA elastomers. The LVE properties of the PGA elastomers were investigated based on the master curves of the storage (G') and loss (G") modulus. The differences in DP and BD for the pPGA prepolymers translated into the PGA elastomer crosslinking densities and the rubbery plateau modulus (G 0). It was shown for the first time that the minor differences in the structure of the pPGA prepolymers (DP of 13 and 21, and DB of 8.4% and 9.7% for the pPGA prepolymers synthesized for 8 and 24 h, respectively) translate into significant differences in the LVE of the PGA elastomers (G 0 of 116 kPa and 50 kPa for the PGA elastomers derived from pPGA8 and pPGA24 prepolymers, respectively). [Display omitted] • Poly(glycerol adipate) prepolymers varying in polymerization and branching degrees were obtained in CALB- catalyzed polymerization. • Linear viscoelastic properties (LVE) of poly(glycerol adipate) prepolymers and elastomers were correlated. • From pPGA prepolymer with lower polymerization and branching degrees elastomer with higher plateau modulus was formed. [ABSTRACT FROM AUTHOR]

Thermosetting polymers, commonly used to improve asphalt properties, bring excellent pavement performance, but also pose significant challenges to the healing and recyclability of their modified asphalts after long-term service. An effective approach involves introducing dynamic bonds into the crosslinking networks to synthesize Vitrimer-modified asphalts with self-healing response. Due to the splendid mechanical properties, high elasticity, flexibility and easily controllable cross-linked structure of polyurethane, a novel castor oil-based polyurethane Vitrimer modified asphalt with disulfide bonds and Diels-Alder reactions was developed. Based on the prepared castor oil-based polyurethane Vitrimer (C-PUV, composed of components A and B) and its modified asphalts (C-PUVAs), a comprehensive study was conducted on the crosslinking density, mechanical properties, thermal stability, viscosity, compatibility, rheological properties, and self-healing behavior. The results indicated that C-PUVAs (C-PUVA1/C-PUVA2/C-PUVA3/C-PUVA4) exhibited sufficient retention times for viscosity, uniform phase distribution, and good high-temperature performance. The damaged C-PUVAs achieved a mechanical strength recovery rate more than 75 % after the first cycle of self-healing at 80 ℃ and 0.3 MPa. Furthermore, the crosslinking density of C-PUV was regulated by the segment length of component B. As the crosslinking density increased, C-PUV and C-PUVAs obtained higher mechanical strength while sacrificing part of self-healing efficiency. The mechanism is that increased crosslinking density altered the gel point distributions of the reversible crosslinked networks, resulting in a longer relaxation time and higher activation energy. In conclusion, the above findings provided valuable insights for developing bio-based polyurethane Vitrimer modified asphalts, and endosing thermosetting polymer modified asphalts with self-healing and recyclable properties. • Castor oil-based polyurethane Vitrimer modified asphalts were prepared and regulated by two-components. • Dual dynamic reversible reactions from disulfide bonds and Diels-Alder reactions. • Coordinating the introduction of dynamic bonds and the end-capping of isocyanate groups. • Moderate conditions for self-healing response and good healability. • Regulation and influence mechanism of crosslinking density on properties of the modified asphalts. [ABSTRACT FROM AUTHOR]

Elastomeric biocomposites based on poly(glycerol adipate urethane) and hydroxyapatite were fabricated for tissue regeneration. The poly(glycerol adipate urethane) (PGAU) elastomeric composite matrices were obtained by chemical crosslinking of the poly(glycerol adipate) prepolymer (pPGA) with diisocyanate derivative of L-lysine. Two series of composites varying in the amount of L-lysine diisocyanate ethyl ester (LDI) used as a crosslinking agent were manufactured. As a ceramic filler both unmodified and L-lysine surface-modified hydroxyapatite (HAP) particles were used. The novelty of our research consists in the manufactured elastomeric materials and characterization of their linear viscoelastic (LVE) properties. The LVE properties of the composites were investigated by means of dynamic thermomechanical analysis. Frequency sweep and amplitude sweep measurements were performed in shear mode. The influence of the crosslinking agent (LDI) amount, HAP content and surface modification of HAP on the LVE properties of the composites was determined based on the analysis of the master curves of storage (G′) and loss (G″) moduli and of tanδ of the composites. Depending on the amount of LDI, HAP and surface modification, the materials differ in the values of rubber elasticity plateau modulus (G 0) and G′ and G″ determined at selected shear frequencies and at the glassy state. G 0 ranges from 278 kPa to 3.98 MPa, G′ in the glassy state is within the range of 219 MPa–459 MPa. The G 0 values of the PGAU-based composites are within the stiffness range of soft tissue. In view of the choice of HAP as the ceramic component and the G 0 values, elastomeric composites have the potential to be used as filling materials in small bone defects (due to their mechanical similarity to osteoid) as well as materials for cartilage tissue regeneration. [Display omitted] [ABSTRACT FROM AUTHOR]

Over the past few years, researchers have demonstrated the use of hydrogels to design drug delivery platforms that offer a variety of benefits, including but not limited to longer circulation times, reduced drug degradation, and improved targeting. Furthermore, a variety of strategies have been explored to develop stimulus-responsive hydrogels to design smart drug delivery platforms that can release drugs to specific target areas and at predetermined rates. However, only a few studies have focused on exploring how innate hydrogel properties can be optimized and modulated to tailor drug dosage and release rates. Here, we investigated the individual and combined roles of polymer concentration and crosslinking density (controlled using both chemical and nanoparticle-mediated physical crosslinking) on drug delivery rates. These experiments indicated a strong correlation between the aforementioned hydrogel properties and drug release rates. Importantly, they also revealed the existence of a saturation point in the ability to control drug release rates through a combination of chemical and physical crosslinkers. Collectively, our analyses describe how different hydrogel properties affect drug release rates and lay the foundation to develop drug delivery platforms that can be programmed to release a variety of bioactive payloads at defined rates. [ABSTRACT FROM AUTHOR]

Vitrimers with dynamic covalent bonds are sustainable alternatives for traditional thermosetting polymers. The variations of building block alter the properties of vitrimers, but obstruct detailed structure–property correlations. Besides, a recognizable crosslinking reaction is responsible for the network structures of vitrimers. Herein, we investigated the effects of building block variations (i.e., the content of crosslinker, and the type and molar mass of polymer matrix) on the crosslinking kinetics, as well as on the static and dynamic properties using polyether‐based vinylogous urethane vitrimers as model system. The kinetics of crosslinking tracked by in situ FTIR provides an insightful understanding on the vitrimer yield and formation rate under conditions. And the apparent rate coefficient (k) was linearly correlated to the crosslinking density (νe) in case of different polymer matrixes being used. In addition, based on the classical theoretical models we also realized the correlations between the static property (i.e., glass transition temperature, Tg) and νe, as well as the dynamic property (i.e., topology freezing temperature, Tv) and νe. Tg has been found to be exclusively positively related to νe, while Tv is jointly determined by νe and the content of free amine. This strategy to establish reaction–structure–property relationship bridged by the crosslinking density will guide the preparation of novel, tailor‐made sustainable polymeric materials. [ABSTRACT FROM AUTHOR]

Scaling behavior relating the synthesis conditions with swelling degree, preparation concentration and crosslinking density of multifunctional ternary‐gels consisting of 2‐(dimethylamino)ethyl methacrylate (DMAEMA), hydroxypropyl methacrylate (HPMA) and glycidyl methacrylate (GMA) is reported. Swelling is controlled by DMAEMA portion of terpolymers and DMAEMA‐rich gels tended to swell more than GMA‐rich gels and increase in GMA increase polymer‐solvent interaction parameter, χ Critical conditions required for terpolymers to show phase transition are calculated as χ1=0.4993 and χ2=0.2959. Dependence of swelling degree ϕVon gel preparation concentration ν20$\nu _2^0$ as a function of solvent quality is examined. Good solvent prediction of φV≈(N¯Cν20)0.29${\varphi _V} \approx {({{{\overline N }_C}\nu _2^0})^{0.29}}$ indicates a crossover between entanglement‐dominated case and strongly cross‐linked case corresponding to low and moderate degrees of swelling. Swelling of ternary‐gels in different solvents shows that cyclohexanol is a good solvent and solubility parameter is 23.7 (MPa)1/2. Addition of hydrophobic GMA leads to a significant increase in stiffness of terpolymer gels. Increase in elastic modulus as‐prepared state is not monotonic and can be controlled by adjusting DMAEMA/GMA ratio since tertiary amine groups triggers cascading epoxy ring‐opening reaction. Reduced modulus Gr decreased with increasing swelling degree and scaling relation between modulus and swelling is found as Gr ≈ (ϕV)−0.66 which indicates that ternary‐hydrogels, limited to weak stretching regime, obey Gaussian statistics. [ABSTRACT FROM AUTHOR]

Starch/Acrylic acid (AAc)/4-Styrenesulfonic Acid Sodium Salt (SSS) based hydrogels were prepared by using simple and facile Gamma radiation (Cobalt-60 source) technique. The prepared hydrogels were characterized by Fourier Transform Infrared-Attenuated Total Reflection (FTIR-ATR), Scanning Electron Microscopy (SEM), and Thermogravimetric Analysis (TGA). The influences of different parameters such as radiation dose, constituents of hydrogels, pH, ionic strength on different hydrogel properties like crosslinking density (CD), porosity, gel content, swelling behavior and adsorption kinetics were investigated in detail. Results showed that porous hydrogels were formed with well-interconnected networks. The maximum gel fraction 97.95% was observed for 15% Starch concentration, and the high-water penetration in the hydrogel with a high swelling ratio (3072%) was found at 25% AAc in the hydrogel composition. The dye adsorption capacity of the optimized hydrogel was assessed in terms of different kinetics and isotherm models using methylene blue (MB) as a model dye. Starch/AAc/SSS hydrogel's adsorption behavior followed pseudo-second order and well-matched with the Langmuir isotherm model with maximum dye adsorption of 136.89 mg/g. Hence, the Starch/AAc/SSS hydrogel could be used as a prospective adsorbent to remove dye from the wastewater. [ABSTRACT FROM AUTHOR]

Profound understanding of relationships between molecular networks with properties can make it more convenient for people to prepare and employ shape memory polymers (SMPs). In this paper, a series of epoxy SMPs were developed based on epoxy (E‐51), trimethylolpropane tris(3‐mercaptopropionate) and pentaerythritoltetrakis (3‐mercaptopropionate), and 2‐methylimidazole, through "epoxy‐thiol" click reaction. The effect of networks on key properties (heat resistance and mechanical properties shape recovery performance) was systematically investigated. Experiment results indicated that there were positive correlations between network regularity with both thermal–mechanical properties and heat resistance. In addition, regularity of network structure had an effect to restrict shape recovery speed. [ABSTRACT FROM AUTHOR]

The polyacrylamide (AAm)-chitosan (CS)-N,N′-methylene bisacrylamide (Bis) bilayer shape memory hydrogel actuators with high mechanical strength were successfully prepared via efficient in-situ free radical polymerization. The hydrogels exhibited high swelling and shape memory bending/unbending properties. Increasing Bis contents enhanced the crosslinking density and maintained the hydrophilic property of hydrogels, providing the microstructure and swelling anisotropy for construction of bilayer structure. Attributed to the enough polymerization on interface, bilayer hydrogels owned high bonding strength. With the increase of Bis content, the swelling bending degree of bilayer hydrogel actuators increased. Based on the mechanism of formation and disappearance of coordination of CS-metal ion, bilayer hydrogel actuators exhibited efficient shape memory property. The intelligent swelling and shape memory properties enriched the multiply deformation patterns of bilayer shape memory hydrogels, providing the potential applications in soft actuators, soft sensors and medical carriers.

Shape-memory polymers (SMPs) show great potential in various emerging applications, such as artificial muscles, soft actuators, and biomedical devices, owing to their unique shape recovery-induced contraction force. However, the factors influencing this force remain unclear. Herein, we designed a simple polymer blending system using a series of tetra-branched poly(ε-caprolactone)-based SMPs with long and short branch-chain lengths that demonstrate decreased crystallinity and increased crosslinking density gradients. The resultant polymer blends possessed mechanical properties manipulable across a wide range in accordance with the crystallinity gradient, such as stretchability (50.5–1419.5%) and toughness (0.62–130.4 MJ m−3), while maintaining excellent shape-memory properties. The experimental results show that crosslinking density affected the shape recovery force, which correlates to the SMPs’ energy storage capacity. Such a polymer blending system could provide new insights on how crystallinity and crosslinking density affect macroscopic thermal and mechanical properties as well as the shape recovery force of SMP networks, improving design capability for future applications.

Shape memory epoxy resin (SMEP) with tailorable properties has gained wide attention. Herein, a series of photocured diphenyl epoxy with excellent shape memory effects, based on the epoxy monomer diglycidyl ether of biphenyl (DGEBP) and M‐xylylenediamine (MXDA), is fabricated. Fourier transform infrared spectrometer (FTIR) is used to verify the mix‐curing process. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), polarized light microscope (POM), and X‐ray diffraction (XRD) are applied to analyze the microstructure of the film. The molar ratio of the raw material and the post‐heating could tune the crosslinking density, which allows the prepared epoxy resin to present a tailorable shape memory effect. Besides, this epoxy resin exhibits an excellent triple‐shape memory effect due to the wide transition temperature. It is worth of noting that post‐heating induced the self‐assembly of biphenyl unit of the lamellar structure due to π–π interactions that causes an adjustable shape memory effect. This work offers a novel SMEP with potential application in the thermal history trace field. [ABSTRACT FROM AUTHOR]