Increase In Degree Of Crosslinking Research Articles

Effects of Chemical Composition and Cross-Linking Degree on the Thermo-Mechanical Properties of Bio-Based Thermosetting Resins: A Molecular Dynamics Simulation Study.

Bio-based epoxy resins have received significant attention in terms of concerns regarding carbon emission. Epoxidized soybean oil (ESO) derived from sustainable feedstock has been widely used to blend with traditional diglycidyl ether of bisphenol-A (DGEBA) to replace some of the petroleum-based components. In this work, molecular dynamics (MD) simulations were applied to track the network formation and predict the performance of methyl hexahydrophthalic anhydride (MHHPA)-cured ESO/DGEBA blend systems. The effects of ESO content and cross-linking degree on the mass density, volumetric shrinkage, glass transition temperature (Tg), coefficient of thermal expansion (CTE), Young's modulus, yield strength, and Poisson's ratio of the epoxy resin were systematically investigated. The results show that systems with high ESO content achieve gelation at low cross-linking degree. The Tg value, Young's modulus, and yield strength increase with the increase in cross-linking degree, but the CTE at the glassy state and Poisson's ratio decrease. The comparison results between the simulated and experimental data demonstrated that the MD simulations can accurately predict the thermal and mechanical properties of ESO-based thermosets. This study gains insight into the variation in thermo-mechanical properties of anhydride-cured ESO/DGEBA-based epoxy resins during the cross-linking process and provides a rational strategy for optimizing bio-based epoxy resins.

Mechanical property tests and physics-informed data-driven modeling of viscoelastic materials subjected to thermal-oxidative aging

The purpose of this paper is to investigate and characterize the performance changes of viscoelastic materials (VEMs) in viscoelastic (VE) devices used for structure vibration control due to the influence of aggressive environmental conditions during long-term service. To this end, a series of thermal-oxidative (TO) aging experiments are conducted on the developed VEMs, variation of microstructure and macroscopic mechanical behaviors of TO aged VEMs are tested and analyzed. The molecular chain network of VEMs that can be simplified into elastic chain network and free chain network is described by the architecture of microsphere model, a physics-informed data-driven model is proposed to reflect the mechanical properties of TO aged VEMs, in which the physics-informed neural network (PINN) theory is incorporated to describe the statistical characteristics of molecular chains and the reaction rate of TO aging. Based on the experimental data, implementation and validation of the proposed model are conducted on the mechanical properties of VEMs under varying aging levels. The experimental results suggest that TO aging could alter molecular chain structure of VEMs from the microscopic level, leading to an increase in crosslinking degree and modulus, and the degeneration of deformation ability and energy-dissipation capacity for VEMs. The verifications reveal that the proposed model possesses high accuracy in characterizing the mechanical properties and energy-dissipation capability of VEMs under different aging levels, which preliminarily proofs the potential of PINN theory in reflecting the complex mechanical behaviors of VEMs.

Designing gelatin microgels by moderate transglutaminase crosslinking: Improvement in interface properties

In this study, gelatin microgels with various crosslinking degrees were facilely fabricated through moderate transglutaminase (TGase) crosslinking. The influences of crosslinking degree on the surface and interface properties of gelatin microgels were investigated. Furthermore, the influence on the stability of gelatin microgels emulsions was also explored. The crosslinking degree of gelatin microgels was gradually increased from 1.85% to 12.25% and positively correlated with the addition amount of TGase. With the increase of crosslinking degree, the high molecular weight components in gelatin microgels increased, and the wettability (32°–81°) and surface hydrophobicity of gelatin microgels gradually increased. The interfacial protein content also increased with the increase of crosslinking degree and reached equilibrium as the crosslinking degree exceeded 6.85%. The results of interfacial dynamic adsorption and interfacial dilatational viscoelasticity demonstrated the formation of a dense and strong interfacial structure, especially at relatively higher crosslinking degree (>6.85%). The liquid chromatography-tandem mass spectrometry (LC-MS/MS) results confirmed that TGase crosslinking increased the crosslinking between α2 chain in gelatin molecular chains, thus enhancing interfacial interaction and regulating interface microstructure. TGase crosslinking effectively improved the physical stabilities of gelatin microgel emulsions, suggesting a dominant contribution of the adjusted interface properties of gelatin microgels by moderate TGase crosslinking.

Highly efficient cross-linking of reactive dyes in the amorphous zones of cotton fabric to simultaneously improve color and wrinkle properties

Dyeing and wrinkle-finishing of cotton fabric is generally achieved through two steps, which consumes a lot of energy and generates a lot of colored wastewaters. In this study, we report an energy-saving and cost-reducing dyeing process that enhanced the cross-linking degree of double reactive group dye in the amorphous zone of cotton fabric, which in turn improved the wrinkle resistance of cotton fabric as well as the utilization of the dye. Compared with the traditional dyeing processes, the method could improve K/S values by 69.7%, dye fixation by 22.4%, and wrinkle recovery angle by 21.5% for dyed cotton fabric. The wrinkle recovery angle of the dyed cotton fabric increased with the increase of cross-linking degree. The cross-section of cotton fibers changed from a flattened girdle shape to an elliptical or rounded shape, and the mid-cavity structure became smaller or even disappeared. In addition, the cross-linked structures could hinder the degradation of fiber macromolecular chains by cellulases. This meant that the better the wrinkle resistance of the dyed cotton fabric, the lower the glucose content in the degradation solution. Compared with the conventional dyeing processes, this dyeing method could be saved 36.5% of dyes and 28.4% of alkali for per 1 kg of cotton fabric, and the cost could be saved by 56.1%. The dye concentration in the waste water was reduced by 43.2%. This method had certain advantages in terms of economic and environmental.

Improving inherent thermal conductivity of epoxy resins based on contribution components of thermal conductivity: A molecular dynamics study

The inherent thermal transport properties of epoxy resin are vital factors to be studied in applications. In the present work, we constructed the amorphous crosslinking structure, the step crosslinking structure and the ordered crosslinking structure based on molecular dynamics theory, and used the reverse nonequilibrium molecular dynamics (RNEMD) method to calculate the thermal conductivity (TC) of different structures. Based on the numerical verification of the reliability of the method, the contribution of bonding to the TC was enhanced by increasing the molecular arrangement orderliness to reduce phonon scattering, taking the composition of the TC contribution as the guide. Meanwhile, the contribution of non-bonding interaction to the TC was enhanced by using compression process to achieve TC increase in the epoxy resin intrinsic structure, followed by exploring the effect of cross-linking degree on the TC of different structures. The results showed that the TC of amorphous epoxy resin decreased with the increase of crosslinking degree, and the relative high-frequency VDOS increased, but the influence of crosslinking degree on TC was minor in the range. For amorphous epoxy resin four-sided pressurization varied in the range of 0–7 Gpa, the non-bond energy percentage increased in the range of 0–60%, and the increase of non-bond energy made its contribution to TC increased significantly. The epoxy resin achieved 215% improvement in intrinsic TC by changing the molecular arrangement orderliness. The ordered structure TC varied from 0.3 to 3.5 W/(m⋅K) in the pressure variation range of 0–7 GPa. As the structure becomes more ordered, the TC increased significantly with the degree of cross-linking. The study provided the theoretical basis for the realization of resin preparation and resin thermal property evaluation with regulated intrinsic TC.

Effects of freezing methods on physicochemical properties, protein/fat oxidation and odor characteristics of surimi gels with different cross-linking degrees

This work investigated the effects of liquid nitrogen immersion freezing (LNF), −35 °C air freezing (AF-35℃) and −18 °C air freezing (AF-18℃) on the physical and chemical characteristics and flavor quality of surimi gels with different cross-linking degrees. Compared to AF-35 °C and AF-18 °C, LNF was shown to considerably delay the texture deterioration and water migration of frozen gels, as well as the accumulation of thiobarbituric acid reactive substance values and carbonyl contents. Additionally, an appropriate increase of cross-linking degree (45.83 to 62.99%) was found able to improve gel properties and inhibit quality deterioration during freezing. Moreover, LNF-treated gels were closer to fresh gels in the amount of volatile compounds, in contrast to most significant negative aroma changes in AF-18℃-treated gels. Furthermore, 29, 29 and 31 key differential volatile compounds were screened for gels with a cross-linking degree of 29.66, 45.83 and 62.99%, respectively, mainly including aldehydes, alcohols and esters.

A novel multifunctional composite film of oxidized dextran crosslinked chitosan and ε-polylysine incorporating protocatechuic acid and its application in meat packaging

Oxidized dextrans (ODex) with different oxidation degrees were prepared by periodate oxidation. Chitosan (CS) and ε-polylysine (PL) were crosslinked with ODex and incorporated with protocatechuic acid (PA) to enhance the properties of the chitosan-based films. FT-IR, XPS and XRD analysis showed that the aldehyde groups of ODex were covalently crosslinked with amino groups of CS and PL through Schiff base reaction, and PA interacted with the film components through non-covalent bonds. The results of SEM and AFM indicated that the film structure became denser with the addition of ODex. Increasing crosslinking degree resulted in deepened color and improved the UV-blocking ability of the films, which was caused by the increase of CN groups. Covalent crosslinking significantly reduced the moisture content, swelling degree and water solubility. The film with 50% ODex had the highest mechanical strength (31.72 MPa) and modulus of elasticity (885.54 MPa). The results of water vapor permeability, thermal properties and PA release experiments also indicated the crosslinking of the films by ODex. The incorporation of PA significantly improved the antioxidant activity of the films, but to some extent, the antibacterial activity of the films decreased due to the consumption of amino groups by crosslinking. In addition, application experiments suggested that crosslinked films could maintain the quality and freshness of chilled meat for a longer period than that of PE plastic films. The study suggested that the new crosslinked films might have good prospects as food packaging materials.

Chlorine resistance property improvement of polyamide reverse osmosis membranes through cross-linking degree increment

Highly permeable polyamide reverse osmosis (RO) membranes are desirable for reducing the energy burden and ensuring future water resources in arid and semiarid regions. One notable drawback of thin film composite (TFC) polyamide RO/NF membranes is the polyamide's sensitivity to degradation by free chlorine, the most used biocide in water purification trains. This investigation demonstrated a significant increase in the crosslinking-degree parameter by the m-phenylenediamine (MPD) chemical structure extending in the thin film nanocomposite (TFN) membrane without adding extra MPD monomers to enhance the chlorine resistance and performance. Membrane modification was carried out according to monomer ratio changes and Nanoparticle embedding into the PA layer approaches. A new class of TFN-RO membranes incorporating novel aromatic amine functionalized (AAF)-MWCNTs embedded into the polyamide (PA) layer was introduced. A purposeful strategy was carried out to use cyanuric chloride (2,4,6-trichloro-1,3,5-triazine) as an intermediate functional group in the AAF-MWCNTs. Thus, amidic nitrogen, connected to benzene rings and carbonyl groups, assembles a structure similar to the standard PA, consisting of MPD and trimesoyl chloride. The resulting AAF-MWCNTs were mixed in the aqueous phase during the interfacial polymerization to increase the susceptible positions to chlorine attack and improve the crosslinking degree in the PA network. The characterization and performance results of the membrane demonstrated an increase in ion selectivity and water flux, impressive stability of salt rejection after chlorine exposure, and improved antifouling performance. This purposeful modification resulted in overthrowing two tradeoffs; i) high crosslink density-water flux and ii) salt rejection-permeability. The modified membrane demonstrated ameliorative chlorine resistance relative to the pristine one, with twice the increase in crosslinking degree, more than four times the enhancement of the oxidation resistance, negligible reduction in the salt rejection (0.83 %), and only 5 L/m2.h flux loss following a rigorous static chlorine exposure of 500 ppm.h under acidic conditions. The excellent performance of new chlorine resistant TNF RO membranes fabricated via AAF-MWCNTs together with the facile membrane manufacturing process offered the possibility of postulating them in the desalination field, which could eventually help the current freshwater supply challenge.

Modification of gelatin from tuna skins by green tea polyphenols

This paper presents the result of study on the effect of modifying conditions on properties and structure of tuna skin gelatin by using green tea polyphenols. The results showed that gelatin modified by green tea polyphenols changed its gel strength, degree of cross-linking, molecular weight and gel structure. The appropriate parameters to modify gelatin by using green tea polyphenols which indicated the best mechanical properties were determined as follows: temperature of 40oC in 40 minutes, polyphenol content of 20 mg/g gelatin and gelatin solution of 20%. As a result, modified gelatin showed a decrease in solubility, an increase in cross-linking degree (16.7%), molecular weight (55÷72 kDa) as well as a larger protein structure in comparison with the natural gelatin. The IR spectrograph of the modified gelatin did not show any evidence of forming new functional groups, only an apparent change in the absorption of infrared spectroscopy at the peak of the amide B region was found. The dried modified gelatin became insoluble in hot water.

Microscopic mechanism of distinct mechanical properties of divinylbenzene reinforced cross-linked polystyrene revealed by molecular dynamics simulations

Molecular dynamics method was employed to establish the model of three-dimensional cross-linked polystyrene (PS) formed by divinylbenzene (DVB) and PS chains. Density, radial distribution function, free volume fraction, mean square displacement of systems with different cross-linking degrees (DVB contents of 0%, 3.8%, 7.1% and 11.1%) were studied, as well as macroscopic properties such as glass transition temperature, elastic mechanical properties, uniaxial tensile deformation. The results showed that cross-linking algorithm proposed was feasible for constructing cross-linked PS. With the increase of cross-linking degree, cross-linked PS became denser, and glass transition temperature increased, indicating an increase of heat resistance. Compared with uncross-linked PS, elastic modulus of three cross-linked systems increased by 19.26%, 29.56% and 40.19%; bulk modulus increased by 2.9%, 20.98% and 44.03%; and shear modulus increased by 21.05%, 29.82% and 42.98%. Tensile stress-strain curves showed that network structure formed by adding DVB improved yield stress and tensile resistance of PS.

Triblock Elastomeric Vitrimers: Preparation, Morphology, Rheology, and Applications

Vitrimer chemistry of boronic transesterification was introduced into a triblock thermoplastic elastomer to allow associative bond exchange in its highly ordered nanoscale domains. The purpose of this localized dynamic cross-linking is to enable reprocessing while also improving creep resistance. Triblock poly(styrene-b-butadiene-b-styrene) (SBS) was used as an elastomeric vitrimer precursor to gain understanding of fundamental structure–property effects of localized dynamical cross-linking. The occurrence of the UV-induced thiol–ene reaction was corroborated by Fourier transform infrared (FTIR) spectroscopy and swelling experiments. Transmission electron microscopy (TEM) images and small-angle X-ray scattering (SAXS) profiles demonstrate a prominently lamellar morphology with domain spacings that decline with the increase of cross-linking degree. Short and long relaxation modes of triblock SBS vitrimers (vSBSs) were found to have opposite temperature dependence, which caused thermorheological complexity in linear viscoelasticity. As compared to its random analog, vSBS shows much slower stress relaxation and higher activation energy, as well as superior resistance to creep under a steady load. The enhanced rheological properties are attributed to the high concentration of localized cross-links in the soft phase and to additional constraints that hinder network strand diffusion in the microphase-separated structure. The mechanical properties of vSBSs at low strain were substantially enhanced by the associative dynamic cross-links between the soft segments, increasing the maximum stress to 7.5 MPa at 100% strain. Its Young’s modulus of 88.1 MPa is about 2.8 times that of neat SBS. Applications of vSBSs as hot-melt adhesives were explored, showing excellent lap shear strength on iron plates. Due to the dynamic exchange of boronic ester bonds, adhesive joints can efficiently rebind at least three times and the lap shear strength remains almost unchanged.

Fortified gelatin-based hydrogel scaffold with simvastatin-mixed nanomicelles and platelet rich plasma as a promising bioimplant for tissue regeneration

Treatment of intervertebral disc (IVD) degeneration includes conservative and surgical strategies that have a high risk of recurrence. Consequently, tissue engineering represents a promising alternative treatment. This study aimed at healing damaged IVD with a bioimplant that can maintain the function of defected IVD. The developed IVD scaffold is composed of a fortified biocompatible gelatin-based hydrogel to mimic the ECM mechanical properties of IVD and to allow a sustained release of loaded bioactive agents. The hydrogel is laden with platelet-rich plasma (PRP) and simvastatin (SIM)-loaded mixed pluronics nanomicelles because of their regenerative ability and anti-inflammatory effect, respectively. The gelatin-based hydrogel attained swelling of 508.9 ± 7.9 % to 543.1 ± 5.9 % after 24 h. Increasing crosslinking degree of the hydrogel improved its mechanical elasticity up to 0.3 ± 0.1 N/mm2, and retarded its degradation. The optimum mixed nanomicelles had particle size of 84 ± 0.5 nm, a surface charge of −10 ± 7.1 mv, EE% of 84.9 %, and released 88.4 % of SIM after 21 days. Cytotoxicity of IVD components was evaluated using human skin fibroblast for 3 days. WST-test results proved biocompatibility of IVD scaffold. Subcutaneous implantation of the IVD scaffold was performed for 28 days to test in-vivo biocompatibility. Histological and histochemical micrographs depicted normal healing signs such as macrophages, T-cells, angiogenesis and granulation reactions. Introducing PRP in IVD improved healing process and decreased inflammation reactions. The developed multicomponent implant could be used as potential IVD scaffold with desirable mechanical properties, biocompatibility and healing process.

AIEgen-Enabled Multicolor Visualization for the Formation of Supramolecular Polymer Networks.

Extensive reports on the use of supramolecular polymer networks (SPNs) in self-healing materials, controlled release system and degradable products have led more researchers to tap their potential owing to the unique properties. Yet, the attendant efforts in the visualization through conventional luminescence methods during the formation of SPNs have been met with limited success. Herein, we designed a special type of SPNs prepared by PPMU polymer chains containing pyrene benzohydrazonate (PBHZ) molecules as AIEgens for the multicolor visualization with naked eyes. The complete detection of the formation process of the networks relied on the PBHZ molecules with aggregation-induced ratiometric emission (AIRE) effect, which enabled the fluorescence of the polymer networks transits from blue to cyan, and then to green with the increasing crosslinking degree derived from the hydrogen bonds between 2-ureido-4-pyrimidone (UPy) units of the polymer chains. Additionally, we certificated the stimuli-responsiveness of the obtained SPNs, and the fluorescence change, as well as observing the morphology transition. The AIEgen-enabled multicolor visualization of the formation of SPNs may provide better understanding of the details of the crosslinking interactions in the microstructural evolution, giving more inspiration for the multifunctional products based on SPNs.

Preparation of FKM/EFEP thermoplastic vulcanizate with excellent heat and oil resistance, gas barrier property and recyclability

Because of the fuel diversification, the harsher use environment and the requirements of environmental protection, the automotive fuel pipeline system requires not only good mechanical properties and high elasticity but also high temperature and oil resistance, high gas barrier property, easy processibility, and recyclability. Nevertheless, the traditional materials cannot simultaneously meet the requirements of these properties. In this study, fluororubber (FKM)/fluoro-thermoplastic (EFEP) TPV (FTPV) with fine dispersion of FKM rubber phase in EFEP matrix was successfully produced through the design and optimization of crosslinking systems of FKM and the processing technology. The as prepared FTPV not only shows good mechanical properties, elasticity, rheological property and recyclability, but also excellent oil resistance and gas barrier properties, outperforms the other TPVs. The mechanism on morphological development of the FKM/EFEP TPV are revealed. Interestingly, a large number of single rubber sub-micron particles (RPs) are formed in the blend during the dynamic vulcanization, which is mainly attributed to an improvement in compatibility and an increase in crosslinking degree of FKM. The combined effect of shearing stress and the improvement of compatibility results in a decrease in the size of rubber sub-micron particles agglomerates and an increase in the content of single rubber sub-micron particles.

The Effect of Crosslinking Degree of Hydrogels on Hydrogel Adhesion.

The development of adhesive hydrogel materials has brought numerous advances to biomedical engineering. Hydrogel adhesion has drawn much attention in research and applications. In this paper, the study of hydrogel adhesion is no longer limited to the surface of hydrogels. Here, the effect of the internal crosslinking degree of hydrogels prepared by different methods on hydrogel adhesion was explored to find the generality. The results show that with the increase in crosslinking degree, the hydrogel adhesion decreased significantly due to the limitation of segment mobility. Moreover, two simple strategies to improve hydrogel adhesion generated by hydrogen bonding were proposed. One was to keep the functional groups used for hydrogel adhesion and the other was to enhance the flexibility of polymer chains that make up hydrogels. We hope this study can provide another approach for improving the hydrogel adhesion generated by hydrogen bonding.

Effect of the crosslinking degree on the microstructure and thermomechanical properties of a polymer grouting material

Polymer grouting materials are widely used in the field of trenchless rehabilitation of infrastructure, and their properties are closely related to the crosslinking process and crosslinking degree between components. In this paper, polyurethane (PU) models with different crosslinking degrees were built based on the molecular dynamics (MD) simulations method and Python script. We investigated the crosslinking reaction process of PU grouting materials and the effect of their crosslinking degree on the glass transition temperature (Tg) and tensile properties. Also, the rationality of the model was verified by comparing the Tg obtained in the simulation and experiment. The results showed that the crosslinking reaction is a volume shrinkage process, and the PU model has a good network structure. The non-bonding energy is the main reason for the variation in the Tg, which increased with increasing crosslinking degree. Besides, the tensile properties of PU are greatly affected by the crosslinking degree, and the elastic modulus increased with increasing crosslinking degree. The results are of great significance for understanding the microstructure and thermomechanical properties of PU grouting materials at the molecular level.

"RHEOLOGICAL PROPERTIES OF SALICYL-IMINE-CHITOSAN HYDROGELS: EFFECT OF CROSSLINKING DENSITY "

The present paper focuses on the rheological behaviour of a series of hydrogels prepared from chitosan and salicylaldehyde. The unusual crosslinking of chitosan with this monoaldehyde was assessed by 1H NMR spectroscopy, which demonstrated the formation of covalent imine bonds. The hydrogels exhibited a super-porous morphology, evidenced by SEM measurements, and the layered supramolecular structure of the hydrogels was sustained by the birefringence texture of the hydrogels, observed by polarized light microscopy (POM). The hydrogel-like behaviour was confirmed by rheologic measurements for the sample containing the highest salicylaldehyde amount. The dynamic flow properties of salicyl-imine-chitosan hydrogels with different crosslinking degrees (NH2/CHO ratios between 2 and 4) were investigated at temperatures in the range of 20–40 °C. The rheological moduli were determined over a wide range of oscillatory frequencies and the experimental results were presented using master curves. In addition, the thixotropic behaviour of the hydrogels based on chitosan and salicylaldehyde was determined and discussed. The measurements of thixotropy were performed by increasing the shear rate to 400 s-1 in an upward sweep, followed by its decreasing in a downward sweep. It was noticed that the hydrogels’ thixotropy increases with an increasing crosslinking degree. In line with this rheological behaviour, the self-healing ability was tested, and it was proved that the hydrogels were able to reshape after applying deformation stress.

Improving the DC Dielectric Properties of XLPE with Appropriate Content of Dicumyl Peroxide for HVDC Cables Insulation

In this research, crosslinked polyethylene (XLPE) is developed with selective content of dicumyl peroxide (DCP), and the influence of microstructural properties and chemical composition on the mechanical and direct current (DC) dielectric properties are investigated. The measurements for the microstructural analysis are taken by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), gel content test and Fourier transform infrared (FTIR). The mechanical properties of XLPE are evaluated by hot-set test. The results of microstructural and chemical composition show that the increase in DCP content increases the crosslinking degree from 74.3% to 81.6%, reduces the crystallinity/lamella thickness from 36.8% to 35.5%/7.6–7.1 nm, reduces the average molecular weight between two crosslinks by 0.01 kg/mol and reduces the oxidation level/carbonyl index. The increase in DCP in XLPE samples decreases the permanent elongation from 2.2% to 0% and elongation rate from 300% to 80% of the cable insulation. The rise in DCP content increases the crosslinking degree due to which the DC resistivity and activation energy is increased. The DC breakdown strength at 30–90 °C is increased due to the increase in crosslinking degree and reduction in carbonyl index/oxidation level. The space charge accumulation is measured at 30 °C under 20–60 kV/mm, resulting in less homo-charges and hetero-charges with the increase in DCP. It is proven that the role of appropriate DCP content is vital in increasing the DC dielectric performance, internal material characteristics and mechanical performance of XLPE.

Microstructural evolution of cross-linked polytetrafluoroethylene under gamma irradiation using ultra-small angle X-ray scattering

Cross-linked polytetrafluoroethylene (XPTFE) with excellent wear and radiation resistant is expected to be applied in strong radiation fields. In this study, synchrotron ultra-small angle X-ray scattering (USAXS) techniques combined with in-situ uniaxial stretching were firstly used to study the mechanical property degradation mechanism of XPTFE in terms of micro-structural evolution in real time. As the increase of cross-linking degree, the size and spacing of the scatterers for XPTFE were more unequal. When XPTFE was uniaxially stretched at room temperature, its molecular chains were tilted, and lamellae crystals were elongated and thinned along the stretching direction. When XPTFE was irradiated by γ-rays under air, with increasing absorbed doses, the long period decreased first and then increased, while the gyration radius was reversed. After high-dose irradiation, the mechanical properties of XPTFE were lost, making their stretching processes were dominated by the orientation of some small broken crystals along the stretching direction and accompanied by the formation of voids. But for XPTFE under constant stress uniaxial stretching at variable temperature, its chain segments and molecular chains cannot be oriented even at elevated temperatures, maintaining a degree of thermal stability.

The effect of temperature and peroxides content on cross-linking and properties of EPDM-based rubber matrix

In this work, dicumyl peroxide (DCP), di-tert-butyl peroxide (DTBP), di(tert-butylperoxyisopropyl)benzene (DTBPIB), and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (DDTBPH) were used for cross-linking of rubber matrix based on Ethylene-propylene-diene monomer rubber (EPDM). The aim was to investigate the influence of curing temperature and the type and content of peroxide on curing process of rubber compounds, cross-link density, and physical-mechanical properties of vulcanizates. The results revealed that higher curing temperatures promote faster decomposition of organic peroxides, leading to the faster reactions of peroxide radicals with rubber chains. The increase in temperature resulted also in the increase of cross-linking degree. Although, side reactions as chain scission take over the main cross-linking reactions at very high temperatures resulting in the decrease of cross-link density. The increasing content of peroxides resulted in the acceleration of curing reactions and to increase of cross-link density. The fastest curing kinetics exhibited rubber compounds cured with DCP. However, vulcanizates cured with DCP were found to have the lowest cross-link density. In generally, the dependences of modulus, hardness, and elongation at break of vulcanizates were in line with the dependences of the cross-link densities, while higher tensile strength were found to have vulcanizates cured at higher temperatures and lower peroxides content.