Physical Cross-linking Points Research Articles

Effect of AC stressed aging on partial discharge, thermal and tensile performance of silicone rubber-based composites

In this paper, partial discharge characteristics of the multi-stressed aged and un-aged high temperature vulcanized silicone rubber (HTV-SR) samples with several concentrations of alumina trihydrate (ATH) and silica are studied. This is to elucidate whether the addition of nano/micro fillers of silica and ATH in silicone rubber enhances the resistance against partial discharges and aging behavior under different weathering conditions. Further, performance assessment is made by thermogravimetric analysis, differential scanning calorimetry and through measurement of tensile strength. Using IEC (b) electrode system, partial discharge measurements are performed according to IEC 60270. From these experimental measurements, it has been observed that by adding filler contents, the multi-stress aging performance improved and partial discharge resistance is enhanced up to 40% as compared to the HTV-SR without any filler. Moreover, through analysis of data obtained by thermogravimetric, indicate that nano doping increases the physical and chemical cross-linking points which confine the motion at the particle-matrix interface. As a result, the enhanced thermal stability of the composites. Considerable improvement in the stress-strain behavior of test samples is also observed with the addition of silica and ATH.

Ultra-stretchable and healable hydrogel-based triboelectric nanogenerators for energy harvesting and self-powered sensing.

The next-generation multifunctional soft electronic devices require the development of energy devices possessing comparable functions. In this work, an ultra-stretchable and healable hydrogel-based triboelectric nanogenerator (TENG) is prepared for mechanical energy harvesting and self-powered sensing. An ionic conductive hydrogel was developed with graphene oxide and Laponite. as the physical cross-linking points, exhibiting high stretchability (∼1356%) and healable capability. When using the hydrogel as the electrode, the TENG can operate normally at 900% tensile strain, while the electrical output of the TENG can fully recover to the initial value after healing the damage. This hydrogel-based TENG is demonstrated to power wearable electronics, and is used as a self-powered sensor for human motion monitoring and pressure sensing. Our work shows opportunities for multifunctional power sources and potential applications in wearable electronics.

The preparation and characterization of a heterocyclic meta-aramid fiber with outstanding thermal stability

A heterocyclic meta-aramid fiber containing benzimidazole structure was prepared by the low-temperature polycondensation of m-phenylenediamine, 2-(4-aminophenyl)-5-benzimidazole, and isophthaloyl dichloride, followed by dry-jet wet spinning. The structure and properties of the heterocyclic modified meta-aramid (h-aramid for short) were explored and compared with poly(isophthaloylmetaphenylene diamine) (m-aramid for short). H-aramid exhibits excellent spinnability. The surface of the fiber is smooth and even, with a partially ordered structure. Mechanical properties, thermal resistance, and flame retardant of h-aramid are significantly enhanced as the introduction of benzimidazole can not only increase the rigidity of molecular chains but also strengthen hydrogen bonds and act as physical crosslinking points. The strength and modulus of h-aramid are up to 0.84 GPa and 16.66 GPa, respectively, with an elongation at break of 20%. The glass transition temperature and melting temperature of h-aramid reach 302°C and 356°C, respectively, about 30°C and 40°C higher than that of m-aramid. Temperature of 10 wt% weight loss under nitrogen and air atmosphere is 17°C and 39°C higher than those of m-aramid. Besides, the thermal degradation behavior of h-aramid was studied, and its degradation process and mechanism was proposed.

Oligoether grafting on cellulose microfibers for dispersion in poly(propylene glycol) and fabrication of reinforced polyurethane composite

Application of cellulose microfibers as a reinforcing agent for high-performance polyurethane composites is rarely mentioned because the inherent hydrophilicity combined with the adverse aspect ratio of the fibers cause difficulties in dispersion within and compatibility with the polymer matrix. Here, we propose a feasible approach to graft oligoethers synthesized by ring-opening polymerization in a heterogeneous reaction to the cellulose microfiber surface. The oligomers composed of ether linkages and flexible hydroxyl groups provide cellulose fibers with good dispersity in poly(propylene glycol) and potential physical and chemical cross-linking points in reaction with isocyanates to form good interfacial adhesion between the fibers and the polyurethane matrix. Consequently, the tensile properties and viscoelasticity of the oligomer-grafted cellulose-filled polyurethane composite were enhanced compared with those of the neat elastomer and composite containing pristine cellulose microfibers. This reinforcement effect is attributed to the natural stiffness of the fiber bundles and the cross-linking network generated by the oligomers grafted on the fiber surface.

Hydrogen Bonding-Derived Healable Polyacrylate Elastomers via On-demand Copolymerization of n-Butyl Acrylate and tert-Butyl Acrylate.

Achieving a desirable combination of good mechanical properties and healing efficiency is a great challenge in the development of self-healing elastomers. Herein, a class of tough and strong self-healing polyacrylate elastomers (denoted as HPs) was developed simply by free-radical copolymerization of n-butyl acrylate (nBA) and tert-butyl acrylate (tBA) and a subsequent hydrolysis reaction rather than direct copolymerization of nBA and acrylic acid (AA). The tiny difference in reactivity between nBA and tBA makes the structural units of the copolymer easy to control. Precise regulation of molecular composition can be realized just by varying the relative monomer content, making its mechanical properties to vary from ductile to robust. Strikingly, when HP samples are cut off within the gauge length, they can heal into coherent and smooth samples and recover at least 79% of the original strength. Hydrogen bond interactions serve as physical cross-linking points, contributing to the high mechanical performance (fracture energy of up to 73.78 MJ·m-3 and tensile strength of up to 17.80 MPa) as well as shape memory function. Moreover, the HP samples emit strong fluorescence when exposed to a 365 nm UV lamp and exhibit an aggregation-enhanced emission effect in the state of dissolution.

Effects of Polyhedral Oligomeric Silsesquioxane (POSS) on Thermal and Mechanical Properties of Polysiloxane Foam.

The thermal and mechanical properties of polysiloxane foam are greatly improved by the addition of acrylolsobutyl polyhedral oligomeric silsesquioxane (MA0701, hereinafter referred to as MAPOSS), which has double bonds. The morphologies and properties of the polysiloxane composite foam were characterized. The average cell diameter of the composite foams decreased, while the cell density increased with increasing MAPOSS. Meanwhile, MAPOSS can enhance thermal conductivity and thermal stability. Thermal conductivity increased by 25%, and the temperature at the maximum weight loss rate increased from 556 °C to 599 °C. In addition, MAPOSS also promoted heterogeneous nucleation by functioning as a nucleating agent, which can increase cell density to improve the mechanical properties. The compressive strength of the composite foam increased by 170% compared with that of pure foam. In the composite, MAPOSS increased the cross-linking density by acting as a physical cross-linking point and limited the movement of the segments.

Molybdenum disulfide enhanced polyacrylamide-acrylic acid-Fe3+ ionic conductive hydrogel with high mechanical properties and anti-fatigue abilities as strain sensors

Existing conductive hydrogels suffer from poor mechanical performance required for flexible wearable sensors. Herein, molybdenum disulfide (MoS2) nanosheet enhanced crosslinked polyacrylamide-acrylic-Fe3+ (PAAM-Fe3+) hydrogels as strain sensors were synthesized by MoS2 catalyzing the autogelation at room temperature. The obtained hydrogels possessed high tensile strength (4.19 MPa), tensile elongation (594.6 %), elasticity (1.5 MPa), and high toughness (14 MJ/m3). The prepared hydrogels also achieved high self-recovery ability; the recovery efficiency of the compression stress was 136 % within 1 min. Based on microstructure observation and mechanical behavior analysis, we demonstrated that the existence of MoS2 and FeCl3 provided reversible physical crosslinking points and that MoS2 utilized adsorption to attract more Fe3+ to enhance ionic coordination in the system, leading to significantly improved mechanical properties and fatigue resistance. The hydrogels were further demonstrated to be strain sensors with high sensitivity (with high gauge factors of 1.50 and 9.06 at subtle (<100 %) and large strains (400 %)). Moreover, the ionic sensors successfully supplied real-time feedback on the movements of the human body. Accordingly, hydrogels offer promising insights for flexible electronic devices, sensors, human health monitoring and soft robots.

Size-dependent phase separation and thermomechanical properties of thermoplastic polyurethanes

Over the years, the manipulation of polymer microphase separation associated with molecular architectures has been holding great promise for rendering structure-property relationship. A series of poly (urea/malonamide) dendrons bearing rigid adamantanes in the periphery were developed to serve as branching hard segments (HS) for thermoplastic polyurethanes (TPUs) either in end-capping or pendant manner. Thermal properties, crystalline structures, and mechanical properties of the TPUs were investigated by DSC, X-ray scattering, and DMA, respectively. The introduction of the branching units with bulky adamantanes would expectedly hinder the crystallization of HS. Intriguingly, the dendrons not only accelerate the crystallization of the soft segment (SS, based on polycaprolactone polyol (PCL)), but promote the phase separation between HS and SS to form distinct microdomains as well. These enhanced effects revealed a size-dependent relationship with growing size (or generation) of the branching HS. With the high SS crystallinity that can freeze the chain mobility and the robust HS domains that work as the physical crosslinking points, the resulting TPUs incorporated with adamantane-bearing units show superior shape memory behaviors. This work demonstrates that the molecular engineering on the architectures can significantly tailor the microstructures and properties of thermal plastic polymers.

Anomalous tensile response of bacterial cellulose nanopaper at intermediate strain rates

Nanocellulose network in the form of cellulose nanopaper is an important material structure and its time-dependent mechanical response is crucial in many of its potential applications. In this work, we report the influences of grammage and strain rate on the tensile response of bacterial cellulose (BC) nanopaper. BC nanopaper with grammages of 20, 40, 60 and 80 g m−2 were tested in tension at strain rates ranging from 0.1% s−1 to 50% s−1. At strain rates le 2.5% s−1, both the tensile modulus and strength of the BC nanopapers stayed constant at ~ 14 GPa and ~ 120 MPa, respectively. At higher strain rates of 25% s−1 and 50% s−1 however, the tensile properties of the BC nanopapers decreased significantly. This observed anomalous tensile response of BC nanopaper is attributed to inertial effect, in which some of the curled BC nanofibres within the nanopaper structure do not have enough time to uncurl before failure at such high strain rates. Our measurements further showed that BC nanopaper showed little deformation under creep, with a secondary creep rate of only ~ 10–6 s−1. This stems from the highly crystalline nature of BC, as well as the large number of contact or physical crosslinking points between adjacent BC nanofibres, further reducing the mobility of the BC nanofibres in the nanopaper structure.

Formation of crystal-like structure and effective hard domain in a thermoplastic polyurethane

Traditionally, the hard domain of polyurethane was considered as the physical crosslinking point and has been studied for several decades. However, due to the hierarchical and metastable nature of the hard phase, its delicate structure hasn't been resolved thoroughly and its formation mechanism was unclear too. In this work, crystal-like structure in an amorphous polyurethane was discovered by conventional DSC experiments, and Flash DSC was used to study the thermodynamics and kinetics of such crystal-like structure by one-step and two-step temperature annealing experiment. It was found that the formation of crystal-like structure included nucleation and growth. However, its equilibrium melting point couldn't be obtained by Hoffman-Weeks’ method and its structural regularity was weaker than the crystal. Moreover, the nucleus had more compact structure than its outer layer and owned higher melting point. With the help of time dependent FTIR experiment, we proved that the hard domain formed much earlier than the crystal-like structure. Such result implied that the crystal-like structure was birthed inside the hard domain and its core was the reorganized hard segments, which acted as nucleus. After that, the reorganization of outer layer began, and was the growth process of crystal-like structure. Furthermore, time dependent oscillatory rheological experiment proved that the physical network in polyurethane formed much later than the formation of crystal-like structure, which meant the growth of hard domain was slow. In conclusion, the formation of effective hard domain, which could act as physical crosslinking point, experiences hydrogen bonding association, crystal-like structure formation and size expansion processes.

Investigation on curing reaction of phthalonitrile resin with nanosilica and the properties of their glass fiber‐reinforced composites

AbstractIn this work, Phthalonitrile containing benzoxazine (BA‐ph) and Bisphenol A based cyanate ester (CE) were chosen as the matrix resin. Various amount of nano‐SiO2 was incorporated into BA‐ph/CE and their glass fiber‐reinforced composite laminates were fabricated. Curing reaction and processability of BA‐ph/CE/SiO2 blends were studied by differential scanning calorimetry and dynamic rheological analysis. Results showed that BA‐ph and CE exhibited good processability and curing reaction of BA‐ph/CE was not obviously affected by SiO2. Scanning electron microscope images of the composites showed that SiO2 particles were well dispersed in BA‐ph/CE matrix. Moreover, SiO2 could act as physical crosslinking points and diluent in matrix as well as between the glass fibers to improve the mechanical properties of composite laminates. As the results of dynamic mechanical analysis and thermogravimetry analysis, composite laminates possessed satisfactory Tg and good thermal stability. With incorporation SiO2 particles into matrix resin, dielectric constant and dielectric loss of BA‐ph/CE/SiO2/GF composites were increased and showed frequency dependence.

Synthesis of polymer networks by means of addition reactions of tri-amine and poly(ethylene glycol) diacrylate or diglycidyl ether compounds

Polymer networks have been synthesized by addition reactions of tri-functional amine, tris(2-aminoethyl)amine (TAEA) or trimethylolpropane tris[poly(propylene glycol)amine terminated] ether (PPGTA), and poly(ethylene glycol) diacrylate (PEGDA), or poly(ethylene glycol) diglycidyl ether (PEGDE) in dimethyl sulfoxide (DMSO) as a solvent. These reactions yielded the gels in which the polymer networks were homogeneously swollen in DMSO. An increase in the monomer concentration increased Young’s modulus and breaking stress of the gels. The gels with PPGTA showed flexible features in comparison with those with TAEA due to the flexible molecular structure of PPGTA. The gels composed of TAEA and PEGDA or PEGDE with high molecular weight tended to show lower Young’s modulus and higher breaking strain than the gels composed of those with low molecular weight due to the decrease in the cross-linking density and/or increase in flexibility of the PEG unit accompanied by the increase in the molecular weight of PEG. The gels with PPGTA showed opposite tendency, indicating entanglement of PEG unit with high molecular weight, which would play a role of physical cross-linking points in the gels. The gels prepared in N,N-dimethylformamide (DMF) were flexible in comparison with the gels prepared in DMSO due to the high miscibility of the network and DMF. The reaction systems of TAEA and PEGDA with low molecular weight in ethanol formed porous polymeric networks derived from the phase separation during the polymerization. The porous polymeric networks were unbreakable by compression and absorbed various organic solvents.

Stiffness switchable supramolecular hydrogels by photo-regulating crosslinking status

Stimuli-responsive hydrogels have been well known as a kind of promising functional material. Among them, intelligent supramolecular hydrogels constructed by covalent and non-covalent bonds have received increasing attention and been extensively researched. In this work we demonstrate a class of supramolecular hydrogels formed via host-guest interactions between coumarin and γ-cyclodextrin. Its stiffness can be modulated by photo-switching the weak physical crosslinking points to strong chemical crosslinking points. It is a simple and effective way to construct a dual-modality supramolecular hydrogel. Owing to the dynamic and reversible nature, the supramolecular hydrogels exhibit a bright future for the usage in self-healing and shape memory materials.

Spatial dependence of ordering process in bulk materials of polylactide and its multiple system during hygrothermal aging

With a growing availability of polylactide (PLA) intended to three-dimensional molding applications, its spatial dependence of structural evolution during hygrothermal aging needs to be further investigated, in order to evaluate the service and degradation of PLA. This work mainly focuses on the spatial dependence of ordering process in the bulk materials of PLA and PLA/poly(butylene adipate-co-terephthalate) (PBAT) multiple system during hygrothermal aging. It is the first time that the microbeam wide angle X-ray diffraction (μWAXD) combined with differential scanning calorimetry and scanning electron microscopy are utilized to quantitatively analyze the spatial dependence of ordering process in the three-dimensional samples. Interestingly, owing to the presence of substantial mesophase and the preferential penetration of water molecules, stronger diffraction intensity in the skin layer can be identified in PLA/PBAT blends at the initial stage of hygrothermal aging, meanwhile neat PLA remains a negligible crystallinity. We ascribe the accelerated crystallization in PLA/PBAT blends to the higher Xmeso (6.3–9.0%) for unaged PLA/PBAT blends. Moreover, at the initial stage of hygrothermal aging, the mesophase can act as active nuclei to promote the formation of crystal, and the PBAT could act as heterogeneous nucleation to facilitate the transformation of mobile amorphous (MAF) to mesophase. However, at the late stage the original mesophase consumes up and the increasing ordered phase could act as physical cross-linking point to hinder the transforming into local ordering, leading to the slower transformation rate in PLA/PBAT blends than that in neat PLA. Notably, the obvious change of phase morphology can be observed at the late stage, and a blurred interface between PLA and PBAT may infer an improved compatibility. The reason for improved compatibility between PLA and PBAT can range a wide combination of factors including lower molecular weight PLA produced by hydrolysis and transesterification.

Thermomechanical-induced polyelectrolyte complexation between chitosan and carboxymethyl cellulose enabling unexpected hydrolytic stability

Natural biopolymers such as chitosan and cellulose have demonstrated huge potential in important and rapidly growing environmental and biomedical applications. However, it is always challenging to create advanced functional biopolymer materials with enhanced hydrolytic stability cost-effectively. Here, we report an advance in preparing biopolymer polyelectrolyte complexed materials based on chitosan and carboxymethyl cellulose (CMC) using a “dry”, thermo-mechanical kneading method. Despite the high hydrophilicity of chitosan and CMC, the resulting films showed excellent dimensional stability and structural integrity (27% dimensional expansion and 94% weight increase after hydration for one day). In comparison, chitosan-only films were swollen dramatically under the same conditions, with a 138% dimensional expansion and a 913% rise in weight, which were also fragile. We propose that our processing method led to polyelectrolyte complexation between chitosan and CMC generating physical crosslinking points in the materials, which stabilised the films in water. Interestingly, the greater hydrolytic stability of chitosan/CMC films is in contrast with their higher surface hydrophilicity, a contribution from CMC. Our simple approach to engineering high-performance biopolymer materials without resorting to complex chemistries can be envisioned to bring about a new direction in the design of advanced functional materials where sustainability and cost-effectiveness are priorities.

Effect of polyether/polyester polyol ratio on properties of waterborne two-component polyurethane coatings

In order to obtain waterborne two-component polyurethane (WB 2K-PUR) with good mechanical properties as well as excellent corrosion resistance, a series of hydroxyl-based polyurethane (HO-WPU) were synthesized using isophorone diisocyanate (IPDI), dimethylolpropionic acid (DMPA), and different proportions of polyether polyol (PTMEG) and polyester polyol (P1012) in this paper. Then WB 2K-PUR coatings were prepared with water-dispersible isocyanates (WDPI) as curing agent. The influence of the proportion of polyester polyols and polyether polyols in soft segments on coatings properties was studied. The characteristics of the coatings were analyzed by Fourier transform infrared spectroscopy (ATR-IR), contact angle, differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA) and atomic force microscopy (AFM). The results showed that the mechanical properties and corrosion resistance of polyester and polyether polyols with appropriate proportion are improved significantly. The mechanism tended to be that with the addition of polyester, more hydrogen bonds were formed between the molecules, which increased the compactness of the coating as a physical crosslinking point. However, the ester groups have a strong affinity to water molecules, so the hydrophilicity of the coating was increased by excessive polyester polyols, and the shielding effect on the corrosion medium was reduced. With the inclusion of 40 % (g/g) P1012 in polyols, the coating has satisfied comprehensive properties.

Effect of weight percentage of B4C reinforcement on physical and mechanical properties of polyamide 6/polystyrene composites

ABSTRACT Blends of polyamide 6/polystyrene (PA6/PS) (50/50 wt./wt. %) were loaded with different concentrations of boron carbide (B4C), (0, 5, 10, 15, 20 and 30 wt./wt. %) separately for EMI shielding. The effects of different weight fractions of fillers on thermal, electrical, as well as, mechanical behavior of the prepared composites were investigated. Thermal stability and hardness (shore D) of composites are studied. Addition of B4C enhances thermal stability of the prepared composites which attributed to the increase in physical and chemical cross-linking points, also to the interactions between the filler and polymer blend. Confirmation of thermal stability was done by determining the activation energy (E a) for the thermal decomposition of all composites. Characterization of the prepared composites was done using SEM. With B4C-loaded PA6/PS blend composites, samples of 2 mm thick, SE values of 5–36 dB were achieved, satisfying the requirement for common commercial applications.

Physical and Rheological Properties of Maleic Anhydride-Incorporated PVDF: Does MAH Act as a Physical Crosslinking Point for PVDF Molecular Chains?

The miscibility and physical and rheological properties of binary poly(vinylidene fluoride)/maleic anhydride (PVDF/MAH) blends have been systematically investigated. MAH was found to be miscible with PVDF by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). Fourier transform infrared (FTIR) investigations provided positive evidence for the specific interaction between the carbonyl groups of MAH and the methylene groups of PVDF. Rheological measurements showed that both the storage modulus and the melt viscosity of PVDF increase with the addition of MAH, followed by a decrease with excess MAH. In addition, the elongation of the PVDF/MAH blend with 10 wt % MAH is 589.7%, which is almost 5 times that of neat PVDF. It is concluded that MAH small molecules act as physical “crosslinking” points for the neighboring PVDF molecule chains due to this specific interaction between PVDF and MAH. Such a physical crosslinking function enhances the storage modulus, viscosity, and mechanical properties of PVDF.

Temperature dependent network properties of amorphous PCT during tensile stretching

Tensile deformation behavior of amorphous Poly (1,4-cyclohexylenedimethylene Terephthalate) (PCT) at 25 °C–108 °C was investigated by means of in situ synchrotron wide-angle X-ray scattering (WAXS). WAXS patterns were processed in a way that the scatterings from isotropic and anisotropic structures were separated. It turned out that the influences of temperature during deformation can be divided into three zones: below 80 °C, only molecular chains oriented; between 80 °C to 90 °C, very few imperfect crystals were induced by strain; above glass transition temperature (92 °C), the triclinic PCT crystalline structure began to form during deformation process. It was found that the Gaussian model of Haward and Thackray described the stress-strain behavior of the tensile stretched amorphous PCT very well. However, when being stretched above 80 °C, the system showed two characteristic network moduli because of strain-induced crystallization. The onset of change in network modulus occurred immediately after the formation of unoriented crystallites which served as physical cross-linking points to reinforce the material.

A polyolefin encapsulant material designed for photovoltaic modules: from perspectives of peel strength and transmittance

A secondary master batch process had been applied to design a polyolefin encapsulant material for photovoltaic modules, in which the polymer blend was composed of polyolefin elastomer (POE) and linear low-density polyethylene (LLDPE) with the addition of the cross-linking agent of tert-butylperoxy 2-ethylhexyl carbonate (TBEC) and silane coupling agent of γ-methacryloxypropyl trimethoxy silane (KH570). The rheological property from Haake torque rheometer illuminated that the chemical cross-linking reaction could take place between the POE and TBEC. The LLDPE was added for further increasing the cohesion of polymer blend by obtaining much more physical cross-linking points from the physical entanglement between the flexible chains of LLDPE and the amorphous zone of POE. However, the transmittance of polymer blend ran down with the rise of LLDPE content within the polymer blend. The number and site of melting points of polymer blend by the DSC analysis show that there was compatibility between the POE and LLDPE within the polymer blend; meanwhile, the resultant molecular structures of the polymer blend also had been explained by the FTIR spectra. These results unanimously illuminated that it was a feasible solution for fabricating the polyolefin encapsulant material for photovoltaic modules.