Network Chain Density Research Articles

Tensile and Elastocaloric Properties of Natural/Devulcanized Waste Rubber Blends.

The need for eco-friendly cooling materials and material recycling are two urgent challenges to address. In this paper, the role of the ground tyre rubber treatment (cryo-grinding and devulcanization) is investigated on the tensile and elastocaloric properties of Natural rubber (NR)/ ground tyre rubber (GTR). The GTR particles that are sieved (<63µm) and devulcanized by microwave irradiation (1 min at 800Watts) exhibit a low network chain density (0.53 × 10-4mol.cm-3) resulting from crosslinks breakage and rubber chains scission, as supported by FTIR showing a decrease of S─S, C─S, and C─C bonds. The NR/GTR blends show a high elastocaloric effect as compared to the pristine NR, which can be ascribed to the high content of carbon black in the GTR (52wt.%) and also the high level of devulcanization of the GTR. NR/GTR blends reach a heating of +8°C and a cooling of >-6°C, resulting in a material's coefficient of performance COPmat = 2.8-3 compared to 2.6 for the pristine NR. The concomitant effect of cryogrinding and microwave devulcanization is proposed as a way to improve the tensile and elastocaloric properties of natural rubber/waste rubber blends for their possible integration into elastocaloric devices for heating/cooling applications.

Control by the curing time of the strain induced crystallization and elastocaloric properties in natural rubber and natural/waste rubber blends

This study provides a systematic investigation of the influence of the curing time on strain-induced crystallization (SIC) and elastocaloric (eC) effect in peroxide-cured natural rubber (NR) and NR/ground tyre rubber (GTR) blends. To do so, materials were prepared with three distinct curing times (t10, t50, and t90). Results reveal that extended curing times correlate with increased network hardness and elastic modulus, promoting the number of elastically active chains. GTR fillers accelerate vulcanization but reduce final network chain density due to their partially devulcanized state and their contribution to the vulcanization of the blends. Both SIC and eC effects improve with curing time, with properties at t50 closely matching those at t90, indicating an optimal crosslink density at such times. The temperature span during mechanical cycling increases linearly with curing time, largely unaffected by GTR presence. The material's coefficient of performance (COP) peaks at t50, suggesting this curing time offers the best balance between mechanical energy dissipation and temperature change, highlighting its potential for eco-friendly heating/cooling applications.

Revealing the Effect of the Length of Chains on Rubber Mechanical Properties by the Two‐Component Quantum Mechanical Gaussian Model

AbstractThe special mechanical properties of natural rubber originate from the stretching of the network chains (i.e., cis‐1,4‐polyisoprene chains) in the rubber network. The chains with various lengths exhibit various relative extensions, leading to various normalized energy contributions to the mechanical properties of rubber. However, the stretching behaviors of cis‐1,4‐polyisoprene chains with different lengths are not studied clearly. Here, the stretching behavior, which is the basis for analyzing the normalized energy contribution, is obtained by the modified freely jointed chain model. Next, using a new statistical model, i.e., the two‐component quantum‐mechanics Gaussian (TCQMG) model, the relative extension–length relationship of network chains is determined. Combining the above results, the energy contributions of network chains with different lengths are analyzed. The results show that long and short chains show different stretching states. The following can be explained by these results: short chains play an important role in the mechanical properties of rubber. As the network chain density increases, the average length of the chains decreases, leading to the variation of the normalized extensions. It is found that the variation of the normalized extensions is responsible for both the effects of the network chain density and network chain length on the mechanical properties of rubber.

Poly(N,N-dimethylacrylamide)-clay nanocomposite hydrogels with patterned mechanical properties

Poly(N,N-dimethylacrylamide)-clay nanocomposite gels (NC gels) with patterned network-chain density were prepared by photoinitiated radical polymerization with photomasks. The exposed region of the obtained gels showed a higher swelling ratio and lower elastic modulus than the masked region, although these regions had the same weight composition. A stripe-patterned gel prepared using a photomask with many slits of several hundred micrometers wide exhibited mechanical anisotropy: the elastic modulus in the direction parallel to the slits was higher than that in the perpendicular direction.

ACTIVATION ENERGIES OF THERMO-OXIDIZED NITRILE RUBBER COMPOUNDS OF VARYING ACRYLONITRILE CONTENT

ABSTRACT The thermo-oxidative behavior of carbon black–reinforced sulfur-cured nitrile rubber compounds with varying acrylonitrile (ACN) content (18–49 wt%) was investigated. Accelerated heat aging was carried out from 40 °C to 115 °C for various aging times. Ambient aging was also included. Samples were tested for hardness, 10% tensile stress, tensile strength, elongation at break, network chain density by equilibrium solvent swell, and toluene-soluble fraction. Diffusion-limited oxidation affected data at high temperatures and was eliminated for time-temperature superposition. Linear Arrhenius kinetic behavior was confirmed throughout the whole temperature range, and calculated activation energies varied from 75 to 93 kJ/mol. Activation energies calculated through the hardness data were found to increase steadily with ACN concentration, whereas the other test responses showed less direct correlation, likely because of the influence of the underlying NBR microstructure, which changes as a function of ACN content. The high-temperature thermo-oxidative process consists of both oxidative crosslinking and chain scission reactions. Sulfur reversion and alkyl radical recombination reactions are likely prevalent at low temperatures during the buildup of hydroperoxides up to 60 °C. The shelf life of nitrile rubbers strongly depends on their ACN level, with lower ACN nitriles being more susceptible to degradation, leading to shorter shelf lives, than higher ACN-containing nitriles.

Research on architecture and composition of natural network in natural rubber

Though the superior properties of natural rubber (NR) have been attributed to its special network architecture, the currently accepted model “naturally occurring network” is far from describing its authentic network structure. In this paper, we focused on the composition of the chain entanglements in the network structure of unvulcanized NR. By using synchrotron wide-angle X-ray diffraction (WAXD), the evolution of its strain-induced crystallization (SIC) behaviors was real-time traced, and the stress-strain behaviors at various strain rates and temperatures were also tested. The results demonstrated that the entanglements can act as crosslinking points to increase the network chain density and lead to the easier SIC behavior. By applying the tube model to analyze the stress-strain curves of unvulcanized NR, we found that the contribution of the entanglement network to the stress is about an order of magnitude larger than that of the network formed by two terminal groups. Via double-quantum nuclear magnetic resonance (1H-DQ NMR), we further detected two types of entanglement in NR and deproteinized NR (DPNR), i.e. transiently trapped entanglements (TTEs) and permanently trapped entanglements (PTEs), among which TTEs are the main composition of the entanglement network. Moreover, the network formed by two terminals causes more PTEs in NR, while due to lacking the constraint of proteins, more TTEs exist in DPNR. Based on the research, we proposed a novel network model for unvulcanized NR.

Experimental determination of the quantity and distribution of chemical crosslinks in unaged and aged natural rubber, part 1: Peroxide vulcanization

A set of unfilled and stabilized natural rubber compounds containing different dicumyl peroxide concentrations were prepared and their chemical crosslink densities were determined using a variety of experimental techniques: stress–strain (Mooney-Rivlin), equilibrium solvent swell (Flory-Rhener) and low field time domain Nuclear Magnetic Resonance (NMR) by the Double Quantum (DQ) method. Testing was carried out on both the unaged and heat aged (96 h at 80 °C) samples. Increasing peroxide level caused a corresponding rise of crosslink density and a decrease in the chain entanglement density, while the relative crosslink distribution width remained unchanged. The molecular weight between entanglement points was in the expected range for natural rubber. Heterogeneous areas of high crosslinking (clusters) were low in concentration. Heat aging caused properties to deteriorate due to chain scission reactions that eliminated network chains creating a more heterogeneous system. Defect levels increased and more toluene soluble uncrosslinked chains were detected. The network chain and chain entanglement densities decreased with heat aging and the molar mass distribution between crosslinks substantially broadened independent of peroxide loading.

INFLUENCE OF THE CROSSLINK STRUCTURE ON THE ACTIVATION ENERGY CALCULATED UNDER THERMO-OXIDATIVE CONDITIONS

ABSTRACT A series of six carbon black reinforced brominated poly(isobutylene-co-isoprene) (BIIR) compounds has been developed varying only in cure system type: sulfur, sulfur donor, zinc oxide, peroxide, phenolic resin, and ionic. Compounds were aged from room temperature up to 115 °C, and hardness, mechanical properties, and network chain density were measured. Non-Arrhenius behavior was observed due to data curvature from 70 to 85 °C. The oxidation process was adequately described by assigning low (23–85 °C) and high (85–115 °C) temperature regimes. Heterogeneous aging due to diffusion limited oxygen (DLO) occurred for heat aging above 85 °C, and all measured responses except tensile strength were strongly affected, causing lower activation energies. The activation energy for the high temperature oxidation process is in the range of 107 to 133 kJ/mol in the following ascending order: zinc oxide, ionic, sulfur donor, sulfur, peroxide, and resin. The midpoint of the high temperature activation energies is of the same order as the BIIR and poly(isobutylene) elastomers. The low temperature activation energy is in the range of 55–60 kJ/mol and is likely due to a combination of oxidative chain scission (crosslink density loss) and crosslinking recombination (network building) reactions. Apart from the crosslink structure stability, the presence of unsaturation along the polymer chain after vulcanization affects the high temperature activation energy.

EFFECT OF THERMO-OXIDATION ON PERMEATION RESISTANCE OF BROMOBUTYL COMPOUNDS

ABSTRACT The impact of thermo-oxidation on the permeation process in a series of BIIR or bromobutyl compounds has been investigated. Methyl salicylate (MS) was used as the simulating chemical agent, and the permeation rate was measured gravimetrically using vapometers. Heat aged samples at 120 °C increased in stiffness with an accompanying decline in ultimate properties and network chain density. A reduction in permeability was found, driven primarily by the reduced solubility of MS in the thermo-oxidized BIIR matrix. The formation of a secondary oxidized chain network has been proposed to explain the decline in MS solubility. The loss of plasticizer during the heat aging process is responsible for the reduction of the MS diffusibility. Diffusion-limited oxidation during thermo-oxidation is also a likely factor in affecting the MS transport process.

Complex dependence on the elastically active chains density of the strain induced crystallization of vulcanized natural rubbers, from low to high strain rate

Strain Induced Crystallization (SIC) of Natural Rubbers (NR) with different network chain densities (ν) is studied. For the weakly vulcanized rubber, the melting stretching ratio λm at room temperature is the lowest. This is correlated with larger crystallites in this material measured by in situ WAXS, suggesting their higher thermal stability. SIC kinetics is then studied via stretching at various strain rates (from 5.6 × 10−5 s−1 up to 2.8 × 101 s−1). For the slowest strain rates, SIC onset (λc) is clearly the lowest in weakly vulcanized rubber. By increasing the strain rate, λc of the different materials increase and converge. For the highest strain rates, λc values still increase but less rapidly for the weakly vulcanized sample. This complex dependence on the elastically active chains (EAC) density of SIC has been confirmed by in situ WAXS during dynamic experiments and interpreted as a consequence of both the polymer chain network topology and of the entanglements dynamics.

Mechanical properties and network structure of hydrophobic association hydrogels prepared from octylphenol polyoxyethylene(7) acrylate and sodium dodecyl sulfate

Hydrophobic association hydrogels(HA-gels) with high mechanical strength were prepared by free radical micellar copolymerization in aqueous solutions of acrylamide(AM), anion surfactant sodium dodecyl sulfate(SDS) and a small amount of hydrophobic monomer octylphenol polyoxyethylene(7) acrylate(OP-7-AC). We found that the molar ratio of SDS to OP-7-AC has a great effect on the tensile strength and other mechanical property parameters. The best ratio point R′ was determined. On the basis of Mooney theory and statistical theory, the critical tensile ratios and critical tensile strengths of the hydrogels were obtained, elastic parameters C 1 and C 2 were calculated via uniaxial tensile equation and structural parameters, such as the effective network chain density and the averaged molecular weight of the chain between cross-linking points of all the hydrogels were evaluated. The results indicate that the variation of mechanical property parameters depends on the number of effective cross-linking points and the match degree of long and short chains.

Temperature dependence of strain-induced crystallization in natural rubber: On the presence of different crystallite populations

The effect of the temperature on strain induced crystallization (SIC) of natural rubber (NR) is studied by in situ wide angle X-rays scattering (WAXS) experiments performed along different thermo-mechanical paths (cyclic deformation at different temperatures or crystallization and melting in the deformed state). The crystallinity index (CI), the average crystallite size (L200) and the average volume of the crystallites (V) increase (decrease) similarly if the sample is cooled (heated) in the deformed state or stretched (unstretched) at fixed temperature. These experiments are analyzed through a thermodynamic description assuming that SIC is controlled by successive nucleation of crystallite populations. Such description predicts that for a stretching ratio λ above (below) 4 and a temperature above (below) room temperature, crystallization first occurs in chains with the highest (lowest) network chain density, leading to the smallest (largest) crystallites.

Strain-Induced Crystallization of Natural Rubber and Cross-Link Densities Heterogeneities

Strain-induced crystallization (SIC) of natural rubber (NR) is characterized during a cyclic deformation at room temperature and low strain rate (∼10–3 s–1) using in situ wide angle X-rays scattering (WAXS) measurements. The crystallinity index (CI) and average size of the crystallites in the three main directions are measured during loading and unloading. A scenario describing SIC is then proposed, assuming that SIC corresponds to the successive appearance of crystallite populations whose nucleation and growth depend on the local network density. From this scenario, a methodology, coupling experimental observations and thermodynamic description is developed to determine the distribution of the network chain density associated with the size of a corresponding crystallite population. Finally, complex cyclic tests are performed. They suggest the existence of a memory effect in the chains involved in crystallization, which eases the nucleation process of the crystallites.

Homogenization of natural rubber network induced by nanoclay

ABSTRACTVulcanization gives birth to the nonuniformity of rubber network, the identification of which is the basis of improvement of performance of rubber products. We established a visco‐hyperelastic constitutive equation to reveal the quantitative distribution of the inhomogeneous network phases according to a three‐phase model. The cross‐linked network was assumed to be composed of the cross‐linking cluster, the low network chain density domain and the fluid‐like mass. The incorporation of clay with high specific surface area induced the effective uniformity of network structure by decreasing the content of the cross‐linking cluster and increasing that of the low network chain density domain. These structural variations were responsible for excellent mechanical properties and strong strain‐induced crystallization ability probed by the in situ synchrotron wide‐angle X‐ray diffraction. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40324.

Synthesis of poly(methylphenylsiloxane)/phenylene-silica hybrid material with interpenetrating networks and its performance as thermal resistant coating

In this study, poly(methylphenylsiloxane) (PMPS) and phenylene-silica based hybrid material with interpenetrating networks was prepared by a two-step sol–gel process. Firstly, in the presence of H2SO4, the phenylene-silica was formed as sol particles with high branching degree by cohydrolysis and condensation of phenylene-bridged monomer, tetraethoxysilane (TEOS), and hexamethyldisiloxane (MM). Then, the intermediate transformed into gel framework in polymer matrix using alkali catalyst, in order to produce a homogenous hybrid material with interpenetrating networks. The structure of prepared hybrid material was characterized by FTIR and NMR, suggesting that phenylene-silica framework was imported into polymer matrix and the hybrid products have a much higher network chain density than neat PMPS. The thermogravimetric analysis (TGA) shows that the prepared materials start to degrade at around 490°C. The results of tensile test indicate that the typical PMPS/phenylene-silica hybrid material has a tensile strength up to 26 MPa and demonstrate a certain degree of flexibility. An increase of phenylene content in phenylene-silica particles tends to produce hybrid materials with improved thermal stability and tensile strength. The hybrid coating films after calcinating at 350 and 400°C for 2 h exhibit a good mechanical performance on adhesion, impact strength and flexibility. Electrochemical impedance spectroscopy (EIS) measurements show that the investigated films have an extremely high electric resistance (1010 Ohm·cm2) and a satisfied impermeability to 3.5 wt % sodium chloride solution. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Large deformation behavior and effective network chain density of swollen poly(N-isopropylacrylamide)–Laponite nanocomposite hydrogels

Hydrogels were often used in aqueous and/or physiological saline media, thus, the mechanical properties of the swollen gels were of particular importance. Until now, these data were lacking due to the most swollen hydrogels being too weak and brittle to undergo the deformation test at all. The mechanical behavior of swollen poly(N-isopropylacrylamide) (PNIPAm)–Laponite nanocomposite hydrogels (NC gels) were studied with compression and elongation under large strains causing deformation. The swollen NC gels were intact after compression and sustained elongation of up to 500%. Strain hardening was observed in the swollen NC gels for the first time, which was considered to be the extensional limitation of the polymer network chains due to the orientation of the Laponite platelets. Therefore, the strain hardening was enhanced by increasing the Laponite content in the NC gels. When the deformation was low, the compression and elongation stress-deformation curves of the as-prepared and swollen NC gels were described quantitatively with the Mooney–Rivlin model. Whilst for high deformation, Creton's model considering the finite extensibility enabled the prediction of these curves almost quantitatively. Stress-deformation hysteresis was found in the as-prepared and swollen NC gels, which was the result of the deorientation of the clay platelets and relaxation of the polymer chains. The orientation process of the clay platelets dissipated energy during deformation, resulting in a high toughness of the NC gels even in the swollen state. The effective network chain density in the swollen NC gels manifested that the swelling in water only expanded the gel volume and did not damage the cross-linking points. The number of effective network chains attached to one Laponite XLS platelet was almost independent of the clay content, so the addition of clay platelets in the NC gel formed new cross-linking junctions.

Ultrahigh Tensibility and Stimuli‐Response of Polymer‐Hectorite Nanocomposite Hydrogels

AbstractSummary: In the field of hydrogels with high mechanical performance, the nanocomposite hydrogel (NC gel) is widely interested due to its unique network structure with extraordinary mechanical, optical, and swelling/deswelling properties. Over the past few years, our group has investigated the fabrication and mechanical properties of a variety of the NC gels with different monomers. The effective network chain density and relaxation were well characterized by dynamic mechanical measurements. From these efforts, we found that the high tensibility of the NC gels came from the low effective network chain density with moderate relaxation. Besides, transparent and ultratensible NC gels with pH response and dual response of pH and temperature have successfully fabricated and characterized. This review will give a brief introduction of our achievements on the understanding of the preparation and properties of NC gels.

Study on Strain-induced Crystallization of Cross-linked Natural Rubber Revealed by Synchrotron X-ray

Characteristics of strain-induced crystallization (SIC) are described on sulfur cross-linked and peroxide cross-linked natural rubber (NR) and synthetic isoprene rubber (IR). Simultaneous tensile and wide-angle X-ray diffraction measurements using synchrotron radiation systems were carried out in order to elucidate SIC. The stretched molecular chains acted as initiating species of the crystallization, while surrounding chains contributed to the crystal growth. Stretching ratio at the onset of SIC (αc) decreased with an increase of network-chain density (ν) for peroxide cross-linked NR (P-NR), while it remained constant for sulfur cross-linked NR (S-NR). But, dependence of relative crystallization rates on ν was similar for both P-NR and S-NR. Calculated entropy differences between the undeformed and the deformed states at αc were equal for P-NR regardless of ν, whereas it became smaller with the increase of ν for S-NR. The SIC behavior of P-NR was in agreement with the prediction on homogeneous or uniform networks by Flory. Network in sulfur cross-linked IR (S-IR) was composed of domains of high ν value embedded in the rubbery network matrix, which gave a characteristic feature in its SIC. Mechanical propeties of S-NR and P-NR are also discussed from the viewpoint of their SIC behaviors on the basis of the network structures.

Large-Scale Orientation in a Vulcanized Stretched Natural Rubber Network: Proved by In Situ Synchrotron X-ray Diffraction Characterization

In situ studies of strain-induced crystallization in unfilled and multiwalled carbon nanotube (MWCNT)-filled natural rubber (NR) were carried out by using synchrotron wide-angle X-ray diffraction (WAXD). Synchrotron WAXD results indicate that more nuclei appear in the MWCNT-filled NR sample, leading to higher crystallinity, lower onset strain of crystallization, and remarkable enhancement in tensile strength. During deformation, despite the amorphous chains remaining in isotropic orientation, the domains of larger scale (10-100 nm) with high network chain density in the NR matrix are oriented. The MWCNTs induce significant variation of this orientational process, and it is monitored by the stearic acid (SA) crystallites, which are effective nanoprobes of the amorphous phase. The results indicate that a small amount of MWCNTs and SA crystallites can be used as new tools to analyze the microstructural orientation of NR during deformation. The results also yield new insight into the strain-induced crystallization mechanism.

Molecular orientation and stress relaxation during strain-induced crystallization of vulcanized natural rubber

Birefringence and tensile stress of vulcanized natural rubber samples were measured simultaneously. The effect of finite chain extensibility could be clearly detected. The increase in birefringence was related to the degree of orientation of amorphous chains to be incorporated into crystals. The normalized stress decreased in a manner that is almost independent of network-chain density. These experimental results could be successfully explained by assuming a certain fraction of network chains that behave in a manner similar to that of a fluid.