Network Chain Density Research Articles

Self-healing Mechanism and Mechanical Behavior of Hydrophobic Association Hydrogels with High Mechanical Strength

Hydrophobic association hydrogels (HA-gels) were prepared through micellar copolymerizationof acrylamide (AM) and a small amount of octyl phenol polyethoxy ether acrylate (OP-4-AC) in an aqueous solution containing sodium dodecyl sulfate (SDS) at 50°C. The resulting HA-gels exhibited excellent mechanical properties and transparency. However, the most striking properties were that HA-gels possessed the capability of self-healing. The mechanical properties of HA-gels were investigated by uniaxial stretching. The results showed that the mechanical properties of HA-gels could be varied greatly by changing OP-4-AC content in the range of OP-4-AC used. On the basis of the rubber elastic theory and using uniaxial stretching data, the effective network chain density and the average distance between the cross-linking points were calculated for HA-gels. For tensile measurements of altering crosshead speed, the modulus of HA-gels increased sharply with increasing crosshead speed. The stress-relaxation behavior of HA-gels consisted of two stages. Similar to rubbery, HA-gels exhibited obvious thermoelastic behavior and also showed a good rubberlike elastic property.

Network structure and compositional effects on tensile mechanical properties of hydrophobic association hydrogels with high mechanical strength

Hydrophobic association hydrogels (HA-gels) were successfully prepared through micellar copolymerization of acrylamide (AM) and a small amount of octylphenol polyoxyethylene acrylate (OP-4-AC) in an aqueous solution containing sodium dodecyl sulfate (SDS). HA-gels exhibited excellent mechanical properties and transparency. Especially, HA-gels possessed the capability of re-forming, such as self-healing and molding. From Fourier transform infrared, swelling behavior and re-forming capability of HA-gels, the network structure was established. On the basis of the micellar copolymerization theory, the statistical molecular theory of rubber elastic, and using uniaxial stretching data, the length of the hydrophobic microblocks, the effective network chain density and the molecular weight of the chain length between cross-linking points were evaluated for all HA-gels; furthermore, they were also evaluated for the region of medium deformation by the Mooney-Rivlin theory. For HA-gels, we investigated in detail the effects of the content of compositions in the initial solution, OP-4-AC, SDS and AM, on their tensile mechanical properties on the basis of the proposed network structure. The results clearly indicate their tensile strength, fracture energy, elastic modulus, and elongation strongly depended on their composition content.

Preferential Adsorption of Poly(ethylene glycol) on Hectorite Clay and Effects on Poly(N-isopropylacrylamide)/Hectorite Nanocomposite Hydrogels

Poly(N-isopropylacrylamide)/Laponite nanocomposite hydrogel (NC gel) was synthesized via in situ polymerization in the Laponite suspension containing PEG. The adsorption of PEG on Laponite platelets was characterized by zeta-potential, which decreased with the PEG adsorption. The tensile strength decreased and elongation at break increased with increasing PEG concentration. The effective network chain density of PNIPAm/Laponite NC gels determined from the equilibrium modulus G(e) decreased upon adsorption of PEG on the Laponite. All of these results revealed the preferential adsorption of PEG on the Laponite platelets occupying the active sites for the PNIPAm chain anchoring, which hindered their cross-linking effect in the NC gels. However, the temperature sensitive swelling behavior still remained in the PNIPAm/Laponite NC gels containing PEG with higher swelling volume below the LCST due to the lower cross-linker density. By adjusting the amount of added PEG, we can easily control the properties of the PNIPAm/Laponite NC gels.

Synthesis and dual response of ionic nanocomposite hydrogels with ultrahigh tensibility and transparence

Ionic nanocomposite hydrogels cross-linked by hectorite Laponite XLS with high tensile strength and ultrahigh tensibility were successfully synthesized for the first time via in situ copolymerization of N-isopropylacrylamide (NIPAm) and sodium methacrylate (SMA). The pH and temperature response, transparency, and mechanical properties of the ionic hydrogels were investigated. The results showed that the addition of only 2mol% of SMA endowed the nanocomposite hydrogels with pH response, while the temperature response remained in the whole pH range. All the as-prepared hydrogels, even with 10mol% of SMA, demonstrated transparency higher than 75%. The tensile strength evidently decreased from 60kPa to 45kPa when the SMA content was higher than 6mol%. The elongation at break increased with increasing SMA content and 2800% was achieved for the sample containing 10mol% of SMA. The effective network chain density was estimated from the tensile stress at elongation of 200% and the equilibrium storage modulus. The low chain density was the intrinsic origin of the ultrahigh tensibility for these ionic NC gels. This work provides a new way to prepare dual responsive hydrogels with ultrahigh tensibility and high transparency.

Network chain density and relaxation of in situ synthesized polyacrylamide/hectorite clay nanocomposite hydrogels with ultrahigh tensibility

Nanocomposite hydrogels (NC gel) D-AM and S-AM were synthesized through in situ polymerization of acrylamide (AM) with hectorite clays of Laponite RD and RDS, respectively. The tensile performance of the NC gel was observed at different crosshead speeds and all of the NC gels exhibited an extremely high tensibility, e.g., the elongation at break even higher than 4000%, except for two samples with the lowest Laponite content of 1 w/v%. Strong tensile hysteresis was observed in the elongation–reversion curve, indicating a slow relaxation in the NC gels. Dynamic moduli G′ and G″ within linear viscoelasticity illustrated that the network structure was formed in these gels with the junction of Laponite platelets. The Laponite RD showed stronger gelation capability than the tetrasodium pyrophosphate modified Laponite RDS. The relaxation modulus G(t) for the NC gels was found to be similar to the slow rubber relaxation with the critical exponent n of about 0.16, much lower than 0.66–0.71 for the critical gel at the sol–gel transition. In comparison, the chemically cross-linked hydrogel showed almost no relaxation during the same time interval. The effective network chain density of the NC gel was determined from equilibrium shear modulus, which was evidently lower than that of the chemically cross-linked hydrogels. The present results reveal that the high deformability of these NC gels comes from their low effective network chain density with moderate relaxation.

Comparative Study on Strain-Induced Crystallization Behavior of Peroxide Cross-Linked and Sulfur Cross-Linked Natural Rubber

Strain-induced crystallization (SIC) behavior of natural rubber (NR) cross-linked by peroxide or sulfur was comparatively studied by time-resolved wide-angle X-ray diffraction measurements at SPring-8. 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 (ΔSdef) 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 is in agreement with the prediction on homogeneous or uniform networks by Flory. Thus, the network structure of S-NR is supposed to be less homogeneous than that of P-NR. The inhomogeneity in S-NR is estimated due to the presence of domains of high ν value embedded in the rubbery network matrix, which is supported b...

Study on Strain-induced Crystallization of Crosslinked Natural Rubber by Synchrotron Radiation

The characteristics of strain-induced crystallization (SIC) are described on sulfur-crosslinked and peroxide-crosslinked 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 rate of which was much faster than so called cold crystallization of NR. Onset strain of SIC of sulfur-crosslinked rubber was almost independent of network-chain density. The stretched molecular chains acted as initiating species of the crystallization, while surrounding chains contributed to the crystal growth. Deformation of crystal lattice with nominal stress was detected, which showed the strain-induced crystallites were responsible for the higher modulus upon stretching. A pantograph model describes the deformation mechanism of the rubber network. The effect of non-rubber components of NR on SIC was clearly detected, where the stress relaxation after high speed stretching gave the significant difference of SIC between NR and IR. Stearic acid did not accelerate SIC of sulfur-crosslinked NR and IR, oppositely with the cold or temperature-induced crystallization of them. The effect of carbon black filler on SIC of sulfur-crosslinked NR is also reviewed. For peroxide-crosslinked NR, SIC occurred when the deformation brought about a definite entropic state, this agrees with the theoretical prediction by Flory and is consistent with the classical theory of rubber elasticity.

Crystallization of stretched network chains in cross-linked natural rubber

Relatively fast kinetics of strain-induced crystallization (SIC) of cross-linked samples with various network-chain densities (ν) of natural rubber and its synthetic analog was examined by the fast time-resolved wide angle x-ray diffraction and simultaneous tensile measurements. The lateral crystallite size was almost unchanged with elapsed time, though the crystallization proceeded considerably during the period. The rate of SIC was faster for the samples having the higher ν during the first tens of seconds. While the development of SIC obviously depends on ν, progress of relative stress relaxation with time was almost independent of ν. The different dependence of the experimental results on ν was explained by assuming coexistence of stretched and relaxed network chains. During SIC at a fixed strain ratio, the intensity of crystalline reflections increased without reducing the intensity of anisotropic amorphous halo on the equator. Accordingly, rather relaxed chains that had shown the off-equatorial scattering were thought to be consumed for the crystal growth.

Strain-induced crystallization of peroxide-crosslinked natural rubber

Effect of network-chain density ( ν) on the strain-induced crystallization of peroxide-crosslinked natural rubber was studied by fast time-resolved synchrotron wide-angle X-ray diffraction. It was observed that the elongation ratio at the onset of strain-induced crystallization ( α c) became smaller with the increase of ν. The difference of the entropy between the undeformed and deformed states at each α c was nearly equal in spite of the variation of ν. The calculated melting temperature at α c of the samples was also almost the same regardless of their ν. These observations mean that the strain-induced crystallization occurred when the deformation brought about a definite entropic state for peroxide-crosslinked natural rubber. They agreed with the prediction by Flory and were consistent with the classical theory of rubber elasticity.

The Effect of Imperfections of the Network on the Properties of Model Polyurethane Networks

The effects of the network imperfections such as dangling chains or abnormally long network chains on the properties of model polyurethane networks were studied. NCO-terminated prepolymer prepared from poly (oxypropylene) glycol with molecular weight 1, 000 and 2, 4-tolylene diisocyanate were end-linked with trimethylolpropane (TMP). The R-values, the molar ratio of OH group of TMP to NCO group of prepolymer, were varied in wide range. It is expected that dangling chains will form for R 1. Sol fraction and network chain densities were determined. Sol-free specimens were obtained by careful drying of those attained equilibrium swelling. Glass transition temperature, stress relaxation, dynamic mechanical property, and energy loss in repeated stretching were measured for both the sol-containing and sol-free specimens. The results are as follows. The values of effective network chain density agreed with those calculated from the consideration of network formation for R 1 region. Though the values of properties are related well with the network chain density, the data lie on different plots for R 1 region. The results were discussed on the basis of network imperfections.

Orientation and Crystallization of Natural Rubber Network As Revealed by WAXD Using Synchrotron Radiation

Strain-induced crystallization of natural rubber samples with various network-chain densities, ν, was investigated by synchrotron X-ray diffraction measurements. It was found that the onset strain of crystallization was almost independent of ν. Lateral crystallite size and degree of orientational fluctuations of crystallites were also evaluated. These results indicated that stretched molecular chains acted as nuclei while surrounding chains could also contribute to the crystal growth. Deformation of crystal lattice with nominal stress was detected, and the strain-induced crystallites were found to be responsible for the increased modulus upon elongation. The unit cell volume decreased almost linearly with nominal stress. By assuming the deformation mechanism of the rubber network as a pantograph, the reinforcement effect of the crystallites is thought to be brought out not directly by crystallites connected in series but indirectly through the surrounding network chains.

An analysis of the influence of the accelerator/sulfur ratio in the cure reaction and the uniaxial stress–strain behavior of SBR

AbstractVulcanized styrene butadiene rubber (SBR) with different cure systems was prepared and analyzed by using the model of rubber elasticity based on the tube concept, applied to the treatment of the stress–strain measurements. Samples with several ratio accelerators to sulfur, Λ, between 0.22 and 3.0 cured at 433 K were studied. The network chain density and the crosslink density of the samples were evaluated. By means of normalized rheometer curves, the kinetics of cure of these samples were evaluated by considering the model of isothermal curing proposed by Kamal and Sourour. In this frame, the parameters of the kinetics model were obtained. A correlation between the order of the kinetic equation, n, and the network chain density of the cure samples was established. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2601–2609, 2004

Hyperelasticity and stress softening of filler reinforced polymer networks

AbstractStarting from an analysis of filler networking in bulk polymers, a constitutive micro‐mechanical model of stress softening and hysteresis of filler reinforced polymer networks is developed. It refers to a non‐affine tube model of rubber elasticity, including hydrodynamic amplification of the rubber matrix by a fraction of hard, rigid filler clusters with filler‐filler bonds in the unbroken, virgin state. The filler‐induced hysteresis is described by an anisotropic free energy density, considering the cyclic breakdown and re‐aggregation of the residual fraction of soft filler clusters with already broken, damaged filler‐filler bonds. Experimental investigations of the quasi‐static stress‐strain behaviour of silica and carbon black filled rubbers up to large strain agree well with adaptations found by the developed model. The microscopic material parameters obtained appear reasonable, providing information on the mean size and distribution width of filler clusters, the tensile strength of filler‐filler bonds and the polymer network chain density.

Graded styrene–butadiene rubber vulcanizates

AbstractStyrene–butadiene rubber (SBR) vulcanizates with graded network chain densities were prepared by a conventional method of rubber science and technology. The characteristics of the gradient and gradient direction of the network chain density were examined from the perspective of the mechanical properties of the graded SBR vulcanizates. In the matrix of the graded SBR vulcanizates, silica particles were generated in situ, and the tensile properties were investigated. © 2002 Wiley Periodicals, Inc. J Appl Polm Sci 87:61–67, 2003

Ionic conductivity and mechanical properties of polymer networks prepared from high molecular weight branched poly(oxyethylene)s

Polymer networks were prepared from high molecular weight branched poly(oxyethylene)s (POE) with di(oxyethylene) units and allyloxymethyl groups as side segments, and their ionic conductivity and mechanical properties were investigated from the viewpoint of polymer solid electrolyte. The optimal crosslinking brought about both high ionic conductivity comparable to the non-crosslinked high molecular weight branched poly(oxyethylene)s and good mechanical properties to give a better dimensional stability, when the network was doped with LiN(SO 2CF 3) 2 in the concentration of [Li]/[-O-]=0.06. This observation implies that the control of network-chain density in the preparation of networks from the high molecular weight branched poly(oxyethylene) is useful for the material design of practical polymer solid electrolyte, as well as their composition, the kind of salt and its concentration.

Thermal Degradation Studies of a Polyurethane Propellant Binder

Abstract The thermal oxidative aging of a crosslinked hydroxy-terminated polybutadiene (HTPB)/isophorone diisocyanate (IPDI) based polyurethane rubber, used as a polymeric binder in solid propellant grain, was investigated at temperatures from 25 to 125 °C. The changes in tensile elongation, polymer network properties and chain dynamics, mechanical hardening and density were determined with a range of techniques including modulus profiling, solvent swelling, NMR relaxation and O2 permeability measurements. We critically evaluated the Arrhenius methodology that is commonly used with a linear extrapolation of high temperature aging data using extensive data superposition and highly sensitive oxygen consumption experiments. The effects of other constituents in the propellant formulation on aging were also investigated. We conclude that crosslinking is the dominant process at higher temperatures and that the degradation involves only limited hardening in the bulk of the material. Significant curvature in the Arrhenius diagram of the oxidation rates was observed. This is similar to results for other rubber materials.

Crosslinked Polycarbosilanes. Synthesis and Properties

New crosslinked polycarbosilanes (PCSs) having biphenyl, biphenyl ether and naphthalene linkages, were synthesized by hydrosilylation reaction between multi-functional vinylsilanes and hydrosilanes. The PCSs were transparent rigid material without cracks or bubbles. The mechanical properties and glass transition temperatures (T g s) of the PCSs were evaluated by flexural tests and differential scanning calorimeter (DSC) or dynamic mechanical analysis (DMA). Crosslinked PCSs VIII-X with high network chain density and rigid-rod spacers had T g s at around 250°C and high flexural strength in the range of 75-96 MPa. 5% weight loss temperatures of PCSs under nitrogen ranged from 490°C to 544°C.

せん断流動場反応制御技術によるゴムの連続再生 第８報 過酸化物架橋ＥＰＤＭの再生条件と加硫再生ゴム物性

Continuous devulcanization by the shear flow stage reaction control technology for vulcanized rubbers has been developed. This technology has been applied to sulfur-crosslinked rubbers such as EPDM, NR/SBR (used tire waste) and IIR (used for tube). In this study, this technology has also been applied to peroxide-crosslinked EPDM. The obtained reclaimed rubber was found to be a mixture of two components; one is a linear chain which is soluble in toluene and whose average molecular weight is one third of that of the original raw rubber, and the other is a very loosely crosslinking network whose effective network chain density is about one fiftieth of that of the peroxide-crosslinked virgin rubber. The ratio of linear chain component and loosely crosslinking network component was approximately 3 to 2. The reclaimed rubber can be cured with sulfur- accelerator curing system. The sulfur-crosslinked reclaimed rubber shows good mechanical properties nearly equal to those of sulfur-crosslinked virgin one.

Synthesis and Properties of Crosslinked Polycarbosilanes by Hydrosilylation Polymerization

The aim of this investigation is the development of crosslinked polycarbosilanes (PCSs) with excellence on mechanical properties. Crosslinked polycarbosilanes were obtained by using a hydrosilylation curing reaction between multi-functional vinylsilanes and hydrosilanes. The PCSs were prepared with a casting method from the monomer solution. The mechanical properties and the glass transition temperatures (Tgs) of the PCSs were measured by a flexural test and Differential Scanning Calorimetry (DSC). 29Si solid-state NMR spectroscopy was employed for estimating the conversion of the vinylsilanes. The effects of the following three points on the mechanical properties of the crosslinked PCSs were evaluated: (1) the molecular weight between the crosslinking points, (2) the introduction of flexural units, and (3) the network chain density. The mechanical properties and Tgs of the PCSs were found to be dependent on the structures of the monomer used. It was clear that both the high network chain density and the rigid-rod unit, [–Me2Si–p–C6H4–Me2Si–CH2CH2–], are favorable to high mechanical properties. The improvement in strength can be related to the structures of vinylsilanes and hydrosilanes used.

新脱硫技術の提出

Continuous devulcanization by shear flow stage reaction control technology for vulcanized rubber has been developed. Screw geometry and alignment, screw rotation, reaction temperature, internal pressure, and addition of process oil and devulcanizing agent are the most important factors for this process. This process can be applied to sulfur vulcanized rubbers such as EPDM, NR, SBR and IIR. This paper described the results of the devulcanization of EPDM rubber. The obtained devulcanized rubber was found to be even mixture of two components; one is linear chain which is soluble in toluene and whose average molecular weight is nearly equal to that of the original raw rubber, and the other is very loosely crosslinking network whose effective network chain density is about one twentieth of that of the virgin vulcanized rubber. The devulcanized rubber can be revulcanized by conventional curing process with sulfur-accelerator vulcanizing system. The revulcanized rubber exhibited excellent mechanical properties and formability nearly equal to virgin vulcanized one.