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The microscopic structures of natural rubber (NR) and deproteinized NR (DPNR) were investigated by means of small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and atomic force microscopy (AFM). They were compared to those of isoprene rubber (IR), which is a synthetic analogue of NR in terms of chemical structure without any non-rubber components like proteins. Comparisons of the structure and mechanical properties of NR, DPNR, and IR lead to the following conclusions. (i) The well-known facts, for example, the outstanding green strength of NR and strain-induced crystallization, are due not much to the presence of proteins but to other components such as the presence of phospholipids and/or the higher stereoregularity of NR. It also became clear the naturally residing proteins accelerate the upturn of stress at low strain. The protein phases work as cross-linking sites and reinforcing fillers in the rubbery matrix. (ii) The microscopic structures of NR were successfully reproduced by SANS intensity functions consisting of squared-Lorentz and Lorentz functions, indicating the presence of inhomogeneities in bulk and thermal concentration fluctuations in swollen state, respectively. On the other hand, IR rubbers were homogeneous in bulk. (iii) The inhomogeneities in NR are assigned to protein aggregates of the order of 200 A or larger. Although these aggregates are larger in size as well as in volume fraction than those of cross-link inhomogeneities introduced by cross-linking, they are removed by deproteinization. (iv) Swelling of both NR and IR networks introduces gel-like concentration fluctuations whose mesh size is of the order of 20 A.

The free volume in the amorphous phase of a simple carbon‐black‐filled peroxide cross‐linked isoprene rubber (IR) is investigated by positron annihilation lifetime spectroscopy to clarify the structural changes in the rubber phase by the carbon black (CB) fillers. The ortho‐positronium (o‐Ps) pick‐off annihilation lifetime measured in the rubber without CB is around 2.8 ns and is found to be almost constant even after CB filling. This result indicates that in a portion of the rubber phase the free volume remains similar in size to that in the cross‐linked IR without CB. The o‐Ps intensity drastically decreases from 37% without CB to 15% with a CB loading as high as 80 phr. This decline is revealed to exceed the contribution, which is expected only from the decrease in the fraction of the amorphous phase by CB addition, by a factor of 0.37 at the highest CB loading. The additional reduction is related to the generation of a bound rubber layer surrounding the CB fillers with low molecular‐chain mobility where positronium formation is inhibited. The decrease rate is found to represent the volume of this rigid rubber phase and strongly depend on the formation of the CB network structure. [ABSTRACT FROM AUTHOR]

The comparison of the reinforcing ability of cuttlebone (CTBO) particles in natural and synthetic isoprene rubber (NR and IR, respectively) composites were investigated. The CTBO particles could reinforce in both NR and IR composites. However, the improvement of tensile properties in CTBO‐filled NR composites was higher than that of IR composites due to the presence of non‐rubber components in NR. The non‐rubber components promoted the strong interfacial adhesions with filler and rubbery matrix to increase the bound rubber content. These results affected the increase of the stress softening degree, hysteresis loss, residual strain during cyclic deformation and the volume fraction of constrained rubber region (immobilized area of rubber) in CTBO‐filled NR composites. The atomic force microscopy evidently revealed good compatibility between the CTBO particles and the NR matrix due to the presence of organic components from CTBO particles and from non‐rubber components in NR. Accordingly, the CTBO particles and non‐rubber components play important roles in improving the properties of green rubber composite materials. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Carbon black (CB) aggregates in CB-filled natural rubber (NR) vulcanizate were joined together and formed CB aggregates in the CB region less than 20 phr. It was found that the viscoelastic behavior was related to the hydrodynamic interaction between CB aggregate and NR matrix. In the CB loading region more than 40 phr, CB aggregates formed CB network structure in it. Using the parallel mechanical model of the two phases of a rubber matrix and an immobilized CB/NR interfacial layer excluding a CB phase, it was possible to quantitatively explain the relationship between the viscoelastic behavior of CB network structure. [ABSTRACT FROM AUTHOR]

Filled rubbers have a long history in rubber technology. Filler morphology in the rubber matrix significantly influences the performance of filled rubber products. Therefore, we have to achieve a certain morphology of filler in the rubbery matrix in order to enjoy good reinforcement. However, it has not been sufficiently known what kind of morphology is necessary for the reinforcement, because preparations of adequate models for the characterization of filler morphology have been difficult. The in situ silica filling of the rubber matrix can be modeled for the preparations. In this paper, the characteristic features of in situ silica generated in diene rubbers are reviewed, citing recent reports. Two important models are presented. One is a homogeneous silica filling in situ, producing soft rubber nanocomposites. The other is the formation of an in situ silica filler network, creating stiff rubber nanocomposites. The controlled in situ silica filling is useful to evaluate the reinforcement effect of filler in a rubber matrix. © 2016 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

Synchrotron in situ zinc K-edge X-ray absorption near-edge structure measurements are conducted during sulfur crosslinking reactions of isoprene rubber at 140 °C. A two-phase network formation via two different reactions is revealed in a benzothiazolesulfenamide-accelerated curing system with zinc oxide and stearic acid. DSC measurements support this observation. The results are in good agreement with previous work using small-angle neutron scattering, where network domains of high network-chain density are proposed to be embedded in a mesh network. A formation of the mesh network is found to occur a little earlier than that of the network domains, and its fraction is unexpectedly observed to be larger than that calculated on the basis of a concentration of zinc stearate. The result is ascribable to amounts of zinc intermediates forming the mesh network. This study may be the first in situ evaluation for the inhomogeneous network formation during vulcanization. [ABSTRACT FROM AUTHOR]

The viscoelastic behavior of carbon black (CB)-filled natural rubber (NR) vulcanizates is explained for the first time by applying a two-phase mixing law, excluding the CB phase, to the volume fraction of the CB/NR interaction layer (CNIL). For CB loadings of 20 phr or less, the CNIL of local CB aggregates induces the reinforcement effect of a series mechanical model of the mixing law. In contrast, for CB loadings of 30 phr or more, the CNIL that forms the CB network (CBN) generates the reinforcement effect of a parallel mechanical model of the mixing law. Therefore, the model of the mixing law presumably changes from a series mechanical model before CBN formation to a parallel one after the network forms. Additionally, employing this fraction, a transition caused by CBN formation was investigated. [ABSTRACT FROM AUTHOR]

The sol-gel reaction of tetraethoxysilane in natural rubber (NR) latex was conducted to produce in situ silica-filled NR latex, followed by adding sulfur cross-linking reagents to the latex in a liquid state. The latex was cast and subjected to sulfur curing to result in a unique morphology in the NR composite of a flexible film form. The contents of in situ silica filling were controlled up to 35 parts per one hundred rubber by weight. The silica was locally dispersed around rubber particles to give a filler network. This characteristic morphology brought about the composite of good dynamic mechanical properties. Synchrotron X-ray absorption near-edge structure spectroscopy suggested that the sulfidic linkages of the sulfur cross-linked composites were polysulfidic, S x ( x ≥ 2), and a fraction of shorter polysulfidic linkages became larger with the increase of in situ silica. The present observations will be of use for developing a novel in situ silica-filled NR composite prepared in NR latex via liquid-phase soft processing. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

To investigate thermal destruction and rearrangement of the carbon black (CB) network consisting of CB aggregates in rubbery matrix, the proper heat-treatment temperature without thermal decomposition of rubber matrix were examined by using differential scanning calorimetry, dynamic mechanical measurements, and thermal expansion measurements. 383 K was chosen as the heat-treatment temperature under vacuum. The volume resistivity (ρ [ABSTRACT FROM AUTHOR]

Abstract: Cuttlebone was proved to be a biomass for new reinforcing filler for natural rubber (NR). The cuttlebone particles were obtained by crushing cuttlebone and followed by sieving. Density and crystal structure of the cuttlebone were 2.70g/cm3 and an aragonite form of CaCO3, respectively. The surface area and average diameter of the cuttlebone particles were measured and the reinforcement effect as filler for NR was investigated. The cuttlebone particles did not prevent a peroxide cross-linking reaction of NR, and mechanical properties of peroxide cross-linked NR filled with cuttlebone particles were found to be comparable with those of peroxide cross-linked NR filled with commercial CaCO3 filler. Presence of chitin on the surface of the cuttlebone particles was speculated to result in a good interaction between cuttlebone particles and NR, which may be ascribed to the mechanical properties of cuttlebone filled NR samples. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

An environmental-friendly chemical recycling process in supercritical carbon dioxide was established for sulfur-cured natural rubber vulcanizates. Diphenyl disulfide was found to be effective as a devulcanizing reagent for this process. The presence of fillers such as carbon black and silica did not prevent the devulcanization reaction. Not only natural rubber but also synthetic rubbers could be subjected to this chemical recycling. The tensile properties of recycled rubber vulcanizates made from virgin rubber and 20-60 phr of recycled rubber were over 90% of those of the original vulcanizates. The devulcanizing reagent was found to be selectively absorbed in the sulfur-cured vulcanizates, which led to the highly efficient devulcanization reaction. This method will be useful for recycling of other conventional sulfurcured rubber vulcanizates. [ABSTRACT FROM AUTHOR]

The free volume in carbon-black filled isoprene rubber is determined by in-situ positron annihilation lifetime measurements on the rubber subject to tensile strain using ortho-positronium as a probe. The dependence of free volume on the elongation and carbon black loading in the strained state is investigated to clarify the reinforcement mechanism of carbon black in synthetic rubber. Although no change in the ortho-positronium lifetime is detected, the ortho-positronium formation probability sharply decreases with increasing elongation and carbon black concentration in the strained state. The percent reduction is enhanced at high elongations and carbon black loadings. This result indicates the formation of both a bound rubber layer around the CB fillers and regions in the strained rubber phase with oriented molecular chains characterized by lower mobility, where ortho-positronium formation is suppressed. • Free volume in carbon-black filled strained rubber is measured using positronium. • Positronium formation is suppressed in regions with low molecular chain mobility. • Formation of bound rubber layer around the fillers by carbon black addition. • Generation of rubber regions with oriented molecular chains by straining. [ABSTRACT FROM AUTHOR]

Abstract: In order to visualize the three-dimensional (3D) state of silica or carbon black dispersion in nanocomposites, 3D-TEM, a technique that combines transmission electron microscopy (TEM) with computerized tomography, is used and the results obtained are discussed. This technique is sometimes called electron tomography. The examples presented here elucidate the dispersion of these nanoparticles in a natural rubber (NR) matrix, which is a technologically very important biomass. It is made clear that such nanoparticles coalesce to form aggregates and they further coalesce to form agglomerates in the rubbery matrix. Once 3D-TEM images are established, one can evaluate all structural parameters, depending on the availability of software for each specific purpose. Here, the volume of each component in the composite, the size, the size distribution, the distance between neighboring aggregates, and the aspect ratio of the aggregates, for example, are evaluated as structural parameters. In the case of silica, 3D images of conventional silica and in situ formed silica are compared. The further coalescing of aggregates of both silica and carbon black particles results in the formation of agglomerates, the visualized images of which show that they form a kind of network structure in the rubber. In the case of carbon black, measurement of volume resistivity or electron conductivity reveals that percolation occurs near a volume fraction of 0.17 as a threshold value. Combining the resistivity results with a structural parameter, i.e., the nearest neighbor distance from 3D-TEM images, it is estimated that electrons hop over the rubber phase (which is an insulator), about 3nm thick, between carbon black aggregates, thereby exhibiting electron conductivity. It is suggested from the results of this study that establishing the correlation between the 3D nanostructures as revealed by 3D-TEM and physical properties can play an important role in advancing the discipline of nanotechnology as an exact science, especially in the area of soft nanocomposites. [Copyright &y& Elsevier]

Abstract: Strain-induced crystallization of crosslinked natural rubber (NR) and its synthetic analogue, cis-1,4-polyisoprene (IR), both mixed with various amounts of stearic acid (SA), were investigated by time-resolved X-ray diffraction using a powerful synchrotron radiation source and simultaneous mechanical (tensile) measurement. No acceleration or retardation was observed on NR in spite of the increase of SA amount. Even the SA-free IR crystallized upon stretching, and the overall crystallization behavior of IR shifted to the larger strain ratio with increasing SA content. No difference due to the SA was detected in the deformation of crystal lattice by stress for both NR and IR. These results suggested that the extended network chains are effective for the initiation of crystallization upon stretching, while the role of SA is trivial. These behaviors are much different from their crystallization at low temperature by standing, where SA acts as a nucleating agent. [Copyright &y& Elsevier]

Abstract: The first visualization of nanofiller networking due to association of carbon black in natural rubber is achieved by a transmission electron microscopy combined with computerised tomography (3D-TEM), and the resulted skeleton displays the characteristics of the filler network, which has long been assumed on the percolation behaviour in electron conductivity and on the mechanical behaviours of filled rubbers. [Copyright &y& Elsevier]