Masatoshi Tosaka

Mechanism of strain-induced crystallization in filled and unfilled natural rubber vulcanizates

Structure evolution during deformation of unfilled natural rubber (NR) vulcanizate and filled ones with carbon black or calcium carbonate was investigated by the synchrotron x-ray diffraction. The crystallization onset strain, α0, was found to decrease by the inclusion of the filler. However, corrected α0 values into the effective strain ratio of deformable rubber portion were almost constant between filled and unfilled samples. Accordingly, our model of strain-induced crystallization of unfilled NR vulcanizates, assuming that melting temperature is independent of network-chain length (n), was applied to the filled samples. The discrepancy between classical theories and experimental results was thought to come from the distribution of n. By the inclusion of filler, the lateral crystallite size was decreased but the orientational fluctuation increased. The lattice of the strain-induced crystallites changed almost linearly with the nominal stress. In addition, the degree of lattice deformation decreased wit...

Effect of network-chain length on strain-induced crystallization of NR and IR vulcanizates

Abstract Strain-induced crystallization of natural rubber (NR) and synthetic isoprene rubber (IR) with various crosslinking densities was investigated by wide angle X-ray diffraction using a synchrotron radiation and simultaneous tensile measurements. The elongation ratio at the onset of crystallization (αc) was almost independent of crosslinking density. IR samples showed larger αc values than NR because of the lower stereoregularity of IR. These results suggest that the onset of crystallization is determined by increased melting temperature by strain due to an entropic reason. The amount of oriented amorphous component changed approximately linearly with strain, and was a little larger in IR than in NR when compared at the same elongation ratio. At small strain (and stress), crystallinity in IR was lower than in NR. These results indicate that, at small strain region, the more stress is assigned to oriented amorphous in IR than in NR.

Strain-induced molecular orientation and crystallization in natural and synthetic rubbers under uniaxial deformation by in-situ synchrotron X-ray study

Abstract In-situ synchrotron wide-angle X-ray diffraction (WAXD) studies and simultaneous measurements of stress and strain during uniaxial stretching of various vulcanized rubbers were carried out (at room temperature and 0°C) to reveal the strain-induced molecular orientation and crystallization relationships. Rubbers evaluated included natural rubber (NR), synthetic poly-isoprene rubber (IR), poly-cis-1,4-butadiene rubber (BR) and butyl rubber (IIR). Some universal features were observed in these systems: (i) At high strains (> 5.0), the majority of the chains (up to 50 ≈ 75%) in natural and synthetic rubbers remained in the un-oriented amorphous state with only a small amount of crystalline fraction formed (10–20%). The rest of the chains were in the oriented amorphous state. (ii) During deformation, the oriented amorphous chains acted as precursors to strain-induced crystallization. A network of micro-fibrillar crystallites is formed within the closely populated vulcanization points, leading to the e...

Effect of silane-coupling agent on natural rubber filled with silica generated in situ

The effect of silane coupling agent was investigated for the novel in situ silica loading to the natural rubber (NR) matrix. The silica was generated in situ by the sol-gel reaction of tetraethoxysilane in the NR matrix before its crosslinking. γ-mercaptopropyltrimethoxysilane (γ-MPS) significantly prevented the delay of sulfur curing and increased the wettability of NR onto in situ silica, which resulted in the increase of reinforcement effect for the NR vulcanizate. γ-MPS decreased the interaction between the in situ silica particles followed by dispersing the in situ silica particles homogeneously and decreasing the hardness, compression set, hysteresis loss and storage modulus at the rubbery state of in situ silica-filled NR vulcanizate. The NR/in situ silica composite with γ-MPS is a promising material for a high performance rubber product.

Chemical recycling of sulfur-cured natural rubber using supercritical carbon dioxide

Sulfur-cured unfilled natural rubber (NR) was devulcanized in supercritical CO2 (scCO2) by using diphenyl disulfide (DD) as a devulcanizing reagent. The product was fractionated into sol and gel components. The fraction of sol component that contains reusable linear polymer increased with the increase in CO2 pressure, especially over the critical pressure. The molar mass of the resulting sol component was about tens of thousands and the crosslink density of the gel component decreased from the initial one.

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 scat...

Devulcanization of sulfur-cured isoprene rubber in supercritical carbon dioxide

Abstract A new devulcanization process that utilizes supercritical CO2 (scCO2) along with devulcanizing reagents was studied. Unfilled polyisoprene rubber samples (vulcanizates) with different crosslink distributions were prepared by controlling the cure time and the curatives. Each of the vulcanizates was subjected to the Soxhlet extraction using azeotropic acetone/chloroform to remove residual curatives. The devulcanization was performed at various temperatures (140–200 °C) in the presence of scCO2 for 60 min. The product was fractionated into sol and gel components, and molecular weight of the sol component and the crosslink density of the gel component were determined. Thiol-amine reagent was found to be effective among several devulcanizing reagents; the molecular weight of the resulted sol component was about tens of thousands and the crosslink density of gel component decreased substantially from the initial ones. Yield of the sol component increased with the increase in the CO2 pressure. In the su...

Supramolecular Fullerene Polymers and Networks Directed by Molecular Recognition between Calix[5]arene and C60

A biscalix[5]arene-C60 supramolecular structure was utilized for the development of supramolecular fullerene polymers. Di- and tritopic hosts were developed to generate the linear and network supramolecular polymers through the complexation of a dumbbell-shaped fullerene. The molecular association between the hosts and the fullerene were carefully studied by using (1) H NMR, UV/Vis absorption, and fluorescence spectroscopy. The formation of the supramolecular fullerene polymers and networks was confirmed by diffusion-ordered (1) H NMR spectroscopy (DOSY) and solution viscometry. Upon concentrating the mixtures of di- or tritopic hosts and dumbbell-shaped fullerene in the range of 1.0-10 mmol L(-1) , the diffusion coefficients of the complexes decreased, and the solution viscosities increased, suggesting that large polymeric assemblies were formed in solution. Scanning electron microscopy (SEM) was used to image the supramolecular fullerene polymers and networks. Atomic force microscopy (AFM) provided insight into the morphology of the supramolecular polymers. A mixture of the homoditopic host and the fullerene resulted in fibers with a height of (1.4±0.1) nm and a width of (5.0±0.8) nm. Interdigitation of the alkyl side chains provided secondary interchain interactions that facilitated supramolecular organization. The homotritopic host generated the supramolecular networks with the dumbbell-shaped fullerene. Honeycomb sheet-like structures with many voids were found. The growth of the supramolecular polymers is evidently governed by the shape, dimension, and directionality of the monomers.

TEM observation of natural rubber thin films crystallized under molecular orientation

Morphology of natural rubber (NR) thin films crystallized under molecular orientation was examined by transmission electron microscopy (TEM). A thin film of NR (RSS#1) was made by casting its n-hexane (or benzene) solution onto the water surface, and prestretched up to a desired amount of strain (%). Each of the specimens thus prepared was introduced into a TEM column and isothermally crystallized at -25 °C with a cryo-transfer specimen-holder. Morphological observations in bright- and dark-field imaging modes revealed directly that the length of the lamellae, which were oriented perpendicular to the stretching direction, decreased with increasing strain. The result seems to be related to the population density of y-filaments (named by Andrews) generated only by prestretching. Time-resolved bright-field observations by in situ TEM of the thin film prestretched up to a strain of 200% revealed the following. Though the film gave crystalline reflections in its selected-area electron diffraction even before isothermal crystallization, no well-defined lamellae were recognized. Subsequent isothermal crystallization for 2 h, however, formed well-defined lamellae oriented perpendicular to the stretching direction. This observation, conceivably, suggests that y-filaments were formed immediately by prestretching and then the lamellae were nucleated on the γ-filaments at -25 °C.

Microscopy of large-scale porphyrin aggregates formed from protonated TPP dimers in water -organic solutions

Abstract Properties of porphyrin aggregates were investigated by absorption spectroscopy (UV–Vis and IR) in water–tetrahydrofuran (THF) solutions in the presence of different concentrations of HCl. The morphology of the aggregates was observed by scanning electron microscopy (SEM) and atomic force microscopy in thin films. A new protonated meso -tetraphenylporphine (TPP) form that shows characteristic absorptions in the UV–Vis spectra was found in the aggregated porphyrin in the presence of 2N HCl. Two types of changes with time were observed in these spectra, one of which is due to sticking together of the porphyrin aggregates. The second is associated with the formation of complexes between the protonated TPP dimer with λ max =465 nm and metal ions that are probably leached out from the support by the acid. IR spectra of porphyrin aggregates prepared in the presence of different concentrations of HCl show huge water contents in the thin films and different characteristics of the water bound in the aggregates. Porphyrin aggregates prepared at different concentrations of HCl exhibited different surface properties. TPP aggregates prepared in the presence of 0.4N HCl and observed by SEM exhibit smooth surfaces over ranges of several micrometers. TPP aggregates prepared in the presence of 2N HCl form a continuous thin layer with 3–5 μm wide domains that consist of submicroscopic grains. These appear to be the result of 200–400 nm wide spherical particles that stick together.