Structural developments in synthetic rubbers during uniaxial deformation by in situ synchrotron X‐ray diffraction

The molecular orientation and strain‐induced crystallization of synthetic rubbers—polyisoprene rubber, polybutadiene rubber, and butyl rubber [poly(isobutylene isoprene)]—during uniaxial deformation were studied with in situ synchrotron wide‐angle X‐ray diffraction. The high intensity of the synchrotron X‐rays and the new data analysis method made it possible to estimate the mass fractions of the strain‐induced crystals and amorphous chain segments in both the oriented and unoriented states. Contrary to the conventional concept, the majority of the molecules (50–75%) remained in an unoriented amorphous state at high strains. Each synthetic rubber showed a different behavior of strain‐induced crystallization and molecular orientation during extension and retraction. Our results confirmed the occurence of strain‐induced networks in the synthetic rubbers due to the inhomogeneity of the crosslink distribution. The strain‐induced networks containing microfibrillar crystals and oriented amorphous tie chains were responsible for the ultimate mechanical properties. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 956–964, 2004     

Synthesis of tripodal aza crown ether calix[4]arenes and their supramolecular chemistry with transition-, alkali metal ions and anions

Tripodal aza crown ether calix[4]arenes, 5a, 5b, 6a and 6b, have been synthesized. The structure of protonated 5a was elucidated by X-ray crystallography to be a self-threaded rotaxane. Complexation studies of 5a and 5b towards anions using Na⁺ as countercation were carried out by ¹H NMR titration in dimethylsulfoxide-d₆ and the mixture of chloroform-d and methanol-d₄, respectively. Ligands 5a and 5b were able to form 1:1 complexes with Br⁻, I⁻ and NO₃⁻ and the complexation stability varied as follows: NO₃⁻>I⁻>Br⁻. The effect of countercation on anion complexation was also investigated. The results showed that the association constants of 5a towards Br⁻ in the presence of various cations varied as K⁺>Bu₄N⁺>Na⁺. The enhancement in anion complexation ability of 5a may result from the rearrangement of the tripodal ammonium unit in the presence of K⁺. The neutral forms, 6a and 6b, were able to form complexes with transition metal ions such as Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺. The stability of the complexes followed the sequence: Ni²⁺2+Cu²⁺Zn²⁺. Compounds 6a and 6b may, therefore, potentially be used as either transition metal ion or anion receptors that can be controlled by pH of the solution.     

In situ silica reinforcement of methyl methacrylate grafted natural rubber by sol–gel process

In situ silica reinforcement of natural rubber (NR) grafted with methyl methacrylate (MMA) (MMA-GNR) was achieved via the sol–gel reaction of tetraethoxysilane (TEOS) by the use of solid rubber and latex solutions. Silica contents within the MMA-GNR as high as 48 and 19 phr were obtained when using the solid rubber and latex solutions, respectively, under optimum conditions. The conversion efficiency of TEOS to silica was close to 95%. The in situ formed silica MMA-GNR/NR composite vulcanizates were prepared. MMA-GNR/NR composite vulcanizates reinforced with the in situ formed silica prepared by either method had similar mechanical properties to each other, but a shorter cure time and higher mechanical properties than those reinforced with the commercial silica at 9 phr. The TEM micrographs confirmed that the in situ formed silica particles were well dispersed within the MMA-GNR/NR composite matrix, whilst the commercial silica particles showed a significant level of agglomeration and a lower level of dispersion.     

Preparation of high performance nanocomposite elastomer: effect of reaction conditions on in situ silica generation of high content in natural rubber

Effect of amines on an in situ silica generation in natural rubber was investigated, and n-hexylamine, n-heptylamine and n-octylamine were found to increase the in situ silica content. The nanometer sized silica particles up to ca. 80 parts per hundred rubber by weight were generated in situ in the rubber matrix via a sol–gel reaction of tetraethoxysilane. Additionally, dispersion of the silica in the rubbery matrix was more homogeneous than that of commercial silica dispersed by a conventional mechanical mixing. In this in situ silica generation, the polarity and solubility in water of amine were influential factors for controlling the in situ silica content in the rubbery matrix. The obtained high in situ silica filled natural rubber was useful to prepare high performance nanocomposite elastomers.     

In situ silica reinforcement of natural rubber by sol–gel process via rubber solution

The in situ silica filling of natural rubber (NR) was carried out via the sol–gel reaction using tetraethoxysilane. The effect of the in situ silica content on the curing, mechanical, dynamic mechanical and thermal properties of the composite vulcanizate materials was investigated in comparison to that with a commercial silica preparation. The Mooney viscosity of the in situ silica filled NR vulcanizates showed a lower value compared with that of the commercial filled ones. The mechanical properties of the in situ silica composite materials, i.e., the moduli and compression set, were improved compared with the commercial silica filler NR vulcanizates. The reinforcement effect of in situ silica did not accord with the Smallwood equation but in contrast was in good agreement with the Guth and Gold equation using a shape factor (f) which itself was in close agreement with estimates derived from independent TEM analysis. The pseudo-network structure of the in situ silica was low, which resulted in a lower storage modulus at 25 °C. By filling NR with in situ silica, the thermal properties of the composite vulcanized material were also improved, and well dispersed in situ silica particles within the NR matrix were also observed.     

Cuttlebone as reinforcing filler for natural rubber

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 2.70 g/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.