Development of internal fine structure in stretched rubber vulcanizates

Small-angle X-ray scattering (SAXS) pattern and tensile stress during relaxation of stretched rubber vulcanizates (synthetic polyisoprene) were measured simultaneously at room temperature and at 0 °C. The samples were quickly stretched to the prefixed strain and then allowed to relax for 1 h. In every SAXS pattern, the intensity distribution was elongated along the equator, indicating the formation of structures elongated in the stretching direction. The so-called two-spots pattern corresponding to the long period of stacked lamellar crystals did not appear even when the critical strain to induce crystallization was exceeded. On the other hand, even below the critical strain, additional development of equatorial streaks was detected in the differential SAXS patterns. This result suggests the growth of the density fluctuation elongated in the stretching direction, which is not directly related to strain-induced crystallization

Real-time crystallization of organoclay nanoparticle filled natural rubber under stretching

An experimental evidence of a remarkable enhancement of strain-induced crystallization in natural rubber nanocomposite under uniaxial stretching, due to the presence of nanoclay particles, is described. Synchroton wide-angle X-ray diffraction (WAXD) have been used to monitor structure changes and crystallinity development during deformation. The results showed a dual crystallization mechanism in nanocomposites, which consists of spatial reorganization of organoclay at low strains, followed by rapid strain-induced crystallization of natural rubber. The presence of nanoparticles introduces new energy-dissipating mechanisms, and the mechanical property enhancement refers to the nanoparticle mobility and orientation during deformation. In-situ experiments were also carried out to relate the stress-strain behavior with structure determination using WAXD.The authors gratefully acknowledge the financial support of the Spanish Ministry of Education (MEC) through its project MAT 2004-00825. J. Carretero-González wishes to thank the Spanish Ministry of Education (MEC) for the concession of a FPI grant and R. Verdejo also acknowledges a Juan de la Cierva contract from the MEC. BH also acknowledges the support by the NSF (DMR-0405432).Peer Reviewe

Molecular dynamics of natural rubber as revealed by dielectric spectroscopy: The role of natural cross-linking

7 p.: gráf.In order to understand the molecular dynamics of natural rubber, the dielectric relaxation behavior of its different components were investigated. These components included: (1) the linear polyisoprene fraction, obtained after deproteinization and transesterification of natural rubber (TE–DPNR), (2) the gel (GEL) fraction, corresponding to pure natural chain–end cross–linked natural rubber, (3)deproteinized natural rubber (DPNR), in which the protein cross–links at the u–end have been removed, and (4) natural rubber (CNR) purified (through centrifugation) but still containing proteins,phospholipids and the sol phases. The dielectric relaxation behaviour of natural rubber revealed a segmental mode (SM) which is not affected by natural chain-end cross-linking (so-called naturally occurring network) and a normal mode (NM) which depends on a naturally occurring network. The dynamics of the NM, which is associated to chain mobility, seems to be strongly affected by natural chain-end cross-linking. We propose a model based on a hybrid star polymer in which the low mobility core (phospholipids) controls the mobility of the polyisoprene arms.© 2010 The Royal Society of Chemistry.Financial support from MAT2007-61116 and MAT2009-07789 (MICINN) Spain for generous support of this investigation. FPI grant, MICINN. Support from the Center of Excellence for Innovation in Chemistry (PERCH-CIC), Thailand. Financial support by the National Science Foundation (DMR-0906512).Peer reviewe