Dynamics of Natural Rubber as a Function of Frequency, Temperature, and Pressure. A Dielectric Spectroscopy Investigation

Abstract

Natural rubber (NR) isolated from Hevea Brasiliensis was investigated by differential scanning calorimetry, dielectric spectroscopy, and high-pressure dielectric spectroscopy. In the range of frequencies (5 × 10−2 to 3 × 106 Hz), temperatures (−120 to 120 °C), and pressures (0.1 to 240 MPa) studied, the dielectric spectra exhibit two overlapped α-processes but no subglass relaxations. Thermal measurements revealed the presence of some moisture in NR. To elucidate the influence of water, dielectric measurements were carried out in dry and wet NR samples. The origin of the two dielectrically active processes was discussed in terms of (i) the apparent activation volume, (ii) the pressure coefficient of the respective glass temperatures, and (iii) the values of the ratio of activation energies, at constant volume and pressure. The latter allowed extracting the relative contribution of thermal energy and volume for each dynamic process. On the basis of these results, the faster α-processes is assigned to the rigidified rubber backbone dynamics whereas the slower to fatty acids (such as stearic acid) that are linked to the rubber chain.

Electrical conductivity of natural rubber–cellulose II nanocomposites

“…It is well known that one of these promising materials is cellulose (cell) [13], which is a renewable raw material, is more environmentally friendly than CB and comes from a sustainable resource. It is for this reason that we have studied a series of NRcell nanocomposite samples in the present work, which is a continuation of previous studies [14][15][16] concerning the morphological, thermal, mechanical and electrical behaviour of these samples. The morphological characterization showed a good dispersion of cell nanoparticles (with size of 50 nm or less in diameter) in the composite.…”

“…In order of increasing temperature, the loss isochrones for all samples present a β-relaxation, located near − 110 °C and related to the local chain dynamics of the cell II polysaccharide chains [28,29]. The high temperature side of the β-process overlaps with the low temperature side of a complex peak, centred near − 60 °C, which can be decomposed into α and α′-relaxations associated, respectively, with the T g s of NR and the lipid component (SA), present in NR not only as a natural impurity but also as an additive [15]. In turn, the high temperature side of the α-relaxation overlaps with a process related to the moisture, only detectable in the wet samples.…”

“…In order to characterize the conductive behaviour it is advisable to represent the obtained dielectric data in terms of the complex conductivity σ * (ω). It is remarkable that σ * (ω) is similar in its temperature and frequency dependence for a wide variety of different materials [15][16][17][18][19][20][21][22].…”

Glass Transition in Rubbery Materials

“…From Figures 2 and 3, we obtain E q /E P ¼0.69 -0.01 at T g and 0.1 MPa, which is consistent with the value reported for natural rubber. 28 Neither of the limiting behaviors of density-or temperature-dominated dynamics apply to polyisoprene, although because E q /E P . 0.5, thermal energy exerts a stronger effect than does q.…”

A contribution to the study of the molecular mobility in polymeric materials by Thermal and Dielectric Analysis

“…Phys., 2003;Janik, et al, 2001;Feldman, et al, 2002;Ortiz-Serna, et al, 2010;OrtizSerna, et al, 2011). This symmetric distribution can be due to the fact that dipole fluctuations associated to slower mechanisms than the segmental relaxation can be presented.…”