No abstract

This paper studies the competition between electric and mechanical force fields simultaneously applied to a polar elastomer that can lead to electric breakdown. The analysis of the system, performed assuming that the free energy of the elastomer is simply the addition of polarizing and stretching energies leads to the classical “thermodynamic” (in this case “electromechanical”) stability.

Understanding the thermal behaviour of lignin is crucial in order to realise its valorisation as an engineering polymer.

Artificial turf is increasingly being used in the construction of football pitches. One of its characteristics is an infill of sand and rubber granules. At present, different materials and layer thicknesses, as well as grain sizes are used for the sand and mainly for the rubber, but they are chosen with little scientific evidence about their influence on the mechanical and biomechanical properties of the pitch. Based on knowledge from materials science, it is reasonable to suggest that grain morphology may have a large influence on pitch performance. This paper presents research conducted to assess the influence of different parameters related to infill grain morphology on the mechanical properties of artificial turf (force reduction (%), vertical deformation (mm) and vertical ball bounce (m)), as well as on their wear with use, measured according to the F ed eration Internationale de Football Association (FIFA) procedures. The results show a significant reduction of pitch performance with use and a significant influence of grain morphology in mechanical response of artificial turf with respect to impact forces and ball rebound.

This work reports a comparative study of the response of poly(2,3-dimethoxybenzyl methacrylate), poly(2,5-dimethoxybenzyl methacrylate), and poly(3,4-dimethoxybenzyl methacrylate) to electrical perturbation fields over wide frequency and temperature windows with the aim of investigating the influence of the location of the dimethoxy substituents in the phenyl moieties on the relaxation behavior of the polymers. The dielectric loss isotherms above T g exhibit a blurred relaxation resulting from the overlapping of secondary relaxations with the glass−rubber or α relaxation. At high temperatures and low frequencies, the α relaxation is hidden by the ionic conductive contribution to the dielectric loss. As usual, the real component of the complex dielectric permittivity in the frequency domain increases with decreasing frequency until a plateau is reached corresponding to the glass−rubber (α) relaxation. However, at high temperatures, the real permittivity starts to increase again with decreasing frequency until a second plateau is reached, a process that presumably reflects a distributed Maxwell−Wagner−Sillars relaxation or α′ absorption. The α and α′ processes appear respectively as asymmetric and symmetric relaxations in the loss electrical modulus isotherms in the frequency domain. To facilitate the deconvolution of the overlapping absorptions, the time retardation spectra of the polymers were computed from the complex dielectric permittivity in the frequency domain using linear programming regularization parameter techniques. The spectra exhibit three secondary absorptions named, in increasing order of time γ′, γ, and β followed by the α relaxation. At long times and well separated from the α absorption the α′ relaxation appears. The replacement of the hydrogen of the phenyl group in position 2 by the oxymethyl moiety enhances the dielectric activity of the poly(dimethoxybenzyl methacrylate)s. The temperature dependence of the relaxation times associated with the different relaxations is studied, and the molecular origin of the secondary relaxations is qualitatively discussed.

The dielectric relaxation spectra of poly­(2-ethoxyethyl methacrylate) in the frequency domain exhibits above T g and at high frequencies a well-developed secondary γ relaxation. This process is followed in decreasing order of frequency for a relatively weak β relaxation and an ostensible glass–rubber relaxation which at high temperatures and low frequencies is dominated by electrode–polymer interfacial processes. By slightly cross-linking the polymer using 2.5% (mol) of 2-ethoxyethyldimethacrylate as cross-linking agent, the β relaxation disappears, the γ relaxation remaining. The activation energy of the γ relaxation for the cross-linked and un-cross-linked polymers is ca. 30 kJ·mol–1, about 10 kJ·mol–1 below that of the β relaxation. Cross-linking shifts the location of the glass–rubber relaxation nearly 10 °C to higher temperatures, without widening the distribution of relaxation times. The X-rays pattern of the cross-linked polymer presents two peaks at q = 5.6 nm–1 and 12.76 nm–1, resembling the X-ray patterns of poly­(n-alkyl methacrylate)­s. The peaks in poly­(n-alkyl methacrylate)­s were attributed to the formation of nanodomains integrated by side chains flanked by the backbone. However, whereas this heterogeneity produces an αPE peak in poly­(n-alkyl methacrylate)­s with n ≥ 2, this microheterogeneity gives rise to a Maxwell–Wagner–Sillars (MWS) relaxation in the cross-linked polymer located at lower frequencies than the glass rubber relaxation. Finally the interfacial-electrode conductive processes of the cross-linked and un-cross-linked polymeric systems are studied in the light of current theories.

: Relaxations in poly(etherimide) PEI Ultem 1000 have been analysed by di †erential scanning calorimetry (DSC), dynamic mechanical spectroscopy (DMS), dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization current (TSDC) measurements. DMTA and DRS results show three distinct relaxations c, b and a in the temperature range [140 to 250¡C. The Ðrst one depends strongly on the water content in the sample as will be discussed in more detail in the second paper of this series. These results are in good agreement with those observed by TSDC of conventionally polarized electrets. In addition to these three relaxations, TSDC measurements show : (1) a peak (o) at which is attributed to space charge temperatures above the a relaxation, (2) indications of structure in the b relaxation zone. In the case of electrets formed by the windowing polarization method, the resulting TSDC spectra allow us to discern the Ðne structure of the b relaxation, which is formed by three subrelaxations. In this work, the activation energies calculated by the di †erent techniques are compared, and a molecular origin for each relaxation is proposed.1998 SCI. Polym. Int. 46, 11È19 (1998)

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.