Ricardo Díaz-Calleja

Polymer viscoelasticity : stress and strain in practice

Structure of polymers crystalline and amorphous states in polymers rubber elasticity stress-strain relations for ideal solids and ideal liquids linear viscoelasticity - viscoelastic functions dynamic viscoelastic functions experimental determination of viscoelastic properties viscoelastic behaviour of polymers above the glass-transition retardation and relaxation spectra viscoelastic models molecular models of the viscoelastic polymers viscoelasticity of glassy and semicrystalline polymers flow behaviour of polymer melts and solutions yield crazing and fracture reinforced polymers multiaxial analysis of linear viscoelastic stress flexion and torsion of viscoelastic rods.

On electromechanical stability of dielectric elastomers

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.

Study of structural relaxation by dynamic-mechanical methods in poly(methyl methacrylate)

Abstract A systematic study of the influence of ageing on the dynamic properties of amorphous poly(methyl methacrylate) (PPMA) has been made. During isothermal annealing a decrease in loss tangent and an increase in modulus with ageing time in a broad zone between 30 and 90°C have been observed. The ageing in the zone of the β peak has been investigated and an interpretation of the results is proposed on the basis of recent theories. The existence of the memory effect after two-jump experiments has been proved.

Comparative dielectric and TSDC studies of molecular mobility in liquid-crystalline side-chain poly(methacrylate)

Abstract The molecular mobility of a LC side-chain polymethacrylate (PM6) was carried out by dielectric spectroscopy and by thermally stimulated discharge current (TSDC) methods. Six relaxation processes γ2, γ1, β, β1, α, and δ at successively increasing temperatures were observed in PM-6. For each of them, a molecular interpretation was proposed. There is a good correlation between the peak temperature positions obtained from the global TSDC spectrum at an equivalent frequency and those obtained from the dielectric method. The TSDC thermal windowing experiments for PM-6 show the high ability to resolve the overlapping processes at low temperatures. Dielectric and TSDC methods giving different details of the observed relaxation processes, allow us to create the overall picture of molecular mobility in the system under investigation. These two methods revealing the differences and similarities of the obtained results could be considered as complementary to each other.

Effect of an electric field on the bifurcation of a biaxially stretched incompressible slab rubber

This paper describes the effect of an electrical field on the bifurcation phenomenon that appears in a biaxially stretched slab of Mooney-Rivlin material (M. Mooney, J. Appl. Phys. 11, 582 (1940)) subjected to equal dead loads. The main conclusion of the analysis is that the stretch ratio at which the bifurcation phenomenon appears crucially depends on the configuration of the system rubber slab-electrodes. The theoretical foundations of the present study are based on a recent formulation on this subject carried out by Dorfmann and Ogden (A. Dorfmann, R.W. Ogden, Acta Mech. 174, 167 (2005); J. Elasticity 82, 99 (2006)).

Permeability of co-extruded linear low-density polyethylene films to oxygen and carbon dioxide as determined by electrochemical techniques

Abstract The mechanical relaxation spectra of co-extruded linear low-density polyethylene (LLDPE) films, prepared from copolymers of ethylene and 1-octene, were measured in parallel and transverse directions to the processing orientation. Both the γ- and β-relaxations do not show a noticeable dependence on the direction in which the measurements were performed. However, whereas the α-relaxation in the measurements performed in the parallel direction appears as two peaks, in order of increasing temperature, which were denoted as α′ and α″, the measurements carried out in the transverse direction only exhibit the α′-peak. The influence of tensile drawing on the permeability of co-extruded LLDPE films to oxygen and carbon dioxide was investigated by electrochemical techniques over the range of temperatures where the α′-relaxation process is located. In general, the permeability coefficients do not show a significant dependence on the drawing direction in the temperature interval corresponding to the low-temperature region of the α′-peak. In this high-temperature zone, the values of the permeability coefficient for O 2 and CO 2 through the oriented films after tensile drawing are significantly lower than those obtained for these gases through undrawn co-extruded LLDPE films. The diffusion coefficients do not show a definite dependence on tensile drawing.

Diffusion coefficients of conductive ions in a copolymer of vinylidene cyanide and vinyl acetate obtained from dielectric measurements using the model of Trukhan

The dielectric dispersion measurements of Furukawa et al. are treated in the light of a previously proposed model of Trukhan. The latter describes the influence of mobile ions on the dielectric dispersion in a slab of material placed between polarizable electrodes. It is shown that the so‐called ‘‘constant phase element’’ is just a crude approximation to the predictions of the theory of Trukhan, an approximation not valid at very low frequencies. At low frequencies macropolarizations appear analogous to the ones observed in asymmetric cellulose acetate membranes by Malmgren‐Hansen et al. The polarizations are much larger in the present case, and this indicates that there are no microheterogeneities in the polymeric film of Furukawa et al. The diffusion coefficient of the most rapidly diffusing ion (presumably H+) may be found as a function of temperature within some uncertainty. The Arrhenius plot shows clearly the change in activation energy around the glass transition temperature (182 °C). Below the gla...

Dielectric and mechanical-dynamical studies on poly(cyclohexyl methacrylate)

Abstract The relaxation processes in poly(cyclohexyl methacrylate) have been studied by transient and dynamic dielectric means as well as by viscoelastic techniques. The study embraces also the effect of the thermal treatment on the α and β relaxations. The α relaxation has been fitted into the empirical model of Havriliak and Negami, and the functional relation between the mean relaxation times and the temperature has been adjusted to the equation of Vogel. The β relaxation is strongly hindered, so it is scarcely perceivable. The secondary γ relaxation was studied mainly by dielectric means. Its apparent activation energy does not depend on the temperature and amounts to 11.2 ± 0.2 kcal mol −1 . It allows an application of the time-temperature superposition principle and its behaviour is thermorheologically simple. A δ relaxation with a temperature-independent apparent activation energy of 8.5 kcal mol −1 has been obtained

Relaxation behavior of acrylate and methacrylate polymers containing dioxacyclopentane rings in the side chains

The synthesis of poly[(2,2-dimethyl-1,3-dioxolan-4-yl) methyl acrylate)] (PACGA) and poly[(2,2-dimethyl-1,3-dioxolan-4-yl) methyl methacrylate] (PMCGA) is reported. Both polymers present dielectric and mechanical β subglass absorptions at −128 and −115 °C, respectively, at 1 Hz, followed by ostensible glass–rubber or α relaxations centered in the vicinity of 0 and 67 °C, respectively, at the same frequency. The values of the activation energy of both the mechanical and dielectric β absorptions lie in the vicinity of 10 kcal mol−1. The critical interpretation of the relaxation behavior of PMCGA suggests that dipolar intramolecular correlations play a dominant role in the response of the polymer to an electric field. The subglass relaxations of PACGA and PMCGA are further compared with the relaxation behavior of poly(1,3-dioxane acrylate), poly(1,3-dioxane methacrylate), and other polymers in the glassy state. The strong conductive processes observed in PMCGA at low frequencies and high temperatures were studied under the assumption that that these processes arise from Maxwell–Wagner–Sillars effects occurring in the bulk combined with Nernst–Planckian electrodynamic effects caused by interfacial polarization in the films.

Permeability of oxygen through membranes of poly(cyclohexyl acrylate)

Abstract The permeability of oxygen through membranes of poly(cyclohexyl acrylate) at temperatures in the vicinity of the glass transition temperature ( T g ) is determined using an electrochemical experiment. The diffusion coefficient of the gas follows Arrhenius behaviour with an activation energy of 22 kcal mol −1 , a surprisingly high value if it is considered that the range of activation energies observed in most systems is 1–20 kcal mol −1 . The interpretation of the free volume of the membrane at T g suggests that the critical volume necessary for permeation of the gas to take place is nearly four times lower than the value of this parameter for the mechanical and dielectric processes. It is suggested that bulky side groups may aid the transport of gas in the glassy state but presumably hinder it in the rubbery state.