Dynamic molecular mobility of polyurethane by a broad range dielectric and mechanical analysis

The dynamic molecular mobility of poly(2-hydroxypropyl methacrylate) (PHPMA) base polyurethane (PU) was studied over a broad range of frequency and temperature by combining dynamic dielectric spectroscopy and dynamic mechanical analysis. Two hydrated levels were considered in this study: dry and room humidity states. In dry state, two secondary relaxations γ and β are identified. And in room humidity state, a βsw mode is pointed out. These modes are well known in poly(hydroxylalkyl methacrylate)s. The main α relaxation is influenced by humidity and crosslinks. The Kramers-Kronig transform is used to reveal an ionic conductivity σionic. Relaxation times extracted from dielectric and mechanical analyses are coherent for PU and PHPMA. This correlation allows us to propose an interpretation of relaxations at a molecular level

Inorganic fillers influence on the radiation-induced ageing of a space-used silicone elastomer

A space-used filled silicone rubber (silica and iron oxide fillers) and its polysiloxane isolated matrix were exposed to high energy electrons in order to determine their ageing mechanisms from a structural point of view. Physicochemical analysis evidenced that both filled and unfilled materials predominantly crosslink under such irradiation. Solid-state 29Si NMR spectroscopy allowed the identification of T-type SiO3 units as the main new crosslinks formed in the polymer network. It also revealed an increase in Qtype SiO4 units in the irradiated filled sample. Thanks to the combination of NMR spectroscopy and ammonia-modified swelling tests, these Q-type units were associated with new crosslinks formed at the silica fillers-matrix interface. While the main interaction between the polysiloxane network and the fillers was shown to proceed mainly through hydrogen bonding in the pristine filled samples, it was suggested that the hydrogen bonds were progressively replaced with SiO4 chemical bonds. These additional chemical crosslinks induced evolutions of the shear modulus on the rubber plateau and crosslink density that were significantly more pronounced in the filled material than in the neat one

Electrical conductivity of a silicone network upon electron irradiation: influence of formulation

In this study, the electrical conductivity of a silicone elastomer filled with inorganic fillers was investigated upon electron irradiation. Neat samples consisting of the isolated polysiloxane matrix (with no fillers) were studied in parallel to identify the filler contribution to this evolution. It was shown that exposure to 400 keV electron doses induced a decrease in electrical conductivity for both the filled and neat materials. This decrease was much more pronounced with the filled samples than with the neat ones. Moreover, the activation energy of electrical conductivity (Arrhenius behaviour) doubled in the filled case, while it varied only weakly for the neat case. In light of these results, structure–property relationships were proposed on the basis of the radiation-induced crosslink processes to which this material is subject. In the framework of electronic percolation theory, it is suggested that the radiation-induced formation of SiO3 crosslinks in the polysiloxane network and SiO4 crosslinks at filler–matrix interfaces affects the percolation path of the material, which can be simply modelled by a network of resistors in series. On one hand, their densification increases the overall resistance of the percolation path, which results in the observed decrease of effective electrical conductivity. On the other hand, the steep increase in activation energy in the filled material attributes to the SiO4 crosslinks becoming the most restrictive barrier along the percolation path. In spite of the misleading likeness of electrical conductivities in the pristine state, this study presented evidence that silicone formulation can affect the evolution of electrical properties in radiative environments. To illustrate this conclusion, the use of this material in space applications, especially when directly exposed to the radiative space environment, was discussed. The decrease in electrical conductivity was associated with a progressively increasing risk for the occurrence of electrostatic discharge and consequent spacecraft failures

Electrical behaviour of a silicone elastomer under simulated space environment

The electrical behavior of a space-used silicone elastomer was characterized using surface potential decay and dynamic dielectric spectroscopy techniques. In both cases, the dielectric manifestation of the glass transition (dipole orientation) and a charge transport phenomenon were observed. An unexpected linear increase of the surface potential with temperature was observed around Tg in thermally-stimulated potential decay experiments, due to molecular mobility limiting dipolar orientation in one hand, and 3D thermal expansion reducing the materials capacitance in the other hand. At higher temperatures, the charge transport process, believed to be thermally activated electron hopping with an activation energy of about 0.4 eV, was studied with and without the silica and iron oxide fillers present in the commercial material. These fillers were found to play a preponderant role in the low-frequency electrical conductivity of this silicone elastomer, probably through a Maxwell–Wagner–Sillars relaxation phenomenon

Correlation between the physical structure of a commercially formulated epoxy paint and its electrochemical impedance response

In the present work, a commercial high solids epoxy-based paint used for the corrosion protection of steel was characterized by electrochemical impedance spectroscopy (EIS). The aim was to analyze the influence of water absorption on the physical structure of the paint, through the impedance data analysis. First, the water uptake was followed by gravimetric measurements on free-standing films as a function of the exposure time to a 0.5 M NaCl solution. Gravimetric measurements indicated a mass loss, linked to a release of a plasticizer from the formulation. This compound was dosed in the NaCl solution. The water volume fraction was also determined from the high-frequency part of the impedance diagrams, employing a linear rule of mixtures. The water fraction values obtained from the two methods were in acceptable agreement. The glass transition temperature (Tg) was determined by differential scanning calorimetry for the dry coating as well as for the coating after different immersion times in the NaCl solution. The use of the dielectric permittivity formalism allowed the mean relaxation times to be obtained from the impedance data analysis. The real part of the electrical conductivity was also considered. The variation of these parameters values as a function of the exposure time to the NaCl solution, was correlated with the Tg variation. They accounted for plasticization at the beginning of immersion (water uptake) and antiplasticization for longer exposure times (plasticizer release). For the studied system, it was possible to extract parameters representative of the molecular mobility from the impedance spectra analysis and to link them to the modification of the coating’s physical structure

Thermal activation of impedance measurements on an epoxy coating for the corrosion protection: 1. Dielectric spectroscopy response in the dry state

In this series, an epoxy varnish for the corrosion protection of carbon steel was analyzed in the dry state by broadband dielectric spectroscopy (part 1) to describe the molecular mobility of the epoxy network. Electrochemical impedance spectroscopy measurements were then performed during immersion in a 0.5M NaCl solution (part 2), with the intent to detect the signature of the molecular mobility in the wet state. The present part 1 focuses on the analysis of the dipolar relaxation times associated with the dielectric manifestation of the glass transition in the dry varnish (a-mode). They displayed the characteristic Vogel-Fulcher-Tammann dependence on temperature, which is the typical signature of the amode. The study of the relaxation times showed an anti-plasticizing process upon heating the sample, attributed to the outgassing of absorbed ambient humidity and plasticizers from the formulation. When these elements were completely removed, through an annealing step, the epoxy network became stabilized. The low-frequency electrical conductivity of the varnish presented a very similar temperature dependence to the a-mode, meaning that the molecular mobility governs the electrical charge transport processes in this system. Absorbed ambient humidity and plasticizers also had a great influence on the electrical conductivity, resulting in three decades higher values compared to the annealed sample without these elements. The strong influence of plasticizers on both the molecular mobility (through plasticization) and the electrical conductivity should be taken into account in view of the barrier properties of the final coating. Part 1 of this series provides a basis for a molecular mobility analysis of the electrochemical impedance measurements performed in part 2

In situ study of the temperature activated kinetics of water sorption in an epoxy varnish

The dielectric permittivity of a polyepoxy varnish with a glass transition temperature (Tg) of 61 °C is monitored during immersion in a 0.5 M NaCl aqueous solution, by electrochemical impedance spectroscopy, at various temperatures in the range [21; 78] °C. The sorption curves are first analysed with a Fickian approach. An Arrhenius-type dependence of the apparent coefficient of diffusion is observed, with no visible influence of Tg. Below 46 °C, the sorption curves clearly do not obey Fick′s second law of diffusion. On another hand, the Kohlrausch-Williams-Watts function (KWW) satisfactorily fits all the isothermal sorption curves. The KWW time constant has the same temperature activation as the apparent coefficient of diffusion, and likewise is unaffected by Tg. The KWW stretching exponent displays a strong dependence on Tg, correlates with the Fickian behaviour observed in the rubbery state, and is thought to be governed by molecular heterogeneities in the polyepoxy network. A linear rule of mixture is used to estimate the water uptake in the polyepoxy varnish as a function of temperature. The originality of this work lies in the in situ analysis of water uptake kinetics (constant immersion), which is performed above (rubbery state) and below (glassy state) the Tg of the polyepoxy coating

Thermal activation of impedance measurements on an epoxy coating for the corrosion protection: 2. electrochemical impedance spectroscopy study

In this series, an epoxy varnish for the corrosion protection of carbon steel was analysed in the dry state by broadband dielectric spectroscopy (part 1) to describe the molecular mobility of the epoxy network. Electrochemical impedance spectroscopy (EIS) measurements were then performed in a NaCl solution (part 2), with the intent to detect the signature of the molecular mobility in the wet state. The dielectric manifestation of the glass transition (a-mode), previously characterized in part 1 of this series for the dry varnish, was evidenced from EIS measurements through the use of the dielectric permittivity formalism. This a-mode showed the characteristic Vogel-Fulcher-Tamman dependence, shifted towards higher frequencies when compared to that of the dry varnish, consistently with the plasticization of the epoxy network due to the water uptake. Moreover, it was shown that taking the a-mode into account when fitting the EIS impedance data with equivalent circuits led to a much better fit in the capacitive-resistive transition region. It was shown that the dc charge transport processes are not only triggered but also governed by the molecular mobility of the epoxy network. In the case of thick coatings (several hundreds of micrometers), this dependence demonstrated the absence of through-pores, as it is often discussed in the literature. Indeed, under the assumption of current flowing through free electrolyte via open pores, the temperature dependence of the electrolyte's dc conductivity (Arrhenius law) should be found instead of the temperature dependence of molecular mobility

Impedance analysis of the barrier effect of coil-coated materials: Water uptake and glass transition variations

In the present work, an industrial polyester coil-coated steel sample was characterized by electrochemical impedance spectroscopy. The diagrams were obtained for various immersion times in a 0.5 M NaCl solution for three different initial states of the same coil coating (as received, dried and dried after the impedance measurements). The aim of the study was to have a better knowledge of how the water uptake influences the coil coating physical properties and to extract relevant parameters of the ageing processes. From the high-frequency part of the impedance diagrams, the water uptake was calculated using a linear rule of mixtures. Two sorption regions were observed for the dried samples suggesting the presence of porosities already filled with ambient moisture for the as-received sample. It was shown that the water uptake was a slow process and, independently of the initial state of the sample, a saturation plateau was never reached, even after 456 h of immersion. A time constant, clearly visible on the phase angle of the impedance diagrams, was analysed through the dielectric permittivity formalism and attributed to the signature of the dielectric manifestation of the glass transition. This time constant was shifted to higher frequencies with increasing water fraction (increasing immersion time), consistent with a plasticization effect. This result was supported by differential scanning calorimetry measurements. Finally, the data obtained for the different initial states of the coating highlighted that, even if the water uptake was reversible, the sorption kinetics was different for the sample dried after the impedance measurements. This could be of importance in the degradation process of the coil coated steel

Dynamic glass transition of filled polysiloxane upon electron irradiation

The influence of radiation-induced crosslinking on the molecular mobility of a filled silicone elastomer near the glass transition (α-relaxation) was analyzed using broadband dielectric spectroscopy. Samples of the isolated polysiloxane matrix (neat) were also studied so as to assess the filler influence on the evolution of the α-relaxation. A slowing-down of the segmental dynamics was observed with increasing ionizing dose. It was ascribed to the relaxing dipoles losing degrees of freedom as a result of network stiffening. An enhancement of intermolecular coupling, associatedwith the cooperativity of the α-relaxation,was deduced fromthe dielectric analysis. Similar observations were made in the past with chemically crosslinked polysiloxanes. This study evidenced that even though the crosslinks formed upon chemical crosslinking (mainly Si\\CH2\\CH2\\Si) differ in nature from those formed upon irradiation (mainly SiO3 and SiO4), they affect the dynamic glass transition in a very similar way. The filler influence on the dynamic glass transition was also studied upon irradiation. One of the main outcomes of this study is the fading of the filler-related effect in themost irradiated samples: both the shape and dynamics of the α-relaxation were identical in the most highly irradiated neat and filled samples