A New Technique for Characterization of Low Impedance Materials at Acoustic Frequencies

Polyurea is a block copolymer that has been widely used in the coating industry as an abrasion-resistant and energy-dissipative material. Its mechanical properties can be tuned by choosing different variations of diamines and diisocyanates as well as by adding various nano- and micro-inclusions to create polyurea-based composites. Our aim here is to provide the necessary experimentally-based viscoelastic constitutive relations for polyurea and its composites in a format convenient to support computational studies. The polyurea used in this research is synthesized by the reaction of Versalink P-1000 (Air Products) and Isonate 143L (Dow Chemicals). Samples of pure polyurea and polyurea composites are fabricated and then characterized using dynamic mechanical analysis (DMA). Based on the DMA data, master curves of storage and loss moduli are developed using time–temperature superposition. The quality of the master curves is carefully assessed by comparing with the ultrasonic wave measurements and by Kramers–Kronig relations. Based on the master curves, continuous relaxation spectra are calculated, then the time-domain relaxation moduli are approximated from the relaxation spectra. Prony series of desired number of terms for the frequency ranges of interest are extracted from the relaxation modulus. This method for developing cost efficient Prony series has been proven to be effective and efficient for numerous DMA test results of many polyurea/polyurea-based material systems, including pure polyurea with various stoichiometric ratios, polyurea with milled glass inclusions, polyurea with hybrid nano-particles and polyurea with phenolic microbubbles. The resulting viscoelastic models are customized for the frequency ranges of interest, reference temperature and desired number of Prony terms, achieving both computational accuracy and low cost. The method is not limited to polyurea-based systems. It can be applied to other similar polymers systems.

Dynamic properties of polyurea-milled glass composites Part I: Experimental characterization

Standard Dynamic Mechanical Analysis (DMA) is generally used to measure the mechanical properties of polymers at frequencies around and below 100 Hz. Ultrasonic (US) techniques measure wave speeds and impedances at higher frequencies. However, both approaches run into issues between the two regimes. DMA systems become less reliable due to the dynamic response of the frames and load path as one tries increasing the frequency. On the other hand, the internal multiple reflections in the wave propagation techniques introduce challenges in clean measurements and require careful analysis. In this presentation, we introduce a robust procedure for determining the storage and loss moduli of low-impedance materials, where a cylindrical sample is placed between two long metal bars, similar to SHPB technique. However, unlike SHPB, the incidence signal is created by a very light impact, to ensure that the sample does not experience permanent or large deformation. Furthermore, due to the length of the specimen, dynamic equilibrium is neither guaranteed nor intended. The reflected and transmitted pulses are measured using semi-conductor strain gages. The wave speed may be determined using a phase spectral analysis of the time-resolved signals. Determination of the material loss requires a more thorough transfer matrix analysis. The method was applied to a soft polyurea elastomer that was tested in a temperature-control chamber and results were compared with DMA and US data using time-temperature superposition (TTS). While the predictions of the storage modulus using DMA and TTS matched very well with the direct measurements, the DMA/TTS predictions generally underestimate the material loss at higher frequencies. We expect that this method may be applied successfully to other low impedance materials including foams and metamaterials.

The control of chemical architecture has been one relevant parameter in the study of glass transition dynamics in macromolecular systems. In this study, two polyester resins differing in the styrene content that was added in the curing process were studied using two complementary mechanical spectroscopy techniques: dynamic mechanical analysis (DMA) and thermally stimulated recovery (TSR). Both techniques showed that the α-relaxation is shifted to higher temperatures (longer times) with increasing styrene content. Master curves were obtained from the DMA data. The shift factors were used to obtain the temperature dependence of the apparent activation energy, Ea(T). The TSR results also permitted to obtain Ea(T) that also exhibited a maximum around Tg. This behaviour, apparently universally observed in thermally stimulated techniques, was explained by the shift from a Vogel-Fulcher-Tamman-Hesse to an Arrhenius regime. The data also allowed to calculate the fragility index of the two materials, which was found to be higher for the one with higher styrene content. Remarks are made on the dependency of the values of this parameter obtained from different techniques.

The effects of physical aging on the viscoelastic behavior of a thermoset polyester

Dynamic mechanical analysis (DMA) is a common way to measure the mechanical properties of materials as functions of frequency. Traditionally, a viscoelastic mechanical model is applied and current DMA techniques fit an analytical approximation to measured dynamic motion data by neglecting inertial forces and adding empirical correction factors to account for transverse boundary displacements. Here, a finite-element (FE) approach to processing DMA data was developed to estimate poroelastic material properties. Frequency-dependent inertial forces, which are significant in soft media and often neglected in DMA, were included in the FE model. The technique applies a constitutive relation to the DMA measurements and exploits a nonlinear inversion to estimate the material properties in the model that best fit the model response to the DMA data. A viscoelastic version of this approach was developed to validate the approach by comparing complex modulus estimates to the direct DMA results. Both analytical and FE poroelastic models were also developed to explore their behavior in the DMA testing environment. All of the models were applied to tofu as a representative soft poroelastic material that is a common phantom in elastography imaging studies. Five samples of three different stiffnesses were tested from 1-14 Hz with rough platens placed on the top and bottom surfaces of the material specimen under test to restrict transverse displacements and promote fluid-solid interaction. The viscoelastic models were identical in the static case, and nearly the same at frequency with inertial forces accounting for some of the discrepancy. The poroelastic analytical method was not sufficient when the relevant physical boundary constraints were applied, whereas the poroelastic FE approach produced high quality estimates of shear modulus and hydraulic conductivity. These results illustrated appropriate shear modulus contrast between tofu samples and yielded a consistent contrast in hydraulic conductivity as well.

Temperature-dependent elastic moduli of epoxies measured by DMA and their correlations to mechanical testing data

Investigation of accelerated aging behaviour of high performance industrial coatings by dynamic mechanical analysis

Compressional and shear velocities of acoustic waves propagating in porous rocks saturated with viscous fluids were measured both in the kHz and MHz frequency range. The experimental results of the velocities were plotted as a function of pore fluid viscosity ranging from 1 to 1010 centipoise. It was shown that the measured velocities increased with increasing pore fluid viscosity in the kHz frequency range, while in the MHz range, the pore fluid viscosity did not have much effect on the acoustic wave velocities. The experimental results were also discussed in terms of theories of wave propagating in viscous fluids and in the saturated porous solids. Applications of the results to oil production and exploration, as well as to the low velocity zone in the earth, were also presented.

Polyurea and polyurea-based composite materials are widely used due to their excellent mechanical properties. In order to facilitate large-scale computational studies for this group of materials, a robust and standard method is needed to extract their viscoelastic constitutive parameters. In this study, frequency-domain master curves which cover a wide range of frequencies are developed using the data of dynamic mechanical analysis through time-temperature superposition (TTS). The quality of the master curves is assessed both by Kramers-Kronig relations and by comparing with the ultrasonic wave testing data. Then the time-domain relaxation modulus is obtained by the high-resolution Prony series approximated from the relaxation spectrum. To reduce computational cost, 4 to 8-term Prony series are then fitted from the time-domain relaxation modulus for a limited frequency range of interest. Both the high and low-resolution Prony series are converted back to frequency domain to compare with the master curves developed by TTS and show good agreements. This method is not limited to polyurea and polyurea-based composites and it can be applied to other similar polymer systems as well.

ABSTRACTSeveral high temperature methods of processing Nafion® have been developed using various alkylammonium ion forms of the ionomer, and the choice of counterion has been shown to have a significant effect on the thermal and mechanical properties of this material. In particular, it has been shown that neutralization gives rises to two high-temperature mechanical relaxations as observed in dynamic mechanical analysis (DMA). While several studies in the literature have attempted to explain the molecular origins of these mechanical relaxations, the assignments were based primarily on limited DMA results and have at times been contradictory. The study presented here is a fundamental investigation into the molecular origins of the thermally induced morphological relaxations and dynamics of alkylammonium forms of Nafion® membranes as studied by variable temperature small-angle x-ray scattering (SAXS) and solid-state 19F NMR spectroscopy. The intensity of the small-angle ionomer peak at ca. q = 2 nm–1 was monitored as a function of temperature for each alkylammonium neutralized sample in unoriented and oriented states. In the case of the oriented samples, the degree of anisotropic scattering from the oriented ionomer morphology was quantified using the Hermann's orientation function and monitored as a function of temperature. Changes in intensity of the ionomer peak and the Hermann's parameter as a function of temperature were shown to correlate well with relaxations observed in DMA. Several variable temperature solid-state 19F NMR techniques (including spin diffusion, side-band analysis and T1ρ experiments) were used to investigate the dynamics of the Nafion® chains. Side band analysis indicated that the side-chain is more mobile than the main chain and that the mobility is greatly affected by the size of the counterion. Changes in side-band intensity as a function of temperature were shown to correlate well with DMA data. Results from T1ρ experiments show strong counterion dependence and suggest coupled main- and side-chain motions. A two-component relaxation process was also observed for the main-chain fluorines. The results of the NMR investigations, along with the SAXS data, have led to the development of a more detailed description of the dynamics of Nafion® and the molecular origins of the mechanical relaxations. With this information, the continuing goal to determine how the strength of the electrostatic interactions in perfluorosulfonate ionomers affects the chain dynamics and developing morphology may be realized for the purpose of controlling the morphology to create more efficient ionomeric membrane materials.

Dynamic mechanical and ultrasonic properties of polyurea

Investigation of the curing behaviour of carbon fibre epoxy prepreg by Dynamic Mechanical Analysis DMA

A series of polyurethane networks were prepared from MDI (4,41-diphenyl methane diisocyanate), ethylene glycol and a polyoxyethylene-tipped polyoxypropylene triol. The phase separation and phase inversion phenomena of these polyurethane networks were investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and measurement of their tensile properties. The DSC and DMA data indicate that the segmented copolyurethanes possess a two-phase morphology comprising soft and hard segments. It can be found from DSC data that the polyether soft segments exhibit a Tg (glass transition temperature) of −60 °C, and the aromatic hard segments display a Tg of about 128 °C. Two Tgs corresponding to the comprised segments can also be found by DMA for some segmented polyurethanes. Varying the content of aromatic hard segments over the range from 0 to 80 wt% changes the material behavior from a soft rubber through a highly extensible elastomer to a brittle semi-ductile glassy material. Based on the property-composition plots, phase inversion appears to occur at a hard segment content of about 50 wt%.

A comparison was carried out of the nature of intermolecular interactions, elastic properties and gas permeability of the crosslinked polyurethanes doped with xanthene dyes and original polyurethane using IR spectroscopy, dynamic mechanical analysis (DMA) and electron paramagnetic resonance (EPR). The introduced dye can be considered as useful microimpurity which, however, can affect the efficiency of the laser. In IR spectra of polyurethanes the complex band of stretching vibrations of C=O groups is sensitive to the nature of intermolecular interaction of urethane groups. From the analysis of that band it is shown that in the presence of dyes, self-association of urethane groups within the hard segment predominates and the interaction of urethane groups with the oligoether component decreases, which can contribute to increasing the mobility of the flexible component. A decrease in the dynamic storage modulus (E’) and a decrease in the glass transition temperature (Tc) of polyurethanes in the presence of dyes is shown by the DMA method. The results of both DMA and IR spectroscopy indicate a greater increase in the mobility of the elastic component with the introduction of the rhodamine B dye, covalently bound to the polyurethane chain. According to nitroxyl paramagnetic probe data the introduction of both rhodamine B and rhodamine 6G dyes into polyurethanes increases their permeability to vapors of low-molecular weight compounds, but rhodamine 6G has a more prominent effect on this characteristic. This is consistent with DMA data indicating a greater increase in the Mc value in the presence of rhodamine 6G in polyurethane. The obtained results make it possible to determine the optimal composition of the active laser medium and are important in assessing the radiation resistance of the polymer matrix. Its increase is facilitated by a decrease in the storage modulus and an increase in the gas permeability of the polymer, leading to a decrease in pressure in the area of local heating.