Higher Relaxation Strength Research Articles

Stretchable High Response Piezoelectric Elastomers Based on Polable Polynorbornene Fillers in a Polydimethylsiloxane Matrix

AbstractTo advance the field of piezoelectrics, it is desirable to find materials that combine high piezoelectricity with high elasticity. Applications such as self‐sensors, stretchable electronics, soft robots, and energy harvesting would benefit tremendously. However, this is not an easy task neither for materials based on ceramics nor for those based on polymers. While ceramics can show strong piezoelectric effects, their mechanical response tends to be weak. Polymers instead are elastic but only few have sizeable piezoelectric effects. Additionally, they cannot maintain the polarization permanently as needed for piezoelectrics. Here, an all‐organic piezoelectric elastomer is synthesized by blending high glass transition temperature (Tg = 104 °C) polar polynorbornene nanoparticles (NPs) with a high relaxation strength (Δε′ = 22.4) into a chemically cross‐linked polydimethylsiloxane matrix. After processing the blends into thin films by doctor blading, they are poled by corona discharge at elevated temperatures. Fifteen days after poling, the materials show a stable and reversible piezoelectric response d31 = 37 pC N−1. This, to the best of the authors’ knowledge, not only is the highest d31 value reported, but the response is three times that of the well‐known polyvinylidene difluoride.

Linear Dynamic Viscoelasticity of Dual Cross-Link Poly(Vinyl Alcohol) Hydrogel with Determined Borate Ion Concentration.

We investigated the linear dynamic viscoelasticity of dual cross-link (DC) poly(vinyl alcohol) (PVA) (DC-PVA) hydrogels with permanent and transient cross-links. The concentrations of incorporated borate ions to form transient cross-links in the DC-PVA hydrogels (CBIN) were determined by the azomethine-H method. The dynamic viscoelasticity of the DC-PVA hydrogel cannot be described by a simple sum of the dynamic viscoelasticity of the PVA gel with the same permanent cross-link concentration and the PVA aqueous solution with the same borate ion concentration (CB = CBIN) as in the DC-PVA gel. The DC-PVA hydrogel exhibited a considerably higher relaxation strength, indicating that the introduction of permanent cross-links into temporary networks increases the number of viscoelastic chains with finite relaxation times. In contrast, the relaxation frequency (ωc) (given by the frequency at the maximum of loss modulus) for the DC-PVA hydrogel was slightly lower but comparable to that for a dilute PVA solution with the same CB. This signifies that the relaxation dynamics of the DC-PVA hydrogels is essentially governed by the lifetime of an interchain transient cross-link (di-diol complex of boron). The effect of permanent cross-linking on the relaxation dynamics was observed in the finite broadening of the relaxation-time distribution in the long time region.

The Effects of the pH‐influenced Structure on the Dielectric Properties of Kaolinite–Water Mixtures

The properties of clay–water mixtures are reflected in their wide‐frequency dielectric responses that result from different polarization mechanisms. This feature is useful in various soil science applications but has not been systematically investigated. The objective of this study was to characterize the broadband dielectric spectrum of kaolinite sediments with different charge properties and structures using a slim‐form open‐ended coaxial probe. The sediment structure in this study was manipulated by changing the pore‐fluid pH. When the pH was below the isoelectrical point of the edge surface, IEPedge, the edge‐to‐face flocculation structure was formed in voluminous sediments with an average porosity of 0.87 (Group A samples). A higher dielectric constant due to bulk water polarization (an average of 65.47) was measured because of the higher water content. As the pH was increased to greater than the IEPedge, a dense sediment with face‐to‐face aggregation was produced in the Group B samples, and a lower dielectric constant (an average of 60.11) was obtained. In bound water polarization, a higher relaxation strength (∼2.7 times higher on average) and a longer relaxation time (∼1.5 times longer on average) were observed in the Group B samples compared with those of the Group A samples. Similar trends can also be found in the results of spatial polarization. These findings can be attributed to more negatively charged surfaces and denser packing. Fluid conductivity dominated the global conductivity of the sediment in the Group A samples so that the β value, i.e., the ratio between the conductivity of the sediment and the fluid, was <1. The β value was >1 in the Group B samples owing to an overcompensation of surface conduction.

Complementary Wave-Based Characterizations of Sedimentation Processes

In this paper, the sedimentation behavior of two kaolinite samples with distinct fabric associations is characterized using mechanical and electromagnetic wave-based techniques. The two different fabric formations, the edge-to-face (EF) flocculated structure (i.e., Sample A) and the dispersed and deflocculated structure (i.e., Sample B), were produced by changing the pH of the pore fluid. The anisotropy of the shear-wave velocity and DC conductivity was not observed in the sediment of Sample A because of EF isotropic fabric associations, but it was detected in Sample B as a result of face-to-face aggregation. An open card-house structure of the Sample A sediment results in a higher relaxation strength of the bulk water, Δ κw owing to a higher water content; the smaller Δ κw measured in the Sample B sediment indicates denser packing. In both samples, sediment consolidation gives rise to a decrease in the bulk-water relaxation strength, but an increase in the bound-water relaxation strength owing to increas...

Analysis on the anelasticity of a superplastic Zn–22% Al eutectoid

The kinetics of anelastic deformation of a superplastic Zn–22%Al was examined to provide the systematic information on the anelastic behavior of fine grained superplastic materials showing the extraordinary high relaxation strength compared to large grained materials. For the present analysis, the strain transient data of a superplastic Zn–22%Al after unloading, which was reported in the literature, were reappraised by taking the instantaneous remaining anelastic strain as a driving force causing anelastic deformation. The results showed that anelastic behavior of the alloy could be described by the empirical description similar to that of forward superplastic deformation. In addition, anelastic strain rate was found to be insensitive to the forward applied stress prior to unloading. The anelastic behavior of the alloy was manifested by the presence of three distinct regions in the double logarithmic plot of anelastic strain rate against remaining anelastic strain according to the values of the dependency of anelastic strain rate on remaining anelastic strain, r, the grain size sensitivity, s, and activation energy, Q. Region I (long time after unloading) was characterized by r≈1, s≈5 and Q≈76 kJ mol −1. In region II (intermediate time after unloading), the values of s and Q were similar with those of region I, but r was about 3. The results of the analysis indicated that region III (short time after unloading) could be attributed to the athermal process. Activation energy close to that for forward superplastic deformation and the high value of grain size sensitivity in region I and II are indicative of the important role of grain boundary diffusion on anelastic deformation of fine grained superplastic materials. In addition, with the aid of the grain boundary back tension model for anelastic deformation, a relationship between remaining anelastic strain and the back stress resulting from grain boundary back tension was derived.

New dielectric relaxation process reveals mesomorphic ordering in rapidly cooled poly(ethylene naphthalate)

A curious, strong dielectric relaxation process (δ) was found in rapidly cooled poly(ethylene naphthalate). This process, which is located between two known β and β* relaxations of PEN, appears predominantly after rapid cooling and remains present even after heating above the glass transition temperature. In view of its very low activation energy of ∼10 kJ/mol, its markedly high relaxation strength of up to Δɛ=5, and its Debye-like peak shape, a collective relaxation mechanism is proposed, which involves collective crankshaft motions of the -O-CH2-CH2-O- sequences in a regular arrangement of the main chains. The analogy between this δ-relaxation and an ultra-slow relaxation recently found in the smectic E phase of a side-chain liquid crystalline polymer suggests a (close-to) hexagonal smectic ordering in PEN. The very existence of liquid-crystalline order in PEN is corroborated by the observation of a thermo-reversible discontinuity in the relaxation parameters around −90 °C, which resembles a broadened LC-LC phase transition. Re-evaluation of experimental data of the β* relaxation, which occurs in the non-crystalline fraction of PEN, suggests that this relaxation is sensitive to the local orientational order, which extends from nematic to isotropic. The shift in the temperature of the β* peak and even the splitting of this peak found by other authors can be ascribed to the lowering of the activation energy by the local ordered packing of the PEN chains in line with a lower activation energy in the nematic order. The coexistence of isotropic and nematic regions in PEN is put in the context of orientational order fluctuations during the induction period of cold crystallisation of semi-flexible polymers.

Viscoelastic characteristics of chain extended/branched and linear polyethylene terephthalate resins

Two chemically modified chain extended/branched polyethylene terephthalate (PET) resins and one unmodified resin, considered to be linear, were characterized in terms of their melt flow, die swell, and viscoelastic properties. The three resins had reportedly similar nominal intrinsic viscosities but exhibited different viscoelastic behavior. The modified resins had lower melt flow index, higher die swell, higher complex viscosity and higher storage modulus than the unmodified one. The Cole–Cole plots of the resins were independent of temperature, and the data for modified resins formed a group that lay below the data group for the unmodified PET. The distribution of relaxation times was determined. The modified resins had higher relaxation strength, Gi, especially at high relaxation times, λi. The mean relaxation times of the chain extended/branched resins were approximately an order of magnitude higher than that of the unmodified resin, implying pronounced elastic character. The modified resins had better foaming characteristics in extrusion foam processing than the unmodified one owing to their elastic nature. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1371–1377, 2000

Linear response theory for thermodynamic properties.

A fluctuation-dissipation theorem, connecting all thermodynamic response functions to equilibrium fluctuations in the microcanonical ensemble, is derived from classical mechanics. This particular problem is not included in the usual linear response scheme, since the relevant perturbations cannot be stated as additional terms in the Hamiltonian. In experiments where the only control parameter is the heat flow, dissipation is present in terms of an entropy flow from the system to the surroundings. As an example, the full frequency-dependent thermodynamic response matrix is extracted from simulations of a supercooled binary Lennard-Jones fluid. This fluid shows rather high relaxation strength of all response functions, except of the adiabatic compressibility. The low frequency limit of all thermodynamic susceptibilities increases as temperature is decreased along an isocore.

High-temperature internal-friction peak in as-quenched bamboo crystalline aluminium

Abstract Soon after the discovery of the bamboo internal-friction peak (the BB peak) in bamboo crystalline aluminium, a new high-temperature internal-friction peak (designated the HT peak) was observed at a higher temperature when the specimen was quenched from high temperatures. Previous experiments with internal friction measured by the free-decay method showed that this peak is characterized by its high relaxation strength and high activation energy and it exhibits an anomalously amplitude-dependent effect. In this paper, these manifestations are critically studied by measuring the internal friction with the forced-vibration method. This HT peak is found to consist of two subpeaks, designated the HT-1 and HT-2 peaks respectively. The activation energy of the HT-1 peak is found to be 1.4 eV and the activation energy of the HT-2 peak is 2.1 eV. On the basis of the results of the transmission electron microscopy observation, it is suggested that the higher-temperature subpeak (the HT-2 peak) is attributed...

Origin of the high temperature internal friction peak introduced by twisting deformation in aluminium single crystals and bamboo crystals

The internal friction peak appeared around 310°C (l Hz) in single crystals and bamboo crystals of high-purity aluminium introduced by twisting deformation as previously reported has been further studied especially concerning its relaxation properties. This peak is thermally-activated having a very high relaxation strength up to 0.3 and a very high activation energy of about 2 eV. The fact that the peak disappears after rapid cooling of the specimen leads to the proposition that dislocation climb does not play a dominating role to the non-elastic motion of the dislocations giving rise to this peak. TEM observations show that the stable dislocation configuration in the twisted specimen after the 600°C anneal always consists of uniformly distributed dislocation networks. Subsequent tensile deformation destructs such dislocation networks and converts it into cell structures so that the twisting peak is suppressed. It is suggested that the dislocation slip enhanced by the non-conservative shift of the node of the dislocation network contributes dominately to the high relaxation strength of the twisting peak and the dislocation climb controls the relaxation time of the relaxation process.

Point defect relaxation in silicon single crystals

Abstract Isothermal internal friction measurements reveal a relaxation peak in the temperature range 360–470 K for vibration frequencies between 10−3 and 10 Hz. The temperature range and the fact that the relaxation parameters are not sensitive to the structural state of the specimens indicate a point defect relaxation mechanism. Since the controlled defects in the bulk cannot explain the relatively high relaxation strength observed, we propose a mechanism based on stress-induced self-interstitial reorientation, this type of defect being the only one to be present in a sufficiently high concentration in our samples.

Debye relaxation equations for a standard linear solid with high relaxation strength

In internal friction measurements, relaxational effects are very often analysed in terms of the classical Debye equations, which are derived for processes with low relaxation strength. In a theoretical study it is shown that, in the case of high relaxation strength processes, deviations from the features of the Debye plots for damping and modulus defect occur. Calculations have been performed as well for the experimental situation of constant frequency as for resonance measurements. Whereas for the former only a shift of the modulus defect with respect to the peak maximum occurs, for the latter an even larger shift of the peak maximum and a narrowing of the peak plotted as a function of relaxation time is observed. Moreover, the influence of a temperature-dependent relaxation strength is studied and seen to yield an asymmetric damping peak when plotted as a function of temperature. Finally, the theoretical results, compared with some experimental observations, are shown to be able to qualitatively explain observed deviations from simple Debye type behaviour.

Absorption of Acoustic Waves in Dilute Solutions and Dilute Suspensions of Polymers at Frequencies 10 to 300 kHz

Acoustic absorption of dilute solutions of polystyrene in xylene is measured at frequencies 10 to 300 kHz by means of the resonance reverberation method. Dynamic volume viscosity as a function of frequency is obtained for solutions of polystyrene of three different molecular weights. The data exhibits relaxational behavior in the covered frequencies. The relaxation has a much higher relaxation strength than shear relaxation predicted by the Rouse theory. The distribution of relaxation times is broad and seems to join with the acoustic relaxation at megahertz range which has been attributed by Hassler and Bauer to crank-shaft motion of polymer segments. Acoustic absorption of dilute aqueous suspensions of styrene-butadiene rubber is measured and the imaginary part of complex bulk modulus of the rubber is calculated. The loss modulus increases with increasing frequency, suggesting the loss peak due to the primary relaxation is positioned at a higher frequency.