Relaxation Of Poly Research Articles

Influence of annealing temperature on ferroelectric and electromechanical responses of P(VDF-TrFE-CFE-FA) tetrapolymer

We study the influences of the annealing temperatures on the ferroelectric and electromechanical (EM) responses of the relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene- fluorinated alkyne) (PVDF-TrFE-CFE-FA) tetrapolymer. The results show that only within a narrow annealing temperature window (<10 °C) around 120 °C, the tetrapolymer exhibits a pure relaxor phase and displays the highest EM response. Films annealed outside of this narrow temperature window contain a small amount of local polar-phase mixed with the relaxor phase. These results reveal that the tetrapolymer composition in the study is near a critical transition region between polar (ferroelectric) and non-polar (relaxor) chain condensed state which has vanishingly small energy barriers between different chain condensed state, thus a small electric field generates a large EM response. The films annealed outside of the narrow temperature window have a broad energy landscape that includes both non-polar and polar chain condensed state. Hence, small changes in annealing temperature result in different ferroelectric and electromechanical responses.

Relaxation‐Induced Significant Room‐Temperature Dielectric Pulsing Effects

AbstractThermo‐responsive dielectric materials are in urgent demand owing to the rapid development of smart electronic/electrical systems. Although different types and structures of thermally responsive dielectric materials have been continuously reported, their dielectric response behaviors all originate from thermodynamic phase transitions. Herein, it is demonstrated that structural relaxation in poly(vinylidene fluoride) (PVDF), a non‐thermodynamic phase transition, can induce a significant thermal dielectric pulse at room temperature. The dielectric pulse strength of up to 6.3 × 105 at 20 Hz, with a dielectric pulsing temperature of 24 °C, is achieved from polyethylene glycol (PEG)‐PVDF coaxial nanofibrous films (PVDF@PEG), fabricated via a continuous blow spinning method. Moreover, the films exhibit excellent flexibility, adjustable strength and toughness, switchable hydrophilicity/hydrophobicity, and effective thermal management capability. The relaxation‐induced dielectric pulsing effect, outstanding multifunctionality, and simple preparation combine to promote further scalability and prospects of PVDF@PEG. In particular, the work contributes to the discovery of the relaxation‐induced dielectric response mechanism, which provides a new strategy for the generation of thermo‐responsive dielectric materials.

Characteristic Features of α and β Relaxations of Poly(diethyl fumarate) as the Poly(substituted methylene)

AbstractRecently the β relaxation draws attention because it may act as a precursor of the α relaxation and potentially influences a wide range of properties. In many cases, the weak intensity of the β relaxation limits the experimental investigation. Herein, the β relaxation of poly(diethyl fumarate) (PDEF) using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and dielectric spectroscopy is investigated. Originating from the absence of methylene spacer in the main chain, polyfumarates possess an intense β relaxation. Unlike common vinyl polymers, PDEF shows two distinct steps in DSC measurement corresponding to α and β relaxations. Interestingly, the relative intensities of α and β relaxations depend on the thermal history. The β relaxation of PDEF obtained by dielectric spectroscopy at high temperature is similar to that of poly(methyl methacrylate) (PMMA). However, the β relaxation of PDEF follows the VFT equation rather than the Arrhenius equation at low temperatures. The analysis of PDEF/PMMA blend suggests the local orientation of PDEF chain plays a vital role in the intense β relaxation.

Electrocaloric cooling over high device temperature span

Summary Electrocaloric cooling demonstrates direct electricity utilization and high energy efficiency, and is often proposed as an environmentally benign, solid-state alternative to the conventional vapor compression cooling technology. As a result, the fabrication of effective electrocaloric materials and engineering of electrocaloric cooling devices have attracted increasing attention in the cooling community. However, the limited material adiabatic temperature change under safe operation field poses a major challenge to practical electrocaloric cooling device development, since real-world applications usually require device temperature span over 20 K. This perspective presents recent efforts devoted to design electrocaloric cooling devices based on active heat regeneration and cascading approaches; both strategies have achieved significant device temperature lift ~10 K with adiabatic temperature change of 2~3 K. The strategies discussed in this work are broadly applicable to the design of efficient and compact cooling systems.

The dynamics of freestanding films: predictions for poly(2-chlorostyrene) based on bulk pressure dependence and thoughtful sample averaging.

In this paper we model the segmental relaxation in poly(2-chlorostyrene) 18 nm freestanding films, using only data on bulk samples to characterize the system, and predict film relaxation times (τ) as a function of temperature that are in semi-quantitative agreement with film data. The ability to translate bulk characterization into film predictions is a direct result of our previous work connecting the effects of free surfaces in films with those of changing pressure in the bulk. Our approach combines the Locally Correlated Lattice (LCL) equation of state for prediction of free volume values (Vfree) at any given density (ρ), which are then used in the Cooperative Free Volume (CFV) rate model to predict τ(T, Vfree). A key feature of this work is that we calculate the locally averaged density profile as a function of distance from the surface, ρav(z), using the CFV-predicted lengthscale, Lcoop(z), over which rearranging molecular segments cooperate. As we have shown in the past, ρav(z) is significantly broader than the localized profile, ρ(z), which translates into a relaxation profile, τ(z), exhibiting a breadth that mirrors experimental and simulated results. In addition, we discuss the importance of averaging the log of position dependent relaxation times across a film sample (〈log τ(z)〉), as opposed to averaging the relaxation times, themselves, in order to best approximate a whole sample-averaged value that can be directly compared to experiment.

Entanglement and Relaxation of Poly(methyl methacrylate) Chains in Imidazolium-Based Ionic Liquids with Different Cationic Structures

A series of dicationic ionic liquids (DILs) with two singly charged imidazolium rings linked by different alkyl chain lengths (C4, C6, and C8) were first synthesized. The relaxation and entanglemen...

Effect of Hydrogenation and Chlorination Reactions on the β Relaxation of Poly(Vinyl Chloride)

The relaxation phenomena of polymers are very important because they play an important role in their physical properties. Dynamic mechanical analysis was used in this work in order to study the effect of tactic and compositional sequences on the relaxation processes observed in poly(vinyl chloride) (PVC). The polymers studied in this work were PVC modified by means of reductive hydrogenation and chlorination reactions at low degrees of modification. The results obtained with hydrogenated poly(vinyl chloride) indicated that the decrease in mmr tetrads at the end of the isotactic sequences in the chain by stereospecific hydrogenation led to concomitant decreases in intensity and temperature of the maximum of the β relaxation. This effect seemed to be stronger the longer the isotactic sequence associated with the said structure. On the other hand, in the case of chlorinated poly(vinyl chloride) (CPVC), the results of the evolution of the β relaxation with the degree of chlorination showed that the predominant factor was the compositional microstructure (substitution of hydrogen atoms by chlorine atoms, increasing the possibility of inter-chain interactions leading to a stiffening of the chain, with the consequent increase in the β temperature).

Enthalpy Relaxation Study of Poly(Vinyl Chloride) Films Based on the Time-Temperature Superposition Principle

In the enthalpy relaxation of poly(vinyl chloride), a decrease in enthalpy upon the isothermal ageing was measured using the differential scanning calorimetry method as a function of ageing time (tA) and ageing temperature. The range of the ageing temperature was from 56 °C (Tg − 25 °C) to 72 °C (Tg − 9 °C) where Tg denotes the glass transition temperature. The limiting value of the decrease in enthalpy was determined by applying a stretched exponential function to the measured enthalpy data. The relaxation function (ϕ) was derived from the measured enthalpy and the construction of a master curve was tried by shifting the ϕ − tA curves of the respective ageing temperatures horizontally. Although there was no agreement between the shift factors (aT) and the relaxation times of the ϕ − tA curves, the superposition was successfully constructed and the aT values obtained for the poly(vinyl chloride) sample were found to be comparable to those reported for viscoelastic experiments over a broad temperature range above and below Tg carried out for different polymers. The origin of the decrease in enthalpy was briefly discussed in terms of the chain dynamics in the isothermal condition.

High cyclic stability of electrocaloric effect in relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymers in the absence of ferroelectric phase transition

The incorporation of bulky defects into ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymers can result in a relaxor behavior, accompanied by outstanding electrocaloric properties near room temperature. However, it remains elusive whether ferroelectric transition occurs in relaxor terpolymers, which is critical for the design of electrocaloric cooling devices. In this work, we study the electrocaloric fatigue in poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (61.8/30.4/7.8 mol. %) terpolymer, in which we revisit its controversial issue of the phase transition. We observe that the electrocaloric response of the terpolymers remains nearly constant (ΔS = 25.5 J K−1 kg−1, ΔT = 5.1 K at 100 MV m−1) within 50 cycles at different temperatures around room temperature. Moreover, we conduct temperature-dependent X-ray diffraction and Fourier-transform infrared spectroscopy measurements on the terpolymer to provide insights into the evolution in intermolecular lattice spacing and intramolecular conformation. We find no clear evidence of the ferroelectric-to-paraelectric phase transition near the temperature range, where differential scanning calorimeter measurement displays an endothermic peak.

Enhancing the electrocaloric effect in a relaxor polymer by including minor normal ferroelectric phase

Generating a large caloric effect under a low applied field is critical for realizing high performance solid state caloric cooling. Here, we report that a large enhancement in the electrocaloric response at a moderate electric field level over a relatively broad temperature range can be achieved in the relaxor poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)] polymers by including the minor normal ferroelectric phase. We show that the terpolymer of P(VDF-TrFE-CFE) 69.7/30.3/6 mol. % which contains a minor normal ferroelectric phase, under a moderate electric field of 90 MV/m, can reach an adiabatic temperature change ΔT = 11 K, compared with ΔT = 6 K of the terpolymers with a pure relaxor phase under the same field. The results reveal that the presence of the minor ferroelectric phase facilitates the polarization switch in a temperature range near the ferroelectric transition while the relaxor phase maintains a large electrocaloric effect (ECE) over a broad temperature range, resulting in a large ECE over a relatively broad temperature range in the material.

Effect of the Degree of Crystallinity on Charge Relaxation in a Polyimide

The method of thermoactivation spectroscopy is used to study effect of the degree of crystallinity on the mechanisms of homocharge relaxation in poly((4,4'-bis(4-aminophenoxy)biphenyl)imido-1,3- bis(3,3'-4,4'-dicarboxyphenoxy)benzene films. Two regions of charge relaxation are found. At a temperature of 177°С (450 K), there is a maximum of the thermally stimulated depolarization current related to the intrinsic conductivity of the dielectric. In samples with degrees of crystallinity of 20 and 40%, a low-temperature maximum is observed at 50°С (323 K), which is related to the release of charge carriers from traps formed at the interface of amorphous and crystalline phases.

Interplay Between Hydroxyl Density and Relaxations in Poly(vinylbenzyltrimethylammonium)-b-poly(methylbutylene) Membranes for Electrochemical Applications.

Anion-exchange membranes (AEMs) consisting of poly(vinyl benzyl trimethylammonium)-b-poly(methylbutylene) of three different ion exchange capacities (IECs), 1.14, 1.64, and 2.03 mequiv g-1, are studied by High-Resolution Thermogravimetry, Modulated Differential Scanning Calorimetry, Dynamic Mechanical Analysis, and Broadband Electrical Spectroscopy in their OH- form. The thermal stability and transitions are elucidated, showing a low temperature Tg and a higher temperature transition assigned to a disorder-order transition, Tδ, which depends on the IEC of the material. The electric response is analyzed in detail, allowing the identification of three polarizations (only two of which contribute significantly to the overall conductivity, σEP and σIP,1) and two dielectric relaxation events (β1 and β2), one associated with the tolyl side groups (β1) and one with the cationic side chains (β2). The obtained results are integrated in a coherent picture and a conductivity mechanism is proposed, involving two distinct conduction pathways, σEP and σIP,1. Importantly, we observed a reordering of the ion pair dipoles which is responsible for the Tδ at temperatures higher than 20 °C, which results in a dramatic decrease of the ionic conductivity. Clustering is highly implicated in the higher IEC membrane in the hydroxyl form, which reduces the efficiency of the anionic transport.

Study of H1 spin lattice relaxation and dielectric relaxation in Poly(propylene glycol) system

Proton NMR spin-lattice relaxation time (T1) and dielectric relaxation time (τ0) have been used to correlate the experimental and calculated values of T1 for Poly (propylene glycol) at 300 K. Also, the contributions of dielectric relaxation time (τ0) of each constituent group of PPG-425 to the effective dielectric relaxation time due to the whole chain of the polymer have been calculated using NMR spin–lattice relaxation time. The H1 NMR data and dielectric data of Poly (propylene glycol) HO [CH(CH3)CH2O]nH with number average molecular weight 425 (n = 7) g mol−1have been used to understand the conformation and basic mechanism of relaxation in polymer in its pure liquid state. Conformational analysis using Hartree-Fock formalism was performed for the polymer of n = 7 and the average bond distances obtained from the computation were used to assign NMR relaxation times to specific subgroups. Free energy and enthalpy of activation associated with methyl and other groups in the chain of PPG molecules have been calculated. BPP (Bloembergen Purcell Pound) model gives better correlation for molecular tumbling motion for PPG-425 in pure liquid state than Kubo & Tomita equation.

Effects of silicon Interface and frequency dependence in solution-processed high-K poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) dielectric characteristics

A solution-based ferroelectric relaxor poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) terpolymer, P(VDF–TrFE–CTFE), is employed in a metal-insulator-semiconductor (MIS) capacitor as a high-k dielectric material. Based on experimental characterizations, the capacitance of the MIS structure is decreased with P(VDF-TrFE-CTFE) thickness reduced. Examined by electrical characterizations, an equivalent interface layer at the P(VDF-TrFE-CTFE)/silicon interface is proposed in this work. This shows that the P(VDF–TrFE–CTFE) film thickness is an important factor while using it as a high-k dielectric material in devices. At the same time, experimental results also show a low remnant polarization and low coercive field of 1.23μC/cm2 and 0.176MV/cm, respectively. This indicates operating frequency is also a factor of its effective dielectric constant. In our experimental results, the maximum dielectric constant is 32.4 at 1kHz with 2900nm film thickness. For applications in different organic electronics, therefore, this study demonstrates that P(VDF–TrFE–CTFE) is a frequency/thickness dependent high-k dielectric material.

Monitoring the structural relaxation in poly(ethylene terephthalate) fibers: Dye sorption and FTIR considerations

This work is aimed to monitor the physical aging (structural relaxation) of polyester fibers in terms of changes in the number of dye molecules absorbed by aged specimens as a new method and to make some aspects of this complex phenomenon more clear which rarely considered in the literature. Specimens, aged thermally and hygrothermally at the specified temperature and periods, are, thus, conducted to the dye sorption experiments to monitor their physical aging. Both purified and commercial dyes are employed. Independent from dye purities, the number of absorbed dye molecules reduces for both aged thermally and hygrothermally specimens with increasing the aging time because of decreasing of the free volume as well as increasing the crystallinity, revealed by WAXD measurement. However, the rate of aging of hygrothermally aged specimen is faster than that of the thermal one due to the role of water as a plasticizer in the polymeric bulk. The non-exponential character of the relaxation is exhibited by fitting of KWW (Kohlrausch-Williams-Watts) stretched exponential function and two additive exponential models into the relaxation data, constructed from the measuring of the changes in the number of absorbed dye molecules during aging. Using FTIR spectroscopy a correlation between the number of absorbed dye molecules and the amount of trans conformers for the sample upon thermal aging is found which shows the method yields reliable information on physical aging of the polymer.

Studies on the Temperature and Time Induced Variation in the Segmental and Chain Dynamics in Poly(propylene glycol) Confined at the Nanoscale

The effect of 2D confinement on the dynamics of the normal mode (chain mobility) and segmental relaxation in poly(propylene glycol) (PPG) has been studied with the use of broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). It is shown that both processes become faster with increasing degree of confinement. Interestingly, the crossover from VFT to the Arrhenius-like behavior of chain and segmental dynamics, observed in the examined system, is strictly related to the vitrification of the adsorbed polymers. We also report that the mean relaxation times of the normal, τNM, and segmental modes, τα, depend on the thermal history of confined PPG and can be significantly modified using different thermal treatments. It is demonstrated that annealing of the samples below the crossover temperature, Tc, leads to a systematic shift of the segmental relaxation and normal mode toward lower frequencies, resulting in an increase in the glass transition temperature of the spatially restrict...

Resonant dipolar relaxation in poly (ε-caprolactone)—A thermally stimulated depolarization current study

Resonant dipolar relaxation in poly(ε-caprolactone) (PCL) is reported using thermally stimulated discharge current spectroscopy. PCL is a bio-medically known shape memory polymer having a well defined γ, β, α, and α′ relaxations, respectively, centered around 125 K, 170 K, 220 K, and 270 K as seen by the measurements. By employing a new protocol variable poling temperature at constant freezing temperature, resonant dipolar relaxation in PCL could be induced, especially in the vicinity of α relaxation. Such a protocol is useful in de-convoluting the features in a more meaningful fashion. By an analysis of activation process, we could show a clear contrast enhancement of the dynamics of the participating dipoles by means of a minimum in the activation energies situated around the glass transition region. The relevant parameters of interest such as activation energies and relaxation times are estimated and discussed.

Dielectric Relaxations in Poly(glycidyl phenyl ether): Effects of Microstructure and Cyclic Topology

Cyclic and linear, isoregic and aregic, and isotactic and atactic poly(glycidyl phenyl ether) (PGPE) with molecular weights up to Mw = 5.5 kg/mol are synthesized by ring-opening polymerization of glycidyl phenyl ether. Initiation with tetrabutylammonium fluoride leads to isoregic linear polymers with ∼95% regular linkages, and initiation with B(C6F5)3 and B(C6F5)3/water leads to aregic cyclic and linear polymers, respectively, with ∼50% regular linkages as quantified by 13C NMR. Local, segmental, and chain dynamics in PGPE is investigated by broadband dielectric spectroscopy (10–2–106 Hz). The β-relaxation for linear PGPE is separated into two contributions arising from the motions of side groups and end groups with activation energies of 35.4 and 23.8 kJ/mol, respectively. The β-relaxation process for cyclic PGPE shows the same activation energy as that shown by the side-group contribution in linear PGPE, indicating that topology does not play a key role on the side-group local dynamics. Moreover, cyclic...

Dielectric and conductivity relaxation in poly(3,4‐ethylenedioxythiophene) nanotubes

The charge carriers relaxation dynamics in synthesized poly(3,4‐ethylenedioxythiophene) nanotubes has been investigated as a function of frequency and temperature through the framework of dielectric permittivity, modulus formalism and impedance formalism. The fitting of experimental data of real part of dielectric permittivity and imaginary part of modulus with Havriliak‐Negami equation and Bergman modified Kohlrausch‐Williams‐Watts function indicates the presence of non‐Debye type relaxation process in the synthesized poly(3,4‐ethylenedioxythiophene) nanotubes. The scaling of imaginary part of modulus and impedance formalisms have been performed to understand the charge carrier relaxation dynamics in the synthesized samples, which suggests the presence of time temperature superposition principle in the nanotubes. The red shifting of the band from 1440 cm−1 to 1430 cm−1 in micro‐Raman spectra indicates the conformational transition from benzenoid to quinoid structure with increasing dopant concentration. The linear increase of imaginary part of dielectric permittivity with decreasing frequency is ascribed to the higher contribution from dc conductivity as compared to that from the electrode polarization effects. POLYM. ENG. SCI., 56:448–457, 2016. © 2016 Society of Plastics Engineers

Properties of Tunability and Stored Energy Density in the Ferroelectric Multilayers

Multilayers consisting of alternating ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer and relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene) (P(VDF-TrFE-CFE)) terpolymer with different periodicities in thickness were prepared by the Langmuir-Blodgett method. Except for the dielectric and ferroelectric properties enhanced, the tunability also was enhanced in the multilayers with thinner periodic thickness. The polymer multilayers can be utilized for high energy density and low loss dielectrics, the multilayers with alternating 3 monolayers get the biggest high energy density. The reasons for the enhanced tunability and high energy density in the multilayer sample are discussed.