Cooperative relaxations/transitions in ferroelectric polymers

Dielectric relaxation and thermal transitions in β-PVDF have been investigated by Thermo-Stimulated Current spectroscopy and Differential Scanning Calorimetry respectively. A comparative study of spectra and thermograms has been performed. The relaxation mode associated with the glass transition of the true amorphous phase is characterized by relaxation times obeying a compensation law due to cooperative molecular movements. A conformationally disordered structure is proposed for β-PVDF to explain thermal events occurring around 60°C. Ageing of ferroelectric properties of β-PVDF has been associated with cooperative molecular movements liberated largely below the melting point.     

Influence of the crystalline phase on the molecular mobility of PVDF

Dielectric relaxation and pyrocurrent of PVDF were studied by thermostimulated current spectroscopy. The transition spectrum of the material was investigated by differential scanning calorimetry. Two well-resolved relaxation peaks have been observed in the temperature range [?100–100°C]. The molecular mechanisms of these phenomena have been discussed, based on a comparative study of α-PVDF. and β-PVDF. The β relaxation mode is located at ?41°C in α-PVDF and is slightly shifted toward higher temperatures in the stretched material. This mode has been ascribed to the dielectric manifestation of the glass transition (T_g) of PVDF. It is comprised of two components corresponding to the free and constrained amorphous phases, respectively, in the order of increasing temperatures. The α_c transition/relaxation has been associated with molecular motions in the crystalline/amorphous interphase. At higher temperatures, a compensation phenomenon corresponding to cooperative movements liberated at the Curie transition has been observed in β-PVDF. © 1993 John Wiley & Sons, Inc.     

The tuning of crystallization behavior of ferroelectric poly(vinylidene fluoride-co-trifluoroethylene)

Poly (vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)] has three crystal forms, including paraelectric α, ferroelectric β, and γ phases. In previous studies, the properties and performances of P(VDF-TrFE) have been the focus of research. However, the formation mechanism and regulation mode of various crystal forms remain unclear. Therefore, it is an important topic for further research to elucidate, summarize, and prospect the polymorphism of P(VDF-TrFE) and regulate the crystal forms. This review systematically summarizes the crystalline structure and phase transition between ferroelectric and paraelectric phase of P(VDF-TrFE) crystals; discusses the influence of annealing, blending and electric field on the crystallinity, selection of polymorphic crystals, and phase transition behavior between them; reviews the effects of annealing, melt-recrystallization, substrate and nanoconfinement on the crystal orientation. Finally, the effects of the crystal structure of P(VDF-TrFE) on its properties are briefly summarized.     

Curie transition,ferroelectric crystal structure,and ferroelectricity of a VDF/TrFE(75/25) copolymer 1. The effect of the consecutive annealing in the ferroelectric state on curie transition and ferroelectric crystal structure

We have completely deconvoluted multiple ferroelectric-to-paraelectric phase transitions of a VDF/TrFE copolymer often observed on DSC by consecutive annealing below the Curie transition temperature and obtained three clear endothermic peaks resolved well from each other. We could also increase the Curie transition temperature by using the consecutive annealing performed in the ferroelectric state. Annealing time, as well as annealing temperature, was found to affect the Curie transition behavior significantly. Each ferroelectric crystalline phase was characterized by IR, Raman scattering, x-ray diffraction, and DSC measurements. The ferroelectric phase, having higher Curie transition temperature, has been found to have more trans sequences and less gauche defects and to require larger thermal energy for the Curie transition. © 1994 John Wiley & Sons, Inc.     

Dielectric behavior of P(VDF-TrFE)/PMMA blends

Binary blends of ferroelectric (vinylidene fluoride-trifluorethylene) copolymer [P(VDF-TrFE)] and amorphous poly(methyl-methacrylate) (PMMA) were investigated over the full range of composition, for the copolymer with 50 mol % of trifluorethylene. Dielectric measurements confirmed that the system becomes amorphous when the PMMA contents exceed 40 wt %. The observed dielectric losses could be attributed to the ferroelectric transition of the copolymer and to molecular relaxations, characteristic of the individual polymers. We demonstrate the thermally activated nature of the β-PMMA and β-P(VDF-TrFE) relaxations, with activation energies of 29.3 and 13.5 kcal/mol, respectively. We also demonstrate that the β-P(VDF-TrFE) relaxation cannot be attributed only to the copolymer glass transition and that its origin is not in the crystalline region, based on experimental evidences. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2996–3002, 1999     

PZT/P(VDF-TrFE)7030 0-3型复合材料的非等温结晶和居里相变行为

采用DSC研究了PZT P(VDF TrFE) 70 30复合材料的非等温结晶和居里相变过程 .结果表明 ,PZT粒子对共聚物基体的结晶行为 ,包括结晶温度、结晶速率和结晶度等影响不大 ;然而共聚物的居里相变过程则受到PZT粒子的影响 ,随PZT粒子含量的增大 ,共聚物基体的居里相变热焓与熔体结晶热焓的比值减小 ,而且其中对应从顺电性的高温相 (HT)转变为铁电性的过冷相 (CL)的相变热焓含量减小 ,意味着PZT粒子的存在能够抑制共聚物铁电晶体中旁式构象缺陷的形成 ,从而提高铁电晶体的完善度     

Thermodynamic and morphological studies of the solid-state transition in copolymers of vinylidene fluoride and trifluoroethylene

Copolymers of vinylidene fluoride and trifluroethylene are presently being considered for many piezoelectric applications. In contrast to the poly(vinylidene fluoride) homopolymer, a transition from the room temperature ferroelectric phase to a paraelectric phase has been observed in these copolymers. The temperature of this transition was determined by measuring the position of the endothermic peak observed in the differential scanning calorimeter. The rate of cooling from the paraelectric to ferroelectric phase was found to affect the temperature at which the transition occurred on subsequent heating. Changing the conditions under which molten polymer initially crystallized into the paraelectric phase had an even greater effect on the resultant transition temperature. In addition to the DSC studies, wide-angle x-ray measurements were performed on samples subjected to different thermal treatments. No significant differences were found in the x-ray scans of these copolymers.     

Cocrystallization in ternary blends of ferroelectric copolymers

In this work, we report evidences of cocrystallization in ternary blends made of crystalline ferroelectric poly(vinylidene fluoride‐trifluorethylene) [P(VDF‐TrFE)] copolymers. Complete cocrystallization has been unequivocally demonstrated by the observation of just one Curie and one Melting temperature in their calorimetric thermograms. These temperatures were intermediary among the respective temperatures of the individual constituents, that is, P(VDF‐TrFE)_72/28, P(VDF‐TrFE)_63/37, and P(VDF‐TrFE)_50/50. Dielectric and X ray diffraction data were used to complement the investigation. The binary blends made of 63/37 and 72/28 copolymers were found to be miscible in the entire range of composition, with the behavior of their Curie temperatures being well fitted by an equation very similar to that proposed by Gordon‐Taylor to describe the behavior of the glass transition temperatures in true binary blends. In the ternary crystalline system, we have found evidences that the complete miscibility of the binary blend made of 63/37 and 72/28 copolymers actually drives the P(VDF‐TrFE)_50/50 copolymer to accommodate their chains in its binary crystalline structure. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 621–626, 2010     

Effect of X-ray radiation on the structure of P (VDF/TrFE) copolymers

Structural changes in poly(vinylidene fluoride)-trifluoroethylene [P(VDF-TrFE)] copolymers caused by X-ray irradiation were investigated by molecular weight determination, EPR analysis, FTIR spectroscopy, gel content, DSC thermal analysis, X-ray diffraction, and piezoelectricity measurements. Samples exhibit radiation-induced conductivity (RIC) due to the formation of radical ions. These radicals are generated by bond cleavage, which could react, leading to structural changes such as oxidation, double bond formation, chain scission, and crosslinking. The increasing gel content with radiation dose indicated that crosslinkings of the polymer chains predominate. Irradiation on P(VDF-TrFE) caused the melting temperature, heat of fusion, and Curie temperature to decrease. These results are consistent with the partial destruction of crystalline domains. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1201–1205, 1997     

The Effect of Substrate on the Properties of Non-volatile Ferroelectric P(VDF-TrFE)/P3HT Memory Devices

Ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE))/semiconducting poly(3-hexyl thiophene) (P3HT) blend systems have drawn great attention with their potential use for electronic applications, particularly non-volatile memory devices. It is essential to grasp a full understanding of the crystallization habits of the two polymers on different substrates for purposeful control of the structures of the blend and therefore the properties of the devices. Here, the effects of structure and morphology of the blend films generated at different substrate surfaces on the ferroelectric and switching properties of related devices are reported. It is identified that P(VDF-TrFE)/P3HT blend films prepared on graphene substrate show not only an obvious optimization in the ferroelectric behavior of P(VDF-TrFE), but also an enhancement of the charge transport within P3HT domains. By employing sandwich structure constructed by silver electrode and P3HT/P(VDF-TrFE) blend film on graphene substrate, high-performance ferroelectric memory devices have been obtained, which exhibit a great electrical switching behavior with high ON/OFF ratio of about 1000 and low coercive voltage of approximately 5 V. These findings provide useful guidance for fabricating high-performance ferroelectric memory devices.

     

Investigation of dielectric relaxations associated with the glass transition of vinylidene fluoride and trifluoroethylene copolymers by thermally stimulated current

Thermostimulated current (TSC) spectroscopy was applied to the characterization of dielectric relaxations associated with the glass transition of a series of P(VDF/TrFE) copolymers. The maximum temperature of the β-mode increases slightly as the TrFE unit content is increased, in the same manner as the glass transition temperature. By using the technique of fractional polarizations to the resolution of this relaxation mode, we have isolated, for all the polymers investigated, a series of elementary relaxation times that obey a compensation law. This behavior is characteristic of the free amorphous phase of polymers. The mean activation energy of this mode increases as the TrFE unit content is increased, due to a stiffening of molecular chains. Annealing of the copolymers above their ferroelectric-to-paraelectric transition induces a strong crystallinity increase of the materials. As a matter of fact, the amorphous phase is squeezed in to dimensions lower than the characteristic length scale associated with the glass transition. © 1995 John Wiley & Sons, Inc.     

Phase Transition and Micellization of Temperature Responsive Dextran—graft—poly（N—isopropylacrylamide）Polymers

Phase behavior and micellization of dextran-graft-poly(N-isopropylacrylamide)(PNIPAAm)polymers in aqueous solution are investigated in this paper using DSC and AFM methods.It is found that with the increase of grafting(G%) of the copolymers the endothermic enthalpy during the phase transition increases significantly and the transition temperature decreases slightly.The phase transition behavior of the copolymers is scanning rate dependent.Micelles are formed whenever the solution temperature is raised above the LCST of the copolymers.It is proposed that by using this thermal responsive property of the copolymers,drugs could be incorporated into the micelles without employing any organic solvent.     

Ferroelectric Poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene)s: Effect of Molecular Weight on Dielectric Property

A series of ferroelectric poly(vinylidene fluoride-trifluoroethylene- chlorotrifluoroethylene), P(VDF-TrFE-CTFE), have been synthesized by a two-step approach. The first step is copolymerization of VDF and CTFE via solution or suspension methods to produce P(VDF-CTFE) copolymers with different molecular weights. The second step is partial de-chlorination to convert copolymers into P(VDF-TrFE-CTFE) terpolymers with precisely controlled compositions. The effect of molecular weight, molecular weight distribution and uniaxially stretching on the dielectric properties has been investigated over a broad range of temperature and frequency. The X-ray diffraction patterns and DSC curves demonstrate the coexistence of the multiple phases in the terpolymers. The dielectric spectra depict the local relaxation processes and relaxor ferroelectric behavior on the basis of the dielectric loss tangent as a function of temperature.     

Enhanced <Emphasis Type="Italic">αγ</Emphasis>′ Transition of Poly(vinylidene fluoride) by Step Crystallization and Subsequent Annealing

Poly(vinylidene fluoride) (PVDF) exhibits pronounced polymorphs.Its γ phase is attractive due to the electroactive properties.The γ-PVDF is however difficult to obtain under normal crystallization condition.In a previous work,we reported a simple melt-recrystallization approach for producing y-phase rich PVDF thin films through selective melting and subsequent recrystallization.We reported here another approach for promoting the αγ'phase transition to prepare γ-phase rich PVDF thin films.To this end,a stepwise crystallization and subsequent annealing process was used.The idea is based on a quick generation of a large amount of α-PVDF crystals with some of their γ-PVDF counterparts at suitable crystallization temperature and then annealing at a temperature above the crystallization temperature for enhancing the molecular chain mobility to overcome the energy barrier of phase transition.It was found that crystallizing the PVDF melt first at 152 ℃ for4 h,then quenching to room temperature and finally annealing the sample at 160 ℃ for 100 h was the most efficient to produce γ-PVDF rich films.This is related to the melting and recrystallization of the α-PVDF crystals produced during quenching in the annealing process at 160 ℃,which favors the formation of γ-PVDF crystals for triggering the αγ'phase transition.     

Investigation of molecular chain orientation change of polymer crystals in phase transitions by friction anisotropy measurement

Direct observation of the molecular orientation change in polymer crystals provides us visible information for understanding their structural phase-transition mechanisms. In this letter, we successfully identified the main-chain orientation of poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) crystals over all directions using friction anisotropy measured by lateral-modulation friction force microscopy (LM-FFM). This technique made possible our investigation of molecular orientation changes caused by a ferroelectric phase transition and also a fabrication process for artificial nanometer-scale structures. These results give us visual information that is directly connected to the transition mechanisms.     

Phase-transitions in Langmuir-Blodgett and cast films of a ferroelectric liquid crystal.

Langmuir-Blodgett films and cast films of a ferroelectric liquid crystal of sec-butyl-6-(4-(nonyloxy)benzoyloxy)-2-naphthoate have been fabricated. Their thermal behavior was investigated using infrared spectroscopy at elevated temperature combined with principal component analysis. The result shows a new phase transition from smectic A to nematic phase, compared to the phase sequence obtained by polarizing optical microscopy. Another solid transition of different isomeric crystals was also found, which was confirmed by calorimetric measurement.     

Poling influence on the mechanical properties and molecular mobility of highly piezoelectric P(VDF‐TrFE) copolymer

The calorimetric, dielectric, and mechanical responses of highly piezoelectric 70/30 P(VDF‐TrFE) displaying homogenous d₃₃ of ?19 pC N^?1 are studied. This work aims at better understanding the influence of poling on the mechanical properties of this copolymer. To explain the one decade mechanical modulus drop observed across the Curie transition, a stiffening process of the amorphous phase due to the local electric fields in the ferroelectric crystals is proposed. In poled P(VDF‐TrFE), these fields are preferentially aligned resulting in a more stable and higher modulus below the Curie transition. This hypothesis accounts for the lower dielectric signals obtained with the poled sample. Through the Curie transition, the vanishing of these local electric fields, stemming from progressive disorientation and conversion of ferroelectric crystals to paraelectric ones, releases the constraints on the amorphous phase, leading to a storage modulus drop typical of a viscoelastic transition. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 1414–1422     

序列结构对苯乙烯/丁二烯共聚物热释电流峰的影响

本文研究聚合方式对苯乙烯/丁二烯共聚物热释电流峰(TSC)的影响。共聚物试样有三个电流峰(β、α和ρ峰)。实验结果表明:聚合方式对β峰(丁二烯局部运动)基本没有影响,但对α峰(丁二烯相的T_g转变)的位置有明显的影响,而且两步法试样有两个α峰;嵌段共聚物试样在高温还有一肩部(苯乙烯相的T_g转变),无规共聚试样没有肩部。由共聚物的序列结构能圆满解释TSC结果,而序列结构决定于聚合方式。因此,本工作将苯乙烯/丁二烯共聚物的序列结构与性能联系起来。     

Unusual high-temperature structural behaviour in ferroelectric Bi2WO6

The crystal structure of Aurivillius phase ferroelectric Bi2WO6 has been studied in detail as a function of temperature by using high-resolution powder neutron diffraction. In agreement with an earlier study, a transition from space group P2(1)ab to B2cb occurs at about 660 degrees C. This transition corresponds to the loss of one octahedral tilt mode within the perovskite-like WO4 layer of the structure. A second, reconstructive, phase transition occurs around 960 degrees C, corresponding to the ferroelectric Curie point; in contrast to previous suggestions, the structure of this high-temperature phase contains layers of stoichiometry WO4, with WO6 octahedra sharing edges and corners, and with the fluorite-like Bi2O2 layers remaining essentially unchanged. This structure is closely related to that of the ambient temperature phase of lanthanide-doped derivatives, for example, Bi0.7Yb1.3WO6 recently reported. This phase-transition behaviour is in stark contrast to that of other members of the Aurivillius family, such as SrBi2Ta2O9 and Bi4Ti3O12, which retain the archetypal Aurivillius connectivity at all temperatures.     

Crystal structure and dielectric properties of the mixed Aurivillius phase Bi7Ti4NbO21

The crystal structure of the ferroelectric mixed Aurivillius phase Bi₇Ti₄NbO₂₁ was determined for the first time by X-ray diffractometry using a single crystal obtained by a flux method in Bi₂O₃ excess. It can be described as a regular stacking of double and triple perovskite-type slabs interleaved by Bi₃O₂ layers along the a axis. Nb is located in the double perovskite-type layers only with a random Nb/Ti distribution whereas the octahedral sites within the triple perovskite-type layers are occupied by Ti only. The thermal evolution of the dielectric constant between room temperature and 900 °C show two phase transitions at 650 and 830 °C. Combined analyses of experimental data support that the highest transition temperature corresponds to the Curie temperature, whereas the lowest one could be assigned to a phase transition within the orthorhombic symmetry by a change of the space group.