Elsevier

Polymer

Volume 204, 9 September 2020, 122811
Polymer

Dielectric relaxation dynamics in poly(vinylidene fluoride)/Pb(Zr0·53Ti0.47)O3 composites

https://doi.org/10.1016/j.polymer.2020.122811Get rights and content

Highlights

  • The dielectric relaxation dynamics of poly(vinylidene fluoride)/Pb(Zr0·53Ti0.47)O3 composites is evaluated.

  • The dielectric behavior is analyzed as a function of polymer phase, filler amount and size.

  • The dielectric response is analyzed by the Havriliak-Negami model.

  • For filler content of 20% or higher, the relaxation dynamics is affected by changes in the polymer nucleation kinetics.

  • The relaxation dynamics is affected by the ceramic filler and the ceramic/polymer interfaces.

Abstract

Polymer-ceramic composites based on poly(vinylidene fluoride) and ceramic particles of the inorganic piezoelectric material Pb(Zr0·53Ti0.47)O3 were prepared with different particle concentrations and size by solution casting in the non-polar (α−) and polar (β-) phases of the polymer. The influence of amount and particle size on the overall dielectric response of α- and β-phase matrix composites was analyzed, focusing on the dielectric relaxation processes. The cooperative segmental motions within the PVDF amorphous phase (low-temperature β-relaxation), are strongly affected by the inclusion of the fillers, both in the α− and β-phase matrix composites. The complex permittivity analyzed by the Havriliak-Negami equation model (NH) and the fragility parameter indicates that the PZT ceramic filler induces heterogeneity in the polymer matrix. For α-PVDF/PZT composites, the strength of the relaxation process increases with increasing the filler amount and it is nearly independent on particle size. The behavior of the HN shape parameters, more noticeable for filler content of 20% or higher, shows that the relaxation dynamics is influenced by the polymer nucleation kinetics. PVDF/PZT composites in β-phase matrix exhibit a strong increase in the relaxation strength for PVDF/PZT composites with 40% of ceramic fillers, and the process becomes more symmetric when the amount of filler increases. The detected variations in the relaxation dynamics in both α- and β-phase matrix composites is strongly affected by the ceramic filler and the interface between the ceramic microparticles and the polymer.

Introduction

Polymer-ceramic composite properties are the result from the combination/mixture of two or more different materials, offering the possibility of tailoring macroscopic materials response [1,2]. Polymer-ceramic composites are increasingly used for applications, as they usually present higher piezoelectric coefficients and dielectric constant when compared to the matrix polymeric material and higher flexibility, lower density and mechanical losses when compared to the ceramic material [3,4]. The ceramic-polymer composite properties depend, together with matrix and filler characteristics, on the connectivity, i.e. how the filler and the matrix are interact with each other [5].

Poly(vinylidene fluoride) (PVDF) is a semi-crystalline polymer with an uncommon polymorphism among polymeric materials. PVDF presents four crystalline phases known as α, β, γ and δ, depending of the processing conditions [6]. Typically, the α-phase is obtained by crystallization from the melt and presents a non-polar crystalline structure TGTG’ [7]. Also, the α-phase can be obtained from solution cast when the solvent evaporation temperature occurs above 120 °C [8,9]. The β-phase shows the highest ferro-, pyro- and piezoelectric properties and is mostly obtained by stretching the α-PVDF at temperatures below 100 °C and draw ratios between 2 and 5 [8,9]. Unoriented films exclusively in the β-phase can be also obtained from the crystallization of the PVDF dissolved in different polar solvents (N,N-dimethyl formamide (DMF) or dimethyl acetamide (DMA)) at temperatures below 70 °C. The resulting material shows high porosity what makes it opaque and fragile [10,11]. Also, it has been also revealed that PVDF in the β-phase can be also obtained under different manufacturing conditions after the inclusion of specific fillers such as ferrites [12], ionic liquids [13] or zeolites [14], among others.

With respect to the dielectric properties, PVDF presents a crystallization phase dependent high room temperature dielectric constant between 6 and 15 [15], and two main relaxation processes: one at temperatures below −20 °C, labeled αa or β and attributed to the amorphous regions and the α or αc-relaxation at temperatures above 80 °C and associated with molecular motions within the crystalline fraction [[16], [17], [18]].

Lead zirconate titanate, with chemical formula of Pb[ZrxTi1-x]O3, 0 < x < 1 (PZT), crystallizes in a perovskite structure (ABO3) [19]. PZT ceramics are widely used in device applications such as micro-mechanical systems, piezoelectric transducers, micro-actuators and pyroelectric sensors, among other [20,21]. The PZT phase diagram is complex due to existence of a morphotropic phase boundary (MPB) that divides the ferroelectric region into two parts: a tetragonal crystalline phase rich in Ti atoms and a rhombohedral phase region rich in Zr atoms. The MPB occurs in the Zr/Ti = 52/48 region and the material is characterized by the highest value of the dielectric constant and the piezoelectric response [22]. Thus, the material in the MPB is widely used in many applications such as sensors [23], transducers [24], energy harvesting [25], among others.

Polymer/PZT composites for technological applications raised early attention due to the interesting properties as smart and multifunctional materials [26]. For PVDF/PZT system, it was detected that the piezoelectric effect is mainly due to the ceramic particles, which was supported by mathematical formalisms developed to predict the elastic modulus, dielectric and piezoelectric values of binary polymer/PZT systems [27]. Generally, the high value of the dielectric constant of the ceramic filler allows a higher complex permittivity for the polymer/ceramic composites for moderate volume fractions of PZT filler and present stronger piezoelectric value than in the neat polymer.

The viscoelastic properties of the polymer/PZT composites depend on the ceramic amount, strain and frequency. Generally, polymer/ceramic composites become stiffer and more brittle with increasing ceramic amount, and exhibit non-linear stress vs strain behavior [28].

The effect of filler volume fraction on PVDF/PZT composites with 0–3 connectivity was performed by Zhang et al. [29]. The PVDF/PZT composites were prepared by two different shaping processes, hot and cold press It was demonstrated that the piezoelectric and dielectric responses of hot-pressed PVDF/PZT composites are superior to those prepared by cold-pressing methods due to both the formation of β-PVDF and the better coupling of the these materials in the hot-press processing [29].

The dielectric properties of PVDF/PZT composites are reported in Ref. [30] as a function of filler content, and this behavior was interpreted in the light of different theoretical models [31].

The present work, on the other hand, focus on the understanding of the dependence of the dielectric relaxation processes through the Havriliak-Negami formalism as a function of the ceramic amount and size, as well as in relation with the main crystalline phases (α and β) of the PVDF polymer.

In this work, PVDF/PZT composite samples with different PZT concentrations and particle size were produced by solution casting technique in the non-polar and polar phases of PVDF: α and β-phase, respectively. The influence of the PZT amount and particle size on the overall dielectric response of two PVDF matrix (α and β-phase) composites was evaluated, focusing in the different relaxations process detected in the dielectric behavior, which is of critical relevance scientifically and for technological applications. It is to notice that despite being performed for PVDF/PZT composites and also, the results are of general interest for related PVDF/ceramic microcomposites such as the ones based on BaTiO3 [32], Sodium niobate (NaNbO3) [33], or lead-free 0.50[Ba(Zr0·2Ti0.8)O3]0·50(Ba0·7Ca0.3)TiO3 (BZT-BCT) [34], among others.

Section snippets

Dielectric relaxation spectroscopy: Theory and analysis

Dielectric relaxation spectroscopy (DRS) is widely applied to assess molecular motions and structural relaxations present in insulator materials possessing permanent dipolar moments [35].

The complex permittivity:ε=ε'iε''can be presented, according to the Debye theory as:ε=ε+ε0ε1+iωτwhere τ is a temperature-dependent relaxation time following an Arrhenius (eq. (3)) or a Vogel-Tammann-Fulcher-Hesse (VTFH) (eq. (4)) law:τ=τ0exp[EactKBT]τ=τ0VTFHexp[BKB(TT0)]where KB is the Boltzmann

Samples preparation

PVDF/PZT composite films were obtained from PVDF polymer and PZT ceramic particles with three different particles sizes (0.84, 1.68 and 2.35 μm) following the method described elsewhere [10,31]. The used solvent was dimethylacetamide (DMA) and the polymer/solvent ratio 20/80 wt%, The thickness of the composites films is ~30 μm.

After the stirring process and polymer dissolution, the solution was spread on a glass plate through bar coating and, in order to allow the crystallization of the β-PVDF

α-phase PVDF matrix

The influence of the processing conditions on the distribution of the ceramic fillers in the PVDF matrix was evaluated in previous works, and it was detected that PZT particles are randomly distributed within the polymeric matrix without aggregates in both α- and β-phase PVDF with a 0–3 connectivity [10,31,41]. The cross-section images for the composites with higher ceramic content are shown in Fig. S1 (supplementary information), in which a good distribution of the ceramic particles in the

Conclusions

PVDF/PZT composite materials with PZT amounts (10–40 wt%) and particle size were prepared in the non-polar and polar phases of the PVDF: α− and β-phase, respectively. The dielectric spectrum displays that the low-temperature β-relaxation of the amorphous phase in PVDF polymer, related to the cooperative segmental motion, is strongly affected by the presence of PZT filler, in both α− and β-polymer matrix. The fitting parameters determined by the HN model, together with the ‘fragility’ parameter

CRediT authorship contribution statement

C.M. Costa: Methodology, Investigation, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. R. Sabater i Serra: Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. A. Andrio Balado: Investigation, Validation, Methodology, Writing - original draft, Writing - review & editing. J.L. Gómez Ribelles: Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Writing -

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank the FCT (Fundação para a Ciência e Tecnologia) for financial support under the framework of Strategic Funding grants UID/FIS/04650/2019, and UID/EEA/04436/2019; and project PTDC/FIS-MAC/28157/2017. The author also thanks the FCT for financial support under grant SFRH/BPD/112547/2015 (C.M.C.). Financial support from the Spanish State Research Agency (AEI) and the European Regional Development Fund (ERFD) through the project PID2019-106099RB-C43/AEI/10.13039/501100011033 and

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