Ferroelectric transitions and relaxation processes of vinylidene fluoride/trifluoroethylene copolymers

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Ferroelectric transitions and relaxation processes of vinylidene fluoride/trifluoroethylene copolymers

R.M. Faria, M. Latour

To cite this version:

R.M. Faria, M. Latour. Ferroelectric transitions and relaxation processes of vinylidene fluoride/trifluoroethylene copolymers. Journal de Physique, 1988, 49 (12), pp.2089-2093.

�10.1051/jphys:0198800490120208900�. �jpa-00210890�

Ferroelectric transitions and relaxation _processes of vinylidene fluoride/trifluoroethylene copolymers

R. M. Faria (*) and M. Latour

Laboratoire de Physique Moléculaire et Cristalline G.D.P.C., ^LA 223, U.S.T.L., place Eugène Bataillon, 34060 Montpellier Cedex, ^France

(Reçu ^{le 24 mai 1988,} révisé le 30 août 1988, accepté le 30 aoat 1988)

Résumé.

Des mesures de dépolarisation thermostimulées associées à un bruit piézoélectrique ^ont permis

d’identifier la transition ferroélectrique-paraélectrique ^de copolymères fluorure de vinylidène/trifluoroéthy- lène, de diverses concentrations molaires. D’autre part, ^la présence d’un deuxième pic indiquerait ^une

nouvelle transition, laquelle ^ne modifie pas le comportement ferroélectique du matériau. Des processus de relaxations lents accompagnent toujours ^ces transitions. Les énergies d’activations associées à ces transitions ont été calculées par la méthode de la pente initiale. Les valeurs de 0,73 ^et 0,33 ^{eV ont} été trouvées

respectivement pour la transition ferro/para ^et pour l’ autre transition. Deux transitions ^ont également ^été

observées pour le copolymère VDF/TrFE 90/10 mol %, ^{considéré comme} ayant ^une conformation non

ferroélectrique. ^Une grande quantité ^de charge observée par des ^mesures de TSC, ^{relaxent aux} températures

de transitions.

Abstract.

Thermally Stimulated Depolarization (TSD) measurements with a superimposed piezoelectric

noise allowed us to identify ^the ferroelectric-to-paraelectric phase transition in vinylidene fluoride/trif-

luoroethylene (VDF/TrFE) copolymers, with different molar ratios. A second peak ^was also detected in TSD measurements which maintain the ferroelectric features of the material. Long relaxation processes always accompanied ^both peaks. Using the initial rise method we calculated the activation energies ^{for these}

relaxations. We found 0.73 eV for the ferroelectric-to-paraelectric peak transition and 0.33 eV for the other.

Two peaks ^{were also} observed in the 90/10 mol % VDF/TrFE copolymer. ^{However it is} considered, ^{hitherto as} having ^{a non} ferroelectric conformation. A great ^{amount of} charge ^was recorded in TSC measurements, around the transition temperatures.

Classification

Physics ^Abstracts

72.20 - 77.30 - 77.80 - 77.80B

1. Introduction

The discovery of ferroelectric polymers ^{in the last}

decade a great impulse ^{on the} investigation ^{of these}

materials. In particular, ^the polyvinylidene ^fluoride (PVDF) ^{was the most studied, seeing that in its 8}

conformation it presents considerable piezoelectric

and pyroelectric ^effects 1-4]. ^These properties pro- vide several technological applications [5]. ^Mean- while, ^{its low} melting point ^{makes difficult a com-} plete study ^{of its} ferroelectric properties.

The vinylidene fluoride/trifluoethylene (VDF/TrFE) copolymers ^have piezoelectric ^and pyroelectric properties ^{similar to those} presented by

the Q-PVDF’[6, 7]. Besides, they ^{have the} advantage

to present, ⁱⁿ appropriate molar ratios of VDF and

TrFE, ^{the Curie} temperature [8, 9]. ^{This ferroelec-} tric-to-paraelectric (F-P) transition has a first-order behaviour for copolymers ^{with a VDF molar concen-}

tration bigger ^{than 60 %} [10, 11]. Structural studies carried out in the 52/48 mol % copo-

lymer ^VDF/TrFE by Lovinger et ^al. [12] ^{showed that}

before the transition it exhibits two crystalline phases. One of them was identified to have a

hexagonal packing ^of 3/1 helical chains (non polar),

and the other a similar packed trans-planar ^chains (polar). ^Above the Curie temperature, only ^the

disordered 3/1 helical conformation was detected.

Horiuchi et al. [13] detected three peaks by X-ray

diffractometric measurements at b6 °C, ^on samples

of 54/46 mol % VDF/TrFE copolymer. ^To explain

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198800490120208900

2090

their results, they ^assumed the presence of ^an intermediate phase, which would take place ^between

66 and 90 °C.

In the present ^{work we} study phase transitions on

90/10, 80/20, 70/30, ^{and 60/40 mol % VDF/TrFE} copolymers by Thermally Stimulated Processes.

Relaxation processes and space charge joined ^to

these transitions are also studied.

2. Experimental procedures.

Vinilidene fluoride/trifluoroethylene copolymers with compositions 90/10, 80/20, 70/30, ^and 60/40 mol % of VDF/TrFE, ^{as well as} pure PVDF,

were provided by ^Atochem (France). ^{Resins were}

melted and pressed ^{at a} temperature ^{above the} melting point, ^then quenched ^{to room} temperature.

Samples ^{were circular with a diameter of 4 cm and a}

thickness of 50 >m. Rectangular aluminium elec- trodes (1.0 by ^2.5 cm) ^were high ^vacuum deposited

on both sides.

Thermally Stimulated depolarization (TSD) ^and Thermally Stimulated Current (TSC) ^{were carried}

out following ^the conventional procedure. ^{A linear} heating ^{rate of 1.9 °C/min was} used for all measure-

ments. During ^{the TSC} measurements a weak elec- tric field of 50 kV/cm was applied ^{to the} samples,

and glow peaks ^{were observed} superimposed ^{to the} conductivity ^{curves. For the} TSD measurements,

samples ^{were first} polarized by ^an electric field of 500 kV/cm at 65° C during ^{20 min.} Afterwards they

were cooled to room temperature ^{under the} applied

field. The samples ^were short-circuited and several minutes were waited before starting ^{the measure-}

ments, ^{to ensure a} negligible value of the dielectric

absorption discharge ^current.

A slight mechanical vibration (around ¹ Hz) ^was imposed ^{to the} sample, ^{in order to} superimpose ^a piezoelectric ^{noise on the TSD} recording ^measure-

ments. It did not appear ^{for non} piezoelectric samples, ^{so it} helped ^{us to} identify unambiguously

the F-P transition.

3. Results.

3.1 TSD MEASUREMENTS. - Figure ^{1 shows the} Thermally Stimulated Depolarization ^{curves of the}

VDF/TrFE copolymers, ^{and the a-PVDF} homopolymer. ^We classify by ^{A and B the} peaks according respectively ^to the permanence ^{or the}

disappearing ^{of the} piezoelectric noise. To present ^a clear picture, ^{the noise was removed of all the curves}

drawn in figure ^{1. The} figure ² presents ^{the measure-}

ments of the 70/30 and the 90/10 copolymers ^{with the}

noise. As we can see it increased with the tempera- ture, in accordance with the piezoelectric ^effect presented by ^{the VDF/TrFE} copolymers [6]. ^Mean-

while it began ^{to decrease} as soon as the B peak

Fig. 1.

Thermally Stimulated Depolarization ^measure-

ments with a Ep500 ^{kV /cm,} Tp

^{65 °C, and} tp

^{20 min.}

(-..-..) 60/40 mol % VDF/TrFE copolymer; ( ) ^70/30 copolymer ; (...) ^80/20 copolymer ; (-

-) ^90/10 copolymer ; ^and (-*-*-) ^a-PVDF homopolymer.

Fig. ^2. - Thermally stimulated depolarization ^measure-

ments superimposed by ^the piezoelectric ^{noise. 70/30} copolymer ((a) curve) ^{and 90/10} copolymer (b) curve).

started, and vanished completely when its maximum

was reached.

The 60/40 copolymer presented only ^{the B} peak ^at

80° C. While it exhibited a long relaxation process for temperatures ^{below its} maximum, after it the current quickly ^{fell down. The} activation _{energy of} this relaxation _process was calculated by ^{the initial}

rise method, and the found value was 0.73 eV

(Fig. 3a). ^The charge density ^{of this} peak ^was

obtained from the integration ^versus time, beeing Q

^5.6 J.LC / cm 2. ^The pyroelectric ^{current for this} copolymer ^was estimated, ^{in the} present experimen-

tal conditions, ^{to be} approximately 0.1 nA. This estimation was based on a linear relation of the

pyroelectric ^{constant with the remanent} polarization

obtained by Furukawa et al. [7]. ^{The A} peak ^{of the}

70/30 copolymer appeared ^at 102 °C, ^{while its B} peak ^{at 108 °C. This B} peak ^{had a similar behavior}

as that exhibited by ^{the 60/40} copolymer, ^since

showed a similar abrupt ^current decay. ^The charge

associated with this TSD curve was 6.0 J.LC/cm2,

Fig. ^3.

(a) Monolog ^curve of the initial rise of the TSD

peak of the 60/40 copolymer, ^and (b) monolog ^{curves of}

the initial rise of the TSD peaks of the 70/30 (o) ^{and 80/20} (0) copolymers used for their activation energies ^calcu-

lations.

almost the same value as that obtained from the 60/40 copolymer. ^{The TSD} measurement of the 80/20 copolymer presented only ^{the A} peak ^{at 87} °C,

but a new one started around 110 °C.

Figure 3b shows the monolog ^curve derived of the

beginning ^{of the A} peaks ^{of the 70/30 and 80/20} copolymers. The calculated activation energies ^of

these relaxation processes presented ^{the same value :}

E

0.33 eV. This indicates that they ^{have the same} origin. ^The charge density variations (corresponding

to the remanent polarization) ^with temperature ^for the 60/40 and 70/30 copolymers, ^were obtained from their TSD _curves, and are presented ⁱⁿ figure ^4.

From this result were verified that the ferro-to- ferroelectric transition would not be detected by ^a

direct measurement of the remanent polarization, ⁱⁿ spite ^of its evidence in the TSD measurements. For the other copolymers ^{it was not} possible ^{to outline}

this _curve, since the TSD peaks remained unfinished.

The 90/10 copolymer presented ^{two weak and}

broad peaks ^{in TSD} measurements (Fig. 1). ^{The A} peak appeared ^{at 63 °C, and the B at 120 °C. For this} copolymer ^{the B} peak ^{exhibited a slow down} decay, showing ^a different characteristic from that pre- sented by ^{the 60/40 and} 70/30. In this measurement ^it

was not possible ^to calculate the activation energy of the A peak, seeing ^{that a} superimposed leakage

current of about 0.1 nA (subtracted ^{from the curve} presented ⁱⁿ Fig. 1) ^was detected before the heating

process. The overlapping ^{between the} peaks ^also

hindered the calculation for the 8 peak. Finally, figure ^{1 shows a TSD} measurement with a «-phase

JOURNAL DE

PHYSIQUE. -

49,

12, DECEMBRE

Fig. ^4.

^Remanent polarization decays ^{with the} tempera-

ture for 60/40 (- - -) ^and (-) ^70/30 copolymers, ^derived

from TSD measurements showed in figure ^1.

PVDF homopolymer sample. ^{It exhibited} only ^a

broad and very weak peak ^{around 65 °C.}

3.2 TSC MEASUREMENTS. - Thermally ^Stimulated

Current measurements were carried out with the

same copolymer samples used in the TSD _exper- iments reported above. The applied ^{field was weak} enough (50 kV /cm) ^to avoid either switching phenomenon [14, 15], ^or transformation of the crys- talline phase of the material [12]. Figures 5(a), (b),

and (c) show the results of the TSC measurements.

As it can be _seen, each TSC recording ^curve presented ^a similarity ^{with the} corresponding ^TSD

.

curve, since the peaks appeared ^at approximately

the same temperatures. ^In spite ^{of this, the} charge

associated with the TSC peaks ^{were about two order}

of magnitude highers. ^{We can} also observe that the storage charge increased for the copolymers ^with higher quantity ^{of TrFE} component. ^{All the TSC} thermograms ^{had a} good reproductibility, ^{even if the}

measurements were carried out one after the other.

We present ^a complete ^TSC cycle (heating ^and cooling) ^{with the 70/30} copolymer (Fig. 5b). During

the cooling ^{it was also recorded two} peaks, ^with

maxima around 72 and 67 °C. So it presented ^a hysteresis ^{in the} measurements. The charge ^associat-

ed with these peaks ^{was of the same order of} magnitude ^{of that obtained} from the TSD measure- ment.

4. Discussion.

4.1 FERROELECTRIC TRANSITIONS.

Recent meas- urements of Differential Scanning Calorimetry [16]

showed that two proeminent peaks ^take place ^{in non} polarized samples ^of 70/30 mol % of VDF/TrFE

copolymer. ^Their temperatures ^were very close to those of the TSD peaks ^{showed in} figures ^{1 and 2.}

We interpret ^{the B} peak ^{as due to the} ferroelectric-

to-paraelectric transition, since the piezoelectric

noise disappeared just after its maximum. Due to the

136

2092

Fig. 5. - Thermally stimulated current measurements with an electric applied ^{field of} kV/cm. (a)

60/40 copolymer. (b) ^70/30 copolymer (-) during ^heat- ing ^and cooling ; 80/20 copolymer (- - -). (c) ^90/10 copolymer.

similarities the B peak ^{of the 60/40} copolymer ^must

have the same origin. ^Their abrupt ^current decays

identified the first-order character of this transition.

The thermal hysteresis ^observed in the TSC measure- ments of the 70/30 sample (Fig. 5b) ^{is another} proof

of the first-order character of the phase transition.

The charge density decay curves, derived from the TSD thermograms showed in the figure ^{4, are in} good accordance with the remanent polarization decay ^{curves of VDF/TrFE} copolymers [6].

The B peak ^showed by the 90/10 copolymers ^is

also probably connected with a F-P transition. How-

ever its polarization ^is relatively weak, showing ^that

this material has a small fraction of the ferroelectric

phase. ^{Moreover, it did not} present ^a first-order transition as the previous copolymers.

Related to the A peak ^{a basic} question arises : is it

simply ^{due to a chain molecular} motion, ^{or due to a}

new structural transition ? Dilatometry ^studies

showed that the F-P transition took place ^{in two} stages, ^{or that even there are two} transitions [17].

Based on the TSD measurements presented here,

and also in the recent DSC results [16] ^we suppose that the A peaks ^{of the 70/30 and 80/20} copolymers

could be connected to a ferro-to-ferroelectric phase

transition. Figure ^{2 shows that the} piezoelectric activity ^{of the} samples ^remained always present during ^this transition, ^{and as} it is well accepted, ^we

assume here that the piezoelectric activity ^of

VDF/TrFE copolymers ^{comes from} their ferroelec- tric phase. ^This supposed ^F-F phase transition takes

place ^at higher temperatures ^for copolymers ^with

small quantity ^{of VDF,} while in the F-P transition as

the VDF/TrFE molar ratio decreases the Curie temperature ^becomes lower. The combination of the two temperatures dependence ^{on the molar ratios} explains ^{the absence of the A} peak in the TSD and TSC measurements of 60/40 copolymer, and that of the B peak ^{in the 80/20} copolymer.

4.2 RELAXATION PROCESSES.

The long ^relax-

ation process connected with the F-P transition had

already been studied by dielectric relaxation [18]. ^It

has been explained ^{to be due to} molecular motions associated with the crystalline regions, ^{and it was}

described in terms of a single relaxation process. Its activation _energy was calculated to be equal ^to

16 kcal/mol [18]. Using ^{the initial rise method for the}

TSD peak ^{of 60/40} copolymer ^{we obtained the value}

of 0.73 eV (equivalent ^{to 16.6} kcal/mol.). ^{A com-} plete study of the ferroelectric relaxation requires ^a

correct relation of the relaxation time ^T with the temperature, seeing ^{that near} the transition tempera-

ture it does not obey the Arrhenius law.

The other peak (A) of the 70/30 copolymer ^was

also joined ^{with a} structural transition of the material

(corroborated by the results presented ^{in refer-}

ence [16]). ^{The 80/20} copolymer ^also presented ^the

same transition, since the activation energy calcu- lated for both A peaks, gave the ^same value : 0.33 eV (Fig. 3b).

We suppose that the A peak ^{of the 90/10} copolymer ^{and of the} a-homopolymer should have the same origin, seeing ^that only ^a small fraction of TrFE would not cause an evident change ^{on these} properties. The likeness between these peaks ^con-

firm this supposition. ^Several papers studied ^a relaxation in a-PVDF which takes place ^{in the 50 to}

100 °C temperature range [19], and it had been associated with molecular motions. These motions

are believed to be executed both by molecules in the chain folds and in the crystalline regions.

4.3 SPACE CHARGE. - The present paper showed that beside the dipolar orientation, the ferroelectric transitions are also accompanied by ^a large ^amount

of released charge (Figs. 5). Thermally ^Stimulated

Current measurements of a-PVDF owing ^to storage

charge had been recorded in several papers [20-22],

and it is well accepted that molecular motions of this material is accompanied by charge liberation [23].

However, ^{in the} present study ^the charge appeared only ^{if the} samples ^{remained under an} applied field ,

since the TSD measurements (carried ^{out in short-} circuit) ^recorded only ^the dipolar depolarization.

The reproductibility ^{of the TSC} measurements is a

strong ^result against ^the storage space charge hypothesis. On the other hand, ^{a recent} study ^with

the purpose ^to investigate ^the origin ^of storage charges ^{in PVDF, concluded that bulk} dissociated ions of absorbed water by ^{the material} give ^a

transient electrical current, ^{under an external} applied

field [25]. ^The charge associated with this transient current had the same value as that reported ^here.

5. Summary and conclusions.

In this paper ^we show that Thermally ^Stimulated Depolarization ^{method becames a} good ^{tool to} explore ferroelectrical phase transitions in polymers,

if a mechanical vibration is imposed ^{to the elec-}

trodes. The Curie point ^{of the} ferroelectric-to-para-

electric phase transition of VDF/TrFE copolymers,

were well determined. At the same time this tech-

nique provided ^{us the} recording ^{of the} phase ^tran-

sition relaxation process, and the temperature ^evolu- tion of the remanent polarization.

We also detected an unknown TSD peak ^{in the}

70/30 and 80/20 mol % VDF/TrFE samples. We sup-

posed that it is probably ^{due to a} ferroelectric-to- ferroelectric phase transition since a very proeminent peak ^{was also recorded in DSC} thermogram [16].

The activation energies calculated from the initial rise method were respectively ^{0.33 and} 0.73 eV for the A and the B peaks.

Thermally Stimulated Conductivity measurements

showed that there is a large space charge ^released during ^each phase transition. During ^the heating ^and cooling ^TSC cycle ^{of the 70/30} copolymer, ^{we ob-}

served a hysteresis phenomenon ^{in the} phase ^tran-

sitions indicating their first-order characteristics.

Acknowledgments.

R. M. Faria gratefully acknowledges the research grant supported by the Brazilian Institutions : Cob- selho Nacional de Desenvolvimento Cientifico e

Technol6gico (CNPq).

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