Polymorphism of poly(vinylidene fluoride) induced by poling and annealing

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Polymorphism of poly(vinylidene fluoride) induced by poling and annealing

B. Servet, J. Rault

To cite this version:

B. Servet, J. Rault. Polymorphism of poly(vinylidene fluoride) induced by poling and annealing. Jour-

nal de Physique, 1979, 40 (12), pp.1145-1148. �10.1051/jphys:0197900400120114500�. �jpa-00209201�

Polymorphism ^of poly(vinylidene fluoride) ^induced by poling ^and annealing

B. Servet

L.C.R. Thomson C.S.F. Corbeville, ⁹¹⁴⁰¹ Orsay, ^France

Laboratoire de Physique ^des Solides, Université Paris-Sud, 91405 Orsay, ^France and J. Rault

Laboratoire de Physique ^des Solides, Université Paris-Sud, 91405 Orsay, ^France

(Roçv le l U roui 1979, ut-c,el)ié ^{!017 uoüt} 1979)

Résumé. 2014 Dans les films étirés de fluorure de polyvinylidène, l’application ^d’un champ électrique ^{de 1} MV/cm produit ^une transformation structurale 03B1 non polaire ~ _03B1p polaire. ^La phase 03B1p orientée, ^avec les mêmes para- mètres cristallins _que la phase 03B1, est toujours accompagnée de taches de Bragg diffuses. Cette structure disparaît

par recuit à 170 °C _pour donner une nouvelle phase ^orientée (03B1’) plus ^{stable dans} laquelle ^{la chaîne a une confor-}

mation T3GT3G-.

Abstract.

In stretched poly(vinylidene fluoride) films, ^the transition from unpolar ^{03B1-form to a} polar ^form (03B1p)

is produced by pohng ^at 80 °C by ^an electric field of 1 MV/cm. This oriented form has the same lattice constants as 03B1-form but is associated with extra diffuse reflections. This form and the diffuse reflections disappear by ^anneal- ing above 170 °C and give ^an oriented and more stable form (03B1’) in which the chain has the T3GT3G- ^confor-

mation.

Classification Physics ^Abstracts

80.20S-61.40

Introduction.

It has been established for several years that poly(vinylidene fluoride), PVDF, ^{exists in}

at least three crystalline ^forms designated ^{as oc,} fl, y

or respectively II, I, ^{III and the} transitions _among them under various conditions (high pressure, heat treatment, drawing) ^{have been} extensively ^studied [1-8].

The chain in polar fl-form ^{has the all trans confor-}

mation (planar zigzag ^{like in} polyethylene) ^{with a} high dipole ^moment perpendicular ^{to the} c-axis. The films of fl-form ^PVDF prepared by stretching ^and poling ^{exhibit strong pyro- and} piezoelectric ^effects.

In unpolar ^{a-form the molecule} have the TGTG - conformation and the unit cell consists of two anti-

parallel dipoles, which leads to large interactions with an applied ^field.

Recently pyro- and piezoelectricity ^{have been}

found [9] ⁱⁿ poled a-form of PVDF. Experimental

results obtained with isotropic ^and biaxially ^oriented samples ^{have indicated a} change ^{in the} crystal ^struc-

ture of a-form, ^{after the} poling process. It has been

reported [9] ^{that fields near 1} MV/cm ^{cause a} phase

transition to a polar ^form (ap) ^{with no} significant change in chain conformation (TGTG-) ^{and that} higher ^{fields near 5} MV/cm produce ^the p-form.

This note presents preliminary results of further ’

investigations ^{on the effect of} poling. ^We report ^new structural changes ⁱⁿ uniaxially ^oriented samples

induced by poling ^and subsequent annealing.

Experimental. ^{- The} 100 J.1m thick PVDF films

containing exclusively a-form ’with 5 % of defects of head-to-head and tail-to-tail type (HHTT) ^are sup-

plied by ^{Kureha chemical} industry ^{Co LTD} of Japan.

They ^{are extended to} drawing ^{ratios ô = 4 at}

145 °C and b ⁼ 5.5 at 155 °C. This treatment results in the orientation of the c-axes of the crystallites parallel ^{to the} drawing direction. Aluminium elec- trodes are evaporated ^on both surfaces and the films

are subjected ^{to a} high d.c. field of ¹ MV/cm ^{at 80 °C}

for 10 min, ^{and then cooled to room} temperature with the field applied. Subsequently, ^the poled ^films

are heated and annealed at high temperatures (T. > ^{170 °C) below the} melting point (178 °C).

Differential thermal analysis measurements are per- formed with a Mettler 2000 equipment.

X-Ray ^fibre diagrams ^{are taken with a} cylindrical

camera, with a radius of 58 mm and CuKa mono-

chromatic radiation (Â ^{= 1.54} A) is used with point

collimation.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0197900400120114500

1146

Results and discussion. 2013 1. Poled samples ^exhibit’

a strong piezoelectric effect. The measurements of the

piezoelectric coefficient d33 ^have given ^8.5 pC/N ^for

ô ⁼ 4 and 13 pC/N ^{for b = 5.5. A} structural analysis

of these samples ^{was carried out in order to} explain

the effect of poling. ^The X-Ray ^fibre diagrams ^were

taken in the directions : a) perpendicular ^and b) parallel ^{to the} sample (Fig. 1), ^{and then} compared ^to

those of unpoled ^films.

Fig. ^1.

X-Ray ^fibre diagrams ^{of drawn} (5 ⁼ 5.5) ^and poled

PVDF films taken with the incident beam perpendicular ^{to a)} (b*, c*) plane ^{and to} b) (a*, c*) plane. ^{c* is} parallel ^{to the draw-} ing direction and a* is parallel ^{to the} poling ^field. Dl ^and D2 ^reflec-

tions are not seen in the unpoled ^a-form.

[Diagrammes ^{de fibre} d’échantillons étirés (5 ⁼ 5.5) ^et polarisés.

L’axe c* est en moyenne parallèle ^à la direction d’étirement et l’axe a* parallèle ^au champ électrique. ^Les taches diffuses D, ^et D2 ^{ne sont} pas observées avec les échantillons non polarisés.]

The relative change ⁱⁿ intensities of the Bragg

reflections indicates that the lattice of a-form is somewhat oriented with the a-axis parallel ^{to the} applied field : in particular, ^the (020) reflection is much more intense in case a) ^{than in case} b).

Furthermore the (100), (120)’and (121) reflections tend to disappear ^{in both} orientations. They ^are pointed ^out by ^{arrows in} figure ^2b.

Finally, annealing ^{of these} poled ^{films for an hour}

at 100 °C results in a decreasing ^{of about 50} % ⁱⁿ piezoelectric activity, ^{but no} significant change ^is

seen on the X-Ray diagram. ^{Most of the} trapped charges injected during ^the poling process have relaxed at 100 °C (10), therefore it is _very likely ^that

the residual permanent polarisation essentially ^arises

from the oriented crystalline phase. These results confirm the previous observations in poled biaxially

oriented samples ^{and are} consistent with a transition from a-form to a polar ^form (ap) involving ^{the rota-}

tion of TGTG- molecules [9]. ^The permanent pola-

risation in ocp-form ^{is due to the} identical orientations of all TGTG- molecules.

Two types of additional diffuse reflections desi-

gnated Dl 1 and D2 have been observed out of the

layer ^{lines of the} poled ^{a-form. No} corresponding

upper orders appear ^on the X-Ray spectrum. ^Both reflections are located in the reciprocal plane ^{(a*, c*).}

D 1 reflections ^{are seen in} figure ^{1 a when the inci-}

dent beam is perpendicular ^{to the film, but are not}

located on the c*-axis. In orientation lb, ^{the maxi-}

mum intensity is obtained by tilting ^the fibre axis by

15° on the incident beam.

D2 reflections are seen in figure ^{1 b near the} equator.

Their shape suggests ^a decorrelation along ^{the c-axis}

and their angular extension is about of 30° with respect

to the direction of the poling field. This extension does not vary significantly ^{with the} drawing ^{ratio for}

ô = 4 and 5.5. Its high ^value prevents ^us from deter-

mining ^{the exact} type of the disorder giving ^{rise to}

the diffusion (planar ^{or linear).}

No similar reflections have been observed with the

poled p-form ^of PVDF ; ^{therefore these diffuse} reflections are presumed ^to arise from a particular arrangement within the a-crystallites : packing and/or

distorsions of TGTG - chains.

2. In this second part, ^our interest is focused ^on the behaviour of the poled ^{a-form at} high tempera-

tures. The samples ^{are fixed at} both ends and then annealed above room temperature. Annealing ^at

100 °C for several hours does not cause any modifi- cation to the X-Ray spectrum ^{of the} poled ^a-form.

After annealing ^{at 170} °C the diffuse reflections tend to decrease whereas new layer ^lines appear ^on the

X-Ray ^fibre diagrams (Fig. 2).

Changes in intensities of the Bragg reflections are

also observed in both orientations. The (100) ^reflec-

tion _reappears in case b. The new period along ^the

c-axis is found to be twice the period in a-form. The results indicate a phase transition to a new oriented form designated ^{as a’. The} doubling ^{of the c} period

can be interpreted ^{as a} change in chain conformation

arising ^{from a} cooperative motion of molecules in

the poled ^a-form.

Fig. ^2.

X-Ray ^fibre diagrams ^{of drawn} (5 ⁼ 5.5) ^and poled

PVDF films annealed at 170 °C. a) (b*, c*) plane, b) (a*, c*) plane :

arrows point ^to (100), (120) ^and (121) reflections which vanish in figures ^1.

[Diagrammes ^{de fibre} d’échantillons étirés, polarisés ^{et recuits à}

170 °C. Les flèches indiquent les taches (100), (120) ^et (121) ^éteintes

sur les figures 1.]

We suggest ^a T3GT3G- conformation. Figure ³

shows the molecular conformations of TGTG- type and T3GT3G- type. ^{In these exact models the values} of the identity period ^{are 4.4 A for TGTG-} type ^and

8.8 A for T3GT3G- type. ^The proposed ^{chain con-}

formation of a’-form is similar to that of Rubber

hydrochloride ^found by ^Bunn and Garner [11].

The transitions at high temperatures ⁱⁿ poled

a-form have been studied by ^DTA technique. ^The application of Differential Thermal Analysis ^{in the}

case of PVDF is made difficult by :

a) the presence of numerous phases (a, fl, _{ap, OC,)}

in our samples,

b) the effect of annealing which results in thicken-

ing of lamellae and therefore increases the tempera-

tures of the transitions [12],

Fig. ^3.

Molecular conformations of a) TGTG- _type and b) T3GT3G- type. ^The period ^of T3GT3G- ^type in a’-form is exactly

twice the period ^{of TGTG-} type in a-form. Both conformations have a glide plane along the chain axis.

[Modèles de conformation de chaîne. a) TGTG- et b) T3GT3G-.

Les deux conformations ont en commun un plan ^de glissement ^le long ^{de la} chaîne.]

c) ^{and a} possible ^surface phase transition which involves a change ^of folding plane [13].

The unannealed samples (with drawing ^ratio

ô ⁼ 4) ^are heated from 1400C to 200°C. Figure ⁴

shows the melting behaviour of the unpoled ^and poled

a-form at various heating ^rates. Two endotherm

peaks ^at 177-178 OC and 180-181 OC are observed for

poled ^{films whereas} only ^{one exists in} unpoled ^films.

The first peak ^{is common to both} types ^of samples

and coincides with the melting point ^of a-form. The second peak increases with decreasing heating ^rate,

which indicates a conversion from the low temperature

Fig. ^4.

Differential thermal analysis ^of poled (full lines) ^and unpoled (dashed lines) samples ^{at various} heating ^rates.

[Endothermes d’échantillons polarisés ( et non polarisés

(- - - -) ^à différentes vitesses de chauffe.]

1148

Fig. ^5.

X-Ray diffraction pattern ^{of the} poled a-form heated at 0.2 °C/min ^{to 179 °C and} quenched ^{to room} temperature. ^The rings ^of diffraction are due to unoriented a-form recrystallised

from the melt.

[Diagramme ^{de RX} d’un échantillon polarisé, ^{chauffé à} 0,2 °C/min jusqu’à ^{179 °C et} refroidi à la température ambiante. Les anneaux

de diffraction sont caractéristiques ^{de la} phase ^{a non} orientée.]

form to the high temperature form, following ^an

activated _process. The small shift of both peaks ^to

the higher temperatures ^is explained by ^{the well}

known phenomenon of lamellae thickening [12]. ^In

order to identify the second peak, ^a sample ^{has been}

heated at 0,2 °C/min ^{to 170 °C} (just between both

melting peaks) ^{and then} quenched ^{to room} tempe-

rature. The X-Ray spectrum is consistent with the presence of oriented a’-form and unoriented a-form

(i.e. rings ^of diffraction) ^obtained by recrystallisation

from the melt (Fig. 5). ^This result confirms the exis-

tence in PVDF of a new form (a’) ^{more stable than}

a-form (i.e. higher melting). ^{This form must be com-} pared ^{to the} y-form [5-9] which is also more stable than a-form but has only ^been obtained in unoriented

specimen.

Conclusion.

Changes ^{in the} reciprocal space have been examined for the poled ^{a-form of PVDF.}

The results are consistent with a phase ^transition

from a-form to an oriented polar ^form (ap) ^involving

the rotation of the TGTG- chains, ^as reported ⁱⁿ

reference [9]. The additional diffuse reflections are

presumed ^to arise from a particular packing and/or

distorsions of the TGTG - chains.

DTA and X-Ray diffraction analysis ^{have revealed}

the existence of a ^{new more} stable form (a’) ^obtained by annealing ^the poled ^{a-form at} high temperature.

The proposed chain conformation (T3GT3G-) ^is

similar to that of Rubber hydrochloride. ^Further investigations ^are necessary ^to determine the origin

of the diffuse reflections and the mechanisms of molecular deformation during poling ^and annealing.

Present works are in progress ^to determine the crystal

structure of this new form.

Acknowledgments.

^{The authors are} particularly grateful ^to C. Lemonon and F. Micheron of L.C.R.

Thomson C.S.F. Corbeville and M. Lambert,

R. Comès and L. Deschamps of Laboratoire de

Physique des Solides Orsay ^for help ^{and useful}

discussions.

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