Polymer Degradation and Stability 35 (1992) 99-104

X-ray induced degradation of poly(vinylidene fluoride) films

Yoshio Kawano & Sandra Soares lnstituto de Qufmica, Universidade de S~o Paulo, Caixa Postal 20780, CEP 01498, S~o Paulo, Brazil

(Received 10 October 1990; accepted 29 October 1990)

The X-ray induced degradation of poly(vinylidene fluoride) (PDVF) films has been investigated by IR, X-ray diffraction, UV/visible and differential scanning calorimetry (DSC) techniques. The IR spectra show a significant decrease in intensity of bands characteristic of crystalline regions, namely a moderate decrease in intensity of bands corresponding to the tr form and a small decrease in intensity of bands of the fl form. The X-ray results show an increase in crystallinity after the first 3 h of irradiation; at longer exposure times, there is a decrease in crystallinity, the 0: form being preferentially attacked relative to the fl form, as shown by IR spectra. The loss of crystallinity under X-ray irradiation was demonstrated by DSC measurements.

The appearance of a new IR band at 1714cm -1, whose intensity increases with X-ray exposure time, is evidence of the formation of conjugated double bonds by dehydrofluorination. The UV/visible spectra show several absorp- tion bands associated with diene, triene and tetraene, the diene band being dominant.

The DSC thermograms show changes in the shape of the curve and the appearance of new melting peaks at lower temperatures with increasing exposure time. The reactions taking place in PVDF films irradiated in vacuum by X-rays are thus apparently similar to those in films irradiated by y-rays or electron beams and films subjected to high-energy ion implantation.

INTRODUCTION

The study of the radiation chemistry of polymers has been of considerable interest. It is well known that crosslinking, bond scission, gas desorption, changes in crystallinity, unsaturation and degradation can be induced by ionizing radiation, although the mechanisms of these effects are still not very well understood. 1,2

Poly(vinylidene fluoride) (PVDF) is a semi- crystalline high molecular weight polymer with the repeat unit (-----CH2 CF:w) and is ap- proximately 50% crystalline and 50% amor- phous. The polymer contains 3-6% of inverted monomer units ( - - - C H 2 - - - C H 2 - - ) and ( - - - -CF2-- CFz--) as structural defects in an otherwise completely head-to-tail structure. 3

PVDF exists in at least four different crystalline phases: 3 phase I (fl form), phase II (a~

Polymer Degradation and Stability 0141-3910/91/$03.50 © 1991 Elsevier Science Publishers Ltd.

99

form), phase III (y form) and phase IV (6 form). Two of these stable crystalline forms have been well characterized. Phase I has a planar zigzag chain conformation, producing a polar crystal, and is the most important of the PVDF polymorphs. Phase II is the most common polymorph of PVDF, with TGTG' chain conformation, producing a nonpolar crystal.

PVDF has been the subject of much investigation in recent years because of its piezoelectric and pyroelectric properties. The piezoelectric activity in the polymer is thought to arise from the polar (presumably/3-form) crystals and several models have been developed to explain the phenomenon. 4 It has also been established that the piezoelectric and pyroelectric activity of PVDF is directly related to the structure of the main carbon chain of the polymer. 5

The crystallinity of PVDF plays a vital role in piezoelectric activity. High-energy radiation has

100 Yoshio Kawano, Sandra Soares

been claimed to increase the crystallinity without extensive degradation of the mechanical properties. 5 Several investigators have studied the radiation chemistry of PVDF. 6-9

This work presents an investigation on the reactions which take place when PVDF films are exposed in vacuum to X-ray radiation.

E X P E R I M E N T A L

Commercial Kureha PVDF films ( 6 # m in thickness) were used; an My value of 105 was obtained viscometrically 9 from measurements obtained using an Ubbelohde viscometer at 37°C with dimethylacetamide as solvent.

The films were irradiated for various periods with X-ray beams at room temperature in vacuum using a Philips X-ray spectrometer (PWl410) equipped with a tungsten tube, operating under 40 kV and 20 mA, as previously described. 1°

IR transmission spectra of the PVDF films before and after irradiation were taken on a Perkin-Elmer PE-283 spectrophotometer in the range of 200-4000 cm -1.

UV/visible spectra of the PVDF films were obtained using a Cary 17 spectrophotometer and a HP 8451A diode-array spectrophotometer in the range of 200-400 nm.

X-ray diffraction scans were obtained using a Philips X-ray diffractometer (PW 1410) with Cu K,, monochromatic radiation.

DSC measurements were performed on a DuPont 1090 thermal analyzer using approxi- mately 5 mg of PVDF film for each thermogram and a heating rate of 10°C/min.

RESULTS A N D DISCUSSION

IR

Figure 1 compares the IR spectra of PVDF films in the ranges of 3100-2800cm -~ and 1800- 400 cm -1 before and after X-ray irradiation. The spectral profile of the irradiated film shows a significant decrease of the relative intensity of one set of IR bands, and a uniform decrease of the intensity of another set.

The IR spectra of PVDF polymer in different phases have been assigned by several authors. 11-17 Only two bands assignable to the CH2 stretching

3200 1800 1400 1000 600 2800 1600 1200 800 400

Wavenumber (¢m -1 )

Fig. 1. IR spectra of PVDF films (6 #m): irradiation times; A, Oh; B, 3 h; C, 6h; and D, 9h.

modes are observed, namely at 2980 and 3020cm -1. A gradual decrease in the relative intensity of both bands with exposure time is observed, possibly due to the loss of hydrogen atoms in the dehydrofluorination process. Kise and Ogata, TM studying dehydrofluorination of PVDF by aqueous sodium hydroxide solution, observed an almost complete disappearance of the C - - H stretching bands at 2980 and 3020 cm -1.

No IR band was detected in the region of 2000-2100 cm -1 (not shown in Fig. 1), indicating the absence of allenic compounds and carbon- carbon triple bonds. In PVDF bombarded with oxygen ions, Le Moel et al. 19 observed a weak band at 2040cm -1, which they assigned to an allenic compound or a carbon-carbon triple bond. In PVDF bombarded with krypton ions, they observed a band at 2038cm-L Kise and Ogata TM detected a weak band at 2100 cm -1 in the dehydrofluorination of PVDF by aqueous sodium hydroxide solution and assigned it to stretching of carbon-carbon triple bonds.

The spectra of the irradiated films show the

X-ray induced degradation o f films 101

appearance of a new band at 1714cm -1 (a shoulder at 1755cm -1 is present in the film before irradiation and in all irradiated spectra) (Fig. 1). The intensity of this 1714cm -~ band, increases with exposure time. It can be assigned to the stretching modes of conjugated carbon- carbon double bonds with a fluorine substituent, such as ,,-,,CH---CF,~, formed as a result of dehydrofluorination. The exposure of PVDF samples to high-energy radiation is well known to give rise to new IR bands in the 1600-1800 cm -~ region, assigned to different stretching vibrations by several authors.

II'Icheva et al. 2° studying )'-irradiated PVDF in vacuum, observed two bands at 1755 and 1715 cm -1, which they assigned to stretching of the carbon-carbon double bonds of vinylidene (---CH----CF2) and vinylene (---CH---CF--) groups, respectively.

Hagiwara et al. 2~ observed two bands at 1750 and 1850 cm -1 in the IR spectrum of PVDF after ),-irradiation in air. They assigned the former band to the stretching mode of carbonyl in ketones, with F-substituents on the a~-carbon, and the latter to carbonyl fluoride. These authors also observed the appearance of a band at 1710 cm -~ on ),-irradiation of PVDF in vacuum, which they assigned to the formation of double bonds such as - - C H - - C F - - .

Said et al. 22 observed two new IR bands at 1714 and 1759cm -~ under high-energy ion implanta- tion in PVDF films. They assigned both bands to carbonyl stretching vibrations arising from two different carbonyl environments: - - -CHz--CO-- CH2-- (1714 cm -1) and --CF2---CO---CF2-- (1759 era-l).

The IR spectrum of PVDF films in the fingerprint region shows bands at 767, 796, 855, 978 and 1072cm -~ which, according to several authors a7,z3'24 are characteristic of the two crystalline forms (c~ and fl). Upon X-ray irradiation, they show significant changes in intensity.

The bands at 532 and 615cm -~ decrease in intensity upon X-ray irradiation. These bands are among the bands characteristic of the ~r phase. ~ ~-~5.25

The bands at 470 and 510cm -~ show a small intensity decrease with exposure time. These bands are characteristic of the/3 phase. H-14,z3,z5

The very weak bands at 1452, 1333 and 675 cm -~, which are assigned to the head-to-head irregularities ~2'24 still appear in the spectrum after

irradiation for 9 h, indicating that segments with defects in the polymer chain are not affected by the X-ray radiation.

The IR spectra generally indicate that the crystalline phases are affected most in irradiated PVDF films, the a~ phase more than the fl phase.

The IR spectra show the appearance of conjugated carbon-carbon double bonds pro- duced by dehydrofluorination of the polymer.

X-ray diffraction

Figure 2 shows X-ray diffractometer scans of PVDF films before irradiation and after irradia- tion for 3, 6 and 9 h.

Unirradiated films show a strong diffraction peak at 20 = 19.8 °, two medium peaks at 17.6 and 20.7 °, and a weak peak at 18.4 °. The peaks at 17.6 and 19.8 ° are characteristic of the tr phase 5'26-29 and correspond to the (100) and (110) planes, respectively. These peaks increase slightly in intensity during the first 3h of exposure but have decreased slightly after 6 h, and significantly after 9 h of exposure.

The peak at 20.7 °, corresponding to the (110) plane of the fl phase, 26-29 shows a small increase in intensity during the first 3 h of exposure, remains unchanged after 6 h, and decreases to a small extent after 9 h.

c

A %

%

1 1 |

C

i

t i

22 20

| i

A i

18 16 22 Degree (20)

13

| i

D

i i

20 18 16

Fig. 2. X-ray diffractograms of PVDF film (6~m): irradiation times; A, 0 h; B, 3 h; C, 6 h; and D, 9 h.

102 Yosh io Kawano , Sandra Soares

The X-ray data show that an increase of crystallinity occurs during the first 3 h of X-ray irradiation, but it decreases at longer irradiation times. This confirms the conclusion drawn from the IR data that the crystalline region is destroyed, the tr form being more strongly affected than the fl form.

Pae et al. 5 have shown that the degree of crystallinity of films of PVDF and several other polymers can be enhanced by electron beam radiation. Kosmynin and Gal'perin 26'27 have shown that ),-irradiation of PVDF samples with a 1000 Mrad dose in vacuum or in air reduces the percentage crystallinity and that the stability of the fl form towards the disintegrating effects of y-irradiation is superior to that of the te form. Finally the X-ray scans show no evidence of a phase transition from the a~ to the fl form of the type observed by Davis et al. 2s upon application of electric fields to PVDF samples.

UV/visible

The UV/visible spectra of PVDF films (6/~m thickness) before and after irradiation are shown in Fig. 3. The unexposed PVDF film absorbs very weakly above 225 nm, but in films irradiated for 3, 6 and 9 h absorption bands appear at 227, 273 and 314nm. The absorption intensity increases with exposure time and a small red-shift of the 227 nm peak with increasing exposure time is observed.

The absorptions at 227, 273 and 314 nm -1 can be assigned to diene, triene and tetraene

functions, respectively, the double bonds appear- ing in the PVDF chain as a result of the dehydrofluorination reaction induced by X-ray radiation.

As shown in Fig. 3, the absorbance at 227 nm grows significantly with time, whereas the absorbances at longer wavelengths grow very slowly, indicating preferential formation of diene units in the skeletal chain. The degree of conjugation slowly increases with exposure time, but the longest polyenes are found not to exceed four conjugated double bonds. Figure 3 also shows spectra (dotted curves) of the films irradiated for 3, 6 and 9 h but recorded 8 months after irradiation. It is clear from a comparison of the two sets of spectra that irradiated PVDF films show a minimal ageing effect, the species formed upon irradiation being relatively stable at room temperature.

The UV/visible spectra of PVDF films irradiated with X-rays are quite similar to those of y-irradiated films. 6'9"2° Thus, Makuuchi e t al . 9

observed absorption bands at 227 and 272nm, Komaki 6 observed bands at 227, 274 and 315 nm, and II'Icheva et al. 2° observed bands at 230, 270 and 310 nm for ),-irradiated PVDF films. These bands were also assigned to diene, triene and tetraene, respectively. Absorption bands at 224 and 272nm have been observed in the UV/visible spectra of helium implanted PVDF films and assigned to diene and triene absorption, respectively. 3°

DSC

3.2 , "~v ' ' '

"' C ::.

: ". . . . . . . ", 273

I l l

n 314

' A """.. ""%'... ""'"'..

2 0 0 250 3 0 0 350 4 0 0

nrn

Fig. 3. UV/visible absorption spectra of PVDF films (6 ~m): irradiation times; A, 0 h; B, 3 h; C, 6 h; and D, 9 h. The corresponding dotted spectra were obtained after 8

months.

DSC scans of PVDF films (6/ ,m thickness), before and after irradiation are compared in Fig. 4. The unirradiated PVDF shows a single melting peak at 177.0°C. After a 3 h exposure, two melting peaks were located at 177.0°C and 165-3°C, while after 6 h, a weak peak at 177.0°C, a shoulder at 161-0°C, and a medium peak at 150.0°C, are observed. The appearance of these new peaks is possibly due to an increase in the number of imperfections in the crystalline regions.

A melting peak at 179°C was observed by Said et al. 22 in unirradiated commercial PVDF film. Pae et al. 5 observed two melting peaks located at 162°C and 168°C, respectively, for unorientated Kynar PVDF film (tr form). Bartos and Vacek 31 observed two peaks at 169 and 172°C, respec- tively, for their PVDF sample. They observed a

X-ray induced degradation of films 103

O

A-1 E

O

-3'

"r_4

-5

I I ! I I | I I ! ! ! I v I I I |

.... .. , y : : ~ ............

- 65.2/, ~ ~ " \ J ~

177.0 °-"

Fig. 4. DSC irradiation times; (

I i t i 1 t i 1 i i I I 1 , , i I i i i !

40 60 80 100 120 140 160 180 200

Temperature (*(2)

thermograms of PVDF films (6/zm): ), Oh; ( . . . . . ), 3h; and ( . . . . . ), 6h.

gradual disappearance of the upper peak and a gradual decrease of the melting temperature with electron beam irradiation dose. For PVDF foils irradiated in an electron accelerator, an increased concentration of chemical crosslinking was found to lower the melting temperature. 32

The difference between the melting tempera- ture (177.0°C) observed for thin PVDF films of the Kureha resins and those obtained by Pae et al. 5 and Bartos and Vacek 31 might be due to differences in the commercial resins or in the relative proportion of inverted monomeric units (head-to-head, tail-to-tail structures) within the macromolecules. 33

From measurements of the total melting peak area, heats of fusion of 69-1, 57-6 and 48-2 kJ/kg for PVDF before irradiation, after 3 h and 6 h irradiation, respectively, were calculated. As- suming a heat of fusion of 139 kJ/kg for a 100% crystalline PVDF film, 34 crystallinities of 50%, 41% and 35% are calculated for these three samples, respectively. This loss of crystallinity on X-ray exposure is in sharp contrast to the gain in crystallinity during the first 3h of exposure observed in the present X-ray results and that reported for exposure to electron beam radiation. 5 On the other hand, single crystals of PVDF exposed to electron beam radiation were found to undergo a loss of crystallinity. 31 A loss of crystallinity is also observed upon high-energy ion implantation in PVDF f i l m y It is clear from the DSC data that a destruction of the crystalline phases has occurred under X-ray radiation. This result is confirmed by X-ray and IR measure- ments on the PVDF films irradiated for longer times.

At present, the increase of crystallinity during

the first 3h of X-ray exposure which was demonstrated by the X-ray diffractometer scan, cannot be explained.

CONCLUSION

Prolonged exposure to X-rays causes both physical and chemical changes in PVDF films. The main physical change is a loss of crystallinity, in agreement with all but one previous report of the effect of ionizing radiation. Of the two crystalline forms present in the samples used, the nonpolar tr form was found to be more vulnerable to X-rays than the polar fl form. No radiation induced phase transition between the two forms was observed.

The irradiated material shows an increase in UV/visible absorption, acquiring a distinct yellow coloration which increases with exposure time; at long exposure times, it becomes brittle.

The main chemical change observed is dehydrofluorination, leading to the formation of both nonconjugated and, at lower concentra- tions, conjugated double bonds. No evidence was found for adjacent (allenic) double bonds or for triple bonds. In general, these results are very similar to those observed with other forms of ionizing radiation, such as 7 rays and ion beams.

ACKNOWLEDGEMENTS

The authors are grateful to Dr Guilherme Leal Ferreira of the Instituto de Fisica e Quimica de Sao Carlos, Universidade de S~o Paulo, for providing the sample used in this work. Thanks also to Mr Ricardo Knudsen for the DSC measurements. SS thanks CAPES/PICD for a fellowship grant and YK thanks CNPq for a research fellowship and PADCT and FINEP for financial support.

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