Effect of irradiation on the dielectric properties and structure of a polyimide

EFFECT OF IRRADIATION ON THE DIELECTRIC PROPERTIES AND STRUCTURE OF A POLYIMIDE G . M . :BARTENEV, S. N . KAR1MOV, B. ,N. NARZULLAYEV, Z. A . KABILOV, V. K .

MATVEY~.V and V. I. SARm~

',

V. I. Lenin State University, Tajik

(Received 10 November 1976) The dielectric properties and structure of poly-4,4'-diphenyloxidepyromellitimide film in the initial and irradiated conditions have been investigated dielectrically and b y mass spectrometry. As distinct from cases described in the literature, the material has been investigated after subjection to the effect of various forms of radiation (7, electron and proton radiation) under different conditions (air or vacuum) and the results are compared. I t is shown t h a t the thermal decomposition of the polyimide structure investigated occurs principally though the rupture of imide rings. During preliminary irradiation, rupture of imide rings occurs with the subsequent formation o f erosslinks between macromolecules, the d a t a for the change in dielectric relaxation time after irradiation also being evidence of this.

MARYproblems connected with the study of the dielectric properties and structure of rigid chain polymers have now been elucidated in detail [1,4]. However, the class of heat resistant polymers, to which polyimides (PI) in particular belong and which are widely used, has still not been adequately studied with respect to their resistance to radiation [5-10]. In connection with this, the effect of ionizing radiation on the structure and thermal stability of PI, as well as the effect on certain of their electrophysical properties, have been investigated. The present paper presents the results of investigation of the dielectric losses of poly-4,4'-diphenyloxidepyromellitimide (PI-1) having a degree of crystallinity of 10% (from the data of X-ray structural analysis), i n the initial condition and after irradiation: 0 C--i 7

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PI - I * Vysokomol .soyed. AI9: No. 10, 2217-2223, 19"/7. 2540

Dielectric properties and structure of a polyimide

254]~'

Chemical structure is known to be the factor determining dielectric loss. behaviour and, to elucidate the changes introduced by ?-irradiation in the PI-1 structure, the general pattern of pyrolysis in the initial and irradiated polymer was also considered. Measurements were made on film specimens with a bridge circuit conforming to GOST 10405-63. Silver layers deposited on to the specimen surface by high vacuum sputtering were used as electrodes. The PI-1 films were subjected to thermal degradation in a special thermal degradation rig secured to the inlet pipe of the gas ion source of a MI-1305 mass spectrometer: the temperature range up to 730°C was used with continuous heating at the rate of 0.5-0.7 dog C/min. All the volatile products with mass numbers in the range 10-200were thus recorded. Figure la, shows how t a n 6 and the dielectric permeability e' depend on temperature for PI-1 at various frequencies. Two maxima in the relaxation dielectric losses m a y be seen on the temperature dependence shown, the activation energy for the first maximum being 8.3 kcal/mole and 29.3 kcal/mole for the second. According to the generally adopted point of view of amorphous polymers [11-13], the first maximum m a y be attributed to losses of t h e dipole group type on the basis of its low activation energy. This maximum in the dielectric losses m a y be connected with the absorption of water [14], which also follows from our mass spectrometry data, or with the movement of carboxyl groups in uncyclized links of the polyimide, which follows from the data of I R spectroscopy. The secGr.d maximum also clearly relates to a dipole group process since its activation energy is also small. Its value is, however, greater t h a n t h a t generally found for a dipole group process. This maximum in the dielectric losses is smaller t h a n the first maximum in height and is appreciably broadened. I t m a y be suggested that, for this process, the movement of the dipole occurs in a volume greater than t h a t of a monomer link. This suggestion is not contradicted by the chemical structure of the PI-1 monomer link since there are sufficient possible relaxation elements in it [6, 9, 10]. The presence of a non-imidized fraction (32%), t h a t is, polyamido-acid (PAA) links, and the solvent (acetamide) in the PI-1 structure also creates the conditions for the appearance of the relaxation region mentioned. I n fact, it follows from analysis of the mass spectrometry data t h a t the temperature range for the second maximum in the dielectric losses oi PI-1 is characterized by the evolution of volatile products whose formation is also connected with the presence of solvent residues. I t will be further shown t h a t still another loss maximum is observed for PI-1 in the higher temperature region, and this must belong to the dipole-segmental type (Fig. 2). Figure lb-e, shows how the dielectric characteristics depend on temperature for PI-1 in the initial condition and after irradiation with various forms of radiation (?, electron, proton) with various doses and under various conditions. I t m a y be seen t h a t a common regularity is found for all the forms and conditions o f irradiation, t h a t is, the t a n 6 maxima in the irradiated specimens are shifted,

G. M. B ~ a ~ N ~ v a a/.

~2542

slightly towards the higher temperature regions as compared with the initial specimen. An increase in the absolute value of tan Jmax occurs in the case of 7 a n d electron radiation. As regards proton irradiation, here the pattern is rather ~lifferent in that the intensity of the peaks is found to decrease. This is evidently IunS~lO: r

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~FIa. 1 Temperature dependence off 1-5--tan J a n d l ' - 5 " - - g for P I - 1 . a - - u n i r r a d i a t e d ; b a n d e--y-irradiated i n air (4e and 4'e are for irradiation in vacuum); c--irradiation b y ~lectrons in air; d--irradiation b y 6 Meg protons in air. Frequencies: 1-- 200; g-- 500; 3-- 1500; 4--6000; b a n d e--500 Hz. Dose: b-e; 1 and 1"--0; g a n d 2'--100; 3, 3", 4 a n d 4'--300; 5 and 5 ' - - 1000 Mrad.

Dielectric properties and strtlcture of a polyimide

2648

connected with the fact that proton irradiation was carried out at higher ternperatures (80-90°C) and this may have substantially altered the yield of t h e radiation chemical processes. I,'/ I00 /7

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FIG. 2 Temperature dependence of t a n $ in its high value "region at a frequency of 500 Hz for: 1 - - u n i r r a d i a t e d P I - 1 a n d 2 - - P I - 1 irradiated with 7-rays to a dose of 30O Mrad. FIG. 3 Mass spectra of the volatile products from the thermal degradation of P I - I polyimide films at 550°C: a--O; b--300 a n d e--lO00 Mrad.

Since PI-1 belongs to the class of highly heat resistant polymers, it was of interest to follow the dielectric properties in the region of higher temperatures. It follows from Fig. 2 that tan $ has yet another loss maximum in the temperature interval 220-275°C, a maximum that we attribute to a dipole-segmental process since it is positioned slightly above the glass temperature and has an activation energy of 117 kcal/mole. The shift in this tan $ maximum for the irradiated material towards higher temperatures and the reduction in its absolute maximum are caused by crosslinking of the polymer chains and by the slowing down of t h e corresponding relaxation elements because of this. At higher temperatures (above 275°C), tan J increases sharply. We explain this increase as being due

2fi44

G.M. BARTENEVeta/.

an increase in the electrical condictivity of the material because of pyrolysis products. T h u s this increase in tan g is found at lower temperatures in the irradiated material because the concentration of pyrolysis products is greater in t h e irradiated material than in the unirradiated. to

W E I G H T LOSSES A T T R I B U T A B L E TO I N D I V I D U A L COMPONENTS OF T H E MASS-SPECTRA.

m/e

12 16 17 18

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0"98 1.58 1-25 2.90

1.84

1.79 0.85 2.76 15.00 14.10 0.76 1.14

2.91 0.31 6.00

0-30 3.36

58.25

55.26

42.81

1.88

The structural changes occurring in the polymer investigated after irradiation were assessed b y mass spectrometry. For this purpose, the kinetics of thermaI degradation and the composition of the volatile products were studied for PI-1 films in the initial condition and after irradiation with doses of 300 and 1000 Mrad. Analysis of the results obtained showed that, when the initial and irradiated films are heated, three characteristic intervals for the evolution of volatile products are observed: because of this, one m a y speak of three different processes occurring during the heating of the specimens. At temperatures from 150 to 250°C, volatile products are found to be evolved whose formation is explained b y the presence of solvent residues. The second temperature range (250-650°C) is connected with decomposition of the basis polymer structure. Mass spectra of the volatile products evoled at these temperatures from the initial and irradiated films are shown in Fig. 3. The principal volatile products from the thermal degradation of the polyimide .studied are seen to be as follows: water (m/e=18); CO (re~e=28); CO2 (re~e=44) and phenol (m/e=94). The remaining products are fragments of these principal products and are formed during electron bombardment in the ion source. Carbon (m/e=12), oxygen (m/e=16) and hydroxyl (mien--IT) are formed from CO,CO2 and H~O and the products with m/e-=39, 40, 50, 55, 63, 65, 66, 93 and 94 are phenol fragments [15]. It m a y thus be considered that, under the action of heat, the decomposition of the macromolecules

Dielectric properties and structure of a polyimide o f the polyimide investigated occurs by the rupture of the imide ring a n d t h e subsequent formation of phenol. Quantitative analysis of the thermal decomposition products of unirradiated ,and irradiated polyimide films (see Table) showed that the proportion of oxygen containing products evolved in the decomposition o£ bhe imide ring decreases as the absorbed dose is raised. This is evidence that it is also the imide ring t h a t is ruptured in the first instance under t h e action of ionizing radiation.

W/%,%

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100 300 500 700 T,*C Fro. 4 Relative weight change W/W0--ealculated from mass-spectroscopic da~a, during the heating of PI-1 films: 1--0; 2--300 and 3--1000 Mrad. There is also evidence of the rupture of imide rings from the IR-spectroseopy data: comparison of the IR spectrum of an initial specimen with that of PI-1 irradiated to the maximum dose (1000 Mrad) shows that they do not differ very much in the range 700-1900 cm -1, which is evidence of the high resistance of PI- 1 to the effect of y-radiation. Slight changes in the intensities of the absorption peaks at 730, 1720 and 1780 cm -1 (vibrations of CO groups in the imide ring) are observed; the fact that these intensities decrease in the irradiated material may be caused by the partial rupture of imide rings during irradiation. It may be seen from the data presented in the Table relating to the overall weight loss that the specimen fraction undergoing degradation decreases as the ,dose of y-radiation is increased. This fact suggests that the rupture of imide rings under the action of r-quanta is accompanied by crosslinking between t h e PI-1 macromolecules. The way in which the weight loss of initial and irradiated PI-1 films depends on temperature is shown in Fig. 4, in which the curves W/Wo=f (T) were obtained by calculation from the mass spectrometry data for the rata of evolution of the principal volatile products during thermal degradation. It may be seen from this Figure that preliminary y-irradiation does not have any appreciable effect on the temperature for the start of decomposition despite the fact t h a t crossl i n k i n g between macromolecules probably occurs during irradiation. This m a y :be explained by the fact that the thermal degradation of the polymer studied i s principally represented by the decomposition of imide rings and a reduction

G. M. BARTINEV ~ ~.

in their ooneentration after irradiation affects only-the amount of volatile products evolved but not the temperature for the start of decomposition. Rupture of PAA of the type --CONH--, also occurs during irradiation, the reduction in intensity of the absorption bands at 920, 940 and 1020 cm -~ being evidence of this. The increase in the intensity of the maxima at 1200-1300 cm -1 in the IR spectra of PI-1 may also be due to the formation of aromatic type ether bonds, and the increase in the background in the region 1550-1680 cm -~ OO

II II

may be a consequence of the appearance of transverse links of the type --C--C-after the rupture of imide rings or PAA. It may be seen from the foregoing that the dielectric losses and dielectrio permeability are closely connected with the structure of the polymer. The presence OC

\

at elevated temperatures of such possible relaxation elements as C----0,

/

1~

OC and others, which are members of the imide hetero-ring under ordinary conditions and whose internal rotation is slowed down, becomes explicable when the m a n y aspects of PI-1 are considered. When PI-1 is further heated in the range 700-730°C, the evolution of volatile products is also observed, in particular, the intensity of the peak with m/e----28, N~, increases. Thus the evolution of the nitrogen molecule in the ease of an initial film occurs abruptly and at a high rate; in the case of a film irradiated with a doseof 300 Mrad, the evolution of N~ is less and, for a film irradiated with a dose of 1000 Mrad, the evolution is hardly observed at all. This feature is explicable if one bears in mind the crosslinking of the macromolecules during the prelimimary y-irradiation. Since a carbonized residue enriched with nitrogen is formed after the thermal decomposition of the imide rings, the formation of a graphitic structure accompanied by the ejection of a nitrogen molecule becomes 10ossible at teml0eratures above 700°C. An incr'ease in the density of the material thus occurs, as found in [16]. W~nereas such restructuring can occur comparatively freely in the case of an initial film, the process will be impeded because of the partial crosslinking of the macromolecules in the case of a film irradiated with a dose of 300 Mrad, a fact which also causes a reduction in the rate and amount of nitrogen evolved. For films irradiated with a dose of 1000 Mrad, a three dimensional network is developed to such an extent that re-ordering of the structure hardly occurs a~ all and no appreciable evolution of N2 is observed. The data obtained thus give evidence that the thermal decomposition of the

Dielectric propertieA and mtructure of a polyimide

2547

p o l y i m i d e film i n v e s t i g a t e d occurs p r i n c i p a l l y t h r o u g h t h e r u p t u r e o f i m i d e rings R u p t u r e o f i m i d e rings also occurs d u r i n g p r e l i m i n a r y 7-irradiation, w i t h t h e s u b s e q u e n t f o r m a t i o n o f erosslinks b e t w e e n m a c r o m o l e c u l e s .

Translated by G. F. MODLEI~ REFERENCES

1. V. Y. KORSHA]K, Khimieheskoye stroyeniye i temperaturnye kharakteristiki polimerov (Chemical Structure and Temperature Characteristics of Polymers). Izd. "Nauka", 197@. 2. M. M. KOTON and Yu. N. SAZANOV, Vysokomol. soyed. A15: 1654, 1973 (Translated in Polymer Sci. U.S.S.R. 15: 7, 1857, 1973) 3. Z. V. GERASHC]TRRK0, Ya. S. YYGODSKII, G. L. SLONIMSgH, A. A. ASKADSKH, V. S. PAPKOY, S. V. YINOGRADOVA, Y. G. DASHEYSKII, F. B. SHE]~MAN, V. A. ]RL]MOVA and V. V. KORSHAK, Vysokomol. soyed. A15: 1718, 1973 (Translated in Polymer Sci. U.S.S.R. 15: 8, 1927, 1973) 4. A. Ya. ARDASHNIKOV, I. Ye. KARDASH and A. N. PRAVEDNIKOV, VysokomoL soyed. BI6: 349, 1974 (Not translated in Polymer Sci. U.S.S.R.) 5. V. V. KORSHA]£, Termostoiklye pollmery (Heat-resistant Polymers). Izd. "Nauka"~ 1969 6. N, A. ADROVA, M. I. BESSONOV, L. A. LAIUS and A. P. RUDAKOV, Polllmidy--novyi~ klass termostoikikh pollmerov (Polylmides--a new Class of Heat-reslstant Polymers). Izd. "Khimiya", 1971 7. A. H. FRAZER, Vysokotermostoikiye polimery (Highly Heat-resistant Polymers). Izd. "Khhm'ya", 1971 (Russian translation) 8. A. N. PRAVEDNEKOV, I. Ye. KARDASH, N. P. GLUKHOYEDOV and A. Ya. ARDASHNIKOV, Yysokomol. soyed. A15: 349, 1973 (Translated in Polymer Sci. U.S.S.R. 15: 2, 399, 1973) 9. T. I. BORISOVA, M.I. BESSONOV and A. P. RUDAKOV, Sb. Sintez, struktura i svoistva polimerov (Collection: Synthesis, Structure and Properties of Polymers). p. 88, Izd. "Nauka", 1970 10. N. A. ADROVA, A. I. ARTYUKWOV, Yu. G. BAKLAGINA, T. L BORISOVA, M. M. KOTON, N. V. M1KHAILOVA, V. N. NIKITIN and A. V. SIDOROVICH, Vysokomol. soyed. A16: 1658, 1974 (Translated in Polymer Sei. U.S.S.R. 16: 7, 1921, 1974) 11. G. P. MIKHAXLOV, Uspekhi khlmli 24: 876, 1955 12. G. P. MTIgH&ILOV and T. 1K. BORISOVA, Uspekhi fiz. nauk 83: 623, 1964 13. Elektricheskiye svoistva polimerov, pod obshchei redaktsei B. I. Sazhlna (Elcetriea~ Properties of Polymers: B. I. Sazhin, general editor). Izd. "Khimiya", 1970 14. S, D. BRUCK, Polymer 6: 49, 1965 15. A. CORNU and R.e~IASSOT, Compilation of mass spectral data published by H e y d e ~ and Son Lhnited in cooperation with Presses universitates de France, 1966 16. S. D. BRUC]K, Polymer Preprints 6: 28, 1965