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Dielectric properties of certain irradiated polymers
3032
V. K. MA~'EYEV et aZ.
12. Ya. A. ZUBOV and D. Ya. TSVANKIN, Vysokomol. soyed. 7: 1848, 1965 (Translated in Polymer Sei. U.S.S.R. 7: 11, 2028, 1965) 13. K. O'ZEARY and P. H. GEIL, J. Makromolek. Sei. BI: 147, 1967 14. V. I. GERASIMOV and D. Ya. TSVANKIN, Pribory i tekhnika eksperimenta, No. 2, 204, 1968 15. R. BONART, Kolloid Z. und Z. ftir Polymere 194: 97, 1964 16. J. L. HAY and A. KELLER, J. Mater. Sci. 1: 41, 1966; 2: 538, 1967
DIELECTRIC PROPERTIES OF CERTAIN IRRADIATED POLYMERS * V. K. MATVEYEV, S. E. VAISBERG and V. L. K ~ P o v L. Ya. Karpov Physical Chemistry Institute (Received 29 November 1968)
THE m a r k e d increase in the dielectric losses in p o l y e t h y l e n e (PE) as a result of its ~- or electron irradiation in air has been described in a n u m b e r of p a p e r s [1-5], a n d this effect has been c o n n e c t e d with radiation-chemical o x i d a t i o n (chiefly o f the surface layers [5]). This effect is n o t present when p o l y s t y r e n e (PS) is irradiated in an electrofi accelerator [4] or in a r e a c t o r [6]. W e h a v e m a d e a detailed investigation of the change in the t a n g e n t of the dielectric loss angle of a n u m b e r of irradiated polymers (various grades of polyethylene, p o l y p r o p y l e n e (PP), p o l y s t y r e n e a n d poly-a-methylstyrene), the radiation dose a n d dose r a t e being varied over a wide range, the specimen thickness also being varied in a n u m b e r o f cases. Measurements were carried out at a frequency of approximately 1010Hz by the resonator method on an IVD-1 apparatus [7] at room temperature. The error in measuring tan J did not exceed 20 ~/o.The polymer specimens, in the form of plates 10 × 30 mm and from 0.3 to 1"2 mrs thick, were obtained by hot pressing from powder or granules. Irradiation was carried out in a cobalt installation and in an electron accelerator ( ~ 1.3 MeV) at room temperature, in air and i n vacuo ( ~ 10-4 mmHg). Figure 1 shows our results for the w a y in which t a n J depends on ?-irradiation dose for various t y p e s o f P E (dose rate, 200-300 rad/sec). The following were selected as materials for investigation: high-pressure p o l y e t h y l e n e ( H P P E ) , a n d also stabilized a n d unstabilized low-pressure p o l y e t h y l e n e s (LPPEst a n d LPPEunst ). I t m a y be seen from the Figure t h a t for irradiation in air the c u r v e showing how t a n J depends on dose has, in all cases, a s h a r p initial rise (up to a dose of 50-60 Mrad) with a s u b s e q u e n t slight increase. The same characteristics * Vysokomol. soyed. A l l : No. 12, 2666-2670, 1969.
Dielectric properties of certain irradiated polymers
3033
were shown b y the w a y in which the increase in the concentration of carbonyl groups varied during the irradiation of the same LPPEst specimens in air, the measurements being made by I R spectroscopy (curve 4, Fig. 1). For irradiation i n v a c u o , the value of tan 6 increases slightly in a linear fashion over the entire range of doses investigated (up to 800 Mrad); as had been expected, no changes were found in the concentration of carbonyl groups in this case. The data presented confirm that the increase in tan 6 during the irradiation of P E in air is connected with the formation of carbonyl groups b y oxidation. The presence of an antioxidant in LPPEst gives rise to less oxidation and consequently to a lower value of tan 6 during irradiation as compared with LPPEu,~t. The slight rise in tan 5 of P E during irradiation i n v a c u o is evidently connected with the specimen's becoming more amorphous, since it is known that dielectric losses in unirradiated P E are caused chiefly b y CO groups in the amorphous phase [8, 9]; the different proportions of the amorphous phase in L P P E (degree of crystallinity, 75%) and H P P E (50~) also give rise to corresponding differences in these materials in the value of tan 6 both for the initial and also for irradiated specimens. A special experiment with L P P E slowly cooled from the melt, having a degree of crystallinity (determined b y the X < a y method) of 90%, established a corresponding reduction in tan 6 in the case of specimens irradiated to various doses. We should dwell particularly on the break (at a radiation dose of 50-60 Mrad) which is found in the kinetics of the build-up of the carbonyl group concentration lan d xI0 ~ ~0
3...~
32 rand, tO•
Ii5 ,
1OF
--o---~-.-.~-~
o
FIG. 1
..xjx/×
6~ _
2L..
100 200 300 z400 500 600 700 800
Dose , Mpad
...~--
300
600
900
f200
Dose , Mead
FIG. 2
FIG. 1. Dependence of tan ~ on 7-radiation dose for various grades of P E (specimen thickness, d =0-7-0.8 mm): 1--LPPE~t, irradiated in air; 2,2'--LPPEunst irradiated in air and in vacuo respectively; 3 , 3 " - - H P P E irradiated in air and i n vacuo respectively; 4--rise in the concentration of CO groups (in relative units) in H P P E with irradiation dose in air. FIG. 2. Dependence of tan 6 on the dose of electron radiation for LPPEst in air. Irradiation dose rate, 7.3 × 10 4 rad/sec, d =0.7 ram.
in P E irradiated in air, and also on the break, associated with it, in the rate at which tan 6 increases (Fig. 1). I f this change is connected with a transition from oxidation kinetics limited b y the accumulation of free radicals to oxidation kinetics
3034
V.K. MATV~.¥~.Vet al.
limited by oxygen diffusion, then the initial rapid rise should be determined by the radiation dose, and the subsequent slow rise should be determined by the time of contact with oxygen. We carried out experiments on the irradiation of P E (LPPEst) by electrons (~ 1.3 MeV) in air at a high dose rate, 7.3 × 104 rad/sec, which is 300 times greater than for y-irradiation. The results shown in Fig. 2 show t h a t from the very start the increase in tan 5 is caused not so much by the radiation dose as by the time of contact with oxygen, i.e. the oxidation kinetics exhibit diffusion characteristics from the very start. The break on the curves shown in Fig. 1 is clearly connected with a transition from kinetics of oxidation of the specimen surface, limited by the function of oxygen within micropores, to kinetics limited by the diffusion within internal layers of the specimen. I t is thus surprising t h a t the oxidation of the surface tends to saturation for whatever cause. (The break cannot be explained by initial oxidation involving oxygen dissolved in the initial specimen, because of the low oxygen concentration as compared with the initial concentration of CO groups, and also because experiments with degassed and undegassed specimens gave identical results.) The diffusion characteristics of the kinetics of the radiation-chemical oxidation of PE, which causes oxidation only in the surface layers, should have a marked effect on the way in which tan 5 of PE irradiated in air depends on specimen thickness. However, no such dependence was observed in reference [1], in contradiction to other data [5]. We carried out experiments to study the effect of the thickness of the irradiated PE specimen on tan 5. The results are shown in Fig. 3. A comparison is made below between tan 5 and the CO group concentration for LPPEst specimens of various thicknesses, irradiated in air to a dose of 15 Mrad Specimen thickness, mm CO-group concentration, arbitrary units tan ~ × 10-4
0'88 0.72 0.55 0.25 0.30 0.40 3.8 4.5 '5.5
The results point to the existence of a specimen thickness effect, confirming the considerations put forward above and refuting the data of reference [1]. From the data shown in Figs. 1 and 3, we have derived an empirical formula connecting tan 5 for any type of PE with irradiation dose in air (D, Mrad), specimen thickness (d, mm) and with the degree of crystallinity (a, %): tan 5 ~ t a n 50-f-
0 . 0 0 1 + 4 × 10 -5 (lO0--a) ( 0-~8D) d °'25 • 1--e ,
where tan 50 is the initial value of tan 5. The difference between the experimental values and those calculated from the formula does not exceed 20%, which is the value of the experimental error. The formula is applicable to dose rates of the order of 200-300 rad/sec and has been checked for thicknesses from 0.4 to 3 mm. We have also checked the effect t h a t the time for which the irradiated P E specimens are held in air has on tan 5. The data shown in Fig. 4a, point to a gradual rise in tan 5 as specimens irradiated i n v a c u o beforehand at 15 Mrad are held
D i e l e c t r i c p r o p e r t i e s of c e r t a i n i r r a d i a t e d p o l y m e r s
3035
in air: in five months' storage, tan 6 of L P P E increases b y a factor of 3.6, the corresponding figure for H P P E being 2.2. Clearly, we are dealing with an extremely slow process of oxidation and migration of long-lived radicals from the crystaline phase into the amorphous phase. It should also be noted that the ratio of the effects involved in this increase of tan 5 for L P P E and H P P E corresponds to the ratio of their degrees of crystallinity. The considerations expressed above are also confirmed b y the data we obtained, which are shown in Fig. 4b: it follows from these that the effect involved in the increase of tan 6 when irradiated P E specimens have been stored in air for one year becomes greater as the preliminary radiation dose is increased only up to doses at which the concentration of free radicals in P E reaches a limiting value [10]. The data shown in Fig. 4b, were obtained b y heating specimens for 5 hr at 80-90°C directly after irradiation. i,./O~
jX~
>,/
~
x
-
tan 6", 10~
1
8f
a
Io
18
12
o
2 4 T i m e , months
6i
C I
0
;00
l
1
200 300 000 Dose , ,Hrad
Fro. 3
14
-
200
"
._ I
#00 600 Dose , tYnad
2• I
800
FzG. 4
FIG. 3. E f f e c t of t h e t h i c k n e s s o f t h e i r r a d i a t e d L P P E s t s p e c i m e n on t h e w a y in w h i c h t a n 6 d e p e n d s o n t h e y - i r r a d i a t i o n dose in air, for: 1 - - d = 0 . 4 2 ram, a n d 2 - - d = 0 . 9 8 r a m . FIG. 4. D e p e n d e n c e o f t a n 5 for a P E s p e c i m e n , i r r a d i a t e d i n vacuo, o n t h e t i m e of s t o r a g e in air. a: 1 - - H P P E , 2 - - L P P E ( d = 0 . 7 m m ) ; b: 1 - - L P P E s t t e s t e d one y e a r a f t e r i r r a d i a t i o n , 2 - - L P P E s t t e s t e d d i r e c t l y a f t e r i r r a d i a t i o n (d = 0 . 7 r a m ) .
I t follows from this that even heating of this type does not lead to annihilation of free radicals in the crystalline phase, which is in agreement with other work [11]. In addition to the investigation of the changes in tan 5 during the irradiation of PE, we also carried out similar experiments with P P (Fig. 5), PS and poly-amethylstyrene (Fig. 6). The data obtained correlate well with all the considerations put forward above in relation to PE: this gives evidence that there are analogous causes for the change in tan 6 during the irradiation of these polymers. The data shown in Fig. 5 point to corresponding effect of the stabilizer, specimen thickness and radiation environment on the change in tan 8 of P P with dose. The data shown in Fig. 6 also indicate that there is no effect of holding time in air for PS specimens after preliminary irradiation i n vacuo: this was to be expected
3036
V . K . MATVEYEVet al.
because of the absence of a n y crystalline p h a s e in a t a c t i c PS; this also e x p l a i n s t h e c o m p l e t e absence o f a n y change in t a n 5 of PS w i t h dose d u r i n g i r r a d i a t i o n iTb v a c u o .
T h e i r r a d i a t i o n of P S a n d p o l y - a - m e t h y l s t y r e n e which we carried o u t in air a t high dose r a t e s ( ~ 7 . 3 × 1 0 t rad/sec) in a n electron a c c e l e r a t o r (1.3 MeV) CHANGE
IN
tan $
FOR
POLYSTYRENE
POLY-6f-METHYLSTYREIqE
AND
DURING
IRRADIATIOI~
Polymer PS, tan 5x 10 -4 Poly-a-methylstyrene, tan 5 x 10 -4
50 4.6
-
6.8
Dose, Mrad 100 i 500
1500
5000
4.6
5"8
6.4
6"8
9.5
-
6"8
showed a slight rise in t a n 5 u p to a dose of ~ 1000 M r a d (Table): this is in agreem e n t w i t h o t h e r w o r k [6] a n d agrees w i t h considerations p u t f o r w a r d p r e v i o u s l y .
toncLfO'
ton cY. I0 ~ 20 F
32 2z~
~
2 1
lg 8
16 12 ~-
2
*
8 :
.
_
~4
-
3 4
-
-
-
a A
F
#0
120 Dose, M p a d
200
n
Fie. 5
10~:
200 300 Dose, Mnod
#00
5OO
Fie. 6
Fio. 5. PP effect of stabilizer and specimen thickness on the way in which tan ~ depend,~ on 7-radiation dose. 1--PPst (d=l-0 mm), 2--PPst (d=0.7 mm), 3--PP, free of stabilizer (d=0.7 mm), 4--PPst, irradiated i n v a c u o ( d = l ' 0 nun). FIG. 6. Dependence of tan efon 7-radiation dose (dose rate, 200-300 rad/sec, d =0.7-0-8 mm): 1--poly-~-methylstyrene, irradiated i n vacuo; 2, 3--PS irradiated in air and i n v a c u o respectively; 4--PS tested after the specimens had been stored one year in air. I n all t h e investigations which we m a d e , m e a s u r e m e n t s of t h e dielectric p e r m e a b i l i t y were also carried o u t a t t h e s a m e t i m e as t h e m e a s u r e m e n t of t a n 5; no changes within t h e limits of error were f o u n d a t a f r e q u e n c y of ~ 101° H z , in a g r e e m e n t w i t h the v e r y m u c h lower s e n s i t i v i t y of this q u a n t i t y (as c o m p a r e d w i t h t a n &) to dipole concentration. CONCLUSIONS
(1) A b r e a k has been f o u n d in the curves relating t a n 5 to 7-irradiation dose in air for v a r i o u s grades o f p o l y e t h y l e n e , p o l y p r o p y l e n e , p o l y s t y r e n e a n d poly-a-
Dielectric properties of certain irradiated polymers
3037
m e t h y l s t y r e n e . I n t h e case o f p o l y e t h y l e n e it has been confirmed, a n d for the remaining materials it has been shown, t h a t the increase in t a n J during irradiation in air is c o n n e c t e d w i t h the f o r m a t i o n of CO groups b y radiation-chemical oxidation. (2) I t has been shown t h a t radiation-chemical o x i d a t i o n has diffusion characteristics. This leads to the fact t h a t t h e increase in t a n J during irradiation in air is d e t e r m i n e d n o t so m u c h b y t h e radiation dose as b y the time t h a t the material is in c o n t a c t with oxygen. (3) An empirical f o r m u l a has been derived for various grades o f p o l y e t h y l e n e which, for dose rates o f 200-300 rad/sec, connects t a n J with the r a d i a t i o n dose in air, the specimen thickness a n d its degree of crystallinity. (4) I t has been shown t h a t t a n J for p o l y e t h y l e n e i r r a d i a t e d i n vacuo increases w i t h its t i m e o f storage in air. I t is suggested t h a t t h e increhse in t a n 5 is caused b y the oxidation o f free radicals migrating from t h e crystalline phase. Translated by G. F. MODLEN
REFERENCES
1. E. RUSHTON, Teehn. Rept. Electr. Res. Assoc. NL/T 377, 14 pp., ill., 1958 2. K. A. VODOP'YANOV, B. N. VOROZHTSOV, G. L POTAKHOVA and N. I. OL'SHANSKAYA, Elektrichestvo, No. 5, 60, 1960 3. H. SUKAKURA, J. Appl. Phys. Japan 2: 66, 1963 4. J. V. PASKALE, D. B. HERRMAN and R. I. MINERR, Mod. plastics 41: 239, 1963 5. B. I. SAZHIN, A. M. LOBANOV, A. L. GOL'DENBERG et al., Zh. tekhn, fiziki 28: 1991, 1958 6. A. CHARLESBY, Yadernyye izlucheniya i polimery (Atomic Radiation and Polymers). Forei~ql Lit. Publ. House, 1962 (Russian translation) 7. P. F. VESELOVSKII, Dissertation, 1965 8. G. P. MIKHAILOV, A. M. LOBANOV and B. I. SAZHIN, Zh. tekhn, fiziki 24: 1553, 1954 9. G. P. MIKHAILOV and T. L BORISOVA, Zh. tekhn, fiziki 23: 2159, 1953 10. A. T. KORITSKII, Yu. N. M/tIJINSKII, V. N. SHAMSHEV, P. N. BUBEN and V. V. VOYEVODSKII, Vysokomol. soyed. 1:1182, 1959 (Translated in Polymer Sci. U.S.S.R. 1: 3, 458, 1960)
V. K. MA~'EYEV et aZ.
12. Ya. A. ZUBOV and D. Ya. TSVANKIN, Vysokomol. soyed. 7: 1848, 1965 (Translated in Polymer Sei. U.S.S.R. 7: 11, 2028, 1965) 13. K. O'ZEARY and P. H. GEIL, J. Makromolek. Sei. BI: 147, 1967 14. V. I. GERASIMOV and D. Ya. TSVANKIN, Pribory i tekhnika eksperimenta, No. 2, 204, 1968 15. R. BONART, Kolloid Z. und Z. ftir Polymere 194: 97, 1964 16. J. L. HAY and A. KELLER, J. Mater. Sci. 1: 41, 1966; 2: 538, 1967
DIELECTRIC PROPERTIES OF CERTAIN IRRADIATED POLYMERS * V. K. MATVEYEV, S. E. VAISBERG and V. L. K ~ P o v L. Ya. Karpov Physical Chemistry Institute (Received 29 November 1968)
THE m a r k e d increase in the dielectric losses in p o l y e t h y l e n e (PE) as a result of its ~- or electron irradiation in air has been described in a n u m b e r of p a p e r s [1-5], a n d this effect has been c o n n e c t e d with radiation-chemical o x i d a t i o n (chiefly o f the surface layers [5]). This effect is n o t present when p o l y s t y r e n e (PS) is irradiated in an electrofi accelerator [4] or in a r e a c t o r [6]. W e h a v e m a d e a detailed investigation of the change in the t a n g e n t of the dielectric loss angle of a n u m b e r of irradiated polymers (various grades of polyethylene, p o l y p r o p y l e n e (PP), p o l y s t y r e n e a n d poly-a-methylstyrene), the radiation dose a n d dose r a t e being varied over a wide range, the specimen thickness also being varied in a n u m b e r o f cases. Measurements were carried out at a frequency of approximately 1010Hz by the resonator method on an IVD-1 apparatus [7] at room temperature. The error in measuring tan J did not exceed 20 ~/o.The polymer specimens, in the form of plates 10 × 30 mm and from 0.3 to 1"2 mrs thick, were obtained by hot pressing from powder or granules. Irradiation was carried out in a cobalt installation and in an electron accelerator ( ~ 1.3 MeV) at room temperature, in air and i n vacuo ( ~ 10-4 mmHg). Figure 1 shows our results for the w a y in which t a n J depends on ?-irradiation dose for various t y p e s o f P E (dose rate, 200-300 rad/sec). The following were selected as materials for investigation: high-pressure p o l y e t h y l e n e ( H P P E ) , a n d also stabilized a n d unstabilized low-pressure p o l y e t h y l e n e s (LPPEst a n d LPPEunst ). I t m a y be seen from the Figure t h a t for irradiation in air the c u r v e showing how t a n J depends on dose has, in all cases, a s h a r p initial rise (up to a dose of 50-60 Mrad) with a s u b s e q u e n t slight increase. The same characteristics * Vysokomol. soyed. A l l : No. 12, 2666-2670, 1969.
Dielectric properties of certain irradiated polymers
3033
were shown b y the w a y in which the increase in the concentration of carbonyl groups varied during the irradiation of the same LPPEst specimens in air, the measurements being made by I R spectroscopy (curve 4, Fig. 1). For irradiation i n v a c u o , the value of tan 6 increases slightly in a linear fashion over the entire range of doses investigated (up to 800 Mrad); as had been expected, no changes were found in the concentration of carbonyl groups in this case. The data presented confirm that the increase in tan 6 during the irradiation of P E in air is connected with the formation of carbonyl groups b y oxidation. The presence of an antioxidant in LPPEst gives rise to less oxidation and consequently to a lower value of tan 6 during irradiation as compared with LPPEu,~t. The slight rise in tan 5 of P E during irradiation i n v a c u o is evidently connected with the specimen's becoming more amorphous, since it is known that dielectric losses in unirradiated P E are caused chiefly b y CO groups in the amorphous phase [8, 9]; the different proportions of the amorphous phase in L P P E (degree of crystallinity, 75%) and H P P E (50~) also give rise to corresponding differences in these materials in the value of tan 6 both for the initial and also for irradiated specimens. A special experiment with L P P E slowly cooled from the melt, having a degree of crystallinity (determined b y the X < a y method) of 90%, established a corresponding reduction in tan 6 in the case of specimens irradiated to various doses. We should dwell particularly on the break (at a radiation dose of 50-60 Mrad) which is found in the kinetics of the build-up of the carbonyl group concentration lan d xI0 ~ ~0
3...~
32 rand, tO•
Ii5 ,
1OF
--o---~-.-.~-~
o
FIG. 1
..xjx/×
6~ _
2L..
100 200 300 z400 500 600 700 800
Dose , Mpad
...~--
300
600
900
f200
Dose , Mead
FIG. 2
FIG. 1. Dependence of tan ~ on 7-radiation dose for various grades of P E (specimen thickness, d =0-7-0.8 mm): 1--LPPE~t, irradiated in air; 2,2'--LPPEunst irradiated in air and in vacuo respectively; 3 , 3 " - - H P P E irradiated in air and i n vacuo respectively; 4--rise in the concentration of CO groups (in relative units) in H P P E with irradiation dose in air. FIG. 2. Dependence of tan 6 on the dose of electron radiation for LPPEst in air. Irradiation dose rate, 7.3 × 10 4 rad/sec, d =0.7 ram.
in P E irradiated in air, and also on the break, associated with it, in the rate at which tan 6 increases (Fig. 1). I f this change is connected with a transition from oxidation kinetics limited b y the accumulation of free radicals to oxidation kinetics
3034
V.K. MATV~.¥~.Vet al.
limited by oxygen diffusion, then the initial rapid rise should be determined by the radiation dose, and the subsequent slow rise should be determined by the time of contact with oxygen. We carried out experiments on the irradiation of P E (LPPEst) by electrons (~ 1.3 MeV) in air at a high dose rate, 7.3 × 104 rad/sec, which is 300 times greater than for y-irradiation. The results shown in Fig. 2 show t h a t from the very start the increase in tan 5 is caused not so much by the radiation dose as by the time of contact with oxygen, i.e. the oxidation kinetics exhibit diffusion characteristics from the very start. The break on the curves shown in Fig. 1 is clearly connected with a transition from kinetics of oxidation of the specimen surface, limited by the function of oxygen within micropores, to kinetics limited by the diffusion within internal layers of the specimen. I t is thus surprising t h a t the oxidation of the surface tends to saturation for whatever cause. (The break cannot be explained by initial oxidation involving oxygen dissolved in the initial specimen, because of the low oxygen concentration as compared with the initial concentration of CO groups, and also because experiments with degassed and undegassed specimens gave identical results.) The diffusion characteristics of the kinetics of the radiation-chemical oxidation of PE, which causes oxidation only in the surface layers, should have a marked effect on the way in which tan 5 of PE irradiated in air depends on specimen thickness. However, no such dependence was observed in reference [1], in contradiction to other data [5]. We carried out experiments to study the effect of the thickness of the irradiated PE specimen on tan 5. The results are shown in Fig. 3. A comparison is made below between tan 5 and the CO group concentration for LPPEst specimens of various thicknesses, irradiated in air to a dose of 15 Mrad Specimen thickness, mm CO-group concentration, arbitrary units tan ~ × 10-4
0'88 0.72 0.55 0.25 0.30 0.40 3.8 4.5 '5.5
The results point to the existence of a specimen thickness effect, confirming the considerations put forward above and refuting the data of reference [1]. From the data shown in Figs. 1 and 3, we have derived an empirical formula connecting tan 5 for any type of PE with irradiation dose in air (D, Mrad), specimen thickness (d, mm) and with the degree of crystallinity (a, %): tan 5 ~ t a n 50-f-
0 . 0 0 1 + 4 × 10 -5 (lO0--a) ( 0-~8D) d °'25 • 1--e ,
where tan 50 is the initial value of tan 5. The difference between the experimental values and those calculated from the formula does not exceed 20%, which is the value of the experimental error. The formula is applicable to dose rates of the order of 200-300 rad/sec and has been checked for thicknesses from 0.4 to 3 mm. We have also checked the effect t h a t the time for which the irradiated P E specimens are held in air has on tan 5. The data shown in Fig. 4a, point to a gradual rise in tan 5 as specimens irradiated i n v a c u o beforehand at 15 Mrad are held
D i e l e c t r i c p r o p e r t i e s of c e r t a i n i r r a d i a t e d p o l y m e r s
3035
in air: in five months' storage, tan 6 of L P P E increases b y a factor of 3.6, the corresponding figure for H P P E being 2.2. Clearly, we are dealing with an extremely slow process of oxidation and migration of long-lived radicals from the crystaline phase into the amorphous phase. It should also be noted that the ratio of the effects involved in this increase of tan 5 for L P P E and H P P E corresponds to the ratio of their degrees of crystallinity. The considerations expressed above are also confirmed b y the data we obtained, which are shown in Fig. 4b: it follows from these that the effect involved in the increase of tan 6 when irradiated P E specimens have been stored in air for one year becomes greater as the preliminary radiation dose is increased only up to doses at which the concentration of free radicals in P E reaches a limiting value [10]. The data shown in Fig. 4b, were obtained b y heating specimens for 5 hr at 80-90°C directly after irradiation. i,./O~
jX~
>,/
~
x
-
tan 6", 10~
1
8f
a
Io
18
12
o
2 4 T i m e , months
6i
C I
0
;00
l
1
200 300 000 Dose , ,Hrad
Fro. 3
14
-
200
"
._ I
#00 600 Dose , tYnad
2• I
800
FzG. 4
FIG. 3. E f f e c t of t h e t h i c k n e s s o f t h e i r r a d i a t e d L P P E s t s p e c i m e n on t h e w a y in w h i c h t a n 6 d e p e n d s o n t h e y - i r r a d i a t i o n dose in air, for: 1 - - d = 0 . 4 2 ram, a n d 2 - - d = 0 . 9 8 r a m . FIG. 4. D e p e n d e n c e o f t a n 5 for a P E s p e c i m e n , i r r a d i a t e d i n vacuo, o n t h e t i m e of s t o r a g e in air. a: 1 - - H P P E , 2 - - L P P E ( d = 0 . 7 m m ) ; b: 1 - - L P P E s t t e s t e d one y e a r a f t e r i r r a d i a t i o n , 2 - - L P P E s t t e s t e d d i r e c t l y a f t e r i r r a d i a t i o n (d = 0 . 7 r a m ) .
I t follows from this that even heating of this type does not lead to annihilation of free radicals in the crystalline phase, which is in agreement with other work [11]. In addition to the investigation of the changes in tan 5 during the irradiation of PE, we also carried out similar experiments with P P (Fig. 5), PS and poly-amethylstyrene (Fig. 6). The data obtained correlate well with all the considerations put forward above in relation to PE: this gives evidence that there are analogous causes for the change in tan 6 during the irradiation of these polymers. The data shown in Fig. 5 point to corresponding effect of the stabilizer, specimen thickness and radiation environment on the change in tan 8 of P P with dose. The data shown in Fig. 6 also indicate that there is no effect of holding time in air for PS specimens after preliminary irradiation i n vacuo: this was to be expected
3036
V . K . MATVEYEVet al.
because of the absence of a n y crystalline p h a s e in a t a c t i c PS; this also e x p l a i n s t h e c o m p l e t e absence o f a n y change in t a n 5 of PS w i t h dose d u r i n g i r r a d i a t i o n iTb v a c u o .
T h e i r r a d i a t i o n of P S a n d p o l y - a - m e t h y l s t y r e n e which we carried o u t in air a t high dose r a t e s ( ~ 7 . 3 × 1 0 t rad/sec) in a n electron a c c e l e r a t o r (1.3 MeV) CHANGE
IN
tan $
FOR
POLYSTYRENE
POLY-6f-METHYLSTYREIqE
AND
DURING
IRRADIATIOI~
Polymer PS, tan 5x 10 -4 Poly-a-methylstyrene, tan 5 x 10 -4
50 4.6
-
6.8
Dose, Mrad 100 i 500
1500
5000
4.6
5"8
6.4
6"8
9.5
-
6"8
showed a slight rise in t a n 5 u p to a dose of ~ 1000 M r a d (Table): this is in agreem e n t w i t h o t h e r w o r k [6] a n d agrees w i t h considerations p u t f o r w a r d p r e v i o u s l y .
toncLfO'
ton cY. I0 ~ 20 F
32 2z~
~
2 1
lg 8
16 12 ~-
2
*
8 :
.
_
~4
-
3 4
-
-
-
a A
F
#0
120 Dose, M p a d
200
n
Fie. 5
10~:
200 300 Dose, Mnod
#00
5OO
Fie. 6
Fio. 5. PP effect of stabilizer and specimen thickness on the way in which tan ~ depend,~ on 7-radiation dose. 1--PPst (d=l-0 mm), 2--PPst (d=0.7 mm), 3--PP, free of stabilizer (d=0.7 mm), 4--PPst, irradiated i n v a c u o ( d = l ' 0 nun). FIG. 6. Dependence of tan efon 7-radiation dose (dose rate, 200-300 rad/sec, d =0.7-0-8 mm): 1--poly-~-methylstyrene, irradiated i n vacuo; 2, 3--PS irradiated in air and i n v a c u o respectively; 4--PS tested after the specimens had been stored one year in air. I n all t h e investigations which we m a d e , m e a s u r e m e n t s of t h e dielectric p e r m e a b i l i t y were also carried o u t a t t h e s a m e t i m e as t h e m e a s u r e m e n t of t a n 5; no changes within t h e limits of error were f o u n d a t a f r e q u e n c y of ~ 101° H z , in a g r e e m e n t w i t h the v e r y m u c h lower s e n s i t i v i t y of this q u a n t i t y (as c o m p a r e d w i t h t a n &) to dipole concentration. CONCLUSIONS
(1) A b r e a k has been f o u n d in the curves relating t a n 5 to 7-irradiation dose in air for v a r i o u s grades o f p o l y e t h y l e n e , p o l y p r o p y l e n e , p o l y s t y r e n e a n d poly-a-
Dielectric properties of certain irradiated polymers
3037
m e t h y l s t y r e n e . I n t h e case o f p o l y e t h y l e n e it has been confirmed, a n d for the remaining materials it has been shown, t h a t the increase in t a n J during irradiation in air is c o n n e c t e d w i t h the f o r m a t i o n of CO groups b y radiation-chemical oxidation. (2) I t has been shown t h a t radiation-chemical o x i d a t i o n has diffusion characteristics. This leads to the fact t h a t t h e increase in t a n J during irradiation in air is d e t e r m i n e d n o t so m u c h b y t h e radiation dose as b y the time t h a t the material is in c o n t a c t with oxygen. (3) An empirical f o r m u l a has been derived for various grades o f p o l y e t h y l e n e which, for dose rates o f 200-300 rad/sec, connects t a n J with the r a d i a t i o n dose in air, the specimen thickness a n d its degree of crystallinity. (4) I t has been shown t h a t t a n J for p o l y e t h y l e n e i r r a d i a t e d i n vacuo increases w i t h its t i m e o f storage in air. I t is suggested t h a t t h e increhse in t a n 5 is caused b y the oxidation o f free radicals migrating from t h e crystalline phase. Translated by G. F. MODLEN
REFERENCES
1. E. RUSHTON, Teehn. Rept. Electr. Res. Assoc. NL/T 377, 14 pp., ill., 1958 2. K. A. VODOP'YANOV, B. N. VOROZHTSOV, G. L POTAKHOVA and N. I. OL'SHANSKAYA, Elektrichestvo, No. 5, 60, 1960 3. H. SUKAKURA, J. Appl. Phys. Japan 2: 66, 1963 4. J. V. PASKALE, D. B. HERRMAN and R. I. MINERR, Mod. plastics 41: 239, 1963 5. B. I. SAZHIN, A. M. LOBANOV, A. L. GOL'DENBERG et al., Zh. tekhn, fiziki 28: 1991, 1958 6. A. CHARLESBY, Yadernyye izlucheniya i polimery (Atomic Radiation and Polymers). Forei~ql Lit. Publ. House, 1962 (Russian translation) 7. P. F. VESELOVSKII, Dissertation, 1965 8. G. P. MIKHAILOV, A. M. LOBANOV and B. I. SAZHIN, Zh. tekhn, fiziki 24: 1553, 1954 9. G. P. MIKHAILOV and T. L BORISOVA, Zh. tekhn, fiziki 23: 2159, 1953 10. A. T. KORITSKII, Yu. N. M/tIJINSKII, V. N. SHAMSHEV, P. N. BUBEN and V. V. VOYEVODSKII, Vysokomol. soyed. 1:1182, 1959 (Translated in Polymer Sci. U.S.S.R. 1: 3, 458, 1960)