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Stability of the dielectric behaviour of polymeric films formed in a glow discharge
Stability of dielectric behaviour of pol~Tneric films
961
REFERENCES
1. S. S. CHANG, J. A. HORMAN and A. B. BESTUL, J. ~es. l~at. Bur. Standards A71: 293, 1967 2. B. V. LEBEDEV, I. B. RABINOVICH and V. A. BUDARINA, Vysokomol. soyed. A9: 488, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 3, 545, 1967) 3. M. M. POP0V and V. P. KOLESOV, Zh. obshch, khimii 26: 2385, 1956 4. P. N. NIKOLAYEV and B. V. LEBEDEV, Trudy po khimii i khimicheskoi tekhnologii (Trans. Chem. and Chem. Technol.) Gor'kii, No. 2, 332, 1966 5. M. M. POPOV, Zh. obshch, khimii 21: 2220, 1951 6. S. AFORD and M. DOLE, J. Amer. Chem. Soc. 77: 4774, 1955 7. M. R. CARPENTER, D. B. DAVIES and A. J. MATHESON, J. Chem. Phys. 46: 2451, 1967 8. P. N. NIKOLAYEV, I. B. RABINOVICH and B. V. LEBEDEV, Zh. fiz. khimii 41: 1294, 1967
STABILITY 0 F THE DIELECTRIC BEHAVIOUR OF POLYMERIC FILMS FORMED IN A GLOW DISCHARGE * L. S. Tuzov, V. M. KOLOTYRKII~and N. N. TUNITSKII L. Ya. Karpov Physicochemical Institute (Received 24 March 1969) POLYMERIC films formed in a glow discharge and during electron bombardment of the film surface possess good dielectric properties making them suitable for use as insulators in thin film circuits [1-4]. However, the stability of the dielectric properties of films, which is a subiect of great practical importance, has barely attracted the attention of investigators. At the same time it was found [3, 5] t h a t changes occurred in air in the dielectric behaviour of films formed in a glow discharge and during electron bombardment; moreover the changes in question depended to a considerable extent on the method of film preparation [6, 7]. Mann studied the ageing of films obtained by bombarding film surfaces with electrons in an atmosphere of silicone oil vapour and found t h a t the dielectric constant, e and also the dielectric loss angle tan J were higher in air [5]. According to Mann these changes were due to the absorption of water vapour by the film, but no detailed investigation of this was carried out. Similar results were obtained by authors [3] studying the ageing of films obtained from hexamethyldisiloxane (HMDS) in a glow discharge. Here also the authors found t h a t polar oxygen-containing groups appeared in the films [3] owing to the reaction of free radicals with atmospheric oxygen, and it was thought t h a t changes in and t a n J in air migh~ be related to the appearance of these groups. * Vysokomol. soyed. A12: No. 4, 849-854, 1970.
962
L.S. Tuzov etal.
I n this p a p e r we are r e p o r t i n g on a more detailed s t u d y of the reasons underlying the unstable b e h a v i o u r o f films in air, t o g e t h e r with possible m e t h o d s o f stabilizing t h e m , t a k i n g as an example p o l y m e r i c films o b t a i n e d f r o m H M D S in a glow discharge. EXPERIMENTAL Experiments in the preparation of polymeric films in a glow discharge were carried out at room temperature under dynamic conditions using the apparatus described in reference [3]. The pressure in the working chamber was measured with a specially designed manometer [8] and was varied within limits of 0-05-0-35 mm, with HI~DS partial pressure not exceeding 0.1 ram. The current density of the discharge was varied from 0.4 to 1-5 mA]cm I, but th~ current was kept constant during each experiment. The combustion voltage of the discharge did not exceed ll00V. The discharge was fed by a ZG-12 generator with a power amplifier (working frequency 20 ke/s) or by an ISN-1 source of stabilized voltage (frequency 1 kc/s). Polished stainless steel plates, rolled aluminium, and glass containing a thermally sprayed layer of aluminium were used as substrates. The film thickness was measured with an MII-4 microinterferometer [9] or by the capacitance method [9]. The measuring error by the first method was N 15~, and considerably more than this with the second method, as it was found that the dielectric constant of the film depends on its thicknes. For this reason the latter method was generally used only to determine the film thickness. A VUP-2 apparatus (N 10-~ mm vacuum) was used to measure the temperature dependence of capacitance c and tan 8. The capacitance measurements were carried out on a special indicator with a doubly coordinated PDS-021 recorder attached to ensure simultaneous recording of capacitance and temperature. The measurements of tan ~ were carried oub at a frequency of 1 kc/s on a UM-3 general-purpose measuring bridge which was intermittently connected. The thermal degradation of the films was studied under isothermal conditions under vacuum with constant evacuation by means of a silica balance using film portions weighing not more than 10-12 mg. The readings were taken from a KIWI-6 cathetometer, accurate to 1 × 10-~ mm, corresponding to a change in weight amounting to 1.1 × 10 -5 g. DISCUSSION OF RESULTS
To ascertain the v a l i d i t y of the a s s u m p t i o n t h a t changes in e a n d in t a n were due to the presence o f polar o x y g e n - c o n t a i n i n g g r o u p s in the films we studied t h e effect of a d d i n g different gases d u r i n g t h e process of film p r e p a r a t i o n , as well as the effect of t h e r m a l processing o f the p r o d u c t films, in relation to changes in 8 a n d t a n ~. To do so H M D S films were p r e p a r e d with the i n t r o d u c t i o n of gases (At, Iq~, 02 a n d H2) into the discharge zone, a n d t h e g r o w t h rate of t h e films was measured, a n d e a n d t a n ~ were d e t e r m i n e d for the films stored u n d e r different conditions. Figure 1 a n d Table 1 show h o w the g r o w t h rate of the films is affected b y the i n t r o d u c t i o n of gases. I t i.s seen f r o m these d a t a t h a t t h e g r o w t h rate is c o n s t a n t for each m i x t u r e (except in t h e initial stages of the process), a n d it is m u c h reduced in the presence of all t h e gases used in the experiments. W e k n o w from a previous investigation [3] based on gass-spectroscopic analysis of t h e gas phase of the discharge, t h a t the i n t r o d u c t i o n o f a r g o n into the discharge zone causes a higher r a t e o f toluene d e c o m p o s i t i o n a c c o m p a n i e d b y t h e evolution of
Stability of dielectric behaviour of polymeric films
963
large amounts of low molecular weight substances, mainly hydrogen. This same effect probably occurs in the case of the HMDS molecules in the presence of the gases used by us. If this is so, it will mean that in the stationary state, represented by the rectilinear portion of the curve of film growth vs. time, the partial pressure of the initial substance in the discharge chamber must differ consider-
1/c x 10a 5 z
!
!
20OO1
I
I
0"5
I
I'0 1"5 Time, rnin
!
2.0
FIG. 1. Film thickness d (2, 4, 5) and inverse capacitance c -z (1, 3) versus time; dynamic method. Content of Ar in initial gas mixture: 1, 2--75; 3, 4--50; 5 - - 7 5 ~ (in the process of film preparation the gas phase was completely changed several times to m a i n t a i n the original HMDS partial pressure, curve 5).
ably from its partial pressure in the initial mixture; this will reduce the concentration of HMDS molecules in the sorption layer, with the result that the growth rate will also be reduced. The validity of this assumption is proved by the result of an experiment which was carried out in such a way that the partial HMDS pressure in the mixture remained approximately constant during the entire process of film preparation (Fig. 1, curve 5). Note that the reduction in the rate of film growth vs. time in the early stages of the process could also be due to reduction in the partial pressure of the vapour of ~he initial substance during this period of time. TABLE
1. G R O W T H RATE OF FILI~IS I N RELATION TO THE COMPOSITION OF THE GAS PtL%SE
Composition of gas phase,
% HMI)S (pure) Ar--50 75 N=-- 50 75
-~tot~l
Ppartial
R,
mmHg
mmHg
A/sec
0"04 0"15 0"25 0"16 0"2
0"04 0"075 0-06 0"08 0"05
55 28 13 37 17
Composition of gas phase,
% O2-- 50 75 H=-- 50 75
Rp
P~tal,
"~par tia],
mmHg
mriLKg
A/sec
0"16 0"23 0"13 0"16
0-08 0"057 0"065 0-04
35 21 28 20
No~: Ptotal is the Initial total pressure in the gas mixture; Pparual Is the Initial partial ~ 4 D S in the gas mixture; R is the rate of film ~ o w t h .
964
L.S. Tvzov et al.
Using these d a t a we d e t e r m i n e d t h e dielectric c o n s t a n t o f the films a n d its relation to t h e film thickness (Fig. 2). I t is seen t h a t increase in film thickness is a c c o m p a n i e d b y a m a r k e d rise in e, a n d no a d e q u a t e e x p l a n a t i o n for this has y e t been found. TABLE
2. E F F E C T
OF T H E GAS P H A S E COM:PO-
SITIO~T D U R I N G FIL~-~[ PREPA.RATIOIq 01~TC H A ~ G E S I ~ CA-PACITA~CE I1~" T H E ~,VET C H A M B E R UI~DER S T E A D Y - S T A T E CONDITIONS Composition
of
gas phase, ~ HMDS (pure) AT--50 75 0,--50
d,/~ ~5000 ~4000 N4100 3000-5000 N4000 3000-5000
N.--50
H2--50
c, 130 145 195 165 140 117
A s t u d y of the stability o f the films based on m e a s u r e m e n t of their dielectric c o n s t a n t showed t h a t in t h e absence o f w a t e r v a p o u r t h e r e is practically no change in ~, b u t the dielectric c o n s t a n t in air rises b y N 3 0 ~ , the rise in e being
200 150
2
I00 I
1000
3O00 :FIG. 2
d,A
5000
5 I
l
I f5' z 7Zm%d~s 2 J T i m e , hn (~) /0
:FIG. 3
FZG. 2. Dependence of e on film thickness. Content, ~o: 1--HM~DS---25, lq,--75; 2--]~MI)S--
25, AT--75; 3--HMDS--50, O,--50; 4--HlVIDS--50, AT--50. FIG. 3. Dependence ofz on time of keeping in dry chamber (1), in air (2), and in wet chamber (3) for films obtained from a gas mixture with an Ar content of 75 Yo; change in e in the course of evacuation (4) p a r t i c u l a r l y m a r k e d in a w e t chamber, where it reaches N 2 0 0 ~ (Fig. 3). T h e reversible n a t u r e of these changes is a p p a r e n t f r o m t h e f a c t t h a t on e v a c u a t i n g t h e sample for ~ 3 hr (see Fig. 3, curve d) its dielectric c o n s t a n t reverts to t h e original value. Similar changes were observed for t a n $ also. I n t h e wet c h a m b e r the value o f t a n 6 f r e q u e n t l y rose f r o m 6-8 × 10 -8 t o 1 × 10 -1 a n d e v e n higher, this change being similarly o f a reversible n a t u r e .
Stability of dielectric behaviour of polymeric fllrrLq
965
Table 2 gives t h e d a t a for changes, in t h e w e t chamber, in the capacitance of films o b t a i n e d f r o m H M D S in t h e presence o f Ar, N2, 02 a n d H 2. I t will be seen t h a t t h e greatest changes in c a p a c i t a n c e occur on i n t r o d u c i n g 02, Ar a n d N2 into t h e discharge zone, a n d the smallest changes are f o u n d with the i n t r o d u c t i o n of H2. A similar effect was n o t e d in a s t u d y [6] o f t h e ageing of films o b t a i n e d c,%
"1
tan#
400
~09
300
0.05
/
~01
200
r
Z
e
1.1 3
I00 I
5
I
p
~B
I
10 15 Time, days
10o
300
:FIo. 4
5O0 T,°C
PIe. 5
:FIG. 4. Dependence of capacitance of films obtained from HMDS (HI~)S-- 25, Ar-- 75 Yo) on time of keeping in wet chamber: /--after thermal treatment in air at N350 ° for N 1 hr; 2--without thermal treatment; 3--after thermal tJreatment at N1 × 10-4 mm and N350 ° for ~1 hr. :FIG. 5. Temperature dependence of tan J (1) and e (2) for films, measured under vacuum ( ~ 1 × 10 -4 m m ) .
f r o m s t y r e n e with the i n t r o d u c t i o n of 03, lq2, a n d H~; in this p a p e r [6] the changes in capacitance are a t t r i b u t e d to the possible emergence of polar g r o u p s in the film with the i n t r o d u c t i o n o f 03 a n d N2. TABLE 3. ELEMENTAL COMPOSITION OF t t M D S
Method of preparation and subsequent treatment H_~DS (pure) Ar--75~o Ar--75 ~o, heat treatment in vacuo A.r--75 Yo heat treatment in air H2--5Oyo
rrLMs
Found, Yo Si 38.26 36.05 40.36 40.32 41.96
H 33.52 26.92 27.35 14.15 28.02
0 (ealc)
O*
24.75 32.72 27-87 42.95 25.16
21.59 16.35 31.46 13.16
3"47
4.31 4.42 2-58 4-86 \
13.75
/
* The oxygen content of the samples is given after deducting the amount of oxygen in the -- Si-- O-- Sl-- group.
/
\
I t is also a p p a r e n t f r o m analysis of t h e d a t a in Table 2 t h a t t h e change in is identical in respect to films o b t a i n e d u n d e r identical conditions b u t v a r y i n g in thickness. This m e a n s t h a t t h e a b s o r p t i o n of w a t e r v a p o u r proceeds b y a v o l u m e process as distinct f r o m surface absorption.
966
L . S . T u z o v et ¢~.
A study was made of the change in ~ in relation to thermal treatment of the films following their preparation. Figure 4 shows plots of capacitance vs. time of keeping in the wet chamber for film samples immediately after their preparation (curve 2), and also after thermal treatment under vacuum at 350 ° (curve 3), and in air at the same temperature (curve 1). We know that thermal treatment of films under vacuum leads to the destruction of free radicals [3] and to thermal degradation, the latter process involving the removal of the oxygen-containing polar groups that have already appeared in the film [10], while on the other hand in the case of thermal treatment in air the formation of polar groups is due to oxidation of the films [11]. This was confirmed b y elemental chemical micro-analysis of the film samples (see Table 3). I t will be seen that the oxygen content is higher in those samples which showed considerable changes in capacitance in the wet chamber. These data therefore indicate that the greatest change in e is found with the polar films, and the smallest one with the nonpolar films, while the values of e measured for the same film samples in the absence of water differ considerably from one another. The weak dependence of ~ on film polarity in the absence of water m a y be explained b y the fact that films formed in a glow discharge are highly crosslinked polymers in which the orientation of polar groups is greatly inhibited in an electric field. This is also confirmed b y the data obtained b y studying the temperature dependence of e and tan & On the other hand the big difference in the values of s and tan ~ for films of different polarity, observed in a humid atmosphere, is due to their sorption capacities in respect to water, and also to the high dielectric constant of water [12, 13]. The temperature dependence of e and tan ~ was studied i n vacuo over a wide temperature range (500---190°). The results obtained for the interval 0-500 ° are presented in Fig. 5. It was found that in the region of 0- --190 ° t a n ~ and 8 are temperature-independent. As is seen from Fig. 5, there is a reduction in 8 at ~ 200 °, while at N 400 ° there is a slight rise. This change is probably due to structural transitions in the film, and must be related to the recombination of free radicals and to thermal degradation taking place in this temperature region, particularly as during the cooling of the sample there is a monotonic change in e, b u t during repeated heating there is a change in e corresponding to the cooling curve.
The constant values ofs over a wide temperature range (excluding the region of structural changes at 200-400 °) confirms the assumption already made as to the absence of orientation of polar groups in an electric field. A marked rise in tan ~ starts at approximately 150 °, and the same monotonic rise is observed whether this curve is plotted during the heating or cooling of the sample. The rise in tan ~ is attributable to increased losses due to rise in the electrical conductivity of the films [7]. The thermal degradation of the films was inves/dgated b y the thermogravimetric method on the basis of weight loss in the region of 20-600 °. The measure-
Stability of dielectric behaviour of polymeric films
967
ments were carried out in a vacuum ( ~ 1 × 10-4ram). Figure 6 shows the thermal degradation curves for two film samples obtained with different discharge currents. These data show t h a t the thermal degradation of the films is strongly zim,% 15
I0 5
I~.....,.~ o
O
2 400
800
T,°C Fro. 6. Temperature dependence of weight loss ztrn for films of HMDS (50~) and Ar (50~) with current densities of the discharge equal to 0.4 (1) and 1.3 mA/cm I (2}. dependent on the method of film preparation: in the case of the sample obtained with a low current density (0.4 mA/cm z) the thermal degradation starts at ~ 200 °, and at 550 ° the weight loss amounts to ~ 13%, while the thermal degradation of the sample obtained with a current density of 1.3 mA/cm ~ starts at 300°, and the maximum loss of weight even at 600 ° is not more than 2 . 5 ~ . These results m a y be explained by the fact t h a t the films obtained with high current densities have a higher degree of crosslinking and the number of siloxane bonds in the polymer is relatively higher, as was shown by I R analysis in reference [14]. CONCLUSIONS
(1) I t has been shown t h a t the reduced growth rate of hexamethyldisiloxane (HM-DS) films observed on introducing At, 0~, N2 and H2 in the discharge zone is due to decrease in the HMDS partial pressure owing to the higher degree of decomposition of the starting material in the gas phase. (2) I t has been shown t h a t changes in e and tan @ in air are due to water sorption by HMDS films, and the sorption increases with rise in the film polarity. The most stable films are obtained on introducing H2 into the discharge zone, or through subsequent thermal t r e a t m e n t of the films under vacuum. (3) The thermal degradation of the films has been studied by the thermogravimetric method in the temperature interval 20-600 °. Translated by R. J. A. ]~ENDRY REFERENCES
1. R. CHRISTY, J. AppI. Phys. 31: 1680, 1960 2. A. K. TSAPUK and V. M. KOLOTYRKIN,Vysokomol. soyed. 7: 1802, 1965 (Translated in Polymer Sci. U.S.S.R. 7: 10, 1985, 1965) 3. L. S. TUZOV, A. B. GIL'MAN, A. N. SHCHUROV and V. M. KOLOTYRKIN,Vysokomo]. soyed. Ag: 2414, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 11, 2728, 1967)
968
M. I. MUSTAFAYEVet al.
4. D. A. BUCK and K. R. SHOULDERS, Proc. of the Eastern Joint Computer Cozlf. 1958, p. 55, Philadelphia, N. Y., 1959 5. H. T. MANN, J. Appl. Phys. 35: 2173, 1964 6. M. STUART, Proc. I E E 112: 1614, 1965 7. A. N. SHCHUROV, L. 8. TUZOV, A. B. GIL'MAN, V. M. KOLOTYRK~ and N. N. TUNITSKII, Vysokomol. soyed. A l l : 582, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 3, 660, 1969) 8. N. G. ALEXEYEV, V. A. PROKHOROV and K. V. CHlVIUTOV, Elektronnye pribory i skhcmy v fiziko-khimicheskom issledovanii (Electronic Apparatus and Systems in Physicochemical Investigations). Goskhimizdat, p. 430, 1961 9. S. METFESSEL, Tonkie plenki, ikh izgotovlenie i izmerenie (Thin Films, their Preparation and Measurement). Gosenergoizdat, 1963 10. B. V. TKACHUK, V. M. KOLOTYRKIN and G. G. KIREI, Vysokomol. soyed. A1O: 585, 1968 (Translated in Polymer Sci. U.S.S.R. 10: 3, 683, 1968) 11. B. V. TKACHUK, Thesis, 1968 12. I. M. MAIOFIS, Osnowy khimii dielektrikov (Basis of Dielectric Chemistry). Izd. "Vysshaya shkola", 1963 13. M. M. MIKHAILOV, Vlagopronitsayemost' organicheskikh dielektrikov (Moisturepermeability of Organic Dielectrics). Gosenergoizdat, 1960 14. B. V. TKACHUK, V. V. BUSHIN, V. M. KOLOTYRKIN and N. P. SMETANKINA, Vysokomol. soyed. A9: 2018, 1967 (Translated iu Polymer Sci. U.S.S.R. 9: 9, 2281, 1964)
MOLECULAR WEIGHTS AND MOLECULAR WEIGHT DISTRIBUTION OF THE PRODUCTS OF SPONTANEOUS POLYMERIZATION OF QUATERNARY SALTS OF 4-VINYLPYRIDINE AND ETHYL BROMIDE IN BENZENE AS MEDIUM* M. I. MUSTAFAYEV, K. V . / b x ~ v and V. A. KABA~OV A. V. Topchiev Institute of Petrochemical Synthesis, U.S.S.R. Academy of Sciences M. V. Lomonosov Moscow State University (Received 26 Maroh 1969)
IT WAS shown in reference [1] that 4-vinylpyridine (VP) polymerizes spontaneously, when alkylated with alkyl halides (AH), to form polymeric quaternary salts of the general formula:
(I) * Vysokomol. soyed. AI2: No. 4, 855-864, I970.
961
REFERENCES
1. S. S. CHANG, J. A. HORMAN and A. B. BESTUL, J. ~es. l~at. Bur. Standards A71: 293, 1967 2. B. V. LEBEDEV, I. B. RABINOVICH and V. A. BUDARINA, Vysokomol. soyed. A9: 488, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 3, 545, 1967) 3. M. M. POP0V and V. P. KOLESOV, Zh. obshch, khimii 26: 2385, 1956 4. P. N. NIKOLAYEV and B. V. LEBEDEV, Trudy po khimii i khimicheskoi tekhnologii (Trans. Chem. and Chem. Technol.) Gor'kii, No. 2, 332, 1966 5. M. M. POPOV, Zh. obshch, khimii 21: 2220, 1951 6. S. AFORD and M. DOLE, J. Amer. Chem. Soc. 77: 4774, 1955 7. M. R. CARPENTER, D. B. DAVIES and A. J. MATHESON, J. Chem. Phys. 46: 2451, 1967 8. P. N. NIKOLAYEV, I. B. RABINOVICH and B. V. LEBEDEV, Zh. fiz. khimii 41: 1294, 1967
STABILITY 0 F THE DIELECTRIC BEHAVIOUR OF POLYMERIC FILMS FORMED IN A GLOW DISCHARGE * L. S. Tuzov, V. M. KOLOTYRKII~and N. N. TUNITSKII L. Ya. Karpov Physicochemical Institute (Received 24 March 1969) POLYMERIC films formed in a glow discharge and during electron bombardment of the film surface possess good dielectric properties making them suitable for use as insulators in thin film circuits [1-4]. However, the stability of the dielectric properties of films, which is a subiect of great practical importance, has barely attracted the attention of investigators. At the same time it was found [3, 5] t h a t changes occurred in air in the dielectric behaviour of films formed in a glow discharge and during electron bombardment; moreover the changes in question depended to a considerable extent on the method of film preparation [6, 7]. Mann studied the ageing of films obtained by bombarding film surfaces with electrons in an atmosphere of silicone oil vapour and found t h a t the dielectric constant, e and also the dielectric loss angle tan J were higher in air [5]. According to Mann these changes were due to the absorption of water vapour by the film, but no detailed investigation of this was carried out. Similar results were obtained by authors [3] studying the ageing of films obtained from hexamethyldisiloxane (HMDS) in a glow discharge. Here also the authors found t h a t polar oxygen-containing groups appeared in the films [3] owing to the reaction of free radicals with atmospheric oxygen, and it was thought t h a t changes in and t a n J in air migh~ be related to the appearance of these groups. * Vysokomol. soyed. A12: No. 4, 849-854, 1970.
962
L.S. Tuzov etal.
I n this p a p e r we are r e p o r t i n g on a more detailed s t u d y of the reasons underlying the unstable b e h a v i o u r o f films in air, t o g e t h e r with possible m e t h o d s o f stabilizing t h e m , t a k i n g as an example p o l y m e r i c films o b t a i n e d f r o m H M D S in a glow discharge. EXPERIMENTAL Experiments in the preparation of polymeric films in a glow discharge were carried out at room temperature under dynamic conditions using the apparatus described in reference [3]. The pressure in the working chamber was measured with a specially designed manometer [8] and was varied within limits of 0-05-0-35 mm, with HI~DS partial pressure not exceeding 0.1 ram. The current density of the discharge was varied from 0.4 to 1-5 mA]cm I, but th~ current was kept constant during each experiment. The combustion voltage of the discharge did not exceed ll00V. The discharge was fed by a ZG-12 generator with a power amplifier (working frequency 20 ke/s) or by an ISN-1 source of stabilized voltage (frequency 1 kc/s). Polished stainless steel plates, rolled aluminium, and glass containing a thermally sprayed layer of aluminium were used as substrates. The film thickness was measured with an MII-4 microinterferometer [9] or by the capacitance method [9]. The measuring error by the first method was N 15~, and considerably more than this with the second method, as it was found that the dielectric constant of the film depends on its thicknes. For this reason the latter method was generally used only to determine the film thickness. A VUP-2 apparatus (N 10-~ mm vacuum) was used to measure the temperature dependence of capacitance c and tan 8. The capacitance measurements were carried out on a special indicator with a doubly coordinated PDS-021 recorder attached to ensure simultaneous recording of capacitance and temperature. The measurements of tan ~ were carried oub at a frequency of 1 kc/s on a UM-3 general-purpose measuring bridge which was intermittently connected. The thermal degradation of the films was studied under isothermal conditions under vacuum with constant evacuation by means of a silica balance using film portions weighing not more than 10-12 mg. The readings were taken from a KIWI-6 cathetometer, accurate to 1 × 10-~ mm, corresponding to a change in weight amounting to 1.1 × 10 -5 g. DISCUSSION OF RESULTS
To ascertain the v a l i d i t y of the a s s u m p t i o n t h a t changes in e a n d in t a n were due to the presence o f polar o x y g e n - c o n t a i n i n g g r o u p s in the films we studied t h e effect of a d d i n g different gases d u r i n g t h e process of film p r e p a r a t i o n , as well as the effect of t h e r m a l processing o f the p r o d u c t films, in relation to changes in 8 a n d t a n ~. To do so H M D S films were p r e p a r e d with the i n t r o d u c t i o n of gases (At, Iq~, 02 a n d H2) into the discharge zone, a n d t h e g r o w t h rate of t h e films was measured, a n d e a n d t a n ~ were d e t e r m i n e d for the films stored u n d e r different conditions. Figure 1 a n d Table 1 show h o w the g r o w t h rate of the films is affected b y the i n t r o d u c t i o n of gases. I t i.s seen f r o m these d a t a t h a t t h e g r o w t h rate is c o n s t a n t for each m i x t u r e (except in t h e initial stages of the process), a n d it is m u c h reduced in the presence of all t h e gases used in the experiments. W e k n o w from a previous investigation [3] based on gass-spectroscopic analysis of t h e gas phase of the discharge, t h a t the i n t r o d u c t i o n o f a r g o n into the discharge zone causes a higher r a t e o f toluene d e c o m p o s i t i o n a c c o m p a n i e d b y t h e evolution of
Stability of dielectric behaviour of polymeric films
963
large amounts of low molecular weight substances, mainly hydrogen. This same effect probably occurs in the case of the HMDS molecules in the presence of the gases used by us. If this is so, it will mean that in the stationary state, represented by the rectilinear portion of the curve of film growth vs. time, the partial pressure of the initial substance in the discharge chamber must differ consider-
1/c x 10a 5 z
!
!
20OO1
I
I
0"5
I
I'0 1"5 Time, rnin
!
2.0
FIG. 1. Film thickness d (2, 4, 5) and inverse capacitance c -z (1, 3) versus time; dynamic method. Content of Ar in initial gas mixture: 1, 2--75; 3, 4--50; 5 - - 7 5 ~ (in the process of film preparation the gas phase was completely changed several times to m a i n t a i n the original HMDS partial pressure, curve 5).
ably from its partial pressure in the initial mixture; this will reduce the concentration of HMDS molecules in the sorption layer, with the result that the growth rate will also be reduced. The validity of this assumption is proved by the result of an experiment which was carried out in such a way that the partial HMDS pressure in the mixture remained approximately constant during the entire process of film preparation (Fig. 1, curve 5). Note that the reduction in the rate of film growth vs. time in the early stages of the process could also be due to reduction in the partial pressure of the vapour of ~he initial substance during this period of time. TABLE
1. G R O W T H RATE OF FILI~IS I N RELATION TO THE COMPOSITION OF THE GAS PtL%SE
Composition of gas phase,
% HMI)S (pure) Ar--50 75 N=-- 50 75
-~tot~l
Ppartial
R,
mmHg
mmHg
A/sec
0"04 0"15 0"25 0"16 0"2
0"04 0"075 0-06 0"08 0"05
55 28 13 37 17
Composition of gas phase,
% O2-- 50 75 H=-- 50 75
Rp
P~tal,
"~par tia],
mmHg
mriLKg
A/sec
0"16 0"23 0"13 0"16
0-08 0"057 0"065 0-04
35 21 28 20
No~: Ptotal is the Initial total pressure in the gas mixture; Pparual Is the Initial partial ~ 4 D S in the gas mixture; R is the rate of film ~ o w t h .
964
L.S. Tvzov et al.
Using these d a t a we d e t e r m i n e d t h e dielectric c o n s t a n t o f the films a n d its relation to t h e film thickness (Fig. 2). I t is seen t h a t increase in film thickness is a c c o m p a n i e d b y a m a r k e d rise in e, a n d no a d e q u a t e e x p l a n a t i o n for this has y e t been found. TABLE
2. E F F E C T
OF T H E GAS P H A S E COM:PO-
SITIO~T D U R I N G FIL~-~[ PREPA.RATIOIq 01~TC H A ~ G E S I ~ CA-PACITA~CE I1~" T H E ~,VET C H A M B E R UI~DER S T E A D Y - S T A T E CONDITIONS Composition
of
gas phase, ~ HMDS (pure) AT--50 75 0,--50
d,/~ ~5000 ~4000 N4100 3000-5000 N4000 3000-5000
N.--50
H2--50
c, 130 145 195 165 140 117
A s t u d y of the stability o f the films based on m e a s u r e m e n t of their dielectric c o n s t a n t showed t h a t in t h e absence o f w a t e r v a p o u r t h e r e is practically no change in ~, b u t the dielectric c o n s t a n t in air rises b y N 3 0 ~ , the rise in e being
200 150
2
I00 I
1000
3O00 :FIG. 2
d,A
5000
5 I
l
I f5' z 7Zm%d~s 2 J T i m e , hn (~) /0
:FIG. 3
FZG. 2. Dependence of e on film thickness. Content, ~o: 1--HM~DS---25, lq,--75; 2--]~MI)S--
25, AT--75; 3--HMDS--50, O,--50; 4--HlVIDS--50, AT--50. FIG. 3. Dependence ofz on time of keeping in dry chamber (1), in air (2), and in wet chamber (3) for films obtained from a gas mixture with an Ar content of 75 Yo; change in e in the course of evacuation (4) p a r t i c u l a r l y m a r k e d in a w e t chamber, where it reaches N 2 0 0 ~ (Fig. 3). T h e reversible n a t u r e of these changes is a p p a r e n t f r o m t h e f a c t t h a t on e v a c u a t i n g t h e sample for ~ 3 hr (see Fig. 3, curve d) its dielectric c o n s t a n t reverts to t h e original value. Similar changes were observed for t a n $ also. I n t h e wet c h a m b e r the value o f t a n 6 f r e q u e n t l y rose f r o m 6-8 × 10 -8 t o 1 × 10 -1 a n d e v e n higher, this change being similarly o f a reversible n a t u r e .
Stability of dielectric behaviour of polymeric fllrrLq
965
Table 2 gives t h e d a t a for changes, in t h e w e t chamber, in the capacitance of films o b t a i n e d f r o m H M D S in t h e presence o f Ar, N2, 02 a n d H 2. I t will be seen t h a t t h e greatest changes in c a p a c i t a n c e occur on i n t r o d u c i n g 02, Ar a n d N2 into t h e discharge zone, a n d the smallest changes are f o u n d with the i n t r o d u c t i o n of H2. A similar effect was n o t e d in a s t u d y [6] o f t h e ageing of films o b t a i n e d c,%
"1
tan#
400
~09
300
0.05
/
~01
200
r
Z
e
1.1 3
I00 I
5
I
p
~B
I
10 15 Time, days
10o
300
:FIo. 4
5O0 T,°C
PIe. 5
:FIG. 4. Dependence of capacitance of films obtained from HMDS (HI~)S-- 25, Ar-- 75 Yo) on time of keeping in wet chamber: /--after thermal treatment in air at N350 ° for N 1 hr; 2--without thermal treatment; 3--after thermal tJreatment at N1 × 10-4 mm and N350 ° for ~1 hr. :FIG. 5. Temperature dependence of tan J (1) and e (2) for films, measured under vacuum ( ~ 1 × 10 -4 m m ) .
f r o m s t y r e n e with the i n t r o d u c t i o n of 03, lq2, a n d H~; in this p a p e r [6] the changes in capacitance are a t t r i b u t e d to the possible emergence of polar g r o u p s in the film with the i n t r o d u c t i o n o f 03 a n d N2. TABLE 3. ELEMENTAL COMPOSITION OF t t M D S
Method of preparation and subsequent treatment H_~DS (pure) Ar--75~o Ar--75 ~o, heat treatment in vacuo A.r--75 Yo heat treatment in air H2--5Oyo
rrLMs
Found, Yo Si 38.26 36.05 40.36 40.32 41.96
H 33.52 26.92 27.35 14.15 28.02
0 (ealc)
O*
24.75 32.72 27-87 42.95 25.16
21.59 16.35 31.46 13.16
3"47
4.31 4.42 2-58 4-86 \
13.75
/
* The oxygen content of the samples is given after deducting the amount of oxygen in the -- Si-- O-- Sl-- group.
/
\
I t is also a p p a r e n t f r o m analysis of t h e d a t a in Table 2 t h a t t h e change in is identical in respect to films o b t a i n e d u n d e r identical conditions b u t v a r y i n g in thickness. This m e a n s t h a t t h e a b s o r p t i o n of w a t e r v a p o u r proceeds b y a v o l u m e process as distinct f r o m surface absorption.
966
L . S . T u z o v et ¢~.
A study was made of the change in ~ in relation to thermal treatment of the films following their preparation. Figure 4 shows plots of capacitance vs. time of keeping in the wet chamber for film samples immediately after their preparation (curve 2), and also after thermal treatment under vacuum at 350 ° (curve 3), and in air at the same temperature (curve 1). We know that thermal treatment of films under vacuum leads to the destruction of free radicals [3] and to thermal degradation, the latter process involving the removal of the oxygen-containing polar groups that have already appeared in the film [10], while on the other hand in the case of thermal treatment in air the formation of polar groups is due to oxidation of the films [11]. This was confirmed b y elemental chemical micro-analysis of the film samples (see Table 3). I t will be seen that the oxygen content is higher in those samples which showed considerable changes in capacitance in the wet chamber. These data therefore indicate that the greatest change in e is found with the polar films, and the smallest one with the nonpolar films, while the values of e measured for the same film samples in the absence of water differ considerably from one another. The weak dependence of ~ on film polarity in the absence of water m a y be explained b y the fact that films formed in a glow discharge are highly crosslinked polymers in which the orientation of polar groups is greatly inhibited in an electric field. This is also confirmed b y the data obtained b y studying the temperature dependence of e and tan & On the other hand the big difference in the values of s and tan ~ for films of different polarity, observed in a humid atmosphere, is due to their sorption capacities in respect to water, and also to the high dielectric constant of water [12, 13]. The temperature dependence of e and tan ~ was studied i n vacuo over a wide temperature range (500---190°). The results obtained for the interval 0-500 ° are presented in Fig. 5. It was found that in the region of 0- --190 ° t a n ~ and 8 are temperature-independent. As is seen from Fig. 5, there is a reduction in 8 at ~ 200 °, while at N 400 ° there is a slight rise. This change is probably due to structural transitions in the film, and must be related to the recombination of free radicals and to thermal degradation taking place in this temperature region, particularly as during the cooling of the sample there is a monotonic change in e, b u t during repeated heating there is a change in e corresponding to the cooling curve.
The constant values ofs over a wide temperature range (excluding the region of structural changes at 200-400 °) confirms the assumption already made as to the absence of orientation of polar groups in an electric field. A marked rise in tan ~ starts at approximately 150 °, and the same monotonic rise is observed whether this curve is plotted during the heating or cooling of the sample. The rise in tan ~ is attributable to increased losses due to rise in the electrical conductivity of the films [7]. The thermal degradation of the films was inves/dgated b y the thermogravimetric method on the basis of weight loss in the region of 20-600 °. The measure-
Stability of dielectric behaviour of polymeric films
967
ments were carried out in a vacuum ( ~ 1 × 10-4ram). Figure 6 shows the thermal degradation curves for two film samples obtained with different discharge currents. These data show t h a t the thermal degradation of the films is strongly zim,% 15
I0 5
I~.....,.~ o
O
2 400
800
T,°C Fro. 6. Temperature dependence of weight loss ztrn for films of HMDS (50~) and Ar (50~) with current densities of the discharge equal to 0.4 (1) and 1.3 mA/cm I (2}. dependent on the method of film preparation: in the case of the sample obtained with a low current density (0.4 mA/cm z) the thermal degradation starts at ~ 200 °, and at 550 ° the weight loss amounts to ~ 13%, while the thermal degradation of the sample obtained with a current density of 1.3 mA/cm ~ starts at 300°, and the maximum loss of weight even at 600 ° is not more than 2 . 5 ~ . These results m a y be explained by the fact t h a t the films obtained with high current densities have a higher degree of crosslinking and the number of siloxane bonds in the polymer is relatively higher, as was shown by I R analysis in reference [14]. CONCLUSIONS
(1) I t has been shown t h a t the reduced growth rate of hexamethyldisiloxane (HM-DS) films observed on introducing At, 0~, N2 and H2 in the discharge zone is due to decrease in the HMDS partial pressure owing to the higher degree of decomposition of the starting material in the gas phase. (2) I t has been shown t h a t changes in e and tan @ in air are due to water sorption by HMDS films, and the sorption increases with rise in the film polarity. The most stable films are obtained on introducing H2 into the discharge zone, or through subsequent thermal t r e a t m e n t of the films under vacuum. (3) The thermal degradation of the films has been studied by the thermogravimetric method in the temperature interval 20-600 °. Translated by R. J. A. ]~ENDRY REFERENCES
1. R. CHRISTY, J. AppI. Phys. 31: 1680, 1960 2. A. K. TSAPUK and V. M. KOLOTYRKIN,Vysokomol. soyed. 7: 1802, 1965 (Translated in Polymer Sci. U.S.S.R. 7: 10, 1985, 1965) 3. L. S. TUZOV, A. B. GIL'MAN, A. N. SHCHUROV and V. M. KOLOTYRKIN,Vysokomo]. soyed. Ag: 2414, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 11, 2728, 1967)
968
M. I. MUSTAFAYEVet al.
4. D. A. BUCK and K. R. SHOULDERS, Proc. of the Eastern Joint Computer Cozlf. 1958, p. 55, Philadelphia, N. Y., 1959 5. H. T. MANN, J. Appl. Phys. 35: 2173, 1964 6. M. STUART, Proc. I E E 112: 1614, 1965 7. A. N. SHCHUROV, L. 8. TUZOV, A. B. GIL'MAN, V. M. KOLOTYRK~ and N. N. TUNITSKII, Vysokomol. soyed. A l l : 582, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 3, 660, 1969) 8. N. G. ALEXEYEV, V. A. PROKHOROV and K. V. CHlVIUTOV, Elektronnye pribory i skhcmy v fiziko-khimicheskom issledovanii (Electronic Apparatus and Systems in Physicochemical Investigations). Goskhimizdat, p. 430, 1961 9. S. METFESSEL, Tonkie plenki, ikh izgotovlenie i izmerenie (Thin Films, their Preparation and Measurement). Gosenergoizdat, 1963 10. B. V. TKACHUK, V. M. KOLOTYRKIN and G. G. KIREI, Vysokomol. soyed. A1O: 585, 1968 (Translated in Polymer Sci. U.S.S.R. 10: 3, 683, 1968) 11. B. V. TKACHUK, Thesis, 1968 12. I. M. MAIOFIS, Osnowy khimii dielektrikov (Basis of Dielectric Chemistry). Izd. "Vysshaya shkola", 1963 13. M. M. MIKHAILOV, Vlagopronitsayemost' organicheskikh dielektrikov (Moisturepermeability of Organic Dielectrics). Gosenergoizdat, 1960 14. B. V. TKACHUK, V. V. BUSHIN, V. M. KOLOTYRKIN and N. P. SMETANKINA, Vysokomol. soyed. A9: 2018, 1967 (Translated iu Polymer Sci. U.S.S.R. 9: 9, 2281, 1964)
MOLECULAR WEIGHTS AND MOLECULAR WEIGHT DISTRIBUTION OF THE PRODUCTS OF SPONTANEOUS POLYMERIZATION OF QUATERNARY SALTS OF 4-VINYLPYRIDINE AND ETHYL BROMIDE IN BENZENE AS MEDIUM* M. I. MUSTAFAYEV, K. V . / b x ~ v and V. A. KABA~OV A. V. Topchiev Institute of Petrochemical Synthesis, U.S.S.R. Academy of Sciences M. V. Lomonosov Moscow State University (Received 26 Maroh 1969)
IT WAS shown in reference [1] that 4-vinylpyridine (VP) polymerizes spontaneously, when alkylated with alkyl halides (AH), to form polymeric quaternary salts of the general formula:
(I) * Vysokomol. soyed. AI2: No. 4, 855-864, I970.