The properties of thin organo-tin polymer films formed by glow discharge

2314 13. 14. 15. 16. 17.

B . V . TKACm~X et al.

S. BYWATER and D. J. WORSFOLD, J. Organomet. Chem. 10: 1, 1967 J. E. R. ROOVERS and S. BYWATER, Macromolccules 1: 328, 1968

D. J. WORSFOLD and S. BYWATER, Canad. J. Chem. 42: 2884, 1964 J. VIALLE, Rcv. gen. caoutchouc et plast. 47: 731, 1970 M. MORTON, L. J. FETTERS, R. A. PETT and J. F. MEIER, Macromolecules 3: 327,

1970 18. D. J. WORSFOLD, J. Polymer Sci. 5, A - I : 2783, 1967

THE PROPERTIES OF THIN ORGANO-TIN POLYMER FILMS FORMED BY GLOW DISCHARGE* ]3. V. TKACHUK, N . YA. MARUSII a n d YE. P . LAURS (Received 23 December 1971)

Thin (0.2-2/zm) polymeric films have been obtained b y the polymerization of organo-tin compounds with the general formula (CHs)sSnR under the action of a glow discharge. A s t u d y has been made of certain kinetic rules for the formation of the films together with their structure and properties. THE polymerization of organo-silicon compounds on the surface of a solid b o d y under the action of a glow discharge has been investigated previously [1, 2]. I t was shown t h a t this m e t h o d could be used to obtain thin (0.1-2/Jm) polymeric films having a high thermal stability, enhanced resistance to the effects of low temperatures, y-radiation and good dielectric properties [3, 4]. The present paper is devoted to an investigation of the formation of thin polymeric films from organo-tin compounds with the general formula (CHs)3SnR, where R------CHa, --C2H6, --CH---~CH2, - - C - C H on the surface of aluminium under the action of a glow discharge. The effect of the composition a n d structure of the initial organo-tin compounds on the properties and structures of the polymeric films formed was investigated. f

EXPERIMENTAL

The initial organo-tin compounds synthesized as in [5] b y the use of a Grignard reagent had the following properties: tetramethyl-tin (TMSn), boiling point 76-77°C/750 m m H g , n~s 1.4386; trimethylethyl-tin (TMESn), boiling point 104-106°C/722 mmHg, n~° 1.4527; ¢rimethylvinyl-tin (TMVSn), boiling point 99-100°C, n~6 1.4543. Trimethylethinyl-tin (TNETSn) was obtained as in [6] b y saturating hexamethyldistanoxane with acetylene under pressure; boiling point 98°C/145 mmHg, n~)° 1.4626. The polymer films were obtained in a UVR-4 vacuum installation b y the method described previously [1]. The glow discharge was formed between two fiat parallel electrodes positioned a t a distance of 25 m m from each other in the reaction chamber. Vapour pressure of the monomer in the reaction chamber, 0.5 mm/:ig; voltage on the electrodes, 400600 V; current density on the substrate, 0.1-1 mA/cm2; current frequency, 1000 Hz. The supply to the discharge was from a stabilized voltage source t y p e ISN-1. * Vysokomol. soyed. A15: No. 9, 2046-2051, 1973.

Properties of thin organo-tin polymer films

2315

The I R spectra of the initial organo-tin compounds and of the polymeric films based on t h e m were recorded with a IKS-22 spectrophotometer. The spectra were recorded in cells of constant thickness; usually the thickness of the specimens was 2-3/lm. I n order to record the I R spectra of the polymers, films 1-2/~m thick were deposited on polished plates of high-ohmic silicon. The electro-physical properties of the polymeric films were investigated on metaldielectric-metal (M-D-M)structures where thermally evaporated aluminium films, approximately 0"5/~m thick, were used as the electrodes and the dielectric was a film of the organotin polymer under investigation. The capacity C a n d the tangent of the dielectric loss angle t a n 5 of the specimens were measured with E12-2 and R589 bridges and with a E9-4 Q-factor meter. The voltagecurrent characteristics of the films were recorded b y the usual" method with a EK6-11 pico-ammeter. The low-temperature measurements were made in a specially developed helium cryostat capable of regulating the temperature in the range 300-4.2°K with an accuracy of 0.1°K. RESULTS A N D

DISCUSSION

The thickness of the polymer films formed from the organo-tin compounds increases linearly with the time of polymerization, and the rate of formation of the films increases in agreement with the activity of the monomers in the fol[7 £//77

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Fie. I. Dependence of the polymer-film thickness, l, on polymerization time for: 1 - - T M E T S n , 2--TMVSn; 3--TMESn, 4--TMSn. FIG. 2. Voltage-current characteristics of M - D - M structures for polymer films of: 1--TMVSn, 2--TMESn, 3 - - T M E T S n . Film thickness, pro: 1--1, 2--0.97, 3--1.8.

lowing series: T M E T S n ~ T M V S n ~ T M E S n ~ T M S n (Fig. 1). This regularity may be connected with the fact that, on going from the symmetrical TMSn to TMETSn, the bond energies are reduced because of the introduction of bulkier

2316

B.V. TXAeHU~ret al.

substituents. Thus the Sn--C bond energy in TMSn is 52.14-1.1 kcal/mole, whereas in compounds with the Sn--C2H5 group, it is 46.24-2.1 kcal/mole [7]. On the other hand, a definite part may be played b y the interaction of the uelectrons of the vinyl or ethyl group with the d-electron shell of the tin atom and b y the transfer of this interaction to the bond in adjacent organic radicals [8, 9]. I t should also be noted that the rate of growth of the polymer films changes only b y 35-60% on going from TMSn to TMETSn. A similar phenomenon has been found in the polymerization of organosilanes in a glow discharge, where the replacement of a saturated compound b y a vinyl or ethinyl compound also causes the rate to increase b y 30-60~/o [2]: this unambiguously points to the absence of chain processes during the formation of polymeric films on the surface of a solid body in a glow discharge. TABLE 1. C~EMICALcoYreOSITmNO~ ~ M S BASEDON TMSn, TMESn AND TMVSn Found,

Polymer TMSn TMESn TMVSn

16.32 17.26 17.26

H

Sn

3.4 3.66 3.99

65.49 66.57 66.05

Ocalc ~ ~o

Gross formula

Sn/C

14.99

C2.sH6,2SnOl,~ C2.56Hs,55SnO1.4 C~,54HT.13SnO1,41

4.0

12-51 12-7

3"9

3.8

Table 1 presents the results of chemical analysis of the polymers obtained: it follows from these that the carbon content in the films after polymerization decreases b y 10-15% on going from TMSn to TMVSn, b u t the Sn/C ratio changes very little. The data in Table 1 illustrate the very slight effect that the structure of the initial monomer has on the composition of the polymer film obtained in a glow discharge. The general rule, namely, that oxygen is present in the polymer films, should be noted. A similar phenomenon was found in [10] and m a y be connected with oxidation of the films in air as well as with the absorption of water vapour b y the film. It is not ruled out that adsorbed water m a y affect the occurrence of secondary processes in the film that m a y lead to a change in the structure of the primary polymer. Calculations show that in order to compensate for the oxygen deficiency in the case of a film 1 × 1 cm ~ in size and 1/~m thick, 0.01 mg of water would be sufficient and the complete removal of this from the system would be a very complex task. A comparison of the I R spectra of the initial organo-tin compounds with those of the polymer films based on them indicates that the initial compounds have an absorption band with a frequency of 1190-1195 cm -1 characteristic of deformational vibrations of the Sn--C bond [11]; the absorption bands at 29502980 cm -1 characterize the valency vibrations of C - - H bonds in CH 8 groups [12]. The presence in the I R spectra of absorption bands in the 700-800 cm -1 region

Properties of thin organo-tin polymer films

2317

that characterize the rocking vibrations of CHs and C2H s groups additionally confirms the presence of the Sn--CH 8 bond [11]. The intensity of the absorption bands corresponding to the valency and deformational vibrations of the methyl groups are, of course, redistributed and reduced. The absorption bands corresponding to the vibrations of the vinyl group in films based on TMVSn are most strongly altered; the absorption band connected with the symmetrical and asymmetrical valency vibrations of the carbon-hydrogen bond in the Ctt~ group (3030 cm -1) disappears and there is also a considerable reduction in the absorption band with a frequency of 1005 cm -1 corresponding to the out-of-plane vibrations of the --CH---- bond [13]. This is evidence that the formation of a polymer from TMVSn clearly occurs through polymerization at the double bond. Similar changes occur in the IR spectra of the TMETSn polymeric film: the intensities of the absorption bands corresponding to the valence and deformational vibrations of the methyl groups are considerably reduced and the absorption bands corresponding to valency vibrations of the C---CH (2040, 3300 cm -1) [14] disappear from the spectrum of the polymer. It may thus be concluded from the data of the I R spectra that the polymerization of TMVSn and TMETSn occurs at the functional unsaturated bonds (double and triple bonds). The study of the degradation of polymeric organo-tin films showed that the thermal stability of the polymers is affected by the structure of the organic groups located directly at the tin atom. Thus the weight loss at 530°C amounts to 52% for the TMSn polymeric films, whereas the corresponding figure for TMETSn is 64O/o. It should also be noted that the organo-tin polymer films may be arranged in the following order with respect to resistance against thermooxidative degradation: CHs >Call5 >CH----CH~ >C~CH

The two-stage nature of the thermal degradation of organo-tin polymer films became apparent when thermogravimetric investigations were carried out [15]. The TMESn, TMVSn and TMETSn polymers begin to undergo appreciable weight losses at 150-160°C, the corresponding figure for TMSn being 180°C; the TMESn, TMVSn and TMETSn polymers lose 16-18~ of their weight at 180-200°C, the corresponding figure for TMSn being 12~, and the first stage, which is accompanied by further structure-formation in the polymer leading to the formation of a network structure, is then completed. As the temperature is further raised, oxidation processes begin to make a contribution and these lead to the final product of thermo-oxidative degradation, namely, tin dioxide. In order to investigate the phase state of the polymers and the effect of the conditions of synthesis on the structure of the films formed, an electron diffraction analysis was made of polymer films obtained from TMVSn under "harsh',' (current density, lmA/cm z) and "mild" (0.2 mA/cm ~) conditions of glow discharge.

2318

B.V. TKAOHu~et al.

A pattern characteristic of a crystalline phase was found in the electronogram of the TMVSn polymer film obtained under "harsh" conditions. The diffraction pattern in this case consists of diffuse maxima and reflection spots characteristic TABLE

2.

ELECTRO-PHYSICAL PROPERTIES

OF

ORGANO-Tllq POLYMER FILI~S

Polymer

e*

tan 6 X l0 s

EbCrX 10-6, V/cm

p~ X 10-lz, ohm. cm

TMSn TMVSn TMETSn

3.0 2.7 3.0

7-9 6 7-9

3-5 2-3 2

1-2 1-3 2-5

* e is the relative dielectric permeability.

t Eb, is the breakdown voltage of the electric field. $Pv is the specific volume resistance.

of coarse crystals are also recorded. A large proportion of the reflection spots correspond to the crystal structure of #-tin. The existence of a diffraction pattern with quite clear lines of low intensity and the ring characteristics of the electronogram for the polymer formed under "mild" conditions give evidence of a poly-

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Fz,equencu,Hz FIG. 3. Frequency dependence of: a - - C and b--~an 6 for: 1--TMVSn, 2--TMESn, 3--TMETSn, d--TMSn. crystalline phase having approximately the same ordered arrangement. The low intensity of the interference lines on the electronogram of the TMVSn polymer obtained under "mild" conditions is evidently connected with the very small

2319

Properties of thin organo-tin polymer films

amount of this phase in the film investigated. Thus a crystalline phase is present in the TMVSn polymer films obtained under "harsh" and "mild" glow discharge conditions. It may be seen from Table 2 that, for the TMSn, TMVSn and TMETSn polymer films, the electrophysica! characteristics have approximately the same values, in agreement with the chemical analysis results and with the structural investigations. tan d~lO"~ lSg

I

I

I

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200

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og

FIG. 4. Temperature dependence of tan& 1--TMESn; 2--TMETSn, 3--TMSn, 4 -- TM-VSn. Investigations of the stability of the dielectric properties of the polymer films showed that storing the specimens in air led to an increase in the capacity and tangent of the dielectric loss angle. Thus the capacity of TMSn, TMESn and TMVSn specimens increases by 3-4% in 3-5 days, the figure for TMETSn being 8%; for TMSn and TMESn, tan ~ increased by 10%, and for TMVSn and TMETSn films by 20-50%. After this, the dielectric properties of the specimens became stabilized and did not change thereafter during 30 days. These processes are connected with ageing of the polymer films and are caused by the presence of free radicals in the polymer, by interaction with atmospheric oxygen and by absorption of atmospheric moisture [10, 16, 17]. Specimens were heated in vacuum at 150°C with the aim-of stabilizing the dielectric properties of the organo-tin polymer films. Stabili2ation of the electrophysical properties of the polymer films occurs under this treatment. Investigations of the voltage-current characteristics were carried out in the temperature range 20-100°C: the characteristics thus remained linear (Fig. 2) and the temperature relationships between log I and T~ (1/I) were represented by straight lines. The value of the residual current did not change with prolonged holding of the specimens under a potential difference. The results obtained make it possible to assume that the films' resistance is determined by the injection of carriers from the electrodes, that is, by the Sehottky effect [18].

2320

B.V. TKACHUKet al.

.

I t follows from Fig. 3 t h a t the c a p a c i t y of the specimens remained u n a l t e r e d in the r a n g e 102-10 e t I z whereas the increase in t a n 6 a t the u p p e r b o u n d a r y o f m e a s u r e m e n t was clearly caused b y the s t r u c t u r e of the p o l y m e r formed. The t e m p e r a t u r e dependence of t a n 6 for the p o l y m e r films is s h o w n in Fig. 4. I t m a y be seen t h a t , as the t e m p e r a t u r e is reduced from 300 to 150°K, a r e d u c t i o n in the dielectric losses is f o u n d in all the specimens, the greatest change in t a n 6 (by a f a c t o r of 2) occurring for the film based on T M E S n , a n d t h e least (by a factor of 1-1-1.4) for the films based on T M E T S n a n d TMVSn. M a x i m a c o n n e c t e d with dipole-relaxation losses are, moreover, f o u n d a t 200-250°K in the o r g a n o - t i n polymers. The m a x i m a are thus f o u n d to become more diffuse on going f r o m T M S n to T M E T S n a n d t h e y are f o u n d to be shifted t o w a r d s higher t e m p e r a t u r e s . The t e m p e r a t u r e coefficient of the specimens' c a p a c i t y was 6 × 10-3-10-4°K-1. Translated by G. F. MODLE~-

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