Radiation polymerization in the solid phase—I. The polymerization of acrylonitrile

Radiation polymerization in the solid phase—I. The polymerization of acrylonitrile

108 I . M . BARKALOVeta/. REFERENCES 1. A. N. NESMEYANOV, R. Kh. FRErDLINA and A. B. BELYAVSI{II, Dokl. Akad. Nauk SSSR 1|2: 821, 1958 2. 8. A. PAVL...

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108

I . M . BARKALOVeta/. REFERENCES

1. A. N. NESMEYANOV, R. Kh. FRErDLINA and A. B. BELYAVSI{II, Dokl. Akad. Nauk SSSR 1|2: 821, 1958 2. 8. A. PAVLOVA, T. A. SOBOLEVA and A. P. SUPRUN, Vysokomol. soyed. 6: 122, 1964 3. T. A. SOBOLEVA, A. P. SUPRUN and G. S. KOLESNIKOV, Vysokomol. soyed. 5: 487. 1963 4. T. A. SOBOLEVA, A. P. SUPRUN and G. S. KOLESNIKOV, Vysokomol. soyed. 5: 639, 1963 5. S. Ye. BRESLER and S.~a. FRENKEL', Zh. tekh. flz. 25: 2163, 1955 6. S. R. RAFIKOV, Vysokomol. soyed. 1: 1558, 1959

R A D I A T I O N P O L Y M E R I Z A T I O N IN ~ SOLID P H A S E - - I . P O L Y M E R I Z A T I O N OF A C R Y L O N I T R I L E *

THE

I. M. BARKALOV, V. I. GOL'DANSKII, ~T. S. YENIKOLOPYAN, S. F . T E R E K H O V A and G. M. TROFIMOVA Institute of Chemioal Physics, U.S.S.R. Academy of Sciences

(Received 10 August 1962)

IN RECENT years many experimental reports have appeared, in which it is demonstrated that under the influence of penetrating radiations a high rate of polymerization can occur in the solid phase [1-4]. This problem has now acquired considerable interest both from the point of view of possible practical applications and for the development of general theoretical ideas in chemical kinetics. Underlying the usual method of investigation of the kinetics of radiation polymerization in the solid phase is the fact that the yield of polymer and rate of polymerization are determined at the end not only by the radiation itself but also by the subsequent strong heating of the monomer, which is accompanied by melting and often by phase transitions (before melting of the irradiated system). Consequently the information obtained is very inde•nite, because it is not clear whether polymerization of the given monomer occurs in the solid phase (and if so whether it is during the irradiation period or as a result of post-effects) or during the defreezing process (for example at the phase transition points, the special role of which has been shown in the interesting work of Kargin and his collaborators [5, 6])or at the melting point [7-9]. Meanwhile, in order to establish the mechanism of polymerization of any given monomer the question of when it actually occurs is of decisive importance. * Vysokomol. soyed. 6: No. 1, 92-97, 1964.

Polymerization ef acrylonitrile

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W i t h this in m i n d we h a v e u n d e r t a k e n a detailed i n v e s t i g a t i o n o f t h e solidphase, r a d i a t i o n p o l y m e r i z a t i o n o f a n u m b e r o f m o n o m e r s , including a s t u d y o f t h e t e m p e r a t u r e d e p e n d e n c e o f t h e initial r a t e of p o l y m e r i z a t i o n , calorimetric m e a s u r e m e n t s o f t h e e v o l u t i o n (absorption) o f h e a t d u r i n g defreezing o f t h e i r r a d i a t e d m o n o m e r s , o b s e r v a t i o n o f E S R signals (during a n d after irradiation) t h e kinetics of p o s t - p o l y m e r i z a t i o n a n d d e t e r m i u a t i o n of t h e molecular weights o f t h e polymers. T h e r a d i a t i o n p o l y m e r i z a t i o n o f acrylonitrile (AN) (m.p. m 8 2 °, b.p. 78.5 °) has been studied. I t has been s h o w n b y Bensasson a n d M a r x [10] a n d b y Sobue a n d T a b a t a [11] t h a t this occurs efficiently in t h e solid phase. EXPERIMENTAL The polymerization of AN was initiated by high-speed electrons of energy 1.6 Me¥, accelerated in the accelerator of the Institute of Chemical Physics of the U.S.S.R. Academy of Sciences. The monomer was first dried and distilled through a fractionating column, degassed under vacuum and distilled over into special irradiation cells [12]. Irradiation was carried out in an automatic cassette (holding eight cells) which enabled the cells to be transferred by remote control under the beam (without switching off the latter). Thermostatic control in the temperature region from --86 ° to --196 ° was attained within ± 2 ° by means of the vapour from liquid nitrogen, and within ±0.5 ° in the region from --36 ° to 0 ° by means of an alcohol thermostat. After irradiation the cells were heated rapidly to room temperature. I t was found that the addition of a solution of an inhibitor (hydroquinone) has no effect on the yield of polymer, consequently in subsequent work no inh{bitor was added. The yield of polymer was determined by evaporating a weighed quantity of irradiated monomer to constant weight/n vacuo. The relative radiation dosage was determined from the electron current on a thin foil placed in front of the cell. The absolute dose rate was measured by means of an ionization chamber and a ferrous sulphate dosimeter [18]. The dose rate in different experiments was varied between 0.2 and 10 Mrs~/mln~ I n order to determine whether polymerization occurs in the solid phase or during defreezing, a dlathermal calorimeter was designed and used [14]. The monomer was irradiated directly in the cell of the calorimeter. The specimen was then lmfrozen and by means of an EPP-09 automatic recorder a simultaneous recording was made of the temperature of the specimen and of the temperature drop between the inner and outer walls of the aliathermal jacket (by means of a battery of differential thermooouples), which is.proportional to the heat flow. This apparatus enabled heat effects in the region of 10-40 oal/g to be me. asurod with a precision of -4-1 cal/g. The kinetics of the formation of active oe~tres during irradiation of AN at various temperatures were studied by the ESR method. Theas me~urements were made in the apparatus described in reference [15] in collaboration with Y. N. Shamshev, to whom the authors express their sincere gratitude. For the study of the kinetics of post.polymerization the irradiated specimens were kept for eho~n periods of time in a eryostat (preeim'on of temperature control -4-2°), were the~ rapidly unfrozen and the yield of polymer was determined. The intrinsic viscosity of each sample of polymer was determined (in dimethylform~mlde) and the molecular weight was calculated from the fermula [~]=2.43 x 10-4 (1~)°'vs [16]. RESULTS R a t e curves for t h e p o l y m e r i z a t i o n o f A N in the liquid a n d solid phases are s h o w n in Fig.. 1. I n t h e liquid p h a s e the r a t e o f p o l y m e r i z a t i o n decreases with decrease in t e m p e r a t u r e , a n d close t o t h e m e l t i n g p o i n t ( m 7 5 °) t h e r a t e is practi-

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cally zero. At the same time in the solid phase a very high reaction rate is observed close to the melting point (at --90°). The rate curves for the polymerization of AN at --196 ° and --170 ° (and to a lesser extent at --150 ° also) approach a limit, which increases with increasing temperature. This is not observed at higher temperatures in either the solid or liquid phases. The rate curve for - - 1 9 6 ° shows this limiting maximal yield most clearly. The limiting points on this curve were obtained at various dose rates. I n some experiments irradiation, with a dosage corresponding to the mammal yield (8 Mrads), was repeated several times with warming to room temperature, melting and then freezing to --196 ° after each irradiation. In these experiments each new irradiation gave an ad-

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FIG. 1. Rate curves for the radiation polymerization of AN at variotm temperatures in the liquid and solid phases (at Iffi=~3 Mrad/min): 1--196°; 2 - - - - 1 7 0 ° ; 3 - - - - 1 5 0 ° ; 4 - - - - 1 0 0 ° ; 5 - - - - 9 0 ° ; 6 - - - - 3 0 ° ; 7--0% On curve 1 point a was obtained at 1----0.5, b at I=1"0 and c at 1----3 ~ / m l n , ditional yield of polymer equal to the maTimal yield for a ~ingle irradiation (for example, 4 . 6 ~ after one irradiation, 9.3% after two and 1 2 . 6 ~ after three). The relationship between the initial rate of polymerization of AN and dose rate I was wo ~ 1 °'8 (similar to the relationship found for the radiation polymerization of AN in the liquid phase [17]). The temperature dependence of the initial rate is shown in Fig. 2. In the liquid phase El ~ 3 kcal/mole, in the solid phase E is practically zero. Determination of the molecular weights of the polyacrylonitrile obtained at all the studied temperatures showed t h a t there is a regular fall in M with increase in dosage, with approach to a definite limiting value. The limiting value of M is independent of the dose rate (see Fig. 3).

111

Polymerization of acrylonitrile

An investigation of post-polymerization in the solid phase showed t h a t this occurs only at temperatures above D 1 4 0 °. A rate curve for post-polymerization at D 9 0 ° (the specimen was irradiated with a dosage of 2.5 Mrads) is given in ~o ~ rmn~

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Fie. 2. Temperatures dependenoe of initial rate of polymerization of AN. (Zffi~3 Mrad/m~). Fie. 8. I)epondenee of molecular weight of polyaerylonitrile on dotage at various doee rateo at --196°; I (M~d/mln): 1--0-3; 2--1.0; 3--3.

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FIG. 4. Rate ourves for poet.po|ymerization of AN: 1 - - - - 1 9 6 ° ; 2 - - - - 9 0 ° ;

dosage 2.5 Mradi. Fig. 4, where it is also shown t h a t there is no post-polymerization at m 1 9 6 °. The energy of activation for post-polymerization in the solid phase was found to be 3 kcal/mole, i.e. equal to the energy of activation for radiation polymerization in the liquid phase. I t was found t h a t the molecular weight of the polymer increases sharply during the course of post-polymerization and after prolonged st~ndlng a polymer insoluble in dimethylformamide is formed.

112

I.M. B ~ r s ~ o v e~ a/.

Measurement of the ESR spectra of the solid AN during and after irradiation showed that the form of the ESR signals is the same in both cases. The strength of the signal did not decrease when the specimen was kept for ~ 30 minutes at --172 after irradiation. Calorimetric measurements showed that the heat of fusion of AN is 3 5 ~ 1 cal/g. Since the heat of polymerization of this monomer is 327 cal/g [18] the formation of 1 ~ of polymer at the moment of melting should give a decrease in the measured heat of fusion of ~ 10% . Meanwhile in measurement of the heat of fusion of samples of AN irradiated at low temperatures, in which the yield of polymer was about 12°/o (irradiation temperature --100 °, dosage 8 Mrads) no change in the measured heat of fusion, within the given limits Of error, was observed. From the data on the rate of post-polymerization it is clear also that no significant polymer formation can occur during rapid defreezing of the irradiated samples. DISCUSSION

The most important result of these experiments is the demonstration by the calorimetric method that polymerization occurs actually in the solid phase, both during irradiation and as post-polymerization. The rate of polymerization in the solid phase under irradiation is considerably higher than the rate obtained by extrapolation of the values obtained in the liquid phase. Thus close to the melting point, at the transition at the same dose rate from the liquid to the solid phase, the polymerization rate increases by more than an order of magnitude (and since the dependence of the polymerization rate on dose rate is the same in the solid and liquid phases, w0 ~ I 04, in the case of AN this break is independent of I and is of an absolute nature). In contrast to polymerization under irradiation, post-polymerization is a slow process with an activation energy equal to that for polymerization in the liquid phase, which suggests that here there is a common mechanism. I t is interesting to note that the post-effect disappears below the phase transition point (m140°). The phenomenon of a limiting maximal yield of polymer under irradiation disappears in the same temperature region. The mechanism of polymerization in the solid phase under irradiation is specific and differs substantially from the processes mentioned above. I t is possible that the main cause of the high rate of polymerization in the solid phase during the irradiation period is the formation of short-lived, excited molecules. which naturally perish rapidly in the solid phase after removal of the beam, and in the liquid phase accumulate at a much lower stationary concentration as a result of the higher rate at which they are "extinguished". Such active centres can be much more efficient than ions or radicals in the propagation of energy chains in the mechanism of polymerization discussed by Semenov [19, 20] (see also [21]). A second reason for the acceleration of radiation polymerization in the solid phase during the irradiation period could be a change of state of the solid substance during irradiation.

Polymerization of aerylonitrfle

113

For electrons of energy 1.6 MeV local beating along the paths of high specific ionization [23, 29] does not exceed a few degrees, as can be shown on the basis of reference [22]. and therefore caunot play a significant part. A possible effect of much greater importance is "loosening" of the substance along the paths of the primary particles and ~-electrons, which is evidently also responsible for acceleration of diOhmion under irradiation [24, 25]. Th~ possibility of displacement of molecules in the solid body by electron impact, short-term unfreezing of internal rotation and the excitation of all the possible vibrations bring the properties of the solid body close to those occuring close to the phase tran~tion or melting points, which, as is well known, promote rapid polymerization [5, 6, 19, 20]. The absence of an energy of activation for polymerization in the solid phase is simply explained within the framework of Semenov's theory (an energy chain, for example of the excitation transfer type). It is necessary however to take note of another, more trivial explanation, for which we have no special basis to suppose that it applies, but which we are not able to refute unequivocally at present. We refer to a polymerization mechanism in which the chain length is determined not by competition between termination and propagation at each repeating unit but by certain spatial conditions, i.e. when the chain length is "predetermined" by the properties of the material. Then in the expression for the rate of radiation polymerization, w=kolv, where the only temperature dependent term is usually the chain length v, all dependence on temperature disappears. However, the time of chain growth will be dependent on the existence of an energy of activation for each chain unit and the order of magnitude of this will be 10-~eRl~rsec. Experiments on irradiation in impulses should provide definite proof of whether or not the absence of an energy of activation is explained by a "predetermined" chain, length. The limiting maximal yield of polyacrylonitrile observed at temperatures below --140 ° and also the increase in the maTimal yield in proportion to the number of times the specimen is frozen in repeated irradiation can be explained by the presence of certain "stores" of future polymer in the original monomer. When these "stores" have been consumed in the production of polymer further irradiation causes only reduction in the molecular weight of the polymer. The authors express their deep gratitude to N. N. Semenov for his great interest and advice in this work and for valuable discussions in connection with this and subsequent eommnnications. CONCLUSIONS

(1) A study has been made of the radiation polymerization of acrylonitrile (AN) in the temperature internal from m196 ° to 0 ° and the energies of activation for the polymerization of AN in the solid phase ( E = 0) and in the liquid phase ( E ~ 3 kcal/mole) have been determined. (2) At temperatures below m140 ° there is no post-effect and at high dosages a limiting, maximal yield of polymer is observed. In repeated irradiations with

114

I . M . Ba~A~OV e¢ a/.

high dosages, w h e n the specimen is m e l t e d between the periods o f irradiation, the yield o f p o l y m e r increases in p r o p o r t i o n to the n u m b e r of irradiations. T h e molecular weight o f the p o l y m e r decreases with increase in dosage. (3) T h e e n e r g y o f a c t i v a t i o n for p o s t - p o l y m e r i z a t i o n ~ 3 kcal/mole (i.e. t h e same as in t h e liquid phase). T h e molecular weight o f t h e p o l y m e r increases s u b s t a n t i a l l y during post-polymerization. (4) C a l o r i m e t r i c m e a s u r e m e n t s showed t h a t t h e p o l y m e r i z a t i o n of AN occurs in t h e solid phases a n d n o t d u r i n g t h e s u b s e q u e n t d e f r e ~ i n g . (5) T h e results are i n t e r p r e t e d as a n indication t h a t p o l y m e r i z a t i o n in t h e solid phase u n d e r irradiation is o f a specific t y p e (for e x a m p l e w i t h a n e n e r g y - c h a i n mechanism [21], which favours b o t h the f o r m a t i o n o f short-lived, e x c i t e d molecular states a n d "loosening" o f t h e s u b s t a n c e u n d e r irradiation. T ~

by E. O. PK~uPs

REFERENCES

I. R. B. M]~ROBIAN, P. ADL]~,, D. 8. B,.AT.T.b~..%yrJ[N]~and J. J. DIENS, J. Chem. Phys. SS: 565, 1934 2. E. I. LAWTON, W. T. GRUBB and L S. BALWIT, J. Polymer Sei. 19: 455, 1956 3. S. OKAMURA and K. HAYASHI, Isotopes and Radiation, 8: 346, 540, 1960 4. A. CHAPIRO ~nd V. 8TANNETT, J. Chim. Phys. 57: 35, 1960 5. V. A. KARGIN, V. A. KABANOV and V. N. ZUBOV, Vysokomol. soyed. 1: 265, 1959 6. V. A. KARGIN, V. A. KABANOV, V. N. ZUBOV and L M. PAPISOV, VysokomoL soyed. 3: 426,. 1961 7. H..3,. RIGRY, C. J. DANBY and C. N. HINSHELWOOD, J. Chem. Soe. 234, 1948. 8. R. E. W. NORRISH and J. ¢. BEVINGTON, Proe. Roy. Soc. 196: 363, 1949 9. M. LETORT and A. J. RICHARD, J. Chlm. Phys. 57: 752, 1960

10. R. BENSASSON and R. MARX, International symposium on mseromoleoular chemistry, Section II, p. 420, Moscow, June 1960 11. H. 80BUE and I. TABATA, J. Polymer Sci. 48: 459, 1960 12. L M. BARKA/~V, A. A. BERLIN, V. I. GOL'DANSKII and B. G. DZANTIEV, Vysokotool soyed. 2: 1024, 1960 13. Radiatsionnaya dozimetriya. (Radiation dosimetry.) Foreign Literature Publishing House, 1959 14. A. F. K A P U S T I N S ~ and Yua P. BARSKII, Trudy soveshchaniya po termografii. (Transactions of conference on thermography.) U.S.S.R. Academy of Sciences, 1955 15. Yu. N. MOLIN, A. T. KORITSKH, N. Y&. BUBEN and V. V. VOYEVODS~i~ Dokl. Akad. Nauk SSSR. 123: 882, 1958 16. P. F. ONYON, Trans. Faraday Soc. 52: 80, 1956 17. E. COLT.1NSONand F. DAINTON, Disc. Faraday Soe. 12: 212, 1956 18. J. TIMMERMANS, Physieochemiedl constants of pure organic compounds, Elsevier Publishing Co. New York, 1950 19. N. N. SEMENOV, Tqhirnlya i tekhnologiya polimerov, Nos. 7-8, 136, 1960 20. N. N. SEMENOV, Report of the International Congress on Pure and Applied Chemistry, Montreal, 1960 21. E. I. ADIROVICH~ Dokl. Akad. Nauk SSSR, 136:117 22. V. I. G O L ' D A N S ~ and Yu. M. KAGAN, Internat. J. Appl. Pad. and Isotopes, 11: 1, 1960

Polymerization of vinyl a c e t ~ e

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23. I. lVL BARKALOV, V. I. GOL'DANSKII, V. G. DZANTIEV and E. V. YEGOROV, Vysokernel, soyed. 2: 1901, 1960 24. M. A. MOKUL'S]gII~ Yu. S. LAZURKIN, N. B. FIVEIBKH and V. I. KOZIN, Dokl. Akad. N a u k SSSR. 125: 1007, 1959 25. N. S. TIKHOMIROVA, Yu. M. M/tLINSKII a n d V. L. KARPOV, Dold. Akad. Nauk. SSSR. lS0: 1081, 1960

RADIATION POLYMERIZATION IN ~ SOLID P H A S E ~ I L POLYMERIZATION OF V I N ACETATE. ~ VARIABILITY OF TEMPERATURE DEPENDENCE OF ~ POLYMERIZATION RATE* I . M. B A R K A L O V , V. I . G O L ' D A N S K H , N . S. Y E N I K O L O P Y A N S. F . T E R E K H O V A a n d G. M. T R O F I M O V A I n s t i t u t e of Chemieal Physics, U.S.S.R. A c a d e m y of Sciences

(R¢oeit~ 10 A ~

1962)

IN CONTINUATION of our systematiestudy of radiation polymerization in the solid phase [1], we have undertaken a detailed study of the polymerization of vinyl acetate (VA), and also a comparison of the temperature dependence of the initial rate of radiation polymerization of a number of monomers (m.p. of vinyl acetate - 9 3 °, b.p. 72.5°). The polymerization of this monomer in the solid phase has not been studied previously, however its polymerization induced by various free-radical initiators in the liquid phase has been studied fairly extensively [2]. At low temperatures (below - 129 °) VA can exist in two form= the glassy and crystalline forms. This provides further possibilities for the study of the effect of structural factors on polymerization kinetics. In addition to the detailed study of the polymerization of acrylonitrile (AN) and VA we have also maple observations of the temperature dependence of the initial rate of radiation polymerization of a number of other monomers as an approach to the problem of the relationship between the temperature dependence and the ability of the monomers to polymerize by the radical or ionic mechanism. EXPERIMENTAL The monomers were irradiated b y electrons of energy 1.6 MeV in the electron accelerator a t the I n s t i t u t e of Chemical Physics of the U.S.S.R. A c a d e m y of Sciences. A general description of our m e t h o d was given in reference [1]. Therefore we shall deal here only * Vysokomol. soyed. 6: No. 1, 98 102, 19ti4,