Radical formation during radiolysis of solid rubbers

lO38 2. 3. 4. 5.

6. 7. 8, 9. 10. 11. 12. 13. 14.

V. T. KozT.ov and Z. ~ . TARASOVA

V. A. KARGIN and V. A. KABANOV, Ibid,, 602 P. V. KOZLOV, Ibid., 660 N. A. PLATE and V. P. SHIBAYEV, Ibid., 638 G. L. SLONIMSKII, V. V. KORSHAK, S, S. VINOGRADOVA, A. I. KITAIGORODSKII, A. A. ASKADSKII, S. N. SALAZKIN and Ye. M. BELAVTSEVA, Dokl. Akad. N a u k SSSR 156: 924, 1964 E. JENCKEL, and E. KLEIN, KoUoid.-Z. 129: 19, 1952 J. ROH:LEDER and H. A. STUART, _~iakromol. Chem. 41: 110, 1960 H. G. KILIAN, Kolloid.-Z. 176: 49, 1961 B. V. VASIL'EV and O. G. TARAKANOV, Vysokomol. soyecl. 6: 2189, 1964 B. E. MYULLER, N. P. APUKHTINA a n d A. L. KLEBANSKII, Vysokomol. soyed. 6: 1330, 1964 V. V. KORSHAK, Yu. A. STREPIKHEYEV and A. F. MOISEYEV, Plast. massy, 6, 10, 1961 C. S. MARVEL and J. H. JOHNSON, J. Am. Chem. Soc. 78: 3207, 1956 O. G. TARAKANOV and A. I. DEMINA, Vysokomo]. soyed. 7: 224, 1965 T. TANAKA, T. YOKOYAMA a n d K. KAKU, Memoirs, Fae. Eng. K y u s h i Univ. 23: 113, 1963

RADICAL FORMATION DURING RADIOLYSIS OF SOLID RUBBERS* t V. T. KOZLOV and Z. I~. TXRASOVA Scientific Research I n s t i t u t e of the Tyre I n d u s t r y

(Received 26 May 1965)

A STUDY is reported in this paper of certain aspects of the effect of molecular rubber structure on the formation of reactive particles by high energy radiations. This problem is of more general importance in studying the reactivity of rubbers. The following technical rubbers were investigated: natural (NK), polyisoprene (SKI-3), polychloroprene (l~airit A-KhK), SKS-30ARM, butadiene-methylstyrene (SKMS-30ARKM) and saturated ethylene-propylene (SKEP). Rubbers which had been purified twice were simultaneously investigated: !~K, SKI-3 and SKEP. The purification was carried out as follows: the rubber was at first extracted with cold acetone under argon (40 hours), then dissolved in benzene, the solution filtered and the rubber precipitated with methyl alcohol. All the radical measurements were caxried out by electron paramagnetio resonance in radiospeetrometer RE-1301 at liquid nitrogen temperature. Th~ * Vysokomol. soyed. 8: No. 5, 943-948, 1966. 1st paper from the series " R e a c t i v i t y of rubbers of different structures".

Radical formation during radiolysis of solid rubbers

1039

rubbers were irradiated by a e0Co source with dose rates of 522 r/see at liquid nitrogen temperature in glass capillaries "Luch-2" which at very high doses only gives a small electron paramagnetie resonance signal having practically no effect on the measurements [1]. The capillaries containing the samples were evacuated to 10-a ram. Figure 1 presents E P R spectra of free radicals formed during low temperature radiolysis of the polymers investigated. The radicals of each have a characteristic E P R spectrum. For several rubbers a superfine structure (SFS) could be observed and interpreted. These rubbers in the glass state [2] are mainly formed by one type of radicals by H atom abstraction.* The g-factor within the limits of error of measurements appeared to be the same: g----2.003~0-002; in all cases the line of diphenylpicrylhydrazyl was situated on the central decrease. The spectnlm of S K E P (Fig. 1, I) consists of 8 components of SFS and has a central decrease which is characteristic of the interaction of an uncoupled spin with an odd n u m b e r of protons, i.e. with 7. This spectrum can be related to an alkyl radical of the type CHa ~ CH,--~--CH,

~

•

With an increase of irradiation dose the radical concentration increases and a widening of components of SFS is observed. With concentrations of the order of (0.5-1.0) × 10 ~0radicals/g SFS gradually becomes a singlet line. A similar widening //

"~ -n7

Ir

ii~

/

J F i e . 1. E P R spectra of rubber polymers irradiated with a dose of 14 Mr at a temperature of liquid nitrogen a n d probable systems of certain radicals: 1 - - S K E P , II--SKI-3, dotted c u r v e - narrow signal which is retained after heating the specimen, I I I - - N K , I V - - N K previously erosslinked wish a dose of 100 Mr, V - - K h K (polychloroprene), VI, VII, VIII--SKD, SKS-30ARM, SKMS-30ARKM. * Polychloroprene rubber is a n exception, in which b y the reaction RC1A-e--->RCI-, etc. [3] (the halides have a significant electron airmity) stable negative ionic radicals are apparently formed, of which She spectrum is superimposed on the spectrum of radicals of the m a i n type.

1040

V; T. KozLov and Z. :~.. TA~ASOvA

of components and merging into a singlet line takes place also in the spectra of other rubbers, but at lower radical concentrations. This is due to the fact t h a t the radicals of other rubbers, except for SKEP, are of the allyl nature, i.e. the uncoupled spins are conjugated with a double bond and are therefore more delocalized. Consequently, the distance between the components of the allyl radicals is 2-2.5 times less than t h a t of alkyl radicals. Allyl radicals SKI and N K (Fig. 1, II, and III) are somewhat different. Each of these polymers has an isopentene grouping 1

2

3

4

~ CH~--C = CH--CH~ ~ ,

,

I

CH3 in which there are two a-methylene H atoms at C1 and C4, more mobile than the others. From the investigation of deuterated rubbers it is well known t h a t the H atom is separated from C4 three times more easily than from C1 [4]. Apparently, both processes take place during irradiation. When t t atom abstraction from C4 predominates, E P ~ spectrum should consist of 5 components of SFS (interaction with 4 protons) similarly to N K (Fig. 1, / / I ) , where the 5-component structure, probably, takes the form: 2

3

4

1

2

--C = CH--CH--CH2--C

3

= CH--.

]

CHs

CH3

I f the t I atom is separated from Ca, the uncoupled spin will already interact with 6 protons, i.e. the spectrum will consist of 7 components as in the case of SKI-3 (Fig. 1, / / ) - - 7 - c o m p o n e n t structure which probably takes the form: CH8

I

--CH2--CH----C~-CH--CH2--. 1

~

s

4

During unfreezing the 7-component structure of SKI changes into a narrow, comparatively intensive singlet (Fig. 1, / / ) (concentration in the range of irradiation doses of 10-150 Mr is (2-8)× 101Sradicals/g). This singlet is very stable and is retained even at 50 ° . According to literature data [5], similar stable signals are formed in conjugated polymeric systems. These differences in I~K and SKI are most probably due to the less regular structure of the latter and to the possibility of other structures (particularly conjugated) being present. For SKD, SKS and SKMS the E P R spectra are very similar and have a symmetrical line without SFS. The merging of superfine components at SKI) is apparently due to the symmetrical character of butene groups and the equal probability of separation of a-methylene hydrogen both from C~ and from C4. Radical concentration was determined from the area of E P R spectra. A CuC12. 2I-I20 singie crystal was used as standard. Figure 2 gives data on radical accumulation in solid rubbers with increase of irradiation dose. From the initial linear sections of the curves showing accumulation the values of radiation-

1041

Radical formation during radiolysis of solid rubbers ._2-°1

×2

8 x~

I

o

zoo

i

200

300 Dose,MPad.

Fz~. 2. Accumulation of radicals during radiolysis of solid rubbers: 1--SKEP; 2--a : NK, b : SKI-3, 3--KhK (polychloroprene); d--SKD; 5--SKMS-30ARKM; 6-- SKS-30ARM. chemical radical yields were calculated (G~). The difference in GR for various polymers is apparently due to the difference in electron structures and consequently, electron conditions in which the forces capable of breaking the interatomic bond in the molecule axe formed. I t follows from Fig. 2 t h a t for N K and S K I the curves showing radical accumulation fully agree. Radical yield apparently depends on the electron structure of the monomer unit and is independent of the macro-structure of the polymer molecule. E P R spectra of raw and crosslinked (up to ~ 1 - 2 × 1030 crosslinks) rubbers do not show a n y difference: SFS in both cases agrees (Fig. 1, I I I and IV). I t can be seen in the Table t h a t the values of G~ are arranged as follows: GR(SKEP) ~G/~(NK,SKI) ~GR (KhK) ~GR (SKD) :~GR(SY,M)~GR (SKS)" The character of several values of GR apparently corresponds to the scheme of radical formation by radiolysis of hydrocarbons, proposed by a group of research workers, including Voyevodskii, Buben, Molin, and others [7] according to whom, the higher the energy of the first level of activation, El, compared with the energy of the C-- H bond to be ruptured (for the glass state), i.e. DcE ~ 4 eV, the higher the probability of decomposition to radicals. Several Ej values* are given for the polymers studied in a qualitative form. The numerical values of E 1 [8-10] and Emax [10, 11] tabulated are normally accepted in the literature. These results indicate t h a t saturated hydrocarbons * The numerical values of E 1 known at present for multi-atomic molecules, determined from the centre position of long-wave absorption bands (Emax),are approximate. A particularly approximate value of E1 is assumed for polymers and for hydrocarbons with multiple bonds, the absorption bands of which are very diffuse.

V. T. KozLov

1042

and

Z. N . TARASOVA

have the highest position of the first activation level. The replacement of the hydrocarbon radical by a halogen (e.g. replacement of the CHa group with chlorine) in any saturated, unsaturated or aromatic substituted molecule reduces E 1. The appearance of double bonds in the molecule markedly reduces the activation level. However, among molecules with a system of u-shells, the aromatic molecules have a special liability to conversion by irradiation. The aromatic molecules have a more general property, symmetry, by which the effect of molecular geometry on the formation of an unstable condition is determined during activation. COMI~ARISOI~ OF VALUES OF E

Emax, i El' EV EV

Compound

Saturated

... - CH~-

Alkyl

CH(CH~)

- CH~-

CH~

with

r~on-conjugated

C =

C

bonds

(rubbers)

~6 ~5-6

0-55 0-4-9

CH = CH - CH:-

i0.28 ~0.20 i0'19 0.113

...

...CIt 2- CIt = CH - CH~- CH(CHs)- CH - CH~ - CH = CH - CH2- ... /\

[6]

--

...- CH~- C (CH3) = C H - C H ~ . . . ... - CH~- CCI = CH - CH2- ... ... - CH~-

Literature

reference

- ...

halides

~Iydrocarbons

GR

10-7

(polymers)

hydrocarbons

A]STD G ~

1

\// /\,

I \/

L l

...CH ~- CH = CH - CH~-

CH - CH2-

CH.-

CH = CH - CH~-

0-096

...

/2\

~_/_cl /=\ %

/=\

4.7

4.8

4-7

4.7

4.6

[ 0.3

5-0

4-2

!0.045

4.5

4.2

[0-045

,

[73

2.8

2"8

0.03

~

[7]

/=',,

%_//--%_,//

/=\

4.9

/=\

/=\ //--~_//--'%

N

-"

//

/=\

0.2 i

,

[7]

0.3

'[7]

!

[7]

[7]

One of the characteristic features of cyclic and linear molecules with conjugated bonds which depends on the delocalization of u-electrons along the whole chain of conjugation [5] is the low value of E1 and ionization potential [12, 18] of these molecules, compared with corresponding saturated and unsaturated molecules with isolated multiple bonds. For the latter, in the case of rubbers, the conjugation chain is ruptured by two CH~ groups which excludes the collection of u-electrons in the molecule but explains their delocalization inside each monomer unit. As a result of the effect of conjugation [14] the bonds are weakened on the boundaries of monomeric ,units for carbon atoms which are in the a-posi-

Radical formation during radiolysis of solid rubbers

1043

tion in relation to the double bond. Localization of the activation energy will thus be assumed to be E1 and to take place with a greater probability during pre-dissociation on the bond for e-methylene hydrogen (in the glass state). Apparently, during pre-dissociation the role of molecular geometry, in this case--the geometry of the monomerie unit, considerably increases. The butane group in SKD (~CIt2--CI:[=CH--CtI2 ~) is symmetrical, compared with the isopentene group of N K or SKI ( ~ C H 2 - - C ( C H a ) = C H - - C H 2 ~ ) which will probably hinder energy concentration during pre-dissociation and increase the probability of energy consumption without decomposition into radicals in the case of SKD. As can be seen in the Table, an increase in the s y m m e t r y of the electron molecular structure does not too markedly reduce the value of E 1 (for rubbers in the range of 5-6 eV), but then very considerably reduces GR, in particular, GR(NK,SK~) > G•(SKD), and GRCSK~S) > GR(s•s). The same is observed for aromatic molecules (see Table). It can thus be assumed that the experimental order of Ga values corresponds to the order of E1 values presented in a qualitative form considering the fact that for very similar values of E~ the geometry of electronic structures has an important role in processes of decomposition into radicals. CONCLUSIONS

(1) Using E P R a study was made of processes of radical formation under conditions of low-temperature (--196 °) radiolysis in rubbers NK, SKI-3, Nairit A, SKD, SKS-30ARM, SKMS-30ARKI~I and SKEP. (2) The spectra of rubbers indicate t h a t allyl type radicals are present except for S K E P which forms radicals of allcyl type. The distance between the components of the latter is 2-2.5 times greater t h a n in the case of allyl radicals. (3) Comparison of the spectra of N K and SKI indicates that in the case of N K the more probable process of H atom separation from C~ predominates and, in the case of SKI, from C1. This difference m a y indicate the less regular structure of SKI, in comparison with NK. (4) E P R spectra of raw and crosslinked (to N1.2× 10 ~0 crosslil~ks/g) rubbers are the same. The curves showing radical accumulation in N K and SKI agree. (5) All these facts indicate that radical formation depends on the electronic structure of the monomer unit and is independent of the presence of steric structures in the system. (6) For the rubbers studied the curves showing radical accumulation with the increase of irradiation dose, were determined. From the initial linear sections of the curves showing the accumulation radiation-chemical yields (G~) were can cu]ated and the following order observed: G/~(SKEP)~-0"55~GR(NK.SKI)~0"28~":>GI~(KhX)~-O"20 > GR (SK]))----~O'19> GR (SKMS)--~O.113> GR (SKS)-----O'096. (7) It can be assumed that the experimental order of GR values corresponds to the qualitative order of E1 values considering that, for very similar values of

1044

T . A . KO~v.TS~AYA et al.

El, the geometry (symmetry) of electronic structures has a considerable role in processes of breakdown to radicals. Translated by E. Sv.~EP,~ REFERENCES 1. S. M. BREKHOVSKIK•, I. V. VERESHCHINSKII, S. A. ZELENTSOVA and A. D. GRISHINA, Auth. Cert. No. 22767, 1961 2. V. V. VOYEVODSKII, The V Intern. Syrup. on Free Radicals, Uppsala, 1961 3. J. L. MAGEE and M. BURTON, J. Amer. Chem. Soc. 73: 523, 1951 4. J. J. SHIPMAN and M. A. COLUB, J. Polymer Sci. 58: llb, 1962 5. L. A. BLYUMENFEL'D, A. A. BEKLIN, A. A. SHNKIN and A. E. KALMANSON, Strukt. khimiya 1: 103, 1960 6. P. B. AYSCOUGH, K. V. IVIN, J. M. O'DONNEL, and C. THOMSON, The V Intern. Symp. on Free Radicals, Uppsala, paper 4, 1961 7. Yu. N. MOLIN, I. I. CHKttEIDZE, Ye. N. KAPLAN, N. Ya. BUBEN and V. V. VOYEVODSKII, Kinetika i kataliz 3: 674, 1962 8. LANDOLT-B~RNSTEIN, Zahlenwerte und Funktionen, Bd. I, Tell 3, 1951 9. Primenenie spektroskopii v khimii. (Use of Spectroscopy in Chemistry.) Izd, inostr. lit. 1959 10. V. L. BROUDE, Ye. A. IZRALEVICH, Optika i spektroskopiya 5: 113, 1958 11. A. GILEM and E. SHTERN, Elektronnye spektry pogloshcheniya organieheskikh soyedinenii. (Electronic Absorption Spectra of Organic Compounds.) Izd. inostr, lit., 1957 12. Ya. K. SYRKIN and M. Ye. DYATKINA, Khimicheskaya svyaz' i stroyenle molekul. (Chemical Bonds and Molecular Structure.) Goskhimizdat, 1946 13. M. V. VOL'KENSHTEIN, Stroyenie i fizicheskie svoistva molekul. (Structure and Physical Properties of Molecules.) Izd. Akad. Nauk SSSR, 1955 14. E. G. LAWTON, G. S. BALWIT and R. S. POWELL, Amer. Chem. Soe. Meeting, Miami, 1957; J. Polymer Sci. 32: 257, 277, 1958

ELECTRON MICROSCOPIC STUDY OF THE CRYSTALLIZATION OF POLYMERS IN THE PRESENCE OF ARTIFICIAL STRUCTUREFORMING AGENTS* T. A. KORETSKAYA, T. I. SOGOLOV~.and V. A. K.tROr~ L. Ya. Karpov Physicochemical Institute

(Received 29 May 1965) IT WAS f o u n d relatively r e c e n t l y t h a t t h e i n t r o d u c t i o n into crystallizing polymers of solid particles w h i c h do n o t r e a c t chemically, has a g r e a t effect on processes o f s t r u c t u r e formation. * Vysokomol. soyed. 8: No. 5, 949-951, 1966.