Effect of light on free radicals stabilized in γ-irradiated polyvinyl acetate and polymethyl-methacrylate

1778

V. K. MILINCHUK and S. YA. PSHEZttETSKn

6. S. E. BRESLER, V. V. KORSHAK and F. A. PAVLOVA and P. A. FINOGENOV, Dokl. Akad. N a u k SSSR 87: 961, 1952; Izd. Akad. N a u k ser. khim. 344, 354, 1954 7. L. MANDELKERN and P. J. FLOI~Y, J. Chem. Phys. 26: 212. 1952 8. S. Ya. FRENKEL, Vysokomol. soyed. 2: 731, 1960 9. S. Ya. FttENKEL, Dissertation 1953; S. E. BRESLER, S. Ya. FRENKEL, Zh. tekh. fiz. 23: 1502, 1953; S. E. BRESLER, I. Ya. PODDUBNYL and S. Ya. FRENKEL, Zh.tekh. fiz. 23: 1521, 1953 10. P. J. FLORY, J. Amer. Chem. Soc. 58: 1877, 1936 11. L. C. CASE, J. Polymer Sci. 39: 175, 1959

EFFECT OF LIGHT ON FREE RADICALS STABILIZED IN y-IRRADIATED POLYVINYL ACETATE AND POLYMETHYLMETHACRYLATE* V. K. MILINCHUK

and S. YA. PSHEZI-IETSKII

L. Ya. Kharpov Physico-Chemical Institute (Received 17 October 1963) I N [1, 2] f i g u r e s w e r e d e r i v e d f o r t h e v a r i a t i o n i n t h e E S R s p e c t r a o f y - i r r a d i a t e d polypropylene exposed to ultraviolet and visible light. These changes were due to photochemical conversions of the radicals formed in polypropylene as a result of the preliminary y-irradiation, and stabilized at liquid nitrogen temperature. To find out to what extent the photochemical conversion of radicals in polym e r s e x p o s e d t o ? - i r r a d i a t i o n is g e n e r a l , w e p e r f o r m e d a n a l o g o u s s t u d i e s o n o t h e r

/

a

?

bj i

75 oe

i

FIG. 1. a - - E S R spectrum of polyvinyl acetate exposed to 7-rays at 77°K; b--spect r u m of the same specimen after exposure to visible light (light filter SZS-22); c--after exposure to UV light (light filter UVS-1) at 77°K. Measuring t e m p e r a t u r e 77°K. * Vysokomol. soyed. 6: 9, 1605-1611, 1964.

Effect of light on free radicals

1779

solid polymers as well. The present report gives figures for polyvinyl acetate (PVA) and polymethylmethacrylate (PMMA). Polyvinyl acetate. A faint triplet was found in the E S R spectrum of ?-irradiated PVA by Abraham and Whiffen [3], and b y Bresler and collaborators [10] when vinyl acetate was photopolymerized. The structure of the free radicals was not studied. The E S R spectrum of ?-irradiated PVA which we obtained (on a R E 1301 radiospectrometer) consists of three clearly defined superfine structure components (Fig. la), (some faint lines can also be seen in the spectrum). When these polymer specimens were exposed to light of different wavelengths, there was a change in shape of the E S R spectra. While red (light filter KS-10) and yellow (light filter ZhS-12) light destroy the radicals with only slight change in the shape of the E S R spectrum, shorter wave light (2min~<3800 A, light filter SZS-22) caused a considerable change in the shape of the spectrum (Fig. lb). After exposure to UV light (light filter UVS-1) the ratio of the STS line intensities is different than after exposure to visible light (Fig. lc).

a

S

/

b

I

FIG. 2. Temperature dependence of the ESR spectrum of polyvinyl acetate exposed to y-rays at 77°K: a--178°K; b--227°K; c--261°K. Measuring temperature 77°K. I f specimens exposed to ?-irradiation at 77°K are heated, there is a change in the shape of the E S R spectrum and also radical recombination. The ratio of the STS component intensities changes and the resolution of the lines deteriorates (Fig. 2). Heating to room temperature gives a spectrum similar to that obtained in [3, 4] (Fig. 3a). If specimens exposed to light at 77°K are brought up to the temperature they had before, the shape of the E S R signal is restored. This means that the processes of radical conversion which occur due to exposure to light and heat are reversible, in the same way as in polypropylene [1]. It is interesting to note

1780

V. K. MILINCHUKand S. YA. PSHEZHETSKII

that recovery of the original shape of the E S R spectrum after exposure to light even occurs at liquid nitrogen temperature albeit rather slowly. For instance, in a polymer exposed to visible light (light filter SZS-22) (Fig. lb), after holding for 23 hr at 77°K the same shaped spectrum was recorded as that found before (Fig. la). All the changes observed in the shape of the E S R signal due to its exposure to light, only occurred at liquid nitrogen temperature. At room temperature light destroys the radicals b u t does not change the E S R spectra. As observed in [1, 2] the reversible conversion of radicals b y light is mainly due to the migration of an unpaired electron (along the polymer chain). The light makes it easier for the potential barrier to be overcome in the process of migration. Migration of the unpaired electron is irreversible. We note that ia polypropylene light absorption in the visible range is due, of course, to the unpaired electron and not to the polymer as a whole [5]. But UV light carmot be absorbed b y the entire polymer, and it is transmitted to the unpaired electron along the polymer chain.

/

a

b

75 oe

)

100 oe

FIG. 3. a - - E S R spectrum of polyvinyl acetate after heating at room temperature; b--spectrum of the same specimen after illumination through light filter SZS-22 for 17 rain; c--through light filter UVS-1 for 13 rain. Measuring temperature 77°K.

After exposure to visible and UV light the E S R spectrum of y-irradiated PVA has nine STS lines which can be split up into three groups of 4, 3 and 2 components. The splitting between components in the triplet and quartet is 25=kl oe. The distance between the two edge components of the spectrum is approx. 129 oe. After exposure to visible light the STS line intensities in the quartet are related as 1 : 2.8 : 2.8 : 1 (after exposure to UV light as 1 : 3 : 3 : 1), and the relation of the intensities in the triplet is 1 : 1.8 : 1). Let us consider the probable structure of the radicals. Radical formation b y

Effect of light on free radicals

1781

the 7-irradiation of P V A m a y be t h e result of separation of h y d r o g e n a t o m s in the main chain or in t h e side group: _CH2--~--CH 2-

(1);

--CH--CH--CH--

(2);

OCOCH a

--CI-L~--CH--CH.,-I

(3)

0 [, . O ~ C--CH~

Besides this, the p o l y m e r chains m a y b r e a k d o w n (4),

--CH,--CH

--CH2--CH--~I-I 2

(~COCH3

(5)

I OCOCH 3

or a n entire side group m a y be detached. --CH2--CI-I--CH~--- etc.

(6)

The E S R of four lines w i t h the ratio 1 : 3 : 3 : 1 gives the e q u i v a l e n t interaction of an u n p a i r e d electron with t h r e e protons, a n d it can be ascribed to the alkyl radical (2). F o r e q u i v a l e n t i n t e r a c t i o n w i t h four p r o t o n s of the m e t h y l e n e group the s p e c t r u m of radical (1) should h a v e 5 STS components. Assuming t h a t it is impossbile for the u n p a i r e d electron to i n t e r a c t w i t h two p r o t o n s of t h e four, due to the steric obstacles, the s p e c t r u m of radical (2) would consist of t h r e e STS lines. I n this case the s p e c t r u m of the seven SFS lines can be f o u n d b y superim-

f *

75 oe

J

i

d

J FIG. 4, ESR

spectrum of polymethylmethacrylate exposed to y-rays at 77°K; a-- spectrum of same specimen after irradiation through light filter ZhS- 12; the same, SZS-22; d--the same SS-6. Measurements were made in liquid nitrogen.

1782

V. K. MILINCHUK and S. YA. PSHEZHETSKn

posing the (1) and (2)radical spectra. The asymmetric doublet with splitting of approximately 139 oe probably belongs to the radical HCO [9], although the question of its origin is not clear. Since the intensity of the doublet is poor, it must be formed in rather low concentrations. After exposure to light followed by a holding period, we m a y get the reversible radical (1) to radical (2) transformation. Depending on which of these is dominant under the given conditions, particular SFS lines will be predominant in the total ESR signal of a y-irradiated polymer. Exposure to light seems to increase the concentration of type (2) radicals. Due to exposure to UV light (which can be absorbed by the polymer), the intensity of the triplets which can be ascribed as radical (1), increases. An increase in the intensity of the triplet lines due to UV light can be seen in Fig. 3c. Recorded after the polymer had been heated up to 178°K, the spectrum of the free lines with the relation 1 : 2 : 1 (Fig. 2a) probably belongs to radical (1). The reversibility indicates that equilibrium is gradually being established between (1) and (2) radicals, and this depends on the temperature. At 77°K (1) radicals are observed; when exposed to light this equilibrium is displaced towards the (2) radical. As we said, the conversion mechanism appears to consist in the migration of an unpaired electron up the molecular chain; its stabilization at any particular carbon atom decides the shape of ESR spectrum. This in its turn depends on the relation between the number of radicals of the structure in question. Polymethylmethacrylate. At liquid nitrogen temperature the ESR spectrum of ?-irradiated PMMA has a very poorly defined hyperfine structure. Schneider [6] claims to have received a signal without any kind of hyperfine structure. But in a carefully recorded spectrum eleven faint STS lines can be found (Fig. 4a). Subsequent exposure in liquid nitrogen in visible light with ~min~4400 ~k (light filter ZhS-12) transforms the original narrow signal into a broader line of the same intensity (Fig. 4b). This exposed to light with )~min~50,800 A (light filter SZS-22) there was a further change in the shape of the spectrum (Fig. 4c); in particular, the resolution of the STS components is rather improved. After using light filter FS-6 (3200-4200/~) eleven STS lines are quite clearly revealed (Fig. 4d). The radical concentration remains practically unchanged when exposed to visible light. UV radiation slightly reduces it. If PMMA ?-irradiated at 77°K is heated, the shape of the spectrum changes and there is also a drop in the concentration of radicals. Figure 5 shows that between 77 and 204°K the spectrum is mainly of four STS lines (intensity ratio 1 : 4 : 6 : 4 : 1). I f heating is brought up to higher temperatures this improves the resolution times, and four more SFS lines appear (Fig. 5d). The spectrum, with which after exposure through light filter FS-6 has the shape shown in Fig. 4d, after subsequent heating to approx. 230°K consists of two groups of four and five components with approximately the same intensity. I f the temperature is raised the intensity of the quartet falls, while that of the quintet rises (Fig. 5d). If

Effect of light on free radicals

1783

exposed to UV light there is an increase in the intensity of the group of four lines (Fig. 6a). Heating to room temperature again gives a spectrum consisting of nine lines (Fig. 5d). The radical concentration remains constant.

O.

S

b

.J

F 150 oe

Y

d

FIG. 5. Temperature dependence of ESR spectrum of polymethylmethacrylate exposed to 7-rays at 77°K: a--77°K; b--178°K; c--234°K; d--290°K. Measuring temperature 77°K. I f the specimen is held for two weeks in liquid nitrogen after exposure, the intensity of the quartet falls and the side components disappear (Fig. 6b, c). Subsequent exposure of this specimen to the unfiltered light of mercury lamp DRSh-250 again restores the spectrum which was observed after soaking in liquid nitrogen (Fig. 6a). Thus, as in the other cases, the changes observed in the spectrum as a result of exposure to light at 77°K, are completely reversible. According to the components, the ESR spectrum of PMMA after exposure can be divided into groups consisting of nine STS lines and a doublet. In the honer the distances between the components of the quartet and quintet are 25 =~ 1 oe. Splitting in the doublet is equal to approx. 139 oe. Due to exposure to the light there is an increase in the intensity of the group of four ]ines, though this immediately after the 7-irradiation at 77°K, and also

1784

V. K. MILINCHUK

and

S. YA. PSHEZHETSKII

after heating. After photo-irradiated specimens are heated the intensity of the quartet drops. Bresler and his collaborators [4], who heated PMMA specimens containing stabilized radicals from 25 to 60°C, also found an increase in the intensity of the quartet lines and drop in the intensity of the quintet lines.

J

75oe

b

f

:/ C

FIG. 6. a--ESR spectrum of PMMA first heated at 290°K, after exposure to combined light or UV light; b--ESR spectrum of the same specimen after keeping for 12 hr in liquid nitrogen; c--after 15 days. Measuring temperature 77°K. The spectrum of the nine STS lines in PMMA is attributed by Ingram and collaborators [7], and also by Abraham and collaborators [8], to the presence of two conformations of the structure radicals CH3 --CH~--C"

I

(7)

I

COOCHs in which an unpaired electron is part of the time interacting with three equivalent protons, and the other part with four. I n [7] and [8] different types of interaction were observed between the unpaired electron and the proton of the methyl and methylene groups. I t might also be thought t h a t the changes in the ESR spectrum in PMMA are due to the influence of the methyl and methylene group rotations. But the relaxation time of this kind of process should be much less than t h a t observed in the experiment. As the relaxation time was very long, this must show t h a t the changes in the ESR spectra are associated with radical conversions of the follo~ngt~e CH 3 --CH2--C"

l i

COOCI-I3

6H~ --> --CH2--CH

l

(s)

l

COOCH~

In (8) radical the free valence m a y only interact with three protons. The appropriate ESR spectrum consists of four components which, together with the five lines of radical (7), give the total spectrum of line STS lines observed.

Effect of light on free radicals

1785

As t h e c o n c e n t r a t i o n of (8) increases t h e r e should also be a n increase in the i n t e n s i t y of t h e q u a r t e t . I t seems t h a t t h e (7) radical w o u l d be m o r e energetically f a v o u r a b l e . T h e r e f o r e a f t e r e x p o s u r e to light, u n s t a b l e e q u i l i b r i u m should arise b e t w e e n t h e radicals CI.I~

CI'I2

i

--CHs--C"

and

I

--CH2--CH

i

COOCH3

I

COOCH a

a n d in a s y s t e m w i t h o u t e n e r g y s u p p l y in t h e f o r m of light q u a n t a this equilibriu m will shift t o w a r d s t h e energetically stable (7) radicals. A t a low t e m p e r a t u r e (77°K) t h e t i m e of this k i n d of r e l a x a t i o n will be greater, for which r e a s o n it is possible to " f r e e z e " a n d r e c o r d this k i n d of e q u i l i b r i u m b e t w e e n (7) a n d (8) radicals b y t h e E S R m e t h o d . As for t h e t w o e x t r e m e a s y m m e t r i c a l c o m p o n e n t s w i t h splitting of a p p r o x . 139 oe., as in t h e case of P V A , it seems t h a t t h e y should be ascribed to t h e f o r m a l radical H C O . CONCLUSIONS

(1) Visible a n d U V light h a s b e e n f o u n d to cause a change in t h e STS E S R s p e c t r u m in 7-irradiated p o l y v i n y l a c e t a t e a n d p o l y m e t h y l m e t h a c r y l a t e a t 77°K, due to p h o t o c o n v e r s i o n s of free radicals. (2) T h e t r a n s f o r m a t i o n s of t h e stabilized radicals b y t h e light is f o u n d to be reversible; a f t e r p r o l o n g e d e x p o s u r e a t 77°K t h e original s h a p e of t h e E S R s p e c t r u m is restored. I t is concluded t h a t t h e r e is d y n a m i c e q u i l i b r i u m b e t w e e n t h e isomeric shapes of t h e radicals. (3) Possible s t r u c t u r e s are considered for t h e radicals, a n d possible m e c h a n i s m s of t h e reversible t r a n s f o r m a t i o n observed. Translated by V. AL~'ORD REFERENCES

1. V. K. MELINCHUK and S. Ya. PSHEZHET.SKII, Vysokomol. soyed. 5: 946, 1963 2. V. K. MELINCHUK and S. Ya. PSHEZHETSKII, Dokl. Akad. Nauk SSSR 152: 665, 1963 3. R. J. ABRAHAM and D. H. WHIFFEN, Transaction of the Faraday Society 54: 1297, 1958 4. S. E. BRESLER, E. N. KAZBEKOV and E. M. SAMINSKII, Vysokomol. soyed. 1 : 132, 1959 5. S. N. USHAKOV, Polivinilovyi spirt i ego proizvodnye. (Polyvinyl Alcohol and its Derivatives.) 2, Izd. Akad. IVauk SSSR 1960 6. E. E. SCHNEIDER, Discussions Faraday Society 19: 158, 1955 7. D. INGRAM, N. SYMONS and M. TOWNSEND, Transactions Faraday Society 54: 409. 1958 8. R. J. ABRAHAM, H. V. MELVILLE, D. W. OVENALL and D. H. WHIFFEN, Transactions :Faraday Society 54: 1133, 1958 9. F. J. ADRIAN, E. L. COCHRANE and V. A. BOWERS, J. Chem. Phys. 36: 1661, 1962 10. S. E. BRESLER, E. I. KAZBEKOV and E. M. SAMINSKII, Vysokomol. soyed. 1 : 1374~ 1959 11. A. CHARLESBY, Nuclear radiations and polymers, 1962