Radiothermoluminescence of ionomers: Copolymers of methacrylic acid with styrene and ethylene, containing UO22+ ions. Thermal deactivation of UO22+ excited states

Polymer Science U.S.S.R. VoL 21, pp. 1070-1076.

0032-3950/79[0501-1070507.50[0

(~) Pergamon Press Ltd. 1980. Printed in Poland

RADIOTHERMOLUMINESCENCE OF IONOMERS: COPOLYMERS OF METHACRYLIC ACID WITH STYRENE AND ETHYLENE, CONTAINING UO 2+ IONS. THERMAL DEACTIVATION OF UO~+ EXCITED STATES* S. R. I~APIKOV, V. N. KOROBEINIKOVA, D. D. APONICHEV, V. P. KAZAKOV, '

G.V.

LEPLYANIN a n d L. F. ANTOI~OVA

Institute of Chemistry of the Bashkir Branch of the U.S.S.R. Academy of Sciences

(R~eived 6 September 1977) The radiothermohuninescence of copolymers of styrene with methacrylic acid, ionomer I, and of ethylene with methacrylic acid, ionomer I I , in which the protons of the carboxyl groups are replaced by UOI + ions, has been studied. The radiothermoluminescence curves of ionomers I and I I contain a maximum and a numbe~ of inflexions, which are associated with the freeing of molecular motion of chain fragments and of the functional groups. The temperature dependence of the energy of activation for radiothermoluminescence of ionomer I was found. I t was found from the photo- and radiothermoluminescence spectra that the emitters of photons in radio%hermoluminescence of ionomer I are UO~+ ions over the entire range of experimental temperatures. The "bleaching" of radiothermoluminescence by visible light leads to irregular reduction in intensity. The r~aximum after "bleaching" appears a t the temperature of one of the inflexions in the "unbleached" radiothermoluminescence curve. The temperature dependence of the life, v, of excited UO.2+ ions in ionomers I and ] I was found. The v = f ( T ) curve contains saveral steps, corresponding to relaxational transitions. I t is concluded that the temperature of deactivation of excited UO :+ ions is determined by a series of different molecular fragments of the matrix entering successively into electronic-vibrational interaction with the (UOI+)* ions. I t is suggested that in the 60-120°K range interaction with vibrations of fragments of the hydrocarbon chain makes the major contribution to deactivation, while at 130° K and 180°K the contribution of motion of phenyl groups becomes important, and at 160°K that of carboxyl groups.

UO~ + i o n s d i s p l a y clear fluorescence a n d t h e life of t h e i r e x c i t e d s t a t e is e a s i l y m e a s u r a b l e [1, 2]. T h e s t r u c t u r e d fluorescence s p e c t r u m of UO~ + m a k e s p o s s i b l e i d e n t i f i c a t i o n of t h e e m i t t e r of r a d i o t h e r m o l u m i n e s c e n c e ( R T L ) . I n m a n y c o m p o u n d s t h e r e is a c h a r a c t e r i s t i c , s t r o n g d e p e n d e n c e of t h e i n t e n s i t y of fluorescence on t e m p e r a t u r e , a n d we e x p e c t e d t h a t t h i s m a y b e r e l a t e d to f r e e i n g o f m o l e c u l a r m o t i o n , on which, as is well k n o w n [3], t h e R T L of p o l y m e r s is d e pendent. * Vysokomol. soyed..tJ21: No. 5, 979-984, 1979. 1070

l~adiothermoluminescence of ionomers

1071

A styrene-MAA copolymer with a ratio of monomer units of 9 : 1 (ionomer I) was prepared by radical polymerization. After neutralization of the solution with a salt of uranyl acetate the ionomer precipitated out, was washed free from solvent and dried. Discs of thickness 0.3 mm and diameter 7 mm were cold moulded under a pressure of about 300 kg/cm 2 from the resulting yellowish green powder. Preparation of the ethylene-MAA eopolymer with a ratio of monomer units of 9 : 1 (ionomer II), was described in reference [4]. The ionomers were irradiated either in a URS-55 X-ray apparatus (dose rate 30 krad/hr) or in an "Issledovatel" y-ray apparatus (dose rate 0-8 Mrad/hr, 60Co). The life period of the UO: + ions was measured by a T-metre with photographic recording of the quenching curve from the oscillograph screen. The source of excitation light was an ISSh-100 lamp with a light-pulse time of 10-e see, or an LGT-21, pulsed nitrogen laser ( ~ 337 nm). The RTL curves were recorded by the apparatus described in reference [5]. The energies of activation for the relaxational processes in ionomer I were found from the temperature dependence of the RTF intensity, by the method described in references [6-10]. In these experiments the temperatures to which each successive heating cycle (cessation of heating, cooling to 77°K then the next heating stage) was taken, correspond to the points in curve 3, Fig. 1. T h e R T L curve of i o n o m e r I has a m a x i m u m a t l l 0 ° K a n d a n u m b e r o f inflexions, n a m e l y a t 100°K on the l e f t h a n d side a n d a t 130, 160 a n d 200°K on t h e right. An e x t r a o r d i n a r i l y n a r r o w p e a k is r e c o r d e d a t 93°K, its w i d t h being a few gauss. I l l u m i n a t i o n of i o n o m e r I a f t e r 7-irradiation, b y the l i g h t of a n inc a n d e s c e n t l a m p ("bleaching"), reduces the R T L intensity, b u t n o t to t h e same e x t e n t as for t h e c o p o l y m e r into which UO~ + ions h a v e n o t been i n t r o d u c e d . W h e r e a s in t h e l a t t e r case illumination w i t h a 400 w a t t laInp for 15 min reduces t h e R T L i n t e n s i t y a t h o u s a n d times, in the first instance the same p r o c e d u r e reduces t h e i n t e n s i t y b y a f a c t o r of only fifty. I t m a y be concluded f r o m this t h a t UO~ + ions in t h e c o p o l y m e r e x e r t a p r o t e c t i v e action against t h e " b l e a c h i n g " o f R T L b y visible light. I n o t h e r respects " b l e a c h i n g " produces the same results w i t h the c o p o l y m e r w i t h o u t UO~ + ions as for the ionomer. As well aS falling off in i n t e n s i t y t h e R T L is redistributed, t h e m a x i m u m appearing a f t e r " b l e a c h i n g " a t one of the inflexions in the " u n b l e a c h e d " R T L curve, n a m e l y a t 130°K (Fig. 1). I o n o m e r I containing UO~ + has t h e slight yellowish-green colour characteristic of u r a n i u m salts. S t y r e n e - M A A copolymers fluoresce in t h e blue region o f t h e spectrum. I t s R T L emission occurs in the same region of the spectrum. T h e blue fluorescence disappears c o m p l e t e l y after i n t r o d u c t i o n of UOI + ions into the c o p o l y m e r . T h e fluorescence of UO~ + ions, p r o d u c e d b y ionomer I a t 300°K also does n o t occur, ~.owever. T h e green fluorescence of UO~ + appears on cooling a n d a t 77°K t h e s t r u c t u r e d s p e c t r u m characteristic of this ion (Fig. 2) is easily recorded. T h e UO~ + ion in i o n o m e r I becomes the only e m i t t e r of p h o t o n s o v e r t h e e n t i r e r a n g e of t e m p e r a t u r e s (Fig. 2). T h e luminescence in t h e blue p a r t o f t h e s p e c t r u m disappears completely. T h u s o v e r the whole R T L range of temp e r a t u r e s , t h e UO~ + ion is t h e a c c e p t e r of the e n e r g y of the r e c o m b i n a t i o n processes. W h e n t h e h y d r o g e n of the c a r b o x y l groups is c o m p l e t e l y replaced, the n r a n y ] ions are s i t u a t e d a t a distance of n o t more t h a n t w e n t y CHz groups f r o m

:1072

S . R . RAFIXOV e t a l .

~ne another along the hydrocarbon chain. Then if the R T L emitter luminescing i n the blue region of the spectrum in the case of the copolymer t h a t does not c o n t a i n UO~ + ions was removed by the introduction of the uranyl ion, this means

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T,°K Fie. 1. Ionomer I containing UOl + ions: RTL curve after X-irradiation at 77°K •(1) and after X-irradiation at 77°K and illumination by a 400 W incandescent lamp for 15 rain (dosage 7 krad, rate of heating 7 deg/min) (2), and the dependence of the energy of activation for RTL (3) and the life of the UO~+ ion in ionomer I in the excited state (4) on temperature. • h a t in recombination of electrons with hole centres the energy is transferred ~)ver a distance of ten monomer units along the chain. Voids in the polymer, aromatic impurities and the benzene rings of the styrene units can act as traps f o r electrons. Benzene rings, foreign aromatic molecules and the carbonyl part ~ f acid groups can also act as hole traps. I t is evident t h a t among the p r i m a r y -traps electrons trapped in structural defects are released, as occurs in T H F solid ~olutions for example [11, 12]. Recombination in hole traps leaves benzene rings :and aromatic impurities, after which follows emissioniess transfer of energy b y u r a n y l ions, obviously by an inductive resonance mechanism. I n view of the pre~ponderance of styrene units in ionomer I, relaxational processes in the polystyrene fragments obviously make a considerable contribution to the RTL. The isothermal luminescence at 77°K can be attributed to freeing of charges in the ~-relaxation process [13-16].

1073

Radiothermolumineseence of ionomers

The energy of activation, E, for freeing of the molecular motion of ionomer I in the temperature range of 77-160°K increases in steps (Fig. 1), as is characteristic of m a n y polymers [10]. At the beginning of the R T L curve E is about 1.5 kcal/mole, which is close to E for the g-transition in PS ( ~ 1.6 kcallmole [13]). I n the temperature region of the first, narrow R T L maximum at 93°K E in_T mLun. /0 I

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]FIG. 2. The fluorescence spectrum with excitation by light with 4=313 nm (77°K) (1) and the RTL spectrum (2) of ionomer I containing UO~+. FIG. 3. Dependence on dosage of the RTL intensity of ionomer I containing UO] ~. creases abruptly. In PS there is a relaxational transition at about 100°K, with E-----2.7 kcal/mole [13]. The region of increasing R T L intensity on the low temperature side of the maximum corresponds to an energy of activation of a b o u t 3 kcal/mole. Then in the 120°K ' region we find a steeper rise in E which changes to a shoulder where it varies between 5 and 6 kcal/mole. In this region (125-140°K) investigation b y the dielectric method discloses no relaxational processes in PS, b u t the mechanical method shows a transition in the region of 130°K, which is attributed to the motion of chain fragments [13]. There is an inflexion in the R T L curve in this region. Then E again rises, approaching a value of 8-9 keal/ ]mole, which is near to the energy of activation for the y-relaxation process with E----9 kcal/mole, assigned in reference [13] to rotation of phenyl groups. Calculation of the energy of activation for rotation of phenyl groups about the bond with the main chain gave a value of ~ 7 kcal/mole [16], which is close to E for the y-relaxation. The dependence of the R T L intensity of ionomer I on dosage, rapidly reaches saturation (Fig. 3). This is not connected with irreversible change in the valence state of the uranium, since the colour of the ionomer is restored after it has been warmed to room temperature. Evacuation of the polymer at 5 × 10 -~ torr before

1074

S. R. R~IKOV e$ a/.

?-irradiation, for the purpose of removal of dissolved oxygen, has no effect on the intensity or shape of the R T L curve. Similarly increase in dosage up to 2.5 Mrad does not produce any new features, apart from the fact that the structural details of the R T L curve become weaker. The fall in R T L intensity after the maximum occurs not only because of decrease in the concentration of electron and hole centres b y recombination, b u t also because of thermal deactivation of the electronically excited UO~ + ion emitter. I t is usually considered that fall in the intensity of fluorescence and corresponding life period of the excited state of the molecules, follows a smooth curve described b y the Arrhenius equation [1, 17-19]. In investigation of vitrified solutions of UO~ + in HaSO 4 we found however that the curve of the dependence of of UO~ + on temperature is b y no means smooth, b u t contains a number of inflexions and maxima, caused b y structural transitions in the vitrified matrix [20]. A similar effect occurs in thermal deactivation of UO~ + present in ionomer I (Fig. 1). Several sections can be seen where ~_varies more rapidiy, and the steps and inflexions in the ~-T curve correspond well with the relaxational transitions displayed in the R T L curve and found b y other, independent methods. Over the range of 180-220°K the value of r becomes halved. This step is evidently related to the mobility of phenyl groups. The freeing of molecular mobility of small fragments of the main chain, evidently caused b y relaxational transitions close to 120°K, has a smaller effect on deactivation of UO~ +. A small drop in z can be seen close to the first, very narrow R T L peak, and there is a surprisingly rapid increase in ~ below 77°K. Thus as well as reduction in the rate of recombination processes, quenching is a substantial cause of fall in R T L intensity. I f in obtaining the R T L curve a correction is introduced for thermal deactivation, we find that excited UOzs+ ions are formed almost as efficiently a t 240°K as at lower temperatures. Thus thermal deactivation of excited UO~ + ions does not correspond to a single activation process, b u t is brought about b y various molecular fragments of the matrix, taking part successively (as the temperature rises) in electronic, vibrational interaction. In the 60-120°K region the main contribution is apparently made b y interaction with vibrations of fragments of the hydrocarbon chain, and at 130°K and 180°K motion connected with phenyl groups is the major factor. At 160°K it is evident that motion of carboxyl groups is decisive

[21]. Similar results were obtained with ionomer' I I (Fig. 4). We first see a rise in v as the temperature is raised from 77°K to 100°K, then coinciding with emergence of mobility of segments of the hydrocarbon chain, there is a downward step in v. The next two steps correspond to the temperatures 165°K and 190°~i~, where deactivation is possibly caused b y motion of carboxyl groups [21]. Thermal deactivation of fluorescing compounds can be a profitable subject for investigation of the nature of electronic, vibrational interaction and of relaxational transitions in polymers. Here account must be taken of the fact t h a t

Radiothermoluminescence of ionomers

1075;

various physical effects can lie at the basis of the temperature dependence of l luminescence in polymers or solid solutions. These include, firstly, change in t h e transparency of the medium as a result of inhomogeneity brought about by a relaxational or structural transition [22], secondly change in the rate of diffusion of particles that are either quenching agents for the excited states [23], o r one of the participants in a recombination process [24], and thirdly true electronic, vibrational interaction with the various fragments of the polymer t h a t have predominant mobility at the different temperatures of relaxational and structural transitions, and causing thermal deactivation. wlO! $ec 1,re/.un.

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FIG. 4. R T L curve of ionomer I containing UO~+ after X-irradiation at 77°K (dosage 7 krad).

In real systems each of these effects plays a larger or smaller part [25]. In., the system investigated by us, deactivation of the UO~+ obviously occurs by electronic, vibrational interaction with fragments of the polymer, because the fluorescence of UO~+ ions is not quenched by dissolved gases and during the life period of ions in the excited state there is no significant change in transparency. Regardless of which of the above processes predominates, the intensity and other temperature-dependent characteristics of fluorescence can serve as useful (either major or supplementary) means of investigation of relaxational processes. in polymers. Translated by E. O. PHILLIPS.

~:1076

S . R . RAFIKOV et at. REFERENCES

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