Makromol. Chem. 181,1021 -1027(1980) 1021

p-Carotene Inhibited Photooxidation of Polystyrene, 2 a)

Effect of p-Carotene on Anthracene Sensitized Photooxidation of Polystyrene Films

Maria Nowakowska

Department of Physical Chemistry and Electrochemistry, Institute of Chemistry, Jagiellonian University, Karasia 3, 30-060 Krak6w, Poland

(Date of receipt: March 5 , 1979)

SUMMARY: The inhibiting effect of 8-carotene on anthracene sensitized photooxidation of polystyrene

was observed and a kinetic scheme of the photophysical and photochemical processes in the system was proposed. The rate constants of several processes occurring in the system were evaluated. The indexes of reactivity for anthracene and /3-carotene in polystyrene were determined.

Introduction

In previous studies it has been shown that anthracene can sensitize photooxidation of polystyrene1--3). The authors try to explain this fact as connected with the reaction of polystyrene with singlet oxygen which can be generated in the quenching of the anthracene excited triplet state.

8-Carotene quenches singlet oxygen very effectively in the liquid phase4-- ’). Besides, it has been found that 8-carotene inhibits photodegradation of polystyrene in benzene solution*). Consumption of 8-carotene was observed during irradiation of a polystyrene film with light at A = 313 nm, absorbed by a polystyrene-oxygen complex9).

This paper describes the attempts to evaluate the effect of 8-carotene on anthracene sensitized photooxidation of polystyrene.

Experimental Part

Polystyrene (PS) was prepared by thermal polymerization of styrene at 85 “C for 450 h in the absence of oxygen. The resulting polymer was purified by dissolving it in chloroform and precipitating with methanol. This process was repeated three times.

Anthracene (A) was Analar reagent. 8-Carotene CS) was obtained from BDH Chemicals, Poole, England. Anthracene and /3-carotene were incorporated to polystyrene films by prep- aring a chloroform solution of these compounds and dissolving an appropriate amount of poly- styrene in this solution. The films were made by pouring the resulting solution onto a Petri dish floating freely on mercury in the dark.

a) Part 1: cf.’’).

1022 M. Nowakowska

The samples were irradiated at A = 365 nm with an HBO-200 high-pressure mercury lamp, and a Carl Zeiss interference filter. Incident radiation intensity at A = 365 nm was of 1 . lo-' einst . s - - ' . ~ r n - ~ *) (3,28 . lo-' J . s - I . ern-.').

UV, VIS, and IR spectra of the films after various irradiation times were recorded with a Zeiss Specord UV-VIS and UR 10 spectrophotometer, respectively.

Results and Discussion

In order to observe the effect of 8-carotene on anthracene sensitized photooxida- tion of polystyrene the following experiments were performed. Polystyrene films of the same thickness, containing anthracene of the same initial concentration and /3-carotene of various initial concentrations were irradiated with light at I = 365 nm. Fig. 1 shows as an example the UV-VIS absorption spectra of the polystyrene film (15 - cm thick) containing anthracene (initial concentration c = 6,6. m ~ l . d m - ~ ) and 8-carotene (initial concentration c = 3,l mol * dm-3) after various periods of irradiation with light at I = 365 nm in the presence of oxygen under atmospheric pressure.

36000 32000 28000 24000 20000 Wave number in crn-'

Fig. 1 . UV-VIS absorption spectra of polystyrene film containing anthracene and 8-carotene, after various periods of irradiation at L = 365 nm in the presence of oxygen under atmospheric pressure: film thickness I = 15 . mol-dm--3, initial /.-carotene concentration c = 3,l * mol.dm-3. 1: 0 min; 2: 1 min; 3: 3 min; 4: 4min; 5: 8 min; 6: 12 min; 7: 25 min; 8: 40 min; 9: 60 min

cm; initial anthracene concentration c = 6 ,6 .

The decrease of absorption within the range of 26000-20000 cm-' is caused by 8-carotene consumption during irradiation of the system. Under these experimental conditions anthracene is also consumed (decrease of absorption within the range of

~

*) In SI-unit: 1 einstein = N L . h . v = 328 kJ.

/?-Carotene Inhibited Photooxidation of Polystyrene, 2 1023

31 OOO- 26000 cm--'). The increase of absorption at wave numbers > 33 OOO cm--' is due to the formation of photochemical reaction products from anthracene and poly- styrene. The comparison of UV-VIS absorption spectra of polystyrene containing anthracene and B-carotene (Fig. 1) with the absorption spectra of a polystyrene film containing anthracene of the same concentration (see Fig. 1 3)) indicates that B-carotene inhibites anthracene sensitized photooxidation of polystyrene. This conclusion was confirmed by IR analysis (Fig. 2). It can be seen that D-carotene, present in the polystyrene film, irradiated with light absorbed by anthracene (A =

365 nm) decreases the concentration of carbonyl products.

Fig. 2. IR absorption spectra of polystyrene film containing anthracene, before 8 20 irradiation (curve 1) and .- c

5 40 after irradiation at 1 = 365 nm for 3 h (curve 2),

f 60 and polystyrene film containing anthracene

irradiation (curve l), 80 after irradiation at 1 = 365 nm for 3 h (curve 3)

- u) u) .-

g ; and D-carotene, before I-

"?500 1600 1700 1 8 0 0 1 9 0 0

Wave number in cm?

The results obtained here and in previous papers3s9) suggest that B-carotene can act as an inhibitor of polystyrene photooxidation because of its ability to singlet oxygen quenching.

One can propose the kinetic scheme of photophysical and photochemical processes similar to that given in the preceding paper"). In addition to Eqs. (i) - (xix) which were considered, the possibility of occurrence of two others processes (Eqs. (xx) and (xxi)) must be taken into account.

3A* __t '4 + hv" (xx) k:[3A*] Phosphorescence

PS + '02 - P (mi) 4 s "Otl Reaction between PS and singlet oxygen

PS: polystyrene, P: reaction products.

Thus, the modified expression for the quantum efficiency of anthracene excited triplet state quenching by oxygen is as follows

the quantum efficiency of the reaction of anthracene with singlet oxygen

1024 M. Nowakowska

and the quantum efficiency of the physical deactivation of singlet oxygen by b-carotene and their chemical reaction

The rate of anthracene consumption (V) in this system can be expressed as

v = z,A(y;ge". . @; + @A) ( 5 )

where: y;? is the quantum yield of singlet oxygen generation in the system poly- st yrene/m%hracene/p-carotene.

0;

The rate of p-carotene consumption V' in the system is given by Eq. (6).

vp = ZaA(@ET @p + Y i g e I b p @p + Y q v p J + Z!@p (6) 0;

In the previous papers3," it has been found that

and

(kpo + 6 - kp) dm3. mol- ' . s- ' = (6 ki) dm3 - mol- ' - s - '

It is known that the process of singlet oxygen quenching by &carotene is controlled by diffusion5). The rate constant of the quenching process controlled by diffusion is given by Eq. (10).

NL: Avogadro's number. W 3 ; RAB: radius of interaction; DAB = DA + DB: sum of diffusion coefficient of reagents; p : probability of quenching in a collison; tN: lifetime of excited state quenching molecule.

The mean value of the rate constant of singlet oxygen in the polystyrene film deactivation was found to be

kd = 232.10' dm3. mol- ' . s - ' (1 1)

B-Carotene Inhibited Photooxidation of Polystyrene, 2 1025

when: p = 1 , R,, = 10. assuming that the lifetime of singlet oxygen is in the range of 1 (because the lifetime of '0; in polystyrene is still unknown).

physical deactivation process and in a chemical reaction. Therefore

cm, DAB = D, = Do, = 3,2 lo--' cm2 s--I 'I), and to ' S

In the kinetic scheme, proposed here, singlet oxygen is quenched by 8-carotene in a

k + k, = k, = 2,52.108dm3.mol-1.s-1 (12) fl0

Substituting the value of r$fi = 6 - in Eq. (6) and transforming it, the following Eqs. were obtained:

Under the experimental conditions the concentration of 8-carotene is low. Thus, eET can be neglected and it can be assumed that y ; ':". = y,';. = 0,69".

0 2 Then

Vp - Z,8.6.10-3 0,69 . I,"

= 6 . @, + @so

Transformation of Eq. (14) and substitution of expressions given in Eqs. (3) and (4) lead to Eq. (15).

Z," . 0,69

- Z,8.6.10-3 - 1 1 ) V S . T the following relation- I," * 0,69

from the graph (Fig. 3) of ships were found

k, [A] + ki + kbs = 0,170dm~-3.mol

6 . kp + kso

Solution of Eqs. (7), (8), (9) and (16), (17) gives the values of the rate constants ki, k,, k i S , and kso.

ki = 4,66 .lo5 -- = 2 (18)

k, = 2 ,5 . lO5drn3 .mol - ' . s~~ ' (19)

kbs = 1,3. 104s-' (20)

k, = 2 , 5 . 1 0 8 d m 3 . m o l ~ ~ 1 . s ~ ~ 1 (21)

kso = 1 , 3 ~ 1 0 6 d m 3 ~ m o l - 1 ~ s ~ ~ 1 (22)

2

The reactivity indexes 8 for anthracene were determined to be

1026

N

'0 A - 3 0 -

M. Nowakowska

?g . =..<

6.- ; g s

' D " ? %

m

ki kr

/IA = - = 1,86m0l.dm-~

and for /?-carotene in the polystyrene film:

(24)

In the same manner the reactivity of polystyrene was evaluated. The "pseudo re- activity'' index /?is was defined as:

ki pS =- = 3 , 6 . 1 0 ~ ~ ' r n 0 l . d m - ~ kSO

(25) ki 4%

/3' PS - - - = 36rn0l.dm-~

Conclusions

8-Carotene inhibites the photooxidation of polystyrene sensitized by anthracene. This can be explained by the assumption that /?-carotene quenches singlet oxygen in the system. The physical quenching process is much more efficient than the chemical reaction of the singlet oxygen with /?-carotene:

8-Carotene Inhibited Photooxidation of Polystyrene, 2 1027

3)

G . RSimme, J. F. Rabek, Eur. Polym. J. 13, 855 (1977) M. Kryszewski, B. Nadolski, in “Singlet Oxygen. Reactions with Organic Compounds and Polymers”, edited by B. RBnby, J. F. Rabek, Wiley, London 1978, p. 244 M. Nowakowska, Makromol. Chem. 179, 2959 (1978)

ibid. 90, 6234 (1968) 4, C. S. Foote, R. W. Denny, J. Am. Chem. SOC. 90,6233 (1968); C. S. Foote, R. W. Denny,

’) C. S. Foote, Y. C. Chang, R. W. Denny, J. Am. Chem. SOC. 92, 5216 (1970) 6, A. Farmilo, F. Wilkinson, Photochem. Photobiol. 18, 447 (1973) ’) I . B. C. Matheson, J. Lee, Chem. Phys. Lett. 14, 350 (1972) ’) J . F. Rabek, B. RBnby, J. Polym. Sci., Part A-1, 12, 273 (1974) 9, M. Nowakowska, in “Singlet Oxygen. Reactions with Organic Compounds and Polymers”,

edited by B. RBnby, J. F. Rabek, Wiley, London 1978, p. 254 lo) M. Nowakowska, Makromol. Chem. 181,1013 (1980)

M. Nowakowska, J. Najbar, B. Waligora, Eur. Polym. J. 12, 387 (1976)