Photodecomposition of commercial polysulphones and model diaryl sulphones

European Polymer Journal. Vol. 13, pp. 1019 to 1023. Pergamon Press 1977. Printed in Great Britain.

P H O T O D E C O M P O S I T I O N OF COMMERCIAL POLYSULPHONES A N D M O D E L DIARYL SULPHONES F. ABDuL-RASOUL,C. L. R. CATHERALL,J. S. HARGREAVES,J. M. MELLOR and D. PHmUPS Department of Chemistry, The University, Southampton SO9 5NH, England (Received 31 January 1977)

Abstract--The absorption and emission characteristics of commercial polysulphones have been compared with those of model diaryl sulphones. Quenching studies with furan and 2-methyl furan show that quenching of excited states is efficient. INTRODUCTION

photophysical and photochemical properties of both model compounds and commercial polysulphones our principal concern being possible retardation of photodegradation.

The polyaryl sulphones are characterized by high thermal stability [1] permitting their use at temperatures above 150 °. Typical polysulphones, e.g. poly/(ether sulphone) 200P (1) or bakelite polysulEXPERIMENTAL phone (2), are characterised [2] by low photostability. On exposure to sunlight, rapid changes in appearance Materials and in physical properties occur. Yellowing occurs Polysulphone (1) and polyethersulphone (2) were used readily on brief exposure outdoors and is even evident as obtained. Bis (4-chlorophenyl) sulphone (K and K after more prolonged exposures indoors. The develop- Laboratories) was recrystallized from petroleum ether ment of colour is associated with marked reduction (100-120 °) and had m.p. 147-148 °. Diphenyl sulphone (m.p. 126-128°) and bis (4-methylphenyl) sulphone (m.p. in tensile strength of the polymer. 159-161 °) were prepared by standard methods [7] and rePreliminary studies of the photodecomposition [2] crystallized from ethanol. Bis (4-hydroxyphenyl) sulphone of sulphones have been made both with commercial (m.p. 249-251 °) and bis (4-methoxyphenyl) sulphone (m.p. polymers and with model compounds. Unusually for 129-131 °) were prepared by standard methods [8] and recommercial polymers, the onset of photodecomposi- crystallized from ethanol. Reference 4-substituted tion is brought about by the absorption of light by biphenyls were prepared by standard methods [9] and rethe polymer itself and not by impurities, thus polybu- crystallized from ethanol. The m.p. of 4-methylbiphenyl, tadiene, for example, undergoes photodegradation [3] 4-methoxybiphenyl, 4-chlorobiphenyl and biphenyl were by absorption of light by impurities such as ketones 47-49 °, 89-91 °, 78-80 ° and 68-70 ° respectively. Solvents, viz acetonitrile (Aldrich), cyclohexane (Hopkins or hydroperoxides. Hence an understanding of the and Williams) isopentane (Aldrich), methylcyclohexane photodegradation of commercial polysulphones has (B.D.H.), and ethyl acetate (Koch Light) were of spectrocome both by study of the polymers and of diaryl scopic quality and showed no emission. The quenchers sulphones. Study of polymer film shows that irradia- furan (Hopkins and Williams) and 2-methylfuran (Hopkins tion at ~ 330 nm leads to photolysis [2] with con- and Williams) were distilled before use and showed no comitant loss of physical strength and increased emission when irradiated at 255 nm. Similarly cyclo-octa-1, brittleness, and formation of complex degradation 3-diene, purified by preparative GLC showed no emission products. Degradation [2] as shown in Scheme 1 has when irradiated at 255 nm. been suggested for bakelite polysulphone with chain Apparatus and methods scission at a number of possible site~ Although cerInfra-red spectra were measured for solutions in chlorotain aspects of the polymer degradation have been form or using nujol mulls with a Unicam SP 200 spectrostudied [4], there is no knowledge concerning the photometer. Ultra-violet spectra were measured in cyclomechanistic features of the photodegradation and hexane and methanol using a Perkin-Elmer 402 spectrolittle appreciation of the relative importances of the photometer. NMR spectra were measured for solutions in possible degradation modes. Some clarification of the CDCI 3 with TMS as internal standard using a Perkinprobable modes of degradation has come from study Elmer R 12 spectrometer. Mass spectra were obtained of model compounds. Thus photolysis of a series of using an A.E.I. MS 12 mass spectrometer. Most fluorsymmetrical and unsymmetrical aromatic sulphones escence and all phosphorescence studies were performed in benzene [5] established that breakage [3] of a car- using a Farrand Mark I spectrofluorimeter fitted with an bon-sulphur bond was the dominant primary mode Aminco--Bowman phosphorescence dewar attachment, and of photolysis. Isotopic dilution studies [6] established with corrected excitation and emission modes. Fluorescence spectra were recorded in cyclohexane at room temthat a minor photoproduct from diphenylsulphone perature after repeated degassing by a freeze-thaw cycle. was benzenesulphinic acid. However, these early Oxygen has a marked quenching effect upon the fluorreports have given no indication of possible methods escence of sulphones. Phosphorescence spectra were for control of the rate of photodegradation of polysul- recorded at liquid nitrogen temperatures using isopentanephones. We report here our detailed analysis of the methylcyclohexane (1 : 1) glasses. 1019

1020

F, ABDUL-RASOUL et al.

Fluorescence decay times were measured by the single photon counting technique [10] as described elsewhere. Fluorescence quantum yields were measured using doubly distilled 'Analar' grade p-cresol as standard (~$ for p-cresol [11] in cyclohexane is 0,09) as it absorbs and emits in the same regions as the sulphones. Fluorescence quenching was studied at six concentrations of quencher. In all cases [6] linear Stern-Volmer plots passing through the origin were obtained. Phosphorescence decay was recorded photographically from an oscilloscope trace, and phosphorescence quenching was studied similarly as a function of quencher concentration. Linear Perrin plots were obtained. Preparative photolyses were run in sodium dried benzene, previously purged with nitrogen, using a medium pressure mercury lamp and a quartz immersion reactor. After photolysis, products were separated by column chromatography on 'Merck' neutral alumina using cyclohexane-ethyl acetate mixtures as eluents. Typically diphenylsulphone (400 rag) in benzene (50 nil) after irradiation for 2 hr gave biphenyl (66 mg) m.p. 68-70 °. Other more polar fractions obtained as gums were not characterized.

Relative photoreactivities were determined using a merry-go-round reactor. Solutions of sulphones in benzene were irradiated through quartz after purging with nitrogen. Analysis of biphenyl products was made using GLC with n-pentadecane as an internal standard (analysis on a column of 5% SE 30 on Chromasorb G at 130°). Retention times of products were compared with standard reference samples,

RESULTS AND DISCUSSION

Absorption, fluorescence, and phosphorescence spectra of commercial polysulphones (1) and (2) and diaryl sulphones (3-6) in cyclohexane are shown in Figs. 1 and 2. The spectra establish fluorescence emission in all cases between 300 and 350 nnx Change of substituent within the series (3-6) substantially affects neither the absorption characteristics nor the position of the fluorescence emission. Phosphorescence emission is observed for sulphones (3-6) at

(I)

(2)

o%

~o

× (31 x = H ( 4 ) X = Ct ( 5 ) X = Me (6) X = 0Me

CH 3 I

•

- "-o-~c-~.

0 II

+.o ~ S

CH 3

0

CH3 )

0 Ii

CH 3

0

~--%--O--

~H3

oJ

•- ' - -

i ~.=/ CH 3

~Jil

0

0"-~

") • + oC--"~'

~)-0--('

~)--S --~/

\%-- O ~

~

o ÷oCH 3 CH 3

0

ICH2 I

0 [i

CH 3

0

+° H

Photodecomposition of commercial polysulphones

(

I

300

250

1021

I

400

350

450

tlt/~

Fig. 1. Fluorescence spectra of: polysulphone

; poly(ethersulphone) ~ .

useful because it permits an investigation of quenching of this excited state to be made. A possible method of stabilisation of sulphones to u.v. irradiation is the addition of quenchers capable of bimolecular quenching of those excited states responsible for photoreaction. Our preparative photolyses with (3-6) establish that in all cases photoreaction gives biphenyls. GLC analysis of solutions exposed to the same light intensity in a merry-goround reactor showed the following order of reactivity (5)> (4)> (3)> (6) but differences in photoreactivity were small. In benzene, products are mainly 4-substituted bipbenyls [5] formed by intermolecular reaction of aryl radicals with solvent. In acetonitrile 4,4' disubstituted biphenyls are formed by radical recombination of two aryl radicals. Yields here are

~ 4 5 0 n m and, again within this series, the position of phosphorescence remains essentially constant. Fluorescence quantum yields were established relative to p-cresol [11] as standard. Values obtained in methanol and in cyclohexane are reported in Table l along with the absorption characteristics of (3-6). As expected for a rt-Tt transition, solvent effects lead to only very minor shifts in 2~,xvalues in the absorption spectra. Solvent effects do not greatly effect ~b;. In all cases fluorescence is slightly more important in the non-polar cyclohexane than in polar methanol. However, in all cases fluorescent decay is clearly a minor pathway for the excited singlet state. Further the quantum yield measurements show an absence of profound substituent effects. Observation of this emission from the excited singlet states is particularly

I-..~.S-" ~, .. ~ : , , , ~ . - , " ~ - x\ ~;...

//"C--:/" "

.../

\

'~

\

,'"..

, '"\?

t ~ko

'~@o

' 350

I 510

5~0

nm

Fig. 2. Absorption, fluorescence and phosphorescence spectra of: diphenylsulphone ; bis-(4-chlorophenyl) sulphone - - - - - - ; bis-(4-methylphenyl) sulphone . . . . . ; bis-(4-methoxyphenyl) sulphone ..... E.P.I 13q2---G

F. ABDUL-RASOULet al.

1022

Table 1. Absorption and fluorescence data for diaryl sulphones

Compound 3 4 5 6

Absorption spectra Cyclohexane Methanol Log ~ox(nm) Log c 2max(nm) 236 268 275 223 250 244.5 236

4.17 3.28 3.11 4.12 4.39 4.27 4.29

~/Cyclohexane

~s Methanol

0.069

0.032

0.02

negligible

0.096 0.085

0.062 0.055

4.21 3.36 3.24 4.11 4.39 4.30 4.36

236 266 273 226 247 243.5 235

low and no other products have been characterised apart from benzenesulphinic acid [6] from diphenyl sulphone. Again with the study of the photodecomposition of the commercial polymers, isolation of discrete primary products has not been possible. Hence in an analysis of the effect of potential quenchers, the use of product yields as a criterion of quenching is difficult. Our observation of emission however affords a ready tool for examining quenching efficiency. Irradiation at 255 nm permits study of the characteristics of the excited singlet state, Accordingly, we have examined the quenching of fluorescence from polymers (1) and (2) and sulphones (3-6) by furan and 2-methylfuran which do not absorb significantly at 255 nm, the wavelength of the exciting light. Stern-Volmer plots, obtained using cyclohexane as solvent, were linear and passed through the origin. Similarly quenching of emission by furan from polymer (2) in ethyl acetate gave a linear plot. From the Stern-Volmer plots, values of the slope kqz were determined (see tables 2 and 3): values of the fluorescence lifetime were measured by single photon counting. Substituting values for the

Fluorescence quantum yields

singlet state lifetime z permitted calculation of the quenching rate constants kq shown in Tables 2 and 3. The polymers had similar fluorescence lifetimes [10nsec for (1) and 3.7 nsec for (2)]. A number of interesting points emerge. Neither fluorescence lifetimes nor quenching rate constants are grossly changed by substituent effects. The diffusion controlled rate constant in cyclohexane at 25 ° has been determined as 1.1 x 10 t° I mole - t sec - t . Hence quenching approaches the diffusion controlled limit. Although the mechanism of quenching is not clear from these data, it is apparent that furan and 2-methylfuran are very efficient in quenching the excited singlet state. As photolysis of the polymer involves excitation to this singlet state, protection of the polymer by suitable quenchers is clearly possible. Although polymer (2) is insoluble in cyclohexane, a quenching study in ethyl acetate shows that furan acts as an equally efficient quencher of the polymer. From the Stern-Volmer plot, quenching by furan of the excited state of the polymer gave a kqr value of 15 1 mole - t , similar to that obtained with the model compounds.

Table 2. Quenching data and fluorescence lifetimes for sulphones (3-6) using furan as quencher Compound

x (nsec)

3 4 5 6

8.7 3.2 14.9 3.9

kq (1 mole- 1 sec- 1) 0.85 1.04 0.73 1.02

x x x x

101° 101° 10 l° 101°

kq x(l mole- 1) 73.2 33.3 110 40.0

Table 3. Quenching data and fluorescence lifetimes for sulphones (3-6) using 2-methylfuran as quencher Compound

~ (nsec)

3 4 5 6

8.7 3.2 14.9 3.9

kq(l mole -1 sec -1) 0.81 0.93 0.71 0.92

x x x x

l01° 101° 101° l0 I°

k~(l mole -1) 70.0 30.0 106 36.0

Table 4. Data concerning the triplet states of diaryl sulphones Compound 3 4 5 6

% (reset) 2.21 52.1 17.5 66.6

x x x x

10-3 l0 -3 10-3 10-3

E r (keal mole- i) 73.9 71.9 72.9 72.3

Ro

(A)

10.9 12.0 11.5 11.6

Photodecomposition of commercial polysulphones 0.6

~

0.3

I'*~Yt v

I

I

O

I

I 5O

I

I

I

t

I

[ 0 ] m-I.-' xlO 3 Fig. 3. Pen'in phosphorescence quenching plot for compound 4 using 1,3 cy¢l~octadiene as a quencher.

In addition to the study of the quenching of the excited singlet states, observation of the phosphorescence permitted a study of the quenching of the triplet states. Phosphorescence lifetimes % were measured conventionally at 77°K using the relationship: (lp),

= e_,/,,"

(Iplo

Intensities of phosphorescence (lp)t were measured by photography of oscilloscope traces from a photomultiplier-oscilloscope detection system and plots of In (lp)~ as a function of t gave the slope -1/~p (see Table 4). Using 1,3-cyclo-octadiene, known to be an efficient quencher of the triplet state, we have measured in isopentane-methylcyclohexane the phosphorescence intensity at different quencher concentrations. Using Perrins relationship [12] I0 e vN"I-Q] I where V is the volume of a quenching sphere, N' equals 6.02 x 102o and Q is the quencher concentration we have determined Ro, the radius of the quenching sphere for the model sulphones. Results are shown in Table 4 and Fig. 3. Again these results suggest that, at a modest quencher concentration ( ~ 1 0 - 2 m o l e l - ~ ) , significant quenching of the excited triplet of a diaryl sulphone chromophone is possible at least at reduced temperatures. As in the study of the singlet excited state, the effect of susti-

1023

tuents in the aryl moiety are not profound. The values of Ro accords well with quenching proceeding by the exchange triplet-triplet energy transfer mechanism. In summary, our results establish that efficient quenching of the excited states of polysulphones is possible and suggest that systematic investigation of various quenchers might lead to one which significantly protects polysulphones against photodegradation. Acknowledgements--We thank the Ministry of Defence for financial assistance and permission to publish these results and Dr. A. Davis, Waltham Abbey for stimulating discussions.

REFERENCES

1. A. Davis, Makromolek. Chem. 128, 242 (1969); A. Davis, ibid. 132, 23, 0970); W. F. Hale, A. G. Farnham, R. N. Johnson and R. A. Glendinning, J. Polym. Sci AI, 5, 2399 (1967). 2. B. Ranby and J. F. Rabek. Photodegradation, Photooxidation and Photostabilisation of Polymers. p. 223. John Wiley, New York (1975~ 3. S. W. Beavan and D. Phillips, Europ. Polym. J. 10,

593 (1974)~ 4. A. Davis, G. H. W. Deane and B. L. Diffey, Nature, Lond 261,169 (1976); B. D. Gesner and P. G. Kelleher, J. appl. Polym. Sci. 12, 1199 (1968); B. D. Gesner and P. G, Kelleher, ibid. 13. 2183 (1969): I. I. Khukhreva, S. V. Derevyagina and A. S. Barkov, Vysokomolek. Soedin. A, 14, 1122 (1972). 5. N. Kharasch and A. I. Khodair, J. C. S. Chem Comm. 98 0967); A. I. Khodair. T. Nakabayashi and N. Kharasch, Int. J. Sulphur Chem. 8, 37 (1973). 6. N. Nakai, N. Furukawe, S. Oae and T. Nakabayashi. Bull. che~ Soc. Japan 45, I117, (1972). 7. G. Holt and B. Pagdin, J. chem. Soc. 506 (1960). 8. L. E. Hinkel and G. H. R. Summers, J. chem. Soc. 2854 (1949). 9. Or.q. Synth. 8. 42 (1928). 10. P. A. Hackett and D. Phillips, J, phys. Chem. 78, 665 (1974). I I. I Berlman Handbook of Fluorescence Spectra of Aromatic Molecules. Academic Press, New York (1971). 12. F. Perrin, C. R. Acad. Sci Ser. C. 178, 1978 (1924).