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Photodegradation and photo-oxidation of poly(vinyl chloride) in solution
European Polymer Journal, VoL 14. pp. 995 to I000 © Pergamon Press Ltd 1978. Printed in Great Britain
0014-3057/78/1201-0995502.00/0
PHOTODEGRADATION AND PHOTO-OXIDATION OF POLY(VINYL CHLORIDE) IN SOLUTION M. BALANDIER a n d C. DECKER Laboratoire de Photochimie G6n6rale, Equipe de Recherche Associ6e au CNRS, Ecole Nationale Sup6rieure de Chimie, 68093 Mulhouse Cedex, France (Received 21 March 1978) Abstract--Quantum yields of dehydrochlorination and of main chain scission were determined for photolysis of PVC in solution in tetrahydrofuran and 1,2-dichloroethane. The observed auto-accelerated degradation results from an increased absorption of light by the growing polyene structures. The presence of oxygen enhances the efficiency of dehydrochlorination, chain scission and crosslinking processes. From the constant quantum yields, it is inferred that energy transfer occurs from the absorbing polyenes and that initially present unsaturations are responsible for initiation of the degradation.
INTRODUCTION
A l t h o u g h b o t h p h o t o d e g r a d a t i o n a n d photo-oxidation of poly(vinyl chloride) (PVC) have been extensively investigated [1-10], the m e c h a n i s m of P V C photolysis is still the subject of m u c h speculation; additional work is required for a better understanding of the photochemical processes occurring in the irradiated polymer. Due to the wide variety of applications of PVC, most of the work has been carried out on the solid material mainly as powder a n d as cast or pressed films. Investigation of the photolysis of P V C in solution may nevertheless provide useful information regarding the initiation step a n d the q u a n t u m efficiency of the various photochemical processes involved. F u r t h e r m o r e , it may also be useful to determine the effect of the solvent on the U V stability since films, cast from solution of P V C in solvents which are difficult to remove completely, are often used for kinetic studies of P V C degradation. While p h o t o d e g r a d a t i o n of P V C has already been studied on samples containing traces of solvent I-11, 121 or in suspension in different liquids [13], n o quantitative investigation on the photolysis of P V C in solution has been reported so far. The present study deals with the effect of the solvent on the competitive reactions in P V C solutions irradiated in the presence of b o t h nitrogen a n d pure oxygen. Q u a n t u m yields of d e h y d r o c h i o r i n a t i o n and of main chain scission have been determined for P V C photolyzed in tetrahydrofuran (THF) a n d in 1,2-dichloroethane (DCE) at r o o m temperature. These two solvents were selected since they are c o m m o n l y used in the casting of P V C films and represent typical classes of solvent of PVC. EXPERIMENTAL
Materials The polymer used in our experiments was an unstabilized commercial material (Solvic 229 from Solvay) synthesized by suspension polymerization at 60 °, with number and weight average molecular weights of 39,000 and 83,000 respectively, as determined by osmometry and light scattering. The polymer was purified by extensive extraction of
995
the PVC powder in water, applying vigorous stirring, followed by several washings with methanol and drying under vacuum at 30 ° for two days. THF (Fluka puriss) was purified, just before use, by 3 hr refluxing over KOH under nitrogen followed by two distillations. This treatment was shown to eliminate completely the peroxides and antioxidants as well as the u.v. absorption at 280nm. Dichloroethane (Fluka puriss) was used without further purification.
Irradiation PVC solutions were irradiated at 25 ~ in a quartz cell under 1.2 atmosphere of pure nitrogen or oxygen with a Philips (HPK 125 W) high pressure mercury lamp. The light was passed through a 35~o cobalt sulphate-50~/;, nickel sulphate aqueous solution to isolate the 250-350 nm wavelength region. The potassium ferrioxalate system developed by Hatchard and Parker [14] and slightly modified by Bowman and Demas [15] was used for determination of the light intensity ( 4 10-VE/s.cm2). The fraction of incident light absorbed by the polymer was calculated taking the difference of the light intensities determined with the cell containing the solvent and the solution, respectively. Determination of functional 9roups The u.v. spectrum of the PVC solution was determined by using a Cary 15 spectrophotometer. Figure l shows how the absorption moves towards longer wavelengths with longer exposure to light; this shift is more pronounced for irradiations carried out in the presence of oxygen than for those with nitrogen. Infrared spectroscopic studies were made on the irradiated solution in a 0.01 cm cell using a Beckman IR 12 spectrophotometer. PVC photolysed in THF in the presence of oxygen exhibits new bands at 1727, 1775 and 3550cm-~ (Fig. 2). When the oxidized polymer was recovered by precipitation in methanol, the 1775 cm - ~ disappeared completely while the 1727 and 3550cm-~ absorptions, although significantly reduced, still remained even after reprecipitation. The 1775 cm - t band is thus related to the oxidation of THF and corresponds probably to the absorption of),-butyrolactone. The carbonyl band centered at 1727cm -~ can be attributed to various groups: aldehyde, ketone, ~ and fl-chloroketones. From investigation on model compounds, the carbonyl content in the oxidized PVC was estimated by taking an average molar extinction coefficient of 400 for these groups. The broad hydroxyl band centered at 3550 cm-1, corresponding to the absorption of hydroperoxide and alcohol groups, was too weak
996
M. BALANDIERand C. DECKER I.O
I1 t\'\ .+7 ! ~ 06
t'
'
\
ot
--
"" - . . . . . . . . . . . 250
300
350
400
Woveleng't h, nm Fig. I. Ultraviolet absorption spectra of PVC in DCE; 0--unirradiated solution; 1, 3 and S--solution irradiated during I, 3 and 5 hr in the presence of oxygen; (...) unirradiated pure DCE; ( - - - ) DCE irradiated for 5 hr.
to permit accurate quantitative determination of these functions.
Detection and measurement of hydrogen chloride The irradiated solution was treated with 0.1 ml of a 0.5 N A g NO3 aqueous solution and 1 ml of N NH4NO3 aqueous solution 1-161. The white precipitate of ARC1 was recovered by filtration on a cellulose acetate Sartorius membranfilter SM 116 (pore size 0.2/~), washed several times with THF to eliminate any polymer which might have precipitated and dried to constant weight. For photolysis experiments carried out in DCE solution, the amount
,3oi
I
'
I
I
'
I ".7.,.. 4
c
.
,.
'-x
60
E m I-.40
2G
I 2000
,
I
1800
Wovenum ber,
,
I
1600
c m -=
Fig. 2. Infrared absorption spectra of PVC samples; l--unexposed PVC solution in THF; 2--PVC solution after 20 hr of ultraviolet irradiation in the presence of oxygen; 3--film of oxidized PVC recovered by precipitation from solution 2; 4---unexposed PVC film.
of HCI resulting from the photodegradation of PVC was calculated by substracting from the HCI content of the irradiated solution the amount of HCI evolved in the photolysis of pure DCE.
Determination of random chain scission Viscosity measurements were carried out at 25 ° on the initial and on the irradiated solutions. The value of the intrinsic viscosity I-~/] was determined before and after photolysis by measuring the specific viscosity, r/,v, of the solution at various concentrations and by extrapolating tl,v/c to zero concentration. This procedure was used rather than the calculation of I-r/] by one-point measurement of r/,p using the Huggins equation since photolysis of both THF and DCE solutions of PVC induces an increase of the H_uggins constant. Number-average molecular weights, M,, were calculated from intrinsic viscosities by using the Mark-Houwink relation for PVC in THF [17]: [r/],,,/~ = 2.4 x 10 -2 ~ 0.~7. The average number of chain scission S per initial macromolecule was then calculated from the relation: S = (M,0/M,) - 1 where M.o and M, are the number-average molecular weights of PVC before and after degradation respectively. This equation is based on the assumptions that chain fracture is a random process, that no intermolecular crosslinking occurs, and that the initial molecular weight distribution is random, i;e. 5 . 0 = 2M.o. Since the Mark-Houwink relation was not known for PVC in DCE, the relation between [7] in THF and ~ ] in DCE was established by using PVC samples with M,, between 57,000 and 133,000: [q]ra~ = 1.5 [~/]D~?~ o s7• From this equation we deduced the Mark-Houwink relation for PVC in DCE: [q]ml/, = 8.6 x 10-3M. °'88 which then allowed calculation of S. Gel-permeation chromatography analysis was performed at 25° on a Water Associates chromatograph (GPC 301 model) by using a styragel column. Figure 3 shows the broadening of the molecular weight distribution of the photolysed PVC towards the low molecular weights, as expected from the decrease in viscosity, but also towards the higher molecular weights, thus indicating a crosslinking process which competes with chain scission. Consequently, the calculated values of S correspond to net chain scissions and constitute the lower limits of the true amount of main chain scissions. An accurate quantitative determination of the contribution of each of these processes is difficult as shown by the study of David et al, [18] who applied statistical theories of main chain scission and crosslinking to the photolysis of polymers.
Photodegradation and photo-oxidation of PVC in solution
997
I
I
O_
NI
x~--~o I C - /
E
s
_O .'9_ _~ 5 u
o~ -1-
I
~
40
~ ,
,
I ,
,
,
,
I
35
,
,
,
,
•
25
30
O
Counts
Fig. 3. GPC molecular weight distribution of PVC samples; ( ) before degradation; ( - - - ) after 10hr of ultraviolet irradiation in the presence of nitrogen; (...) after
10hr of ultraviolet irradiation in the presence of oxygen.
PHOTOLYSIS
I
I0
lO-aeinsfein/g PVC Fig. 5. Amount of HCI evolved from the polymer as a function of the number of photons absorbed by the
polymer in photolysis of DCE solutions of PVC in nitrogen (O) and in oxygen (O); (---) HCI evolved in photolysis of pure DCE in nitrogen (@) and in oxygen (O).
O F P V C I N DCE
Dehydrochlorination When DCE solutions of PVC (1.5g/100ml) are photolysed in a nitrogen atmosphere, HC1 is evolved with an increasing rate, as shown by Fig. 4. This auto-accelerating process results from an increased absorption of light by the polymer since a plot of the amount of HCI evolved by the photolysed PVC as a function of the number of photons absorbed by the polymer yields a straight line, as shown by Fig. 5. The slope of this line is the quantum yield of dehydrochlorination which remains thus constant throughout the photolysis: number of HC1 molecules evolved/g PVC number of quanta absorbed/g PVC
/O2J
= 0.08. I
I
5
I
I
I
O.. IO
I
I
Irradiations carried out in the presence of pure oxygen lead to a similar autoaccelerating kinetic, the dehydrochlorination process being slightly enhanced by the presence of oxygen: q~HO= 0.10. Figure 5 also shows that the amount of HCI evolved in the photolysis of pure DCE increases linearly with the number of photons absorbed by DCE, the quantum yield of dehydrochlorination remaining constant at 0.060 in the presence of both nitrogen and pure oxygen.
Change in molecular weight The viscosity of the PVC solution drops rapidly with irradiation time, indicating the occurrence of main chain scission. The auto-accelerated photodegradation appears to result again from the increasingly enhanced light absorbance by PVC since S increases linearly with the number of photons absorbed (Fig. 6). The quantum yield of chain scission was calculated to be 0.0011 in the presence of nitrogen and
0.6 -
/02
, J
~r i
_o ~ 0.4 o
t,
5
u •-~ 0.2
--
2,
o "o n-
0
I
2
3
Time,
4
,5
6
hr
Fig. 4. Kinetics of dehydrochlorination of DCE solutions of PVC photolysed in the presence of nitrogen O or oxygen O.
0
I
I
2
4
10 2 Einstein / M a c r o m o l e c u l e
Fig. 6. Chain scission formation as a function of the number of photons absorbed by the polymer in photolysis of DCE solutions of PVC.
M. BALANDIER and C. DECKER
998 t.) >
a. a,
-
our result is in marked constrast with some other observations [19] on the photolysis of PVC in the solid state which show, from gel fraction measurements, crosslinking to occur more intensively in the presence of nitrogen than in oxygen.
I
o
E ,7 _O
Oxidation products -o
o .c ta
,= °~
I
~0
5
"i-
10 - 3
Einstein/g
I0 PVC
Fig. 7. Amount of HCI evolved as a function of the number of photons absorbed by the polymer in photolysis of THF solutions of PVC. 0.0014 in the presence of oxygen. These values must be underestimates since our GPC analysis has shown that simultaneous crosslinking occurs during photolysis of PVC in DCE. PHOTOLYS|S OF PVC IN THF
Dehydrochlorination A similar investigation was carried out on the photolysis of THF solutions of PVC. The rate of dehydrochlorination increases steadily with irradiation time, but remains constant if plotted as a function of the number of photons absorbed by the polymer (Fig. 7). In the presence of oxygen, the photoinduced dehydrochlorination of PVC proceeds faster than in the presence of nitrogen. The calculated ratio of the amounts of HCl evolved after 7 hr of irradiation in oxygen and in nitrogen respectively, [HCI]o,/[HCI]N, = 1.6, is close t o the value of 1.71 determined by Gibb and McCallum J6] by irradiating PVC films for 7 hr at a similar light intensity. This difference in rate of dehydrochlorination originates mostly from a faster increase of the fraction of light absorbed by the polymer upon irradiation in the presence of oxygen since the quantum yields of HCI evolved are close for experiments carried out in n i t r o g e n (~H('I = 0.14) and in oxygen (q~HC~= 0.16).
Chain scission Degradation of the polymer by scission of the main chain proceeds effectively on u.v. irradiation of PVC in THF, both in the presence of nitrogen and of oxygen; the molecular weight of the polymer drops to one third of its original value after 20 hr of irradiation in Oz. The quantum yield of chain scission is significantly higher than in DCE solution, with a marked effect of oxygen on the extent of the degradation process since q%~on increases from 0.0022 in nitrogen to 0.0035 in oxygen (Fig. 8). GPC analysis reveals that crosslinking proceeds more efficiently in the presence of oxygen than in nitrogen. Consequently, values of ~c~+o. are more underestimated in 02 than in N2 so that the true oxygen effect on the chain scission process is even more pronounced than indicated by the values of the net chain scission quantum yield. Although crosslinking is also apparent from the GPC curves reported by Rabek et al. [13] and by Mori et al. [-9] in the photo-oxidation of PVC,
As shown by the infrared spectrum of the oxidized polymer (Fig. 2), hydroxyl and carbonyl groups are formed by ultraviolet irradiation of PVC solutions in the presence of oxygen. As for dehydrochlorination and chain scission, the kinetic of formation of these products is clearly auto-accelerating, in agreement with the observations of Scott [7] on the photo-oxidation of rigid PVC. The carbonyl content increases linearly with the energy absorbed by the polymer with an overall quantum yield of about 0.006.
Light intensity and wavelength dependence In order to determine the effect of light intensity on the quantum efficiency, solutions of PVC in THF were irradiated for up to 50 hr at intensities between 10 -7 and 10 -s E/s'cm 2. No major change could be noticed in the values of the quantum yields of dehydrochlorination and chain scission, implying that second order chain reactions play no significant part in the photodegradation of PVC in solution. Investigation carried out at different initial concentrations revealed no substantial effect of the PVC concentration on the quantum efficiency of the photodegradation process. Some experiments were also performed with a pyrex filter between the chemical filter and the cell in order to isolate the 300-350 nm region. The fraction of incident light absorbed by the polymer is then very low (~ 1~o) and does not allow a reliable evaluation of the quantum yields. Upon prolonged u.v. irradiation, the evolution of HC1 and a drop in viscosity could be observed, thus indicating that photodegradation proceeds even at wavelengths where very little radiation is absorbed by the undegraded polymer. Since traces of catalyst or additive in the polymer may influence the degradation, similar experiments were carried out with a PVC sample purified by precipitation in methanol from a THF solution. Quantum yields of dehydrochlorination and chain scission were identical to those determined with the unpurified
E
-
--
g_ ffl
0
2
I
I
4
6
10 2 E i n s t e i n I m o c r o m o l e c u l e
Fig. 8. Chain scission formation as a function of the number of photons absorbed by the polymer in photolysis of THF solutions of PVC.
Photodegradation and photo-oxidation of PVC in solution PVC, both in the presence of nitrogen and pure oxygen• External impurities that might have been present in the commercial sample appear thus to take no significant part in the photodegradation process. M E C H A N I S M O F P H O T O L Y S I S O F PVC IN S O L U T I O N
Photolysis in the absence of oxygen The main novel feature of our investigation on the photolysis of PVC in solution is that the chain scission process which was known to occur in photooxidation of PVC develops also in the absence of oxygen and competes successfully with the crosslinking process. This behaviour must be related to a phase effect since no chain scission process has been reported so far in the photolysis of solid PVC. Due to the low mobility of the polymeric chain in the solid state, recombination of the alkyl radicals resulting from C---C cleavage is highly probable while in solution the two molecular fragments are expected to separate easily. Consequently, scissions of the main chain as well as C - - H and C--C1 cleavages are expected to occur when PVC is photolysed in solution. Compared to the extent of the dehydrochlorination, chain scission appears as a minor process since up to 70 molecules of HCI are evolved for each scission of the polymer backbone and since only one photon over 500 absorbed by the polymer induces a scission of the PVC chain. Although photodegradation of PVC appears to be auto-accelerating, quantum yields of dehydrochlorination and of chain scission remain strictly constant during all the irradiation. Since light is increasingly absorbed by the build-up of the polyene structures, this result suggests that some unsaturated structures initially present in the PVC sample are responsible for the initiation process. Commercial PVC samples have indeed been shown to contain some unsaturations, the highest double bond content being observed in samples obtained by suspension polymerization 1-20]. As suggested by Gibb and McCallum [5], a significant proportion of the energy absorbed by the growing polyene structures is likely to be transferred along the polymer chain to cause reactions in the non-absorbing parts of the polymer, thus promoting further dehydrochlorination and scission of the backbone chain. The indicated values of the quantum yields should not depend on the initial amount of unsaturation in the polymer sample since our kinetic results clearly indicate that the quantum efficiency of PVC degradation is not affected by the increasing build up of polyene structures. The effect of the solvent on the degradation of PVC 2 ~ C H 2 - - C C I ~ C H 2 ~ --~ 2
I
OO is difficult to evaluate. According to Gibb and McCallum [4], traces of residual T H F in cast PVC films do not affect the extent of degradation but do affect the relative concentration of the conjugated polyenes produced. Our investigations show that photodegra-
dation of PVC proceeds twice as efficiently in THF than in DCE probably as a result of the difference in the solvent photosensitivity and in the reactivity of the solvent radicals.
Photo-oxidation of PVC Our kinetic results indicate that oxygen enhances the photodegradation of PVC in both THF and DCE solutions. They can be accounted for by the overall reaction scheme proposed recently for the ?-initiated oxidation of PVC [21]. Some differences may however appear since the physical state of the polymer and the type of radiation were not the same in the two investigations. For instance, the free radicals formed in the photolysis of the solvent are expected to take a part in the initiation process and to interact with the macroradicals. THF, in particular, is known to react rapidly with oxygen to yield a charge-transfer complex which absorbs towards longer wavelengths [22]. Photolysis of this complex leads ultimately to reactive hydroxy and peroxy radicals [12] which will abstract hydrogen mostly from the weakest tertiary C - - H bonds of the polymer. Destruction of this THF---O 2 complex may explain why the u.v. absorption of THF is shifted towards shorter wavelengths upon irradiation. Another difference between S' and u.v.-initiation arises from the selectivity of these two types of radiation. While the PVC oxidation products are not selectively destroyed by ?-rays, some of them such as hydroperoxides and carbonyl groups are expected to be photolysed with formation of propagating radicals, thus increasing the efficiency of the photo-oxidation. However, since the quantum efficiency of the oxidation appears to remain strictly constant, we must assume that, during the early stages of the degradation, decomposition of photoproducts does not interfere significantly with the major oxidation process which involves primarily the reactions of secondary and tertiary peroxy macroradicals (PO~). Besides reacting with PVC or with the solvent to give hydroperoxides, these radicals are expected to disappear by terminating bimolecular interactions to yield ketone and alcohol groups in the case of secondary PO~ radicals (Russell mechanism [23]) and dialkylperoxides in the case of tertiary POz radicals. This last reaction is assumed to be responsible for the observed increase of the crosslinking process in the presence of oxygen. Cross termination by the various types of PO) radicals and with the peroxy radicals of the solvent are also possible. Tertiary peroxy radicals may disappear also by non-terminating interactions with formation of free alkoxy radicals: C H 2 - - C C I - - C H 2 ~ + O2,
]
(1)
O The principal fate of the resulting tertiary alkoxy radical is either an hydrogen abstraction from the polymer (PH) or from the solvent (SH) to yield a chain carrier alkyl radical and a tertiary alcohol which converts easily in a ketone by releasing hydrochloric acid:
~ C H 2 - - C C I - - C H 2 ~ + PH or SH -+ ~ C H 2 - - C C 1 - - C H 2 ~ + P" or S"
I
O"
999
I
OH
(2)
I000
M. BALANDIER and C. DECKER
or a decomposition by fl scission 1,24] which may occur not only by cleavage of the C---C bond [25] but also by cleavage of the C - - C ! bond:
production of Ci" by reaction (5) will thus account for the higher quantum efficiency of the dehydro' ~ C H 2 - - C - - C I + "CH2---CHCI ~
(3)
l[
O CH2--CCI~H
2~ __
I
O"
(4)
' ~ CH2"--C--CH2 ~ + CI'.
II
O Reaction (3) may play an important role in the backbone scission of chains and thus account for the significant increase of the quantum yield of net chain scission observed when irradiations of PVC were carried out in the presence of oxygen. According to our previous observations 1,21], these ct-chloro alkoxy radicals decompose preferentially by splitting off a C1 atom and formation of a ketone, i.e. reaction (4) is favoured over reaction (3). CI' evolved in reaction (4) is then expected to react with the polymer to yield HCI and a propagating secondary or tertiary alkyl radical. Due to polar effects, hydrogen abstraction from a methylene group is, more likely to occur than the abstraction of a tertiary bonded hydrogen 1-26, 27] : "CI + ~ C H 2 ~ C H - - C H 2 ~ C H
I
C1
chlorination process and for the subsequent larger increase of the polymer u.v.-absorption observed in the photo-oxidation of PVC. An opposite effect of oxygen was expected since 0 2 is known to react effectively with allylic radicals and should thus reduce the extent of the chain dehydrochlorination process. The net increase of ~HcJ in the presence of oxygen is thus the result of two opposing effects, the scavenging by oxygen of the growing polyenyl radicals being more than offset by the additional formation of HCI through the sequence of reactions (1), (4) and (5). The observed formation of relatively large amounts of carbonyl groups on the polymer backbone (q~>C=O "" 2tP~cission) is also consistent with the pro-
~ ---, ~ ' C H ~ C H - - C H 2 - - C H ~
I
CI
I
Cl
+ HCI.
(5)
I
C1
Since the fl-chloro-alkyl radicals formed in reaction (5) are known to expell easily CI" with formation of a double bond 1-28], dehydrochlorination is expected to proceed further along the macromolecule to yield polyene structures by a chain reaction. The additional
posed reaction scheme since ketones result both from reactions involving scission of the main chain, such as the formation of ~-chloroketone (reaction 3), and from reactions not affecting the PVC molecular weight (reaction 4).
REFERENCES
13. J. F. Rabek, G. Canb~ick, J. Lucky and B. Ranby, J. Polym, Sci., Polym. Chem. Ed. 14, 1447 (1976). 14. C. G. Hatchard and C. A. Parker, Proc. R. Soc. A235, 518 (1956). 15. W. D. Bowman and J. N. Demas, J. phys. Chem., Ithaca 86, 243 (1976). 16. V. Karaoglanov, Z. Analyt. Chem. 115, 385 (1939). 17. M. Bohdanecky, K. Solc, P. Kratochvil, M. Kolinsky, M. Ryska and D. Lim, J. Polym. Sci. A2 5, 343 (1967). 18. C. David and D. Baeyens-Volant, Europ. Polym. J. 14, 29 (1978). 19. H. Sobue and Y. Tajima, J. Chem. Ind., Tokyo 62, 1149 (1959). 20. J. Boissel, J. appl. Polym. Sci. 21, 855 (1977). 21. C. Decker, J. appl. Polym. Sci. 20, 3321 (1976). 22. V. I. Stenberg, R. D. Olsen, C. T, Wang and N. Kulevsky, J. Org. Chem. 32, 3227 (1967). 23. G. A. Russell, J. Am. chem. Soc. 79, 3871 (1957). 24. E. R. Bell, J. H. Raley, F. F. Rust, F. H. Seubold and W. E. Vaughan, Discuss. Faraday Soc. 10, 242 (1951). 25. D. E. Winkler, J. Polym. Sci. 35, 3 (1959). 26. A. A. Miller, J. phys. Chem., Ithaca 63, 1755 (1959). 27. J. M. Fedder, Quart. Rev. 14, 336 (1960). 28. R. Kh. Friedlina, Advances in Free-Radical Chemistry (Edited by G. H. Williams) Vol. l, Chap. 6. Logos, London (1965).
I. R. F. Reinish, H. R. Gloria and D. E. Wilson, Am.
Chem. Soc., Div. Polym. Chem., Preprints 7(!), 372 (1966). 2. K. P. S. Kwei, J. Polym. Sci. AI 7, 1075 (1969). 3. W. H. Gibb and J. R. MacCallum, Europ. Polym. J. 7, 1231 (1971). 4. W. H. Gibb and J. R. McCallum, Europ. Polym. J. 9, 77 (1973). 5. W. H. Gibb and J. R. McCallum, Europ. Polym. J. 10, 529 (1974). 6. W. H. Gibb and J. R. McCallum, Europ. Polym. J. 10, 533 (1974). 7. G. Scott and M. Tahan, Europ. Polym. J. 11, 535 (1975). 8. E. D. Owen, Ultraviolet Light Induced Reactions in Polymers, p. 208. Labana, ACS (1976). 9. F. Mori, M. Koyama and Y. Oki, Angew. Makromolek. Chem. 64, 89 (1977). 10. B. Ranby and J. F. Rabek, Photodegradation, Photooxidation and Photostabilization of Polymers, Wiley, New York (1975). 11. M. R. Kamal, M. M. EI-Kaissy and M. M. Avedesian, J. appl. Polym. Sci. 16, 83 (1972). 12. J. F, Rabek, Y. J. Shur and B. Ranby, J. Polym. Sci., Polym. Chem. Ed. 13, 1285 (1975).
0014-3057/78/1201-0995502.00/0
PHOTODEGRADATION AND PHOTO-OXIDATION OF POLY(VINYL CHLORIDE) IN SOLUTION M. BALANDIER a n d C. DECKER Laboratoire de Photochimie G6n6rale, Equipe de Recherche Associ6e au CNRS, Ecole Nationale Sup6rieure de Chimie, 68093 Mulhouse Cedex, France (Received 21 March 1978) Abstract--Quantum yields of dehydrochlorination and of main chain scission were determined for photolysis of PVC in solution in tetrahydrofuran and 1,2-dichloroethane. The observed auto-accelerated degradation results from an increased absorption of light by the growing polyene structures. The presence of oxygen enhances the efficiency of dehydrochlorination, chain scission and crosslinking processes. From the constant quantum yields, it is inferred that energy transfer occurs from the absorbing polyenes and that initially present unsaturations are responsible for initiation of the degradation.
INTRODUCTION
A l t h o u g h b o t h p h o t o d e g r a d a t i o n a n d photo-oxidation of poly(vinyl chloride) (PVC) have been extensively investigated [1-10], the m e c h a n i s m of P V C photolysis is still the subject of m u c h speculation; additional work is required for a better understanding of the photochemical processes occurring in the irradiated polymer. Due to the wide variety of applications of PVC, most of the work has been carried out on the solid material mainly as powder a n d as cast or pressed films. Investigation of the photolysis of P V C in solution may nevertheless provide useful information regarding the initiation step a n d the q u a n t u m efficiency of the various photochemical processes involved. F u r t h e r m o r e , it may also be useful to determine the effect of the solvent on the U V stability since films, cast from solution of P V C in solvents which are difficult to remove completely, are often used for kinetic studies of P V C degradation. While p h o t o d e g r a d a t i o n of P V C has already been studied on samples containing traces of solvent I-11, 121 or in suspension in different liquids [13], n o quantitative investigation on the photolysis of P V C in solution has been reported so far. The present study deals with the effect of the solvent on the competitive reactions in P V C solutions irradiated in the presence of b o t h nitrogen a n d pure oxygen. Q u a n t u m yields of d e h y d r o c h i o r i n a t i o n and of main chain scission have been determined for P V C photolyzed in tetrahydrofuran (THF) a n d in 1,2-dichloroethane (DCE) at r o o m temperature. These two solvents were selected since they are c o m m o n l y used in the casting of P V C films and represent typical classes of solvent of PVC. EXPERIMENTAL
Materials The polymer used in our experiments was an unstabilized commercial material (Solvic 229 from Solvay) synthesized by suspension polymerization at 60 °, with number and weight average molecular weights of 39,000 and 83,000 respectively, as determined by osmometry and light scattering. The polymer was purified by extensive extraction of
995
the PVC powder in water, applying vigorous stirring, followed by several washings with methanol and drying under vacuum at 30 ° for two days. THF (Fluka puriss) was purified, just before use, by 3 hr refluxing over KOH under nitrogen followed by two distillations. This treatment was shown to eliminate completely the peroxides and antioxidants as well as the u.v. absorption at 280nm. Dichloroethane (Fluka puriss) was used without further purification.
Irradiation PVC solutions were irradiated at 25 ~ in a quartz cell under 1.2 atmosphere of pure nitrogen or oxygen with a Philips (HPK 125 W) high pressure mercury lamp. The light was passed through a 35~o cobalt sulphate-50~/;, nickel sulphate aqueous solution to isolate the 250-350 nm wavelength region. The potassium ferrioxalate system developed by Hatchard and Parker [14] and slightly modified by Bowman and Demas [15] was used for determination of the light intensity ( 4 10-VE/s.cm2). The fraction of incident light absorbed by the polymer was calculated taking the difference of the light intensities determined with the cell containing the solvent and the solution, respectively. Determination of functional 9roups The u.v. spectrum of the PVC solution was determined by using a Cary 15 spectrophotometer. Figure l shows how the absorption moves towards longer wavelengths with longer exposure to light; this shift is more pronounced for irradiations carried out in the presence of oxygen than for those with nitrogen. Infrared spectroscopic studies were made on the irradiated solution in a 0.01 cm cell using a Beckman IR 12 spectrophotometer. PVC photolysed in THF in the presence of oxygen exhibits new bands at 1727, 1775 and 3550cm-~ (Fig. 2). When the oxidized polymer was recovered by precipitation in methanol, the 1775 cm - ~ disappeared completely while the 1727 and 3550cm-~ absorptions, although significantly reduced, still remained even after reprecipitation. The 1775 cm - t band is thus related to the oxidation of THF and corresponds probably to the absorption of),-butyrolactone. The carbonyl band centered at 1727cm -~ can be attributed to various groups: aldehyde, ketone, ~ and fl-chloroketones. From investigation on model compounds, the carbonyl content in the oxidized PVC was estimated by taking an average molar extinction coefficient of 400 for these groups. The broad hydroxyl band centered at 3550 cm-1, corresponding to the absorption of hydroperoxide and alcohol groups, was too weak
996
M. BALANDIERand C. DECKER I.O
I1 t\'\ .+7 ! ~ 06
t'
'
\
ot
--
"" - . . . . . . . . . . . 250
300
350
400
Woveleng't h, nm Fig. I. Ultraviolet absorption spectra of PVC in DCE; 0--unirradiated solution; 1, 3 and S--solution irradiated during I, 3 and 5 hr in the presence of oxygen; (...) unirradiated pure DCE; ( - - - ) DCE irradiated for 5 hr.
to permit accurate quantitative determination of these functions.
Detection and measurement of hydrogen chloride The irradiated solution was treated with 0.1 ml of a 0.5 N A g NO3 aqueous solution and 1 ml of N NH4NO3 aqueous solution 1-161. The white precipitate of ARC1 was recovered by filtration on a cellulose acetate Sartorius membranfilter SM 116 (pore size 0.2/~), washed several times with THF to eliminate any polymer which might have precipitated and dried to constant weight. For photolysis experiments carried out in DCE solution, the amount
,3oi
I
'
I
I
'
I ".7.,.. 4
c
.
,.
'-x
60
E m I-.40
2G
I 2000
,
I
1800
Wovenum ber,
,
I
1600
c m -=
Fig. 2. Infrared absorption spectra of PVC samples; l--unexposed PVC solution in THF; 2--PVC solution after 20 hr of ultraviolet irradiation in the presence of oxygen; 3--film of oxidized PVC recovered by precipitation from solution 2; 4---unexposed PVC film.
of HCI resulting from the photodegradation of PVC was calculated by substracting from the HCI content of the irradiated solution the amount of HCI evolved in the photolysis of pure DCE.
Determination of random chain scission Viscosity measurements were carried out at 25 ° on the initial and on the irradiated solutions. The value of the intrinsic viscosity I-~/] was determined before and after photolysis by measuring the specific viscosity, r/,v, of the solution at various concentrations and by extrapolating tl,v/c to zero concentration. This procedure was used rather than the calculation of I-r/] by one-point measurement of r/,p using the Huggins equation since photolysis of both THF and DCE solutions of PVC induces an increase of the H_uggins constant. Number-average molecular weights, M,, were calculated from intrinsic viscosities by using the Mark-Houwink relation for PVC in THF [17]: [r/],,,/~ = 2.4 x 10 -2 ~ 0.~7. The average number of chain scission S per initial macromolecule was then calculated from the relation: S = (M,0/M,) - 1 where M.o and M, are the number-average molecular weights of PVC before and after degradation respectively. This equation is based on the assumptions that chain fracture is a random process, that no intermolecular crosslinking occurs, and that the initial molecular weight distribution is random, i;e. 5 . 0 = 2M.o. Since the Mark-Houwink relation was not known for PVC in DCE, the relation between [7] in THF and ~ ] in DCE was established by using PVC samples with M,, between 57,000 and 133,000: [q]ra~ = 1.5 [~/]D~?~ o s7• From this equation we deduced the Mark-Houwink relation for PVC in DCE: [q]ml/, = 8.6 x 10-3M. °'88 which then allowed calculation of S. Gel-permeation chromatography analysis was performed at 25° on a Water Associates chromatograph (GPC 301 model) by using a styragel column. Figure 3 shows the broadening of the molecular weight distribution of the photolysed PVC towards the low molecular weights, as expected from the decrease in viscosity, but also towards the higher molecular weights, thus indicating a crosslinking process which competes with chain scission. Consequently, the calculated values of S correspond to net chain scissions and constitute the lower limits of the true amount of main chain scissions. An accurate quantitative determination of the contribution of each of these processes is difficult as shown by the study of David et al, [18] who applied statistical theories of main chain scission and crosslinking to the photolysis of polymers.
Photodegradation and photo-oxidation of PVC in solution
997
I
I
O_
NI
x~--~o I C - /
E
s
_O .'9_ _~ 5 u
o~ -1-
I
~
40
~ ,
,
I ,
,
,
,
I
35
,
,
,
,
•
25
30
O
Counts
Fig. 3. GPC molecular weight distribution of PVC samples; ( ) before degradation; ( - - - ) after 10hr of ultraviolet irradiation in the presence of nitrogen; (...) after
10hr of ultraviolet irradiation in the presence of oxygen.
PHOTOLYSIS
I
I0
lO-aeinsfein/g PVC Fig. 5. Amount of HCI evolved from the polymer as a function of the number of photons absorbed by the
polymer in photolysis of DCE solutions of PVC in nitrogen (O) and in oxygen (O); (---) HCI evolved in photolysis of pure DCE in nitrogen (@) and in oxygen (O).
O F P V C I N DCE
Dehydrochlorination When DCE solutions of PVC (1.5g/100ml) are photolysed in a nitrogen atmosphere, HC1 is evolved with an increasing rate, as shown by Fig. 4. This auto-accelerating process results from an increased absorption of light by the polymer since a plot of the amount of HCI evolved by the photolysed PVC as a function of the number of photons absorbed by the polymer yields a straight line, as shown by Fig. 5. The slope of this line is the quantum yield of dehydrochlorination which remains thus constant throughout the photolysis: number of HC1 molecules evolved/g PVC number of quanta absorbed/g PVC
/O2J
= 0.08. I
I
5
I
I
I
O.. IO
I
I
Irradiations carried out in the presence of pure oxygen lead to a similar autoaccelerating kinetic, the dehydrochlorination process being slightly enhanced by the presence of oxygen: q~HO= 0.10. Figure 5 also shows that the amount of HCI evolved in the photolysis of pure DCE increases linearly with the number of photons absorbed by DCE, the quantum yield of dehydrochlorination remaining constant at 0.060 in the presence of both nitrogen and pure oxygen.
Change in molecular weight The viscosity of the PVC solution drops rapidly with irradiation time, indicating the occurrence of main chain scission. The auto-accelerated photodegradation appears to result again from the increasingly enhanced light absorbance by PVC since S increases linearly with the number of photons absorbed (Fig. 6). The quantum yield of chain scission was calculated to be 0.0011 in the presence of nitrogen and
0.6 -
/02
, J
~r i
_o ~ 0.4 o
t,
5
u •-~ 0.2
--
2,
o "o n-
0
I
2
3
Time,
4
,5
6
hr
Fig. 4. Kinetics of dehydrochlorination of DCE solutions of PVC photolysed in the presence of nitrogen O or oxygen O.
0
I
I
2
4
10 2 Einstein / M a c r o m o l e c u l e
Fig. 6. Chain scission formation as a function of the number of photons absorbed by the polymer in photolysis of DCE solutions of PVC.
M. BALANDIER and C. DECKER
998 t.) >
a. a,
-
our result is in marked constrast with some other observations [19] on the photolysis of PVC in the solid state which show, from gel fraction measurements, crosslinking to occur more intensively in the presence of nitrogen than in oxygen.
I
o
E ,7 _O
Oxidation products -o
o .c ta
,= °~
I
~0
5
"i-
10 - 3
Einstein/g
I0 PVC
Fig. 7. Amount of HCI evolved as a function of the number of photons absorbed by the polymer in photolysis of THF solutions of PVC. 0.0014 in the presence of oxygen. These values must be underestimates since our GPC analysis has shown that simultaneous crosslinking occurs during photolysis of PVC in DCE. PHOTOLYS|S OF PVC IN THF
Dehydrochlorination A similar investigation was carried out on the photolysis of THF solutions of PVC. The rate of dehydrochlorination increases steadily with irradiation time, but remains constant if plotted as a function of the number of photons absorbed by the polymer (Fig. 7). In the presence of oxygen, the photoinduced dehydrochlorination of PVC proceeds faster than in the presence of nitrogen. The calculated ratio of the amounts of HCl evolved after 7 hr of irradiation in oxygen and in nitrogen respectively, [HCI]o,/[HCI]N, = 1.6, is close t o the value of 1.71 determined by Gibb and McCallum J6] by irradiating PVC films for 7 hr at a similar light intensity. This difference in rate of dehydrochlorination originates mostly from a faster increase of the fraction of light absorbed by the polymer upon irradiation in the presence of oxygen since the quantum yields of HCI evolved are close for experiments carried out in n i t r o g e n (~H('I = 0.14) and in oxygen (q~HC~= 0.16).
Chain scission Degradation of the polymer by scission of the main chain proceeds effectively on u.v. irradiation of PVC in THF, both in the presence of nitrogen and of oxygen; the molecular weight of the polymer drops to one third of its original value after 20 hr of irradiation in Oz. The quantum yield of chain scission is significantly higher than in DCE solution, with a marked effect of oxygen on the extent of the degradation process since q%~on increases from 0.0022 in nitrogen to 0.0035 in oxygen (Fig. 8). GPC analysis reveals that crosslinking proceeds more efficiently in the presence of oxygen than in nitrogen. Consequently, values of ~c~+o. are more underestimated in 02 than in N2 so that the true oxygen effect on the chain scission process is even more pronounced than indicated by the values of the net chain scission quantum yield. Although crosslinking is also apparent from the GPC curves reported by Rabek et al. [13] and by Mori et al. [-9] in the photo-oxidation of PVC,
As shown by the infrared spectrum of the oxidized polymer (Fig. 2), hydroxyl and carbonyl groups are formed by ultraviolet irradiation of PVC solutions in the presence of oxygen. As for dehydrochlorination and chain scission, the kinetic of formation of these products is clearly auto-accelerating, in agreement with the observations of Scott [7] on the photo-oxidation of rigid PVC. The carbonyl content increases linearly with the energy absorbed by the polymer with an overall quantum yield of about 0.006.
Light intensity and wavelength dependence In order to determine the effect of light intensity on the quantum efficiency, solutions of PVC in THF were irradiated for up to 50 hr at intensities between 10 -7 and 10 -s E/s'cm 2. No major change could be noticed in the values of the quantum yields of dehydrochlorination and chain scission, implying that second order chain reactions play no significant part in the photodegradation of PVC in solution. Investigation carried out at different initial concentrations revealed no substantial effect of the PVC concentration on the quantum efficiency of the photodegradation process. Some experiments were also performed with a pyrex filter between the chemical filter and the cell in order to isolate the 300-350 nm region. The fraction of incident light absorbed by the polymer is then very low (~ 1~o) and does not allow a reliable evaluation of the quantum yields. Upon prolonged u.v. irradiation, the evolution of HC1 and a drop in viscosity could be observed, thus indicating that photodegradation proceeds even at wavelengths where very little radiation is absorbed by the undegraded polymer. Since traces of catalyst or additive in the polymer may influence the degradation, similar experiments were carried out with a PVC sample purified by precipitation in methanol from a THF solution. Quantum yields of dehydrochlorination and chain scission were identical to those determined with the unpurified
E
-
--
g_ ffl
0
2
I
I
4
6
10 2 E i n s t e i n I m o c r o m o l e c u l e
Fig. 8. Chain scission formation as a function of the number of photons absorbed by the polymer in photolysis of THF solutions of PVC.
Photodegradation and photo-oxidation of PVC in solution PVC, both in the presence of nitrogen and pure oxygen• External impurities that might have been present in the commercial sample appear thus to take no significant part in the photodegradation process. M E C H A N I S M O F P H O T O L Y S I S O F PVC IN S O L U T I O N
Photolysis in the absence of oxygen The main novel feature of our investigation on the photolysis of PVC in solution is that the chain scission process which was known to occur in photooxidation of PVC develops also in the absence of oxygen and competes successfully with the crosslinking process. This behaviour must be related to a phase effect since no chain scission process has been reported so far in the photolysis of solid PVC. Due to the low mobility of the polymeric chain in the solid state, recombination of the alkyl radicals resulting from C---C cleavage is highly probable while in solution the two molecular fragments are expected to separate easily. Consequently, scissions of the main chain as well as C - - H and C--C1 cleavages are expected to occur when PVC is photolysed in solution. Compared to the extent of the dehydrochlorination, chain scission appears as a minor process since up to 70 molecules of HCI are evolved for each scission of the polymer backbone and since only one photon over 500 absorbed by the polymer induces a scission of the PVC chain. Although photodegradation of PVC appears to be auto-accelerating, quantum yields of dehydrochlorination and of chain scission remain strictly constant during all the irradiation. Since light is increasingly absorbed by the build-up of the polyene structures, this result suggests that some unsaturated structures initially present in the PVC sample are responsible for the initiation process. Commercial PVC samples have indeed been shown to contain some unsaturations, the highest double bond content being observed in samples obtained by suspension polymerization 1-20]. As suggested by Gibb and McCallum [5], a significant proportion of the energy absorbed by the growing polyene structures is likely to be transferred along the polymer chain to cause reactions in the non-absorbing parts of the polymer, thus promoting further dehydrochlorination and scission of the backbone chain. The indicated values of the quantum yields should not depend on the initial amount of unsaturation in the polymer sample since our kinetic results clearly indicate that the quantum efficiency of PVC degradation is not affected by the increasing build up of polyene structures. The effect of the solvent on the degradation of PVC 2 ~ C H 2 - - C C I ~ C H 2 ~ --~ 2
I
OO is difficult to evaluate. According to Gibb and McCallum [4], traces of residual T H F in cast PVC films do not affect the extent of degradation but do affect the relative concentration of the conjugated polyenes produced. Our investigations show that photodegra-
dation of PVC proceeds twice as efficiently in THF than in DCE probably as a result of the difference in the solvent photosensitivity and in the reactivity of the solvent radicals.
Photo-oxidation of PVC Our kinetic results indicate that oxygen enhances the photodegradation of PVC in both THF and DCE solutions. They can be accounted for by the overall reaction scheme proposed recently for the ?-initiated oxidation of PVC [21]. Some differences may however appear since the physical state of the polymer and the type of radiation were not the same in the two investigations. For instance, the free radicals formed in the photolysis of the solvent are expected to take a part in the initiation process and to interact with the macroradicals. THF, in particular, is known to react rapidly with oxygen to yield a charge-transfer complex which absorbs towards longer wavelengths [22]. Photolysis of this complex leads ultimately to reactive hydroxy and peroxy radicals [12] which will abstract hydrogen mostly from the weakest tertiary C - - H bonds of the polymer. Destruction of this THF---O 2 complex may explain why the u.v. absorption of THF is shifted towards shorter wavelengths upon irradiation. Another difference between S' and u.v.-initiation arises from the selectivity of these two types of radiation. While the PVC oxidation products are not selectively destroyed by ?-rays, some of them such as hydroperoxides and carbonyl groups are expected to be photolysed with formation of propagating radicals, thus increasing the efficiency of the photo-oxidation. However, since the quantum efficiency of the oxidation appears to remain strictly constant, we must assume that, during the early stages of the degradation, decomposition of photoproducts does not interfere significantly with the major oxidation process which involves primarily the reactions of secondary and tertiary peroxy macroradicals (PO~). Besides reacting with PVC or with the solvent to give hydroperoxides, these radicals are expected to disappear by terminating bimolecular interactions to yield ketone and alcohol groups in the case of secondary PO~ radicals (Russell mechanism [23]) and dialkylperoxides in the case of tertiary POz radicals. This last reaction is assumed to be responsible for the observed increase of the crosslinking process in the presence of oxygen. Cross termination by the various types of PO) radicals and with the peroxy radicals of the solvent are also possible. Tertiary peroxy radicals may disappear also by non-terminating interactions with formation of free alkoxy radicals: C H 2 - - C C I - - C H 2 ~ + O2,
]
(1)
O The principal fate of the resulting tertiary alkoxy radical is either an hydrogen abstraction from the polymer (PH) or from the solvent (SH) to yield a chain carrier alkyl radical and a tertiary alcohol which converts easily in a ketone by releasing hydrochloric acid:
~ C H 2 - - C C I - - C H 2 ~ + PH or SH -+ ~ C H 2 - - C C 1 - - C H 2 ~ + P" or S"
I
O"
999
I
OH
(2)
I000
M. BALANDIER and C. DECKER
or a decomposition by fl scission 1,24] which may occur not only by cleavage of the C---C bond [25] but also by cleavage of the C - - C ! bond:
production of Ci" by reaction (5) will thus account for the higher quantum efficiency of the dehydro' ~ C H 2 - - C - - C I + "CH2---CHCI ~
(3)
l[
O CH2--CCI~H
2~ __
I
O"
(4)
' ~ CH2"--C--CH2 ~ + CI'.
II
O Reaction (3) may play an important role in the backbone scission of chains and thus account for the significant increase of the quantum yield of net chain scission observed when irradiations of PVC were carried out in the presence of oxygen. According to our previous observations 1,21], these ct-chloro alkoxy radicals decompose preferentially by splitting off a C1 atom and formation of a ketone, i.e. reaction (4) is favoured over reaction (3). CI' evolved in reaction (4) is then expected to react with the polymer to yield HCI and a propagating secondary or tertiary alkyl radical. Due to polar effects, hydrogen abstraction from a methylene group is, more likely to occur than the abstraction of a tertiary bonded hydrogen 1-26, 27] : "CI + ~ C H 2 ~ C H - - C H 2 ~ C H
I
C1
chlorination process and for the subsequent larger increase of the polymer u.v.-absorption observed in the photo-oxidation of PVC. An opposite effect of oxygen was expected since 0 2 is known to react effectively with allylic radicals and should thus reduce the extent of the chain dehydrochlorination process. The net increase of ~HcJ in the presence of oxygen is thus the result of two opposing effects, the scavenging by oxygen of the growing polyenyl radicals being more than offset by the additional formation of HCI through the sequence of reactions (1), (4) and (5). The observed formation of relatively large amounts of carbonyl groups on the polymer backbone (q~>C=O "" 2tP~cission) is also consistent with the pro-
~ ---, ~ ' C H ~ C H - - C H 2 - - C H ~
I
CI
I
Cl
+ HCI.
(5)
I
C1
Since the fl-chloro-alkyl radicals formed in reaction (5) are known to expell easily CI" with formation of a double bond 1-28], dehydrochlorination is expected to proceed further along the macromolecule to yield polyene structures by a chain reaction. The additional
posed reaction scheme since ketones result both from reactions involving scission of the main chain, such as the formation of ~-chloroketone (reaction 3), and from reactions not affecting the PVC molecular weight (reaction 4).
REFERENCES
13. J. F. Rabek, G. Canb~ick, J. Lucky and B. Ranby, J. Polym, Sci., Polym. Chem. Ed. 14, 1447 (1976). 14. C. G. Hatchard and C. A. Parker, Proc. R. Soc. A235, 518 (1956). 15. W. D. Bowman and J. N. Demas, J. phys. Chem., Ithaca 86, 243 (1976). 16. V. Karaoglanov, Z. Analyt. Chem. 115, 385 (1939). 17. M. Bohdanecky, K. Solc, P. Kratochvil, M. Kolinsky, M. Ryska and D. Lim, J. Polym. Sci. A2 5, 343 (1967). 18. C. David and D. Baeyens-Volant, Europ. Polym. J. 14, 29 (1978). 19. H. Sobue and Y. Tajima, J. Chem. Ind., Tokyo 62, 1149 (1959). 20. J. Boissel, J. appl. Polym. Sci. 21, 855 (1977). 21. C. Decker, J. appl. Polym. Sci. 20, 3321 (1976). 22. V. I. Stenberg, R. D. Olsen, C. T, Wang and N. Kulevsky, J. Org. Chem. 32, 3227 (1967). 23. G. A. Russell, J. Am. chem. Soc. 79, 3871 (1957). 24. E. R. Bell, J. H. Raley, F. F. Rust, F. H. Seubold and W. E. Vaughan, Discuss. Faraday Soc. 10, 242 (1951). 25. D. E. Winkler, J. Polym. Sci. 35, 3 (1959). 26. A. A. Miller, J. phys. Chem., Ithaca 63, 1755 (1959). 27. J. M. Fedder, Quart. Rev. 14, 336 (1960). 28. R. Kh. Friedlina, Advances in Free-Radical Chemistry (Edited by G. H. Williams) Vol. l, Chap. 6. Logos, London (1965).
I. R. F. Reinish, H. R. Gloria and D. E. Wilson, Am.
Chem. Soc., Div. Polym. Chem., Preprints 7(!), 372 (1966). 2. K. P. S. Kwei, J. Polym. Sci. AI 7, 1075 (1969). 3. W. H. Gibb and J. R. MacCallum, Europ. Polym. J. 7, 1231 (1971). 4. W. H. Gibb and J. R. McCallum, Europ. Polym. J. 9, 77 (1973). 5. W. H. Gibb and J. R. McCallum, Europ. Polym. J. 10, 529 (1974). 6. W. H. Gibb and J. R. McCallum, Europ. Polym. J. 10, 533 (1974). 7. G. Scott and M. Tahan, Europ. Polym. J. 11, 535 (1975). 8. E. D. Owen, Ultraviolet Light Induced Reactions in Polymers, p. 208. Labana, ACS (1976). 9. F. Mori, M. Koyama and Y. Oki, Angew. Makromolek. Chem. 64, 89 (1977). 10. B. Ranby and J. F. Rabek, Photodegradation, Photooxidation and Photostabilization of Polymers, Wiley, New York (1975). 11. M. R. Kamal, M. M. EI-Kaissy and M. M. Avedesian, J. appl. Polym. Sci. 16, 83 (1972). 12. J. F, Rabek, Y. J. Shur and B. Ranby, J. Polym. Sci., Polym. Chem. Ed. 13, 1285 (1975).