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Primary hydroperoxidation in photooxidation of polyolefins and polyamides
Polymer Photochemistry 5 (1984) 243-265
Primary Hydroperoxidation in Photooxidation of Polyolefins and Polyamides
Jacques Lemaire and Ren6 Arnaud Laboratoire de Photochimie Mol6culaire et Macromol6culaire, ERA CNRS 929, Universit6 de Clermont II, Ensemble Universitaire des C6zeaux, BP 45, 63170 Aubiere, France
ABSTRACT Special attention is focussed on the various hydroperoxides which appear as primary products in the photooxidation of polyolefins and polyamides. In polyethylene, hydroperoxidation in ot position to the vinylidene group affords monomeric hydroperoxides (absorbing at 3550 cm-1), thermostable up to 85°C and easily photolyzed. Hydroperoxidation on the saturated chain affords hydrogen-bonded hydroperoxides which are fairly unstable at 85°C under vacuum. The polyethylene hydroperoxides are not able to induce significantly new oxidation processes. Isolated and associated hydroperoxides also exhibit very different behaviour in an ethylene-propylene-norborneneethylidene terpolymer. In polyamides (PA 11, PA 12, P A 6), hydroperoxidation ot to the nitrogen atom is predominant. Hydroperoxides are revealed only by chemical titration at room temperature. Thermally unstable above 60°C, photounstable at )t < 3 0 0 nm, they decompose into imides and hydroxylated groups ct to the nitrogen. Up to now, the photoinductive ability of the hydroperoxides cannot be completely excluded.
INTRODUCTION H y d r o p e r o x i d i c groups which a p p e a r in t he oxi dat i on of p o l y m e r s are key c o m p o u n d s for u n d e r s t a n d i n g t he p r i m a r y m e c h a n i s m of oxidation as well as for d e v e l o p i n g a b e t t e r insight into t he funct i on of 243
Polymer Photochemistry 0144-2880/84/$3.00 (~ Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Northern Ireland
244
Jacques Lemaire and Ren~ Arnaud
photostabiliser systems. Studies have been devoted to hydroperoxidation in most materials. However, many basic questions can be still considered: (1) Are there any specific sites for hydroperoxidation on a nonabsorbing polymer? After the initial formation of radicals (mostly unknown), hydrogen abstraction is generally observed in polyolefins, polyamides and polyurethanes. This reaction is essentially controlled by the lability of the corresponding bond. As shown later, many specific hydroperoxidation sites have been identified, due to large differences in bond dissociation energies. (2) What is the thermal stability of hydroperoxides in the matrix in the time-scale of the experiment? It is generally noted that the thermal stability of any oxidation products formed on a polymer chain is dependent on the matrix. Thermal stability cannot be considered as an intrinsic property of the products but as a property of the products in interaction with their close neighbourhood. Large differences in the thermal stability of tertiary hydroperoxides are observed in various matrices, for example. (3) What are the photochemical properties of hydroperoxides, i.e. absorption and photolysis quantum yield ? Hydroperoxides present an no-* transition band, localized on the O - O bond, almost completely forbidden but which extends to very long wavelengths (up to 360 nm, for example); the molar extinction coefficients are generally low, sometimes below 1 M-1 c m -1. The quantum yield of photolysis is generally very high, but most measurements have been carried out in solution and not in the solid state. 1-3 (4) What is the reactivity of hydroperoxides witl~ radicals? This question is especially important in photocatalyzed oxidation. The reactive species formed on a photoactive pigment are able to induce the decomposition of hydroperoxide. The reactivity of hydroperoxide with radicals has often been recognized, due to lability of the O O - H bond. Such a process accounts for the chain mechanism observed in the photolysis of R - O O H . 1 (5) What is the photoinductive ability of hydroperoxides in the polymer matrix? The hydroperoxides formed are potential
Hydroperoxidation in photooxidation of polyolefins and polyamides
245
sources for radicals, especially reactive radicals such as O H . . H o w e v e r , the radical pairs f o r m e d in thermal or p h o t o t h e r m a l dissociation can react together in the solid state (in 'cage' reaction) or can migrate apart. The ability to initiate a new oxidation chain depends on this competition as far as the hydroperoxide can dissociate. If the first question is related to the structure of hydroperoxides, answers to the four o t h e r questions are n e e d e d to account for the behaviour of these intermediates in the overall mechanism of the oxidation. U p to now, these basic questions have not been satisfactorily resolved although much useful information has been found by m a n y research groups. Such a situation is not really surprising since hydroperoxides usually appear at a low stationary concentration (apart from a few exceptions) and, before the F T I R development, analytical techniques were limited to chemical titrations in solutions and conventional IR spectroscopy. Chemical titrations present high sensitivity (even after dissolution) but are unspecific. IR spectrometry was poorly sensitive but m o r e informative on the structure of hydroperoxides and on their interactions (e.g. hydrogen bonding). In recent years, our group has developed the use of photoactive pigments as tools for the understanding of the chemical evolution of 'non-absorbing' polymers (c.f. Ref. 4, for example). The fundamental advantages of TiO2, Z n O and CdS pigments are: (1) T h e y afford an efficient control of the absorption of the light and of the initiation rate when added to a polymer in which the absorbing species are not identified. (2) T h e y act as inner screens to protect the photoproducts and prevent their disappearance in further photochemical steps (except when the products are photocatalytically oxidized). (3) In the presence of pigment, oxidative p h e n o m e n a are limited to the surface of sample. Oxygen diffusion is never a controlling factor. The use of pigments is especially interesting for the study of hydroperoxides since they ensure controlled formation and sometimes controlled disparition of these intermediates. Moreover, it is often observed that in the presence of pigment hydroperoxides
246
Jacques Lemaire and Ren~ Arnaud
accumulate until a stationary concentration is reached which is much higher than in a transparent matrix. Therefore for several years we have been comparing the formation and disappearance of hydroperoxides in thermal, photothermal and photocatalytic oxidations. Each initiation mechanism has its own advantages and disadvantages and comparison is often fruitful. In the present paper, we report the main observed properties of hydroperoxides in polyolefins and polyamides.
PRIMARY HYDROPEROXIDATION
IN P O L Y O L E F I N S
Atactic polypropylene would be the most simple, but academic, sys-
tem as far as hydroperoxidation alone is concerned. In this completely amorphous matrix, hydroperoxides accumulate at a very high concentration ( - 0 . 1 4 mol litre-1) 4"5 in photothermal oxidation (60°C,)t > 300 nm). Only hydrogen-bonded hydroperoxides were observed in the IR spectra (gma~--~3400 cm -a) together with alcoholic groups, and their contribution to the total absorption was kept fairly small. It is important to stress that no m o n o m e r form of hydroperoxides absorbing at 3550 cm -1 could be observed. In the presence of Z n O (of various types), the initial rate of hydroperoxidation was far higher than in unpigmented samples; this observation was easily explained by the increase in the rate of light absorption in the pigmented sample. However, as shown in Table 1, the mean photostationary concentration in pigmented samples was only 60% higher than in unpigmented samples. The rate of disappearance of hydroperoxides was therefore largely increased by the photoactive pigment. The decomposition of R O O H induced by the reactive species formed on the pigment leads to higher concentrations of peroxy radicals and therefore favours rupture processes. Scheme 1 summarizes the processes implied in the ZnO-photocatalyzed oxidation of atactic polypropylene. The photocatalyzed decomposition of hydroperoxide groups has been controlled on ten-butyl hydroperoxide in solution in n - h e p t a n e containing up to 14% by weight of photoactive pigment (TiO2, ZnO). Total absorption of the light in the pigment was checked by using different pigment concentrations. Direct excitation of the hydroperoxide is therefore prevented and the dissociation of the c o m p o u n d is only induced by the radical species formed on the pigment.
Hydroperoxidation in photooxidation of polyolefins and polyamides
247
TABLE 1
Variations in Hydroperoxide Concentration with Irradiation Time in Photothermal or ZnO-Photocatalyzed Oxidation of Atactic Polypropylene Irridiation time ( h ) 50
93
160
202
273
326
350
Hydroperoxide concentration (mol kg 1) in atactic PP
0.010 0.010 0.052 0-075 0.096 0.120 0.140 +0.010 +0.010 ±0-010 +0.010 ±0.010 :e0.012 ±0.010
Hydroperoxide concentration (mol kg-l) in atactic PP +3% ZnO A
0.037 0.063 0-100 0.147 +0.010 +0.010 ±0.010 +0-014
--
0.193 0.220 ±0-019 +0-022
Isotactic p o l y p r o p y l e n e appears to be m o r e complex as a semicrystalline matrix. H y d r o p e r o x i d e s also accumulate to give high concentrations (for example, 0.1 mol litre -1 in photochemical oxidation at )t > 300 nm). Only h y d r o g e n - b o n d e d hydroperoxides are o b s e r v e d (at 3400 cm -1) but again the large contribution of alcoholic and acidic groups overlaps this absorption even in the early stages of the oxidation. N o m o n o m e r form has ever been observed in I R spectra (at 3550 cm-~); as o p p o s e d to polyethylene, no local defect plays an important role in the primary hydroperoxidation (see later). P r o p e r ties of the tertiary h y d r o p e r o x i d e forms are known following the basic work of Wiles et al. and others 5-~2 and of M a y o et al. 13-is These hydroperoxides generally appear in close proximity and sequences of groups have been o b s e r v e d (through reduction and dehydration of the corresponding tertiary alcohols16). A c o n s e q u e n c e of such an association has b e e n recently observed in p o l y p r o p y l e n e stabilized with H A L S . The effectiveness of H A L S has been at least partially explained by interactions of H A L S with several hydroperoxidic groups. 17 The thermal and photochemical decompositions of hydroperoxides lead, after the homolytic cleavage of the O - O bond, to alcohols and various ketones (Scheme 2) (although the /3-scission of alkoxy radicals at r o o m t e m p e r a t u r e has recently b e e n questioned by G e u s k e n s and K a b a m b a ) . TM
248
Jacques Lemaire and Ren~ Arnaud
Pigment
~
hv, 02, H20
(R') Reactive species Radicals PH (atacticpolypropylene)
~
---C~CH2-I
CH3
OO" I --C--CH2-- ,~
' CH3 Propagation~
Ru~ Peroxides~
I(R') ~Pigment
OOHI --C---CHE-hv (pigment-inhibited),'"[ ,/ CH3
OI C--CH2 I CH~
P i
'ii
O
i i
L .+Z J " ~
+ CH2--
(R'
X
by'. inhibited)-,.
(pigment
-~" ,
+HO"
," t~--CH2 ' I : CH3
OH I
--C--CH2-I CH3
Lactones Acids (Norrish type I) Unsaturations (Norrish type II)
Sehane 1
Hydroperoxidation in photooxidation of polyolefins and polyamides
249
CH3
....
I C--CH2 .... I OOH
1 ....
CH3 I C--CH2 .... I O-
+ OH-
CH3
I
....
C---CH2,
I OH
(1726 cm-1)
(1718 cm 1)
Sdaeme 2
Introduction of a photoactive pigment into such a semi-crystalline p o l y m e r is not so demonstrative as in atactic polypropylene. T h e Z n O - or TiO2-photocatalyzed oxidation of isotactic polypropylene appears as a superposition of the pigment-catalyzed oxidation of a m o r p h o u s zones (similar to atactic polypropylene photocatalyzed oxidation) and of the p h o t o t h e r m a l oxidation of u n p i g m e n t e d zones. H y d r o p e r o x i d e s and ketones are not only reacting with the species f o r m e d onto the pigment but direct excitation is also occurring and final photoproducts such as acids and esters are f o r m e d as opposed to the atactic case. A n amorphous copolymer PP/PE has recently been examined by Geuskens and Kabamba. 18 H y d r o g e n - b o n d e d h y d r o p e r o x i d e was observed at 3400 cm -1 and chemically titrated. A n original photochemical property of the f o r m e d hydroperoxides (essentially secondary hydroperoxides) was proposed. Excitation of the hydroperoxide c h r o m o p h o r e in interaction with ketones was proposed to afford directly an acid group H .... O I II R___oJO pjC~p
hv
H O I II ~ O--C~p+
R--O---P
250
Jacques Lemaire and Ren~ Arnaud
According to Geuskens and Kabamba, this explains why vacuum photolysis at 365 n m of a prephotooxidized sample in the presence of b e n z o p h e n o n e (which is consumed in the prior irradiation) led to acid formation through the decomposition of hydroperoxide, although no ketone photolysis proceeded (as demonstrated by the absence of a Norrish type II process). The situation is especially controversial in p o l y e t h y l e n e where only the general lines of the overall oxidation mechanisms can be sketched. The stationary concentrations of hydroperoxides are far lower than in polypropylene and difficulties obviously exist for the determination of the primary hydroperoxidation sites in L D P E as in H D P E . In 1976, Cheng et al. 19 reported an N M R study of the hydroperoxides formed in the thermooxidation at 140°C of two L D P E ' s (17 and 22 branching points per 2000 secondary hydrogen atoms). These authors observed essentially secondary hydroperoxides and rationalized by estimating the ratio of rate constants for abstraction of tertiary and secondary hydrogen atoms as 9 . 8 + 1-0. Tertiary hydroperoxidation can only be a minor route. In 1975, A m i n et al. 2° compared the evolution of hydroperoxides (chemically titrated) with the evolution of vinylidene, vinyl and carbonyl groups in samples of L D P E exposed to U V after different times of mild processing. Two main conclusions were derived: (1) During the process at 165°C, hydroperoxidation occurs in the a position of the vinylidene. (2) T h e hydroperoxides formed in the processing operation are able to photoinduce the oxidation of L D P E . In a preliminary paper, 21 we have shown that the hydroperoxides formed during the thermooxidation at 90°C of L D P E have no photoinductive effects as opposed to the tertiary hydroperoxides formed in polypropylene oxidation. We have recently studied six low-density polyethylenes, as described in Table 2. They have been used in the form of films (thickness 150 ~m). The initial vinylene ( - - C H - - C H - - ) content is low (less than 1 - 2 × 10 -3 M). T h e CH3 branching ratio indicated in Table 2 is the conventional 'methyl index' measured from the IR absorbance at 1378 cm -1 ( A S T M D 2 2 3 8 - 6 4 T ) . The polymer used for film 3 is an L D P E obtained with propylene as the transfer agent. The usual methyl index cannot be measured since the branch points
Hydroperoxidation in photooxidation of polyolefins and polyamides
251
TABLE 2
Characteristics of LDPE Film Density CH3 branching number (d) ratio
Vinylidene
Vinyl
(M)
(M)
(per 1000 C) 1 2 3
0.921 0.922 0-923
23 21 --
1.7 x 10 -2 1.7x10 2 1 . 6 x 10 -2
7 ' 2 x 10 -3 5"0x10 3 2 . 1 × 1 0 -2
4 5 6
0-918 0-916 0"912
30 42.5 --
1 . 9 x 10 -2 3.3x10 2 3 " 4 x 10 2
1 . 7 x 10 -2 1.6xlO 2 8 . 6 x 1 0 -3
c o r r e s p o n d essentially to p e n d a n t methyl groups (and not to C4 branches as assumed in A S T M D 2 2 3 8 - 6 4 T). As shown in Table 2, a typical L D P E presents, per 1000 carbon atoms in the chain, a r o u n d 2 0 - 4 0 tertiary hydrogen atoms (branch points) and a r o u n d 0-5-1 unsaturated groups such as vinylidene, vinyl and vinylene sites. A n y radical f o r m e d in L D P E , under U V exposure for example, would either abstract a hydrogen a t o m or add to an unsaturated group. Considering the hydrogen abstraction on the three different sites (saturated chain, branch point, a position to an unsaturated group), it is expected that abstraction of a secondary hydrogen a t o m from the chain w o u l d prevail on the abstraction of a tertiary atom. Allara and E d e l s o n 22 r e p o r t e d that the ratio of the rate constant of the abstraction of a tertiary hydrogen a t o m to the rate constant of the abstraction of a secondary a t o m was close to 8 in m o d e l h y d r o c a r b o n oxidation. According to Cheng et al., 19 the reactivity ratio of branch points to linear chain is indeed 9- 8 + 1 in L D P E . H y d r o p e r o x i d a t i o n on the 2 0 - 4 0 tertiary carbon atoms would represent only 1 0 - 2 0 % , at most, of the total hydroperoxidation. It has b e e n shown z3 in the oxidation of acyclic alkenes of low molecular weight that peroxy radicals can either abstract an allylic hydrogen or add to the double bond. Except when the allylic hydrogen is of tertiary nature (as in 3-methyl 1-butene), the rate constants of abstraction and addition are of the same o r d e r of magnitude. In L D P E , radical reactions with vinylene, vinyl or vinylidene groups can occur either by an abstraction or by an addition mechanism. Summarizing the data obtained up to n o w on model c o m p o u n d s and in L D P E , it appears impossible to predict w h e t h e r the formation
Jacques Lemaire and RenO Arnaud
252
of hydroperoxidic groups on the saturated chain or the hydroperoxidation of unsaturated groups prevails. Considering the 3 5 5 0 c m -1 band, previously attributed to a m o n o m e r i c f o r m of secondary hydroperoxides f o r m e d on the saturated chain, zl the following r e m a r k s should be noted: (1) No 3550 cm -1 band was observed in the 95°C thermooxidation of H D P E in which only vinyl groups are present initially. (Film 7: d = 0 - 9 4 8 , v i n y l i d e n e = 6 . 5 x l 0 - 3 M , v i n y l = 7 - 6 X l 0 - Z M . Film 8: d = 0 . 9 6 4 , vinylidene=l'2xl0-3M, vinyl=7"6X 10 -2 M.) (2) In thermooxidation of L D P E at 85°C, the m a x i m u m intensity of the 3550 cm -1 band was not related to the initial content of the branch point. In film 3 containing m a n y tertiary carbon atoms the m a x i m u m intensity of 3550 cm -~ was about the same as in L D P E with a low branching ratio (samples 1, 2,, 4). In contrast, the m a x i m u m intensity of the 3550 cm -~ band was clearly related to the initial vinylidene content. In Fig. 1, it appears that the concentration of vinylidene-type
,[FreeROOH],~C:CH2]or r-HC:CH2-1x 102(M) O
,~,,~ a. v\
2-
/"
0
o o
o
\~o~ / +/ ./_._~_._. . "'~..~ o-~ "-7---,-.---+-.t..+_ .... - - ~ . - - . % . "-+-T~-.. '" " C" % \o
0
0
160
260
a60
""
IRRADIATION TIME ( h r s l 460
Fig. 1. Variations of isolated hydroperoxides and vinylidene concentrations (10 -2 mol litre -~) during the thermooxidation of L D P E at 85°C. O, Sample 5; +, sample 1.
Hydroperoxidation in photooxidation of polyolefins and polyamides
[FreeROOH-]max x10 2(M) "
sample 3
sample 5 --~ O~, - ' /~~'-sample
253
6
/ ~ ' ~ / / O ' ~ sample 4
/vii_" sample I
,i
7~
"" sample 7
O./".~ sample 8 0
r~r-_-~l-i "1 x 102 (M) L.a_-L,n2Jo
I
I
I
I
1
2
3
4
)
Fig. 2. Variations of maximum concentration (10 2 mol litre-1) of hydroperoxide monomer as a function of initial concentration (10 2 tool iitre 1) of vinylidene in the thermooxidation of LDPE and HDPE (for definition of samples 1-8 see Table 2 and subsequent text). unsaturations decreases as soon as t h e r m o o x i d a t i o n begins. Simultaneously, the m o n o m e r hydroperoxide concentration, m e a s u r e d at 3550 cm 1 (e = 80 M l cm-1) increases and reaches a limiting value well before the unsaturation has completely disappeared. T h e r e f o r e h y d r o p e r o x i d a t i o n precedes the disappearance of vinylidene. However, the w a v e n u m b e r of the I R absorption of vinylidene groups was not modified by peroxidation. On the other hand, in Fig. 2 the m a x i m u m concentration of isolated hydroperoxides [ROOH]ma~ has been plotted as a function of the initial vinylidene [ ~ C = C H 2 ] 0 content in various L D P E ' s and for the H D P E ' s 7 and 8. It can be pointed out that [ROOH]max is proportional to [~C~-CH2]0. T h e slope of the experimental straight line is close to 1. In each type of polyethylene, [ROOH]max is almost equal to [ ~ C = C H 2 ] 0 . As has been pointed out earlier, hydroperoxidation of vinylidene groups can imply an abstraction or an addition mechanism and the structure of hydroperoxides f o r m e d would d e p e n d on the reaction pathway
254
Jacques Lemaire and Rend Arnaud
Abstraction mechanism --C--CH2--
II CH~
+ r-
, --C--CH-II
( ,--C=CH-t
CH2
C.H2
02, PH~ O O H
1o:, PH
I --C C-II I CH2 H
--C-----CH I CH2OOH II
I
Addition mechanism
- - C - - C H 2 - - + r. II CH2
--C--CH2-I CH2r
~o2
OOH I --C--CH2-~H2r Ill
Formation of primary hydroperoxides (type II), which are probably very unstable, implies simultaneously the transposition of vinylidene into vinylene groups. Increase of absorption at 810-830 cm -1 of such a group was not observed. Formation of tertiary hydroperoxide (type I!I) implies a parallel disappearance of the double bond and hydroperoxidation, which contradicts the observed facts. Formation of secondary hydroperoxides without any transposition of the double bond and without any shift of the IR absorption band at 888 cm -1 accounts for the experimental results
--C--CH2--
It
CH2
OOH l a, o½ - - C - - C H II CH2 (absorbs at 888 and 3550 cm-1)
Saturation of the oxidized vinylidene sites is provoked afterwards by radical attack. These radicals are formed either primarily or through the decomposition of any hydroperoxides (and not necessarily those
Hydroperoxidation in photooxidation of polyolefins and polyamides
255
which are f o r m e d in the a position of the double bond) OOH I --C C - - + 2r- ~ II I CH2 H r
r I --C I rCHe
OOH I C I H
OOH
I I rCH2 H
,
0>85oc
.
.
.
.
In the early stages of thermooxidation of L D P E at 85°C, the carbonyl c o m p o u n d s (ketonic, then acid) were readily f o r m e d at a concentration which was soon higher than the initial concentration of vinylidene groups. Meanwhile, no significant decomposition of the hydroperoxides f o r m e d on the vinylidene sites was observed. The same observation can be m a d e in H D P E , in which no significant 3550 cm 1 absorption was detected when the concentration of carbonyl c o m p o u n d s largely e x c e e d e d the initial concentration of unsaturations. Obviously, hydroperoxidation of the saturated chain proc e e d e d simultaneously leading only to h y d r o g e n - b o n d e d h y d r o p e r o x ides (observed at 3400 cm -1 in I R spectrophotometry). As suggested previously, 21 these hydroperoxides d e c o m p o s e rapidly at 85°C, directly into ketones OOH I --C-I H
O" I ' - - C - - + OH" I H
O II ~ --C-- + HzO
In photooxidation of L D P E , the stationary concentration of hydroperoxide absorbing at 3550 cm -1 was kept low unless a photoactive pigment afforded photochemical protection. 4 The vinylidene site disappears rapidly on radical attacks. Many sources of radicals could be considered (primary radicals, R O O H on saturated chain or R O O H a to the vinylidene). The carbonyl c o m p o u n d s appear in concentrations much higher than the initial unsaturations. In Fig. 3 the variations of the absorbance at 1 7 1 3 c m -1 were plotted as a function of the variations of the absorbance at 888 cm I for L D P E with high initial vinylidene content (samples 5 and 6) at different temperatures. In the early stages of the oxidation, the concentration of carbonyl comp o u n d s (essentially ketonic groups with ~1735--~250M - 1 c m -1) largely
256
Jacques Lemaire and RenE Arnaud
E
E
O f~
U
K <3
o,s samplle~ e
sample 6
0.5
~t
~
0
I
i
i
i
1
2
3
4
2
0
1
2
Fig. 3. Variations in absorbance AD at 1713 cm-1 versus variations in absorbance at 888 cm-1 in polychromatic photooxidation (SAIREM SEPAP 12.24; ~ > 300 nm: sample temperature 60°C) of LDPE films (thickness 150/xm). e x c e e d e d the concentration of the d e s t r o y e d vinylidene groups (e388 = 160 M-1 cm-1). This was also valid for L D P E with low initial vinylidene content (samples 1 and 4). Vinylidene groups are not, therefore, the only precursors of carbonyl compounds, since no chain reaction has b e e n seen in L D P E . T h e r m o o x i d a t i o n and photooxidation of L D P E t h e r e f o r e p r o c e e d through hydroperoxidation in a position to the vinylidene group and on the saturated chain affording m o n o m e r i c hydroperoxides absorbing at 3550 cm -~ and t h e r m o s t a b l e at 85°C, and h y d r o g e n - b o n d e d hydroperoxides fairly unstable at 85°C, respectively. B o t h hydroperoxides f o r m e d present a s e c o n d a r y structure and, through a cage direct decomposition into ketones and water, it is expected that these hydroperoxides have a p o o r photoinductive ability as suggested previously. 21 T h e most striking result o b t a i n e d in polyethylene oxidation is the thermal instability of h y d r o g e n - b o n d e d hydroperoxides c o m p a r e d to the relative stability of free hydroperoxides. This could be explained by high local concentrations of b o n d e d hydroperoxides, favouring chain reactions in the decomposition similar to those o b s e r v e d in highly concentrated solutions.1 This u n e x p e c t e d observation leads us
Hydroperoxidation in photooxidation of polyolefins and polyamides
257
to study a polyolefin containing a high and controlled a m o u n t of vinylidene-like sites. 300-/xm films of different e t h y l e n e - p r o p y l e n e norbornene-ethylidene terpolymers have been submitted to photothermal oxidation. Purposely, these terpolymers have not been chemically crosslinked; the concentrations of ethylidene sites are therefore equal to the initial concentration of the diene. On exposure under different sources (SEPAP 12.24 unit, h > 3 0 0 n m ; Westinghouse 125W, 365 nm; monochromatic light at 365 nm), a fast photooxidation proceeds; free and hydrogen-bonded hydroperoxides are directly observed at 3550 and 3380 cm l, respectively (as opposed to a polyolefin with a low initial content of vinylidene sites). Free hydroperoxides reach photostationary concentrations (around 0-015 m o l k g -1 at 50°C, around 0 . 0 3 m o l k g -1 at 20°C) as hydrogen-bonded hydroperoxides accumulate at higher concentrations (0.04 versus 0.015 mol kg-1). These hydroperoxide concentrations are directly related to the initial content of ethylidene sites. H O 0 ~
In the terpolymer samples, the evolution of the ethylidene sites observed at 8 0 7 c m -1 presents two different phases. Initially the initial concentrations of ethylidene remain practically constant (around 0 - 2 7 m o l kg -1) while the build-up of free and hydrogenb o n d e d hydroperoxides is important (up to 60% of the stationary concentrations of free hydroperoxides). Then, as hydrogen-bonded hydroperoxides develop further, the disappearance of ethylidene sites appears to be fast. In a second series of experiments the same samples were submitted to prephotooxidation up to 34% disappearance of ethylidene sites, and then to vacuum thermolysis at 70°C. In the first stages of vacuum thermolysis hydrogen-bonded hydroperoxides and the ethylidene double b o n d disappear simultaneously as the free hydroperoxides remain invariant. After the complete disappearance of associated hydroperoxides, residual ethylidene sites are still observed. In the last stages of vacuum thermolysis, free hydroperoxides vanish as the ethylidene sites remain invariant.
258
Jacques Lemaire and Ren~ Arnaud
From these observations it is deduced that the associated hydroperoxides are less thermally stable than isolated hydroperoxides. The build-up of hydroperoxides precedes the disappearance of ethylidene sites and this disappearance is related to the decomposition of hydrogen-bonded polymers.
PRIMARY HYDROPEROXIDATION ALIPHATIC POLYAMIDES
IN
The behaviour of aliphatic polyamides, especially nylon-type polymers, on exposure to U V irradiation has attracted much attention. However, most of the research in this field concerns either vacuum photolysis of the polymer or photooxidation at wavelengths shorter than 290 nm. Photooxidation of model compounds such as N-alkyl amides RCH2 CO NI-I CH2R' have also been considered. 23-26 Since 1978 we have been studying the photooxidation of polyamides in the solid state (films of 4 0 / x m thickness), either at wavelengths longer than 300 nm or at 254 nm. The first polymers studied were polyundecanamides (PA 11). In the polymer matrix it appeared possible to characterize four intermediate products (hydroperoxides, imides, hydroxyl groups a to the nitrogen atom and aldehydes) and two final groups (amines and acids). The main conclusions on the mechanism of photooxidation at long and short wavelengths reported recently27 are summarized in Scheme 3. In the scope of the present paper, the following properties of the primary hydroperoxides can be noted: (1) The hydroperoxide groups are revealed only by chemical titration in a special solvent. Since, as shown later, these hydroperoxides are fairly unstable at temperatures higher than 60°C, this titration must be carried out at room temperature. Hexafluoro-2-propanol appears to be the only solvent suitable for polyundecanamide at room temperature. The titration is based on the oxidation of Fe 2÷ and complexation of F e 3+ by SCN- anions. 19"28Iodometric methods are unsuitable since they are carried out in a boiling CH3COOH-2-propanol mixture. (2) The hydroperoxides formed in the sample exposed to long wavelength photooxidation at 60°C in a SEPAP 12-24 set-up (this set-up has been described in many papers, for example
P" +
I A (T > 90°C)
--CH2COOH
l ?J2 hv A'
II O
--CH2CH + NH2COCH2--
I OH
--CHzCHNHCOCH2--
O-
OH- +--CHECHNHCOCH2-I
hv (k = 254 nm and A> 3 0 0 y
A (T around 6 0 ° C ) /
J (T around 60"C) "~k = 254nm and ,~> 300 nm)
+p.
I -c~
hv
(PH)
Sehele 3
--CH2CO"
•NFICOCH2-- ,c
[--c~coN--~ I + --CH~COO~
A
I A (T > 90°C)
--CH2NH- +.COCH2--
hv (,k = 254 nm and X>300 nm)
I--c-~cooH 1
hv, 02
I + i.i~i~i~:.i + [ --c~cHo 1
"~
--CH2CONHCOCH2-- + H20 ~-]
~ A
--CH2CHNHNOCH2-I OOH
OO"
--CH2CHNHCOCHz--
--CH2C. HNHCOCHE--(P-)
--CH2CH2NHCOCH2--(PH)
tO t.~
g~
t~
260
Jacques Lemaire and Ren~ Arnaud
Refs 2 8 - 3 0 ) accumulated until a stationary concentration close to 1-5 × 10 -2 mol kg -1 was achieved. In contrast, on exposure at short wavelengths ( 2 5 4 n m ) the stationary concentration of hydroperoxides is far lower, a r o u n d 0 . 3 x 1 0 - 2 m o l kg -1. T h e photolysis of R O O H at 254 nm was directly o b s e r v e d after pre-irradiation at long wavelengths through successive titrations. (3) H y d r o p e r o x i d e s were found to b e fairly stable in the matrix at 60°C, in the dark. Chemical titrations in pre-irradiated samples ((ROOH)0-~ 1-5 x 10 -2 mol kg -1) maintained at 60°C s h o w e d that the lifetime of R O O H was a b o u t 3 0 h . A t l l 0 ° C , the lifetime was f o u n d to be a b o u t 5 min. T h e thermal decomposition of h y d r o p e r o x i d e led to imides (revealed in the I R spectra at 1735 and 1690 cm -1) and to hydroxylated groups ct to the nitrogen a t o m (to a lesser extent). (4) T h e p h o t o t h e r m a l decomposition of h y d r o p e r o x i d e at 254 nm led directly to acid groups without any accumulation of intermediates (such as imides). (5) T h e hydroperoxides were readily r e d u c e d in the matrix by reducers such as phosphites. In the presence of 3% by weight of photoactive pigment (such as o r T i O 2 R E 90,* rutile forms which have received a surface t r e a t m e n t with alumina and silica, or type C - Z n O t ) P A samples undergo, u p o n U V exposure, a p h o t o c a t a l y z e d oxidation. 31 T h e h y d r o p e r o x i d e groups, f o r m e d in the sample photocatalytically oxidized at 60°C in a S E P A P 12.24 set up, accumulated to 2 . 4 × 10 -2 mol kg -~. Although this mean concentration was not corrected for light absorption profile (in the presence of 3% T i O 2 m o r e than 9 0 % of the light, at A < 380 nm, is a b s o r b e d in the first 10 txm of the 40 p.m film), it clearly a p p e a r e d that the photostationary concentration was higher in the presence of pigment than in its absence. At r o o m t e m p e r a t u r e (25°C), on the same U V exposure, the p h o t o stationary concentration of hydroperoxides was f o u n d to b e the same (2.35 x 1 0 - 2 mol kg-~). Although the hydroperoxides were fairly unstable at 60°C, it a p p e a r e d that the stationary concentrations of hydroperoxides were essentially controlled by photocatalysis at 60°C. TiO2 R L 6 5
RL 65 and RL 90 from Produits Chimiques, Thann et Mulhouse, France. t Type C-ZnO from Soci6t6 des Mines et Fonderies de Zinc de la Vieille Montagne, Belgium. * TiO2
Hydroperoxidation in photooxidation of polyolefins and polyamides
261
In photocatalytic oxidation, the primary formation of reactive species is wavelength-independent, as long as the energy of the radiation is higher than the width of the forbidden band of the photoactive pigment (3.5 eV in TiO2 to 3-2 eV in ZnO). Excitation in the range 300-400 nm or at 254 nm must therefore induce the same phenomena. Titration of hydroperoxide has been carried out in a sample containing 3% TiO2 RL65 and exposed to 254 nm irradiation. The mean photostationary concentration of hydroperoxides appeared to be somewhat lower at 254 nm than under long wavelength irradiation (1-2 x l0 -2 versus 2"5 x 1 0 - 2 mol kg -~, respectively). The photostationary concentration of hydroperoxide groups in an unpigmented polyundecanamide sample photooxidized under the same experimental conditions has been shown to be about 0 . 2 x 1 0 - 2 mol kg -~. Two conclusions can be drawn: (1) The inner filter effect of the pigment at 2 5 4 n m is clearly observed, the photolysis of R O O H being inhibited by the absorbing pigment; (2) A residual wavelength effect appeared since long wavelength and 254 nm irradiation did not correspond to the same mean stationary concentration of ROOH. Differences in profile light absorption at 254 nm and the polychromatic radiation may account for the difference. Moreover, as isotactic polypropylene P A 11 is a semi-crystalline polymer, the wavelengthdependent photochemistry (and especially the photolysis of ROOH) occurring in the unpigmented zones is expected to be superposed on the photocatalytic oxidation of amorphous and pigmented zones. In the pigmented zones the formation and decomposition of hydroperoxides are induced by the reactive species appearing on the excited pigment, which form stationary concentrations of hydroperoxide. At short wavelengths the direct photolysis of the primary hydroperoxides proceeds faster than at long wavelengths, the direct photolysis in the unpigmented zones is an additional route of disappearance of the hydroperoxides and lower stationary concentrations are observed. The photothermal oxidation at long wavelengths and at 254 nm of PA 12 (polydodecanamide) and P A 6 (polyhexanamide) is presently being studied in our group and is being compared with the photooxidation of P A 11 under the same experimental conditions. Very similar intermediates (hydroperoxides, imides, hydroxylated groups o~
262
Jacques Lemaire and Rend Arnaud
to the nitrogen atom, aldehydes) have been observed throughout both photothermal oxidations. Therefore the same oxidation mechanism implying a primary direct photoscission at short wavelengths ()t < 330 nm) and a photoinduced oxidation at wavelengths longer than 2 5 4 n m is valid for every aliphatic polyamide studied. P A 11, P A 12 and P A 6 differ only on quantitative grounds. For example, the initial rates of the direct photoscission (measured as the rates of appearance of aldehyde groups) are in the ratio 1 : 3 : 4-5 respectively for P A 12, P A 11 and P A 6. Dealing with hydroperoxides, chemical titrations in hexafluoroisopropanol supplied the results represented in Fig. 4 on long wavelength exposure ()t > 300 nm). T h e photostationary concentrations of hydroperoxides are, respectively, 45, 20 and 1 2 m m o l k g -1 for P A 12, P A 11 and P A 6. At 254 n m the photostationary concentrations are much lower (7, 2-5 and 3.8, respectively, for P A 12, P A 11 and P A 6).
[ROOH] (mmol.kg -1 } 60-
40-
90-
TIME 0 0
I
I
I
I
50
100
150
900
{hrs~
Fig. 4. Variations in hydroperoxide concentration throughout photooxidation (at )t > 300 nm) of aliphatic polyamides carried out in: O, PA 12; A, PA 11 (new data); and x, PA 6.
Through chemical titrations hydroperoxides have been shown to decompose above 70°C. First-order kinetics are obeyed, the lifetime of P A 12, P A 11 and P A 6 hydroperoxides being, respectively, 5, 6 and 1 h. Photolysis of hydroperoxides proceeded rapidly at 254 nm, or more slowly at )t > 300 nm. The decomposition stoichiometry of hydroperoxides is to be examined. Competition between processes
Hydroperoxidation in photooxidation of polyolefins and polyamides
263
(a) and (b) has to be determined H20 + --CO---NH--CO-
OOH I -C--NH---CO-I H
o. I ~ - - C - - N H - - C O - - - + OH. I H H
--C--NH--CO I OH Some difficulties arise related to the instability of the decomposition products in the matrix. The imide groups are photochemically unstable, especially at 254 nm, or are hydrolyzed; the hydroxylated groups are thermally (above 90°C) and photochemically unstable. The decomposition of the hydroperoxide groups provokes an increase in the imide concentration only after thermal treatment at 90°C. At lower temperatures imides hydrolyze faster than hydroperoxides decompose, the activation energy of the homolytic cleavage of hydroperoxidic bonds being higher than the activation energy for hydrolysis. Even at 90°C the importance of imide formation has still not been quantitatively assessed. (IR detection of imide groups is, however, easier than IR detection of hydroxylated groups.)
CONCLUSIONS Although the various questions appearing in the introduction have not been completely answered in the polymers under investigation, special attention to the behaviour of hydroperoxides has given rise to some useful information. Most polymer materials exhibit specific hydroperoxidation sites. Although the corresponding hydroperoxides formed present qualitatively similar properties, large differences on quantitative grounds are observed. Mostly attributed to the molecular structure of the various hydroperoxidized groups, these differences must be accounted for by matrix effects. Studies on model compounds would therefore hardly afford any relevant information on the properties of hydroperoxides in the matrix. Improvements in analytical techniques are certainly positive factors towards the understanding of the role of hydroperoxides (and maybe for collecting better information on their molecular
264
Jacques Lemaire and Ren~ Arnaud
structure and their interactions with their neighbourhood). Such progress would be even more fruitful in systematic comparisons of various oxidative conditions. As shown in the previous sections, it is interesting to compare the formation and the evolution of hydroperoxide groups in low-temperature thermooxidation, in short- and longwavelength photooxidation and in photocatalyzed oxidation.
REFERENCES 1. Martin, J. T. and Norrish, R. G. W., Proc. Roy. $oc., A220 (1953) 322. 2. Guillet, J., Stabilization and degradation o[ polymers, Allara, D. L. and Hawkins, W. L. (eds), Advances in Chemistry Series 169, Washington, 1978, pp. 1-10. 3. Mill, T., Richardson, H. and Mayo, F. R., J. Polym. Sci., Chem. Ed., 11 (1973) 2899. 4. Lemaire, J., Pure & Appl. Chem., 54(9) (1982) 1667. 5. Penot, G., Amaud, R. and Lemaire, J. Makromol. Chem., 183 (1982) 2731. 6. Carlsson, D. J. and Wiles, D. M., Macromolecules, 2 (1969) 597. 7. Carlsson, D. J., Garton, A. and Wiles, D. M., Macromolecules, 9 (1976) 695. 8. Carlsson, D. J. and Wiles, D. M., J. Macromol. Sci., Rev. Macromol. Chem., 14 (1976) 65. 9. Carlsson, D. J, and Wiles, D. M., Ultraviolet light induced reactions in polymers, Labana, S. S. (ed), A.C.S. Symposium Series 25, 1976, p. 321. 10. Carlsson, D. J. and Wiles, D. M., J. Macromol. Sci., Rev. Macromol. Chem., 14 (1976) 155. 11. Garton, A., Carlsson, D. J. and Wiles, D. M., Macromolecules, 12 (1979) 1071. 12. Garton, A., Carlsson, D. J. and Wries, D. M., Makromol. Chem., 181 (1980) 1841. 13. Niki, E., Decker, C. and Mayo, F. R., J. Polym. Sci., Chem. Ed., 11 (1973) 2813. 14. Decker, C. and Mayo, F. R., J. Polym. Sci., Chem. Ed., 11 (1973) 2847. 15. Decker, C., Mayo, F. R. and Richardson, H., J. Polym. Sci., Chem. Ed., 11 (1973) 2879. 16. Chien, J. C. W., Vandenberg, E. J. and Jabloner, H., J. Polym. Sci., Part A1, 6 (1968) 381. 17. Carlsson, D. J. and Wiles, D. M., Proc. 28th Macromolecular IUPAC Symposium, Amherst, July 1982, p. 325. 18. Geuskens, G. and Kabamba, M. S., Polym. Deg. and Stability, 4 (1982) 69. 19. Cheng, M. N., Schilling, F. C. and Bovey, F. A., Macromolecules, 9 (1976) 363.
Hydroperoxidation in photooxidation of polyolefins and polyamides
265
20. Amin, M. U., Scott, G. and Tillikeriane, L. M. K., Eur. Polym. J., 11 (1975) 85. 21. Ginhac, J.-M., Gardette, J.-L., Arnaud, R. and Lemaire, J., Makromol. Chem., 182 (1981) 1017. 22. Allara, D. L. and Edelson, E., Rubber Chem. Technol., 45 (1972) 437. 23. Van Sickle, D. E., Mayo, F. R., Arluck, R. M. and Syz, M. G., J. Am. Chem. Soc., 89 (1967) 967. 24. Lock, M. V. and Sagar, B. F., J. Chem. Soc. B, (1966) 690. 25. Sagar, B. F., J. Chem. Soc. B, (1967) 1047. 26. Sharkey, W. H. and Mochel, W. E., J. Am. Chem. Soc., 81 (1959). 3000. 27. Tang, L., Sallet, D. and Lemaire, J., Macromolecules, 15 (1982) 1432. 28. Petruj, J. and Marchal, J., J. Radiat. Phys. Chem., 16 (1980) 27. 29. Tang, L., Sallet, D. and Lemaire, J., Makromol. Chem., 182 (1981) 3467. 30. Tang, L., Sallet, D. and Lemaire, J., Macromolecules, 15 (1982) 1437. 31. Schollenberger, C. S. and Stewart, F. D., Advances in urethane science and technology, Frish, K. C. and Reegen, S. L. (eds), Technonic Publishing Co. Inc., Westport, Vol. 2, 1973, p. 71; Vol. 4, 1976, p. 68. 32. Beachell, H. C. and Chang, I. L., J. Polym. Sci., Part A1, (1972) 503. 33. Schultze, H., Makromol. Chem., 172 (1973) 57. 34. Gardette, J.-L. and Lemaire, J., Makromol. Chem., 182 (1981) 2723. 35. Mair, R. D. and Graupner, A. J., Anal. Chem., 36 (1964) 194.
Primary Hydroperoxidation in Photooxidation of Polyolefins and Polyamides
Jacques Lemaire and Ren6 Arnaud Laboratoire de Photochimie Mol6culaire et Macromol6culaire, ERA CNRS 929, Universit6 de Clermont II, Ensemble Universitaire des C6zeaux, BP 45, 63170 Aubiere, France
ABSTRACT Special attention is focussed on the various hydroperoxides which appear as primary products in the photooxidation of polyolefins and polyamides. In polyethylene, hydroperoxidation in ot position to the vinylidene group affords monomeric hydroperoxides (absorbing at 3550 cm-1), thermostable up to 85°C and easily photolyzed. Hydroperoxidation on the saturated chain affords hydrogen-bonded hydroperoxides which are fairly unstable at 85°C under vacuum. The polyethylene hydroperoxides are not able to induce significantly new oxidation processes. Isolated and associated hydroperoxides also exhibit very different behaviour in an ethylene-propylene-norborneneethylidene terpolymer. In polyamides (PA 11, PA 12, P A 6), hydroperoxidation ot to the nitrogen atom is predominant. Hydroperoxides are revealed only by chemical titration at room temperature. Thermally unstable above 60°C, photounstable at )t < 3 0 0 nm, they decompose into imides and hydroxylated groups ct to the nitrogen. Up to now, the photoinductive ability of the hydroperoxides cannot be completely excluded.
INTRODUCTION H y d r o p e r o x i d i c groups which a p p e a r in t he oxi dat i on of p o l y m e r s are key c o m p o u n d s for u n d e r s t a n d i n g t he p r i m a r y m e c h a n i s m of oxidation as well as for d e v e l o p i n g a b e t t e r insight into t he funct i on of 243
Polymer Photochemistry 0144-2880/84/$3.00 (~ Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Northern Ireland
244
Jacques Lemaire and Ren~ Arnaud
photostabiliser systems. Studies have been devoted to hydroperoxidation in most materials. However, many basic questions can be still considered: (1) Are there any specific sites for hydroperoxidation on a nonabsorbing polymer? After the initial formation of radicals (mostly unknown), hydrogen abstraction is generally observed in polyolefins, polyamides and polyurethanes. This reaction is essentially controlled by the lability of the corresponding bond. As shown later, many specific hydroperoxidation sites have been identified, due to large differences in bond dissociation energies. (2) What is the thermal stability of hydroperoxides in the matrix in the time-scale of the experiment? It is generally noted that the thermal stability of any oxidation products formed on a polymer chain is dependent on the matrix. Thermal stability cannot be considered as an intrinsic property of the products but as a property of the products in interaction with their close neighbourhood. Large differences in the thermal stability of tertiary hydroperoxides are observed in various matrices, for example. (3) What are the photochemical properties of hydroperoxides, i.e. absorption and photolysis quantum yield ? Hydroperoxides present an no-* transition band, localized on the O - O bond, almost completely forbidden but which extends to very long wavelengths (up to 360 nm, for example); the molar extinction coefficients are generally low, sometimes below 1 M-1 c m -1. The quantum yield of photolysis is generally very high, but most measurements have been carried out in solution and not in the solid state. 1-3 (4) What is the reactivity of hydroperoxides witl~ radicals? This question is especially important in photocatalyzed oxidation. The reactive species formed on a photoactive pigment are able to induce the decomposition of hydroperoxide. The reactivity of hydroperoxide with radicals has often been recognized, due to lability of the O O - H bond. Such a process accounts for the chain mechanism observed in the photolysis of R - O O H . 1 (5) What is the photoinductive ability of hydroperoxides in the polymer matrix? The hydroperoxides formed are potential
Hydroperoxidation in photooxidation of polyolefins and polyamides
245
sources for radicals, especially reactive radicals such as O H . . H o w e v e r , the radical pairs f o r m e d in thermal or p h o t o t h e r m a l dissociation can react together in the solid state (in 'cage' reaction) or can migrate apart. The ability to initiate a new oxidation chain depends on this competition as far as the hydroperoxide can dissociate. If the first question is related to the structure of hydroperoxides, answers to the four o t h e r questions are n e e d e d to account for the behaviour of these intermediates in the overall mechanism of the oxidation. U p to now, these basic questions have not been satisfactorily resolved although much useful information has been found by m a n y research groups. Such a situation is not really surprising since hydroperoxides usually appear at a low stationary concentration (apart from a few exceptions) and, before the F T I R development, analytical techniques were limited to chemical titrations in solutions and conventional IR spectroscopy. Chemical titrations present high sensitivity (even after dissolution) but are unspecific. IR spectrometry was poorly sensitive but m o r e informative on the structure of hydroperoxides and on their interactions (e.g. hydrogen bonding). In recent years, our group has developed the use of photoactive pigments as tools for the understanding of the chemical evolution of 'non-absorbing' polymers (c.f. Ref. 4, for example). The fundamental advantages of TiO2, Z n O and CdS pigments are: (1) T h e y afford an efficient control of the absorption of the light and of the initiation rate when added to a polymer in which the absorbing species are not identified. (2) T h e y act as inner screens to protect the photoproducts and prevent their disappearance in further photochemical steps (except when the products are photocatalytically oxidized). (3) In the presence of pigment, oxidative p h e n o m e n a are limited to the surface of sample. Oxygen diffusion is never a controlling factor. The use of pigments is especially interesting for the study of hydroperoxides since they ensure controlled formation and sometimes controlled disparition of these intermediates. Moreover, it is often observed that in the presence of pigment hydroperoxides
246
Jacques Lemaire and Ren~ Arnaud
accumulate until a stationary concentration is reached which is much higher than in a transparent matrix. Therefore for several years we have been comparing the formation and disappearance of hydroperoxides in thermal, photothermal and photocatalytic oxidations. Each initiation mechanism has its own advantages and disadvantages and comparison is often fruitful. In the present paper, we report the main observed properties of hydroperoxides in polyolefins and polyamides.
PRIMARY HYDROPEROXIDATION
IN P O L Y O L E F I N S
Atactic polypropylene would be the most simple, but academic, sys-
tem as far as hydroperoxidation alone is concerned. In this completely amorphous matrix, hydroperoxides accumulate at a very high concentration ( - 0 . 1 4 mol litre-1) 4"5 in photothermal oxidation (60°C,)t > 300 nm). Only hydrogen-bonded hydroperoxides were observed in the IR spectra (gma~--~3400 cm -a) together with alcoholic groups, and their contribution to the total absorption was kept fairly small. It is important to stress that no m o n o m e r form of hydroperoxides absorbing at 3550 cm -1 could be observed. In the presence of Z n O (of various types), the initial rate of hydroperoxidation was far higher than in unpigmented samples; this observation was easily explained by the increase in the rate of light absorption in the pigmented sample. However, as shown in Table 1, the mean photostationary concentration in pigmented samples was only 60% higher than in unpigmented samples. The rate of disappearance of hydroperoxides was therefore largely increased by the photoactive pigment. The decomposition of R O O H induced by the reactive species formed on the pigment leads to higher concentrations of peroxy radicals and therefore favours rupture processes. Scheme 1 summarizes the processes implied in the ZnO-photocatalyzed oxidation of atactic polypropylene. The photocatalyzed decomposition of hydroperoxide groups has been controlled on ten-butyl hydroperoxide in solution in n - h e p t a n e containing up to 14% by weight of photoactive pigment (TiO2, ZnO). Total absorption of the light in the pigment was checked by using different pigment concentrations. Direct excitation of the hydroperoxide is therefore prevented and the dissociation of the c o m p o u n d is only induced by the radical species formed on the pigment.
Hydroperoxidation in photooxidation of polyolefins and polyamides
247
TABLE 1
Variations in Hydroperoxide Concentration with Irradiation Time in Photothermal or ZnO-Photocatalyzed Oxidation of Atactic Polypropylene Irridiation time ( h ) 50
93
160
202
273
326
350
Hydroperoxide concentration (mol kg 1) in atactic PP
0.010 0.010 0.052 0-075 0.096 0.120 0.140 +0.010 +0.010 ±0-010 +0.010 ±0.010 :e0.012 ±0.010
Hydroperoxide concentration (mol kg-l) in atactic PP +3% ZnO A
0.037 0.063 0-100 0.147 +0.010 +0.010 ±0.010 +0-014
--
0.193 0.220 ±0-019 +0-022
Isotactic p o l y p r o p y l e n e appears to be m o r e complex as a semicrystalline matrix. H y d r o p e r o x i d e s also accumulate to give high concentrations (for example, 0.1 mol litre -1 in photochemical oxidation at )t > 300 nm). Only h y d r o g e n - b o n d e d hydroperoxides are o b s e r v e d (at 3400 cm -1) but again the large contribution of alcoholic and acidic groups overlaps this absorption even in the early stages of the oxidation. N o m o n o m e r form has ever been observed in I R spectra (at 3550 cm-~); as o p p o s e d to polyethylene, no local defect plays an important role in the primary hydroperoxidation (see later). P r o p e r ties of the tertiary h y d r o p e r o x i d e forms are known following the basic work of Wiles et al. and others 5-~2 and of M a y o et al. 13-is These hydroperoxides generally appear in close proximity and sequences of groups have been o b s e r v e d (through reduction and dehydration of the corresponding tertiary alcohols16). A c o n s e q u e n c e of such an association has b e e n recently observed in p o l y p r o p y l e n e stabilized with H A L S . The effectiveness of H A L S has been at least partially explained by interactions of H A L S with several hydroperoxidic groups. 17 The thermal and photochemical decompositions of hydroperoxides lead, after the homolytic cleavage of the O - O bond, to alcohols and various ketones (Scheme 2) (although the /3-scission of alkoxy radicals at r o o m t e m p e r a t u r e has recently b e e n questioned by G e u s k e n s and K a b a m b a ) . TM
248
Jacques Lemaire and Ren~ Arnaud
Pigment
~
hv, 02, H20
(R') Reactive species Radicals PH (atacticpolypropylene)
~
---C~CH2-I
CH3
OO" I --C--CH2-- ,~
' CH3 Propagation~
Ru~ Peroxides~
I(R') ~Pigment
OOHI --C---CHE-hv (pigment-inhibited),'"[ ,/ CH3
OI C--CH2 I CH~
P i
'ii
O
i i
L .+Z J " ~
+ CH2--
(R'
X
by'. inhibited)-,.
(pigment
-~" ,
+HO"
," t~--CH2 ' I : CH3
OH I
--C--CH2-I CH3
Lactones Acids (Norrish type I) Unsaturations (Norrish type II)
Sehane 1
Hydroperoxidation in photooxidation of polyolefins and polyamides
249
CH3
....
I C--CH2 .... I OOH
1 ....
CH3 I C--CH2 .... I O-
+ OH-
CH3
I
....
C---CH2,
I OH
(1726 cm-1)
(1718 cm 1)
Sdaeme 2
Introduction of a photoactive pigment into such a semi-crystalline p o l y m e r is not so demonstrative as in atactic polypropylene. T h e Z n O - or TiO2-photocatalyzed oxidation of isotactic polypropylene appears as a superposition of the pigment-catalyzed oxidation of a m o r p h o u s zones (similar to atactic polypropylene photocatalyzed oxidation) and of the p h o t o t h e r m a l oxidation of u n p i g m e n t e d zones. H y d r o p e r o x i d e s and ketones are not only reacting with the species f o r m e d onto the pigment but direct excitation is also occurring and final photoproducts such as acids and esters are f o r m e d as opposed to the atactic case. A n amorphous copolymer PP/PE has recently been examined by Geuskens and Kabamba. 18 H y d r o g e n - b o n d e d h y d r o p e r o x i d e was observed at 3400 cm -1 and chemically titrated. A n original photochemical property of the f o r m e d hydroperoxides (essentially secondary hydroperoxides) was proposed. Excitation of the hydroperoxide c h r o m o p h o r e in interaction with ketones was proposed to afford directly an acid group H .... O I II R___oJO pjC~p
hv
H O I II ~ O--C~p+
R--O---P
250
Jacques Lemaire and Ren~ Arnaud
According to Geuskens and Kabamba, this explains why vacuum photolysis at 365 n m of a prephotooxidized sample in the presence of b e n z o p h e n o n e (which is consumed in the prior irradiation) led to acid formation through the decomposition of hydroperoxide, although no ketone photolysis proceeded (as demonstrated by the absence of a Norrish type II process). The situation is especially controversial in p o l y e t h y l e n e where only the general lines of the overall oxidation mechanisms can be sketched. The stationary concentrations of hydroperoxides are far lower than in polypropylene and difficulties obviously exist for the determination of the primary hydroperoxidation sites in L D P E as in H D P E . In 1976, Cheng et al. 19 reported an N M R study of the hydroperoxides formed in the thermooxidation at 140°C of two L D P E ' s (17 and 22 branching points per 2000 secondary hydrogen atoms). These authors observed essentially secondary hydroperoxides and rationalized by estimating the ratio of rate constants for abstraction of tertiary and secondary hydrogen atoms as 9 . 8 + 1-0. Tertiary hydroperoxidation can only be a minor route. In 1975, A m i n et al. 2° compared the evolution of hydroperoxides (chemically titrated) with the evolution of vinylidene, vinyl and carbonyl groups in samples of L D P E exposed to U V after different times of mild processing. Two main conclusions were derived: (1) During the process at 165°C, hydroperoxidation occurs in the a position of the vinylidene. (2) T h e hydroperoxides formed in the processing operation are able to photoinduce the oxidation of L D P E . In a preliminary paper, 21 we have shown that the hydroperoxides formed during the thermooxidation at 90°C of L D P E have no photoinductive effects as opposed to the tertiary hydroperoxides formed in polypropylene oxidation. We have recently studied six low-density polyethylenes, as described in Table 2. They have been used in the form of films (thickness 150 ~m). The initial vinylene ( - - C H - - C H - - ) content is low (less than 1 - 2 × 10 -3 M). T h e CH3 branching ratio indicated in Table 2 is the conventional 'methyl index' measured from the IR absorbance at 1378 cm -1 ( A S T M D 2 2 3 8 - 6 4 T ) . The polymer used for film 3 is an L D P E obtained with propylene as the transfer agent. The usual methyl index cannot be measured since the branch points
Hydroperoxidation in photooxidation of polyolefins and polyamides
251
TABLE 2
Characteristics of LDPE Film Density CH3 branching number (d) ratio
Vinylidene
Vinyl
(M)
(M)
(per 1000 C) 1 2 3
0.921 0.922 0-923
23 21 --
1.7 x 10 -2 1.7x10 2 1 . 6 x 10 -2
7 ' 2 x 10 -3 5"0x10 3 2 . 1 × 1 0 -2
4 5 6
0-918 0-916 0"912
30 42.5 --
1 . 9 x 10 -2 3.3x10 2 3 " 4 x 10 2
1 . 7 x 10 -2 1.6xlO 2 8 . 6 x 1 0 -3
c o r r e s p o n d essentially to p e n d a n t methyl groups (and not to C4 branches as assumed in A S T M D 2 2 3 8 - 6 4 T). As shown in Table 2, a typical L D P E presents, per 1000 carbon atoms in the chain, a r o u n d 2 0 - 4 0 tertiary hydrogen atoms (branch points) and a r o u n d 0-5-1 unsaturated groups such as vinylidene, vinyl and vinylene sites. A n y radical f o r m e d in L D P E , under U V exposure for example, would either abstract a hydrogen a t o m or add to an unsaturated group. Considering the hydrogen abstraction on the three different sites (saturated chain, branch point, a position to an unsaturated group), it is expected that abstraction of a secondary hydrogen a t o m from the chain w o u l d prevail on the abstraction of a tertiary atom. Allara and E d e l s o n 22 r e p o r t e d that the ratio of the rate constant of the abstraction of a tertiary hydrogen a t o m to the rate constant of the abstraction of a secondary a t o m was close to 8 in m o d e l h y d r o c a r b o n oxidation. According to Cheng et al., 19 the reactivity ratio of branch points to linear chain is indeed 9- 8 + 1 in L D P E . H y d r o p e r o x i d a t i o n on the 2 0 - 4 0 tertiary carbon atoms would represent only 1 0 - 2 0 % , at most, of the total hydroperoxidation. It has b e e n shown z3 in the oxidation of acyclic alkenes of low molecular weight that peroxy radicals can either abstract an allylic hydrogen or add to the double bond. Except when the allylic hydrogen is of tertiary nature (as in 3-methyl 1-butene), the rate constants of abstraction and addition are of the same o r d e r of magnitude. In L D P E , radical reactions with vinylene, vinyl or vinylidene groups can occur either by an abstraction or by an addition mechanism. Summarizing the data obtained up to n o w on model c o m p o u n d s and in L D P E , it appears impossible to predict w h e t h e r the formation
Jacques Lemaire and RenO Arnaud
252
of hydroperoxidic groups on the saturated chain or the hydroperoxidation of unsaturated groups prevails. Considering the 3 5 5 0 c m -1 band, previously attributed to a m o n o m e r i c f o r m of secondary hydroperoxides f o r m e d on the saturated chain, zl the following r e m a r k s should be noted: (1) No 3550 cm -1 band was observed in the 95°C thermooxidation of H D P E in which only vinyl groups are present initially. (Film 7: d = 0 - 9 4 8 , v i n y l i d e n e = 6 . 5 x l 0 - 3 M , v i n y l = 7 - 6 X l 0 - Z M . Film 8: d = 0 . 9 6 4 , vinylidene=l'2xl0-3M, vinyl=7"6X 10 -2 M.) (2) In thermooxidation of L D P E at 85°C, the m a x i m u m intensity of the 3550 cm -1 band was not related to the initial content of the branch point. In film 3 containing m a n y tertiary carbon atoms the m a x i m u m intensity of 3550 cm -~ was about the same as in L D P E with a low branching ratio (samples 1, 2,, 4). In contrast, the m a x i m u m intensity of the 3550 cm -~ band was clearly related to the initial vinylidene content. In Fig. 1, it appears that the concentration of vinylidene-type
,[FreeROOH],~C:CH2]or r-HC:CH2-1x 102(M) O
,~,,~ a. v\
2-
/"
0
o o
o
\~o~ / +/ ./_._~_._. . "'~..~ o-~ "-7---,-.---+-.t..+_ .... - - ~ . - - . % . "-+-T~-.. '" " C" % \o
0
0
160
260
a60
""
IRRADIATION TIME ( h r s l 460
Fig. 1. Variations of isolated hydroperoxides and vinylidene concentrations (10 -2 mol litre -~) during the thermooxidation of L D P E at 85°C. O, Sample 5; +, sample 1.
Hydroperoxidation in photooxidation of polyolefins and polyamides
[FreeROOH-]max x10 2(M) "
sample 3
sample 5 --~ O~, - ' /~~'-sample
253
6
/ ~ ' ~ / / O ' ~ sample 4
/vii_" sample I
,i
7~
"" sample 7
O./".~ sample 8 0
r~r-_-~l-i "1 x 102 (M) L.a_-L,n2Jo
I
I
I
I
1
2
3
4
)
Fig. 2. Variations of maximum concentration (10 2 mol litre-1) of hydroperoxide monomer as a function of initial concentration (10 2 tool iitre 1) of vinylidene in the thermooxidation of LDPE and HDPE (for definition of samples 1-8 see Table 2 and subsequent text). unsaturations decreases as soon as t h e r m o o x i d a t i o n begins. Simultaneously, the m o n o m e r hydroperoxide concentration, m e a s u r e d at 3550 cm 1 (e = 80 M l cm-1) increases and reaches a limiting value well before the unsaturation has completely disappeared. T h e r e f o r e h y d r o p e r o x i d a t i o n precedes the disappearance of vinylidene. However, the w a v e n u m b e r of the I R absorption of vinylidene groups was not modified by peroxidation. On the other hand, in Fig. 2 the m a x i m u m concentration of isolated hydroperoxides [ROOH]ma~ has been plotted as a function of the initial vinylidene [ ~ C = C H 2 ] 0 content in various L D P E ' s and for the H D P E ' s 7 and 8. It can be pointed out that [ROOH]max is proportional to [~C~-CH2]0. T h e slope of the experimental straight line is close to 1. In each type of polyethylene, [ROOH]max is almost equal to [ ~ C = C H 2 ] 0 . As has been pointed out earlier, hydroperoxidation of vinylidene groups can imply an abstraction or an addition mechanism and the structure of hydroperoxides f o r m e d would d e p e n d on the reaction pathway
254
Jacques Lemaire and Rend Arnaud
Abstraction mechanism --C--CH2--
II CH~
+ r-
, --C--CH-II
( ,--C=CH-t
CH2
C.H2
02, PH~ O O H
1o:, PH
I --C C-II I CH2 H
--C-----CH I CH2OOH II
I
Addition mechanism
- - C - - C H 2 - - + r. II CH2
--C--CH2-I CH2r
~o2
OOH I --C--CH2-~H2r Ill
Formation of primary hydroperoxides (type II), which are probably very unstable, implies simultaneously the transposition of vinylidene into vinylene groups. Increase of absorption at 810-830 cm -1 of such a group was not observed. Formation of tertiary hydroperoxide (type I!I) implies a parallel disappearance of the double bond and hydroperoxidation, which contradicts the observed facts. Formation of secondary hydroperoxides without any transposition of the double bond and without any shift of the IR absorption band at 888 cm -1 accounts for the experimental results
--C--CH2--
It
CH2
OOH l a, o½ - - C - - C H II CH2 (absorbs at 888 and 3550 cm-1)
Saturation of the oxidized vinylidene sites is provoked afterwards by radical attack. These radicals are formed either primarily or through the decomposition of any hydroperoxides (and not necessarily those
Hydroperoxidation in photooxidation of polyolefins and polyamides
255
which are f o r m e d in the a position of the double bond) OOH I --C C - - + 2r- ~ II I CH2 H r
r I --C I rCHe
OOH I C I H
OOH
I I rCH2 H
,
0>85oc
.
.
.
.
In the early stages of thermooxidation of L D P E at 85°C, the carbonyl c o m p o u n d s (ketonic, then acid) were readily f o r m e d at a concentration which was soon higher than the initial concentration of vinylidene groups. Meanwhile, no significant decomposition of the hydroperoxides f o r m e d on the vinylidene sites was observed. The same observation can be m a d e in H D P E , in which no significant 3550 cm 1 absorption was detected when the concentration of carbonyl c o m p o u n d s largely e x c e e d e d the initial concentration of unsaturations. Obviously, hydroperoxidation of the saturated chain proc e e d e d simultaneously leading only to h y d r o g e n - b o n d e d h y d r o p e r o x ides (observed at 3400 cm -1 in I R spectrophotometry). As suggested previously, 21 these hydroperoxides d e c o m p o s e rapidly at 85°C, directly into ketones OOH I --C-I H
O" I ' - - C - - + OH" I H
O II ~ --C-- + HzO
In photooxidation of L D P E , the stationary concentration of hydroperoxide absorbing at 3550 cm -1 was kept low unless a photoactive pigment afforded photochemical protection. 4 The vinylidene site disappears rapidly on radical attacks. Many sources of radicals could be considered (primary radicals, R O O H on saturated chain or R O O H a to the vinylidene). The carbonyl c o m p o u n d s appear in concentrations much higher than the initial unsaturations. In Fig. 3 the variations of the absorbance at 1 7 1 3 c m -1 were plotted as a function of the variations of the absorbance at 888 cm I for L D P E with high initial vinylidene content (samples 5 and 6) at different temperatures. In the early stages of the oxidation, the concentration of carbonyl comp o u n d s (essentially ketonic groups with ~1735--~250M - 1 c m -1) largely
256
Jacques Lemaire and RenE Arnaud
E
E
O f~
U
K <3
o,s samplle~ e
sample 6
0.5
~t
~
0
I
i
i
i
1
2
3
4
2
0
1
2
Fig. 3. Variations in absorbance AD at 1713 cm-1 versus variations in absorbance at 888 cm-1 in polychromatic photooxidation (SAIREM SEPAP 12.24; ~ > 300 nm: sample temperature 60°C) of LDPE films (thickness 150/xm). e x c e e d e d the concentration of the d e s t r o y e d vinylidene groups (e388 = 160 M-1 cm-1). This was also valid for L D P E with low initial vinylidene content (samples 1 and 4). Vinylidene groups are not, therefore, the only precursors of carbonyl compounds, since no chain reaction has b e e n seen in L D P E . T h e r m o o x i d a t i o n and photooxidation of L D P E t h e r e f o r e p r o c e e d through hydroperoxidation in a position to the vinylidene group and on the saturated chain affording m o n o m e r i c hydroperoxides absorbing at 3550 cm -~ and t h e r m o s t a b l e at 85°C, and h y d r o g e n - b o n d e d hydroperoxides fairly unstable at 85°C, respectively. B o t h hydroperoxides f o r m e d present a s e c o n d a r y structure and, through a cage direct decomposition into ketones and water, it is expected that these hydroperoxides have a p o o r photoinductive ability as suggested previously. 21 T h e most striking result o b t a i n e d in polyethylene oxidation is the thermal instability of h y d r o g e n - b o n d e d hydroperoxides c o m p a r e d to the relative stability of free hydroperoxides. This could be explained by high local concentrations of b o n d e d hydroperoxides, favouring chain reactions in the decomposition similar to those o b s e r v e d in highly concentrated solutions.1 This u n e x p e c t e d observation leads us
Hydroperoxidation in photooxidation of polyolefins and polyamides
257
to study a polyolefin containing a high and controlled a m o u n t of vinylidene-like sites. 300-/xm films of different e t h y l e n e - p r o p y l e n e norbornene-ethylidene terpolymers have been submitted to photothermal oxidation. Purposely, these terpolymers have not been chemically crosslinked; the concentrations of ethylidene sites are therefore equal to the initial concentration of the diene. On exposure under different sources (SEPAP 12.24 unit, h > 3 0 0 n m ; Westinghouse 125W, 365 nm; monochromatic light at 365 nm), a fast photooxidation proceeds; free and hydrogen-bonded hydroperoxides are directly observed at 3550 and 3380 cm l, respectively (as opposed to a polyolefin with a low initial content of vinylidene sites). Free hydroperoxides reach photostationary concentrations (around 0-015 m o l k g -1 at 50°C, around 0 . 0 3 m o l k g -1 at 20°C) as hydrogen-bonded hydroperoxides accumulate at higher concentrations (0.04 versus 0.015 mol kg-1). These hydroperoxide concentrations are directly related to the initial content of ethylidene sites. H O 0 ~
In the terpolymer samples, the evolution of the ethylidene sites observed at 8 0 7 c m -1 presents two different phases. Initially the initial concentrations of ethylidene remain practically constant (around 0 - 2 7 m o l kg -1) while the build-up of free and hydrogenb o n d e d hydroperoxides is important (up to 60% of the stationary concentrations of free hydroperoxides). Then, as hydrogen-bonded hydroperoxides develop further, the disappearance of ethylidene sites appears to be fast. In a second series of experiments the same samples were submitted to prephotooxidation up to 34% disappearance of ethylidene sites, and then to vacuum thermolysis at 70°C. In the first stages of vacuum thermolysis hydrogen-bonded hydroperoxides and the ethylidene double b o n d disappear simultaneously as the free hydroperoxides remain invariant. After the complete disappearance of associated hydroperoxides, residual ethylidene sites are still observed. In the last stages of vacuum thermolysis, free hydroperoxides vanish as the ethylidene sites remain invariant.
258
Jacques Lemaire and Ren~ Arnaud
From these observations it is deduced that the associated hydroperoxides are less thermally stable than isolated hydroperoxides. The build-up of hydroperoxides precedes the disappearance of ethylidene sites and this disappearance is related to the decomposition of hydrogen-bonded polymers.
PRIMARY HYDROPEROXIDATION ALIPHATIC POLYAMIDES
IN
The behaviour of aliphatic polyamides, especially nylon-type polymers, on exposure to U V irradiation has attracted much attention. However, most of the research in this field concerns either vacuum photolysis of the polymer or photooxidation at wavelengths shorter than 290 nm. Photooxidation of model compounds such as N-alkyl amides RCH2 CO NI-I CH2R' have also been considered. 23-26 Since 1978 we have been studying the photooxidation of polyamides in the solid state (films of 4 0 / x m thickness), either at wavelengths longer than 300 nm or at 254 nm. The first polymers studied were polyundecanamides (PA 11). In the polymer matrix it appeared possible to characterize four intermediate products (hydroperoxides, imides, hydroxyl groups a to the nitrogen atom and aldehydes) and two final groups (amines and acids). The main conclusions on the mechanism of photooxidation at long and short wavelengths reported recently27 are summarized in Scheme 3. In the scope of the present paper, the following properties of the primary hydroperoxides can be noted: (1) The hydroperoxide groups are revealed only by chemical titration in a special solvent. Since, as shown later, these hydroperoxides are fairly unstable at temperatures higher than 60°C, this titration must be carried out at room temperature. Hexafluoro-2-propanol appears to be the only solvent suitable for polyundecanamide at room temperature. The titration is based on the oxidation of Fe 2÷ and complexation of F e 3+ by SCN- anions. 19"28Iodometric methods are unsuitable since they are carried out in a boiling CH3COOH-2-propanol mixture. (2) The hydroperoxides formed in the sample exposed to long wavelength photooxidation at 60°C in a SEPAP 12-24 set-up (this set-up has been described in many papers, for example
P" +
I A (T > 90°C)
--CH2COOH
l ?J2 hv A'
II O
--CH2CH + NH2COCH2--
I OH
--CHzCHNHCOCH2--
O-
OH- +--CHECHNHCOCH2-I
hv (k = 254 nm and A> 3 0 0 y
A (T around 6 0 ° C ) /
J (T around 60"C) "~k = 254nm and ,~> 300 nm)
+p.
I -c~
hv
(PH)
Sehele 3
--CH2CO"
•NFICOCH2-- ,c
[--c~coN--~ I + --CH~COO~
A
I A (T > 90°C)
--CH2NH- +.COCH2--
hv (,k = 254 nm and X>300 nm)
I--c-~cooH 1
hv, 02
I + i.i~i~i~:.i + [ --c~cHo 1
"~
--CH2CONHCOCH2-- + H20 ~-]
~ A
--CH2CHNHNOCH2-I OOH
OO"
--CH2CHNHCOCHz--
--CH2C. HNHCOCHE--(P-)
--CH2CH2NHCOCH2--(PH)
tO t.~
g~
t~
260
Jacques Lemaire and Ren~ Arnaud
Refs 2 8 - 3 0 ) accumulated until a stationary concentration close to 1-5 × 10 -2 mol kg -1 was achieved. In contrast, on exposure at short wavelengths ( 2 5 4 n m ) the stationary concentration of hydroperoxides is far lower, a r o u n d 0 . 3 x 1 0 - 2 m o l kg -1. T h e photolysis of R O O H at 254 nm was directly o b s e r v e d after pre-irradiation at long wavelengths through successive titrations. (3) H y d r o p e r o x i d e s were found to b e fairly stable in the matrix at 60°C, in the dark. Chemical titrations in pre-irradiated samples ((ROOH)0-~ 1-5 x 10 -2 mol kg -1) maintained at 60°C s h o w e d that the lifetime of R O O H was a b o u t 3 0 h . A t l l 0 ° C , the lifetime was f o u n d to be a b o u t 5 min. T h e thermal decomposition of h y d r o p e r o x i d e led to imides (revealed in the I R spectra at 1735 and 1690 cm -1) and to hydroxylated groups ct to the nitrogen a t o m (to a lesser extent). (4) T h e p h o t o t h e r m a l decomposition of h y d r o p e r o x i d e at 254 nm led directly to acid groups without any accumulation of intermediates (such as imides). (5) T h e hydroperoxides were readily r e d u c e d in the matrix by reducers such as phosphites. In the presence of 3% by weight of photoactive pigment (such as o r T i O 2 R E 90,* rutile forms which have received a surface t r e a t m e n t with alumina and silica, or type C - Z n O t ) P A samples undergo, u p o n U V exposure, a p h o t o c a t a l y z e d oxidation. 31 T h e h y d r o p e r o x i d e groups, f o r m e d in the sample photocatalytically oxidized at 60°C in a S E P A P 12.24 set up, accumulated to 2 . 4 × 10 -2 mol kg -~. Although this mean concentration was not corrected for light absorption profile (in the presence of 3% T i O 2 m o r e than 9 0 % of the light, at A < 380 nm, is a b s o r b e d in the first 10 txm of the 40 p.m film), it clearly a p p e a r e d that the photostationary concentration was higher in the presence of pigment than in its absence. At r o o m t e m p e r a t u r e (25°C), on the same U V exposure, the p h o t o stationary concentration of hydroperoxides was f o u n d to b e the same (2.35 x 1 0 - 2 mol kg-~). Although the hydroperoxides were fairly unstable at 60°C, it a p p e a r e d that the stationary concentrations of hydroperoxides were essentially controlled by photocatalysis at 60°C. TiO2 R L 6 5
RL 65 and RL 90 from Produits Chimiques, Thann et Mulhouse, France. t Type C-ZnO from Soci6t6 des Mines et Fonderies de Zinc de la Vieille Montagne, Belgium. * TiO2
Hydroperoxidation in photooxidation of polyolefins and polyamides
261
In photocatalytic oxidation, the primary formation of reactive species is wavelength-independent, as long as the energy of the radiation is higher than the width of the forbidden band of the photoactive pigment (3.5 eV in TiO2 to 3-2 eV in ZnO). Excitation in the range 300-400 nm or at 254 nm must therefore induce the same phenomena. Titration of hydroperoxide has been carried out in a sample containing 3% TiO2 RL65 and exposed to 254 nm irradiation. The mean photostationary concentration of hydroperoxides appeared to be somewhat lower at 254 nm than under long wavelength irradiation (1-2 x l0 -2 versus 2"5 x 1 0 - 2 mol kg -~, respectively). The photostationary concentration of hydroperoxide groups in an unpigmented polyundecanamide sample photooxidized under the same experimental conditions has been shown to be about 0 . 2 x 1 0 - 2 mol kg -~. Two conclusions can be drawn: (1) The inner filter effect of the pigment at 2 5 4 n m is clearly observed, the photolysis of R O O H being inhibited by the absorbing pigment; (2) A residual wavelength effect appeared since long wavelength and 254 nm irradiation did not correspond to the same mean stationary concentration of ROOH. Differences in profile light absorption at 254 nm and the polychromatic radiation may account for the difference. Moreover, as isotactic polypropylene P A 11 is a semi-crystalline polymer, the wavelengthdependent photochemistry (and especially the photolysis of ROOH) occurring in the unpigmented zones is expected to be superposed on the photocatalytic oxidation of amorphous and pigmented zones. In the pigmented zones the formation and decomposition of hydroperoxides are induced by the reactive species appearing on the excited pigment, which form stationary concentrations of hydroperoxide. At short wavelengths the direct photolysis of the primary hydroperoxides proceeds faster than at long wavelengths, the direct photolysis in the unpigmented zones is an additional route of disappearance of the hydroperoxides and lower stationary concentrations are observed. The photothermal oxidation at long wavelengths and at 254 nm of PA 12 (polydodecanamide) and P A 6 (polyhexanamide) is presently being studied in our group and is being compared with the photooxidation of P A 11 under the same experimental conditions. Very similar intermediates (hydroperoxides, imides, hydroxylated groups o~
262
Jacques Lemaire and Rend Arnaud
to the nitrogen atom, aldehydes) have been observed throughout both photothermal oxidations. Therefore the same oxidation mechanism implying a primary direct photoscission at short wavelengths ()t < 330 nm) and a photoinduced oxidation at wavelengths longer than 2 5 4 n m is valid for every aliphatic polyamide studied. P A 11, P A 12 and P A 6 differ only on quantitative grounds. For example, the initial rates of the direct photoscission (measured as the rates of appearance of aldehyde groups) are in the ratio 1 : 3 : 4-5 respectively for P A 12, P A 11 and P A 6. Dealing with hydroperoxides, chemical titrations in hexafluoroisopropanol supplied the results represented in Fig. 4 on long wavelength exposure ()t > 300 nm). T h e photostationary concentrations of hydroperoxides are, respectively, 45, 20 and 1 2 m m o l k g -1 for P A 12, P A 11 and P A 6. At 254 n m the photostationary concentrations are much lower (7, 2-5 and 3.8, respectively, for P A 12, P A 11 and P A 6).
[ROOH] (mmol.kg -1 } 60-
40-
90-
TIME 0 0
I
I
I
I
50
100
150
900
{hrs~
Fig. 4. Variations in hydroperoxide concentration throughout photooxidation (at )t > 300 nm) of aliphatic polyamides carried out in: O, PA 12; A, PA 11 (new data); and x, PA 6.
Through chemical titrations hydroperoxides have been shown to decompose above 70°C. First-order kinetics are obeyed, the lifetime of P A 12, P A 11 and P A 6 hydroperoxides being, respectively, 5, 6 and 1 h. Photolysis of hydroperoxides proceeded rapidly at 254 nm, or more slowly at )t > 300 nm. The decomposition stoichiometry of hydroperoxides is to be examined. Competition between processes
Hydroperoxidation in photooxidation of polyolefins and polyamides
263
(a) and (b) has to be determined H20 + --CO---NH--CO-
OOH I -C--NH---CO-I H
o. I ~ - - C - - N H - - C O - - - + OH. I H H
--C--NH--CO I OH Some difficulties arise related to the instability of the decomposition products in the matrix. The imide groups are photochemically unstable, especially at 254 nm, or are hydrolyzed; the hydroxylated groups are thermally (above 90°C) and photochemically unstable. The decomposition of the hydroperoxide groups provokes an increase in the imide concentration only after thermal treatment at 90°C. At lower temperatures imides hydrolyze faster than hydroperoxides decompose, the activation energy of the homolytic cleavage of hydroperoxidic bonds being higher than the activation energy for hydrolysis. Even at 90°C the importance of imide formation has still not been quantitatively assessed. (IR detection of imide groups is, however, easier than IR detection of hydroxylated groups.)
CONCLUSIONS Although the various questions appearing in the introduction have not been completely answered in the polymers under investigation, special attention to the behaviour of hydroperoxides has given rise to some useful information. Most polymer materials exhibit specific hydroperoxidation sites. Although the corresponding hydroperoxides formed present qualitatively similar properties, large differences on quantitative grounds are observed. Mostly attributed to the molecular structure of the various hydroperoxidized groups, these differences must be accounted for by matrix effects. Studies on model compounds would therefore hardly afford any relevant information on the properties of hydroperoxides in the matrix. Improvements in analytical techniques are certainly positive factors towards the understanding of the role of hydroperoxides (and maybe for collecting better information on their molecular
264
Jacques Lemaire and Ren~ Arnaud
structure and their interactions with their neighbourhood). Such progress would be even more fruitful in systematic comparisons of various oxidative conditions. As shown in the previous sections, it is interesting to compare the formation and the evolution of hydroperoxide groups in low-temperature thermooxidation, in short- and longwavelength photooxidation and in photocatalyzed oxidation.
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