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ESCA studies of the photo-oxidation and the γ-radiation-induced oxidation of low density polyethylene
Polymer Degradation and Stability 12 (1985) 249-259
ESCA Studies of the Photo-oxidation and the y-Radiation-Induced Oxidation of Low Density Polyethylene
H. S. M u n r o Department of Chemistry, University of Durham, South Road, Durham, Great Britain (Received: 3 October, 1984)
ABSTRACT ESCA has been used to monitor the changes in the surface chemistry of low density polyethylene during exposure to uv light (2 > 300 nm) and 7" radiation. In photo-oxidation, oxygen uptake is reflected by C_--O, ~C_-m--O and O--C_--O functional group jbrmation and the rate of reaction is shown to decrease gradually in the top 150,4 of the exposed surface. By the use of sandwich layers, the extent of oxygen uptake in the front surface is found to be double that at a depth of 20 #m. v-irradiation shows similar trends to photo-oxidation although the extent of oxidative functionalisation is less.
I N T R O D U C T I ON The oxidative degradation of polyethylene (low and high density) has been the subject of numerous investigations which directly reflect the importance of this material in everyday life. The photo-oxidation 1- l o and, to a lesser extent, the v-radiation-induced oxidation, lx-15 have received attention in the literature with respect to the attainment of an understanding of the degradative pathways and subsequent stabilisation for the enhancement of in-service lifetime. It is generally acknowledged that the photo-oxidative process is initiated in the surface regions but, as 249 Polymer Degradation and Stability 0141-3910/85/$03-30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain
250
H. S. Munro
yet, a comprehensive examination of the nature of the changes in surface chemistry has not been reported. Electron Spectroscopy for Chemical Application has been shown to be the single most powerful technique for the interrogation of composition and bonding in polymer surfaces 16 and has been successfully applied to surface photo-oxidation studies of Bisphenol A polycarbonate, 17-~9 Bisphenol A polysulphone, 2° polyether sulphone 2~ and polystyrene. 22- 24 It is for this reason that the studies discussed below have utilised ESCA to follow the changes in surface chemistry during the irradiation of low density polyethylene (LDPE) with uv (2 > 290 nm) and y-radiation.
EXPERI MENTAL Samples of LDPE film (20 nm) were exposed in air (relative humidity ,-,55%) to the output of a black lamp (2> 300nm, Photoflux ,-,0.25 W/m 2) over various periods of time. Similarly, samples were exposed to ionising radiation from a 6°Co source (90 k rads/h). The films were placed in the low vacuum insertion lock of a Kratos ES300 electron spectrometer within 10min of the termination of exposure. ESCA spectra were recorded with MgK~ and TiKu excitation radiation where ,-, 95 ~o of the signal arises from depths of 40/~ and 130/~ for the C ~~core level. 16 Binding energies were referenced to the C - - H component at 285.0 eV. Core level area ratios are quoted to + 5 %. For the component peak analysis in the MgK~ spectra, gaussian peaks were fitted to known carbon oxygen binding energies. 16- 24
RESULTS A N D DISCUSSION UV irradiation Wavelengths lower than 290nm are not normally found in natural sunlight and, in order to obtain a laboratory approximation of the degradation process that occurs in sunlight, uv light below this wavelength should be avoided. We have previously used a black lamp in photo-oxidation studies of Bisphenol A polycarbonate 19 and have found
Photo- and ?-radiation-induced oxidation oJ LPDE
251
that the induced changes in surface and bulk chemistry closely follow those for natural weathering. This lamp has therefore been utilised again for the LDPE studies. Although the low photon flux output results in long exposure times, it is a very cheap source of uv radiation, particularly compared with the filtered Xenon arc lamp which is perhaps more representative of sunlight. Typical MgKot and TiK0~ core level spectra for unexposed and photooxidised LDPE films are shown in Figs l and 2, respectively. In Fig. 1 the unexposed film reveals a single Cls peak centred at 285-0 eV arising from the ~ H 2 ~ H 2 - units in the polymer. A very low level of oxygen is also present on the surface, as shown by the Ols signal, but it is not reflected in the Cls envelope. The corresponding TiK0c spectra have characteristic doublets in the ratio 1:2 separated by ,-,6eV. The respective sampling depths for MgK0c and TiKot spectra are ~40 and 130A. 16 After exposure to uv radiation the Cls spectrum in Fig. 1 has become more complex in that a distinctive shoulder to the higher binding energy side of the main photo-ionisation peak at 285.0 eV is now evident. This arises from the presence of oxidative functionalities such as C - - O , C_=O and O - ~ _ = O and, with the increase in the Oxs signal, is indicative of extensive photo-oxidation in the outermost region of the film. The TiK~
XI
L_ Ohrs / 290
[
I×I \ 285
280
BE.
(e.V)
Fig. 1. C ~sand O~s MgKctcore levelspectra for unexposedand photo-oxidised LD PE.
252
H . S . Munro
133~ h X2
Ohrsh~./~~'
Fig. 2.
XI
BE. IN eV C h and O~, TiK~ core level spectra for unexposed and photo-oxidised LDPE.
spectra in Fig. 2 also reveal changes compared with the unexposed material and indicate that oxidation extends to at least 130,~,. The oxygen uptake for L D P E as a function of time at the two sampling depths is shown in Fig. 3. The data are displayed as carbon to oxygen stoichiometries. These may be obtained from the experimentally determined area ratios and a knowledge of the relevant instrument sensitivity factors. From the MgK~ data, oxygen uptake occurs rapidly over the first ,~ 800 h of exposure and then remains relatively constant at a carbon:oxygen atomic ratio of ~ C 1 : O o.ls. This plateau may well arise from an equilibrium of competitive processes involving further photooxidation and the absorption of low molecular weight species. 17 - 24 The corresponding data for the TiK~ spectra shows that this equilibrium is not reached until ,-, 1500 h of exposure and suggests that the rates of reaction at the two sampling depths are very different. It is also evident from a comparison of the carbon: oxygen stoichiometrics that the extent of photo-oxidation at the TiKct sampling depth is not as great as that for MgK~. However, a straightforward comparison of the stoichiometrics at sampling depths of 40 and 130 A is not completely valid. As alluded to above, the stoichiometrics are calculated from the area ratios and the appropriate instrument sensitivity factors. These sensitivity factors
Photo- and v-radiation-induced oxidation o/LPDE
253
02
/
C~:Ox STOICHIOMETRY
___~i__.-----
~
----~
Mg Anode
0, z//i/y
x
/
/
I
00
1000
Tt Anode
I
I
2000
3000
I
4000
IRRADIATION TIME (HOURS)
Fig. 3.
O:C atomic ratios For photo-o×idised LDPE as a Function of time. LDPE irradiated in air (2 < 2 9 0 n m , I o ~ 0.26
Wh/m2/h).
correspond to a vertically homogeneous material. It is evident from the data in Fig. 3 that the photo-oxidation of LDPE does not give rise to a vertically homogeneous material over the depth of the material sampled in the ESCA experiment. The large oxygen uptake detected by MgK~t excitation will also contribute to the Ols core level in the TiK~ operation. Before considering in further detail a comparison between the Mg and Ti C:O stoichiometrics it is worth examining the data obtained with MgKct radiation in greater detail. The observed plateau in oxygen uptake (Fig. 3) occurs after --~800h exposure and, at first, might suggest that within the top 40/k, an equilibrium stage has been reached, as alluded to earlier. However, on examining the data for the changes in Cls components as a function of time, as shown in Fig. 4, it becomes evident that this is not completely true. The contribution of oxidised carbon species to the Cls envelope (i.e. C - - O , C ~ O and O - - C ~ O ) continues to increase after 800h exposure and does not begin to plateau until after 1300 h. This is demonstrated further by consideration of the total per cent contribution to the C~s envelope. After ---800h exposure this is ,~8 % and, at 1330 h, 14 %, yet the C: O stoichiometrics of these films are the same (1:0.17). To explain these observations it is useful to compare the inelastic mean free paths for the MgK~ C~s and O~ photoelectrons and examine in greater detail the sampling depths from which the core level signals arise. The contribution to the overall intensity of a core level, dL arising from a depth, dz, is given b y : 25 dl = Ke( - Z/2
cos O) dz
H. S. Munro
254 100
[~X~x~x.....~
80
" " ~ x
10
x
.,, C-H
~
__x
x ~
/
%Cls
c_=o
x/
0
c_-o
x--
o-c_:o
I
I
I
I
1000
2000
3000
/-.,000
IRRADIATION TIME (HOURS)
Fig. 4.
MgK~t C 1, component analysis for photo-oxidised LDPE (irradiated in air as for Fig. 3).
where: K is a composite function of photon flux, number density, photoionisation cross section and spectrometer factors which, for a given core level, are assumed to be constant; z is the depth of interest; 2 is the inelastic mean free path of the photoelectron and 0 is the electron take off angle. Integration of this equation for the Cls and Ols core levels over TABLE 1 Contributions to Signal Intensity as a Function of Depth into the Sample
Depth (,4)
Mg Kct C1.~ Ols
Cl~
Ol~
0-5 5.-10 10 15 15-20 20-25 25 -30 30 35 35--40
1 0"75 0-56 0"42 0'32 0.24 0.18 0.13
1 0'9 0"81 0"72 0"65 0"58 0.52 0'47
1 0"89 0'80 0-71 0-63 0"56 0-51 0'45
1 0'65 0"42 0"27 0'18 0.12 0"07 0.05
Ti K~t
Photo- and 7-radiation-induced oxidation o[ LPDE
255
successive 5 A depths for MgK~ and TiKe radiation and normalising the contribution in the first 5 A gives rise to the data in Table 1. For MgKe radiation it can be seen that the contributions to the Ols signal arising from depths greater than ~ 20 A are small whilst those for the Cls signal are larger. As a result, extensive oxidation below a depth of ~ 2 0 A will not result in a significant increase in the Ols of the original. The increase in percentage of oxidised carbon components observed in the C ~ in the 800-1300 h irradiation period is therefore indicative of the photo-oxidation process extending to greater depths than the O1~ sampling depth but occurring at slower rates compared with the outermost regions of the polymer. In the photo-oxidation of aromatic polymers 17-24 the rate of oxygen uptake appears to be essentially homogeneous over the first ~ 40 A, as evidenced by angular dependence studies and because the percentage of oxidised carbon functionalities closely follows the changes in oxygen uptake. The observed variation in rate of photo-oxidation as a function of depth over the first 40 A in the present case for LDPE is thus very different and may well be a feature of the surface photo-oxidation of polyolefins. As shown by the data in Table 1 for TiK~ radiation, the contributions for the C is and O1~ core levels are similar at each depth. From these data it is evident that the oxidation in the top layers of the sample will make significant contributions to the observed area and derived atomic ratios for the ESCA spectra recorded with TiK~ radiation. Further, from the preceding discussion, the level of oxygen is not homogeneous over the C 1~ sampling depth of ,-~ 130 A and it is thus unlikely that the real O :C ratio at 130 A is as high as 0.13 indicated by the data in Fig. 2. This emphasises the decreasing rate of photo-oxidation as a function of depth in the sample. Recent studies of the photo-oxidation of 3 m m thick plaques and multilayered sandwiches of LD PE have indicated that the oxygen uptake in the first 40.-60/~m after extended exposure is reasonably uniform. In a similar experiment a 40 ~m thick sample composed of two 20 I~m films pressed together was exposed to black lamp irradiation for 3384 h. From the data in Table 2 it can be seen that the overall bulk oxygen uptake for the two layers is similar. However, comparison of the MgK~ ESCA data observed for the front and back surfaces of the first layer reveals a distinct contrast. The oxygen uptake at the back surface is half of that at the front and highlights the extensive nature of the photooxidation processes in the latter surface. The data for the back surface may well be representative of the bulk oxidation of 40 ~m thick films but
H.S. Munro
256
TABLE 2 LDPE Irradiated in Air for 3384h
Total Cls Front surface Back surface Carbonyl index 1st 20#m 2nd 20/~m
C--H
C_--O
C_-~-O
O--C--O
01,/Cl~
82 89
10 6
4 3
4 2
0.34 0.17
100 100 0.9 0.83
should be treated with some caution due to the unknown effects on surface composition caused by separating the two layers.
?-Irradiation Previous studies of the oxidative radiolysis of LDPE have been generally confined to considerations of the changes in bulk chemistry. For example, the formation of hydroperoxide and ketone functionalities has been the basis for the proposal of possible mechanisms. 15 However, the nature of the changes occurring in the surface during the oxidative radiolysis of LDPE may not necessarily be the same as those observed in the bulk. The data in Figs 5 and 6 reveal the changes in O :C atomic ratios (from 0.15
MgK,,~ --x
0.10
O:C
Ti K~ x
0.05
0.0(3
~
X
--
I
I
I
I
10
20
30
z.0
50
Exposure Time (Doys}
Fig. 5.
O:C atomic ratios for y-radiation-induced oxidation of LDPE (80 krads/h)as a function of time.
Photo- and ;,-radiation-induced oxidation of LPDE
257
100 95
89
C-H x
%Cls C-O 5
x ~
z.
_C=O
0 0
Fig. 6.
10
= 20 30 Exposure Time (Doys)
I ~0
J 50
MgKct Cls component analysis for ";-radiation-induced oxidation of LDPE (80 krads/h) as a function of time.
MgK~ and TiK~ spectra) and Cls components (MgKe spectra) as a function of exposure time to a 6°Co source (80krads/h). The oxygen uptake by the polyethylene film at the MgKe sampling depth is more extensive than at the TiKe sampling depth. Comparison of these data with those obtained for photo-oxidation reveals some interesting differences. Whereas, in photo-oxidation, oxygen uptake at the TiK~ sampling depth continues to increase with time although the MgKe data indicates a plateau, the data obtained in the radiolysis experiment shows the pattern of oxygen uptake at both sampling depths to be similar, i.e. the O:C atomic ratio does not change to any great extent after 28 days exposure. Further, for radiolysis there is a larger difference in the oxygen uptake at the two sampling depths on extended exposure than was observed for photo-oxidation. This indicates that the extent of oxidation during the radiolysis of the surface of the polymer (MgKe) is considerably greater than observed in the subsurface (TiK~). The discussion in the section on photo-oxidation on the contribution to the TiK~ signal of the oxygen within the outermost tens of angstroms of the surface is applicable to the present data and reinforces the differences in the degree of oxidation at the two sampling depths. The C1s component analysis for the MgK7 spectra in Fig. 6 reveals the formation of similar oxidative functionalities as observed in photooxidation, i.e. C - - O , C----O and O--C~---O. The formation of the last of these; namely, the carbonyl moiety, which occurs in the early stages of
258
H. S. Munro
exposure, has not been accounted for in the mechanisms for bulk oxidation where only ketone production has been considered. The main difference between the Cls components for radiolysis and photooxidation arises in the intensity of the functionality in which carbon is singly bonded to oxygen. For an extended exposure to photo-oxidation the intensity of C_--O is ~ 1 0 ~ of the Cls envelope whereas, for radiolysis, it is ~ 6 ~ . A consideration of the O:C atomic ratios and the percentage contribution to the Cls envelope arising from oxidative functionalities as a function of exposure time reveals that they follow the same trend, a priori; as the O:C ratio levels out, so does the percentage contribution to the Cls envelope. Again, this is in contrast to the photo-oxidation data and, in the present case, indicates that within the MgK~ sampling depth the rate of photo-oxidation is homogeneous.
CONCLUSION Extensive oxygen uptake occurs in the surface on exposing LDPE films to uv- (2 > 300nm) and y-irradiation. The formation of C_--O, C_=O and O---~_~O oxidative functionalities is observed for both types of irradiation although, for radiolysis, the intensity of the carbon singly bonded to the oxygen component is less than that observed for photooxidation. From a comparison of the O:C atomic ratios obtained from MgK~ and TiK~ spectra and the MgK~CIs component analyses, the rate of photo-oxidation is more rapid in the outer tens of angstroms of the film than in the subsurface and is also not homogeneous over the first 40 A. During radiolysis the reaction rate in the top 40 A is homogeneous.
ACKNOWLEDGEMENTS Thanks are due to Professor D. T. Clark for helpful discussions.
REFERENCES 1. K. Tsuji and T. Seiki, Polymer J., 2, 606 (1971). 2. K. Tsuji and T. Seiki, J. Polym. Sci., Polym. Letts. Edn., 10, 139 (1972).
Photo- and y-radiation-induced oxidation oj LPDE
259
3. G. Scott, J. Polym. Sci., Part C, 57, 357 (1976). 4. J. F. Heacock, F. B. Mallory and F. B. Gary, J. Polym. Sci., Polym. Chem. Edn., 6, 2921 (1968). 5. F. H. Rugg, J. J. Smith and R. C. Bacon, J. Polym. Sci., Polym. Chem. Edn., 13, 335 (1954). 6. M. U. Amin, G. Scott and L. M. K. Tillakeratue, Eur. Polym. J., 11, 85 (1975). 7. J. H. Adams, J. Polym. Sci., Polym. Chem. Edn., 8, 1279 (1970). 8. K. B. Chakraborty and G. Scott, Eur. Polym. J., 13, 731 (1977). 9. G. C. Furneaux, K. J. Ledbury and A. Davis, Poly. Deg. andStab., 3,431 (1981). 10. A. V. Cunliffe and A. Davis, Poly. Deg. and Stab., 4, 17 (1982). 11. C. Decker, F. R. Mayo and H. Richardson, J. Polym. Sci., Polym. Chem. Edn., 11, 2879(1973). 12. K. Arakawa, T. Seguchi, Y. Watanabe, N. Hayakawa, I. Kuriyama and S. Machi, J. Polym. Sci., Polym. Chem. Edn., 19, 2123 (1981). 13. K. Arakawa, T. Seguchi, Y. Watanabe and N. Hayakawa, J. Polym. Sci., Polym. Chem. Edn., 20, 2681 (1982). 14. K. Arakawa, T. Seguchi, N. Hayakawa and S. Hachi, J. Polym. Sci., Polym. Chem. Edn., 21, 1173 (1983). 15. J. Petriy and J. Marchal, Radiat. Phys. Chem., 16, 27 (1980). 16. D. T. Clark, Pure and Appl. Chem. (1980). 17. D. T. Clark and H. S. Munro, Poly. Deg. andStab., 4, 441 (1982). 18. D. T. Clark and H. S. Munro, Poly. Deg. andStab., 5, 227 (1983). 19. D. T. Clark and H. S. Munro, Poly. Deg. and Stab., 8, 195 (1984). 20. H. S. Munro and D. T. Clark, Poly. Deg. andStab., II, 211 (1985). 21. H. S. Munro and D. T. Clark, Pol),. Deg. andStab., II, 225 (1985). 22. D. T. Clark and H. S. Munro, Poly. Deg. and Stab., 8, 213 (1984). 23. H. S. Munro and D. T. Clark, Poly. Deg. and Stab. (In press.) 24. H.S. Munro, D. T. Clark and J. Peeling, Poly. Deg. and Stab., 9, 185 (1984). 25. A. Dilks, in Electron spectroscopy, theory, techniques and applications, Vol. 4 (C. R. Brundle and A. D. Baker (Eds)), Academic Press, New York (198 l).
ESCA Studies of the Photo-oxidation and the y-Radiation-Induced Oxidation of Low Density Polyethylene
H. S. M u n r o Department of Chemistry, University of Durham, South Road, Durham, Great Britain (Received: 3 October, 1984)
ABSTRACT ESCA has been used to monitor the changes in the surface chemistry of low density polyethylene during exposure to uv light (2 > 300 nm) and 7" radiation. In photo-oxidation, oxygen uptake is reflected by C_--O, ~C_-m--O and O--C_--O functional group jbrmation and the rate of reaction is shown to decrease gradually in the top 150,4 of the exposed surface. By the use of sandwich layers, the extent of oxygen uptake in the front surface is found to be double that at a depth of 20 #m. v-irradiation shows similar trends to photo-oxidation although the extent of oxidative functionalisation is less.
I N T R O D U C T I ON The oxidative degradation of polyethylene (low and high density) has been the subject of numerous investigations which directly reflect the importance of this material in everyday life. The photo-oxidation 1- l o and, to a lesser extent, the v-radiation-induced oxidation, lx-15 have received attention in the literature with respect to the attainment of an understanding of the degradative pathways and subsequent stabilisation for the enhancement of in-service lifetime. It is generally acknowledged that the photo-oxidative process is initiated in the surface regions but, as 249 Polymer Degradation and Stability 0141-3910/85/$03-30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain
250
H. S. Munro
yet, a comprehensive examination of the nature of the changes in surface chemistry has not been reported. Electron Spectroscopy for Chemical Application has been shown to be the single most powerful technique for the interrogation of composition and bonding in polymer surfaces 16 and has been successfully applied to surface photo-oxidation studies of Bisphenol A polycarbonate, 17-~9 Bisphenol A polysulphone, 2° polyether sulphone 2~ and polystyrene. 22- 24 It is for this reason that the studies discussed below have utilised ESCA to follow the changes in surface chemistry during the irradiation of low density polyethylene (LDPE) with uv (2 > 290 nm) and y-radiation.
EXPERI MENTAL Samples of LDPE film (20 nm) were exposed in air (relative humidity ,-,55%) to the output of a black lamp (2> 300nm, Photoflux ,-,0.25 W/m 2) over various periods of time. Similarly, samples were exposed to ionising radiation from a 6°Co source (90 k rads/h). The films were placed in the low vacuum insertion lock of a Kratos ES300 electron spectrometer within 10min of the termination of exposure. ESCA spectra were recorded with MgK~ and TiKu excitation radiation where ,-, 95 ~o of the signal arises from depths of 40/~ and 130/~ for the C ~~core level. 16 Binding energies were referenced to the C - - H component at 285.0 eV. Core level area ratios are quoted to + 5 %. For the component peak analysis in the MgK~ spectra, gaussian peaks were fitted to known carbon oxygen binding energies. 16- 24
RESULTS A N D DISCUSSION UV irradiation Wavelengths lower than 290nm are not normally found in natural sunlight and, in order to obtain a laboratory approximation of the degradation process that occurs in sunlight, uv light below this wavelength should be avoided. We have previously used a black lamp in photo-oxidation studies of Bisphenol A polycarbonate 19 and have found
Photo- and ?-radiation-induced oxidation oJ LPDE
251
that the induced changes in surface and bulk chemistry closely follow those for natural weathering. This lamp has therefore been utilised again for the LDPE studies. Although the low photon flux output results in long exposure times, it is a very cheap source of uv radiation, particularly compared with the filtered Xenon arc lamp which is perhaps more representative of sunlight. Typical MgKot and TiK0~ core level spectra for unexposed and photooxidised LDPE films are shown in Figs l and 2, respectively. In Fig. 1 the unexposed film reveals a single Cls peak centred at 285-0 eV arising from the ~ H 2 ~ H 2 - units in the polymer. A very low level of oxygen is also present on the surface, as shown by the Ols signal, but it is not reflected in the Cls envelope. The corresponding TiK0c spectra have characteristic doublets in the ratio 1:2 separated by ,-,6eV. The respective sampling depths for MgK0c and TiKot spectra are ~40 and 130A. 16 After exposure to uv radiation the Cls spectrum in Fig. 1 has become more complex in that a distinctive shoulder to the higher binding energy side of the main photo-ionisation peak at 285.0 eV is now evident. This arises from the presence of oxidative functionalities such as C - - O , C_=O and O - ~ _ = O and, with the increase in the Oxs signal, is indicative of extensive photo-oxidation in the outermost region of the film. The TiK~
XI
L_ Ohrs / 290
[
I×I \ 285
280
BE.
(e.V)
Fig. 1. C ~sand O~s MgKctcore levelspectra for unexposedand photo-oxidised LD PE.
252
H . S . Munro
133~ h X2
Ohrsh~./~~'
Fig. 2.
XI
BE. IN eV C h and O~, TiK~ core level spectra for unexposed and photo-oxidised LDPE.
spectra in Fig. 2 also reveal changes compared with the unexposed material and indicate that oxidation extends to at least 130,~,. The oxygen uptake for L D P E as a function of time at the two sampling depths is shown in Fig. 3. The data are displayed as carbon to oxygen stoichiometries. These may be obtained from the experimentally determined area ratios and a knowledge of the relevant instrument sensitivity factors. From the MgK~ data, oxygen uptake occurs rapidly over the first ,~ 800 h of exposure and then remains relatively constant at a carbon:oxygen atomic ratio of ~ C 1 : O o.ls. This plateau may well arise from an equilibrium of competitive processes involving further photooxidation and the absorption of low molecular weight species. 17 - 24 The corresponding data for the TiK~ spectra shows that this equilibrium is not reached until ,-, 1500 h of exposure and suggests that the rates of reaction at the two sampling depths are very different. It is also evident from a comparison of the carbon: oxygen stoichiometrics that the extent of photo-oxidation at the TiKct sampling depth is not as great as that for MgK~. However, a straightforward comparison of the stoichiometrics at sampling depths of 40 and 130 A is not completely valid. As alluded to above, the stoichiometrics are calculated from the area ratios and the appropriate instrument sensitivity factors. These sensitivity factors
Photo- and v-radiation-induced oxidation o/LPDE
253
02
/
C~:Ox STOICHIOMETRY
___~i__.-----
~
----~
Mg Anode
0, z//i/y
x
/
/
I
00
1000
Tt Anode
I
I
2000
3000
I
4000
IRRADIATION TIME (HOURS)
Fig. 3.
O:C atomic ratios For photo-o×idised LDPE as a Function of time. LDPE irradiated in air (2 < 2 9 0 n m , I o ~ 0.26
Wh/m2/h).
correspond to a vertically homogeneous material. It is evident from the data in Fig. 3 that the photo-oxidation of LDPE does not give rise to a vertically homogeneous material over the depth of the material sampled in the ESCA experiment. The large oxygen uptake detected by MgK~t excitation will also contribute to the Ols core level in the TiK~ operation. Before considering in further detail a comparison between the Mg and Ti C:O stoichiometrics it is worth examining the data obtained with MgKct radiation in greater detail. The observed plateau in oxygen uptake (Fig. 3) occurs after --~800h exposure and, at first, might suggest that within the top 40/k, an equilibrium stage has been reached, as alluded to earlier. However, on examining the data for the changes in Cls components as a function of time, as shown in Fig. 4, it becomes evident that this is not completely true. The contribution of oxidised carbon species to the Cls envelope (i.e. C - - O , C ~ O and O - - C ~ O ) continues to increase after 800h exposure and does not begin to plateau until after 1300 h. This is demonstrated further by consideration of the total per cent contribution to the C~s envelope. After ---800h exposure this is ,~8 % and, at 1330 h, 14 %, yet the C: O stoichiometrics of these films are the same (1:0.17). To explain these observations it is useful to compare the inelastic mean free paths for the MgK~ C~s and O~ photoelectrons and examine in greater detail the sampling depths from which the core level signals arise. The contribution to the overall intensity of a core level, dL arising from a depth, dz, is given b y : 25 dl = Ke( - Z/2
cos O) dz
H. S. Munro
254 100
[~X~x~x.....~
80
" " ~ x
10
x
.,, C-H
~
__x
x ~
/
%Cls
c_=o
x/
0
c_-o
x--
o-c_:o
I
I
I
I
1000
2000
3000
/-.,000
IRRADIATION TIME (HOURS)
Fig. 4.
MgK~t C 1, component analysis for photo-oxidised LDPE (irradiated in air as for Fig. 3).
where: K is a composite function of photon flux, number density, photoionisation cross section and spectrometer factors which, for a given core level, are assumed to be constant; z is the depth of interest; 2 is the inelastic mean free path of the photoelectron and 0 is the electron take off angle. Integration of this equation for the Cls and Ols core levels over TABLE 1 Contributions to Signal Intensity as a Function of Depth into the Sample
Depth (,4)
Mg Kct C1.~ Ols
Cl~
Ol~
0-5 5.-10 10 15 15-20 20-25 25 -30 30 35 35--40
1 0"75 0-56 0"42 0'32 0.24 0.18 0.13
1 0'9 0"81 0"72 0"65 0"58 0.52 0'47
1 0"89 0'80 0-71 0-63 0"56 0-51 0'45
1 0'65 0"42 0"27 0'18 0.12 0"07 0.05
Ti K~t
Photo- and 7-radiation-induced oxidation o[ LPDE
255
successive 5 A depths for MgK~ and TiKe radiation and normalising the contribution in the first 5 A gives rise to the data in Table 1. For MgKe radiation it can be seen that the contributions to the Ols signal arising from depths greater than ~ 20 A are small whilst those for the Cls signal are larger. As a result, extensive oxidation below a depth of ~ 2 0 A will not result in a significant increase in the Ols of the original. The increase in percentage of oxidised carbon components observed in the C ~ in the 800-1300 h irradiation period is therefore indicative of the photo-oxidation process extending to greater depths than the O1~ sampling depth but occurring at slower rates compared with the outermost regions of the polymer. In the photo-oxidation of aromatic polymers 17-24 the rate of oxygen uptake appears to be essentially homogeneous over the first ~ 40 A, as evidenced by angular dependence studies and because the percentage of oxidised carbon functionalities closely follows the changes in oxygen uptake. The observed variation in rate of photo-oxidation as a function of depth over the first 40 A in the present case for LDPE is thus very different and may well be a feature of the surface photo-oxidation of polyolefins. As shown by the data in Table 1 for TiK~ radiation, the contributions for the C is and O1~ core levels are similar at each depth. From these data it is evident that the oxidation in the top layers of the sample will make significant contributions to the observed area and derived atomic ratios for the ESCA spectra recorded with TiK~ radiation. Further, from the preceding discussion, the level of oxygen is not homogeneous over the C 1~ sampling depth of ,-~ 130 A and it is thus unlikely that the real O :C ratio at 130 A is as high as 0.13 indicated by the data in Fig. 2. This emphasises the decreasing rate of photo-oxidation as a function of depth in the sample. Recent studies of the photo-oxidation of 3 m m thick plaques and multilayered sandwiches of LD PE have indicated that the oxygen uptake in the first 40.-60/~m after extended exposure is reasonably uniform. In a similar experiment a 40 ~m thick sample composed of two 20 I~m films pressed together was exposed to black lamp irradiation for 3384 h. From the data in Table 2 it can be seen that the overall bulk oxygen uptake for the two layers is similar. However, comparison of the MgK~ ESCA data observed for the front and back surfaces of the first layer reveals a distinct contrast. The oxygen uptake at the back surface is half of that at the front and highlights the extensive nature of the photooxidation processes in the latter surface. The data for the back surface may well be representative of the bulk oxidation of 40 ~m thick films but
H.S. Munro
256
TABLE 2 LDPE Irradiated in Air for 3384h
Total Cls Front surface Back surface Carbonyl index 1st 20#m 2nd 20/~m
C--H
C_--O
C_-~-O
O--C--O
01,/Cl~
82 89
10 6
4 3
4 2
0.34 0.17
100 100 0.9 0.83
should be treated with some caution due to the unknown effects on surface composition caused by separating the two layers.
?-Irradiation Previous studies of the oxidative radiolysis of LDPE have been generally confined to considerations of the changes in bulk chemistry. For example, the formation of hydroperoxide and ketone functionalities has been the basis for the proposal of possible mechanisms. 15 However, the nature of the changes occurring in the surface during the oxidative radiolysis of LDPE may not necessarily be the same as those observed in the bulk. The data in Figs 5 and 6 reveal the changes in O :C atomic ratios (from 0.15
MgK,,~ --x
0.10
O:C
Ti K~ x
0.05
0.0(3
~
X
--
I
I
I
I
10
20
30
z.0
50
Exposure Time (Doys}
Fig. 5.
O:C atomic ratios for y-radiation-induced oxidation of LDPE (80 krads/h)as a function of time.
Photo- and ;,-radiation-induced oxidation of LPDE
257
100 95
89
C-H x
%Cls C-O 5
x ~
z.
_C=O
0 0
Fig. 6.
10
= 20 30 Exposure Time (Doys)
I ~0
J 50
MgKct Cls component analysis for ";-radiation-induced oxidation of LDPE (80 krads/h) as a function of time.
MgK~ and TiK~ spectra) and Cls components (MgKe spectra) as a function of exposure time to a 6°Co source (80krads/h). The oxygen uptake by the polyethylene film at the MgKe sampling depth is more extensive than at the TiKe sampling depth. Comparison of these data with those obtained for photo-oxidation reveals some interesting differences. Whereas, in photo-oxidation, oxygen uptake at the TiK~ sampling depth continues to increase with time although the MgKe data indicates a plateau, the data obtained in the radiolysis experiment shows the pattern of oxygen uptake at both sampling depths to be similar, i.e. the O:C atomic ratio does not change to any great extent after 28 days exposure. Further, for radiolysis there is a larger difference in the oxygen uptake at the two sampling depths on extended exposure than was observed for photo-oxidation. This indicates that the extent of oxidation during the radiolysis of the surface of the polymer (MgKe) is considerably greater than observed in the subsurface (TiK~). The discussion in the section on photo-oxidation on the contribution to the TiK~ signal of the oxygen within the outermost tens of angstroms of the surface is applicable to the present data and reinforces the differences in the degree of oxidation at the two sampling depths. The C1s component analysis for the MgK7 spectra in Fig. 6 reveals the formation of similar oxidative functionalities as observed in photooxidation, i.e. C - - O , C----O and O--C~---O. The formation of the last of these; namely, the carbonyl moiety, which occurs in the early stages of
258
H. S. Munro
exposure, has not been accounted for in the mechanisms for bulk oxidation where only ketone production has been considered. The main difference between the Cls components for radiolysis and photooxidation arises in the intensity of the functionality in which carbon is singly bonded to oxygen. For an extended exposure to photo-oxidation the intensity of C_--O is ~ 1 0 ~ of the Cls envelope whereas, for radiolysis, it is ~ 6 ~ . A consideration of the O:C atomic ratios and the percentage contribution to the Cls envelope arising from oxidative functionalities as a function of exposure time reveals that they follow the same trend, a priori; as the O:C ratio levels out, so does the percentage contribution to the Cls envelope. Again, this is in contrast to the photo-oxidation data and, in the present case, indicates that within the MgK~ sampling depth the rate of photo-oxidation is homogeneous.
CONCLUSION Extensive oxygen uptake occurs in the surface on exposing LDPE films to uv- (2 > 300nm) and y-irradiation. The formation of C_--O, C_=O and O---~_~O oxidative functionalities is observed for both types of irradiation although, for radiolysis, the intensity of the carbon singly bonded to the oxygen component is less than that observed for photooxidation. From a comparison of the O:C atomic ratios obtained from MgK~ and TiK~ spectra and the MgK~CIs component analyses, the rate of photo-oxidation is more rapid in the outer tens of angstroms of the film than in the subsurface and is also not homogeneous over the first 40 A. During radiolysis the reaction rate in the top 40 A is homogeneous.
ACKNOWLEDGEMENTS Thanks are due to Professor D. T. Clark for helpful discussions.
REFERENCES 1. K. Tsuji and T. Seiki, Polymer J., 2, 606 (1971). 2. K. Tsuji and T. Seiki, J. Polym. Sci., Polym. Letts. Edn., 10, 139 (1972).
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3. G. Scott, J. Polym. Sci., Part C, 57, 357 (1976). 4. J. F. Heacock, F. B. Mallory and F. B. Gary, J. Polym. Sci., Polym. Chem. Edn., 6, 2921 (1968). 5. F. H. Rugg, J. J. Smith and R. C. Bacon, J. Polym. Sci., Polym. Chem. Edn., 13, 335 (1954). 6. M. U. Amin, G. Scott and L. M. K. Tillakeratue, Eur. Polym. J., 11, 85 (1975). 7. J. H. Adams, J. Polym. Sci., Polym. Chem. Edn., 8, 1279 (1970). 8. K. B. Chakraborty and G. Scott, Eur. Polym. J., 13, 731 (1977). 9. G. C. Furneaux, K. J. Ledbury and A. Davis, Poly. Deg. andStab., 3,431 (1981). 10. A. V. Cunliffe and A. Davis, Poly. Deg. and Stab., 4, 17 (1982). 11. C. Decker, F. R. Mayo and H. Richardson, J. Polym. Sci., Polym. Chem. Edn., 11, 2879(1973). 12. K. Arakawa, T. Seguchi, Y. Watanabe, N. Hayakawa, I. Kuriyama and S. Machi, J. Polym. Sci., Polym. Chem. Edn., 19, 2123 (1981). 13. K. Arakawa, T. Seguchi, Y. Watanabe and N. Hayakawa, J. Polym. Sci., Polym. Chem. Edn., 20, 2681 (1982). 14. K. Arakawa, T. Seguchi, N. Hayakawa and S. Hachi, J. Polym. Sci., Polym. Chem. Edn., 21, 1173 (1983). 15. J. Petriy and J. Marchal, Radiat. Phys. Chem., 16, 27 (1980). 16. D. T. Clark, Pure and Appl. Chem. (1980). 17. D. T. Clark and H. S. Munro, Poly. Deg. andStab., 4, 441 (1982). 18. D. T. Clark and H. S. Munro, Poly. Deg. andStab., 5, 227 (1983). 19. D. T. Clark and H. S. Munro, Poly. Deg. and Stab., 8, 195 (1984). 20. H. S. Munro and D. T. Clark, Poly. Deg. andStab., II, 211 (1985). 21. H. S. Munro and D. T. Clark, Pol),. Deg. andStab., II, 225 (1985). 22. D. T. Clark and H. S. Munro, Poly. Deg. and Stab., 8, 213 (1984). 23. H. S. Munro and D. T. Clark, Poly. Deg. and Stab. (In press.) 24. H.S. Munro, D. T. Clark and J. Peeling, Poly. Deg. and Stab., 9, 185 (1984). 25. A. Dilks, in Electron spectroscopy, theory, techniques and applications, Vol. 4 (C. R. Brundle and A. D. Baker (Eds)), Academic Press, New York (198 l).