Thermal degradation of polymethacrylonitrile

Polymer Degradation and Stability 40 (1993) 143-150

Thermal degradation of polymethacrylonitrile David J. T. Hill, Limin Dong, James H. O'Donnell, Graeme George & Peter Pomery Polymer and Radiation Group, Department of Chemistry, University of Queensland, Brisbane, Australia 4072 (Received 20 March 1992; accepted 6 April 1992)

The thermal degradation properties of polymethacrylonitrile (PMAN) have been studied by isothermal heating and thermogravimetric analysis. There are two initiation processes for weight loss from PMAN degraded in nitrogen, namely chain-end and random scission initiation. There is also an internal cyclization reaction which forms a thermally stable residue during the thermal degradation process. The activation energies of the weight loss and formation of stable residues have been calculated. X-ray photoelectron spectroscopy has been used to investigate the structure of the stable residue and thus to confirm the degradation mechanism.

INTRODUCTION

sample will contain information about the significance of each of these processes, and may clarify their relative importance at various stages of the degradation. In this study, the thermal degradation of PMAN in nitrogen was followed by thermogravimetric analysis (TGA) at various heating rates and also by isothermal heating experiments at various temperatures. The residue from PMAN degradation was analysed by X-ray photoelectron spectroscopy. The values of the apparent kinetic rate constants for degradation reactions of PMAN were calculated from the experimental data. Here we consider the mechanism of the thermal degradation of PMAN in terms of these observed kinetic rate parameters.

Polymethacrylonitrile (PMAN) has been studied frequently because of its potential uses as a photoresist material, 1-5 and there have been some previous investigations of its thermal degradation properties. 6-1° In common with other polymers having the ( - - C H : - - C X Y - - ) repeat structure, such as poly(methyl methacrylate), P M A N depolymerizes to monomer in high yield when heated above 220°C. In the previous studies of the thermal degradation mechanism, it has been inferred that there are two types of initiation reactions for P M A N degradation. One is chain-end initiation and the other is initiation at random by chain scission. The depolymerization reactions involve unzipping of the chain to form monomer. However, there have been few reported studies of the kinetics of the thermal degradation of PMAN. The weight loss from PMAN is the final result of a complex degradation process consisting of chain initiation reactions, depolymerization reactions, termination reactions, transport of the decomposition products through softened PMAN, and internal chemical reactions in the polymer chain. The apparent kinetic rate constants determined from the weight loss of the

EXPERIMENTAL Materials Methacrylonitrile was purified by the usual procedures. 1~ The mixture of monomer and initiator benzoyl peroxide was freeze-thaw degassed and then sealed under vacuum. The polymerization was carried out at 60°C. The conversion was limited to 15%. The molecular weight of the polymer was measured by gel

Polymer Degradation and Stability 0141-3910/93/$06.00 (~) 1993 Elsevier Science Publishers Ltd. 143

David J. T. Hill et al.

144

permeation chromatography M w / M n = 2-2).

(Mn = 25 000,

1oo

Thermogravimetric analysis (TGA) and isothermal heating analysis

8O

The weight loss from the samples was measured by TGA and isothermal heating using a Perkin-Elmer (Norwalk, Connecticut, USA) TGA7 Thermogravimetric Analyzer. The samples were placed into an evacuable die using a pressure of 6000 kg/cm 2 for 10 min, and thus moulded into a disk of about 0.5 mm thickness. The experiments were conducted using 5-8 mg samples held in a platinum pan under a pure nitrogen flow of 40 cm3/min. Derivative thermogravimetry (DTG) curves were obtained using the Perkin-Elmer TGA7 Multitasking Software Kit.

60

I

I

c-~

I

4O

2O

0 0

'

'

~

'

200

400

600

800

1000

Time (min) Fig. 1. Isothermal weight loss of P M A N at 250, 285, 300, 330, 350, 380 and 400°C (from top to bottom).

X-ray photoelectron spectroscopy (XPS) extent of weight loss and is given by: The XPS data were determined using a Perkin-Elmer PHI-560 ESCA/SIMS Spectrometer. All spectra were collected by using Mg Kte excitation (1253-6 eV). The samples for XPS analysis had undergone isothermal degradation in the Perkin-Elmer TGA7 Thermogrametric Analyzer prior to analysis by XPS. For every sample a survey spectrum and high-resolution spectra of C ls and N ls (25 eV pass energy) were collected.

RESULTS AND DISCUSSION Isothermal weight loss of PMAN was studied at 250, 285, 300, 330, 350, 380 and 400°C. The dependence of the weight fraction remaining, ( 1 - D), on heating time is shown in Fig. 1. To interpret these results, the method recommended by MacCallum '2 was used to calculate the apparent activation energy. The rate of the global degradation process is given by a Dakin '3 type kinetic relationship:

- d(1 - D) dt

- k f ( 1 - D)

(1)

where t is the time, k is the rate constant of the global degradation process and f ( 1 - D ) is a function of the degree of degradation. D is the

D = Wv/Wo--

Wo-W Wo

(2)

where Wo is the initial weight of the polymer, Wv is the weight of the polymer evolved as volatile fragments and W is the weight of the sample during the thermal degradation process. We also consider that the variation of k with temperature is of the Arrhenius type: (3)

k = A exp(-E/RT)

where E is the activation energy, R is the universal gas constant, T is the absolute temperature and A is the pre-exponential factor. Integrating eqn (1), we obtain: F(1 - D) = kt

(4)

where F ( 1 - D) is the integrated expression for f(1 - D) F(1 - D) = - f

d(1

D)

f(1-----~

(5)

It can be assumed that for isothermal experiments performed over a range of temperatures F(1-D) has the same value for a given conversion D. Taking eqn (3) into account, eqn (4) can be rewritten in the form of eqn (6): In(t) = In(F(1 - D)) - In(A) + E / R T

(6)

For a range of temperatures, the logarithm of the time taken to reach a fixed conversion D

145

Thermal degradation of polymethacrylonitrile

plotted against the reciprocal of the temperature of the experiment will yield the activation energy, E. Using the data in Fig. 1, In(t) was plotted v e r s u s 1 / T as shown in Fig. 2 for various degrees of degradation. The regression coefficients were calculated for these lines (ordinate intercept a and gradient b) and the global activation energies, E, for the degradation were estimated as in Table 1. It can be seen that E increases with increasing conversion, O. This variation can be interpreted in terms of the Degradation Compensation Effect (DCE, also called the Ageing Compensation Effect, ACE). 14 Montanaris has investigated several polymer materials in accelerated thermal ageing studies. 15 For all the materials investigated, he found the following linear relationship between the regression coefficients a and b: (7)

a = oL + f i b

where tr and/3 are constants. Equation (7) describes the DCE for thermal degradation, which is an extension of the kinetics compensation effect (CE), well known in chemical reaction kinetics. 16-~8 Using the data in Table 1, a was plotted v e r s u s b as shown in Fig. 3. This plot clearly indicates that eqn (7) can satisfactorily account for the observed experiment data. A thermally stable residue was formed in all of the isothermal degradation studies (see Fig. 1). 4

I

--

Table 1. Regression coefficients a and b of In(t) = a + b/T and activation energies (E, kJ/mol) for various degrees of degradation of PMAN 1- D

-a

b(103T)

E

0.98 0"95 0.90 0-80 0.70

26.48 32.74 40.56 43-44 50-89

14.93 19-32 24.48 26-70 31-69

124 160 203 221 263

The yield of this residue was greater at lower temperatures. A possible explanation for the stability of the residue would be internal cyclization with formation of a stable ring structure without weight loss. This stable ring is similar to the structure reported for polyacrylonitrile. 2s Thus there would appear to be at least two degradation pathways, one leading to formation of volatile fragments and one leading to the formation of ring structures. The kinetic equations of the processes may be expressed as: (a) For formation of volatiles d(W -

-

dt

M) -

kw(W

-

(8)

M) ~

(b) For the cyclic component dM

(9)

d t = k m ( W -- M ) n'

-20

I

3 \

-50

2

x, \.

\

-40 v c!

o \ -1

-50

•

10N 05% 02%

-2 -3 1.55

1.60

t.65

I

I

I

I

1,70

1.75

1.80

1.85

I

-60 1.0

.90

IOOO/T (K-') Fig. 2. Plot of In(time to fixed conversion) versus 1/T according to eqn (6).

Fig.

3. The

1.5

I

I

2.o 2.s b (lo ~ K)

degradation compensation according to eqn (7).

I

s.o effect

s.s (DCE)

146

D a v i d J. T. Hill

where W is the weight of the sample, M is the weight of the nonvolatile component formed during the degradation process, t is time, kw and km are the rate constants of the weight loss and the formation of the nonvolatile component, respectively; n and n' are the apparent orders, of the degradation and cyclization reactions, respectively. If we make the simplest assumption that these reactions are first-order, and combine eqns (8) and (9), d(W-M)= dM

kw

(10)

km

W - M - Wo = - M ( k w / k m )

(11)

where W0 is the initial weight of the polymer. At the end of the degradation process, the actual weight of the sample We is equal to the weight of the stable, cyclic residue formed, Me. Thus, under this condition eqn (11) may be expressed as =

(12)

W¢(kw/km)

According to eqn (2), We may be expressed in terms of Dr, the extent of conversion of the volatiles at the end of the degradation process: We = 14:o(1- De)

(13)

If kw and km are assumed to obey the Arrhenius relationship, the temperature dependence of these constants is given by

ki Ai exp(-Ei/nz) =

(14)

where subscript i represents w or m. Taking eqns (13) and (14) into account, eqn (12) can be rewritten as eqn (15): I n ( l - D r ) =In

0 -I -2 cm

-.3

I

"~

-4 -5 -6 -7

Integrating eqn (10), we obtain:

Wo

et al.

+

RT

Thus I n ( 1 - De) plotted against the reciprocal of the temperature should be linear and yield the difference of the activation energies ( E w - Era) from the gradient of the plot. The data obtained from Fig. 1 over the temperature range 285-400°C have been analysed according to eqn (15) and are shown in Fig. 4. The figure indicates that there are two stages during the thermal degradation of PMAN. In the first stage the activation energy difference (Ew- Era) is approximately 72 kJ/mol (temperature range 285-350°C) and in the second stage

t 1.4

t i 1.6 1.8 1000/T (K -I)

2.0

Fig. 4. Plot of In(1- De) versus l I T accordingto eqn (15). the activation energy difference is approximately 183 kJ/mol (temperature range 350-400°C). This seems to suggest that there may be a change at about 350°C in the predominant mechanism for the reaction involving weight loss. NcNeill used Thermal Volatilization Analysis (TVA) to study the degradation of PMAN.19 The TVA curves for PMAN shows that there are two stages in the thermal degradation. McNeill proposed that in the first stage, the degradation is initiated at chain ends, and that in the second stage, the decomposition is initiated by random scission followed by unzipping of the chain. On this basis, our results indicate that the activation energy for the random scission initiation of PMAN is 111 kJ/mol higher than the value for the chain-end initiation. We have calculated the global activation energy for the thermal degradation of PMAN (see Table 1). At low conversion, the nonvolatile component formed by the internal cyclization reaction may be neglected, and E may be considered to be the activation energy for the weight loss in the first stage, Ew. Thus, the activation energy for the weight loss of PMAN is about 124kJ/mol in the first stage and 235kJ/mol in the second stage. The activation energy of the formation of the nonvolatile component by internal cyclization, Em, would therefore be about 52 kJ/mol. The thermal degradation mechanism of PMAN is similar to that of poly(methyl methacrylate) (PMMA) 2°, but generally it is not necessary to consider the formation of nonvolatile residues during the PMMA degradation process, so that

Thermal degradation of polymethacrylonitrile its kinetic analysis is more straightforward. Hirata et al. have studied the kinetics of thermal degradation PMMA in nitrogen. 21 They found that the activation energies for thermal degradation of P M M A in the first stage (chain-end initiation) and the second stage (random scission initiation) are 31 kJ/mol and 224 kJ/mol, respectively. In general, chain-end initiation for different polymers follows different mechanisms, and Hirata et al. have explained that the low activation energy of chain-end initiation obtained in their experiments is due to the effect of impurities, such as unreacted monomer. It is interesting that the activation energy of thermal degradation P M M A in the second stage (224 kJ/mol) is close to our experimental result, 235kJ/mol, for PMAN thermal degradation. Thus it may be concluded that for both PMAN and PMMA the thermal degradation in the second stage is related to random scission of carbon-carbon bonds in the repeat structure ( - - C H 2 - - C X Y - - ) , followed by unzipping. TGA measurements of PMAN were also carried out at several heating rates to investigate further the reaction mechanism. The results for TGA and DTG of PMAN in nitrogen are shown in Figs 5 and 6. The DTG curves for the samples degraded at higher heating rates (20, 15 and 10°C/min) indicate that two main reaction stages occur during the degradation process. The first reaction stage has peaks appearing from about 300 to 370°C and the second reaction stage from

I O0

80

10c ,::x

6O

Heating Rate:

?

~

'",

40 E'5oC(min 5.0[C/rain 2O

\

"~"~ \,\ \, t\ ',\

..... lSo-C/

,

0 200

\

250

q

,

................... ,

,

500 350 Temperature

400 (°C)

450

500

Fig. 5. T G A curves of P M A N at various heating rates.

t

t-

147 t

F---T

1

--

I

..................... : ...........

-~~'1 "~

50

!ii

. ...................

2o

. ....,

oC/mi~

15 o C/min ..... 10 oC/min ___ 5.0 oC/min 2.5 C/rnin 0 -1.0 C/min i

I

I

I

I

J

f

100

150

200

250

,300

,350

400

450

500

Temperature(°C) Fig. 6. D T G curves of P M A N at various heating rates.

about 370 to 450°C. These results agree well with the TVA curves reported by McNeill, ~9 and are consistent with the kinetic analysis from our isothermal heating experiments. When the heating rate is decreased, the two peaks in the DTG curves gradually approach one another and finally become one peak at a heating rate of 1.0°C/min. Therefore, the chain-end initiation and random scission initiation cannot be distinguished in the DTG curve at a heating rate of 1.0°C/min. In the T G A experiments with various heating rates, a thermally stable residue was formed, as was found in the isothermal studies. When the samples were heated at higher rates, the proportion of this residue formed was smaller. The formation of cyclized structures within the chain has the effect of blocking the depolymerization reaction, ~9 thus impairing the formation of monomer. At the lower heating rates, more cyclized product is formed at lower temperatures, so the proportion of cyclized product formed is greater under these conditions. There are many methods for determining kinetic rate constants from T G A and DTG data, but these are difficult to apply to PMAN degradation because the general methods for dealing with thermal degradation do not consider the formation of nonvolatile fragments. However, we may consider, as an approximation, that the internal cyclization can be neglected in the kinetic equation for weight loss at low conversion. For the T G A experiments, in the first stage

David J. T. Hill et al.

148

at higher heating rates it may be considered that the nonvolatile component does not effect the kinetic equation for weight loss, so that the general kinetic analysis methods can be applied. In this study, the Kissinger method was used to determine the kinetic constant. 22 According to the expression derived by Kissinger: ln(r/T2m) = I n ( n R A W ~ - a / E ) - E / R T m

(16)

where r is the heating rate in the TGA experiment, Tm is the temperature at the maximum rate of weight loss, R is the universal gas constant, E is the activation energy, A is the pre-exponential factor, Wm is the weight of the sample at the maximum rate of weight loss, and n is the apparent order of the reaction with respect to the sample weight. Thus, the value of E can be determined from a plot of ln(r/T2m) versus 1~Tin for the various heating rates. Using the data from the first peaks of the DTG curves (heating rates 20, 15 and 10°C/min) in Fig. 6, ln(r/T2m) was plotted versus 1~Tin as shown in Fig. 7. The activation energy obtained for weight loss in the first stage (chain-end initiation stage) is about l l 6 k J / m o l , which is close to the value obtained from the isothermal heating experiments (124 kJ/mol). In order to confirm the mechanism of thermal degradation of PMAN, X-ray photoelectron spectroscopy (XPS) was used to investigate the molecular structure of PMAN and its degradation residues. XPS can provide the information to differentiate the chemical states of elements by --8

--

measuring the binding energies of the valence electrons in the elements. In this study, XPS analyses were performed on three samples. Sample 1 was pure PMAN, sample 2 was a residue of PMAN after isothermal heating at 250°C for 1000 min, and sample 3 was a residue of PMAN after isothermal heating at 330°C for 1000 min. The C ls XPS spectra for the samples are given in Fig. 8. Figure 8(a) is the spectrum of PMAN. The peak is relatively broad and symmetric. The full width at half maximum (fwhm) for this peak is 3.8eV, which is larger than the expected standard peak width. Therefore, the observed peak may be composed of contributions from at least two overlapping peaks. Clark and Harrison 23 have indicated that the binding energy of C ls for both carbon atoms in the --C---CN structure suffer a shift of about 1-4 eV above the binding energy for the carbon atoms in a hydrocarbon environment. Thus, for the repeat structure (--CH2---C(CH3)(CN)--), it may be I

I

I

f

f

b

I

-9

-10 [.-.

_=

n

-11

-12

I 292

I

-15 1.58

1.60

I

I

1.62

1.64

.66

IO00/Tm (K-l) Fig. 7. Kissinger plot of PMAN at the first stage of the TGA higher heating rates (20, 15 and 10°C/min).

290

~

I

(C)

I

I

288 286 284 Binding E n e r g y (eV)

I 282

280

Fig. 8. XPS of C ls: (a) polymethacrylonitrile, (b) residue of PMAN after isothermal heating at 250°C for 1000 rain, (c) residue of PMAN after isothermal heating at 330°C for 1000 min.

Thermal degradation of polymethacrylonitrile assumed that the spectrum is composed of two peaks, one contribution from the ""-'CH3 and -----CH2-- groups and the other from the ---C----CN group. These two component peaks should have the same area. The C Is spectra of degradation residues after isothermal heating at 250°C for 1000min and 330°C for 1000 min are shown in Fig. 8(b) and (c), respectively. Comparing these spectra with that in Fig. 8(a), the spectra are observed to be asymmetric. There is a tail in the higher binding energy region. This can be explained by contributions from other carbon species. It has been assumed that the thermally stable residues are made up of a conjugated structure formed by cyclization of PMAN: 24 CH3

CH~

CH3

CH3

C

C

C

C

f

I

I

I

I

I

149 f

I

I

1 ~

(a)

(b) >

CH 3

(c)

\C

I

/ c \ N/c N/C N/C N/C 406

The tail in the higher binding energy region of the C ls spectrum may provide information to confirm this structure. First, the C ls in the - - N - - - C - - N ~ - structure may have a higher binding energy, which has been indicated in Fig. 8(c) by the arrow n. Second, we may consider the shake-up satellite, which has been indicated in Fig. 8(c) by the arrow m. Generally, polymers containing unsaturation have a shake-up satellite, but it is very weak. The conjugation will obviously increase the intensity of the shake-up satellite. 25 For a conjugated system, the shake-up satellite is typically 6 - 7 e V to high binding energy from the primary C ls peak. Previous research has shown that the satellite structure of a conjugated system is due to J r ~ J r * transitions? 6 The ring structure formed by PMAN is a conjugated system, so that its shake-up satellite may be detected in the higher binding energy region. In addition, the length of the conjugated sequence will determine the excitation energy difference between the :r and ~r* orbitals, which will cause the shake-up satellite to be a broad signal. Figure 9 shows the N ls spectra for these three samples. The spectra of the thermally stable residues have a broad peak which indicates the presence of different nitrogen groups, such as ~---C--N=C--, ---CN and --NH2. The other important piece of information

I

I

I

404

402

400

598

I

I

396

394

392

Binding Energy (eV) Fig. 9. XPS of N is: (a) polymethacrylonitrile, (b) residue of PMAN after isothermal heating at 250°C for 1000 min, (c) residue of PMAN after isothermal heating at 330°C for 1000 min.

obtained from the XPS spectra is the area ratio of the N ls and C ls peaks, which will be directly proportional to the ratio of the atomic concentrations. Table 2 shows the area ratios of N ls and C ls spectra for the three experimental samples, which indicate that the thermally stable residues have a lower ratio of N ls/C ls than PMAN. It may be assumed that the conjugated polyimine formed by internal cyclization of PMAN will undergo end-group reactions to produce ammonia and hydrogen cyanide, which has been observed in the thermal degradation of polyacrylonitrile. 27 This will lead to lower proportions of nitrogen in the stable residues. No oxygen signal could be detected in the survey spectrum of PMAN, but there was a small amount of oxygen in the survey spectrum of the stable residue. The area ratio of O ls/C ls in Sample 3 (330°C isothermal heating for 1000 min) Table 2. The area ratios of N l s / C Is for XPS of P M A N

Samples N ls/C ls

Sample 1 0.37

Sample 2 0.29

Sample 3 0.30

150

David J. T. Hill et al.

is about 0.09. Studies conducted by Grassie and Scott 28 have shown that the presence of traces of some impurities can induce P M A N to undergo internal cyclization. For example, internal cyclization may be initiated by methacrylic acid, so that oxygen may be concentrated in the final thermally stable residue, and thus be detected by XPS. For example: CH3

CH3

CH3

CH3

C

C

C

C

/CH2\ ICH2\ ]zCH2\ zCH2\ I

o C.o. CH3

I

CH3 /CH2

C I

I

N

N

CH3 /CH2

CH3 /CH2

CH3

I

\C

I

o//C~o/C%N H C N

/CHz~

REFERENCES

N

/CH2~ /CH2 C \C P

f

\

C] /CH2\ I

C N

CH3 CH3 CH3 /CH2~ /CH2~

\C

J

o~C~o~C~

/CH2,~

C

C

N~C~NH

C

I

reaction was found to be 52 kJ/mol. XPS studies of the thermally stable residue suggest that it is made up of a conjugated polyimine structure, which may undergo endgroup reactions to produce ammonia and hydrogen cyanide, so that the atomic concentration of nitrogen in the residues is lower than the value in PMAN.

I

N

CONCLUSIONS There are two reaction stages in the weight loss from the thermal degradation of P M A N in nitrogen. The first reaction stage is due to chainend initiation. The activation energy of the first reaction stage is 124 kJ/mol as determined by isothermal heating and 116kJ/mol as determined by TGA. The second reaction stage is due to random chain scission initiation. The activation energy of the second reaction stage is 235 kJ/mol, as determined by isothermal heating. A thermally stable residue is formed during the thermal degradation of PMAN. The yield of this residue is greater at low temperatures of isothermal heating and at the low heating rates in TGA. A possible explanation for the stability of the residue is internal cyclization of P M A N with the formation of a stable ring structure. The activation energy of this internal cyclization

1. Ho, B. C., Chin, W. K. & Lee, Y. D., J. Appl. Polym. Sci., 42 (1991) 99. 2. Schlege, L. & Schnabel, W., J. Vac. Sci. Technol., B6 (1988) 82. 3. Helbert, J. H., Cook, C. F., Chen, C. Y. & Pittman, C. U., J. Electrochem. Soc., 126 (1979) 694. 4. Helbert, J. H., Inflate, G. J., Pittman, C. U. & Lai, J. H., Polym,. Engng Sci., 20 (1980) 1077. 5. Acar, M. H. & Yagci, Y., Macromol. Reports, A28 (Suppl. 2) (1991) 177. 6. Buniyat, A. A., Plast. Massy., 5 (1990) 71. 7. Grassie, N. & McNeill, I. C., J. Polym. Sci., 39 (1959) 211. 8. Grassie, N. and McGuchan, R., Europ. Polym. J., 7 (1971) 1503. 9. Grassie, N. & McNeill, I. C., J. Polym. Sci., 30 (1958) 37. 10. Shaul, F. C., David, G. & Albert, Z., J. Polym. Sci., Polym. Chem. Ed., 10 (1972) 3109. 11. Braun, D., Cherdron, H. & Kern, W., Techniques of Polymer Syntheses and Characterization. John Wiley and Sons, New York, 1971. 12. MacCallum, J. R., In Comprehensive Polymer Science, Vol. 1, ed. G. Allen. Pergamon Press, Oxford, 1989, p. 903. 13. Dakin, T. W., Trans. AIEE, 67 (1984) 113. 14. David, P. K., IEEE Trans. Electrical Insulation, EI-22 (1987) 229. 15. Montanaris, G. C., IEEE Trans. Electrical Insulation, EI-26 (1988) 1057. 16. Galway, A. K., Advances in Catalysis, 26 (1977) 247. 17. Segal, E., Thermochimica Acta, 48 (1989) 127. 18. Agrawal, R. K., J. Thermal, Anal., 31 (1986) 73. 19. McNeill, I. C., Europ. Polym. J., 3 (1967) 409. 20. Grassie, N. & McGuchan, R., Europ. Polym. J., 8 (1972) 243. 21. Hirata, T., Kashiwagi, T. & Brown, J. E., Macromolecules, 18 (1985) 1410. 22. Kissinger, H. E., Anal. Chem., 29 (1957) 1702. 23. Clark, D. T. & Harrison, A., J. Polym. Sci., Polym. Chem. Ed., 19 (1981) 1945. 24. McNeill, I. C., In Comprehensive Polymer Science, Voi. 6, ed. G. Allen. Pergamon Press, Oxford, 1989, p. 452. 25. Briggs, D. & Riviere, J. C. In Practical Surface analysis by Auger and X-ray Photoelectron Spectroscopy, ed. D. Briggs, and M. P. Seah. John Wiley and Sons, New York, 1983. 26. Clark, D. T. & Dillks, A., J. Polym. Sci., Polym. Chem. Ed., 15 (1977) 15. 27. Peebles, L. H., Encycl. Polym. Sci. Technol. Suppl., 1 (1976) 1. 28. Grassie, N. & Scott, G., Polymer Degradation and Stabilization. Cambridge University Press, Cambridge, 1985, p. 53.