- Email: [email protected]
Chemiluminescence spectral evolution along the thermal oxidation of isotactic polypropylene
Polymer Degradation and Stability 65 (1999) 113±121
Chemiluminescence spectral evolution along the thermal oxidation of isotactic polypropylene Pilar Tiemblo a, Jose Manuel GoÂmez-Elvira a,*, Gilbert Teyssedre b, Francoise Massines b, Christian Laurent b a Instituto de Ciencia y TecnologõÂa de PolõÂmeros, CSIC, Juan de la Cierva, 3, 28006 Madrid, Spain Laboratoire de GeÂnie Electrique, Universite Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex, France
b
Received 24 September 1998; received in revised form 26 October 1998; accepted 30 October 1998
Abstract Several isotactic polypropylene samples have been thermally oxidised and their chemiluminescence recorded. The experimental device employed has allowed the simultaneous measurement of the integral and the spectral light emitted by the samples during oxidation. It has been found that the spectrum of chemiluminescence remains unchanged along the auto-acceleration stage of oxidation, while a gradual broadening and shift to longer wavelengths occurs as the kinetic curve approaches its intensity maximum, a shift which continues as long as oxidation does so. The shift to longer wavelengths originates in the disappearing of the lowwavelength emission (around 415 nm in isotactic polypropylene) which dominates the spectrum along the auto-acceleration stage, concomitant to a growing of longer wavelength components. Correlated stages exist in the kinetics and in the spectral evolution of the chemiluminescent emission along the thermal oxidation of polypropylene. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Polypropylene; Thermal oxidation; Chemiluminescence spectrum
1. Introduction The thermal degradation of polyole®ns has for long been studied with the aim of understanding its kinetics and its mechanism as this reaction, even at very low conversion levels, is responsible for the marked failure of almost every polymer property. The degradation mechanism is heterogeneous, as has been shown by chemiluminescence imaging [1] and microscopic viewing of stained oxidised domains [2] and several models have been proposed recently trying to describe the time evolution of the dierent degradation products [3] and the related chemiluminescence (CL hereafter) phenomenon [4±6] but there is not a conclusive description of how the degradation occurs. In fact, the kinetic equation of an homogeneous reaction has been successfully applied [7] usually to predict the time evolution of the concentration of the oxygenated species formed, although it is well known that this condition is not consistent with the very true nature of the degradation. Some
* Corresponding author. Tel.: +34-1-5622900; fax:+34-1-5644853; e-mail: [email protected]
heterogeneous models [3,7] only ®t the kinetic experimental data concerning the induction and the autoaccelerated periods of both the hydroperoxides in the degradation of LDPE or the CL-time curves in the case of polypropylene, but they do not account for the decrease observed in the content of these functional groups or in the CL intensity at long degradation times. In contrast, the assumptions made on Celina and coworkers [4,5] infectious spreading model are consistent with the evolution time of the CL intensity in the degradation of polypropylene. Making use of a removal coecient rate, the various chemiluminescence intensity behaviours after the auto-acceleration stage [8±16] are partly described, though a quantitative ®t has not been achieved. In short, the time evolution of the CL intensity has been thoroughly studied for a long time, however, this has not been so for the spectral features of the light, due to the diculties posed by the very low intensity of CL. The possibility of obtaining a spectral analysis of the light emitted during the oxidation, contemporary to the following of the total light emitted along the oxidation time, is an interesting tool which allows the investigation of virtual qualitative changes in the nature and
0141-3910/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0141 -3 910(98)00226 -2
114
P. Tiemblo et al. / Polymer Degradation and Stability 65 (1999) 113±121
distribution of the dierent emitting species. In this work, making use of the possibility to measure simultaneously the time evolution of the CL intensity and its spectra, we have undertaken the study of the CL emitted during the thermooxidation of a series of polypropylene samples, and its spectral analysis, from the induction period up to well after the end of the autoacceleration stage has been obtained. 2. Experimental 2.1. Sample preparation Three polypropylene samples have been used in the chemiluminescence experiments: Sample B is a commercially available additive-free polypropylene supplied by Repsol (PP050) which has been puri®ed with the pair solvent/non-solvent (o-dichlorobencene/methanol), at 125 C, with the stabiliser Irganox 1010 and under N2. Samples A and C are polypropylenes resulting from PP050 through consecutive fractionations with the pair of solvents 2-ethoxyethanol/xylene [17]. This method allows the low molecular weight fractions of the PP050 to be separated according to their isotacticity degree. Thus, sample C is the polymer obtained in the sixth fractionation step corresponding to the volume ratio 30/ 70 in the mixture of solvents, and sample A is the insoluble fraction which remains after the last step is performed. The ®lms have been made by hot pressing in a Collin-200 press under the temperature, pressure, time and number of processing cycles shown in Table 1. It is worth remarking that two cycles were needed to get an homogeneous ®lm of the insoluble fraction (sample A), causing this polypropylene to have a relative higher oxidation level (Fig. 1). 2.2. Sample characterisation 2.2.1. Molecular weight The viscosimetric average molecular weight was determined from the intrinsic viscosity of polymers dissolved in decaline and stabilised with Irganox 1010 under N2 at 135 C in a Ubbelhode viscosimeter (Table 2). 2.2.2. Tacticity The isotacticity has been measured by means of the absorbance ratio of the IR bands at 998ÿ1 and 973 cmÿ1 Table 1 Film preparation Sample
P (bar)
T ( C)
t (min)
No. of cycles
A B C
190 190 50
190 190 170
3 3 1
2 1 1
Fig. 1. Initial oxidation state of the samples as measured by FTIR. (a) Sample A, (b) sample C, and (c) sample B.
Table 2 Physico-chemical characteristics of the samples as used in the chemiluminescence experiment Sample Tm Tm Xc Mm (peak) (onset) (DSC) ( C) ( C) (%) A B C
± 167 165
± 157 157
46.0 42.2 38.7
Isotacticity (IR) (%)
215 000 96.6 186 000 94.2 68 000 93.2
Isotacticity (RMN) (%) 100 100 90
(A998 cmÿ1/A973 cmÿ1) using a quantitative approach in terms of the percentage of isotactic triads (%mm). Results appear in Table 2. It has been assumed that the calibration curve IR/13C-RMN given by Bur®eld et al. [18] is instrument independent. Spectra were recorded on hot-pressed ®lms using a Nicolet 520 FTIR spectrometer, 32 scans and 2 cmÿ1 resolution. 2.2.3. Crystallinity and melting point The values of crystallinity and melting point are presented in Table 2. They have been calculated from the enthalpy and the maximum of the melting peak obtained by calorimetry in a Perkin±Elmer DSC-7, after two runs, scanning at 10 C/min. The standard heat of melting for 100% crystalline polypropylene was H 209J=g [19]. 2.2.4. Initial oxidation state of the samples The carbonyl and hydroperoxide concentration of the ®lms have been measured by FTIR in the 1600± 1800 cmÿ1 and 3300±3600 cmÿ1 regions, respectively, under the same IR recording conditions as for the isotacticity measurement. The hydroperoxide and carbonyl content increases in the following order: Sample B
P. Tiemblo et al. / Polymer Degradation and Stability 65 (1999) 113±121
2.3. Chemiluminescence measurement 2.3.1. Technical features The experimental set-up is depicted in Fig. 2. It is designed in such a way that low level light emission from polymers may be measured in situ for various kinds of excitation, being either electrical ®eld [20], cold plasma [21], UV or temperature. The temperature regulation is achieved by means of a heating resistor associated with a liquid nitrogen reservoir with continuous ¯ow. The available temperature range is [ÿ180, +180 C]. Gas evacuation is made with a pumping unit composed of a roughing pump and a turbo-molecular pump producing a vacuum of the order 10ÿ7 mbars. For low temperature measurements (phosphorescence for example), gaseous helium is introduced at atmospheric pressure as exchange gas. The chamber is also equipped with electrical feedthroughs allowing high voltage input and measurement of parameters such as temperature or current if needed. The light tight chamber is organised around three optical axes equipped with windows and lenses made of quartz. The UV excitation for photoluminescence measurement is on axis 1. The available range of excitation wavelength is [220±800 nm]. In the con®guration used, the bandwidth of excitation is 2 nm. The wavelength dependence of the power of excitation produced on the sample holder has been estimated by means of a calibrated silicon photodiode. This calibration allows a correction for the responses of the xenon source, the monochromator and the optical coupling. It may be used to determine the relative value of photoluminescence for variable excitation wavelength.
115
On axis 2, integral light detection is made by means of a cooled (ÿ40 C) photomultiplier working in photon counting mode. Wide bandwidth (40 nm) ®lters may be inserted on the optical path. They are used for example to determine the phosphorescence lifetime or for acquiring rough spectra in case of very low emission level. The spectral analysis of the emitted light is made on axis 3 using a grating monochromator coupled to a liquid nitrogen cooled CCD camera. A mirror is moved on axis 2 for de¯ecting the light along axis 3 when spectra are recorded. The analysis range is [220±840 nm] and the resolution is 4.5 nm in the con®guration used. For both integral and wavelength resolved detection, the light is collected along a direction perpendicular to the plane of the sample ®lm (axis 2). 2.3.2. Procedure For CL measurements, samples were heated either in inert atmosphere (nitrogen) or in air. Films of 100 mm thickness, 22 cm2 area were deposited in a glass vessel in contact with the heating resistor. Samples have been kept in isothermal conditions for times up to 8 h and temperatures up to 165 C (Table 3). The total light emission was continuously registered with the photomultiplier (Fig. 2, axis 2) except in the time interval when spectra were recorded (Fig. 2, axis 3). Photoluminescence emission spectra have been recorded (Fig. 2, axis 1) using preferentially excitation wavelengths of 230 and 280 nm, corresponding to the usual excitation bands for polyole®ns [22]. The numbers inserted in the CL curves correspond to the spectra performed along the oxidation process. The
Fig. 2. Schematics of the experimental set-up.
116
P. Tiemblo et al. / Polymer Degradation and Stability 65 (1999) 113±121
Table 3 Chemiluminescence test conditions Sample
Oxidation atmosphere
Previous heating in N2
Temperature ( C)
A B B C
Air Air Air Air
No Yes No Yes
165 152 145 145
overall intensity shown by the spectra evolves with time in the same way as in the integral CL curves, as has been tested by integrating the spectra and representing the resulting intensity as a function of time. Elimination of peroxides by previous heating in nitrogen has been done in some selected tests (see Table 3). 3. Results According to the literature the typical CL curve consists of an initial intensity rise accompanying the temperature increase, a rise which stops as soon as isothermal conditions are achieved. The CL level remains then constant during a period of varying length which is known as the induction time. During this period no clear changes are seen in the IR spectra of the sample, even if a constant level of CL implies the formation of triplet carbonyl through oxidation reactions. This period ends with a rapid increase of CL, varying with time as a sigmoidal and the attaining of a plateau or a maximum. This sigmoidal increase of CL intensity, which is called the autoaccelerating period, can also be followed by IR spectroscopy, both by monitoring carbonyl concentration or peroxide build-up. Further oxidation leads to the appearance of other intensity maxima, which correspond to high oxidation levels. In Figs. 3±6 the CL kinetic curves and the spectral evolution of the CL emission of the samples tested in this work are shown. Each ®gure illustrates an example of the possible behaviours regarding kinetics, and they have been selected among a series of experiments for they allow good comparison on the eect of several conditions on the kinetic behaviour and on the spectral evolution: (i) the same polymer has been tested at two temperature values (Figs. 3 and 4), (ii) two polymers of dierent initial oxidation levels and dierent tacticity have been tested at the same temperature (Figs. 3 and 5), (iii) two polymers of increasing initial oxidation degree but of very similar structure have been tested at three temperatures (Figs. 3, 4 and 6), and (iv) in two of the tests, previous heating in nitrogen has been performed (Figs. 4 and 5). Bearing in mind the experimental conditions under which each measurement shown in Fig. 3±6 has been performed, a comparison of the kinetic behaviour but
Fig. 3. CL of sample B at 145 C. (a) kinetics (solid) and temperature ramp (dot). The numbered squares indicate the recording of a spectrum. (b) and (c) Spectral evolution of CL. The numbers of the spectra correspond to the numbered squares in (a). Spectrum 11 corresponds to a much longer oxidation time.
specially of the relationship between kinetics and spectral evolution of the light has been done. The simultaneous measurement of the CL overall intensity and of its spectral analysis has allowed accurate comparison of the CL spectrum along the oxidation in each of the four selected examples. In the results which appear in Figs. 3(a) and 5(a) the kinetic curve shows induction period, autoacceleration stage and a ®nal third stage, where intensity variation with time has the shape of a maximum [Fig. 5(a)] or a plateau [Fig. 3(a)]. Samples in Figs. 4(a) and 6(a) show no induction: for these samples the CL intensity quickly increases as soon as the temperature is risen because of
P. Tiemblo et al. / Polymer Degradation and Stability 65 (1999) 113±121
117
Fig. 4. CL of sample B at 152 C (a) kinetics (solid) and temperature ramp (dot). The numbered squares indicate the recording of a spectrum. (b) and (c) Spectral evolution of CL. The numbers of the spectra correspond to the numbered squares in (a).
Fig. 5. CL of sample C at 145 C: (a) kinetics (solid) and temperature ramp (dot). The numbered squares indicate the recording of a spectrum. (b) and (c) Spectral evolution of CL. The numbers of the spectra correspond to the numbered squares in (a).
the high temperature and/or initial concentration of hydroperoxides, and no induction period can be isolated. In addition, the CL continues to increase after the temperature achieves a constant value, a rise in intensity which corresponds to the autoacceleration stage. After autoacceleration ends, again, two distinct behaviours are found: either a plateau is reached [Fig. 4(a)] or a sharp maximum [Fig. 6(a)]. It must be born in mind that the behaviour of the sample in Fig. 4(a) after 10 000 s corresponds to a severe oxidation degree. In Figs. 3±6, together with the kinetics, the spectral evolution of the samples appears. In all cases real intensity comparisons can be made, for it has been possible to keep the CCD integration time the same (10 min per spectrum). The numbers in each spectrum correspond to
the numbers included in the kinetic curves, which indicate the moment at which the spectra were recorded. To enable easy comparison the spectra corresponding to the quick rise in intensity happening during the autoaccelerating period are represented together [3(b)±6(b)], while the spectra corresponding to the arrival at the plateau or to the maximum are represented in Figs. 3(c) to 5(c) and 6(b). Selected spectra have been normalised to the maximum emission in Fig. 7±10 . In the samples shown in Figs. 4 and 6, the separation in time of the end of the induction stage and the initiation of autoacceleration is not possible. This means that the ®rst spectrum recorded will contain light coming from chemiluminescent reactions and chemical environments characteristic of the ending of the induction per-
118
P. Tiemblo et al. / Polymer Degradation and Stability 65 (1999) 113±121
Fig. 6. CL of sample A at 165 C: (a) kinetics (the numbered squares indicate the recording of a spectrum). (b) Spectral evolution of CL. The numbers of the spectra correspond to the numbered squares in (a).
Fig. 7. Sample B's CL spectra at 145 C normalised to the emission maximum.
iod together with that coming from reactions and/or environments characteristic of the autoacceleration period of oxidation sites which have initiated oxidation as soon as the temperature began to rise. Thus, the ®rst spectrum recorded in these samples is not going to be considered in this work. A ®rst look at the spectra corresponding to the acceleration period [3(b)±5(b)] shows that during this period no change is seen in the spectrum of the emitted light. As a matter of fact, it seems that an emission, peaking at about 415 nm is the dominant one along this period. In eect, the normalised spectra which appear in Figs. 7±9 show that there is no evolution in the light emitted by the samples during the autoaccelerating period, for the coincidence of the normalised spectra recorded during this period is complete. As the plateau (or maximum) is reached, the CL spectra gradually begin to change: the low wavelength side emission near 415 nm, which has been the dominant one during the autoaccelerating period, shows a relative
decrease contemporary to a slight increase of the intensity from 490 nm onwards. This diminishing of the low wavelength side produces a broadening of the spectra and a shift in the emission to longer wavelengths (Figs. 7±10). The CL spectrum is exactly the same in Samples B and C during the autoaccelerating period. This is shown in the perfect coincidence of the normalised spectra which appear in Fig. 11(b) where we have also added the phosphorescence spectrum of Sample C obtained on irradiating at lexc 280 nm just before heating the polymer. An interesting feature revealed in Fig. 11(b) is that CL during the autoacceleration is composed mainly of one of the phosphorescence components, that at 415 nm, which is the less conspicuous of the components of the sample's phosphorescence. In fact, though this component is well seen in the second derivative of the spectrum, it is scarcely noticeable in the spectrum of the phosphorescence [Fig. 11(a)], due to its small relative intensity. The CL components which develop as oxidation proceeds and as the 415 nm emission looses intensity, i.e.,
P. Tiemblo et al. / Polymer Degradation and Stability 65 (1999) 113±121
119
Fig. 8. Sample B's CL spectra at 152 C normalised to the emission maximum.
those near 465 and 510 nm, are also components present in the phosphorescence of the original sample [Fig. 11(a)]. On the other hand, when oxidation becomes extreme the CL spectra shows emissions peaking at about 600 nm, which do not appear in the phosphorescence spectrum. This is the case of spectrum 11 in Fig. 3(c) (not included in the kinetic curve in Fig. 3(a) as it corresponds to a much longer oxidation time) and of some of the spectra which appear in Figs. 4(c) and 6(b). In sample A (Fig. 6), though kinetics are very dierent to those of sample B or sample C, the spectral evolution is very similar. The spectrum corresponding to the maximum of intensity [spectrum number 2, Fig. 6(b)] greatly resembles those corresponding to the autoacceleration of B or C. The spectrum of the light after the maximum [spectrum 3 onwards in Fig. 6(b)] evolves as in samples B or C. The intensity decrease is due to a decrease of the light at the low wavelength side, near 415 nm; as this low wavelength side decreases, the spectrum broadens in the high wavelength side, and ®nally a very broad emission peaking over 500 nm is found, very similar to spectrum 11 of sample B at 145 C [Fig. 3(c)] or to spectra 7, 8 and 9 of sample B at 152 [Fig. 4(c)].
Fig. 9. Sample C's CL spectra at 145 C normalised to the emission maximum.
4. Discussion In spite of the various shapes of the kinetic curves shown in Figs. 3(a)±6(a), a general trend exists in the evolution of the CL spectra with oxidation. That CL spectrum red-shifts on prolonged oxidation had already been reported by other authors [23] though the correlation between kinetic stages and CL spectral evolution had not been observed due to the absence of experiments enabling simultaneous integral and spectral light measurements. The evolution of the kinetic curve and of the luminescence spectra is easily explained if the spreading
Fig. 10. Sample A's CL spectra at 165 C normalised to the emission maximum.
model of Celina and co-workers [4,5] is adopted. A natural consequence of this model is that after the intensity maximum is achieved, no more unoxidised fractions are infected, and thus, oxidation is taking place in regions which have been already oxidised. In fact, this model predicts a content of the unoxidised
120
P. Tiemblo et al. / Polymer Degradation and Stability 65 (1999) 113±121
functions (roughly 440±510 nm), thus in the range of the main emissions of the phosphorescence spectrum [Fig. 11(a)], which are known to be due to conjugated carbonyl functions [22]. Finally, when extreme oxidation happens, CL peaks over 510 nm. Now, the exact reason why over-oxidation brings a shift to longer wavelengths is not clear. Some authors believe it is due to the creation of carbonylic functions near already created conjugated groups, while others believe that absorption and re-emission by conjugated species is the reason for the shift. In a previous paper [24] related to the existence of CL in the light emitted by a PP surface after a short treatment with a cold He plasma, and on the evolution of this CL as plasma treatments accumulate, we found that the CL due to the plasma attacks evolves in a way similar to that of the thermal CL : accumulation of treatments, i.e. of oxidation time, leads to the diminishing of roughly the same wavelength emission at 410 nm with the corresponding broadening and shift to higher wavelengths. In the case of the PP surface treated with a cold plasma, the coupling of newly created carbonyls to nearby existing ones, leading to a quenching of the emission was invoked [25] as one of the probable reasons for the diminishing of the emission at low wavelength (near 400 nm), which had been assigned to isolated carbonyl groups. In the thermooxidation, the coupling between carbonyls, which is related to the diculty in creating isolated carbonyls could be also playing a role in the diminishing of the low wavelength side of the spectrum. Fig. 11. (a) Sample C phosphorescence on irradiating at lexc 280 nm prior to heating and its second derivative; (b) normalised sample C phosphorescence (solid) compared to the normalised CL spectra along sample B (145 and 160 C) and sample C (145 C) spreading stage.
polymer fraction almost zero when the CL maximum is attained. Thus, after this model, during the spreading, an oxidation front which propagates in unoxidised parts develops, and from our data the oxidation reactions involved along this stage lead to a sharp emission at 415 nm. This emission must correspond to the generating of carbonylic functions in a mostly unoxidised aliphatic environment. As the plateau is reached, the relative importance of this emission at 415 nm diminishes leading to a gradual broadening and shifting of the spectra. This gradual evolution of the spectra must be due to the fact that, from the arrival to the plateau on, there is no oxidation of fresh parts of the polymer, but the ``over-oxidation'' of parts already infected. The consequence is a diminishing of the emission which originates in isolated carbonyls in an unoxidised aliphatic environment (415 nm), and the increase in the emission at wavelengths which correspond to conjugated carbonyl
5. Conclusion After a simultaneous study of the CL kinetic curve and of its spectral evolution in three isotactic PP during their thermal oxidation at high temperature, a correlation has been found between the well-known stages in the CL kinetic curve and the stages in the spectral evolution of CL. Excluding the induction period, classi®cation according to spectral features of the oxidation process renders the following: a ®rst stage in which the spectra do not evolve, which corresponds to the autoacceleration kinetic stage, a second stage where the lowwavelength emission gradually diminishes, and ®nally a third stage (severe degradation) during which new maxima at wavelengths over 510 nm appear. While the emissions occurring during the ®rst and the second stages are in the range of the phosphorescence spectrum of the samples, that is not the case for the long wavelength emissions which appear on extensive degradation along the third stage. The model proposed by Celina and co-workers, which relates the shape of the kinetic curve to the content of infected and unoxidised fraction of the polymer explains easily the evolution of this spectra during oxidation.
P. Tiemblo et al. / Polymer Degradation and Stability 65 (1999) 113±121
Acknowledgements One of us (P. Tiemblo) wishes to acknowledge ®nancial support from the Consejeria de EducacioÂn y Cultura de la Comunidad de Madrid. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Lacey DJ, Dudler V. Polym Degrad Stab 1996;51:101. Billingham NC. Makromol Chem Macromol Symp 1989:28;145. Gugumus F. Polym Degrad Stab 1996;52:131. Celina M, George GA, Billingham NC. Polym Degrad Stab 1993;42:335. George GA, Celina M, Lerf C, Cash G, Weddell D. Macromol Symp 1997;115:69. Audoin-Jirackova L, Verdu J. J Polym Sci: Chem Ed 1987;25:1205. Verdu S, Verdu J. Macromolecules 1997;30:2262. Matisova-Rychla L, Rychly J, Vavrekova M. Eur Polym J 1978;14:1033. Matisova-Rychla L, Rychly J, Verdu J, Audouin L, Csomorova K. Polym Degrad Stab 1995;49:51. George GA. In: Grassie N, editor. Development series. Developments in polymer degradation-3. Applied Science Publishers, 1981 (Chapter 6).
121
[11] Dudler V, Lacey DJ, KroÈhnke C. Polym Degrad Stab 1996;51:115. [12] Osawa Z, Wu S, Konoma F. Polym Degrad Stab 1988;22:97. [13] Kron A, Stenberg B, Reitberger T, Billingham NC. Polym Degrad Stab 1996;53:119. [14] Zhong X, Yoshii F, Sasaki T, Yagi T, Makuchi K. Polym Degrad Stab 1996;51:159. [15] Zlatkevich L. J Polym Sci: Polym Phys Ed 1985;23:1691. [16] Osawa Z, Kato M, Tenaro M. Macromol Rapid Commun 1997;18:667. [17] Lehtinen A, Paukken R. Macromol Chem Phys 1994;195: 1539. [18] Bur®eld DR, Loi PST. J Appl Polym Sci 1988:36;279. [19] Wunderlich B. Macromolecular physics, vol. II. New York: Academic Press, 1976. p. 149. [20] Albertini M, Mary D, Laurent C. J Phys D 1997;30:171. [21] Massines F, Tiemblo P, Teyssedre G, Laurent C. J Appl Phys 1997;81:937. [22] Allen NS, Homer J, McKellar JF. J Appl Polym Sci 1977;21:2261. [23] Lacey DJ, Dudler V. Polym Degrad Stab 1996;51:109. [24] Tiemblo P, GoÂmez-Elvira JM, Teyssedre G, Massines F, Laurent C. Polymer Degradation and Stability, in press. [25] Tiemblo P, GoÂmez-Elvira JM, Teyssedre G, Massines F, Laurent C. Polym Int 1998;46:33.
Chemiluminescence spectral evolution along the thermal oxidation of isotactic polypropylene Pilar Tiemblo a, Jose Manuel GoÂmez-Elvira a,*, Gilbert Teyssedre b, Francoise Massines b, Christian Laurent b a Instituto de Ciencia y TecnologõÂa de PolõÂmeros, CSIC, Juan de la Cierva, 3, 28006 Madrid, Spain Laboratoire de GeÂnie Electrique, Universite Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex, France
b
Received 24 September 1998; received in revised form 26 October 1998; accepted 30 October 1998
Abstract Several isotactic polypropylene samples have been thermally oxidised and their chemiluminescence recorded. The experimental device employed has allowed the simultaneous measurement of the integral and the spectral light emitted by the samples during oxidation. It has been found that the spectrum of chemiluminescence remains unchanged along the auto-acceleration stage of oxidation, while a gradual broadening and shift to longer wavelengths occurs as the kinetic curve approaches its intensity maximum, a shift which continues as long as oxidation does so. The shift to longer wavelengths originates in the disappearing of the lowwavelength emission (around 415 nm in isotactic polypropylene) which dominates the spectrum along the auto-acceleration stage, concomitant to a growing of longer wavelength components. Correlated stages exist in the kinetics and in the spectral evolution of the chemiluminescent emission along the thermal oxidation of polypropylene. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Polypropylene; Thermal oxidation; Chemiluminescence spectrum
1. Introduction The thermal degradation of polyole®ns has for long been studied with the aim of understanding its kinetics and its mechanism as this reaction, even at very low conversion levels, is responsible for the marked failure of almost every polymer property. The degradation mechanism is heterogeneous, as has been shown by chemiluminescence imaging [1] and microscopic viewing of stained oxidised domains [2] and several models have been proposed recently trying to describe the time evolution of the dierent degradation products [3] and the related chemiluminescence (CL hereafter) phenomenon [4±6] but there is not a conclusive description of how the degradation occurs. In fact, the kinetic equation of an homogeneous reaction has been successfully applied [7] usually to predict the time evolution of the concentration of the oxygenated species formed, although it is well known that this condition is not consistent with the very true nature of the degradation. Some
* Corresponding author. Tel.: +34-1-5622900; fax:+34-1-5644853; e-mail: [email protected]
heterogeneous models [3,7] only ®t the kinetic experimental data concerning the induction and the autoaccelerated periods of both the hydroperoxides in the degradation of LDPE or the CL-time curves in the case of polypropylene, but they do not account for the decrease observed in the content of these functional groups or in the CL intensity at long degradation times. In contrast, the assumptions made on Celina and coworkers [4,5] infectious spreading model are consistent with the evolution time of the CL intensity in the degradation of polypropylene. Making use of a removal coecient rate, the various chemiluminescence intensity behaviours after the auto-acceleration stage [8±16] are partly described, though a quantitative ®t has not been achieved. In short, the time evolution of the CL intensity has been thoroughly studied for a long time, however, this has not been so for the spectral features of the light, due to the diculties posed by the very low intensity of CL. The possibility of obtaining a spectral analysis of the light emitted during the oxidation, contemporary to the following of the total light emitted along the oxidation time, is an interesting tool which allows the investigation of virtual qualitative changes in the nature and
0141-3910/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0141 -3 910(98)00226 -2
114
P. Tiemblo et al. / Polymer Degradation and Stability 65 (1999) 113±121
distribution of the dierent emitting species. In this work, making use of the possibility to measure simultaneously the time evolution of the CL intensity and its spectra, we have undertaken the study of the CL emitted during the thermooxidation of a series of polypropylene samples, and its spectral analysis, from the induction period up to well after the end of the autoacceleration stage has been obtained. 2. Experimental 2.1. Sample preparation Three polypropylene samples have been used in the chemiluminescence experiments: Sample B is a commercially available additive-free polypropylene supplied by Repsol (PP050) which has been puri®ed with the pair solvent/non-solvent (o-dichlorobencene/methanol), at 125 C, with the stabiliser Irganox 1010 and under N2. Samples A and C are polypropylenes resulting from PP050 through consecutive fractionations with the pair of solvents 2-ethoxyethanol/xylene [17]. This method allows the low molecular weight fractions of the PP050 to be separated according to their isotacticity degree. Thus, sample C is the polymer obtained in the sixth fractionation step corresponding to the volume ratio 30/ 70 in the mixture of solvents, and sample A is the insoluble fraction which remains after the last step is performed. The ®lms have been made by hot pressing in a Collin-200 press under the temperature, pressure, time and number of processing cycles shown in Table 1. It is worth remarking that two cycles were needed to get an homogeneous ®lm of the insoluble fraction (sample A), causing this polypropylene to have a relative higher oxidation level (Fig. 1). 2.2. Sample characterisation 2.2.1. Molecular weight The viscosimetric average molecular weight was determined from the intrinsic viscosity of polymers dissolved in decaline and stabilised with Irganox 1010 under N2 at 135 C in a Ubbelhode viscosimeter (Table 2). 2.2.2. Tacticity The isotacticity has been measured by means of the absorbance ratio of the IR bands at 998ÿ1 and 973 cmÿ1 Table 1 Film preparation Sample
P (bar)
T ( C)
t (min)
No. of cycles
A B C
190 190 50
190 190 170
3 3 1
2 1 1
Fig. 1. Initial oxidation state of the samples as measured by FTIR. (a) Sample A, (b) sample C, and (c) sample B.
Table 2 Physico-chemical characteristics of the samples as used in the chemiluminescence experiment Sample Tm Tm Xc Mm (peak) (onset) (DSC) ( C) ( C) (%) A B C
± 167 165
± 157 157
46.0 42.2 38.7
Isotacticity (IR) (%)
215 000 96.6 186 000 94.2 68 000 93.2
Isotacticity (RMN) (%) 100 100 90
(A998 cmÿ1/A973 cmÿ1) using a quantitative approach in terms of the percentage of isotactic triads (%mm). Results appear in Table 2. It has been assumed that the calibration curve IR/13C-RMN given by Bur®eld et al. [18] is instrument independent. Spectra were recorded on hot-pressed ®lms using a Nicolet 520 FTIR spectrometer, 32 scans and 2 cmÿ1 resolution. 2.2.3. Crystallinity and melting point The values of crystallinity and melting point are presented in Table 2. They have been calculated from the enthalpy and the maximum of the melting peak obtained by calorimetry in a Perkin±Elmer DSC-7, after two runs, scanning at 10 C/min. The standard heat of melting for 100% crystalline polypropylene was H 209J=g [19]. 2.2.4. Initial oxidation state of the samples The carbonyl and hydroperoxide concentration of the ®lms have been measured by FTIR in the 1600± 1800 cmÿ1 and 3300±3600 cmÿ1 regions, respectively, under the same IR recording conditions as for the isotacticity measurement. The hydroperoxide and carbonyl content increases in the following order: Sample B
P. Tiemblo et al. / Polymer Degradation and Stability 65 (1999) 113±121
2.3. Chemiluminescence measurement 2.3.1. Technical features The experimental set-up is depicted in Fig. 2. It is designed in such a way that low level light emission from polymers may be measured in situ for various kinds of excitation, being either electrical ®eld [20], cold plasma [21], UV or temperature. The temperature regulation is achieved by means of a heating resistor associated with a liquid nitrogen reservoir with continuous ¯ow. The available temperature range is [ÿ180, +180 C]. Gas evacuation is made with a pumping unit composed of a roughing pump and a turbo-molecular pump producing a vacuum of the order 10ÿ7 mbars. For low temperature measurements (phosphorescence for example), gaseous helium is introduced at atmospheric pressure as exchange gas. The chamber is also equipped with electrical feedthroughs allowing high voltage input and measurement of parameters such as temperature or current if needed. The light tight chamber is organised around three optical axes equipped with windows and lenses made of quartz. The UV excitation for photoluminescence measurement is on axis 1. The available range of excitation wavelength is [220±800 nm]. In the con®guration used, the bandwidth of excitation is 2 nm. The wavelength dependence of the power of excitation produced on the sample holder has been estimated by means of a calibrated silicon photodiode. This calibration allows a correction for the responses of the xenon source, the monochromator and the optical coupling. It may be used to determine the relative value of photoluminescence for variable excitation wavelength.
115
On axis 2, integral light detection is made by means of a cooled (ÿ40 C) photomultiplier working in photon counting mode. Wide bandwidth (40 nm) ®lters may be inserted on the optical path. They are used for example to determine the phosphorescence lifetime or for acquiring rough spectra in case of very low emission level. The spectral analysis of the emitted light is made on axis 3 using a grating monochromator coupled to a liquid nitrogen cooled CCD camera. A mirror is moved on axis 2 for de¯ecting the light along axis 3 when spectra are recorded. The analysis range is [220±840 nm] and the resolution is 4.5 nm in the con®guration used. For both integral and wavelength resolved detection, the light is collected along a direction perpendicular to the plane of the sample ®lm (axis 2). 2.3.2. Procedure For CL measurements, samples were heated either in inert atmosphere (nitrogen) or in air. Films of 100 mm thickness, 22 cm2 area were deposited in a glass vessel in contact with the heating resistor. Samples have been kept in isothermal conditions for times up to 8 h and temperatures up to 165 C (Table 3). The total light emission was continuously registered with the photomultiplier (Fig. 2, axis 2) except in the time interval when spectra were recorded (Fig. 2, axis 3). Photoluminescence emission spectra have been recorded (Fig. 2, axis 1) using preferentially excitation wavelengths of 230 and 280 nm, corresponding to the usual excitation bands for polyole®ns [22]. The numbers inserted in the CL curves correspond to the spectra performed along the oxidation process. The
Fig. 2. Schematics of the experimental set-up.
116
P. Tiemblo et al. / Polymer Degradation and Stability 65 (1999) 113±121
Table 3 Chemiluminescence test conditions Sample
Oxidation atmosphere
Previous heating in N2
Temperature ( C)
A B B C
Air Air Air Air
No Yes No Yes
165 152 145 145
overall intensity shown by the spectra evolves with time in the same way as in the integral CL curves, as has been tested by integrating the spectra and representing the resulting intensity as a function of time. Elimination of peroxides by previous heating in nitrogen has been done in some selected tests (see Table 3). 3. Results According to the literature the typical CL curve consists of an initial intensity rise accompanying the temperature increase, a rise which stops as soon as isothermal conditions are achieved. The CL level remains then constant during a period of varying length which is known as the induction time. During this period no clear changes are seen in the IR spectra of the sample, even if a constant level of CL implies the formation of triplet carbonyl through oxidation reactions. This period ends with a rapid increase of CL, varying with time as a sigmoidal and the attaining of a plateau or a maximum. This sigmoidal increase of CL intensity, which is called the autoaccelerating period, can also be followed by IR spectroscopy, both by monitoring carbonyl concentration or peroxide build-up. Further oxidation leads to the appearance of other intensity maxima, which correspond to high oxidation levels. In Figs. 3±6 the CL kinetic curves and the spectral evolution of the CL emission of the samples tested in this work are shown. Each ®gure illustrates an example of the possible behaviours regarding kinetics, and they have been selected among a series of experiments for they allow good comparison on the eect of several conditions on the kinetic behaviour and on the spectral evolution: (i) the same polymer has been tested at two temperature values (Figs. 3 and 4), (ii) two polymers of dierent initial oxidation levels and dierent tacticity have been tested at the same temperature (Figs. 3 and 5), (iii) two polymers of increasing initial oxidation degree but of very similar structure have been tested at three temperatures (Figs. 3, 4 and 6), and (iv) in two of the tests, previous heating in nitrogen has been performed (Figs. 4 and 5). Bearing in mind the experimental conditions under which each measurement shown in Fig. 3±6 has been performed, a comparison of the kinetic behaviour but
Fig. 3. CL of sample B at 145 C. (a) kinetics (solid) and temperature ramp (dot). The numbered squares indicate the recording of a spectrum. (b) and (c) Spectral evolution of CL. The numbers of the spectra correspond to the numbered squares in (a). Spectrum 11 corresponds to a much longer oxidation time.
specially of the relationship between kinetics and spectral evolution of the light has been done. The simultaneous measurement of the CL overall intensity and of its spectral analysis has allowed accurate comparison of the CL spectrum along the oxidation in each of the four selected examples. In the results which appear in Figs. 3(a) and 5(a) the kinetic curve shows induction period, autoacceleration stage and a ®nal third stage, where intensity variation with time has the shape of a maximum [Fig. 5(a)] or a plateau [Fig. 3(a)]. Samples in Figs. 4(a) and 6(a) show no induction: for these samples the CL intensity quickly increases as soon as the temperature is risen because of
P. Tiemblo et al. / Polymer Degradation and Stability 65 (1999) 113±121
117
Fig. 4. CL of sample B at 152 C (a) kinetics (solid) and temperature ramp (dot). The numbered squares indicate the recording of a spectrum. (b) and (c) Spectral evolution of CL. The numbers of the spectra correspond to the numbered squares in (a).
Fig. 5. CL of sample C at 145 C: (a) kinetics (solid) and temperature ramp (dot). The numbered squares indicate the recording of a spectrum. (b) and (c) Spectral evolution of CL. The numbers of the spectra correspond to the numbered squares in (a).
the high temperature and/or initial concentration of hydroperoxides, and no induction period can be isolated. In addition, the CL continues to increase after the temperature achieves a constant value, a rise in intensity which corresponds to the autoacceleration stage. After autoacceleration ends, again, two distinct behaviours are found: either a plateau is reached [Fig. 4(a)] or a sharp maximum [Fig. 6(a)]. It must be born in mind that the behaviour of the sample in Fig. 4(a) after 10 000 s corresponds to a severe oxidation degree. In Figs. 3±6, together with the kinetics, the spectral evolution of the samples appears. In all cases real intensity comparisons can be made, for it has been possible to keep the CCD integration time the same (10 min per spectrum). The numbers in each spectrum correspond to
the numbers included in the kinetic curves, which indicate the moment at which the spectra were recorded. To enable easy comparison the spectra corresponding to the quick rise in intensity happening during the autoaccelerating period are represented together [3(b)±6(b)], while the spectra corresponding to the arrival at the plateau or to the maximum are represented in Figs. 3(c) to 5(c) and 6(b). Selected spectra have been normalised to the maximum emission in Fig. 7±10 . In the samples shown in Figs. 4 and 6, the separation in time of the end of the induction stage and the initiation of autoacceleration is not possible. This means that the ®rst spectrum recorded will contain light coming from chemiluminescent reactions and chemical environments characteristic of the ending of the induction per-
118
P. Tiemblo et al. / Polymer Degradation and Stability 65 (1999) 113±121
Fig. 6. CL of sample A at 165 C: (a) kinetics (the numbered squares indicate the recording of a spectrum). (b) Spectral evolution of CL. The numbers of the spectra correspond to the numbered squares in (a).
Fig. 7. Sample B's CL spectra at 145 C normalised to the emission maximum.
iod together with that coming from reactions and/or environments characteristic of the autoacceleration period of oxidation sites which have initiated oxidation as soon as the temperature began to rise. Thus, the ®rst spectrum recorded in these samples is not going to be considered in this work. A ®rst look at the spectra corresponding to the acceleration period [3(b)±5(b)] shows that during this period no change is seen in the spectrum of the emitted light. As a matter of fact, it seems that an emission, peaking at about 415 nm is the dominant one along this period. In eect, the normalised spectra which appear in Figs. 7±9 show that there is no evolution in the light emitted by the samples during the autoaccelerating period, for the coincidence of the normalised spectra recorded during this period is complete. As the plateau (or maximum) is reached, the CL spectra gradually begin to change: the low wavelength side emission near 415 nm, which has been the dominant one during the autoaccelerating period, shows a relative
decrease contemporary to a slight increase of the intensity from 490 nm onwards. This diminishing of the low wavelength side produces a broadening of the spectra and a shift in the emission to longer wavelengths (Figs. 7±10). The CL spectrum is exactly the same in Samples B and C during the autoaccelerating period. This is shown in the perfect coincidence of the normalised spectra which appear in Fig. 11(b) where we have also added the phosphorescence spectrum of Sample C obtained on irradiating at lexc 280 nm just before heating the polymer. An interesting feature revealed in Fig. 11(b) is that CL during the autoacceleration is composed mainly of one of the phosphorescence components, that at 415 nm, which is the less conspicuous of the components of the sample's phosphorescence. In fact, though this component is well seen in the second derivative of the spectrum, it is scarcely noticeable in the spectrum of the phosphorescence [Fig. 11(a)], due to its small relative intensity. The CL components which develop as oxidation proceeds and as the 415 nm emission looses intensity, i.e.,
P. Tiemblo et al. / Polymer Degradation and Stability 65 (1999) 113±121
119
Fig. 8. Sample B's CL spectra at 152 C normalised to the emission maximum.
those near 465 and 510 nm, are also components present in the phosphorescence of the original sample [Fig. 11(a)]. On the other hand, when oxidation becomes extreme the CL spectra shows emissions peaking at about 600 nm, which do not appear in the phosphorescence spectrum. This is the case of spectrum 11 in Fig. 3(c) (not included in the kinetic curve in Fig. 3(a) as it corresponds to a much longer oxidation time) and of some of the spectra which appear in Figs. 4(c) and 6(b). In sample A (Fig. 6), though kinetics are very dierent to those of sample B or sample C, the spectral evolution is very similar. The spectrum corresponding to the maximum of intensity [spectrum number 2, Fig. 6(b)] greatly resembles those corresponding to the autoacceleration of B or C. The spectrum of the light after the maximum [spectrum 3 onwards in Fig. 6(b)] evolves as in samples B or C. The intensity decrease is due to a decrease of the light at the low wavelength side, near 415 nm; as this low wavelength side decreases, the spectrum broadens in the high wavelength side, and ®nally a very broad emission peaking over 500 nm is found, very similar to spectrum 11 of sample B at 145 C [Fig. 3(c)] or to spectra 7, 8 and 9 of sample B at 152 [Fig. 4(c)].
Fig. 9. Sample C's CL spectra at 145 C normalised to the emission maximum.
4. Discussion In spite of the various shapes of the kinetic curves shown in Figs. 3(a)±6(a), a general trend exists in the evolution of the CL spectra with oxidation. That CL spectrum red-shifts on prolonged oxidation had already been reported by other authors [23] though the correlation between kinetic stages and CL spectral evolution had not been observed due to the absence of experiments enabling simultaneous integral and spectral light measurements. The evolution of the kinetic curve and of the luminescence spectra is easily explained if the spreading
Fig. 10. Sample A's CL spectra at 165 C normalised to the emission maximum.
model of Celina and co-workers [4,5] is adopted. A natural consequence of this model is that after the intensity maximum is achieved, no more unoxidised fractions are infected, and thus, oxidation is taking place in regions which have been already oxidised. In fact, this model predicts a content of the unoxidised
120
P. Tiemblo et al. / Polymer Degradation and Stability 65 (1999) 113±121
functions (roughly 440±510 nm), thus in the range of the main emissions of the phosphorescence spectrum [Fig. 11(a)], which are known to be due to conjugated carbonyl functions [22]. Finally, when extreme oxidation happens, CL peaks over 510 nm. Now, the exact reason why over-oxidation brings a shift to longer wavelengths is not clear. Some authors believe it is due to the creation of carbonylic functions near already created conjugated groups, while others believe that absorption and re-emission by conjugated species is the reason for the shift. In a previous paper [24] related to the existence of CL in the light emitted by a PP surface after a short treatment with a cold He plasma, and on the evolution of this CL as plasma treatments accumulate, we found that the CL due to the plasma attacks evolves in a way similar to that of the thermal CL : accumulation of treatments, i.e. of oxidation time, leads to the diminishing of roughly the same wavelength emission at 410 nm with the corresponding broadening and shift to higher wavelengths. In the case of the PP surface treated with a cold plasma, the coupling of newly created carbonyls to nearby existing ones, leading to a quenching of the emission was invoked [25] as one of the probable reasons for the diminishing of the emission at low wavelength (near 400 nm), which had been assigned to isolated carbonyl groups. In the thermooxidation, the coupling between carbonyls, which is related to the diculty in creating isolated carbonyls could be also playing a role in the diminishing of the low wavelength side of the spectrum. Fig. 11. (a) Sample C phosphorescence on irradiating at lexc 280 nm prior to heating and its second derivative; (b) normalised sample C phosphorescence (solid) compared to the normalised CL spectra along sample B (145 and 160 C) and sample C (145 C) spreading stage.
polymer fraction almost zero when the CL maximum is attained. Thus, after this model, during the spreading, an oxidation front which propagates in unoxidised parts develops, and from our data the oxidation reactions involved along this stage lead to a sharp emission at 415 nm. This emission must correspond to the generating of carbonylic functions in a mostly unoxidised aliphatic environment. As the plateau is reached, the relative importance of this emission at 415 nm diminishes leading to a gradual broadening and shifting of the spectra. This gradual evolution of the spectra must be due to the fact that, from the arrival to the plateau on, there is no oxidation of fresh parts of the polymer, but the ``over-oxidation'' of parts already infected. The consequence is a diminishing of the emission which originates in isolated carbonyls in an unoxidised aliphatic environment (415 nm), and the increase in the emission at wavelengths which correspond to conjugated carbonyl
5. Conclusion After a simultaneous study of the CL kinetic curve and of its spectral evolution in three isotactic PP during their thermal oxidation at high temperature, a correlation has been found between the well-known stages in the CL kinetic curve and the stages in the spectral evolution of CL. Excluding the induction period, classi®cation according to spectral features of the oxidation process renders the following: a ®rst stage in which the spectra do not evolve, which corresponds to the autoacceleration kinetic stage, a second stage where the lowwavelength emission gradually diminishes, and ®nally a third stage (severe degradation) during which new maxima at wavelengths over 510 nm appear. While the emissions occurring during the ®rst and the second stages are in the range of the phosphorescence spectrum of the samples, that is not the case for the long wavelength emissions which appear on extensive degradation along the third stage. The model proposed by Celina and co-workers, which relates the shape of the kinetic curve to the content of infected and unoxidised fraction of the polymer explains easily the evolution of this spectra during oxidation.
P. Tiemblo et al. / Polymer Degradation and Stability 65 (1999) 113±121
Acknowledgements One of us (P. Tiemblo) wishes to acknowledge ®nancial support from the Consejeria de EducacioÂn y Cultura de la Comunidad de Madrid. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Lacey DJ, Dudler V. Polym Degrad Stab 1996;51:101. Billingham NC. Makromol Chem Macromol Symp 1989:28;145. Gugumus F. Polym Degrad Stab 1996;52:131. Celina M, George GA, Billingham NC. Polym Degrad Stab 1993;42:335. George GA, Celina M, Lerf C, Cash G, Weddell D. Macromol Symp 1997;115:69. Audoin-Jirackova L, Verdu J. J Polym Sci: Chem Ed 1987;25:1205. Verdu S, Verdu J. Macromolecules 1997;30:2262. Matisova-Rychla L, Rychly J, Vavrekova M. Eur Polym J 1978;14:1033. Matisova-Rychla L, Rychly J, Verdu J, Audouin L, Csomorova K. Polym Degrad Stab 1995;49:51. George GA. In: Grassie N, editor. Development series. Developments in polymer degradation-3. Applied Science Publishers, 1981 (Chapter 6).
121
[11] Dudler V, Lacey DJ, KroÈhnke C. Polym Degrad Stab 1996;51:115. [12] Osawa Z, Wu S, Konoma F. Polym Degrad Stab 1988;22:97. [13] Kron A, Stenberg B, Reitberger T, Billingham NC. Polym Degrad Stab 1996;53:119. [14] Zhong X, Yoshii F, Sasaki T, Yagi T, Makuchi K. Polym Degrad Stab 1996;51:159. [15] Zlatkevich L. J Polym Sci: Polym Phys Ed 1985;23:1691. [16] Osawa Z, Kato M, Tenaro M. Macromol Rapid Commun 1997;18:667. [17] Lehtinen A, Paukken R. Macromol Chem Phys 1994;195: 1539. [18] Bur®eld DR, Loi PST. J Appl Polym Sci 1988:36;279. [19] Wunderlich B. Macromolecular physics, vol. II. New York: Academic Press, 1976. p. 149. [20] Albertini M, Mary D, Laurent C. J Phys D 1997;30:171. [21] Massines F, Tiemblo P, Teyssedre G, Laurent C. J Appl Phys 1997;81:937. [22] Allen NS, Homer J, McKellar JF. J Appl Polym Sci 1977;21:2261. [23] Lacey DJ, Dudler V. Polym Degrad Stab 1996;51:109. [24] Tiemblo P, GoÂmez-Elvira JM, Teyssedre G, Massines F, Laurent C. Polymer Degradation and Stability, in press. [25] Tiemblo P, GoÂmez-Elvira JM, Teyssedre G, Massines F, Laurent C. Polym Int 1998;46:33.