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New approach to understanding chemiluminescence from the decomposition of peroxidic structures in polypropylene
Polymer Degradation and Stability 67 (2000) 515±525
New approach to understanding chemiluminescence from the decomposition of peroxidic structures in polypropylene L. MatisovaÂ-RychlaÂ*, J. Rychly Polymer Institute, Slovak Academy of Sciences, 842 36 Bratislava, Slovak Republic Received 9 June 1999; received in revised form 25 August 1999; accepted 31 August 1999
Abstract It is suggested that the chemiluminescence from the thermal oxidation of polypropylene arises predominantly from the decomposition of associated hydroperoxides. Measurements of the chemiluminescence response to heating/cooling cycles applied to polypropylene oxidation in the induction period and in an advanced stage of the process is a novel approach to the extrapolation of the oxidation course to the lower temperature region. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Chemiluminescence; Polypropylene; Peroxides; Decomposition of peroxidic structures
1. Introduction The method of chemiluminescence, which measures the light emission accompanying chemical reactions mostly of oxidation type is not a new method Ð it is widely used in medicine and biochemistry as a very ecient analytical tool for determination of traces of hydrogen peroxide and other compounds [1]. The knowledge of the mechanisms of chemiluminescence reactions is far from being satisfactory but the method is used just because it works. In polymer chemistry, the fact that there exists some connection between the oxidizability of the material and the faint emission of light from it has been known since 1961 and 1964 when Ashby [2] and Schard and Russell [3,4] expressed the idea that chemiluminescence should provide us with a fundamental tool for the study of polymer degradation once the phenomenon is understood. However, the present acceptance of the method by the polymer industry does not correspond to such expectations regardless of the fact that analogue detection of the light signal has been replaced by an extremely sensitive photon counting technique [5,6] and by sensitive cameras enabling monitoring of the development of the oxidation along the sample surface [7±9]. The reasons for that may be summarized as follows: * Corresponding author. Tel.: 4217-5941-2599; fax: +4217-54775923. E-mail address: [email protected] (L. MatisovaÂ-RychlaÂ).
1. From the very beginning the method was developed in polymer research as if it should substitute classical tests of polymer stability like oxygen absorption measurements and oven ageing tests with the measurements of the loss of mechanical strength and the changes of the concentration of carbonyl groups and other oxidized structures. The materials were usually tested as received and measurements on standard and well characterized polymer samples are not very numerous despite the relatively large number of publications on the subject [10±33]. 2. There still is a lack of standard, reliable and easily manageable instruments having high sensitivity, good approach to the optical system and uniform temperature throughout the heating oven. Authors usually use proprietary apparatus designs with dierent sensitivity limits and dierent level of background signal discrimination. 3. It should be pointed out that the method should be developed mainly as an analytical tool in polymer chemistry providing the easy way of determination of e.g. concentration of hydroperoxides and investigation of peculiarities of various stabilizing systems. Only then it may bring a new dimension to understanding of the oxidation process particularly at temperatures close to ambient and ®nd the large acceptance by the industrial environment. This paper presents a new approach to the applicability of the chemiluminescence method in the oxidation of
0141-3910/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(99)00153-6
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polypropylene as well as to the study of degradation of oxidized polymers in an inert atmosphere. The main emphasis is put on the demonstration of signi®cant linkage between the chemiluminescence signal and the presence of associated oxidized structures, presumably of hydroperoxidic character. In combination with the measurement of chemiluminescence response to temperature cycling it might enable us to characterize the oxidation process in its early stages. 2. Experimental 2.1. Materials Polypropylene was an isotactic homopolymer from ATOCHEM. Films of 100 mm thickness were prepared from granules by compression moulding between Te¯on plates. The ®lms were extracted by a mixture of ethanol, chloroform and hexane (1:1:4) in a Soxhlet extractor for 30 h and then dried in a vacuum for 48 h at 30 C. The stabilized polypropylene sample was received from Clariant Huningue SA, France as the 100 mm thick ®lm. Peroxides in polypropylene were determined by iodometry. 2.2. Luminometers Two types of chemiluminescence devices were used which are produced and available as the commercial products at the Polymer Institute, Slovak Academy of Sciences, Bratislava, Slovak Republic. Lumipol 1 has the level of the background discrimination set at 0 counts/s, Lumipol 2 at 2 counts/s at 40 C. Lumipol 2 device, can adopt any reasonable temperature programme, such as e.g. cycling, i.e. heating and cooling of the sample within some temperature interval, etc. The details of the luminometers are available in lea¯ets from the authors. 3. Results and discussion 3.1. Oxidation of polypropylene Polypropylene belongs to the most frequently studied polymers from the viewpoint of its chemiluminescence during oxidation. Typical oxidation runs from 110±150 C for ®lms of unstabilized polymer are shown in the Fig. 1. The typical sigmoidal increase of the chemiluminescence - time curves and relatively strong intensity of the signal which at 120 C attains maximum value somewhere at 16 kHz/1 mg for the 100 mm thick ®lm, predisposes the polymer for detailed kinetic study. This is possible even at 70 C if one is patient enough to wait for the increase of the chemiluminescence intensity following
the so-called induction time. The decay after the maximum may be understood if one realizes that the chemiluminescence intensity re¯ects the rate of the process. If the accumulation of hydroperoxides governed by bimolecular initiation is the leading process of the oxidation, the chemiluminescence is expected to re¯ect the rate of concentration changes of hydroperoxides POOH, i.e. it will be proportional to the term [d[POOH]/dt]. The classical Bolland, Gee scheme [33] of hydrocarbon oxidation which involves the bimolecular decomposition of polymer hydroperoxides is: 2 POOH ! PO:2 PO: H2 O PO: PH ! POH P: P: O2 ! PO:2
kbi
ktr
k2
PO:2 PH ! POOH P: 2PO:2 ! products
k3
k6
(kbi, ktr, k2 and k6 are rate constants of bimolecular decomposition of hydroperoxides, transfer reaction of alkoxyl radicals to the polymer chain and recombination of two peroxy radicals, respectively.) Bearing in mind that such a scheme for the oxidation of solid polypropylene is valid only within some distance from the primary initiation site determined by the mobility of polymer segments and low molecular products we may use it for the further analysis of chemiluminescence-time runs. One should be also aware of the fact that averaging of individual and relatively separated microreactors in the polymer bulk may lead to compensation phenomena among respective parameters determined when using the scheme of homogeneous reaction kinetics. Such an approach, however, appears to us much more descriptive than that based on the solid state chemistry kinetics working with nucleation and spreading of reaction from the nucleation site. In the above scheme, we assume a very fast transfer reaction of alkoxyl radicals to a polymer chain. The analytical equation for the time changes of hydroperoxide concentration is as follows: A ; where A POOH1 ; 1 B exp ÿkt POOH1 ÿ POOH0 B POOH0 s k3 PH kbi POOH1 p and k k3 PH k6 2 k6 kbi POOH
The chemiluminescence intensity I, proportional to the rate of hydroperoxide concentration changes will be as follows:
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Fig. 1. Isothermal chemiluminescence-time curves for oxidation of polypropylene in the temperature interval 110±150 C. The initial mass of the sample 6 mg, oxygen ¯ow 3.4 l/h.
I dPOOH=dt
A0 exp ÿkt 1 B exp ÿkt2
ABk exp ÿkt 1 B exp ÿkt2 1
where and A0 are proportionality constants. However, the ®t of experimental curves by the function (1) is not very good when one wants to include the part of the curve after the maximum. We have tentatively found that excellent ®t may be achieved if we replace the rate constant k in the nominator of the term (1) by a more general constant r (Fig. 2). I
A0 exp ÿrt 1 B exp ÿkt2
2
As r<
0
A exp mt exp ÿkt 1 B exp ÿkt2
const exp mt dPOOH=dt
3
The parameters found by ®tting Eq. (2) to experimental chemiluminescence-time runs are in Table 1. The right-hand side of Eq. (3) is the product of the function describing the rate of concentration changes of hydroperoxides and a function representing an exponential increase. The latter function somewhat modulates the relation between chemiluminescence intensity and the rate of concentration changes of hydroperoxides
or between the integrated chemiluminescence intensitytime or chemiluminescence intensity-temperature curves vs hydroperoxide concentration and may explain the discrepancies observed in correlation between integrated chemiluminescence and initial concentration of hydroperoxides which was reported once to be a straight line [34] with a positive slope once a line with downward curvature from the straight line [16]. This modulating function may be attributed to the gradual appearance of light emitters (carbonyl groups) with higher quantum yield of chemiluminescence in the course of oxidation and their signi®cance will be higher in a more advanced stage of the process. While at the beginning of the process the light emitters are assumed to be isolated carbonyls, in an advanced stage they may be a, b-unsaturated carbonyls.
Table 1 Parameters of Eq. (2) ®tted to the experimental chemiluminescence intensity-time curves from thermal oxidation of unstabilized polypropylene (6 mg) Temperature ( C)
A0 counts
B
r104a (sÿ1)
k104 (sÿ1)
70 90 100 110 120 130 140 150
3 31 55 78 80 82 183 292
148.4 152.9 91.1 88.2 25.8 24.2 17.4 9.3
0.0134 0.12 0.19 0.377 0.394 0.0107 1.08 0.901
0.0986 0.713 1.18 3.29 5.79 10.50 17.79 31.1
a
090 700 800 120 500 400 000 000
This value is sensitive to the development of CL intensity-time curve after the maximum.
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From this viewpoint, the comprehension of the mechanism of chemiluminescence in an oxidized polypropylene should be adopted. Until now it was commonly accepted that the light is emitted from carbonyl triplets being formed in a synchronous way in suciently exothermic reaction like e.g. in recombination of peroxidic radicals according to the Russell scheme, without considering the possibility of the change of quality of light emitter:
2P1 P2 CHO:2 ! O2 P1 P2 CHOH P1 P2 C O 4 P1 and P2 are polymer segments. (According to this scheme oxygen should be formed in a singlet state). This will lead to the linear correlation between the integrated chemiluminescence and concentration. However, in
Fig. 2. The chemiluminescence-time curves for oxidized polypropylene at 90 and 100 C. The initial mass of the polymer Ð 3 mg, oxygen, ¯ow 3.4 l/ h. Points correspond to the theoretical ®t by Eq. (2).
Fig. 3. Logarithmic coordinates of respective chemiluminescence-time runs for the temperature interval 70±150 C.
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an advanced stage of oxidation the quality of the polymer should stepwise change including the quality of the light emitters and the above correlation deviates from linearity. Of interest is also the steady decay of chemiluminescence within the induction period before the ®nal increase takes place (Fig. 3). This occurs regardless of whether the polymer is unstabilized or stabilized and will correspond to the annealing of initially present initiating species like catalyst residues, etc. 3.2. Chemiluminescence at the degradation of preoxidized polypropylene in inert atmosphere Polypropylene ®lms preoxidized to the level of peroxides 208 mmol/kg emit luminescence when heated in nitrogen atmosphere (Fig. 4). The decay of luminescence is apparently faster then one should expect from the rate constant of decomposition of hydroperoxides determined iodometrically. An idea was expressed in a paper [16] that the minority fraction of peroxides in the polymer is responsible for the light emission. One should, however, have in mind that the measured decays of the luminescence are rate curves which re¯ect the governing process of the decay. Provided that this governing process is not a pure ®rst order reaction it is very dicult to approximate the total area below the curve exactly as it is the function of the initial concentration of hydroperoxides in the polymer. We have repeated the experiments from the paper [16] in an attempt to found the proper approximation function of chemiluminescence intensity decay. Because of the zero background of our luminometer
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Table 2 The comparison of the rate constants of hydroperoxides decomposition estimated by iodometry and by CL Temperature ( C)
k from fractional surfaces below CL curve (sÿ1)
70 80 90 100 110 120
3.33 1.02 2.97 9.60 1.92 3.36
10ÿ6 10ÿ5 10ÿ5 10ÿ5 10ÿ4 10ÿ4
kdecomposition iodometry faster process (sÿ1) 4.2 10ÿ6 2.8 10ÿ5 8.7 10ÿ5 3.0 10ÿ4 1.4, 6.8 10ÿ4
Reference
35 35 36 37 37,38
we could also verify the approximation experimentally. The approximation we have used may be expressed as the sum of rates of second and ®rst order process, respectively. I
a c exp ÿdt 1 bt2
where t is time and a,b,c and d are parameters. We have found that this function ®ts the experimental curve of chemiluminescence decay of preoxidized materials for a temperature interval within 70±120 C and time of measurement within 0±72 000 s (20 h). The area (TLI) below the experimental curve for the total decay of chemiluminescence is: TLI Idt a=b c=d
Fig. 4. Chemiluminescence intensity-time runs for polypropylene preoxidized to the level of 208 mmol of peroxides/kg of polymer. The measurements were in nitrogen ¯ow 3.4 l/h.
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and usually it is at least two times higher than that calculated from the measurements for a given time. This shifts the determination of the apparent rate constants of the ®rst order magnitude to the values which are in excellent agreement with rate constants determined
iodometrically referred to the faster process of hydroperoxide decomposition in nitrogen atmosphere. This may be an important indication that chemiluminescence from oxidized polypropylene predominantly re¯ects the kinetics related to the decomposition of associated hydroperoxides.
Fig. 5. The decay of peroxides obtained by the transformation of isothermal chemiluminescence intensity-time runs from the Fig. 4 extrapoled to the in®nity time for the temperature interval 70±120 C.
Fig. 6. RAMP experiments in nitrogen of preoxidized polypropylene, the rate of heating 5 C/min. The oxidation at 140 C measured by chemiluminescence shows the line 1. Time at each nonisothermal curve indicates the instant of interruption of isothermal oxidation before the RAMP experiment. X denotes the polymer melting.
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The comparison of values of kdecomposition from iodometry for faster process of hydroperoxide decomposition and values of determined from the fractional surfaces below the chemiluminescence curve which have both the magnitude of the ®rst order is shown in the Table 2. The metamorphosis of chemiluminescence-time curves (Fig. 4) to those for fractional surfaces vs. time related to the initial concentration of peroxides is in Fig. 5. Another veri®cation of the kinetics of presumably associated hydroperoxides decomposition are so-called RAMP experiments where the sample of preoxidized polymer is heated from a certain lower temperature to higher temperature at a linear rate until the signal Table 3 Parameters of RAMP experiments in nitrogen for samples of polypropylene peroxidized in oxygen at 140 C, the rate of temperature increase in RAMP experiment 5 C/min Time of previous oxidation at 140 C (s)
Area below RAMP curve (counts)
Preexponential factor Ai of the ®rst order rate constant (sÿ1)
500 1000 1515 2641 4764
2 9 15 26 32
3.45 1.09 1.47 2.13 3.72
854 336 480 505 274
200 600 000 600 000
1011 1011 1010 109 108
Activation energy (kJ/mol) 112.9 109.2 101.9 94.9 90.0
521
intensity goes through a maximum and back to a lower value. These experiments were performed on Lumipol 1 with somewhat dierent geometry and zero level of signal discrimination at 40 C. The ®lms of unstabilized polypropylene were oxidized at 140 C in oxygen for a certain time directly in the oven of chemiluminescence instrument, then they were cooled down and a RAMP experiment continued in nitrogen after 30 min of sample ¯ushing by nitrogen at room temperature. Such nonisothermal runs may be seen in Fig. 6. We should like to focus the reader's attention on the residual chemiluminescence at 200 C which is not in any case the chemiluminescence from the background. It is related to the polymer and to the presence of better energy acceptors than single carbonyl groups which appear there as a consequence of hydroperoxides decomposition. As the melting of the polymer somewhat distorted the nonisothermal curves just at the temperature of melting, the ®tting by nonisothermal second order kinetics involving only one second order process, namely the decomposition of associated hydroperoxides was performed within the temperature interval 60±150 C. The experimental curves are ®tted by this nonisothermal second order ®t with a high accuracy and corresponding parameters are given in Table 3. The extrapolated value of rate constant k to the zero area below the RAMP curve (zero level of associated hydroperoxides) calculated from these parameters for 70 C is 2.5 10ÿ6
Fig. 7. Chemiluminescence response for the temperature cycling of unstabilized polypropylene within 60±130 C by the rate of heating and cooling 5 C/min. After the ®rst ®ve cycles (A) the measurement was interrupted, the sample kept at room temperature overnight and then continued as cycles 6±8 (B). The temperature cycles are represented by the line C. The initial mass of the sample 3 mg, atmosphere oxygen, ¯ow 3.4 l/h.
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sÿ1 which again corresponds well with experimental determination of kinetics of polypropylene hydroperoxide decomposition in nitrogen ascribed to the faster process by Gijsman [35]. We recall that the rate constants for both cases have the magnitude of the ®rst order.
3.3. Chemiluminescence response to the cycling of temperature during the oxidation of polymers The eect of dierent shape of chemiluminescence-time curve which is the case of dierent polymers may be eliminated by the measurements of the chemiluminescence
Fig. 8. Arrhenius' plots of 2nd and 7th cycle for the sample of polypropylene from Fig. 7.
Fig. 9. Arrhenius' plots of the chemiluminescence increase corresponding to the increase of temperature during 1±8 cycle for the sample from Fig. 7.
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response when temperature is cycled within some interval at a constant rate. The illustrative case of such an approach may be seen in Fig. 7 where 8 cycles of heating and cooling with the rate 5 C/min were applied to polypropylene in oxygen within 50±130 C. One can realize that each cycle will characterize the oxidative state of polymer within a given short time interval. As it may be seen from the area of the Arrhenius' hysteresis curve for 2nd and 7th cycle (Fig. 8) the concentration of hydroperoxides does not change signi®cantly during one cycle. If we plot the increase of chemiluminescence in Arrhenius' coordinates we can thus follow how the character of the oxidation process changes during the progress of the cycling (Fig. 9). The Arrhenius' curve is not linear but upward curved from the straight line in the lower region of temperatures. Table 4 The change of the activation energy Ea of oxidation for the temperatures above 100 C with the increasing number of heating/cooling cycles within 50±130 C. Polypropylene, the rate of heating and cooling was 5 C/min The sequence number of the cycle
Ea (kJ/mol)
1 2 3 4 5 6 7 8
79.5 94.0 96.9 105.8 102.1 104.5 107.2 106.9
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The curvature becomes less distinct with the increasing number of temperature cycles. The activation energy of higher temperature region goes to the saturated level corresponding to 107 kJ/mol, approximately (Table 4). The ®rst cycle quite obviously re¯ects the eect of sample history on the start of oxidation which manifests itself in the reduction of activation energy to 80 kJ/mol. Polypropylene stabilized with hindered phenol which under isothermal conditions at 150 C in oxygen gave an induction time of oxidation of 5.5 days was cycled in the range 80±160 C under oxygen atmosphere at the same rate of heating and cooling (Fig. 10). This is of particular interest because the Arrhenius' plots depicted e.g. for 2nd and 6th temperature increase show a much more distinct curvature than those for unstabilized samples (Fig. 11); as if the rate of oxidation is less and less dependent on temperature when going to lower temperatures. The rate of photon emission from unstabilized and stabilized samples becomes practically equal at certain temperature. As we do not know the contribution of phenol itself to the total chemiluminescence, we cannot do any conclusion from this. We have e.g. found that chemiluminescence signals from polymer samples containing phenols may be aected essentially by the illumination by visible light. As we see in the Fig. 10 the peak maximum decays with the progress of cycling. It is likely to correspond with the gradual consumption of a stabilizer during the oxidation. The weaker dependence of the induction time of oxidation of stabilized polypropylene in the temperatures below 100 C which eventually converts to a stronger dependence below and at around 80 C was reported by Gugumus [39].
Fig. 10. Nine cycles of temperature within 80±160 C with corresponding chemiluminescence response for stabilized polypropylene. The induction period of the sample at 150 C in oxygen was 5.5 days. The initial mass of the sample Ð 3 mg.
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Fig. 11. Arrhenius' plots for the increase of chemiluminescence for corresponding increase of temperature for 2nd and 6th cycle of stabilized polypropylene from Fig. 10 and for the increase of temperature for the 6th cycle of sample from Fig. 7. Full lines are exponential ®ts of the plots ln I vs 1/T.
Application of the above kinetic schemes to the chemiluminescence response on temperature cycling will be the subject of a future paper. 4. Conclusions 1. The chemiluminescence emitted from thermally treated preoxidized polypropylene in nitrogen atmosphere appears to be due to the decomposition of associated hydroperoxides. 2. The kinetics of the appearance of potential light emitters (carbonyl groups) having changing quality from single carbonyls to e.g. a, b-unsaturated carbonyl aect both the kinetics of isothermal chemiluminescence intensity-time runs as well as the total chemiluminescence signal. 3. The response of chemiluminescence to temperature cycling which cuts the total chemiluminescence event into separate stages provides a new insight into the interpretation of chemiluminescence nonisothermal experiments. This may ®nd its use particularly in the case of stabilized samples. 4. The decay of the steady level of chemiluminescence within the so-called induction period of oxidation before the advanced stage of the process takes place, which was observed for both stabilized and unstabilized polypropylene samples should be taken into account when applying either homogeneous or heterogeneous kinetic approaches.
Acknowledgements The authors acknowledge cooperation with L. Achimsky, ENSAM Paris, who provided the samples of carefully puri®ed unstabilized polypropylene. The paper was prepared with ®nancial support from Grant Agency VEGA, Slovak Academy of Sciences; project No. 2/ 4018/1998.
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New approach to understanding chemiluminescence from the decomposition of peroxidic structures in polypropylene L. MatisovaÂ-RychlaÂ*, J. Rychly Polymer Institute, Slovak Academy of Sciences, 842 36 Bratislava, Slovak Republic Received 9 June 1999; received in revised form 25 August 1999; accepted 31 August 1999
Abstract It is suggested that the chemiluminescence from the thermal oxidation of polypropylene arises predominantly from the decomposition of associated hydroperoxides. Measurements of the chemiluminescence response to heating/cooling cycles applied to polypropylene oxidation in the induction period and in an advanced stage of the process is a novel approach to the extrapolation of the oxidation course to the lower temperature region. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Chemiluminescence; Polypropylene; Peroxides; Decomposition of peroxidic structures
1. Introduction The method of chemiluminescence, which measures the light emission accompanying chemical reactions mostly of oxidation type is not a new method Ð it is widely used in medicine and biochemistry as a very ecient analytical tool for determination of traces of hydrogen peroxide and other compounds [1]. The knowledge of the mechanisms of chemiluminescence reactions is far from being satisfactory but the method is used just because it works. In polymer chemistry, the fact that there exists some connection between the oxidizability of the material and the faint emission of light from it has been known since 1961 and 1964 when Ashby [2] and Schard and Russell [3,4] expressed the idea that chemiluminescence should provide us with a fundamental tool for the study of polymer degradation once the phenomenon is understood. However, the present acceptance of the method by the polymer industry does not correspond to such expectations regardless of the fact that analogue detection of the light signal has been replaced by an extremely sensitive photon counting technique [5,6] and by sensitive cameras enabling monitoring of the development of the oxidation along the sample surface [7±9]. The reasons for that may be summarized as follows: * Corresponding author. Tel.: 4217-5941-2599; fax: +4217-54775923. E-mail address: [email protected] (L. MatisovaÂ-RychlaÂ).
1. From the very beginning the method was developed in polymer research as if it should substitute classical tests of polymer stability like oxygen absorption measurements and oven ageing tests with the measurements of the loss of mechanical strength and the changes of the concentration of carbonyl groups and other oxidized structures. The materials were usually tested as received and measurements on standard and well characterized polymer samples are not very numerous despite the relatively large number of publications on the subject [10±33]. 2. There still is a lack of standard, reliable and easily manageable instruments having high sensitivity, good approach to the optical system and uniform temperature throughout the heating oven. Authors usually use proprietary apparatus designs with dierent sensitivity limits and dierent level of background signal discrimination. 3. It should be pointed out that the method should be developed mainly as an analytical tool in polymer chemistry providing the easy way of determination of e.g. concentration of hydroperoxides and investigation of peculiarities of various stabilizing systems. Only then it may bring a new dimension to understanding of the oxidation process particularly at temperatures close to ambient and ®nd the large acceptance by the industrial environment. This paper presents a new approach to the applicability of the chemiluminescence method in the oxidation of
0141-3910/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(99)00153-6
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polypropylene as well as to the study of degradation of oxidized polymers in an inert atmosphere. The main emphasis is put on the demonstration of signi®cant linkage between the chemiluminescence signal and the presence of associated oxidized structures, presumably of hydroperoxidic character. In combination with the measurement of chemiluminescence response to temperature cycling it might enable us to characterize the oxidation process in its early stages. 2. Experimental 2.1. Materials Polypropylene was an isotactic homopolymer from ATOCHEM. Films of 100 mm thickness were prepared from granules by compression moulding between Te¯on plates. The ®lms were extracted by a mixture of ethanol, chloroform and hexane (1:1:4) in a Soxhlet extractor for 30 h and then dried in a vacuum for 48 h at 30 C. The stabilized polypropylene sample was received from Clariant Huningue SA, France as the 100 mm thick ®lm. Peroxides in polypropylene were determined by iodometry. 2.2. Luminometers Two types of chemiluminescence devices were used which are produced and available as the commercial products at the Polymer Institute, Slovak Academy of Sciences, Bratislava, Slovak Republic. Lumipol 1 has the level of the background discrimination set at 0 counts/s, Lumipol 2 at 2 counts/s at 40 C. Lumipol 2 device, can adopt any reasonable temperature programme, such as e.g. cycling, i.e. heating and cooling of the sample within some temperature interval, etc. The details of the luminometers are available in lea¯ets from the authors. 3. Results and discussion 3.1. Oxidation of polypropylene Polypropylene belongs to the most frequently studied polymers from the viewpoint of its chemiluminescence during oxidation. Typical oxidation runs from 110±150 C for ®lms of unstabilized polymer are shown in the Fig. 1. The typical sigmoidal increase of the chemiluminescence - time curves and relatively strong intensity of the signal which at 120 C attains maximum value somewhere at 16 kHz/1 mg for the 100 mm thick ®lm, predisposes the polymer for detailed kinetic study. This is possible even at 70 C if one is patient enough to wait for the increase of the chemiluminescence intensity following
the so-called induction time. The decay after the maximum may be understood if one realizes that the chemiluminescence intensity re¯ects the rate of the process. If the accumulation of hydroperoxides governed by bimolecular initiation is the leading process of the oxidation, the chemiluminescence is expected to re¯ect the rate of concentration changes of hydroperoxides POOH, i.e. it will be proportional to the term [d[POOH]/dt]. The classical Bolland, Gee scheme [33] of hydrocarbon oxidation which involves the bimolecular decomposition of polymer hydroperoxides is: 2 POOH ! PO:2 PO: H2 O PO: PH ! POH P: P: O2 ! PO:2
kbi
ktr
k2
PO:2 PH ! POOH P: 2PO:2 ! products
k3
k6
(kbi, ktr, k2 and k6 are rate constants of bimolecular decomposition of hydroperoxides, transfer reaction of alkoxyl radicals to the polymer chain and recombination of two peroxy radicals, respectively.) Bearing in mind that such a scheme for the oxidation of solid polypropylene is valid only within some distance from the primary initiation site determined by the mobility of polymer segments and low molecular products we may use it for the further analysis of chemiluminescence-time runs. One should be also aware of the fact that averaging of individual and relatively separated microreactors in the polymer bulk may lead to compensation phenomena among respective parameters determined when using the scheme of homogeneous reaction kinetics. Such an approach, however, appears to us much more descriptive than that based on the solid state chemistry kinetics working with nucleation and spreading of reaction from the nucleation site. In the above scheme, we assume a very fast transfer reaction of alkoxyl radicals to a polymer chain. The analytical equation for the time changes of hydroperoxide concentration is as follows: A ; where A POOH1 ; 1 B exp ÿkt POOH1 ÿ POOH0 B POOH0 s k3 PH kbi POOH1 p and k k3 PH k6 2 k6 kbi POOH
The chemiluminescence intensity I, proportional to the rate of hydroperoxide concentration changes will be as follows:
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Fig. 1. Isothermal chemiluminescence-time curves for oxidation of polypropylene in the temperature interval 110±150 C. The initial mass of the sample 6 mg, oxygen ¯ow 3.4 l/h.
I dPOOH=dt
A0 exp ÿkt 1 B exp ÿkt2
ABk exp ÿkt 1 B exp ÿkt2 1
where and A0 are proportionality constants. However, the ®t of experimental curves by the function (1) is not very good when one wants to include the part of the curve after the maximum. We have tentatively found that excellent ®t may be achieved if we replace the rate constant k in the nominator of the term (1) by a more general constant r (Fig. 2). I
A0 exp ÿrt 1 B exp ÿkt2
2
As r<
0
A exp mt exp ÿkt 1 B exp ÿkt2
const exp mt dPOOH=dt
3
The parameters found by ®tting Eq. (2) to experimental chemiluminescence-time runs are in Table 1. The right-hand side of Eq. (3) is the product of the function describing the rate of concentration changes of hydroperoxides and a function representing an exponential increase. The latter function somewhat modulates the relation between chemiluminescence intensity and the rate of concentration changes of hydroperoxides
or between the integrated chemiluminescence intensitytime or chemiluminescence intensity-temperature curves vs hydroperoxide concentration and may explain the discrepancies observed in correlation between integrated chemiluminescence and initial concentration of hydroperoxides which was reported once to be a straight line [34] with a positive slope once a line with downward curvature from the straight line [16]. This modulating function may be attributed to the gradual appearance of light emitters (carbonyl groups) with higher quantum yield of chemiluminescence in the course of oxidation and their signi®cance will be higher in a more advanced stage of the process. While at the beginning of the process the light emitters are assumed to be isolated carbonyls, in an advanced stage they may be a, b-unsaturated carbonyls.
Table 1 Parameters of Eq. (2) ®tted to the experimental chemiluminescence intensity-time curves from thermal oxidation of unstabilized polypropylene (6 mg) Temperature ( C)
A0 counts
B
r104a (sÿ1)
k104 (sÿ1)
70 90 100 110 120 130 140 150
3 31 55 78 80 82 183 292
148.4 152.9 91.1 88.2 25.8 24.2 17.4 9.3
0.0134 0.12 0.19 0.377 0.394 0.0107 1.08 0.901
0.0986 0.713 1.18 3.29 5.79 10.50 17.79 31.1
a
090 700 800 120 500 400 000 000
This value is sensitive to the development of CL intensity-time curve after the maximum.
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From this viewpoint, the comprehension of the mechanism of chemiluminescence in an oxidized polypropylene should be adopted. Until now it was commonly accepted that the light is emitted from carbonyl triplets being formed in a synchronous way in suciently exothermic reaction like e.g. in recombination of peroxidic radicals according to the Russell scheme, without considering the possibility of the change of quality of light emitter:
2P1 P2 CHO:2 ! O2 P1 P2 CHOH P1 P2 C O 4 P1 and P2 are polymer segments. (According to this scheme oxygen should be formed in a singlet state). This will lead to the linear correlation between the integrated chemiluminescence and concentration. However, in
Fig. 2. The chemiluminescence-time curves for oxidized polypropylene at 90 and 100 C. The initial mass of the polymer Ð 3 mg, oxygen, ¯ow 3.4 l/ h. Points correspond to the theoretical ®t by Eq. (2).
Fig. 3. Logarithmic coordinates of respective chemiluminescence-time runs for the temperature interval 70±150 C.
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an advanced stage of oxidation the quality of the polymer should stepwise change including the quality of the light emitters and the above correlation deviates from linearity. Of interest is also the steady decay of chemiluminescence within the induction period before the ®nal increase takes place (Fig. 3). This occurs regardless of whether the polymer is unstabilized or stabilized and will correspond to the annealing of initially present initiating species like catalyst residues, etc. 3.2. Chemiluminescence at the degradation of preoxidized polypropylene in inert atmosphere Polypropylene ®lms preoxidized to the level of peroxides 208 mmol/kg emit luminescence when heated in nitrogen atmosphere (Fig. 4). The decay of luminescence is apparently faster then one should expect from the rate constant of decomposition of hydroperoxides determined iodometrically. An idea was expressed in a paper [16] that the minority fraction of peroxides in the polymer is responsible for the light emission. One should, however, have in mind that the measured decays of the luminescence are rate curves which re¯ect the governing process of the decay. Provided that this governing process is not a pure ®rst order reaction it is very dicult to approximate the total area below the curve exactly as it is the function of the initial concentration of hydroperoxides in the polymer. We have repeated the experiments from the paper [16] in an attempt to found the proper approximation function of chemiluminescence intensity decay. Because of the zero background of our luminometer
519
Table 2 The comparison of the rate constants of hydroperoxides decomposition estimated by iodometry and by CL Temperature ( C)
k from fractional surfaces below CL curve (sÿ1)
70 80 90 100 110 120
3.33 1.02 2.97 9.60 1.92 3.36
10ÿ6 10ÿ5 10ÿ5 10ÿ5 10ÿ4 10ÿ4
kdecomposition iodometry faster process (sÿ1) 4.2 10ÿ6 2.8 10ÿ5 8.7 10ÿ5 3.0 10ÿ4 1.4, 6.8 10ÿ4
Reference
35 35 36 37 37,38
we could also verify the approximation experimentally. The approximation we have used may be expressed as the sum of rates of second and ®rst order process, respectively. I
a c exp ÿdt 1 bt2
where t is time and a,b,c and d are parameters. We have found that this function ®ts the experimental curve of chemiluminescence decay of preoxidized materials for a temperature interval within 70±120 C and time of measurement within 0±72 000 s (20 h). The area (TLI) below the experimental curve for the total decay of chemiluminescence is: TLI Idt a=b c=d
Fig. 4. Chemiluminescence intensity-time runs for polypropylene preoxidized to the level of 208 mmol of peroxides/kg of polymer. The measurements were in nitrogen ¯ow 3.4 l/h.
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and usually it is at least two times higher than that calculated from the measurements for a given time. This shifts the determination of the apparent rate constants of the ®rst order magnitude to the values which are in excellent agreement with rate constants determined
iodometrically referred to the faster process of hydroperoxide decomposition in nitrogen atmosphere. This may be an important indication that chemiluminescence from oxidized polypropylene predominantly re¯ects the kinetics related to the decomposition of associated hydroperoxides.
Fig. 5. The decay of peroxides obtained by the transformation of isothermal chemiluminescence intensity-time runs from the Fig. 4 extrapoled to the in®nity time for the temperature interval 70±120 C.
Fig. 6. RAMP experiments in nitrogen of preoxidized polypropylene, the rate of heating 5 C/min. The oxidation at 140 C measured by chemiluminescence shows the line 1. Time at each nonisothermal curve indicates the instant of interruption of isothermal oxidation before the RAMP experiment. X denotes the polymer melting.
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The comparison of values of kdecomposition from iodometry for faster process of hydroperoxide decomposition and values of determined from the fractional surfaces below the chemiluminescence curve which have both the magnitude of the ®rst order is shown in the Table 2. The metamorphosis of chemiluminescence-time curves (Fig. 4) to those for fractional surfaces vs. time related to the initial concentration of peroxides is in Fig. 5. Another veri®cation of the kinetics of presumably associated hydroperoxides decomposition are so-called RAMP experiments where the sample of preoxidized polymer is heated from a certain lower temperature to higher temperature at a linear rate until the signal Table 3 Parameters of RAMP experiments in nitrogen for samples of polypropylene peroxidized in oxygen at 140 C, the rate of temperature increase in RAMP experiment 5 C/min Time of previous oxidation at 140 C (s)
Area below RAMP curve (counts)
Preexponential factor Ai of the ®rst order rate constant (sÿ1)
500 1000 1515 2641 4764
2 9 15 26 32
3.45 1.09 1.47 2.13 3.72
854 336 480 505 274
200 600 000 600 000
1011 1011 1010 109 108
Activation energy (kJ/mol) 112.9 109.2 101.9 94.9 90.0
521
intensity goes through a maximum and back to a lower value. These experiments were performed on Lumipol 1 with somewhat dierent geometry and zero level of signal discrimination at 40 C. The ®lms of unstabilized polypropylene were oxidized at 140 C in oxygen for a certain time directly in the oven of chemiluminescence instrument, then they were cooled down and a RAMP experiment continued in nitrogen after 30 min of sample ¯ushing by nitrogen at room temperature. Such nonisothermal runs may be seen in Fig. 6. We should like to focus the reader's attention on the residual chemiluminescence at 200 C which is not in any case the chemiluminescence from the background. It is related to the polymer and to the presence of better energy acceptors than single carbonyl groups which appear there as a consequence of hydroperoxides decomposition. As the melting of the polymer somewhat distorted the nonisothermal curves just at the temperature of melting, the ®tting by nonisothermal second order kinetics involving only one second order process, namely the decomposition of associated hydroperoxides was performed within the temperature interval 60±150 C. The experimental curves are ®tted by this nonisothermal second order ®t with a high accuracy and corresponding parameters are given in Table 3. The extrapolated value of rate constant k to the zero area below the RAMP curve (zero level of associated hydroperoxides) calculated from these parameters for 70 C is 2.5 10ÿ6
Fig. 7. Chemiluminescence response for the temperature cycling of unstabilized polypropylene within 60±130 C by the rate of heating and cooling 5 C/min. After the ®rst ®ve cycles (A) the measurement was interrupted, the sample kept at room temperature overnight and then continued as cycles 6±8 (B). The temperature cycles are represented by the line C. The initial mass of the sample 3 mg, atmosphere oxygen, ¯ow 3.4 l/h.
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sÿ1 which again corresponds well with experimental determination of kinetics of polypropylene hydroperoxide decomposition in nitrogen ascribed to the faster process by Gijsman [35]. We recall that the rate constants for both cases have the magnitude of the ®rst order.
3.3. Chemiluminescence response to the cycling of temperature during the oxidation of polymers The eect of dierent shape of chemiluminescence-time curve which is the case of dierent polymers may be eliminated by the measurements of the chemiluminescence
Fig. 8. Arrhenius' plots of 2nd and 7th cycle for the sample of polypropylene from Fig. 7.
Fig. 9. Arrhenius' plots of the chemiluminescence increase corresponding to the increase of temperature during 1±8 cycle for the sample from Fig. 7.
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response when temperature is cycled within some interval at a constant rate. The illustrative case of such an approach may be seen in Fig. 7 where 8 cycles of heating and cooling with the rate 5 C/min were applied to polypropylene in oxygen within 50±130 C. One can realize that each cycle will characterize the oxidative state of polymer within a given short time interval. As it may be seen from the area of the Arrhenius' hysteresis curve for 2nd and 7th cycle (Fig. 8) the concentration of hydroperoxides does not change signi®cantly during one cycle. If we plot the increase of chemiluminescence in Arrhenius' coordinates we can thus follow how the character of the oxidation process changes during the progress of the cycling (Fig. 9). The Arrhenius' curve is not linear but upward curved from the straight line in the lower region of temperatures. Table 4 The change of the activation energy Ea of oxidation for the temperatures above 100 C with the increasing number of heating/cooling cycles within 50±130 C. Polypropylene, the rate of heating and cooling was 5 C/min The sequence number of the cycle
Ea (kJ/mol)
1 2 3 4 5 6 7 8
79.5 94.0 96.9 105.8 102.1 104.5 107.2 106.9
523
The curvature becomes less distinct with the increasing number of temperature cycles. The activation energy of higher temperature region goes to the saturated level corresponding to 107 kJ/mol, approximately (Table 4). The ®rst cycle quite obviously re¯ects the eect of sample history on the start of oxidation which manifests itself in the reduction of activation energy to 80 kJ/mol. Polypropylene stabilized with hindered phenol which under isothermal conditions at 150 C in oxygen gave an induction time of oxidation of 5.5 days was cycled in the range 80±160 C under oxygen atmosphere at the same rate of heating and cooling (Fig. 10). This is of particular interest because the Arrhenius' plots depicted e.g. for 2nd and 6th temperature increase show a much more distinct curvature than those for unstabilized samples (Fig. 11); as if the rate of oxidation is less and less dependent on temperature when going to lower temperatures. The rate of photon emission from unstabilized and stabilized samples becomes practically equal at certain temperature. As we do not know the contribution of phenol itself to the total chemiluminescence, we cannot do any conclusion from this. We have e.g. found that chemiluminescence signals from polymer samples containing phenols may be aected essentially by the illumination by visible light. As we see in the Fig. 10 the peak maximum decays with the progress of cycling. It is likely to correspond with the gradual consumption of a stabilizer during the oxidation. The weaker dependence of the induction time of oxidation of stabilized polypropylene in the temperatures below 100 C which eventually converts to a stronger dependence below and at around 80 C was reported by Gugumus [39].
Fig. 10. Nine cycles of temperature within 80±160 C with corresponding chemiluminescence response for stabilized polypropylene. The induction period of the sample at 150 C in oxygen was 5.5 days. The initial mass of the sample Ð 3 mg.
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Fig. 11. Arrhenius' plots for the increase of chemiluminescence for corresponding increase of temperature for 2nd and 6th cycle of stabilized polypropylene from Fig. 10 and for the increase of temperature for the 6th cycle of sample from Fig. 7. Full lines are exponential ®ts of the plots ln I vs 1/T.
Application of the above kinetic schemes to the chemiluminescence response on temperature cycling will be the subject of a future paper. 4. Conclusions 1. The chemiluminescence emitted from thermally treated preoxidized polypropylene in nitrogen atmosphere appears to be due to the decomposition of associated hydroperoxides. 2. The kinetics of the appearance of potential light emitters (carbonyl groups) having changing quality from single carbonyls to e.g. a, b-unsaturated carbonyl aect both the kinetics of isothermal chemiluminescence intensity-time runs as well as the total chemiluminescence signal. 3. The response of chemiluminescence to temperature cycling which cuts the total chemiluminescence event into separate stages provides a new insight into the interpretation of chemiluminescence nonisothermal experiments. This may ®nd its use particularly in the case of stabilized samples. 4. The decay of the steady level of chemiluminescence within the so-called induction period of oxidation before the advanced stage of the process takes place, which was observed for both stabilized and unstabilized polypropylene samples should be taken into account when applying either homogeneous or heterogeneous kinetic approaches.
Acknowledgements The authors acknowledge cooperation with L. Achimsky, ENSAM Paris, who provided the samples of carefully puri®ed unstabilized polypropylene. The paper was prepared with ®nancial support from Grant Agency VEGA, Slovak Academy of Sciences; project No. 2/ 4018/1998.
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