Oxyluminescence of polyamide 12

Polymer Degradation and Stability 44 (1994) 335-341 L ~, i " ' /

ELSEVIER

Oxyluminescence of polyamide 12 A . T c h a r k h t c h i , °'* L. A u d o u i n , ° J. M . T r e m i l l o n b & J. Verdu" aENSAM, 151 Boulevard de l'Hopital, 75013 Paris, France bATOCHEM, Groupement de Recherches de Lacq, RN 117, BP 34, 64170 Lacq, France (Received 3 November 1993; accepted 3 January 1994)

The oxyluminescence of unstabilised samples of polyamide 12 was studied in the temperature range 80-250°C. The chemiluminescence intensity was first recorded at constant oxygen pressure and temperature. The kinetic curves reveal the existence of two kinetic stages. The first one is a burst whose intensity is linked to the radical concentration at the onset of oxygen admission. The second one corresponds to the steady-state oxidation chain process in which an autoaccelerated character attributed to branching is observed. A discontinuity is observed in Arrhenius plots of maximum intensities at the melting point, Tf. A possible explanation is that the peroxyl bimolecular terminating combination is governed by segmental motion at T < Tf and by the radical intrinsic reactivity at T > Tf. Perturbed experiments in which oxygen is abruptly suppressed, and programmed temperature experiments in which preoxidised samples are exposed in nitrogen reveal clearly that hydroperoxide decomposition contributes to oxyluminescence.

INTRODUCTION

Indeed, the oxidative attack of other methylenes is not completely excluded and the whole ageing phenomenon can be probably described as a co-oxidation process involving both types of ----CH2-- groups with different propagation rate constants. Most of the published literature deals with PA6 and PA66. It seemed to us interesting to study polyamides with longer polymethylenic sequences, which could bring interesting data on the relative importance of both types of methylenes in the propagation of oxidation chains. The aim of this article is to report the results of a preliminary study in which some important features of the polyamide 12 (PAl2) CL were studied.

It is now well known that aliphatic polyamides emit a relatively strong chemiluminescence (CL) during their oxidative ageing. 1 The fact that they are more emissive than polyethylene is to be related to the fact that the most reactive site towards hydrogen abstraction is the traminomethylene. 2 ~C--NH~H2--(CH2

,~._ l POO)

II O ~ , C - - N H~(~. H--(CH2,)7,~_ ~ II O ,,MC--NH II O

CH--(CH2 ,~._, I OO"

1

EXPERIMENTAL

reactions * To whom all correspondence should be addressed.

Material

This paper was presented at the 18th Annual Meeting of the UK Polymer Degradation Discussion Group held at the Bolton Institute on 15-17 September 1993.

An unstabilised sample of P A l 2 was supplied by A T O C H E M . Its melting point is 181°C. Films of 60 + 5/zm thickness were used for this study.

Polymer Degradation and Stability 0141-3910/94/$07.00 © 1994 Elsevier Science Limited. 335

336

A. Tcharkhtchi et al. 1(mY) 600

-

(a)

400

t

1

(b)

2O0

IIN2~)2 ~O2 t

T l

(c)

0 Tl ~

~ 0

" ~ D'a-t

IjL

TI T2 T Fig. 1. Different methods of chemiluminescence experiments: (a) classical, (b) perturbed, and (c) non-isothermal.

Chemiluminescence measurements

A previously described laboratory-made apparatus 3 was used in this study. Three types of experiments were performed (Fig. 1). (a) The first (classical) one (Fig. l(a)) consists of preheating the sample under nitrogen for 3 min. When thermal equilibrium is reached, air is abruptly admitted at atmospheric pressure and the CL signal is recorded in isothermal conditions. (b) The second (perturbed) type (Fig. l(b)) experiment consists of recording the CL response to very fast atmosphere changes f r o m 0 2 to N2 and vice versa. Such experiments are also performed in isothermal conditions. (c) The third (non-isothermal) type (Fig. 1(c)) experiment was done only on preoxidised samples. It consists of recording the CL signal during a temperature programmed scan from ambient temperature to about 500K, at 10K/min scanning rate, under nitrogen. RESULTS A N D DISCUSSION Classical experiments

Some CL kinetic curves obtained in conditions (a) at different temperatures are presented in

t

I 10

:

20

'l 30

((min)

Fig. 2. Time variations of luminescence intensity in air at (1) 200°C, (2) 180°C, and (3) 170°C.

Fig. 2. They display two peaks revealing the existence of at least two distinct kinetic stages. First stage

A zoom view of the first peak is presented in Fig. 3. This stage is characterised by very fast intensity changes with time constants not longer than a few minutes. It can be attributed to the peculiar behaviour of alkyl radicals accumulated

(,nV)

200 160

Ji

120

5

4

3

8O

400 0

21 5

10

15

t ([hiJ0

Pig. 3. Zoom view of the first stage at (1) 160°C, (2) 165°C, (3) 170°C, (4) 175°C, (5) la0°c, (6) 200°C, (7) 205°C, and (8) 210°C.

Oxyluminescence of polyamide 12 during the preheating stage under inert atmosphere. These alkyl radicals are almost instantaneously transformed into peroxy radicals directly responsible for CL emission, for instance via the well-known Russell mechanism." It could be reasonably supposed that the PO2 maximum concentration (at the peak) is sharply linked to the P" concentration just before 02 admission: [P']o ~ [POz]m

337

I(mV) 4OO

300

200

100 .

(1) o 0

02

02

i\'2

The fast intensity decrease would be due to the fact that [PO2],. is higher than the concentration [PO~]s corresponding to the stationary regime of the chain oxidation of polymer. In the standard, unbranched, non-diffusion-controlled mechanistic scheme, the following would be obtained:

i 10

i

20

30

t (mia)

Fig. 4. Influence of the duration of preheating under nitrogen on the first stage at 180°C after: 2 rain preheating (1), 12 min preheating (2), and 29 rain preheating (3).

(2)

simplified. It was used only to indicate a way of interpretation for the first kinetic stage. The peak intensity increases with the duration of preheating stage (Fig. 4). This shows that the concentration of alkyl radicals resulting of the polymer thermolysis has not reached its stationary state in the time-scale under consideration. A simple kinetic model was already proposed to explain this behaviour. 6 As a consequence, [P']0< (ri/k4) ~/2 and conditions can be imagined where [PO~]m = [P']0 -> [POE],. In this case, no intensity decrease would be expected in the first kinetic stage, which seems to be the case in low temperature experiments (Fig. 3).

After oxygen admission, when the stationary regime of chain oxidation is reached, only processes (I), (II), (III) and (VI) are to be considered, and the corresponding peroxy radical concentration is given by

Second stage This stage can be attributed to the steady-state chain oxidation process. In the standard mechanistic scheme, the CL intensity, I, is expected to vary with initiation rate:

(I) (II) (III) (IV) (V) (VI)

PH (polymer) --* P" (radicals) P" + 02 --, PO; PO2 + PH ~ PO2H + P" P" + P" ~ inactive products P" + PO~ ~ i n a c t i v e products PO~ + PO~ --~ inactive products + hv

r~ k2 k3 k4 k5

k6

During the preheating stage, only processes (I) and (IV) can occur. In a first approach, it will be assumed, for simplicity, that this anaerobic process reached to its stationary regime before the end of the preheating stage. Then [P']o

= ( ri ] ''2 \~/

[PO~]~

= ( ri ~ 1/2 \~/

(3)

According to a former hypothesis, the maximum peroxyl concentration would be such as [P02] m

\~44/

(4)

If [PO~],, is different of [PO2], the system is out of equilibrium at the peak maximum and an intensity decrease is expected if [PO~],, > [PO~]~, i.e. if k6 > k4. The values of these rate constants are unknown in the case under study, but data on model compounds such as tetralin ((k6/k4)~--3 (Ref. 5)) seem to indicate the good trend. The above mechanistic scheme is over-

I = tp

x

k6 × [PO~] z = ~

X ri

(5)

where tp is the CL yield. If the only source of alkyl radicals is reaction (I) (polymer thermolysis), the intensity is expected to remain constant or to decrease at high conversions owing to the depletion of active sites. In fact, an intensity increase is first observed, which suggests that the initiation rate increases with time. The most probable explanation is that branching occurs, so that hydroperoxides participate more and more in initiation events (or directly to CL events, see below). The final intensity decrease is presumably linked to the consumption of reactive c~-aminomethylenes. The Arrhenius plots for maximum intensities at the first and second peak are shown in Fig. 5.

338

A. Tcharkhtchi et al.

collision is the rate-determining step in solid stage, whereas it does not play a kinetic role in liquid state, owing to the high molecular mobility.

In Im

9-

87 6 5 4 3

i

1.9

i

2.0

i

2.1

i

2.2

I

2.3

2.4

Fig. 5. Arrhenius plots for (171) the first, and ( 0 ) the second peak intensity.

They display a slope change at the melting point (181°C). The apparent activation energy and pre-exponential coefficient decrease from solid to liquid state (Table 1). There are many possible reasons for a such discontinuity. (1) The key variable is the overall concentration of active sites in the amorphous phase (crystallites are impermeable to oxygen). In this case, the substrate concentration [PH] must increase from solid to liquid state and an increase of the preexponential factor is expected. Experimental results display the opposite trend (Fig. 5), showing that the reaction rate is not very sensitive to changes of the PH concentration. (2) The increase of polymer reactivity towards oxygen. For instance, chain folds at the crystal surfaces could be especially reactive. But in the case where this effect would be predominant, an intensity decrease would be expected. (3) CL results from an intermolecular process, for instance, the bimolecular peroxy radical combination: PO2 + PO2--* [P=O]* + P - - O H + O2 [ P ~ O ] * --* f ~ O + h v

Table 1. Apparent activation energies for the first and the second peak intensity Ea (kJ/moi)

Perturbed experiments Let us now consider the results of perturbed experiments, especially the intensity decay linked to nitrogen admission (conditions (b)). Various time-scales are to be considered here. (1) The time-scale of the decrease of oxygen partial pressure, Z(O2). The reactor purging conditions have been optimized in order to obtain Z(O2) values of about 5 s. Indeed, kinetic data on phenomena having characteristic times lower than Z(O2) are inaccessible with our apparatus. (2) The time-scale Z(PO2) of CL decay due to the terminating combination of PO2 radicals. It could be in principle determined from rate constants, for instance in the case of the standard mechanistic scheme. It could be equated to the half-life time of PO2 radicals when oxygen supply in stopped whereas oxidation is in steady state. Then O(PO2) _ dt

k6[PO212

1 k6 x Z(PO2) = [PO2]s

(6)

i.e. using eqn (3):

In this case, it could be imagined that the segmental motion allowing the radical

Intensity maximal

Supplementary investigations are needed to elucidate the mechanism of this discontinuity of Arrhenius plots of the CL intensity but, henceforward, CL appears as a very interesting tool to study the changes of oxidation kinetics at transition points.

R2

First peak

110 53

for T < T f for T > Tf

0.959 0.897

Second peak

206 104

for T < Tf for T > Tf

0.994 0-962

1 Z(PO2) = ~/r, x k6

(7)

Although precise data on rate constants is scarce, it can be recalled that k6 is usually very high (for instance~ 107 1/mols for tetralin oxidation at ambient temperatureS). It is not unreasonable to suppose that rlk6 >- 1/s so that Z(PO2) - 1 s, and this phenomenon cannot be observed with our system, k6 is almost temperatureindependent whereas ri is an increasing

Oxyluminescence of polyamide 12

339

(nN) (mV) 150

5

N POIZfAMIDE ~ -

POL~POXY

4 3

100

1

2

50 1

O~ 0

2

4

6

8

10

12

14

t (rain)

o

L ~

O'~

,i ; (b)

(a)

8

[ (nlin)

Fig. 6. Comparison of intensity decay in nitrogen between (a) polyamide 12 and (b) a previously studied epoxy sample."

function of temperature. Thus, it could be expected that below a given temperature, Z(PO2) would become significantly larger than Z(O2), and kinetic data on the termination could be obtained from CL measurements. Unfortunately, [PO2]s, and thus the maximum CL intensity, are also increasing functions of temperature so that such measurements would only be possible with highly sensitive detectors (photon counting systems). (3) The time-scale Z ( P O O H ) of CL decay due to P O O H decomposition. 7 Experiments in conditions (c) (see below) clearly show that such mechanisms are to be taken into consideration. In Fig. 6, the intensity decay observed in identical conditions for P A l 2 and for a previously studied epoxy sample 6 are compared. It appears considerably faster for the latter than for the former. As noted at point (1), the characteristic time for any experimental intensity decay, for instance for expoxy, must be at least equal to Z(O~). It appears therefore clearly that at least one chemical mechanism responsible for CL can be kinetically studied, from the curves of Fig. 6. A detailed study of decay shape for P A l 2 reveals in fact the existence of at least two distinct steps. The first one is very fast and presumably linked to the mechanism of PO~ combination as in the case of epoxy. The second one is considerably slower and can be tentatively attributed to P O O H decomposition.

Pre-oxidised samples In order to check this later hypothesis, CL emission of preoxidised samples in nitrogen atmosphere was studied (in order to avoid the

1 (mV)

30O

200

100

N2

.02 o

lO

i

I

I

20

30

40

t (rain) Fig. 7. Chemiluminescence of a preoxidised sample of polyamide 12 in isothermal conditions at 180°C.

formation of new PO~ radicals). In a first series of experiments, the CL was studied in isothermal conditions and a weak emission was detected during the preheating stage (Fig. 7) as previously found by Billingham for PA66Y A first-order plot was built from the results, taking arbitrarily the peak maximum as the time origin. The results seem to indicate the existence of two distinct mechanisms corresponding to two distinct rate constants (Fig. 8). In order to avoid these complications, it was decided to record the CL intensity during a programmed temperature scan. The results are shown in Fig. 9. They reveal the existence of a very strong emission obviously linked to a pre-existing species which is completely consumed during the test, presumably a hydroperoxide.

A. Tcharkhtchi et ai.

340

In hydrocarbon polymers, such sec-alkoxy radicals must have a low emissivity. In polyamides, however, there is the presence of a formamide: H---CO NH-----CH2--, whose photophysical properties are not well known. (2) The P O O H decomposition is bimolecular:

In 1/1°

1

0 ~

~

~

-1 -2 -3

P O O H + POOH---> PO; + PO" + H20 -4

0

I

2

3

4

5

This reaction opens the way to a wide variety of processes among which several could induce a CL event, for instance:

t (min)

Fig. 8. Kinetic of intensity decay in nitrogen at 180°C for a preoxidised sample.

PO2 + PO2 ~ l(mV)

or

4"

\

PO2(PO') + H---COOH /

3

POOH(POH) + [)~01"

2¸

+ HO"

(hydroperoxide-induced decomposition)

I 0 75

95

~5

~35

155

~75

T(°C)

The CL curves seem to display a shoulder, revealing the existance of at least two kinetic stages, as found in the isothermal experiment. This can be due to the presence of two distinct populations of P O O H groups or to the simultaneous occurrence of both bimolecular and unimolecular P O O H decomposition processes, the first one being predominant only above a certain critical concentration.

195

Fig. 9. Chemiluminescence of preoxidised samples of polyamide 12 under nitrogen after (1) 96 h at 140°C, and (2) 48 h at 140°C.

In a standard mechanistic approach (assuming homogeneous reactions), there are only two possibilities. (1) P O O H decomposition is unimolecular. In this case, CL can result only from the disporportionation of PO" and H O ' . H

products + hv

--OOH

, H - - C - - O " + HO" [%~rC=O] * + H20

This process has been proposed by Vassil'ev and co-workers, ~"' but it remains questionable owing to the small size of hydroxyl radicals which makes relatively unprobable their retention in the cage. An alternative mechanism has been proposed by Audouin-Jirackova and Verdu: 3 H---~C--OOH

In Kd 2 0

-4 -6

.8 0.0023

> H---C--O"

R/ [H % ~ O ] *

Classical kinetic tests to determine the apparent reaction order are highly questionable in this case, owing to the existence of these two mechanisms. Despite of that it seemed interesting to determine the apparent first order rate constants whose Arrhenius plot is shown in Fig. 10. The apparent activation energy is about

+ R" (/3 scission)

i 0.0025

i t 0.002"/ l/I"(l/K) 0.0029

Fig. 10. Arrhenius plots for the rate constants of (O) the first-order, and ( 0 ) the second-order hydroperoxide decomposition.

Oxyluminescence of polyamide 12

40kJ/mol which is noticeably lower than the values found for hydrocarbon polymers (80140 kJ mol-1,11-13 and derived from chemiluminescencelH3). It is noteworthly that such kinetic studies start from the implicit hypothesis that the CL quantum yield is temperature independent, which remains unproven.

CONCLUSIONS It is relatively difficult to extract useful information on oxidation kinetics from the classical CL curves obtained at constant 02 pressure in isothermal conditions. These curves reveal however an autoaccelerating behaviour which has been attributed to branching via POOH decomposition. The first peak, which is attributed to non-stationary conditions, could be used to estimate the radical yield of thermolysis. Non-classical experiments (conditions B and C) bring in contrast interesting elements of discussion concerning especially the contribution of POOH decomposition to CL. These results are

341

relatively difficult to reconcile with the most popular theory according to which CL results from PO2 biomolecular termination.

REFERENCES 1. Ashby, G. E., J. Polym. Sci., 50 (1961) 100. 2. George, G. A., Polym. Deg. & Stab., 1 (1979) 234. 3. Audouin-Jirackova, L. & Verdu, J., J. Polym. Sci., Part A, 25 (1987) 1205-17. 4. Russell, G. A., J. Am. Chem. Soc., 78 (1965) 1047. 5. Bamford, C. H. & Dewar, M. J. S., Proc. Royal Soc., London, A, 198 (1949) 252. 6. Tcharkhtchi, A., Audouin, L. & Verdu, J., J. Polym. Sci., Part A, 31 (1993) 683. 7. Reich, L. & Stivala, S. S., J. Polym. Sci., Part A, 3 (1965) 4299. 8. Billingham, N. C., Burdon, J. W., Kaluska, I. W., Keefe, E. S. & Then, E. T. H., Project Report, University of Sussex, Sussex, UK, 1985, p. 27. 9. Vassii'ev, R. F. & Vichutinskii, A. A., Dolk. Akad. Nauk., SSSR, 142 (1962) 615. 10. Vassil'ev, R. F., Makromol. Chem., 126 (1969) 231. 11. Schard, M. P. & Russell, G. A., J. Appl. Polym. Sci., 8 (1964) 997. 12. O'Keefe, E. S. & Billingham, N. C., Polym. Deg. & Stab., 10 (1985) 137. 13. Matisova-Rychla, L., Rychla, J. & Vavrekova, M., Eur. Polym. J., 14 (1978) 1033.