Oxyluminescence of Cross-Linked Amine Epoxies: Diglycidylether of

Downloaded by PENNSYLVANIA STATE UNIV on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch015

15 Oxyluminescence of Cross-Linked Amine Epoxies: Diglycidylether of Bisphenol A-Diaminodiphenyl Sulfone System L. Audouin, V. Bellenger*, A. Tcharkhtchi, and J. Verdu Ecole Nationale Supérieure d'Arts et Métiers, 151 Bd de l'Hôpital, 75013 Paris, France The oxidation

of a stoichiometric

minodiphenyl

sulfone network was studied from

spectrophotometry

diglycidylether

and chemiluminescence.

of bisphenol 200-240

A-dia-

°C by

Chemiluminescence

IR

reveals

the existence of two kinetic stages. The first one is very brief and corresponds

to the radicals resulting from

the polymer

thermolytic

deg-

radation under nitrogen during the preheating stage. This process leads to a sharp emissive peak whose intensity is an increasing function the temperature

and duration

under nitrogen.

responds to the oxidation propagation sible for carbonyl this mechanism

by hydrogen abstraction

and amide formation.

is that it is practically

An interesting independent

ratio of the first stage process. Some mechanistic

of

The second stage correspon-

peculiarity

of the

implications

of

conversion of these

results are discussed.

INTEREST IN CHEMILUMINESCENCE AS A TOOL for the study of polymer oxidation mechanisms and kinetics is increasing (J, 2). Linear hydrocarbon polymers such as polypropylene have been widely studied. However, the cor responding mechanisms are still the topic of much discussion and research. The research field remains largely open to other polymers, especially tridi mensional, heteroatoms containing polymers for which there is only a scarce literature. Investigations were reported, however, on epoxy networks for which chemiluminescence was tentatively used as a probe to monitor the cross-link ing process (3). W e recently reported (4) a comparative study concerning * Corresponding author

0065-2393/96/0249-0223$12.00/0 © 1996 American Chemical Society In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.


224

POLYMER DURABILITY

oxidation mechanisms of anhydride-cured epoxy systems revealing the strong influence of hardener structure on chemiluminescence and the changes in duced by the cross-linking reaction. Because data obtained by classical (es pecially IR spectrophotometric) analytical methods on oxidation mechanisms and kinetics of cross-linked amine diglycidylether of bisphenol A ( D G E B A ) systems are available (5), we decided to study the same systems by chemilu minescence with the hope of reaching a better understanding of this process and to appreciate the potential of chemiluminescence as a routine tool in this field. The present chapter is devoted to the study of a stoichiometric D G E B A diaminodiphenyl sulfone (DDS) system.

Experimental Materials. The network under study was based on D G E B A having an epox ide index of 5.8 mol k g and cross-linked by D D S in stoichiometric amount. The compounds were weighed, mixed with a stirrer in an oil bath at 130 °C for 15 min, outgassed for 30 min, and then cured for 1 h at 180 °C and postcured for 3 h at 250 °C in a cylindrical mold. The final glass transition temperature (T ), determined by differential scanning calorimetry at 20 Κ m i n , is 225 °C and corresponds to the full conversion of the cure reaction. For spectrophotometric and chemiluminescence measurements, microtomic slides of 25-μιη thickness, 20mm diameter, were cut from the molded cylinder. - 1

g

-1

Infrared Spectrophotometry. IR spectra were recorded on a Perkin-Elmer Fourier transform IR (FTIR) 1710 spectrophotometer between 400 and 4000 c m . The study was focused on peaks of carbonyl groups (v = 1720 c m , ε = 200 kg m o l cm ), amide groups (t> = 1680 c m , (= 470 kg m o l c m ; phenylethers v ^ = 1107 cm ), and alkylethers (v __ = 1035 cm" ). We verified first that no oxidation occurred during processing, then we measured the carbonyl and amide build-up during the chemiluminescence experiments. -1

-1

c=Q

-1

c

-1

0

-1

CON

-1

c

Q

-1

- 1

1

Chemiluminescence. A laboratory made, previously described apparatus (6) was used. The test chamber was swept by preheated gas at a flow rate of 50 L m i n . The sample was heated under nitrogen until thermal equihbrium (for at least 0.5 min and generally 3 min). Air was then admitted. Two kinds of experi ments were carried out: -1

Steady-State Experiments. A l l the exposure conditions (temperature, oxygen pressure) were constant. The chemiluminescence intensity expressed in arbitrary units was recorded versus time. Experiments were made at 200, 210, 220, 230, and 240 °C. Break-Off Experiments. The oxidation was interrupted by switching the gas supply from air to nitrogen. The chemiluminescence decay was observed. After a given period of interruption, air was readmitted and the corresponding change of chemiluminescence intensity was recorded. Pressure measurements showed that the order of magnitude or the time constant of partial pressure change was about 5 s.

In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

15.

AUDOUIN ET AL.

225

Oxyluminescence of Amine Epoxies

Results


Steady-State Chemiluminescence. Kinetic curves of chemilumi nescence are presented in Figures 1 and 2. In Figure 1, the time of preheating in nitrogen was constant at 3 min, and temperatures varied from 200 to 240 °C. In Figure 2, the three curves were obtained at 240 °C, but they differ by the preheating times of 3 min, 15 min, and 1 h.

KmVh

O4

,

!

—ι

τ

i

0

20

40

60

80

100

;

1

t(min)

Figure 1. Kinetic curves of chemiluminescence experiments at various temperatures: 1, 240 °C; 2, 230 °C; 3, 220 °C; 4, 210 °C; 5, 200 °C. lnl

5-

4

0

30

60

90

t(min)

120

Figure 2. Kinetic curves of chemiluminescence experiments at 240 °C with various preheating times: 1, 3 min; 2, 20 min; 3, 60 min.


226

POLYMER DURABILITY

On these curves, it is possible to distinguish two successive stages: • Stage 1. Immediately after admission of oxygen into the test chamber, the intensity of emission increased abruptly and reached a maximum after a period of about 1 to 2 min, and this maximum was nearly independent of temperature. Then, the intensity decreased rapidly. • Stage 2. This stage is characterized by time constants that are at least one order of magnitude higher than those of the stage 1. The peak width and time (t ) corresponding to the maximum intensity were decreasing functions of the temperature. For instance, t was about 10 min at 240 °C and 100 min at 200 °C. Ninety percent of photons were emitted during this stage.


m

m

A gravimetric study at 240 °C (Figure 3) revealed a weight loss process. This loss was about 10% after 40 min. Consequently, it appears that the final decrease of intensity was not linked to the complete volatilization of the ma terial. The spectrophotometric study of samples revealed the build up of carbonyls (1720 cm" ) and amides (1680 cm" ) (Figure 4). Phenyl ether (1107 cm" ) and aliphatic ether groups disappeared. In Figure 5, the integrated intensity of chemiluminescence at t is plotted versus the concentration of amides de termined concurrently by IR for two temperatures. A l l the experimental points are close to a single curve that calls for the following comments: 1

1

1

• The two previously described stages are also present and they can be distinguished by the yield of amides, which is higher in the second stage than in the first one. • The apparent activation energy, E , was determined for maximal ina

(ΔΜ/Μ

0

)%

Figure 3. Weight loss of sample during chemiluminescence experiment at 240 °C.



15.

AUDOUIN ET AL.

227


area(mv.cm)

600 500 400300200 9 >

100- I * 0-i

0

1

5

1

'

I

10

'

1

15

i

1

20

1

»

I

25

»

I

30

ι

ι

ι

35

\

40

amide (10 mol/kg) 2

Figure 5. Integrated luminescence intensity versus amide concentration measu concurrently during experiments at 220 °C (+) and 240 °C (Ώ).


228

POLYMER DURABILITY

tensity of the second peak and for kinetic rate constants of formation of carbonyls and amides. For temperatures below T , £ for lumines cence was 130 kj/mole. For temperatures >T , the E values for l u minescence, carbonyls, and amides were 90, 110, and 130 kj/mole, respectively. N o simple relationship among the different values exists. In the case of second stage of chemiluminescence (second maximum), the Arrhenius plot (Figure 6) displays a discontinuity at T . g

a

a

g


g

Break-Off Chemiluminescence. Chemiluminescence behavior of the sample during a single interruption of variable duration after 6 min of oxidation is presented in Figure 7 and during repeated interruptions of 5 min in Figure 8. To make the discussion easier, we shall call I the stationary intensity just before the nth interruption (time t ) under nitrogen, At the duration of interruption, I the intensity of the peak immediately following the readmission of air at time t + At, and l , the stationary-state intensity immediately after the peak. These results can be summarized as follows: sn

n

mn

s n

n

1. Each change of atmosphere involves a sharp variation of luminescence intensity. It is not possible to say if time constants (maximum few sec onds) are linked to the disappearance kinetics of the radicals that are responsible for luminescence, to the oxygen diffusion in the sample, or to the change in oxygen partial pressure. 2. The oxygen readmission at the end of interruption period, t + At, results in a luminescence peak for which i > i . But the intensity decreases rapidly to reach a stationary value of I ,. Variations of the ratio l /Z follow the same curve as during the nonperturbed, contin uous exposure. The steady-state regime represented by an envelope of n

m

n

s n

sn

sn

sn

ln(Imax) 5Ί

Figure 6. Arrhenius plot of the maximum luminescence intensity. below T ; •, above T . g

R


15. AuDOUiN ET AL.

229


I(mV)


3 A

0 2

0

Q£\\ 02'

~\

2

8

12

Î (min)

Figure 7. Chemiluminescence behavior after a single oxidation interruption various duration.

I(mV)

β

ΙΑ

ι • *

0

20

40

60

ι

80

ι

1

100 t(min)

Figure 8. Chemiluminescence behavior dunng repeated interruptions of 5 mi


230

POLYMER DURABILITY

points (Z , t ) is apparently not affected by interruptions, even when At (duration of interruption) is not negligible compared with the period of the cycle (t + 1 - t ). 3. The intensity of the peak following the readmission of air (!„„) increases with the duration of interruption At. sn

n

n

n


Discussion At least three types of chemiluminescence mechanisms are recognized that are more or less compatible with experimental kinetic data according to the nature of polymer and experimental conditions: 1. Luminescence resulting from hydroperoxide decomposition (7, 8) 2. Luminescence resulting from peroxy radical recombination (9) 3. Luminescence resulting from non identified processes in highly oxi dized microdomains (10-13) In the absence of data concerning eventual oxidation heterogeneity in the case under study, we will temporarily separate case 3 and consider cases 1 and 2, which can be discussed by using classical schemes of mechanisms and kinetics. Let us consider first the hypothesis of luminescence resulting from hy droperoxide decomposition, 1. Earlier papers (14) suggested that the amide formation results from the bimolecular decomposition of a aminohydroperoxides. From this point of view, the eventual existence of a relationship between amide concentration and integrated intensity (Figure 5) seems to be in favor of this hypothesis. Let us consider now the rate of intensity increase during air readmission: If it is induced by the hydroperoxide decomposition, hydroperoxide formation would have to be effective in the time scale of intensity variations (typically P 0 H + Ρ* 2

2

This condition does not comply with data of a previous study about oxidiza bility of epoxy networks (5) and kinetics of carbonyl and amide formation. The most current hypothesis of luminescence originated from terminating combination of peroxy radicals seems to be α priori more compatible with our experimental results: 1. Formation of POO* is very fast. Oxygen addition to alkyl radicals is diffusion controlled. 2. Rate constant of POO* + P O O ' termination can be very high (15). Con sequently, we observed very short transitions in both cases: when N is admitted (time t ) and also when oxygen is readmitted (time t + At). 2

n


n

15.

AUDOUIN ET AL. Oxyluminescence of Amine Epoxies

231

Thermolysis of polymers produces alkyl radicals where the concentration just before 0 admission is [R*] . The luminescence peak linked to this per turbation is undoubtedly induced by the oxidation of those radicals. According to the standard scheme, we can write: 0

2

Polymer ( R H ) ^ U R' R O O H - ^ ROR* + 0 > ROO*

1 > R*

J

n

k


2

ROO- + R H ROO' + ROO'

2

>ROOH + R* > products + hv

fc k

p

t

where h is Planck constant and υ is frequency. The stationary concentration of aklyl radicals is

where λ

=

fcJRH] (2nfc ) t

1/2

is the kinetic chain length of the reaction. We will observe a chemiluminescence sharp peak if the radical concen tration [R-] at the onset of oxygen admission is [R'] > > [R']». In this case, the system is initially far from equilibrium and tends rapidly toward [R'L (corresponding to the steady state when the radical concentration is [R-]J. This condition ([R'] > > [R\U is realized only when alkyl radicals ac cumulate during heating under N ; that is, when the thermolysis of the pol ymer has not reached a stationary state during the experiment duration. This result was justified because the intensity of the first peak increased with the time of exposition under nitrogen (Figure 2). A simplified kinetic scheme was proposed to explain this behavior (4). Information about the nature of R' radicals is scarce, but we can reason ably suppose that they originate principally from network scission (see Mech anisms I and II). Mechanism I can be justified by disappearance of ethers observed by IR spectroscopy. Formation of methylamines via Mechanism II was noted by Paterson-Jones (16), but others mechanisms are not excluded. Indeed, it is easy to imagine that the above kinetic scheme simplifies the phenomenon too much because we can suppose that alkyl radicals resulting from propagation (P) are probably different from radicals R" resulting from polymer thermolysis. We should then consider a new simplified scheme: 0

0

0

2

Polymer (RH) -> R* R- + 0 - » ROO* 2

η (thermolysis) k


21

(1) (2)

POLYMER DURABILITY

232 CH -

3

C CH

-^^-O-CH -ÇH-CH -N^ 2

2

OH

3

CH

3

_

\


— C - ^ ^ - 0 - C H 2 - C H + CH2-N^ CH3

OH

CH3 — C - ^ ^ - O + CH2-ÇH-CH2-N^ CH

3

OH

Mechanisms I and II. Origination of R' radicals via network scission. ROO- + P H -> R O O H + F F + 0 - » POO"

k k

pl

2

POO' ROO* + ROO* + POO* +

+ P H -> ROO* -> POO* POO" ->

POOH + F products products products

22

fc 2 P

k k^ tl

fe s t

(3) (4) (5) (6) (7) () 8

This scheme could probably explain the existence of two distinct kinetic stages observed in experimental curves J = f(t) (Figure 1). Indeed, we can suppose that: 1. Termination 6 predominates just after 0 admission and is respon sible for the corresponding luminescence peak. 2. Termination 8 predominates long term (in the stationary state). 3. Termination 7 predominates in an intermediary term. The experimental results, particularly the existence of an intensity mini mum after the first luminescence peak, can be explained because termination 6 could be responsible for short-term luminescence, and termination 8 could be responsible of long-term luminescence. The fact that reaction 7 is pro gressively induced in competition with reaction 6 could be the second reason for a fast intensity decrease after the first maximum. The necessary condition of that phenomenon is a nonemissive cross termination, 7. Species created during the second stage influence the development of the first stage. The first peak just after preheating under nitrogen is always more intensive than the nth peak whenever η is superior to 1. Here again, a no nemissive cross termination, 7, could explain the phenomenon. More surprising is the following result (Figures 2 and 7): The first stage has no influence on the second stage. Starting from the second kinetic scheme, 2


15.

AUDOUIN ET AL.


233

we can suggest that R 0 * radicals do not propagate the oxidation reaction (fc is very low) and that they participate only in terminations. In the opposite case, it would be difficult to imagine that the hydroperoxides R O O H created during the first stage are not involved in the initiation of oxidation chains during the second stage. It is difficult to determine the nature of R* and P" radicals in the absence of complementary analytical data. It seems logical to suppose that the R* radicals are primary radicals resulting from skeleton break ing reactions, and P* radicals are either secondary when hydrogen splitting takes place on methylene or tertiary when it takes place on secondary alcohol: Downloaded by PENNSYLVANIA STATE UNIV on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch015

2

p

OH

Our interpretation is valid only in the case when long-time anaerobic degra dation (a few dozens of minutes under nitrogen) does not result in noticeably decreasing concentrations of groups involved in oxidation propagation. A thermogravimetric study under N revealed that degradation was weak for Τ < 350 °C, and the weight loss observed by dynamic thermogravimetric analysis begins at 396 °C (17). This fact seems to be compatible (but should be verified) with the hypothesis of a relative polymer stability during exposure of a few hours at 240 °C or lower temperatures. The oxidative degradation mechanism of D G E B A - D D S network is rela tively complex and supposes the coexistence of two reaction mechanisms prac tically independent of one another and the participation of two kinds of alkyl radicals. Intensity variations during nonstationary experiments were extremely fast, and these results provided us with information about luminescence mech anism but did not allow us any quantitative approach to rate constants of termination or hydroperoxide decomposition. The limited sensitivity of our apparatus did not allow us to make the experiments at temperatures noticeably lower than 200 °C, where these values could probably be measurable. The luminescence study of the D G E B A - D D S network revealed relatively little about the mechanism of oxidation in comparison with spectrophotometric or physical methods (14). O n the contrary, our results suggest that the chemiluminescence can be a valuable tool to study the radical production induced by a rhermolytic degradation under an inert atmosphere. The inten sity of the peak accompanying the oxygen admission is direcdy finked to the concentration of radicals R*. The luminescence gives us the ability to measure, with a good sensitivity, parameters finked to the degradation rate when other methods such as thermogravimetry or IR spectrophotometry are not able to detect any changes. 2


234

POLYMER DURABILITY

References

1. Zlatkevich, L. In Luminescence Technique In Solid State Polymer Research; Zlatkevich, L., Ed.; Dekker: New York, 1989; pp 135-197. 2. George, G. Α.; Egglestone, G. T.; Riddell, S. Ζ. Polym. Eng. Sci. 1983, 7, 412418. 3. George, G. Α.; Schweinsberg, D. P.J.Appl. Polym. Sci. 1987, 2281-2292. 4. Tcharkhtchi, Α.; Audouin, L. Verdu, J. J. Polym. Sci. 1993, 31, 682. 5. Bellenger, V.; Verdu, J. J. Appl. Polym. Sci. 1985, 30, 363-374. 6. Audouin, L. Verdu, J. J. Polym. Sci. 1987, A25, 1205-1217. 7. Loyd, R. A. Trans. Farad. Soc. 1965, 61, 2173 and 2182. 8. Reich, L. Stivala, S. J. Polym. Sci. 1965, A3, 4299. 9. Russel, J. A. J. Am. Chem. Soc. 1965, 78, 1047. 10. George, G. Α.; Ghaemi, M. Polym. Degrad. Stab. 1991, 34, 37-53. 11. Billingham, N. C. Makromol. Chem. Symp. 1989, 28, 145. 12. Knight, I. B. Calvert, P. D. Billingham, N. C. Polymer 1985, 26, 1713. 13. Geuskens, G. Polym. Photochem. 1984, 5, 313-331. 14. Bellenger, V.; Verdu, J. J. Appl. Polym. Sci. 1985, 30, 363-374. 15. Reich, L.; Stivala, S. In Autooxidation of Hydrocarbons and Polyolefins; Dekker: New York, 1969;p375. 16. Paterson-Jones, J. C. J. Appl. Polym. Sci. 1975,19, 1539. 17. Bellenger, V.; Fontaine, E.; Fleishmann, Α.; Saporito, J.; Verdu, J. Polym. Degrad. Stab. 1984, 9, 195-208. ;


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RECEIVED for review January 26, 1994. ACCEPTED revised manuscript January 23, 1995.


Oxyluminescence of Cross-Linked Amine Epoxies: Diglycidylether of

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