Low-Temperature Chemiluminescence from - American Chemical

3 Low-Temperature Chemiluminescence from cis-1,4-Polybutadiene,1,2-Polybutadiene,and Downloaded via TUFTS UNIV on July 11, 2018 at 13:06:18 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

trans-Polypentenamer at Temperatures near Ambient R I C H A R D A. N A T H A N , G . D A V I D M E N D E N H A L L , M I C H E L L E A. BIRTS, and C R A I G A. O G L E Battelle-Columbus Laboratories, Columbus, O H 43201 M O R T O N A. G O L U B Ames Research Center, NASA, Moffett Field, C A 94035

The chemiluminescence emission at 25-60°C was measured fromfilmsof cis-1,4-polybutadiene,1,2-polybutadiene,and trans-polypentenamer. The polymers were autoxidized previ ously in air 100°C, or allowed to react with singlet molecular oxygen in solution, and then cast into films. Values of β (or k ( O ->O )/k ( O +polymer->products))were deter mined in benzene for cis-1,4-polybutadiene and cis-1,4-poly isoprene, and for model compounds cis-3-hexene and cis-3 methyl-3-hexene by independent methods. The chemilumi nescence emission from irradiatedfilmsof the polymers containing a dye sensitizer showed a complicated time de pendence, and the results depended on the length of irra diation. 1

d

3

2

1

2

r

2

^Jphe light associated with combustion is a commonplace phenomenon, and the luminescence from cool flames i n the gas phase is well known to specialists i n the oxidation field. A much weaker light emission is associated with the autoxidation process i n organic materials and can be observed frequently i n samples at temperatures near ambient with sufficiently sensitive photomultipliers. I n terms of efficiency of produc tion, this emission does not comprise a significant part of the oxidation, but the chemiluminescence reflects degradation rates i n materials that are often too slow to measure easily by more conventional methods. 0-8412-0381-4/78/33-169-019$05.00/l © 1978 American Chemical Society Allara and Hawkins; Stabilization and Degradation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

20

STABILIZATION A N D DEGRADATION OF P O L Y M E R S

Recently, we reported on the weak chemiluminescence emitted i n the autoxidation of thin films of cis-l,4-polyisoprene and also from a film of cis-l,4-polyisoprene containing hydroperoxide groups that were intro duced through singlet oxygenation ( I ) . Activation energies of 20-25 kcal/mol were obtained for chemiluminescence emission over the tem perature range 2 5 - 6 6 ° C from both partially autoxidized and hydroperoxidized samples of ds-l,4-polyisoprene. These values were taken to represent the activation energy for decomposition of hydroperoxide groups i n either of those polymer systems. The weak emission itself was thought to arise from electronically excited species ( carbonyl-containing structures) formed i n termination reactions involving peroxy radicals ( 2 - 6 ) , whether generated i n the autoxidation or by hydroperoxide de composition. This assumption is consistent with the thermodynamics of the autoxidation process steps and with kinetic evidence. Following Ashby's first observation (7) of chemiluminescence from the oxidative degradation of polymers, a number of papers have appeared dealing with oxidative chemiluminescence from a variety of polymers (8-16). In this chapter we continue the 1,4-polyisoprene work with a study of the low-temperature chemiluminescence emitted in the autoxi dation of three additional elastomers, cis-l,4-polybutadiene, amorphous 1,2-polybutadiene, and frans-polypentenamer. W e also report the chemi luminescence obtained from singlet-oxygenated samples of ds-l,4-polybutadiene and frans-polypentenamer, as well as rate data for singlet oxygen reactions with the 1,4-polyisoprene, 1,4-polybutadiene, and model compounds i n solution. Experimental The B. F . Goodrich Research and Development Center provided the cis- 1,4-polybutadiene; the 1,2-polybutadiene was obtained from the Institut Français du Pétrole; and the frans-polypentenamer was obtained from the Farbenfabriken Bayer G m b H . These polymers were purified by three reprecipitations from benzene solution with methanol as precipi tant. The solvents were reagent grade and were degassed by flushing with argon before use. Operations with the solutions were carried out i n an argon or nitrogen atmosphere, and the final benzene stock solutions ( ~ 15 g/1 ) were stored i n the dark under positive argon pressure. Poly mer films for autoxidation were prepared by adding portions of the stock solutions to petri dishes (9.5-cm diameter) and allowing the solvent to evaporate. The weights of the resulting films were determined, and they were used immediately for chemiluminescence experiments. Triphenyl phosphite ozonide was prepared from base-washed, re distilled triphenyl phosphite (Eastman) and ozone i n redistilled fluorotrichloromethane at — 78 °C. C o l d aliquots of the ozonide ( 1.0 m L ) were injected into solutions of the other reactants at room temperature in 10 m L of benzene containing 0.30 m L of 1:1 v/v pyridine-methanol catalyst

Allara and Hawkins; Stabilization and Degradation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

3.

MENDENHALL E T AL.

Low-Temperature Chemiluminescence

21

(17). The consumption of rubrene (Eastman, recrystallized from benzene-methanol) i n solution was followed by the decrease i n optical density at 492.5 nm. The photosensitizers, dibenzanthrone ( Κ & Κ L a b o ratories) and raeso-tetraphenylporphine (Strem Chemicals), were used as received. The olefins were pure ( > 98% ) commercial samples and were passed through a short column of aluminum before use. The irradiations for the competitive photosensitized oxidation (method (fe), see below) were conducted wtih light from a 150-watt Eimac high-pressure xenon lamp i n conjunction wtih a Bausch and L o m b monochromator centered at 580 nm. The exit slit width was set at 3.5 mm. Under these conditions virtually all of the light was absorbed by the dibenzanthrone. The photosensitized oxidations i n solution for determination of β by the slope-intercept method were carried out i n a constant temperature bath with light from a 250-watt projector lamp with a UV-cutoff filter. Oxygen uptake was monitored with a conventional mercury burette. Polymer films also were irradiated with the same lamp containing a Corning CS 3-69 filter. The apparatus and procedures for carrying out the chemilumines cence measurements, as well as two techniques for autoxidation i n air at 100°C and for singlet oxygenation, were otherwise similar to those de scribed previously (1,2). The weights of the autoxidized and hydroperoxidized films varied between 0.1 and 0.25 g. Results and

Discussion

Low-Temperature Chemiluminescence from Polymers. The activa tion energies for chemiluminescence observed at 2 5 - 6 0 ° C from various autoxidized and singlet-oxygenated polymers are recorded i n Table I. Exemplary Arrhenius plots for the chemiluminescence from two polymers are shown i n Figure 1. The E values range from a low of 9.6 kcal/mol for a sample of singlet-oxygenated frans-polypentenamer to a high of about 32 kcal/mol for a sample of autoxidized 1,2-polybutadiene. The latter sample, however, also gave a lower value of 23 ± 2 kcal/mol from a second experiment, so that the initial high value may have been an artifact caused by morphological changes i n the film on warming. E l u c i dation of this point w i l l require additional work. However, reproducible results were obtained from the other oxidized polymer samples. The oxygen content of the autoxidized samples varied 2 - 1 7 % ; as expected, 1,2-polybutadiene autoxidized at the slowest rate . a

Except for the autoxidized 1,2-polybutadiene values, all of the E values i n Table I were equal to or less than those found earlier for autoxi dized or singlet-oxygenated cis-l,4-polyisoprene ( J ) . The data i n Table I reveal a trend towards higher E values with increasing extent of singlet oxygenation or time of autoxidation. A possi ble interpretation of this observation may be sought i n a consideration of the autoxidation-chemiluminescence mechanism (1-17). Since the rate a

a


22


of free-radical initiation equals the rate of termination, and if d(hv)/dt = = &Rinit where Φ is the quantum yield for light emission (per termination event), which is assumed to be nearly temperature-inde pendent (2), it follows that: d(hv)/dt = Φ [initiator] · k = Φ [initia tor] · A e- ^*^ . In our system, E is approximately E u. A n increase i n E t with extent of oxidation may correspond to a change i n mode of initiation to one with higher activation energy. This is reasonable, since the concen trations of initiating species (principally peroxides and hydroperoxides) are expected to increase, at least initially, i n the samples during the course of the oxidation. The nature of the medium changes also during the oxidations. The influence of the medium i n determining decomposition rates and E 's of hydroperoxides is well established (18). Photosensitized Oxidation of Polymer Films. I n the singlet-oxy genated polymer samples prepared i n solution, the oxygen is expected to be present only as hydroperoxide groups. The increase i n E with an increase i n oxygen uptake i n singlet-oxygenated films suggests that a change i n mechanism from induced-unimolecular ( R O O H + M - » radi cals) to a bimolecular ( 2 R O O H - » R O - + R 0 - + H 0 ) mode of initia tion may account for this trend. Since the intensity of chemiluminescence from films cast from singlet-oxygenated polymer solutions was not very &Rterm

init

init

E

T

a

in

ini

a

a

2

2

Table I. Activation Energies for Chemiluminescence from Autoxidized and Singlet-Oxygenated Polymers 0

Polymer cis- 1,4-Polyisoprene

On per Autoxidized (A, min) 100 Monomer or SingletUnits Oxygenated (S)

cis-1,4-Poly

butadiene A , 22 A , 40 S S S (another film)

1,2-Polybutadiene

b

28.4

A , 65 S S

Percent Oxygen in Sample

8.93 18.9

A , 74

5.46 6.67 6.67 —•

A S S

1.89 4.55

In air at 100°C. * A, from combustion analysis; S, calculated from 0 Previous work (1).

4.0 8.1 — 6.6 3.1 3.8 3.8 2.3 17.1 0.9 2.1

kcal/mol 24 ± 3 ; 23 ±2' C

23.7 ± 2 24.9 18 ± 21 ± 11.5 16 ± 15 ±

1 1

32 ± 4 ; 23 ± 2 20 ± 1 9.6 ±2 16 ± 2

β

2

2 2

uptake.

e


3.

MENDENHALL ET A L .


23

4001

ΙΟ / Τ 3

Figure I.

Arrhenius plots of chemiluminescence from oxidized polymer films (cf. Table I)

reproducible, we attempted to photooxidize polymer films to different extents and measure both chemiluminescence emission and hydroperoxide content i n individual films. Since the photosensitized production of singlet oxygen is expected to occur at a constant rate under uniform illumination, an induced-unimolecular mechanism should show chemi luminescence directly proportional to the time of irradiation. F o r a bimolecular mode of initiation the chemiluminescence should inorease as the square of the irradiation time. Films of cis-l,4-polyisoprene, ds-l,4-polybutadiene, or frans-polypentenamer containing dissolved tetraphenylporphine or dibenzanthrone were prepared from solutions by evaporation and irradiated with filtered incandescent light. F o r all samples studied, the chemiluminescence after a short time of irradiation ( < 60 sec) showed a rapid decrease i n inten sity to the background level after a few minutes. Longer irradiation times that were sufficient to show changes i n the ir spectra of the polymers gave samples with greater chemiluminescence emission, but the emission rates were not stable and decreased, or increased and then decreased with


24

STABILIZATION A N D DEGRADATION O F P O L Y M E R S

time. Typical results for a film of frarw-polypentenamer are shown i n Figure 2. Similar results were obtained from cis-l,4-polybutadiene films containing dibenzanthrone that were irradiated and examined at 0 ° C . Fresh polymer films with or without sensitizers gave no appreciable chemiluminescenoe at room temperature, and a film without sensitizer d i d not show chemiluminescence under the same conditions after 10-sec irradiation with unfiltered incandescent light. 300 f -

200

10 min irrn.

100

2 hr 300

··=

200

5 sec irrn.

12 mm Figure 2. Chemiluminescence at 25°C from a film of trans-poZt/pentenamer (0.0152 g) containing 5.0 X 10~ g of dibenzanthrone after different irradiation times 5

Although we cannot give à detailed interpretation of these results at the present, the short-lived chemiluminescence seen after short-term exposure of polymer-sensitizer films suggested that we were observing the decay of free radicals formed during the irradiation process. The E S R spectrum of a benzene solution containing frans-polypentenamer and dibenzanthrone and a film prepared from the solution, both displayed a


3.


MENDENHALL ET A L .

25

weak, unresolved signal consistent with a peroxy radical, but i n neither case d i d it increase upon irradiation with visible light. The longer-lived chemiluminescence seen i n films irradiated for longer periods (Figure 2) is ascribed to thermal decomposition of a second labile species. Simple first-order kinetics are not observed i n either decay curve i n Figure 2. After 10-min irradiation, the i r spectrum of the frans-polypentenamer film showed the expected weak 2.9 μ peak ( O - H ) without the 5.8 μ absorption ( C = 0 ) , by analogy to 0 -reacted 1,4-polybutadiene (20). Only minor changes i n the spectrum were noted after decay of the chemi luminescence. I n contrast to this result, the chemiluminescence from polymer films oxidized at 100 °C or singlet-oxygenated i n solution and cast into films usually was invariant at 25 °C for several hours. β-Values for Polymer and Model Compounds. 1,4-Polyisoprene and 1,4-polybutadiene are so sufficiently reactive toward singlet oxygen that we can conveniently obtain β-values b y three methods: (a) from oxygenuptake rates by photosensitized oxidation at different polymer concentra tions; ( b ) by the initial disappearance rate of rubrene on photooxygenation of solutions with and without polymer (or model olefin); and (c) for 1,4-polyisoprene, by the amount of rubrene consumed upon addition of aliquots of triphenyl phosphite ozonide i n the presence and absence of olefin or polymer. The treatment of data from methods (b) and ( c ) was modified to give β-values directly from the following equations: F o r method ( & ) , 1

p

k [R] r

0

2

- 1]

+ k [(m/m ) d

A

[A] (k /k )[R] + r

d

(m/m )

0

A

- 1

In these equations, [ A ] and [R] are the respective initial concen trations of polymer (or olefin) and rubrene, Κ and k are the respective rate constants for reaction of rubrene with singlet molecular oxygen and for the unimolecular decay of the latter to triplet oxygen. The quantities m and m are the initial rates of rubrene consumption i n the absence and presence of A , respectively. Under our experimental conditions the first term i n the denominator is small, so we have: 0

d

A


26

STABILIZATION A N D DEGRADATION OF POLYMERS

For method (c) we derive: Γ [ Ρ 0 » ] . - [ Λ ] . + [«]Γ1

H

*-[

k β

~ Κ

Κ

0

Γ[Ρ0 ] 3

[A] I

0

-

[Ε], +

\n[R] /[R] 0

_

kt

J

]n[R) /[R]r

*V

[R) n

k

T

d

J

A

T

fcr

where k refers to the rate constant for reaction of A w i t h Ό . Under our experimental conditions this reduces to: 2

a

Γ 4 1 Γ 1 η ( [ 1 ϋ ] 0 / [ ϋ ! ] ^ ) 1 - l\n([R] /[R] ) J

P

lAi

0

T

In these equations [ P 0 ] is the initial concentration of triphenyl phos phite ozonide i n the reacting solution, [R]ta and [R] are the final con centrations of rubrene i n the solutions with and without olefin acceptor, respectively. The other symbols have the same significance as indicated earlier. 3

0

T

Table II.

Values of β for cis-1,4-Polybutadiene and cw-l,4-Polyisoprene

Competitive Photooxidation

Method

0

Rubrene WM

-m, ίί^Μ sec'

—ηΐχ, ί0*Μ sec'

A M

cis-1,4-Polyisoprene .144 .086

4.13 4.13

8.7 8.2

4.6 6.1

.16 .25

cis-3-Methyl-3-hexene .164 .598

4.13 4.13

9.1 7.0

3.6 1.2

.11 .12

os-3 Hexene .536 .592

4.13 4.13

7.9 19.8

4.9 8.1

.88 .41

cis-1,4-Polybutadiene .825 .660

4.13 4.13

20.5 23.3

15.4 17.7

2.50 2.09

RrV^M

β, M

6.3

10.6

0.10

6.3

10.3

0.18

Hydrocarbon,

M

1

1

Phosphite Ozonide Method" Hydrocarbon,

R t ^ M

M

cis-1,4-Polyisoprene 0.127 cis-3-Methyl-3-hexene 0.218

8.89 8.89

Dibenzanthrone sensitizer (1.53 Χ 10~ ΛΟ. * Initial ozonide concentration 2.28 Χ 10" Λί.

β

4

3


3.

MENDENHALL E T AL.

27


Values of β determined b y the various techniques are summarized i n Table II. A plot of the oxygen-uptake data and ^-values obtained b y the slope-intercept method (21) appear i n Figure 3. T h e reactivity of ds-3-hexane (β ~ 0.6 zfc 0.2M) was found to be about a factor of four less than that of ds-l,4-polybutadiene (β ~ 2.3 ± 0 . 2 M ) , whereas 1,4polyisoprene (β ~ 0.16 ± 0.05M) is almost comparable i n reactivity to its model olefin 3-methyl-3-hexene (β ~ 0.13 db 0.04M). 1.001

1

I / [cone, M] Figure 3. Double reciprocal plot for determination of polymer β-values (benzene, 6°C, 4 X I0~ M porphyrin sensitizer). For 1,4-polybutadiene, ordinate calculated from oxygen absorbed in 37 mL of solution in 95 mm. For 1,4-polyisoprene, ordinate from oxygen absorbed in 40 mL of soltuion in 45 min. 6


28


Thus, the differences between polymer and model olefin reactivities are small. This is reasonable because the small size of the singlet oxygen molecule should allow it to interact with the olefin groups i n a polymer without appreciable steric interference from the rest of the polymer chain. The variability i n chemiluminescence from the photooxidation of solid polymer films prevented measurements of ^-values i n solid media, where the results would be most pertinent to the general question of polymer stability. W e might expect a close correspondence of ^-values to those determined i n solution i n this work, since the lifetimes of singlet oxygen i n benzene and cyclohexane, which should bracket the properties of the polymers in this study, are nearly identical (22).

Acknowledgment This work was supported i n part by the National Aeronautics and Space Administration under Contract N o . NAS2-8195 and by a group program for chemiluminescence studies at Battelle supported by several industrial companies. W e wish to thank Frank Huber for carrying out the combustion analyses and Miles Chedekel for assistance with the E P R experiments.

Literature Cited

1. Mendenhall, G. D., Nathan, R. Α., Golub, Μ. Α., Polym. Prepr., Am. Chem Soc., Div. Polym. (1976) 17, 726. 2. Vassil'ev, R. F., in "Progress in Reaction Kinetics," Vol. 4, pp. 305-352. 3. Shlyapintokh, V. Ya., Karpukhin, O. N., Postnikov, L. M., Tsepalov, C. F., Vichutinskii, Α. Α., Zakharov, I. V., "Chemiluminescence Techniques in Chemical Reactions," Consultants Bureau, New York, 1968. 4. Vassil'ev, R. F., Russ. Chem. Rev. (Eng. Transl.) (1970) 39, 529. 5. Kellogg, R. E.,J.Am. Chem. Soc. (1969) 91, 5433. 6. Beutel, J.,J.Am. Chem. Soc. (1971) 93, 2615. 7. Ashby, G. E.,J.Polym. Sci. (1961) 50, 99. 8. Stauff, J., Schmidkunz, H., Hartmann, G., Nature (1963) 198, 281. 9. Schard, M. P., Russell, C. Α.,J.Appl. Polym. Sci.(1964)9, 985, 997. 10. Barker, Jr., R. E., Daane, J. H., Rentzepis, P. M.,J.Polym.Sci.,A (1965) 3, 2033. 11. Reich,L.,Stivala,S. S.,J.Polym. Sci. (1965) 3, 4299. 12. de Kock, R. J., Hol, P. Α. H. M., Int. Synth. Rubber Symp., Lect., 4t (1969) 2, 53. 13. de Kock, R. J., Hol, P. Α. H. M., Chem. Abstr. (1971) 74, 4425. 14. Isacsson, U., Wettermark, G., Anal. Chim. Acta (1974) 68, 339. 15. Pokholok, T. V., Karpukhin, Ο. N., Shlyapintokh, V. Ya.,J.Polym. Sci., A1 (1975) 13, 525. 16. Mendenhall, G. D., Angew. Chem., Int. Ed. Engl. (1977) 16, 225. 17. Mendenhall, G. D., Barlett, P. D., unpublished results. 18. Hiatt, R., in "Organic Peroxides," Chapter 1, D. Swern, Ed., Wiley-Interscience, N.Y., 1971. Allara and Hawkins; Stabilization and Degradation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

3. MENDENHALL ET AL. Low-Temperature Chemiluminescence 29

19. Golub, M. Α., Gemmer, R. V., Rosenberg, M. L., ADV. CHEM. SER. (1978) 169, 11. 20. Carlsson, D. J., Mendenhall, G. D., Suprunchuk, T., Wiles, D. M.,J.Chem. Amer. Soc. (1972) 94, 8060. 21. Higgins, R., Foote, C. S., Cheng, H., ADV. CHEM. SER. (1968) 77, 102. 22. Merkel, P. B., Kearns, D. R.,J.Amer. Chem. Soc. (1972) 94, 7244. RECEIVED May 12, 1977.


Low-Temperature Chemiluminescence from - American Chemical

Recommend Documents