Polymer Durability - ACS Publications - American Chemical Society

8 Formation and Removal of Singlet (aΔg) Oxygen in Bulk Polymers: Events That May Influence Photodegradation

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch008

1

P. R. Ogilby , M. Kristiansen , D. O. Mártire , R. D. Scurlock , V. L. Taylor , and Roger L. Clough 1

1

2

1

1

2

1Department of Chemistry, University of New Mexico, Albuquerque, NM 87131 Sandia National Laboratories, Albuquerque, NM 87185

2

The lowest excited state of molecular oxygen, singlet oxygen [O (a1 Δg)], 2

is an intermediate

in many photooxygenation

reactions. In polymeric

materials, these reactions may play a role in the photodegradation the macromolecule

of

or of additives (such as dyes) dissolved in the ma

trix. We used time-resolved

spectroscopy

to directly monitor the be

haviors of O (a1Δg) generated in bulk polymer matrices. This 2

chapter

describes some of our recent results concerning

(a) mechanisms and

quantum yields for the photo-induced formation

of O (a1Δg) in poly 2

mers, (b) O (a1Δg) lifetimes in different types of polymers, (c) compar 2

ison of rate constants for the quenching

of O (a1Δg) by additives

in

2

amorphous polymer glasses and in liquid solutions, and (d) character istics of chemical reactions between O (a1Δg) and solute molecules dis 2

solved in a polymer glass.

T H E GROUND STATE of molecular oxygen is a triplet-spin state [0 (X X ~)]. The lowest excited electronic state of oxygen is a singlet, 0 (a A ), which lies 94.3 kj/mol (22.5 kcal/mol) above 0 (X S "). The 0 (^)-0 (Χ%-) energy gap corresponds to a phosphorescent transition in the near IR region (1270 nm), which provides a method by which O (a A ) can be directly monitored (J). This spectroscopic probe can be used in several ways: The kinetics of 0 (a A ) formation and removal can be quantified in time-resolved measure2

2

2

3

g

2

a

2

1

1

1

g

2

g

g

0065-2393/96/0249~0113$12.00/0 © 1996 American Chemical Society In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

3

g

POLYMER DURABILITY

114

merits, and the intensity of the phosphorescence can be used to determine 0 (a A ) yields. Data from 0 (a A ) phosphorescence measurements can be supplemented with luminescence and flash-absorption data from organic mol ecules to provide a unique and informative perspective of processes that occur in the oxygen-organic molecule photosystem. We have applied these tech niques in studies of a variety of organic polymers both above and below the glass transition temperature (T ) (2-6). 0 (a A ) is an intermediate in many photooxygenation reactions (7). In some cases, these reactions may be important in the degradation of organic materials (7-12). For example, the production of an allylic hydroperoxide via the "ene" reaction may precede a variety of radical reactions that result from cleavage of the labile O - O bond (see Scheme I). OcjÂg) may also be important in the photobleaching of dyes or other additives dissolved in the polymer. Polymeric materials susceptible to these reactions are used in a wide range of applications, including new electrooptic systems (e.g., nonlinear optical devices and organic light emitting diodes). However, even if 0 (s^\) is not an intermediate in reactions that result in the degradation of an organic polymer, experiments in which O (a A ) is mon itored can be useful probes of other photoinduced processes which, in turn, may be directly related to the degradation of the material. In this chapter, we discuss the formation and removal of 0 (a A ) in bulk organic polymers from the perspective of events that may influence the photodegradation of poly meric materials. 1

2

g

1

2

g

g


2

1

g

2

1

a

2

1

g

g

0 (α Δ^ Formation 2

λ

O (a A ) can be formed in bulk polymers by several photoinduced processes. It has been shown by several investigators, including ourselves, that O (a A ) can be produced in polymers by energy transfer from an excited-state organic molecule to 0 ( X X " ) . We recently demonstrated that 0 (a A ) can also be formed upon irradiation into the oxygen-organic molecule charge-transfer (CT) band. This C T absorption band derives from the interaction between z

1

a

2

3

g

2

O-OH

1

g

Ο·

Scheme I.

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

·ΟΗ

1

g

8.

Formation and Removal of Singlet Oxygen in Bulk Polymers

OGILBY ET A L .

oxygen and the polymer in which it is dissolved or between oxygen and a solute dissolved in the polymer. Photosensitized Production of 0 ( a A ) . 0 (& Δ ) can be formed by energy transfer from a molecule dissolved as a solute in the polymer matrix, or from a chromophore that is an integral part of the polymer itself, such as the carbonyl group in polycarbonate (2, 3) (Scheme II). 1


2

g

2

ι

&

% i

hv» i

M o

M

l

3^

intersystem»

+

0 (a A ) 2

1

g

CMX'Sg")

crossing Scheme II.

The singlet state O M ^ hfetime of organic-molecule photosensitizers is typically less than 20 ns. [In designating organic molecule (M) electronic states, nu merical superscripts identify the spin state, and subscripts identify either the ground (0) or first excited (1) state.] In liquid solutions where solute diffusion coefficients are on the order of 1-5 X 10~ cm /s, it is possible for 0 (X % ~) to encounter M within the M hfetime. Indeed, the oxygen-induced deac tivation of M is usually limited by diffusion, and it is possible, in certain cases, to produce 0 (a}A ) by energy transfer from M (13-17). In amorphous polymer glasses, however, the oxygen diffusion coefficient is typically less than ~ 1 X 10~ cm /s, and unimolecular decay channels of M are more efficient than bimolecular quenching by 0 (X X ~). Thus, in rigid media, M should rarely, i f ever, be a precursor to 0 (a}A ). Organic molecule triplet state ( Mi) lifetimes, however, are sufficiently long that quenching by 0 (X X ~) is an efficient process, even in amorphous polymers below T . Nevertheless, because solute diffusion coefficients i n the glass are small, M deactivation by 0 (X X ~~), and the corresponding rate of 0 (a A ) production, is significantly slower than in a liquid solvent (2, 3). In aerated or oxygenated polymer glasses, rates of 0 (a A ) decay often exceed the rate of 0 (a A ) formation i n a photosensitized process (2, 3). Thus, i n analyzing time-resolved 0 (a}A ) phosphorescence data from such polymers, we must deconvolute the decay function of the 0 (a}a ) precursor to obtain the intrinsic 0 (a A ) decay function (2, 3). The 0 (o}A ) precursor decay func tion can be obtained in a flash absorption experiment in which M is moni tored. 5

1

X

1

1

2

2

3

g

1

X

2

7

1

g

1

2

1

3

2

1

1

g

2

1

g

3

2

3

g

g

3

2

1

X

3

2

g

g

2

2

1

1

g

g

2

g

2

2

1

g

2

g

g

3

1

T h e Quantum Yield o f 0 ( a A ) i n a Bulk Polymer. In liquid solvents, the quantum yield, φ , of 0 (a A ) i n a photosensitized reaction de pends on the sensitizer, M , and on the solvent, among other variables (18). 2

Δ

1

2

g

1

g


11

116

POLYMER DURABILITY

Under conditions where M is the sole precursor to 0 (a A ), the product φ ι* defines φ . The parameter φ is the M quantum yield, and ί is the fraction of M states that yield O ^ a ^ ) upon interaction with 0 (X S ~). In independent experiments (6), we measured the quantum yield of acridine-sensitized 0 (sÂ ) production in an amorphous polystyrene glass by using a reaction in which 0 (a A ) was chemically trapped and by comparison of the 0 (a A ) phosphorescence intensity to a known standard. Upon 355-nm pulsed-laser irradiation of acridine at ~1.3 mj/pulse, the O (a A ) quantum yield in polystyrene fà (acridine) = 0.56 ± 0.05] is somewhat smaller than that obtained in the liquid solvent analog toluene [φ Ηacridine) = 0.83 ± 0.06]. The φ value also depends inversely on the incident laser power. This latter phenomenon is not uncommon when pulsed lasers are used as the pho tolysis source, where nonlinear effects can contribute at high incident powers (18). 3

τ

Δ

τ

2

1

Δ

3

χ

1

g

Δ

1

X

1

g

g

g

1

a


τ

g

2

1

3

2

2

2

3

g

ps

A

Δ

Δ

το

Ρ δ

Several additional experiments were performed to help i n the interpre tation of these data. Because solute diffusion coefficients are smaller in the glass, it was important to ascertain whether or not the polymer data reflected a less-efficient scavenging of the acridine triplet state by oxygen. Changes i n the triplet state hfetime on the admission of oxygen into the system indicated that in both the glass and liquid, 99.9% of the acridine triplets formed are quenched by 0 (X S ~). [The hfetime of acridine in both media is short enough to preclude quenching by oxygen (13-15)]. It is also possible that the inequality φ ^ Γ ΐ ( Ι ΐ η β ) < φ Μacridine) may reflect a larger component of 0 (a A ) quenching by the sensitizer itself in the solvent cage of the more rigid polymer. If such quenching was more pronounced i n the polymer, one might expect the ratio φ ν φ ^ to be smaller for sensitizers that can moreefficiently quench O ^ a ^ g ) . Results obtained by using different sensitizers, however, do not appear to support this hypothesis. At higher incident-laser powers, despite efficient quenching of acridine by 0 ( X £ ~ ) , the lower 0 (a A ) yields may result (1) from an "inner-filter" effect in which absorption by aeridine effectively shields ground-state acridine from the incident light, and/or (2) from depletion of the initial acridine population by multiphoton absorption to form species that are not 0 (a A ) precursors. 2

Δ

1

2

3

g

ρδ

Δ

το

g

Δ

ρ

Δ

Τ01

3

1

2

2

3

g

g

3

3

1

2

g

One objective of this φ study was to provide more insight into the po tential contribution of O ^ a ^ g ) in photooxygenation reactions in a solid matrix. Specifically, if the 0 (a A ) yield was substantially less in a solid polymer than in a liquid, it would then appear that 0 (a A ) was not a likely intermediate in photoinduced reactions that degrade either the macromolecule itself or low molecular weight additives within the solid matrix. Whatever the reasons for the slightly lower φ value obtained i n the macromolecular matrix, it is clear that a photosensitized process i n a solid, air-saturated polymer can indeed produce a substantial amount of 0 (s}A ). Δ

2

x

g

2

1

g

Δ

2

g


8.

Formation and Removal of Singlet Oxygen in Bulk Polymers 111

OGILBY ET A L .

Production of 0 ( a A ) upon Charge-Transfer Band Irradiation. 0 (a A ) can be produced in polymers that are free of solutes or chromophores, which can otherwise act as photosensitizers, by irradiation into the oxygen-polymer C T absorption band (2, 4). This feature in the ab sorption spectrum usually appears as a red shift in the absorption onset sub sequent to dissolution of oxygen in the material, and it is attributed to a transition from a ground-state oxygen-organic molecule complex to a C T state ( M ^ 0 - ) (19, 20). The C T state is an 0 (a A ) precursor (16, 20). In a triplet-state photosensitized process, photon absorption precedes the rate-limiting encounter between M and 0 (X X ~). Consequently, the rate of signal appearance in a pulsed-laser, time-resolved 0 (a A ) phosphores cence experiment can be comparatively slow in an amorphous polymer glass due to the small diffusion coefficients. On the other hand, the encounter between oxygen and the organic component necessarily precedes the absorp tion of a photon in the transition that produces the C T state. Thus, the rate of 0 (a A ) signal appearance upon pulsed irradiation into the C T band is much faster than that observed in a photosensitized reaction in polymer glasses. This difference in the rates of 0 (a A ) signal appearance in timeresolved measurements has two important ramifications: 1

2

1

2

g

g


2

l

2

3

X

g

3

2

g

1

2

1

2

g

g

1

2

g

1. Upon C T band irradiation, it is not necessary to deconvolute the decay kinetics of the 0 (s}a ) precursor from the observed 0 (a A ) phospho rescence signal to obtain the intrinsic rate of 0 (a A ) decay. 2. 0 (a A ) phosphorescence measurements can be used to monitor the production of 0 ^ Δ ) sensitizers that result from the thermal or pho tochemical degradation of the medium. 2

2

g

2

2

x

1

1

g

g

g

2

δ

A n example of how 0 (a A ) data can be used to follow events associated with the photodegradation of a polymer is shown in Figure 1. Upon pulsed irradiation into the oxygen-polymer C T band of a freshly prepared polystyrene sample, the rate of 0 (a A ) phosphorescence signal appearance is rapid. After prolonged irradiation into the C T band, however, the intensity of the 0 (a A ) phosphorescence signal progressively increases, and the rate of 0 (a A ) signal appearance becomes slower. These results indicate that, subsequent to pro longed irradiation, 0 (a A ) is also being produced by energy transfer from a photosensitizer. These sensitizers arise by the photooxidation of the polymer. If sites of unsaturation are already present in the polymer (as chain defects, for example), the 0 (a A ) produced upon C T band irradiation may, via the "ene" reaction, form sensitizing chromophores (e.g., carbonyls) with a con comitant cleavage of the macromoleeular chain {vide supra). Alternatively, chromophores capable of producing 0 (a A ) may arise at a saturated center through the intermediacy of the C T state. A saturated hydroperoxide, for example, could be formed by the sequence of events in Scheme III (21), 2

1

2

1

g

g

2

2

1

2

2

1

g

g

2

1

g


x

g

1

g

118

POLYMER DURABILITY

o JU

1

L>1

hv ^ ~rV^

•OOH 2

—

^ ]

OOH

U

Proton ^ Transfer

1

"

J"" i ^ J / k y K Combination f

CT State


Scheme HI. Alkyl and aryl hydroperoxides serve as both photochemical and thermal precursors of products such as ketones, aldehydes, and alkenes that charac terize a "degraded" polymer (4). These products may not only sensitize the production of 0 (a A ) during the absorption of subsequent photons, but some may also provide a center suitable for reaction with 0 ( a A ) or for reaction via other mechanisms. 2

1

g

1

2

g

0 (a A ) Removal 1

2

g

0 (a A ) can be removed from a particular system by physical or reactive quenching channels. A physical quencher deactivates O (a A ) to 0 ( Χ Σ ~ ) . 2

l

g

a

0

40

Θ0

TIME

120

ISO

1

g

2

3

8

200

(microseconds)

Figure 1. Time-resolved 0 (a phosphorescence recorded from a polystyrene glass subsequent to pulsed-laser irradiation at 341 nm. Data recorded from a freshly prepared sample (—) that had not previously been irradiated had faster rates of both appearance and disappearance than data recorded from the same sample subsequent to three separate 30-min periods of photolysis at 341 nm (—). Between photolysis sessions, the sample was allowed to stand at 25 °C under 84 kPa of oxygen for 4 days. The data have been scaled to the same intensity to better show rate differences. (Reproduced from reference 4. Copyright 1990 American Chemical Society.) 2

1


8.

OGILBY ET A L .

119


Even in the absence of an added quencher, the host medium or solvent will itself deactivate O ^ A g ) to 0 ( Χ Σ ~ ) ( i , 22). However, the addition of spe cific solutes can often provide a more efficient process by which 0 (a}A^ may be removed. Many amines, for example, are particularly effective at inducing O ( a A ) deactivation (7). 0 (SL \) can also be removed by chemical reaction to form oxygenated products (vide supra). 3

2

8

2

1


a

g

1

2

Intrinsic Lifetime of O ( a A ) in Bulk Polymers. The hfetime of 0 ( a A ) is strongly influenced by the surrounding medium (I, 7). The probability of 0 ( a A ) participating in a chemical reaction depends, in part, on its intrinsic hfetime in that medium. For example, the medium-induced deactivation of 0 (a A ) may be rapid compared with the rate at which 0 ( a A ) can encounter a reaction partner by diffusion. Measuring the hfetime of 0 (a A ) in bulk polymers is thus important in evaluating the potential role of 0 ( a A ) in the photochemistry of polymeric materials. We have determined the hfetime of 0 ( a A ) in a number of common polymer matrices at 25 °C (Table I). The lifetimes are similar to those obtained in hquid solvents of analogous molecular composition. The data are consistent with a model in which the electronic excitation energy of 0 ( a A ) is deposited in vibrational modes of the host medium, particularly in the C - H and O - H stretching modes. Thus, the 0 (& A ) hfetime decreases with increasing con centration of C - H and O - H bonds in the polymer. There is no indication from our results that matrix rigidity has a significant influence on 0 ( a A ) lifetimes. For example, the 0 ^ Δ ) lifetime in a poly(methyl methacrylate) ( P M M A ) glass is —22 μ8. In poly(ethyl acrylate), a rubbery material (i.e., T < 25 °C) that has a molecular composition closely related to that of P M M A , 0 (d}A ) has a hfetime of —31 μς. 1

s

1

2

1

2

2

1

2

2

g

g

g

l

g

g

l

g

1

2

g

2

1

g

2

l

2

1

g

g

2

1

g

2

g

2

g

Rate Constants for 0 ( a A ) Removal in Bulk Polymers. The removal of O ( a A ) by physical and reactive quenching channels can be de scribed, respectively, by the kinetic reactions in Scheme IV 1

2

a

0 &\) 2

+

g

1

Q j Ë E r [ 0 (a A )— Q ] 1

x

2

3

g

[ 0 ( Χ Σ > · Q]

- 0 (X^ ") + Q

3

2

9

0 ( a ' A ) Q ^ z r [ 0 ( a ' A ) - . Q ] JSDBL*.

2

i

2

g

+

J

g

Q

_O

g

2

Scheme IV. where Q is the 0 ( a A ) quencher, k is the bimolecular rate constant for the diffusion-dependent encounter of two solutes, and k_ is the unimolecular 2

1

g

m

m


120

POLYMER DURABILITY

Table I. Singlet Oxygen Lifetimes in Bulk Polymer Matrices


Polymer

Intrinsic Lifetime (μ&)

Structure

Η

Poly(4-methyl-1 -pentene )

•CH -CCH CH(CH )

18 ± 2

2

2

3

2

Η

Polystyrene

19 ± 2

•CH -C2

Ô CH •CH -C .C-OCHJ 3

Poly(methyl methacrylate)

22 ± 3

2

Polycarbonate

29 ± 2

Η Poly(ethyl acrylate)

--CH -Ç.C-OCH CH .c-

o*

2

DuPont Teflon A F

3

3

3

3

Perdeuteriopolystyrene

3

ÇH CH CH CH -ShO| -Sl-0 -Si-CH CH CH CH 3

Poly(dimethyl siloxane)

31 ± 1

2

D .CD -C-

FsCF

— CF C F -

I \ Ο Ο •χ

FC 3

46 ± 1

250 ± 15

2

+ t

3

3

3

CF

1700 ± 100

3


8.

OGILBY ET A L .


rate constant for diffusion-dependent dissociation of an encounter pair. The overall physical quenching rate constant, fc , can be expressed in terms of k , fc-diff, and fc by invoking the steady-state approximation for the Q - 0 en counter pair. The rate constant for quencher-induced intersystem crossing in oxygen is denoted by fc . A similar treatment for the reactive scheme yields the overall rate constant for 0 (a A ) removal by reaction, k , in terms of k , k_ , and k^. The rate constant for reaction from the Q - 0 encounter pair is denoted by k^. q

m

isc

2

isc

1

2

g

r


m

2

m

^diff &isc/(fc-diff

=

^diff ^raïA^-diff

=

+

îsc)

+

^rxn)

The rate constants k and k can be measured by quantifying the rate constant k for 0 (a A ) removal as a function of the quencher concentration [k is the reciprocal of the intrinsic 0 (a A ) hfetime] (5). q

2

à

1

r

g

0

1

2

k

g

or

= k + k [Q]

A

0

q

k = k + k [Q] A

0

r

For very efficient quenchers, k or k approaches k , and we say that the process is limited by diffusion (with the comparatively small molecule oxygen, k 3 X 10 s M " in hquids). At this limit, a change from a liquid solvent to a polymer glass, with the concomitant reduction in diffusion coefficients, should result in a decrease in k or k . The data are indeed consistent with this expectation (Table II). For poor quenchers, a change from a liquid solvent to a polymer glass results in an increase in the quenching rate constant (Table II). This phenom enon is understood by recognizing that in a solid matrix, where comparatively small diffusion coefficients yield correspondingly small values of k_ , 0 (a A ), and Q will undergo more collisions within the solvent cage and thus increase the chance for quenching before dissociation. The data in Table II indicate that rate constants for 0 (a A ) removal in a polymer glass are not directly proportional to those recorded in a liquid solution, and in fact the data span a much smaller range compared with liquid solvents. Thus, in attempts to ascertain whether or not a polymer additive acts as a stabilizer by removing 0 (a A ), ^(polymer) rather than fc(liquid) should be used in interpreting longevity data from quencher-doped polymers. r

m

%

10

- 1

q

m

1

r

q

M{

2

1

g

2

1

2

1

g

g

Removal of O ( a A ) by Reaction: Unique Features of a Solid Matrix. Removal of 0 (a A ) by chemical reaction provides a model for investigating photooxygenations in solid materials. A variety of polycychc ar omatic hydrocarbons, such as 9,10-diphenylanthracene (DPA), react with Oaia^g) generated either chemically (7), by U V M s irradiation (7), or by iona

1

g

2

1

g


121

122

POLYMER DURABILITY

Table II. Rate Constants for Q (a A ) Removal at 25 °C 1

2

Quencher

Poly(methyl Polystyrene methacrylate)

Toluene

^ O ^ P ^ / ^ P ^ O - ^ ^


g

(6.5±0.1) Χ ΙΟ

(2.0±0.1) Χ ΙΟ (1.4±0.2) Χ 10

9

8

(2.4 ±0.1) X 10

(9±1) Χ 10 (3.5 ±0.4) Χ 10

8

7

(1.3 ±0.1) Χ 10

6

(2.4 ±0.3) Χ 10

8

Ph

7

7

OCH

3

(4.5 ±0.3) X 10 (1.6 ±0.15) X 10 7

7

(7.8 ±0.3) X 10

(9.8 ±0.7) X 10

5

(Ph) N

5

(1.6 ±0.1) X 10

(3.0 ±0.4) X 10 (6.2 ±0.5) X 10

5

3

5

5

NOTE: Representative values for the oxygen diffusion coefficient (cm s" ) are as follows: 5 X 10~ for liquid toluene, 2 X 10" for polystyrene glass, and 1 X 10~ for poly(methyl methacrylate) glass. Rate constant units are in M s . The nickel complex and the two amines are physical quenchers. The three aromatic hydrocabons remove O (a A ) by reaction. Errors are two standard deviations in the slope offc vs. [Q]. 2

5

7

1

8

- 1

-1

a

1

g

4

izing radiation (23) via a ^2 + 4 cyeloaddition to yield an endoperoxide (see Scheme V). Data from photolysis experiments involving amorphous polymer glasses doped with an aromatic hydrocarbon are consistent with a process that re moves O (a A ) by chemical reaction (24). In polystyrene samples containing rubrene (5,6,11,12-tetraphenylnaphthacene) or D P A π

a

1

g

1. UV/vis absorption measurements indicate that the aromatic hydrocarbon is being consumed during the experiment. 2. Upon dissolution of the irradiated polymer sample in benzene, the ar omatic endoperoxide can be recovered.


8.


OGILBY ET A L .

Rh

Ph Oata ^) 1

Ph

Ph


Scheme V.

3. O ( a A ) hfetime measurements indicate that 0 ( a A ) is quenched by the aromatic hydrocarbons. 1

a

g

2

1

g

Furthermore, with each successive photolysis pulse, the intensity of the 0 (s}A ) phosphorescence signal and the decay rate of the O ( a A ) precursor both decrease, which is consistent with consumption of the oxygen dissolved in the polymer matrix. The O ^ A g ) phosphorescence intensity and precursor decay rate recover during re-equilibration of the sample in air. In the self-sensitized photooxygenation of rubrene, the quantum yield of the 0 ( a A ) reaction product (rubrene endoperoxide) is greater in the hquid than in the polymer glass, particularly at high rubrene concentrations. This result derives principally from differences between hquid- and solid-phase solute diffusion coefficients. Thus, in the hquid, where oxygen and rubrene are more mobile, the 0 ( a A ) generated by one excited-state rubrene mole cule can more readily interact with other rubrene molecules in the system. Also in the hquid, 0 (X 2 ~) can more readily induce deactivation of rubrene (T 16 ns) to produce both 0 ( a A ) and rubrene, which is itself a 0 (s}A ) precursor. [Because φ for rubrene is inherently small in the absence of ox ygen, the deactivation of ^ b r e n e by 0 (X X ~) ultimately results in a higher yield of 0 (a A )]. In the polymer glass, however, where the mobility of oxygen is reduced and rubrene is essentially immobilized, the reaction of O ( a A ) with rubrene molecules other than the "parent" photosensitizing molecule is restricted on the time scale defined by the O ( a A ) hfetime, and 0 ( Χ Σ " ) induced deactivation of rubrene is precluded. 2

a

g

2

1

1

g

g

1

2

3

2

g

x

g

s

2

1

3

g

2

g

χ

3

2

2

1

g

g

1

a

a

1

g

3

2

g

§

1

Useful information can also be obtained from experiments in which two different quenchers compete for the 0 (a A ) produced. In a sample contain ing both rubrene and the physical quencher l,4-diazabicyclo[2.2.2]octane ( D A B C O ) , the 0 (a A ) produced during rubrene irradiation can either be deactivated to 0 ( X X " ) by D A B C O or react with rubrene to yield the en doperoxide. In such a system, where rubrene sensitizes its own 0 (a A )-mediated photooxygenation, it can be expected that at the limit of high D A B C O concentration, all the O ( a A ) would be quenched except that reacting with the "parent" sensitizer molecule within a solvent cage (see Scheme VI). 2

1

2

2

3

1

g

g

g

2

z

1

g


1

g

123


124

POLYMER DURABILITY

Data obtained by using polystyrene samples are consistent with this ex pectation (Figure 2); an increase in the D A B C O concentration blocks all but a fixed amount of rubrene endoperoxide formation. Furthermore, and as ex pected, the quantum yield of rubrene endoperoxide at high D A B C O concen tration (φ = 0.021) is independent of the rubrene concentration. When these same competitive quenching experiments are performed in toluene, the quantum yield of rubrene endoperoxide at high D A B C O con centration (φ = 0.01) is a factor of 2 smaller than that observed in polystyrene. This difference between the solid and liquid phase data is even more pro nounced for D P A , which has a quantum yield of unquenchable endoperoxide in polystyrene (φ = 0.0022) that is 22 times larger than that in toluene (φ = 0.0001). These results are consistent with a "cage escape" channel (vide supra)

Rubrene Endoperoxide 3

R u b r e n e · · · 0 ( Χ Σ ) — • R u b r e n e · · · 0 (a^)

•

3

2

9

2

" escale» 0»

Ε 3 2

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Figure 2. Quantum yield of rubrene endoperoxide as a function of DABCO concentration in an amorphous polystyrene glass.


8.

OGILBY ET AL.


that is more pronounced in the liquid where diffusion coefficients are much larger. In the polymer, the rigid matrix inhibits the escape of 0 ( a A ) from its parent sensitizer. Thus, when a molecule capable of acting both as a sen sitizer and a reactant is present in a solid polymer at the limit of low concen tration, where an out-of-cage reaction will be insignificant, the quantum yield of reaction with O (o?-A ) can be greater than that observed in an analogous liquid-phase system. These experiments also indicate that in sohd polymer matrices that contain low concentrations of a species that can act both as 0 ( a A ) sensitizer and reactant, the addition of a 0 (a A ) quencher (i.e., stabilizer) may not inhibit the oxygenation reaction. Thus, cage effects can play an important role in reactions that may influence the degradation of additives, or of the macromolecule itself. 1

2


2

2

1

g

g

g

2

1

g

Summary and Conclusions The formation and removal of O ( a A ) in bulk polymers can be monitored using time-resolved spectroscopy. Such experiments indicate that 0 ( a A ) can arise within sohd materials by two photo-induced mechanisms: a photosensi tized process involving either dissolved dye molecules or chromophores on the macromolecule, and subsequent to irradiation into the C T band that arises upon oxygen dissolution in the polymer. The quantum yield of 0 (a A ) in a photosensitized process was determined in one polymer glass (polystyrene) and found to be substantial (~0.6). Lifetimes of 0 (a A ) in common polymers are similar to those in liquid-solvent analogs, and these lifetimes vary inversely with the concentration of C - H bonds in the macromolecule. Rate constants for quenching of 0 {d}A ) by additives can differ substantially in polymers versus hquid solvents. Efficient quenchers exhibit greatly reduced rate con stants in polymer glasses, whereas poor quenchers exhibit somewhat larger rate constants. The quantum yield of 0 (a A )-derived reaction products, aris ing from low concentrations of molecules able to serve as both sensitizer and reaction partner, can be significantly higher in a polymer glass than in a hquid solution. Differences in the diffusion coefficient for oxygen, along with atten dant changes in the dynamics of the solvent cage, play an important role in the photophysics and photochemistry of bulk solids as compared with liquids. a

1

g

1

2

2

2

2

1

1

g

g

g

%

2

1

g

Acknowledgments The assistance of M . Malone (Sandia) is gratefully acknowledged. This work was supported by the Department of Energy under contract numbers D E AC04-4AL85000 and DE-AC04-76DP00789 and by a grant from the Army Research Office.


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References 1. Gorman, Α. Α.; Rodgers, M . A. J. In CRC Handbook of Organic Photochemistry; Scaiano, J. C., Ed.; CRC: Boca Raton, FL, 1989; Vol. 2, pp 229-247 and references cited therein. 2. Ogilby, P. R. Dillon, M . P.; Gao, Y. Iu, K.-K. Kristiansen, M . Taylor, V. L . Clough, R. L. In Structure-Property Relations in Polymers; Urban, M . W.; Craver, C. D., Eds.; Advances in Chemistry Series 236; American Chemical Society: Wash ington, DC, 1993; pp 573-598. 3. Clough, R. L . Dillon, M . P.; Iu, K.-K.; Ogilby, P. R. Macromolecules 1989, 22, 3620-3628. 4. Ogilby, P. R.; Kristiansen, M . Clough, R. L. Macromolecules 1990, 23, 26982704. 5. Ogilby, P. R.; Dillon, M . P.; Kristiansen, M.; Clough, R. L. Macromolecules 1992, 25, 3399-3405. 6. Scurlock, R. D . Mártire, D. O; Ogilby, P. R.; Taylor, V. L . Clough, R. L. Macromolecules 1994, 27, 4787-4794. 7. Singlet Oxygen; Frimer, Α. Α., Ed.; CRC: Boca Raton, FL, 1985. 8. Ranby, B.; Rabek, J. F. Photodegradation, Photooxidation, and Photostabilization of Polymers; Wiley: New York, 1975. 9. Allen, N . S.; McKellar, J. F. Macromol. Rev. 1978, 13, 241-281. 10. Photochemistry of Dyed and Pigmented Polymers; Allen, N . S.; McKeller, J. F., Eds.; Applied Science: London, 1980. 11. Carlsson, D. J. Wiles, D. M . J. Polym. Sci. Polym. Phys. 1973, 11, 759-765. 12. Carlsson, D. J.; Wiles, D. M . Rubber Chem. Technol. 1974, 47, 991-1004. 13. Scurlock, R. D . Ogilby, P. R. J. Photochem. Photobiol. A 1993, 72, 1-7. 14. Iu, K.-K.; Ogilby, P. R. J. Phys. Chem. 1987, 91, 1611-1617. (Erratum:J.Phys. Chem. 1988, 92, 5854.) 15. Iu, K.-K.; Ogilby, P. R. J. Phys. Chem. 1988, 92, 4662-4666. 16. Kristiansen, M.; Scurlock, R. D.; Iu, K.-K.; Ogilby, P. R. J. Phys. Chem. 1991, 95, 5190-5197. 17. Saltiel, J.; Atwater, B. W. Adv. Photochem. 1988, 14, 1-90. 18. Wilkinson, F.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1993, 22, 113-262. 19. Tsubomura, H.; Mulliken, R. S. J. Am. Chem. Soc. 1960, 82, 5966-5974. 20. Scurlock, R. D . Ogilby, P. R. J. Phys. Chem. 1989, 93, 5493-5500. 21. Onodera, K; Furusawa, G.-I.; Kojima, M . ; Tsuchiya, M . ; Aihara, S.; Akaba, R.; Sakuragi, H.; Tokumara, K. Tetrahedron 1985, 41, 2215-2220. 22. Scurlock, R. D . Ogilby, P. R. J. Phys. Chem. 1987, 91, 4599-4602. 23. Clough, R. L. J. Am. Chem. Soc. 1980, 102, 5242-5245. 24. Clough, R. L . Taylor, V. L . Kristiansen, M . ; Scurlock, R. D . Ogilby, P. R., sub mitted for publication. ;

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revised manuscript January 17,


Polymer Durability - ACS Publications - American Chemical Society

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