Role of Excitation Transfer in the Photochemistry and Radiation

Chapter 25

Role of Excitation Transfer in the Photochemistry and Radiation Chemistry of Solid Polymers

Downloaded by PENNSYLVANIA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch025

Studies of Styrene and Vinylnaphthalene Copolymers with Vinyl Ketones 1

J. E. Guillet, S. A. M. Hesp , and Y. Takeuchi Department of Chemistry, University of Toronto, Toronto, Ontario M5S 1A1, Canada

Radiation studies of styrene and vinyl naphthalene copolymers with vinyl ketones show that the presence of the naphthalene group imparts increased radiation stability to the polymers. Photochemical studies on the same polymers and model compounds lead to the conclusion that the naphthalene quenches singlet or triplet excitons migrating in the solid polymer. Mechanisms are proposed to explain this effect. The radiation chemistry of polymers has been investigated extensively, largely as a result of interest in the ionizing radiation obtained from nuclear reactors and because of the extensive work done on nuclear weaponry and power systems. More recently it has become important in the design of materials for space vehicles and the manufacture of microelectronic and electro-optical devices. It is perhaps unfortunate that the early work in this field was done almost exclusively without the benefit of information about the photochemistry of the same macromolecules, and in fact this remains a major problem in the interpretation of the results of the radiation chemistry of polymers, even at the present time. The field of radiation chemistry of polymers has been extensively reviewed (1-4) and in this chapter we will not attempt to summarize the field at its present stage but will simply review briefly a number of accounts of experimental investigations which attempt to draw correlations between radiation chemistry and the conventional photochemistry of macromolecules. In particular, we will restrict our attention to chemical changes caused by electromagnetic and electron-beam radiation which relate to the role of energy transfer in the solid state. Current address: Department of Chemistry, Queen's University, Kingston, Ontario K7L 3N6, Canada 0097-6156/91/0475-0414S06.00/0 © 1991 American Chemical Society

In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

25. GUILLET ET AL.

415

Photochemistry and Radiation Chemistry


Absorption of High Energy Radiation As the wavelength of radiation is reduced below that of the far ultraviolet the energy of the incident photon increases drastically. Table I shows the wavelength ranges for various types of high energy radiation and the associated energy per quantum (i). As a result of the high energy of the x- or 7-ray photons, it is unlikely that the first step of interaction with matter will be with a valence or out er shell electron. In fact, the first step in the absorption of high energy radiation by matter is usually the formation of a high energy ion by ejection of an inner electron and the recombination of this ion with another electron gives rise to a stream of Compton electrons. These electrons cascade by collision with other mole cules in the system until they reach lower energy levels and can be captured by the outer shell electrons. For example, a single 7-ray photon can give rise to many excitations resulting from the colli sion of Compton electrons with the matter surrounding the original absorption site. For this reason chemical events induced by high energy irradiation often follow tracks or spurs as the particle moves through the condensed phase. Table I.

Relation Between Wavelength and Photon Energy (5) Photon energy, e

Wavelength Type of X (nm) radiation 1250 125 12.5 1.25 0.125 0.0125 0.001

Infrared Ultraviolet Soft X-rays X- rays 7-rays ( Co) eo

eV

kcal

1 10 100 1000 10,000

23 230 2300 2.3 χ 2.3 χ 2.3 χ 2.9 χ

105

1.2 χ ΙΟβ

p

kJ

104 105

10* 107

96 960 9600 9.6 χ 10^ 9.6 χ 105 9.6 χ 10* 12.1 χ 107

The interaction of a Compton electron with a molecule AB may take one of two paths, either equation 1, the primary ionization, or some form of excitation, equation 2. +

AB —

•

AB + e"

AB —

•

AB

ionization excitation

(1) (2)

The electrons ejected during the ionization will lose their excess kinetic energy by initiation of further ionization and excitation until the electrons finally reach thermal energy. At this point, neutralization of a cation or attachment can occur: +

e" + AB

—> AB*


(3)

RADIATION EFFECTS ON POLYMERS

416 or attachment to a neutral molecule

e" + AB —• AB"

(4)

Cationic species may also have sufficient excess energy to decompose further by one of two mechanisms: +

—• A + Β

+

—• A + Β- + e"


AB

AB

+

(5)

+

(6)

Energy Transfer Processes (4a,5) It has long been recognized that in certain cases of radiation-in duced reactions, the amount of reaction occurring is much larger than expected (5,6). This phenomenon can be accounted for by the transfer of energy, which can occur by means of mass transfer, charge transfer, excitation transfer or a combination of these. The different mechanisms are not always distinguishable. However, in solid polymer matrices at low temperatures, mass transfer contribu tions are usually negligible. Charge Transfer. Charge transfer can occur by the transport of electrons, positive charges or ions through the solid matrix. Elec tronic transfer occurs when electrons or positive charges are the only migrating entities. Ionic transfer involves a combination of mass and charge transfer, i.e., protons or larger charged entities may migrate through the polymer matrix. The migration of electrons or positive charges can further be divided into localized and non-localized diffusion of charge. Posi tive or negative charges localized on particular groups on the poly mer chain can be exchanged between like groups by resonant charge transfer without the loss of energy. The non-localized diffusion of charge can result in an annihilation of a positive and negative charge pair to result in a highly excited group in the polymer. This excitation energy can then migrate further through the polymer. Excitation Transfer. Excitation energy transfer occurs when an excited molecular group in the polymer loses its excitation energy to an acceptor molecular group (equation 7). This transfer can D* + A

• D + A*

(7)

occur either by the emission and reabsorption of a photon, called "trivial transfer", or by radiationless transfer. Depending on the distance between donor and acceptor and their respective energy levels and spin state, the energy exchange can be described by different models. For the exchange of electronic excitation energy, the mechanisms are exchange transfer, Fôrster transfer and excimer transfer.


25. GUILLET ET A L


417

Exchange Transfer. Exchange transfer occurs only over short dis tances since it requires an overlap of the orbitals of the groups involved. The greater the overlap, the more efficient is the trans fer of electronic excitation. In the region of overlap, an excited electron can be associated with the donor as well as with the acceptor molecule.


According to Dexter (7), the rate of exchange energy transfer is given by W

=

(4r /h)Z J fpi^f^i/Jdi/ 2

2

(8)

2 2 where Ζ = k exp(-2R/L), k is a constant, L is the effective aver age Bohr radius, *S the acceptor absorption spectrum, is the donor emission spectrum, and R is the donor-acceptor distance. From this equation it can be seen that the rate of transfer is expo nentially dependent on the donor-acceptor distance. Fôrster Transfer. Fôrster transfer (also referred to as resonance transfer) is due to the Coulombic interaction between two groups that are at a distance of several times their van der Vails radii. According to Fôrster ( £ ) , the rate of resonance transfer is given by

D

^

A

4

n R° r(D)

J

0

v

1

where kj)*^ is the rate constant for energy transfer (s" ), R is the donor-acceptor distance, κ is an orientation factor. For a random distribution of donors and acceptors κ = 2/3, ^ is the quantum yield for donor emission, η is the refractive index, r(D) is the do nor state lifetime, FTJ(Î') is the normalized spectral distribution of donor emission, and e(v) is the molar extinction coefficient of the acceptor. One can conclude that resonance or Fôrster transfer is less dependent on the donor-acceptor distance than exchange transfer. A

2

2

In the case of Fôrster transfer, one usually defines a distance at which there is a 507· chance of effective tranfer between donor and acceptor. This distance, Ro, can be predicted from R

6

2

=

8.8 , 10-J « ,(D) j -

ν

,

)

φ

)

to

{ 1 0 )

Using equation 10, the Ro value for naphthalene-to-naphthalene transfer, Ro, is about 7 i , whereas for phenyl-to-phenyl it is about 3.5 A. Chemical Processes in Solid Polymers. The radiation chemistry of polymers will depend on the reactions of the ions, radicals or ex cited species created by the primary events. In polymers, not only


418



does radiation induce chemical changes but extensive changes of physical properties may also occur as a result of crosslinking or chain scission. Concern with these two reactions of crosslinking and scission dominates the earliest studies of the radiation chem istry of polymeric materials (9). In general, the chemical struc ture of a polymer determines which of the reactions will predom inate. The efficiency of a radiochemical process is usually described in terms of the G value, which is the number of chemical or physical events per hundred electron volts of energy absorbed (9600 kJ). For a polymer with a normal molecular weight aistribution, scission will predominate over crosslinking if the G value for scission, G , ex ceeds the G value for crosslinking, G , by a factor greater than two (10). Vhen crosslinking predominates, the molecular weight rises until a critical dose, called the gel dose, r , is reached, above which an insoluble gel network of crosslinked polymer is formed in increasing amounts. As a general rule, vinyl polymers on which each atom of the main chain carries at least one hydrogen atom, will tend to crosslink, while, if the polymer contains a tetrasubstituted carbon atom, the polymer will usually degrade (11)- Presumably, this is due to the higher probability of the scission process in tertiary hydrocarbon radicals: s

x

g

,—i—CH„-£—CH —CHg—ν—v^«2— —^"2" 0 Ό

ΙΐΗβ

ΙΐΗβ

lîHg

Η

1 ~~—CH —C—CH —C=CH + 2

2

Η

\

2

H

3

-C—

(11)

C /

There are, however, many notable exceptions to this general rule. From this definition of the G value, it is obvious that the relationship of this to a photochemical quantum efficiency, φ is given by 9

G = *(100/e ) p

(12)

where C p is the energy in eV per photon of the light used. For light oi 313 nm, for example, one photon has an energy of 3.96 eV. The radiation dose absorbed by a polymer is usually expressed in terms of the Rep (Rôntgen equivalent physical) or, more commonly, Rad. The amount oi radiation absorbed per unit time can be calculated from the known energy flux of the source multiplied by the absorbance of the polymer for the particular radiation used, or can be determined directly with a suitable actinometer. Usually a good


25. GUILLET ET AL·


419

approximation for 7-radiation is that the absorbance is proportional to the density of the polymer. Experimental details on these pro cedures can be found in standard reference works on polymer radia tion chemistry (1-4,9).


Comparison of UV and 7-lay Photochemistry of Polymeric letones Studies of lodel Compounds. Only a few studies have been made of the correlations which can be drawn between UV photochemistry and radiation chemistry in polymer systems. Among the earliest of these are studies by Schultz (12) who showed that solid films of poly(methyl isopropenyl ketone) (PMIPK) degraded rapidly in

1=0

PMIPK molecular weight when exposed to both 7-rays and UV photons. How ever, the G value for cobalt-60 7-rays was 1.1, as compared to 4.5 for UV photolysis at 254 nm (φ = 0.221. The 7-ray G value increas ed to 2.4 in the presence of air. Slivinskas and Guillet studied a series of model compounds containing ketone functional groups. The first of these studies involved a series of symmetrical aliphatic ketones subjected to 7-radiolysis ( Co) under vacuum, the products being analyzed by GLC. The results were then compared with the UV photochemistry 01 the same model molecules (IS). 8

s

60

The absorption of high-energy radiation does not follow the same selection rules and spin conservation laws that apply to the forma tion of electronically excited states in UV absorption. Vith highenergy 7-ray photons, the probability of absorption is, in practice, directly proportional to the mass of the stopping atoms and conse quently a rather random absorption of energy should take place in the first instance. Surprisingly, in spite of this, it appears from experimental results, that a significant portion of the energy is localized in the same excited states as are induced by the absorp tion of the UV photon. For example, Pitts and Osborne (U) observed that the major products arising from 7-irradiated aliphatic methyl ketones were those of the classical Norrish type I and II processes. The primary events after interaction of 7-rays with an aliphatic ketone are usually considered to be

(13)


420


+

0 0 R—ϋ—R + R-l—R 0 R—— I RH Downloaded by PENNSYLVANIA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch025

+

0 -53—

ο

R—U—RH* + R .H ' —l!—R (14) +

0H • R-|—R

(15)

The free electrons released by the ionization will undergo addition al collisions and will probably react to neutralize some of the ionic species present by

0

0'

R— — I R + e"

• R—— iR

0 R—ϋ—R + e"

(16)

0 > R—U—R

+

(17)

Additional excited molecules will be formed by reactions between cationic and anionic species present. Thus

0'

0

R—ϋ—R + R—— I RH +

0* 0 R—ϋ—R + R—ϋ—R+

0

O H

• R— — j[ κ + R—— j ι

(18)

0* • 2R—U—R

(19)

Based on the experimental evidence, most of the excited ketone species will react by the four classical processes elucidated in the UV photochemistry of aliphatic and aromatic ketones (15):

Norrish type I

Norrish type II


25. GUILLETETAL


421


Hydrogen abstraction

Cyclobutanol formation

Although the primary excited species (e.g., ions and electrons) formed by radiation processes are seldom those created by selective absorption of UV photons, they have very short lifetimes (16) and the path of the reaction appears to be nearly the same. It seems likely that lower energy excited states such as those accessible by UV absorption are eventually populated by a series of energy transfer processes. In any case, the products produced from the excited ketone molecules are predominantly those expected from the Norrish type I and II processes (13,14)' Polymeric Ketones. In the case of vinyl ketone copolymers with polystyrene, reaction of the carbonyl group competes with that of the methylene groups in the hydrocarbon backbone and the phenyl groups in the side chains. Analysis of the products of the reaction in simple model aliphatic ketones can give an accurate estimate of the relative proportions of the reactions of the carbonyl and methylene groups. Table II shows that the carbonyl group is about four times as reactive towards 7-rays as would be predicted from its mass absorption coefficient. For example, although in 4-heptanone the carbonyl would be expected to absorb only 257. of the radiation, the Norrish type I and II processes accounted for 847. of the total reaction products observed. Similarly, the ratio of Norrish type I and II products to total reaction products remains constant at about 4 for the series from 4-heptanone to 7-tridecanone (IS). It was also noted, both in photochemical and radiochemical investigations, that crystalline ketones, such as 8-pentadecanone and 12-tricosanone, gave no measurable amounts of either type I, or type II or of normal disproportionation products (IS). This is presumably because the molecular motion possible in a crystalline lattice is insufficient to allow escape of the radical products or the formation of the cyclic six-membered ring intermediate required for the Norrish type II process. When the ketones were diluted with hexane or other hydrocarbon solvents it was found that the yield of type I and II products was independent of concentration down to about 507. dilution, after which there was some decrease in the products (Figure 1). This shows that there was an effective mechanism for the In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


e

60

4

4

Role of Excitation Transfer in the Photochemistry and Radiation

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