9- photodegradation in polymer

2

PHOTODEGRADATION OF HIGH POLYMERS

ROBERT B. Fox

u.s. Naval Research Laboratory, Washington, D.C.

CONTENTS

Introduction 47

Experimental Methods in Polymer Photolysis 48

Ultraviolet Irradiation Processes in Polymers 50

General 50

Primary Reactions Following Absorption 50

Reactions Following Bond Dissociation 51

Direct and Indirect Processes 53

Photodegradation of Individual Polymers 54

Poly(methyl Methacrylate) 54

Poly(butyl Methacrylates) 60

Poly(methyl Acrylate) 62

Poly(ethyl Acrylate) 62

Poly(methacrylic Acid) and Poly(acrylic Acid) 63

Polystyrene 63 Poly(~-methylstyrene) 68

Polyethylene 70

Polypropylene 72

Polyketones 72

Polyacrylonitrile 74 Poly(vinyl Chloride) and Other Halogenated Polymers 75

Poly(viny] Alcohol) and its Esters 77

Polyesters 77 Polyethers 78 Polysulfides 79

Silicones 79 Polyamides 80 Cellulose and Cellulose Derivativ~ 82

Concluding Remarks 85

References 86 45

2

P H O T O D E G R A D A T I O N O F H I G H P O L Y M E R S

ROBERT B. FOX

U.S. Naval Research Laboratory, Washington, D.C.

INTRODUCTION

Since man was first burned by an overdose of sunshine, he has been wondering what happened and what he could do about it. As his technology moved into the present-day age of plastics, he has seen these materials grow old before their time from the warming rays of the sun and what he had come to regard as the life-sustaining agency of oxygen. It became economically important that something be done about this seemingly paradoxical situation even while man continued to receive his annual spring burn. A beginning had been made when a technology based on higher energy radiation burst forth. Investigative efforts were forcefully turned toward the effects of this relatively new and important influence on the materials used by man. This effort has burgeoned in the last fifteen years as attested by the enormous amount of research recounted elsewhere in this volume.

Ultraviolet and visible light have not lost their importance as progenitors of destruction in plastics. Increasingly, it is being realized that an understanding of the fundamental processes involved in the interaction between these radiations and polymeric materials is required before major progress can be made in the design of stable polymer-containing systems. This not only applies to photolytic stability but to radiolytic stability as well. For example, it has been demonstrated with poly(methyl methacrylate) (Charlesby and Moore, 1964), that the overt symptoms of degradation due to ionizing and ultraviolet radiation are identical and differ only in the efficiency with which such degradation is brought about. Thus, studies of either type of radiation effect may be applicable to an understanding of both. High energy radiation, however, often gives rise to a broad spectrum of reactive intermediates with consequent complications in the analysis of the total effect. The less energetic ultraviolet radiation, on the other hand, acts directly and in a more selective way in bringing about electronic excitation and subsequent bond homolysis; the results are generally more readily interpreted than those from ionizing radiation.

47

48 R O B E R T B. FOX

In this review, emphasis will be placed on the photolysis of synthetic high polymers and closely related natural products such as cellulose and its derivatives. Jellinek (1962b) has reviewed much of the literature through 1961; reviews of the older work are also available (Jellinek, 1955; Grassie, 1956). In connection with a survey of the effects of ionizing radiation on polymers, Wall and Flynn (1962) have covered some photodegradation work as well. An outstanding review of ionizing radiation effects on polymeric systems has been presented by Chapiro (1962). For a fairly broad survey of a number of topics in the field of photochemistry of small molecules, a continuing series of reviews is available (Noyes, Hammond and Pitts, 1963-5).

EXPERIMENTAL METHODS IN POLYMER PHOTOLYSIS

Two general experimental approaches appear to be followed in the photodegradation field. The first, which might be termed the "practical approach", has given rise to a far larger volume of published papers than the second-- and, as with an iceberg, much more work than appears on the surface may be frozen in laboratory files. Essentially, the practical approach involves some kind of an analytical technique by which a specific property of a degraded system or systems may be assessed. Usually, the property is of utilitarian interest, as with color changes in an organic coating during actual or simu- lated weathering. The object is the solution of a specific problem caused by degrading influences. The results are almost always fragmentary and some- times appear to be pieces from a number of unrelated jigsaw puzzles. None- theless, such information can be useful, provided it can survive the test of intercomparison among different laboratories. Very frequently, it provides the only guides to investigators following the second approach to polymer photodegradation--the mechanistic approach.

It is with work which contributes to the knowledge of the fundamental processes of polymer photolysis that this review is primarily concerned. The immediate problem is the determination of what took place during degradation; the ultimate answers desired are to questions of how the phenomena occurred and what can be done to control them. The phrase "elucidation of mechanism" is often bandied about; "elucidation" as used in polymer science is frequently a euphemism for "speculation".

The methodology used here is the natural outgrowth of a merging of the fields of photochemistry and polymer radiolysis. Ultraviolet radiation sources are far simpler and less hazardous to use than high energy radiation sources. In most work, mercury vapor lamps have been used: the low pressure "germicidal" lamp is a convenient, relatively monochromatic, low-intensity source of 2537 A radiation; medium pressure lamps provide radiation at a number of wavelengths in the 2200-4000A region, and these are readily

P H O T O D E G R A D A T I O N OF H I G H P O L Y M E R S 49

isolated by the use of filters; high pressure lamps give high intensity radiation over a continuum of wavelengths in the same region. These and other ultraviolet radiation sources such as xenon lamps, hydrogen lamps, carbon arcs, etc., have been described in detail (Sch6nberg, 1958; Masson, Boekelheide and Noyes, 1956). A convenient portable radiation source shield has been developed in our laboratory (McDowell, Isaacs, Fox and Saalfeld, 1963). Intensities of radiation from these sources can be varied by the use of screens or by interposing suitable thicknesses of filters between sample and source. Determination of the intensity of the incident radiation can be made by con- ventional uranyl oxalate actinometry (Masson, Boekelheide and Noyes, 1956) or by the more rapid ferrioxalate method (l-[atchard and Parker, 1956; Parker, 1953); many other methods are available. It should be mentioned at this point that all too few investigators trouble themselves to make actino- metric measurements; it is only by this means that quantitative comparisons among different workers can be made. Many contributions shed much light on the subject of polymer photolysis; it is a pity when the authors fail to say how much.

Films, powders, and solutions of polymers have been irradiated under various conditions. Quartz cells are generally used, although secondary effects due to fluorescence from the quartz have not been considered. Polymer purity is always an open question. Jellinek and Bastien (1961) noted that sample history had a marked influence on the results of polyacrylonitrile photolysis in solution; in our own laboratory ([saacs, unpublished), we have observed a one-hundred-fold variation in the quantum yield for hydrogen formation from polystyrene films cast from different chlorinated solvents.

The analytical methods used in the assessment of changes in a polymer during irradiation are generally the same as those used in high energy radiation work (Chapiro, 1962). Viscosity is the most-used technique to evaluate molecular weight changes, although this method is only valid where a "most probable" molecular weight distribution exists before and after degradation and no crosslinking occurs. Light-scattering and osmometry have also been used. Where crosslinking occurs, gel fraction methods are appropriate.

Volatile products from the photolysis of solid polymers are measured by the methods commonly employed in photochemistry. The mass spectrometer (Wall, 1962) is the instrument of choice, although little work has appeared to indicate the actual errors encountered in the measurement of gases from replicate polymer samples. In certain instances, gas chromatography can be fruitfully employed. Manometric techniques have also been used successfully where a single gas is evolved. Grassie and Weir (1965a) have detailed such an apparatus which has proved useful in photooxidation studies. These workers were able to measure the absorption of as little as 1.4 × 10-s moles of oxygen from an initial pressure of 600 torr during the irradiation of a 5 mg sample of polystyrene.

D P.P,S.

50 ROBERT B. FOX

Both optical and magnetic spectroscopic techniques are routinely utilized in the identification of products and intermediates formed during degradation. An apparatus for solution degradation work has been described (Fox and Price, 1965a), in which both ultraviolet spectra and viscosity measurements can be made without opening the irradation cell containing the solution. A four-port cell which allows ultraviolet irradiation of a film sample in one direction and measurement of the infrared spectrum of the evolved gases in the other has also been described (Alexander, Noonan, Cowling and Kagarise, 1959). Searle and Hirt (1962) have developed the technique of ultraviolet microphotometry to determine the effects of different wavelengths of radiation on the absorption spectrum of a polymer sample; the method is said to have a sensitivity of the same order as fluorimetric techniques.

U L T R A V I O L E T I R R A D I A T I O N P R O C E S S E S IN P O L Y M E R S

General It was observed above that the overt effects of ultraviolet and high energy

radiation in polymers are similar. These effects are generally interpreted in terms of free radical reactions ultimately leading to main chain scission, crosslinking, unsaturation, and the formation of small molecule fragments. The major differences between the two types of radiation are felt in the events immediately following absorption of the radiation by the polymer system. The magnitudes of the energy absorbed are such that while a quantum of ultraviolet energy is capable of exciting an orbital electron to a higher level, a quantum of gamma radiation, for example, is capable of both ionization and subionization of orbital electrons. The ejected electrons are in turn capable of excitation of other molecules in their paths before capture by a positive ion. At this site, which must be close to the site of absorption because of cage effects, the available energy is very much in excess of that required for bond homolysis. Thus, the major difference between radiation types is one of selectivity. Although the results are similar, they are almost entirely due to secondary reactions in the case of high energy radiation. In the determination of the primary reactions in the degradation of polymeric substances, lower energy ultraviolet radiation should be the more useful tool. A disadvantage in the use of ultraviolet radiation lies in its attenuation by the absorbing substance. In solid polymers, this often means that chemical changes will be seen primarily in the surface layers.

Primary Reactions Following Absorption Most research effort in degradation has been dedicated to uncovering the

nature of the reactions following the formation of the first free radical in the system. Tremendous strides have been made in recent years by photo-

P H O T O D E G R A D A T I O N OF H I G H POLYMERS 51

chemists studying the events occurring between absorption of an ultraviolet photon and bond dissociation. This fundamental knowledge is now beginning to find application in the study of polymer degradation processes and their control (Gardner and Epstein, 1961 ; Fox and Price, 1965b).

It is therefore appropriate to consider the detailed progress of a polymer (or any other) molecule immediately after absorption of a photon. Most organic molecules lie in a singlet ground state. Absorption of a photon raises the molecule to an excited singlet state. The molecule may revert to the ground state by the emission of a photon (fluorescence) or by radiationless transitions and the generation of heat. In some instances, intersystem crossing can take place, and the molecule will find itself in a lower energy excited triplet level. Again, reversion to the ground state can be accompanied by photon emission (phosphorescence) or heat. If the molecule has sufficient energy in the excited state, whether it be singlet or triplet, bond dissociation may take place. This bond-breaking process, which is relatively slow, must compete with all other deexcitation processes, and it is statistically more probable in the long-lived triplet state. Bond dissociation in a polymeric molecule may manifest itself as a main chain break.

Where an excited molecule is in the vicinity of a second molecule, and this is of course always the case, reversion to the ground state may be accomplished through a transfer of energy between the two. Wilkinson (1964) has recently reviewed this currently exciting area of investigation. Such electronic energy transfers are now thought to be crucial in determining the rates of photodegradation of polymer molecules, particularly where long-lived triplet states are involved (Fox and Price, 1965b). This is an area which has long entertained speculation in radiolysis. It may be expected that energy transfer in polymers will be a fruitful area of investigation in the near future, for it is apparent that much of the fundamental mechanism of polymer photodegradation lies within this sphere.

Reactions Followin9 Bond Dissociation

The general processes of degradation following initial bond dissociation have been discussed at length both qualitatively and quantitatively by other reviewers (Jellinek, 1962, 1955; Grassie, 1956; Wall and Flynn, 1962; Chapiro, 1962). The kinetic analysis differs from that for radiolysis primarily where attenuation of the radiation is involved. Expressions have been derived by Shultz (1958) and Flynn (1958) for average molecular weights of polymers undergoing random chain scission where the initial and final number- to weight-average molecular weight ratio is two and the Beer-Lambert law is obeyed. Jellinek (1962a) has done the same for a polymer sample having an initially homogeneous chain length.

Chain scission and crosslinking are the processes most responsible for the gross effects of ultraviolet irradiation of polymers. It is an empirical rule that

52 ROBERT B. FOX

carbon-chain polymers having quaternary carbon atoms in the main chain do not crosslink in the absence of air or sensitizers. At room temperatures, chain scission is generally random. A commonly used expression for the rate of scission is

1 1 . . . . . kt Pt Po

where Pt and P0 are the number-average degrees of polymerization at time t and initially. This equation assumes a constant absorbed intensity of radiation. More accurately, with random scission a plot of the number of scissions per polymer molecule against the energy absorbed by the system will be linear. The quantum yield for random scission in scissions per quantum absorbed (analogous to the G-value ofradiolysis) is readily evaluated from such a plot if crosslinking is absent, if the molecular weight distribution is random at all stages, and if the attenuation of the radiation is slight.

In most cases, depolymerization is small at room temperature; "zip lengths" increase as the temperature ofphotolysis is increased. Grassie (1956) has considered the type of experimental results to be expected from various degrees of depolymerization and scission.

Where depolymerization and chain mobility are slight, the picture at the termination step appears to be one of a cage in which the two chain ends are surrounded by a cloud of low molecular weight radical fragments. Polymer chain recombination is a relatively rare event, however, since quantum yields for random scission have usually been found to be independent of the intensity of the radiation. Under such conditions, termination of the chain ends could be by combination with, for example, a relatively mobile hydrogen atom, or by expulsion of a radical from the chain itself; both processes would be first order with respect to polymer chains. In those few cases where termination is second order and involves the reaction of two polymer radicals, Jellinek (1964) has shown that the steady-state treatment is still applicable.

Crosslinking is a frequently encountered process in polymer photolysis. It is of practical importance in photo-reproduction processes and has received much attention where an improvement in physical properties of the bulk material results. For the purpose of generalization, it need only be stated here that crosslinking may be largely due to reactions of secondary polymer radicals resulting from cleavage of groups or atoms from the main chain. The crosslinking may be by polymer radical combination or by addition to an unsaturated site in another chain. These phenomena will be dealt with under the discussion of individual polymers.

Many of the volatile products in polymer photodegradation are a consequence of these same side chain cleavages. The nature of these products will, of course, vary with the polymer under irradiation. A complete analysis


is desirable, since such products not only help to delineate mechanisms but are often indicative of impurities, either adventitious or as part of the polymer chain.

Direct and Indirect Processes

As methods of analysis become more refined, it is becoming increasingly apparent that the above reactions are not entirely a consequence of absorption of a photon by the polymer molecule undergoing degradation. It is evident that it is a rare polymer sample that is not accompanied by polymeric or small molecule impurities. These "foreign" materials can affect not only the rates of degradation reactions, but the nature of the reactions as well. Quite possibly, the photodegradation of a pure polymer has yet to be studied.

In the most qualitative sense, and in the terminology of the radiation chemist, the processes occurring in a polymeric system undergoing photolysis can be divided into two groups, "direct" reactions and "indirect" reactions. Direct reactions come about from the absorption of a photon by the polymer followed by bond homolysis and the formation of degradation products. For example, i ra C-Iff bond is broken:

PH + hv ~ Plq*

P H * ~ P" + HI" ~ products

In the indirect reactions, other molecules or even other functional groups in the same polymer become involved. They can similarly be excited and undergo reactions to form free radicals. These excited molecules or fragments may eventually interact with the polymer to give products similar to those arising from the direct reaction. The reverse reactions may also occur: excited polymer or polymer radicals may interact with the "foreign" molecules. These "indirect" processes can lead to degradation, and they may involve energy transfer or free radical processes:

PH* + SH~-~PH + SH*

P" + SH~-PI-I + S"

An obvious example of an indirect reaction is the effect of atmospheric oxygen on polymer photodegradation. In regard to chain scission, oxygen can play the role of an apparent inhibitor (poly(methyl methacrylate)), accelerator (polystyrene), or seemingly be without influence (poly(~-methylstyrene) ). Few investigations have been devoted specifically to the interactions occurring among the constituents of a solid polymer system subjected to ultraviolet irradiation. The effect of residual solvents on polystyrene film photodegradation is under study in our laboratory (Isaacs, unpublished). Considerable attention has been given to the influence of "sensitizers' on the crosslinking of polyolefins (see, for example, Charlesby, Grace and Pilkington

54 R O B E R T B. F OX

1962). Reactions probably involving a transfer of electronic energy are seen in the photolysis of silicones in the presence of naphthalene (Siegel and Judeikis, 1965) and in the effect of styrene on the emission spectrum of polystyrene (Basile, 1962). Charlesby and Partridge (1965a) have identified carbonyl groups as the luminescence centers in several polymers.

The existence of an indirect effect is most readily seen in the photodegradation of polymers in solution. Apparent quantum yields for the random scission of poly(~-methylstyrene) (Fox and Price, 1965a) and poly(methyl methacrylate) (Fox and Price, 1965b), for example, are markedly dependent on the solvent. Additives can exert both an accelerating and inhibiting effect over and above that of an optical filter in systems containing these polymers.

Some external or environmental effects on polymer photodegradation might also be mentioned. Zaitoun (1962, 1964) has studied the influence of the wavelength of the incident radiation on the degradation of several polymers. In general, the lower the wavelength of the radiation absorbed, the greater the damage unless impurities are involved; on this basis, a rate-wavelength plot should correspond to the absorption spectrum of the polymer. It is not unexpected that the nature of the degradation products, most of which are the result of secondary reactions, will also be affected by the radiation.

The influence of atmospheric oxygen has been mentioned. Stephenson and co-workers have compared photolytically-induced changes in physical properties (Stephenson, Moses and Wilcox, 1961 ; Stephenson and Wilcox, 1963), crosslinking and scission rates (Stephenson, Moses, Burks, Coburn and Wilcox, 1961), and gaseous decomposition products (Stephenson, Lacey and Wilcox, 1961) for a number of polymers after irradiation in vacuum, nitrogen, or in oxygen. A wide variation in behavior was encountered, depending on the nature of the polymer.

It is clear that while a large number of degradation processes and phenomena are common to all polymers and polymer systems, there is much yet to be learned. An examination of the detailed results of studies of specific polymers is in order to determine what points are general and correspond to the above discussion and what points emerge as a consequence of the individual properties of the polymers themselves.

P H O T O D E G R A D A T I O N O F I N D I V I D U A L P O L Y M E R S

Poly(methyl Methacrylate) Perhaps more attention has been paid to the quantitative aspects of radio-

lytic, photolytic, and thermal degradation of poly(methyl methacrylate) than of any other polymer. This polymer is of interest to the experimentalist, not only because of its manifold practical applications, but because its properties


are well characterized and the degradation reactions which it undergoes are relatively clean-cut. To the photochemist, the low absorption coefficient of poly(methyl methacrylate) at 2537A means that this useful radiation can penetrate to all parts of a thin film with only slight attenuation, whereas with highly absorbing polymers, a "skin effect" will become important. Finally, poly(methyl methacrylate) is unusual in that the mechanisms for photodegradation at room temperatures and for thermal degradation are entirely different.

Near room temperature in vacuum, films of poly(methyl methacrylate) subjected to ultraviolet irradiation undergo rapid decreases in molecular weight concomitant with the formation of small amounts of volatile material. This is typical behavior for a polymer suffering random cleavage of the main chain without extensive depolymerization.

Thin (15~/) films of poly(methyl methacrylate) formed by the evaporation of methylene chloride solutions were irradiated in vacuum at 25°C with a medium pressure mercury lamp (Fox, Isaacs and Stokes, 1963). These workers reported quantum yields for random scission of 4 x 10 - 2 scission per quantum absorbed at a pressure of 2 × 10-5 torr. A one hundred-fold increase in the air pressure did not affect the results. The quantum yields were essentially independent of the incident intensity of the radiation and appeared to be a function only of the total energy absorbed. It was therefore concluded that recombination of polymer radicals was not a significant termination process. A slight dependence of the quantum yield on molecular weight was also noted; this may only reflect the increasing likelihood of a longer chain to contain a weak link. Some error attended the absolute values of the results because of difficulties in measuring small absorption coefficients in the tail of an absorption band [2~a x ----213 rn/~ for poly(methyl methacrylate)]. The manufacturer's spectral distribution for the lamp was used, and ageing changes in the lamp were not considered.

Charlesby and Thomas (1962) also studied the photolysis of thin (40/0 films of poly(methyl methacrylate) at room temperature. They used both low and medium pressure lamps with similar results. The photolyses were carried out in atmospheres of both air and nitrogen; and the quantum yields obtained were within 5 ~ of each other, 0.4 x 10-2. This value may be compared with those obtained in air by Fox, Isaacs and Stokes (1963), 1.7 × 10- 2, Gardner and Epstein (1961), 3.2 x 10 - 2 , and by Shultz (1961), 2.1 × 10 - 2 . The latter value was recalculated from the data of Shultz (1961) without consideration of the attenuation of the radiation by the film. If the different rates of absorption by succeeding layers in the film are considered (Shultz, 1958), the quantum yield, 2.3 x 10- 3, given in the original paper is obtained. It would appear that both oxygen and nitrogen act as inhibitors to photodegradation. While nitrogen might suppress the diffusion of gases from the film (Stephenson, Lacey and Wilcox, 1961), it is difficult to see how this would change the rate


of scission unless a considerable amount of dissolved oxygen remained in the film. It might be supposed that oxygen itself acts as an inhibitor by scavenging secondary polymer radicals which are precursors to chain scissions.

Crosslinking has not been observed in photodegraded poly(methyl methacrylate) in the absence of sensitizers. The sedimentation pattern of a vacuum- degraded polymer film was unchanged from that of the original material (Fox, Isaacs, Stokes and Kagarise, 1964). Poly(methyl methacrylate) in air has been crosslinked, however, by ultraviolet radiation in the presence of benzophenone (Oster, Oster and Moroson, 1959) or by daylight in the presence ofp-benzo- quinone (Bevington, 1959). It is unlikely that a very small amount of crosslinking by, say, sensitizing impurities, would account for the different quantum yields obtained by the various investigators cited above, since in every case, molecular weight changes were determined by viscometric methods.

In the patent literature may be found a plethora of references in which various substances are added to bulk poly(methyl methacrylate) to act as photodegradation inhibitors. The vast majority of these function primarily as optical filters and only reduce the intensity of radiation reaching the polymer without actually affecting the quantum yield for chain scission. Gardner and Epstein (1961), however, have observed that in air pyrene and p-terphenyl act as inhibitors over and above the optical filter effect; their results were interpreted in terms of an energy transfer mechanism. This will be treated more fully below in the discussion of solution degradation.

While molecular weight changes ordinarily exert an overriding influence on the bulk physical properties of a degraded polymer, these same properties may also be influenced by relatively small chemical changes occurring within the material. These changes may alter the chemical or physical character of the chain through the formation of new functional groups or they may result in the formation of new chemical entities which can act as plasticizers, for example. The identification of the nature of these changes can provide important information regarding the mechanism of the degradation process.

Many investigators have noted the appearance of a new band at about 285m# in the ultraviolet spectrum of ultraviolet-irradiated poly(methyl methacrylate). The same spectrum is obtained in the presence or absence of air (Fox, Isaacs and Stokes, 1963; Frolova, Efimov and Riabov, 1964), and the new absorption remains with the polymer after reprecipitation, indicating a new chain chromophore. Heating an air-irradiated film in vacuum reduced the intensity of the absorption (Shultz, 1961). Whether this is the result of the volatilization of an absorbing compound or due to an accelerated decay of an absorbing species is not known. Charlesby and Thomas (1962) observed such a decay in this band for both ultraviolet- and gamma-irradiated films. It is reasonable to ascribe the 285m# absorption to a carbonyl-containing chromophore formed by secondary reactions after homolysis of the ester C-O bond (Fox, Isaacs and Stokes, 1963). The fact that the absorption shifts


to higher wavelengths during irradiation, however, led Frolova, Efimov and Riabov (1964) to assign it to conjugated unsaturation in the chain. An additional band near 240m# has also been observed in degraded films (Charlesby and Thomas, 1962); the wavelength is somewhat high for monomer, and it has not been identified. The participation of impurities in the overall increase in absorbance at 300mp for poly(methyl methacrylate) irradiated in air with a sunlamp is seen in the fact that polymer containing dibutyl phthalate rapidly became opaque relative to either azobisisobutyro- nitrile- or benzoyl peroxide-initiated material (Frolova, Efimov and Chekmodeeva, 1964).

Rather long exposures are necessary to induce changes which can be seen in the infrared spectrum. Frolova, Efimov and Riabov (1964) observed a broadening of the carbonyl band at 1750 cm-1 and new bands at 1615 cm-1 and 1640cm-1 which they ascribed to olefinic unsaturation in support of their conclusions regarding the ultraviolet spectral changes.

Frolova and Riabov (1959) exposed poly(methyt methacrylate) films under high vacuum to 3030-3130 A radiation at 25 ° and obtained a quantum yield of 2.3 x 10-4 for gas formation. By shorter wavelength irradiation of samples tagged with C 14, it was shown that the ester groups were the primary source of these gases and that methyl formate was a major product (Frolova, Nevskii and Riabov, 1961). Methyl formate, methanol, and methyl methacrylate were formed in quantum yields of 0.14, 0.48, and 0.20, respectively, indicating a depolymerization of about five monomer units per chain break (Fox, lsaacs and Stokes, 1963); methane, hydrogen, carbon monoxide, and carbon dioxide were also detected.

Electron spin resonance spectra of ultraviolet- and gamma-irradiated poly(methyl methacrylate) are identical, indicating that the same free radicals are present in the irradiated polymer (Abraham, Melville, Ovenall and Whiffen, 1958; Charlesby and Thomas, 1962). This radical is probably I, but Milinchuk and Pshezhetskii (1964) have shown that the radicals II and III are formed at 77°K in reversible reactions by the ultraviolet irradiation of a previously gamma-irradiated sample.

CH3 CH~ I I

C H 2 C " ~ CH'2CH I i

C O O C H 3 C O O C H 3

HdO

I II III

As the temperature of irradiation of poly(methyl methacrylate) is increased, marked changes in the rates of certain of the degradation processes are

58 ROBERT B. FOX

observed. Notably, these consist of a greatly increased yield of monomer (the quantum yield is 220 at 163°), an intensity exponent of about 0.5, and a dependence of the rates of initiation on the nature of the chain ends. This outstanding early work (Cowley and Melville, 1952a, 1952b) has been chronicled at length in other reviews and will not be detailed here.

Recently, this work has been extended through a comparison of the results of ultraviolet and gamma irradiation at temperatures up to 180 ° (Charlesby and Moore, 1964) which amplified a similar comparison at room temperatures (Charlesby and Thomas, 1962). No evidence was found for a contribution by ionization reactions at any temperature; at the lower temperatures, the difference between radiation types was primarily one of efficiency. Depolymerization at the higher temperatures was much smaller with gamma than with ultraviolet radiation; no intensity dependence was observed with gamma radiation. This was interpreted to mean that at the higher temperatures, ultraviolet radiation is selectively absorbed at chain ends and that the resulting radicals are far more effective in initiating depolymerization than are the radicals formed by cleavage of a main chain. Charlesby and Moore (1964) also pointed out that at room temperatures, relative to the susceptibility of the main chain to undergo scission, the likelihood of an ester group to undergo homolysis by ultraviolet radiation is about 40 times that with gamma radiation. This is a reflection, again, of selectivity in absorption by ultraviolet light, as well as the tenfold differences in main- chain cleavage efficiencies between the two kinds of radiation.

Photodegradation of polymers in solution has received relatively little attention. This seeming neglect is the natural consequence of the fact that most practical utilization of polymers is in the solid state. It has been repeatedly emphasized, however, that in solid polymers control of composition is difficult, particularly where volatile materials are involved. The use of solution in polymer photolysis allows some freedom in the investigation of the interaction between the polymer and other molecules, since composition can be more accurately controlled. In addition, the elimination of some problems due to bulk polymer properties can be accomplished through the use of solutions; solid phase transitions and small molecule diffusion through a solid, for example, will not be factors in degradation in solution. The probability of crosslinking should be decreased through the separation of polymer chains by solvent molecules unless the chains are agglomerated in a micelle.

Much of the published polymer solution work has concerned poly(methyl methacrylate). A major problem has been that of finding a photolytically "inert" solvent, a problem not likely to be solved. A second question is that of the mechanism of the photodegradation in solution as opposed to the solid. The only detailed answer has been given by Jellinek and Wang (1965), who made a kinetic study of the 2537-A photolysis of poly(methyl methacrylate) in 2-chloroethanol solution under nitrogen at 25 ° to 159°C. The extent to

P H O T O D E G R A D A T I O N OF HIGH POLYMERS 59

which oxygen is present in these solutions is not known. At 25°C, the random scission mechanism was followed. The experimental rate constants in the equation

1 / P t - I/Po = kexj,

however, decreased with increasing polymer concentration, which indicated participation of the solvent in the photolysis. A similar situation has been indicated for chloroform solutions (M6nig, 1958). These results may be due to a cage effect at increased viscosity which operates to increase the rate of combination relative to the disproportionation of polymer radicals. At temperatures up to 159°C, thermal degradation did not play a role. The kinetic chain length (monomer molecules formed per main chain scission), which was negligibly small at 25°C, increased to 14 at 54°C and 312 at 159°C, based on viscosity changes. Energies of activation for monomer formation, 8-9 kcal/mole, were similar for solutions and bulk degradation. Interestingly, at the elevated temperatures random initation is still indicated although a small chain-end initiation may also occur. This is in contrast to the mechanism in hulk, which was primarily chain-end initiation at these temperatures (Cowley and Melville, 1952a). The result in solution may be due to random attack by radicals from photolysis of the solvent. Termination appeared to be diffusion-controlled in bulk and in dilute solution; the intensity exponent approached 0.5 in both cases and the energies of activation for termination were similar.

In contrast to 2-chloroethanol, the quantum yields for random scission at room temperature in methylene chloride (Fox and Price, 1964) and in benzene (Charlesby and Thomas, 1962) appear to be independent of polymer concentration. On the basis of energy absorbed only by the polymer, the quantum yields for random scission in degassed methylene chloride and dioxane solutions at 25 ° were reported to be about 0.14 with some variation due to the radiation source itself and with slight dependence on intensity (Fox and Price, 1965b). In both cases, oxygen acted as an inhibitor. Fox and Price (1965b) have studied the effect of a variety of chemically disparate additives on the quantum yields for scission of poly(methyl methacrylate) in these solvents in the presence and absence of air. In the latter case, they were able to show a rough correlation between the degree of inhibition or acceleration of degradation and lowest excited triplet energy levels of the additives. This was interpreted in terms of electronic energy transfer and indicates that poly(methyl methacrylate) may undergo photodegradation from an excited triplet state, It should be possible to demonstrate singlet or triplet energy transfer in specific cases through sensitized (or inhibited) fluorescence or phosphorescence of the additives. Such experiments would also serve to detail the processes of inhibition with certain polycyclic aromatic hydrocarbons (Gardner and Epstein, 1961).


M6nig and Kriegel (1960a, 1960b, 1964) and Siewert (1964) have investigated the sensitizing effect of polycyclic aromatic hydrocarbons on the photolysis of poly(methyl methacrylate) in solution. Radiation absorbed by the additives but not by the polymer was used. Sensitization was observed with pyrene and 3,4-benzopyrene in benzene in the presence of oxygen. Sensitiza- tion was not seen in chloroform or dioxane solutions in the presence or absence of oxygen. The mechanism of the sensitization is not completely clear, but it must involve an oxygen-transfer reaction which includes both the solvent and a photolysis product from the polycyclic hydrocarbon. Through the use of optical filters and other hydrocarbon additives (which were in- active), it was established empirically that specific energy states in the hydrocarbons are required for activity. Clearly, this work and that outlined in the previous paragraph may lead to a means for the prediction of the photolytic behavior of a polymer system for which the composition is accurately known.

A final example of photodegradation of poly(methyl methacrylate) in solution is provided by the reaction with chlorine. Presumably, the degradation is initiated by the attack of chlorine atoms which in chlorinated solvents abstract hydrogen atoms from the chain (Hahn and Grafmiiller, 1956). The photochlorination reaction has also been studied in aqueous suspensions, where the mechanism is the same as in solution, and the distribution of chlorinated products has been determined (Owens and Zimmerman, 1963).

Poly(butyl Methacrylates) A comparison has been made of the thermal- and photo-initiated (2537 A)

degradation of poly(n-butyl methacrylate) in a molecular still in vacuum (Grassie and MacCallum, 1964). Poly(n-butyl methacrylate) under irradiation at 170 ° was completely converted to monomer, as was poly(tert-butyl- methacrylate), and this appears to be a general property of polymethacryl- lares. Such is not the case in thermal degradation, for while poly(methyl methacrylate) degrades entirely to monomer at 250 °, the n-butyl ester gave only 40 ~ monomer and the tert-butyl ester gave mostly olefin. The latter two polymers became progressively more stable on heating, and this was attributed to the formation of intramolecular anhydride linkages. This idea was supported by the finding that a methacrylic acid-n-butyl methacrylate copolymer was stabilized to thermal degradation at 250 ° and to photodegradation at 170 ° by preheating at 170 ° . It was suggested that in the ester polymer, once an acid group has formed, it can migrate along the chain (p. 61) until a second acid group is encountered and anhydride formation ensues.

Certainly acid groups must form, as has been shown by the photodegradation of poly(n-butyl methacrylate) at 50 ° and 100 ° where, in addition to monomer, l-butene and n-butyl formate were major products (Isaacs and

P H O T O D E G R A D A T I O N OF HIGH POLYMERS 61

CH'2 CH'2CH'2CH 3 /

O O

I \ ~ / I I o I 1 I

CH2C C I \ / I CH2

CH3 CH3

CH2CH2CH2CHa \

0 O o / \

,% / H C C

d > 0

CI-[2 C C ,-~

I CH2 CH 3 CH 3

Fox, 1965b). 1-Butene and acid groups were also products in the polymerization of n-butyl methacrylate at 30 ° (Grassie and MacCallum, 1964). Since the same kind of polymer radicals are undoubtedly present in both the polymerization and depolymerization, the latter authors proposed that a free radical rather than the usual ionic process is involved in the ester decomposition:

CH 3 CH 3 CH3 I I I

CHzC" ~ ~ CH2C ~ ~ C H 2 C

] 11 I~ COOR C--O" C - - O

/ / \ O H O H

"\ / \ CH2CHCHzCH 3 CH2CHCH2CH3

CH 3 CH 3 I

~ CH2C ~ ~ CH2C" + C H 2 = CHCI-[2CH 3

C C . / \ / / \ 0 OH 0 OH

More than one mechanism is possible, however, for Isaacs and Fox (1965) found that acetic acid was a major product from the photolysis of both n-butyl acetate (which also gave 1-butene) and methyl acetate. A question then arises in regard to the reason why poly(methyl methacrylate) does not stabilize itself through acid, and subsequent anhydride, formation as did the n-butyl ester. The answer may lie in the mechanism of anhydride formation where reaction between an acid group and an adjacent ester may be more facile than the acid migration reaction.


MacCallum (1964) further examined the photoinitiated degradation of poly(n-butyl methacrylate) at 170 ° and concluded that, unlike poly(methyl methacrylate), initiation can take place at random sites along the chain since hydrogenation of the unsaturated chain end had no effect on the rate of depolymerization.

Poly(methyl Acrylate) In the light of its technological importance, it is surprising that so little

attention has been paid to poly(methyl acrylate). Partly, this is because both scission and crosslinking take place, and molecular weight changes are therefore more difficult to assess than with its analog, poly(methyl methacrylate).

Fox, Isaacs, Stokes and Kagarise (1964) studied thin (30V) films of poly(methyl acrylate) subjected to 2537-A irradiation at 22 ° in vacuum and in air. No "skin" effect was observed. Benzene-insoluble material was formed early in the vacuum runs, and qualitative comparisons of the sedimentation patterns of degraded and undegraded polymer indicated that crosslinking occurred in both air and vacuum. Oxygen acted to reduce crosslinking, since no benzene-insoluble material was obtained in air runs; based on viscosity changes, an apparent quantum yield for scission of 0.013, independent of intensity, was reported. 2-Methylanthraquinone has been suggested as a sensitizer for the crosslinking of poly(methyl acrylate)-type materials in air for use in photomechanical processes (Oster, 1964).

Spectral changes were minimal, with the formation of a weak band at about 280m/~ the only detectable change. Monomer was not among the volatile products. Formaldehyde, methanol, and methyl formate were formed in quantum yields of 0.02, 0.0019, and 0.008, respectively; these products could well have originated from the ester groups in tbe polymer. Carbon monoxide, methane, and hydrogen were also observed. An interesting result was that carbon dioxide was being accumulated in amounts which increased exponentially with dose. It was suggested that the carbon dioxide was associated with a decomposition of chain ends, since the total number of chain ends resulting from scission also increases with dose.

Poly(ethyl Acrylate) The photodegradation of thin films of poly(ethyl acrylate) in a rough

vacuum or under nitrogen has been investigated by Jacobs and Steele (1960a, 1960b). As with the methyl ester, both chain scission and crosslinking occurred under 2537-A irradiation. Of particular importance in this study was the finding that the time necessary to produce an insoluble gel increased as the temperature of irradiation decreased; below the glass transition temperature (-17°C), no insoluble material was obtained. The explanation for this behavior is seen in the lessened chain mobility at low temperatures which


prevents combination of secondary polymer radicals. It was suggested that the secondary radicals involved are those formed by homolysis of the ethoxy- carbonyl groups rather than by homolysis of the main-chain tertiary hydrogen atoms. If such were the case, it might be wondered why crosslinking does not occur more readily in polymethacrylates, where all the evidence points to the scission of ester groups. Methane and carbon monoxide were the major volatile products, with lesser amounts of carbon dioxide and hydrogen; these products could have come from the ester groups, but there is no explanation for the lack of products analogous to those found in poly(methyl acrylate).

Poly(methacrylic Acid) and Poly(acrylic Acid) Thus far, only the photolytic decomposition of aqueous solutions of these

polymers has been investigated. Anhydride formation, as envisioned in the photolysis of poly(alkyl methacrylates), would be at a minimum under such conditions, and direct comparisons between polymers of the acids and their esters cannot be readily made.

In the presence of hydrogen peroxide, the photolysis of an aqueous poly- (methacrylic acid) solution leads to chain scission with about two hydroxyl radicals required for each break (Baxendale and Thomas, 1958). In the absence of hydrogen peroxide, a half-neutralized polymer solution was degraded with a quantum of about 0.03. Both processes were inhibited by oxygen. By analogy to the photolysis of aliphatic acids, it was suggested that the primary step in the direct photolysis was the cleavage of carboxyl radicals followed by disproportionation of the secondary polymer radical.

VOlker (1961a, 1961b) found that a copolymer from methacrylic acid and methyl methacrylate in weakly acid solution underwent random scission under the influence of ultraviolet radiation or even of daylight and that the cleavage was catalyzed by heavy metal salts. A similar result was observed for poly(acrylic acid) and poly(methacrylic acid). Tile scission rate was related to the coiling of the polymer.

A somewhat more quantitative approach to the photolysis of poly(methacrylic acid) in aqueous solution was taken by Chou and Jellinek (1964). They found that the random scission rate law was obeyed for solutions under nitrogen irradiated by 2537-A light and that the quantum yields for scission decreased with increasing pI-I but were essentially independent of initial chain length, polymer concentration, or the concentration of added electrolytes. As indicated above, the controlling factor in the degradation is the compact- ness of the polymer coil, which in turn is greatly decreased with even a small increase in the degree of ionization of the polymer.

Polystyrene Research effort has been rather limited in the photodegradation of poly-

styrene. The reason is not long in appearing once an investigation of the


polymer is begun: polystyrene is one of the most radiation-stable polymers. This means that any changes taking place during irradiation will be small on a time basis, and these changes may be relatively difficult to measure or to reproduce. Impurities will assume an unusual importance; to make the party a success, the host should be able to outlast the guests.

In some ways, the behavior of polystyrene under irradiation is analogous to that of poly(methyl acrylate). Both crosslinking and scission take place, with an apparent reduction in crosslinking when oxygen is present. Changes in the optical spectrum, especially in the ultraviolet region, are more pro- nounced in irradiated polystyrene than in the acrylates, and they are often attended by post-irradiation phenomena. The volatile products consist primarily of hydrogen.

Polystyrene films exposed to 2537-A radiation in vacuum at 25 ° to 120°C rapidly form benzene-insoluble material which is undoubtedly crosslinked polymer. Since 2537-A radiation is strongly absorbed by polystyrene, most of the degradation must occur at the surface of the film. With increasing radiation dose, however, the benzene-soluble portion of the film shows gradually increasing intrinsic viscosities. The rate at which this increase takes place increases with temperature, as would be expected from increased segmental motion, particularly above the glass-transition temperature, about 100 ° (Fox, Isaacs, Saalfeld and McDowell, 1965). It would be surprising if the crosslinking were not accompanied by some scission, but in films in the absence of air, at least, no direct evidence for C-C bond scission has yet been advanced (Grassie and Weir, 1965b).

The phenomenon of yellowing in aged polystyrene is well known and has usually been associated with oxidation. We (Fox, Isaacs, Saalfeld and McDowell, 1965) have found 2537-A irradiation of polystyrene films in vacuum results in a general featureless increase in absorbance at wavelengths above the cut-off for the film; at 300mp, the absorbance of a typical film changed from 0.027 to 0.11 after the absorption of 28 x 1021 quanta per gram. A similar film irradiated in air underwent a change in absorbance to 0.57 after absorption of 7 x 10 / 1 quanta per gram. The tail of the absorption in the blue region accounts for yellow coloration of the film. Grassie and Weir (1965d) followed the increases in absorbance at 440 m#. For the same exposures of 2537-A radiation, they reported absorbance increases of 0.09, 0.14, and 0.19 for films irradiated in vacuum, 600 mm of oxygen, and 600 mm of nitrogen, respectively.

In vacuum exposures, Grassie and Weir (1965b) also observed a shoulder at 825 cm- 1 in the infrared spectrum and an increase in the ultraviolet absorption at about 240m/~. Both of these points support the formation of an unsaturated linkage in the main chain. The increase in absorbance at 240mp as well as that at 400mp, was a function of increasing nitrogen pressure.

P H O T O D E G R A D A T I O N O F H I G H P O L Y M E R S 65

It thus appears that oxygen is not required for the yellowing reaction. Grassie and Weir found that an irradiated yellow film rapidly turned black on heating above its melting point. Their interpretation was that the initial yellowing was due to short sequences of conjugated unsaturation in the back- bone; heating allowed the coplanarity of longer conjugated sequences of double bonds. The effect of an inert gas was to restrict the mobility of hydrogen atoms formed in the primary photochemical act and to increase the probability of their abstracting hydrogen atoms from adjacent carbon atoms in the chain. This explanation ignores the increased possibility of recombination at the original site of homolysis under the same conditions. Clearly, more work is needed to explain fully the coloration of polystyrene.

Volatile product collection and identification from vacuum irradiations of polystyrene can be difficult because of the small quantities evolved. In an early study by mass spectrometry, Achhammer, Reiney, Wall and Reinhart (1952) identified some twenty-one compounds, most of which could be related to solvents or impurities. Grassie and Weir (1965b), using 2537-A radiation and manometric techniques for gas collection, identified only hydrogen in the evolved gases. They reported a quantum yield for hydrogen of 4 × 10-2, with an intensity exponent close to one and an activation energy of 2.9 kcal/mole. From these and the results given above, Grassie and Weir (1965b) suggest that the most energetically favorable primary act in polystyrene photolysis is

H +H"

CH2C--CH 2 ~ __---~ ~ CH2(~--CH 2 ,~ --+ ~ CHzC~CI-[ I I I

C6H5 C6H5 C6H5

q- H 2 .

Mass spectrometric examination of the volatile products from polystyrene subjected to 2537-A radiation in our laboratory (Fox, Isaacs, Saalfeld and McDowell, 1965) showed some ten compounds, of which only hydrogen could be traced to the polymer. A quantum yield for hydrogen of only 10- 4 was observed, however. Subsequent experiments (Isaacs, unpublished) have shown that the hydrogen yield is markedly dependent on the solvent from which film to be irradiated is cast. While quantum yields for hydrogen from films prepared from cyclohexane are about 10-a and those from methylene chloride solutions are about 10 -4, the yields from films cast from chloroform, chloroform-d, and carbon tetrachloride are closer to 10-2 in the early stages of irradiation. No deuterated gases were found in the irradiation products from films cast from chloroform-d, which indicates that the polymer itself was the source of hydrogen. Thus, although the mechanism suggested by Grassie and Weir for the vacuum photolysis of polystyrene may be essentially

Ii P.P.S.

66 R O B E R T B. F O X

correct, the possibility of a solvent effect (they used chloroform in preparing thin films) is present and may affect the detailed mechanism.

The photodegradation of polystyrene in the presence of oxygen presents quite a different picture from that in vacuum. Both scission and crosslinking occur. Although benzene-insoluble material could be detected at doses of about 1021 quanta per gram, at lower doses decreases in intrinsic viscosities of the irradiated films were observed. In the initial stages of degradation, at least, apparent quantum yields for scission were reported to be 9 x 10- 5 and 7 x 10-4 at 25 ° and 120 ° respectively (Fox, Isaacs, Saalfeld and McDowell, 1965).

Grassie and Weir (1965c) have made a much more complete study of polystyrene photooxidation at 28 ° under 2537-A irradiation by following rates of oxygen absorption by a manometric technique. The only significant products were water and carbon dioxide, although at low pressures of oxygen, carbon monoxide would be expected to form. Both the radiation intensity and the oxidation rate were attenuated exponentially through the film, and therefore the photooxidation took place only in a thin surface layer and rates of oxidation were not affected by film thickness. With a 4.4 # film cast from a chloroform solution, the quantum yield for oxygen uptake was 8.73 x 10-2, independent of initial molecular weight and initiator. Over the range 28- 57°C, the overall energy of activation was 6.0 kcal/mole. At 600 torr of oxygen, the rate of oxygen absorption was proportional to the intensity of the radiation and the intensity exponent was unity, i.e. the quantum yield is independent of intensity. At 20 torr of oxygen, the rate was stated to be independent of intensity. At constant intensity, the rate was proportional to the partial pressure of oxygen. Radical inhibitors such as 2,6-di-tert-butyl-4-methyl- phenol has no affect on the photooxidation. Ultraviolet absorbers such as 2-hydroxy-4-methoxybenzophenone actually accelerated the rate of oxygen uptake, and the curve of rate versus absorber concentration reached a maximum at about 3 ~ absorber.

These results were interpreted by Grassie and Weir in terms of competition among (a) the recombination of polymer radicals and hydrogen atoms formed by direct photolysis; (b) reaction of polymer radicals with oxygen; and (c) diffusion of hydrogen atoms from the site of their formation. Suppression of (c) occurs at high oxygen pressures and allows competition between (a) and (b), while at low oxygen pressures (a) is suppressed because of a more facile movement of hydrogen atoms. Unfortunately, no direct relationship between oxygen uptake and polymer oxidation was established, so that no rationaliza- tion of the finding of only water and carbon dioxide in the products could be made.

The observation of a higher energy of activation for oxygen uptake than for hydrogen formation in the absence of oxygen seems to suggest (1) a suppression of carbon-hydrogen bond scission and an independent, relatively slow,


oxygen-consuming reaction, or (2) the re-formation of oxygen, as by the combination of hydroperoxy radicals as proposed by Grassie and Weir (1965c) to account for the formation of water.

Additional clarification is needed for the results at low pressures of oxygen. If the rates at all oxygen pressures are dependent on the concentration of polymer radicals, it might seem that some intensity dependence should have been found at low oxygen pressures. Thus, polymer radicals must form by some means other than photoactivation if the rate is to be independent of intensity, or a different mechanism than that given applies in the presence of a limited amount of oxygen.

A possible answer lies in the wavelength dependence of the photooxidation reaction. The absorbance of thin films of polystyrene is quite small above 300 mp. Grassie and Weir (1965c) compared the effects of 2537-A and 3650-A radiation, and with the latter wavelength observed an induction period followed by a slow but constant rate of oxygen uptake. In vacuum, no hydrogen formed under 3650-A radiation (Grassie and Weir, 1965b). Zaitoun (1962), using 3023-A, 3662-A, 4047-A, 4348-A, and 5770-A radiation, found that the highest rates of carboxylic acid formation occurred with 3662-A and 4047-A light. If neither the polymer nor oxygen absorb these wavelengths, it is clear that other reactions, possibly with impurities, are involved. Lumines- cence excited by 3660-A light and ascribed to carbonyl-containing impurities (Zapol'skii, 1965) is an indication of such a possibility.

As noted earlier, oxygen exerts a considerable effect on the ultraviolet absorption spectrum of polystyrene during irradiation. With 2537-A radiation at 25°C, bands at 282mp and 340 mp were observed along with a general increase in absorbance in the entire ultraviolet region (Fox, Isaacs, Saalfeld and McDowell, 1965). Reprecipitation of the polymer indicated the 282-mp band to be due to a chromophore which was part of the polymer chain while the 340-mi~ band must be associated with some small molecule. The 340-rap band has also been observed during irradiations with 3130-A light at 60°C in air (Tryon and Wall, 1958; Wall, Harvey and Tryon, 1956). The irradiation of poly(~-methylstyrene) and various deuterated polystyrenes indicated that the c~-hydrogen atom was involved in the reaction. A subsequent dark reaction was also observed. The results were explained on the basis of two concurrent reactions occurring at the chain ends after scission and involving the formation and rearrangement of a benzalacetophenone analog.

Relative rates of photodegradation under broad-spectrum ultraviolet irradiation in air have been determined for a number of ring-substituted polystyrenes and copolymers of polystyrene. All underwent decreases in intrinsic viscosity, which suggests that chain scission may have taken place. Chlorination of the ring of polystyrene generally decreased stability, while copolymers of styrene with ~-methylstyrene or 2-vinylnaphthalene were more stable than polystyrene itself (Kirilova, Matveeva, Leitman and Fratkina,


1964). A copolymer with acrylonitrile was less stable than polystyrene (Kirilova, Matveeva, Leitman and Fratkina, 1962).

An early study of the 2537-A photolysis of polystyrene in benzene solution, probably in the presence of air, indicated a quantum yield for scission of about 10- s, based on energy absorbed by the entire solution (Chen, 1949). A more recent investigation (Price and Fox, 1966) of the 2537-A photodegradation of polystyrene in solution at 25°C shows results which parallel those for radiolysis of polystyrene in solution. In the absence of air, crosslinking was encountered, and only in dioxane and carbon tetrachloride was a measurable decrease in the intrinsic viscosity of the solutions observed. This indicates that scission as well as crosslinking is probably taking place. In the presence of air, molecular weight decreases were seen in all of the solvents used; in Table 1 are shown the apparent quantum yields for random scission based on energy absorbed only by the polymer. Interaction between polymer and solvent is quiet evident; carbon tetrachloride also sensitized the degradation in cyclohexane. The mechanisms of these reactions are obscure, but probably involve scission after the formation ofperoxy or other oxygen-headed radicals.

T A B L E 1

APPARENT QUANTUM YIELDS FOR THE

RANDOM SCISSlON OF POLYSTYRENE IN

SOLUTION IN AIR

Solvent 4'~ × 104

Cyclohexane Dioxane Benzene Methylene chloride* Chloroform Carbon tetrachloride

2 7

34 44

440 1100

* Cone. of polymer 5 g/1; all others were 10g/l.

Poly(~-methylstyrene) Substitution of a methyl group for the ~-hydrogen atoms of polystyrene

results in a polymer whose degradation characteristics little resemble those of the parent. A most notable difference is in the absence of crosslinking in the photolysis of poly(~-methylstyrene), which makes an analysis of molecular weight changes relatively simple.

JeUinek (1962b) has extensively reviewed the work of Stokes and Fox (1962) on the photodegradation of poly(~-methylstyrene) films in vacuum. No photooxidation studies of the solid polymer have yet appeared. In vacuum at 27°C, the quant m yield for random scission was 10- 3 and for monomer formation,


7 x 10- 3. The kinetic chain length was approximately fifteen monomer units for each scission. At 115°C, the quantum yield for scission was 2 x 10- 2 and for monomer formation, 0.5, with a kinetic chain length of 25. The quantum yield for monomer formation at both temperatures was independent of intensity. Among the volatile products, the most prominent were hydrogen, carbon dioxide, and carbon monoxide, formed in quantum yields of 0.017, 0.016, and 0.0068, respectively. A rough correlation was noted between the number of scissions and the number of carbon dioxide plus carbon monoxide molecules evolved, which suggested that a weak link involving oxygen was involved in the breakdown of this polymer chain. It is of interest that at 25°C the efficiency of producing chain scissions by gamma radiation was about ten times that of ultraviolet radiation in both poly(~-methylstyrene) and poly- (methyl methacrylate).

Poly(~-methylstyrene) has also been photolyzed in solution with 2537-A radiation at 25°C (Fox and Price, 1965a). As in films, this polymer undergoes random scission in solution. Quantum yields based on the energy absorbed by the polymer in 1 ~ solutions are given in Table 2. The marked solvent dependency of the quantum yields is clear here, as it has been with every

TABI.E 2

APPARENT QUANTUM YIELDS FOR THE RANDOM

SCISSION OF POLY(c~-METHYLSTYRENE)

4~ × 10 ~ Solvent Air Degassed

Dioxane Cyclohexane Methylene chloride Benzene Chloroform (alcohol-free) Carbon tetrachloride

4 5 - - 6

50 50 95

1600 2110 1850

polymer system studied in a variety of solvents. In solvents such as carbon tetrachloride, in which scission occurs in high yield, solvent involvement is quite evident. Carbon tetrachloride sensitized the degradation in cyclohexane (as it also did with polystyrene). Spectral changes occurred which must have involved the solvent, and, by studying these changes in model compounds, it was concluded that the reaction between solvent and polymer took place at the secondary carbon atom in the polymer chain.

Oxygen did not appear to exert a large effect on the quantum yields for the scission of poly(~-methylstyrene) in these solvents. With polystyrene in the same solvents, oxygen played a definite role, and apparent quantum yields for scission were similar to those for poly(~-methylstyrene). It was suggested that

70 ROBERT B. FOX

oxygen-headed radicals having similar stabilities result from the photooxidation of either polymer, and therefore the rates of degradation are similar under these conditions. In the absence of oxygen, a relatively stable tertiary radical may form with polystyrene, while an unstable secondary radical is the precursor to the more rapid scission occurring in poly(~-methylstyrene).

Polyethylene

Polymethylene, a chain of methylene groups, is the prototype of carbon- chain polymers; nothing has appeared on its photodegradation. Perhaps it is too simple a structure. The next best thing is polyethylene, and much has been published on the effect of light on this material. Alas, the facts accumulate much faster than the understanding, and progress has been slow in uncovering the fundamental processes of photodegradation with this polymer. This may be because polyethylene is too complex a structure, but more likely, it is due to the experimental difficulties of working with this very useful substance.

It has long been known that polyethylene subjected to high energy irradiation in vacuum will undergo crosslinking, evolve hydrogen, and form trans- vinylene unsaturation while vinylidene and vinyl unsaturation disappear. Little chain scission occurs. There appears to be only partial agreement on the actual reactions involved, but it is likely that many of the same processes occur in polyethylene under ultraviolet irradiation. A problem arises concerning the extent and the reasons for radiation absorption in this polymer. High energy radiation is absorbed at random throughout a solid sample. A pure polymethylene chain would not be expected to absorb ultraviolet radiation having wavelengths above 2000A. Polyethylene does absorb some of this higher wavelength radiation, and the absorbing sites can only be impurities in the sample or imperfections in the chain. Since these will vary with the sample and the source of the polymer, it is not surprising that some difficulty attends the interpretation of results of photolytic studies of polyethylene.

In the absence of oxygen, polyethylene is a relatively stable material under ultraviolet irradiation. Ziegler polyethylene underwent no physical change upon irradiation in nitrogen, while in air the same material suffered embrittlement (Matsuda, Kurihara and Mori, 1959). In either oxygen or nitrogen, 2537-A irradiation of polyethylene results in an increased absorbance in the 200-280m# region. On the basis of irradiations of aliphatic hydrocarbons, this absorption has been ascribed to conjugated unsaturation (Charlesby, Grace and Pilkington, 1962). Polyethylene subjected to 2537-A irradiation at - 196°C in vacuum gave a broad ESR spectrum of low intensity (Ohnishi, Sugimoto and Nitta, 1963). Polyethylene irradiated with electrons at -78°C, the hydrogen pumped off, and the sample allowed to stand at room temperature for i week, showed only an allyl-type ESR spectrum. Ohnishi et al. ( 1 9 6 3 ) 2537A-irradiated this material at -196°C and observed a conversion of the allyl-type radicals to alkyl-type radicals with a constant radical con-


centration. They suggested that an intramolecular hydrogen atom shi~ took place:

~ C H 2 C H C H = C H ~ ~ ~ C H C H 2 C H = C H ~

Reillumination indicated the formation of polyene radicals of the type ~ CH2t~H(CHzCH),Clff 2-~. These changes were confirmed by absorption spectra of the radical-containing films.

Charlesby and Partridge (1965a, 1965b) investigated the ultraviolet and gamma-induced thermoluminescence and phosphorescence of polyethylene and showed that in this instance, these phenomena were dependent upon the presence of carbonyl-containing impurities in the polymer. This again demo n- strates the complexities of working with polyethylene.

Much more drastic changes take place in polyethylene when it is irradiated in air. A general embrittlement and deterioration of physical properties occurs. These changes have been followed by measurements of electrical and mechanical properties, absorption spectra, molecular weight, gel content, and crystallinity. Cotten and Sacks (1963) showed that both scission and crosslinking took place, that the ratio of these was dependent on the oxygen concentration at the site of reaction, and that scission was favored in the highly crystalline regions of the polymer. The crystalline regions may themselves be destroyed by ultraviolet radiation (Ozawa and Maekawa, 1963). Since the diffusion rate of oxygen into amorphous regions will be greater than in ordered regions, amorphous material will be more likely to crosslink (Turi, Roldan, Rahl and Oswald, 1964); infrared spectra suggest that the crosslink is an ether linkage (Cotten and Sacks, 1963). Ultraviolet radiation did not affect the rate of thermal oxidation (GrafmiJller and Husemann, 1960).

The nature of impurities may again have much to do with the rat e of the photodegradation of polyethylene. It has been suggested that titanium dioxide and aluminum oxide residues in Ziegler polyethylene can act as oxidation catalysts (Matsuda, Kurihara and Mori, 1959). The crosslinking reaction itself has been put to good purpose as a means of improving mechanical properties. This has generally been done through the use of photosensitizers (Oster, 1956). Benzophenone has been investigated in detail in this regard, but a number of other sensitizers, which are consumed in the process, have been used (Oster, Oster and Moroson, 1959; Wilski, 1959, 1963; Charlesby, Grace and Pilkington, 1962; Liang, Wang, Fang, Hsiang and Hsien, 1962; Hsien, Hsiang, Liao, Liang, Wang and Fang, 1962; and many others). Charlesby (1962) has compared the effects of alpha, gamma, and ultraviolet radiation on the crosslinking and unsaturation formation in polyethylene in the presence of air and sensitizers. In a similar vein, chloronitroso compounds have recently been used to sensitize the photodegradation of cis-l,4-poly- isoprene (Rabek, 1965) and other polymers (Rabek and Rabek, 1963). The


photoisomerization of polybutadiene in solution under nitrogen in the presence of diphenyl disulfide has been investigated by Seely (1962); a rapid degradation ensued in the presence of oxygen.

Polypropylene It will come as no surprise to learn that this important polymer is much like

polyethylene in its behavior under ultraviolet radiation. In the absence of air, the same type of allyl-alkyl radical interconversion

takes place as was observed in the ESR spectra of polyethylene. Admission of oxygen resulted in the formation of peroxy radicals from both the allyl and alkyl types of radicals, and further irradiation produced chain scission (Milinchuk and Pshezhetskii, 1963). The kinetics of the decomposition of atactic polypropylene hydroperoxide in vacuum under near-ultraviolet irradiation have been studied iodometrically (Ershov, Lukovnikov and Baturina, 1964). Direct irradiation of polypropylene with 2537-A light at -196°C has been shown by ESR spectra to give methyl radicals and ,-~ CH2(~HCI~ 2 ,-, (Yoshida and Rhnby, 1964), as well as

"~ CI - - [2CI - [ ( (~H2)CH 2 ,-~ and ,-~ C H ( C H a ) C H C H ( C H 3 ) -,~

structures (Browning, Ackermann and Patton, 1965). A few investigations of the photooxidation and general weathering of

polypropylene have begun to appear. These have mostly been concerned with the effects of additives (Balab~in, 1963), chlorination (Tanaka, 1964), and pigments and dyes (Takahashi and Suzuki, 1964; Balab~in, 1965). Mechanisms have not been determined.

Polyketones The photochemistry of aliphatic ketones has been the subject of intensive

investigation for many years. The transformations occurring in this family of compounds under the influence of light are comparatively well understood. For this reason, the photochemistry of the polymeric analogs of aliphatic ketones

CH3 1

~ CH2CH ,'~ ~ CH2C I I

C~-O C-~O I I

C H 3 C H 3

Poly(methyl vinyl ketone) Poly(methyl isopropenyl ketone)

is of particular interest. The few papers which have appeared have been unusually competent and have emphasized the semantic difference between

PHOTODEGRADATION OF HIGH POLYMERS 73

photochemistry and photodegradation in polymers. Interest in the photodegradation of polyketones has not been great because of their lack of com- mercial application. Yet in polymer structures of this type may lie the key to energy transfer effects, for example, which would open the door to practical polymer stabilization among commercially important materials. It is to be hoped that more work on these polymers will soon be forthcoming.

By analogy, it would be expected that poly(methyl isopropenyl ketone) would behave in a manner similar to that of poly(methyl methacrylate) and poly(~-methylstyrene). Shultz (1960) irradiated films of poly(methyl isopropenyl ketone) at 23°C in air with both 2537-A and gamma radiation. A quantum yield for random scission of 0.22 was observed, which makes this polymer one of the least photolytically stable of carbon-chain polymers. The effect of oxygen was not ascertained, but under gamma irradiation, oxygen increased the rate of main chain scission, an effect opposite to that observed in polymethacrylates. Wissbrun (1959) studied films of this polymer at 150-190°C in vacuum under 3130-A irradiation. After a short induction period which decreased with increasing temperature, monomer was evolved along with small amounts of carbon monoxide and methane. Monomer was formed in quantum yields of about 5 at 177°C, somewhat dependent on sample history. The intensity exponent for monomer formation increased from 0.41 at 150°C to 0.73 at 177°C, indicating a gradual change from radical recombination to some first-order termination step at higher temperatures. A rapid drop in molecular weight coincident with monomer formation indicated a random scission process. These findings were rationalized by the following mechanism:

CH3 CH3 C H 3 C H 3

I I I I C H 2 C - - - - - - C I - [ 2 C C H 2 ~ _ _ . , ~ C H 2 C - - C H 2 C C H 2

I I I C H 3 C ~ O C H 3 C ~ O C H 3 C ~ O

C H 3 C H 3

I I - - - -o ~ C H 2 C ~ C H 2 + ' C C H z ~ + C O + CH~

C H 3 C ~ O

followed by depolymerization of the polymer radical fragment. Wissbrun (1959) also examined the 3130-A photolysis of films of poly-

(methyl vinyl ketone) at 28 ° and 80°C. Initially, the number of scissions (based on viscosity changes) was a linear function of dose and independent of film thickness, intensity, and temperature, although in later stages, the rate of scission became dependent on film thickness. The latter result was said to be due to non-uniform light intensity throughout the film, but could well have been due to a small amount of branching. An apparent initial quantum

74 ROBERT B. FOX

yield for random scission of 0.03 was found, and this agrees very well with the 0.025 found by Guillet and Norrish (1955a) for this polymer in dioxane solution. Crosslinking was not mentioned in either the solid or the solution study. An increased absorbance below 250 m/t was observed, and acetaldehyde, carbon monoxide, and methane were formed in quantum yields of 0.06, 0.003, and 0.0006 at 80°C. In dioxane, the quantum yields for all three products were 0.01. Wissbrun considered the difference to be due to the formation of acetaldehyde by disproportionation of CI-I~ and CH3CO in a cage reaction in the solid, rather than by hydrogen abstraction from the polymer or solvent as might occur in solution. Molecular weight changes in both the earlier solution work and in the solid were well explained by a caged disproportionation reaction such as:

,~ C H 2 C I - I C H 2 C H C H 2 ~ ,~ C H 2 C z C H 2 H 2 C C H 2 ,,,,

I I - - ' I + I C H 3 C ~ O C H a C z O C H 3 C z O CH3C~--~-O

which also accounts for the increased absorbance below 250m#. Some branching probably occurred by reaction between the unsaturated chain end and the secondary polymer radical after cleavage of the CI-I3CO radical. Evidence for this is seen in the formation of graft polymers of acrylonitrile, methyl methacrylate, or vinyl acetate on a poly(methyl vinyl ketone) back- bone (Guillet and Norrish, 1955b).

Polyacrylonitrile In spite of the intense interest in the thermal degradation of this important

polymer, little has appeared on its photodegradation. The longest wavelength absorption maximum of the nitrile chromophore is about 160 mp, and like polyethylene, polyacrylonitrile should be fairly transparent to ultraviolet radiation of wavelengths above the vacuum region. Again like polyethylene, polyacrylonitrile does absorb in this region with a maximum at 265 m# which is associated with the polymer structure itself. The chromophore giving rise to this absorption has not been identified, but it is certain that the photochemistry and, indeed, the photostability, of this polymer depends on its behavior.

Although no thorough film studies have been reported, an interesting investigation has appeared on the photosensitivity of polyacrylonitrile which has been dry-heated in vacuum or nitrogen (Yoshino and Manabe, 1963). The reaction is a complex one and is sensitive to a variety of factors, a description of which is outside the scope of this review. Irradiation of solid polyacrylonitrile in vacuum with 2537-A light results in crosslinking and probable formation of hydrogen cyanide (Jellinek and Bastien, 1961).


The 2537-A photolysis of polyacrylonitrile in an ethylene carbonate propylene carbonate solution at 25°C has been investigated under anaerobic conditions (Jellinek and Schlueter, 1960; Jellinek and Bastien, 1961). An apparent random scission reaction occurred as evidenced by viscosity decreases, with a quantum yield of about 10- 4 and an intensity exponent of one. The rates of scission were markedly dependent on sample history. In the ultraviolet absorption spectrum, a new band at 295 m# was interpreted as indicating extensive double bond formation; greatly increased absorption appeared at the 216 m/~ band of the original polymer. Since similar changes occurred during the irradiation of glutaronitrile, they are probably uncon- nected with the chain scission reaction. A mechanism was suggested which involved a primary C-CN scission and a subsequent main-chain break. The formation of hydrogen cyanide and possibly hydrogen could result in conjugated unsaturation or cyclization; it was noted that these side reactions are probably more important than chain scission from the overall chemical point of view.

Apparently the photolysis ofpolyacrylonitrile in dimethylformamide follows a somewhat different path, since viscosity was unaffected and crosslinking was observed (Urazovskii and Postoeva, 1962).

Poly(vinyl Chloride) and Other Halogenated Polymers The photochemistry of poly(vinyl chloride) and its copolymers is in a

vexed state, the continuing effusion of publications concerning the effects of light on these commercially valuable materials notwithstanding. Bersch, Harvey and Achhammer (1958) have reviewed the earlier work on poly- (vinyl chloride), Winkler (1959) has published a general discussion of the mechanisms involved, and Stepanek and Dolezel (1963) have surveyed the more recent contributions.

There seems to be a common understanding that poly(vinyl chloride) loses hydrogen chloride and becomes colored when it is exposed to heat or to ultraviolet radiation. The extent to which these phenomena are interdependent and the influence of air have been the subject of controversy. Golub and Parker (1965) have recently followed the increased absorbance in the 210-650mp range with poly(vinyl chloride) films at 3 to 4 x 10 -2 torr under broad spectrum ultraviolet and gamma radiation. They assumed that the absorbance changes, in which no specific bands appeared, were due to conjugated polyene sequences formed by the loss of hydrogen chloride. Below the ultraviolet cut-off, the quantum yield for hydrogen chloride, 0.043, was independent of wavelength. Gamma radiation was used about 13 times more efficiently than was ultraviolet radiation, but the overall result was the same. One difference between the two types of radiation was in a falling off of the absorbance increases with longer doses of ultraviolet radiation; this was attributed to intramolecular energy transfer to the polyene system, which therefore acts


as an internal inhibitor. Polymeric additives containing such systems have been used to photo-stabilize poly(vinyl chloride) (Berlin, Popova and Yanovskii, 1965). Since the films used by Golub and Parker (1965) contained residual methyl ethyl ketone, it was thought that ultraviolet radiation was initially absorbed by the ketone and transferred to the polymer followed by homolysis of a C-C1 bond along the lines suggested by Winkler (1959).

In air, ultraviolet irradiation of poly(vinyl chloride) films also produces hydrogen chloride and unsaturation, but with an increased quantum yield, 0.13 (Petit and Zaitoun, 1963); oxidation proceeds via hydroperoxide formation (Karyakin, Grishin and Kurykin, 1965). Novfik (1962) argued that the color changes noted, especially in the presence of oxygen, were due, not to polyene formation, but to carbon black formed by reaction of oxygen with acetylenic groups at the chain ends of the polymer.

Crosslinking takes place during the 2537-/~ irradiation of poly(vinyl chloride) films (but not powders) in either air or nitrogen, but gel contents in air were less than in nitrogen, indicating an inhibition of crosslinking by oxygen (Sobue, Tabata and Tajima, 1958; Sobue and Tajima, 1959). With vinyl chloride-vinylidene chloride copolymers irradiated in air, simultaneous crosslinking and scission occurred, the glass transition temperature increased, and it was shown that the hydrogen chloride generated came from the vinylidene chloride units in the copolymer (Kryszewski and Mucha, 1965). At the same time, the ultraviolet absorption spectrum changed in a complex way, the ESR spectrum indicated the presence of radicals, and the surface conductivity of the polymer increased markedly (Oster, Oster and Kryszewski, 1961, 1962). With vinyl chloride-vinyl isobutyl ether copolymers irradiated in air by a medium pressure mercury lamp, it has been found that the ether portion of the copolymer is the weak point and that irradiation appears to prod uce syndiotactic groups at the cost of isotactic pairs (Hippe, Jablonski and Krzy£anowska, 1965).

Under nitrogen, ultraviolet irradiation of poly(chlorotrifluoroethylene) at 250°C resulted in degradation while only minor degradation was observed with poly(tetrafluoroethylene) at 325°C; no crosslinking was noted. With a copolymer of tetrafluoroethylene and hexafluoropropene, both scission and crosslinking occur and the latter increases with increasing temperature (Bowers and Lovejoy, 1962).

Shultz, Knoll and Morneau (1962) compared the photolysis, radiolysis, and thermal degradation of a copolymer of tetraftuoroethylene and trifluoronitro- somethane having the unit structure ,-~CF2CFzN(CF3)O~. The weak point in the main chain was the N-O bond, and a quantum yield for random scission of 0.91 x 10 -3 was observed; gamma radiation was about 120 times more effective than 2537-• ultraviolet radiation in producing scissions. An analysis of the volatile products indicated that about 3.6 repeating units were decom- posed to COF2 and C F 3 N ~ C F 2 for each ultraviolet-produced scission. For


gamma radiation, this decomposition amounted to 5.3 units peL scission, which suggests that the excess energy from this type of radiation is not effective in sustaining a chain degradation extended from the original site of radiolysis.

Poly(vinyl A lcohol) and its Esters These polymers undergo crosslinking in air under ultraviolet light, par-

ticularly in the presence of a sensitizer. The greatest interest in their photodegradation has been connected with their use as photo-resist materials, and little study has been made of the fundamentals of their photolysis.

As a water-soluble polymer, poly(vinyl alcohol) has been of special interest in photoengraving processes for many years. Ultraviolet irradiation of the polymer in air results in the formation of carbonyl groups, and heating above 120°C results in gel formation (Mori and Kumagai, 1964). If the irradiation is carried out in the presence of potassium dichromate, insolubilization occurs more readily. Duncalf and Dunn (1964) have investigated the details of this commonly used process and have shown that the crosslinking takes place by coordination of hydroxyl groups by chromic ions and that the carbonyl groups which also form are not involved. When diazonium or tetrazonium salts are used, the crosslinks involve an ether linkage (Tsunoda and Yamaoka, 1964).

Trudelle and Neel (1965) irradiated poly(vinyl alcohol) under nitrogen at 25°C in the presence of benzophenone. They found that the conversion of secondary hydroxyl groups to carbonyl groups and concomitant reduction of benzophenone to benzopinacol was accompanied by degradation of the polymer to give terminal carboxyl groups which underwent lactonization. Dehydration occurred, and this accounted for the yellowing which took place.

Although photodegradation of poly(vinyl acetate) has not been studied as such, mention might be made of the investigation of the photochlorination of this polymer carried out by Hahn and Grafmtiller (1956). They postulated the abstraction of a main-chain tert-hydrogen atom by a chlorine atom as the initiation step, and this was followed by chain scission analogous to that which occurs in poly(methyl acrylate) and similar polymers.

Among other esters of poly(vinyl alcohol), the cinnamate types have received the most attention. Again, the object is insolubilization by the action of light. Crosslinking in this case takes place through the double bond of the side chain (see, for example, Tsuda, 1964; Inami and Morimoto, 1962; Minsk, Smith, Van Deusen and Wright, 1959).

Polyesters Poly(ethylene terephthalate) is the only film-forming material of this class

whose photolysis has been studied quantitatively. Osborne (1959) irradiated

78 ROBERT B. FOX

thin films.of this polymer in air with the light from a carbon arc or a fluores- cent sunlamp. Irrespective of the radiation source, the quantum yield for random scission was 5 x 10 -4 at about 55°C, essentially independent of intensity. With the same material, Shultz and Leahy (1961) demonstrated a method for determining the wavelengths of radiation responsible for chain scission where the spectral distribution of the radiation is unknown. The carbon dioxide, nitrogen, and air permeability of poly(ethylene terephthalate) films subjected to solar irradiation has been shown to increase initially as a result of internal stress relief; after the first increase, a subsequent decrease in permeability was ascribed to crosslinking (Kordub and Livshina, 1964).

Irradiation at 30°C of a condensation polymer from phenolphthalein with terephthalic and isophthalic acids gave only carbon monoxide and carbon dioxide as volatile products, as well as a crosslinked residue; the degradation was said to have been self-inhibited by the formation of quinoid compounds (Rode, Yarov and Rafikov, 1964). Maerov (1965) has recently reported a molecular photo-rearrangement which apparently takes places within a polyester chain:

O O

O /ff--'-~ C(CI ._[3)2/~ ~ _ ~ ~___~ I[ A [I _

C--~ . .~ . - -C

This conversion, which is analogous to the photochemical Fries rearrangement, occurred with a quantum yield of about 0.016 and was accompanied by scission at a rate about two and one-half times that of the rearrangement.

Polycarbonates have been only briefly investigated; at higher temperatures under ultraviolet irradiation, they undergo crosslinking and generate carbon dioxide (Sato, 1964).

Polyethers As in the case with most polymers, solutions of poly(ethylene oxide) suffer

a decrease in viscosity on exposure to ultraviolet light in air (McGary, Jr., 1960). Kelleher and Jassie (1965) have followed the photooxidation of poly- (oxymethylene) by means of infrared spectroscopy. Qualitatively, the results were similar whether initation is by ultraviolet, gamma, or thermal radiation. Random scission was accompanied by extensive depolymerization to formaldehyde; complete unzipping could be seen in the formation of methyl formate from the chain ends. A similar photooxidation occurred in an oxyethylene-


oxymethylene copolymer, except that depolymerization appeared to stop when an ethylene unit in the chain was encountered.

Polyacrolein has been crosslinked by u]traviolet light in the presence of sensitizers (Gole and Calvayroc, 1965).

Polysulfides In an investigation which was unusual in that complete photovolatilization

of a polymer was the desired result, Isaacs and Fox (1965a) studied the effect of radiation from a medium pressure mercury lamp on a series of poly- (alkylene polysulfides) in vacuum at 50°C. Poly(methylene di- and tetrasulfide) and poly(ethylene di- and tetrasulfide) underwent random scission and formed hydrogen sulfide and, from the methylene polymers, carbon disulfide, as major low molecular weight volatile products. The majority of the total weight loss, however, was made up of products which condensed on the cell walls or in traps to form polymers. All of the polymers evolved carbon mono- sulfide, which is readily polymerized; in addition, the tetrasulfides generated species which on condensation, reformed the original polymers.

Silicones Only two papers have appeared on the photodegradation of silicones. Both

are exceptional, in that they indicate one of the new directions in which the field of polymer photolysis is heading.

Siegel and Judeikis (1965) observed changes in radical concentrations taking place in poly(dimethylsiloxane) which contained naphthalene in concentrations up to 10-2m. The samples at 77°K in the cavity of an ESR spectrometer were subjected to radiation having wavelengths greater than 2400 A. This polymer was found to be fairly resistant to photodegradation by this radiation in the absence of impurities. In the presence of naphthalene, which absorbed nearly all of the incident ultraviolet radiation, only polymer photolysis took place. The concentration of radicals at the lowest concentration of naphthalene (5 × 10-4m) was about seven times that formed in the absence of naphthalene. In the initial phases of the photolysis, the following reactions of the excited (indicated by the asterisk) polymer molecules took place with approximately equal probability:

oH3

O S i ~ * - - - - I

CH3

CH; I

O S i ~ + H" k

CH3

OSi ~ + CH~ I

C H 3

80 ROBERT B. FOX

All but the hydrogen atoms were identified in the spectrum. Very small volatile gas yields were obtained in preliminary experiments, with the ratios of products near those found for gamma irradiation ( I - ' I 2 : C l f f 4 : C 2 H 6 =

0.95 : 1.62: 0.22). The processes by which the polymer molecules become excited clearly

involve a transfer of electronic energy from excited naphthalene. Siegel and Judeikis (1965) demonstrated that this transfer must be by way of a naphthalene molecule not in the first triplet level, but in an excited triplet level which could only be achieved by a biphotonic mechanism. The mechanism is entirely analogous to that shown to apply to the naphthalene-sensitized decomposition of ethanol or diethyl ether (Siegel and Eisenthal, 1965).

Zhuzhgov, Bubnov and Voevodskii (1965) photolyzed poly(methyl- phenylsiloxane) at 77°K within the cavity of an ESR spectrometer. No sensitizer was added in this case, but the presence of the phenyl chromophore means that 2537-A radiation will be strongly absorbed. The spectrum indicated the presence of CHj and -CH~, and the results pointed to a biphotonic mechanism for the formation of the methyl radicals. The fate of the silicon- headed radicals, which must have formed at the same time as the methyl radicals, was not determined. Volatile products, the same as those noted above for poly(dimethylsiloxane), were hydrogen, methane, and ethane, in the ratio 25: 70: 5.

Polyamides Since the general investigation of the degradation ofpolyamides carried out

by Achhammer, Reinhart and Kline (1951), the literature has been replete with the aches, pains, and strains suffered by various nylons of suspect purity under the influence of light, alone or in combination with other degrading factors. It would be difficult, if not tiresome, to engage in a recitation of the findings cited in these publications. This review will therefore be confined to a few papers which bear directly on the mechanism of the photodegradation ofpolyamides.

Rafikov and Hsu (1961) irradiated films of poly(e-caprolactam) (nylon-6) at 30°C and 10- 5 torr. Under the full range of emission from a medium pressure mercury lamp, which means wavelengths throughout the entire quartz region, about nine C-H bonds were broken for every main-chain C-N bond undergoing scission. Hydrogen and carbon monoxide were the major products. As confirmed by solubility and viscosity measurements, crosslinking and branching predominated over scission. If wavelengths below 3000-A were filtered out of the incident radiation, however, main-chain scission was the predominant process, embrittlement did not occur, and the proportion of carbon monoxide in the volatiles increased. With 2537-/k radiation, the quantum yield for total gas evolution was about 7 × 10-4 (Hsu and Rafikov, 1962). The ESR spectrum of a sample at 77°K irradiated in the spectrometer


cavity with a high pressure mercury lamp showed only the presence of ~CH2(~O radicals or the ~ CHfi resulting from loss of carbon monoxide from those radicals.

The mechanism suggested by Rafikov and Hsu (1961) for the crosslinking reaction involves elimination of a hydrogen atom from the carbon atom adjacent to the nitrogen atom followed by combination of two polymer radicals; hydrogen is formed by abstraction of a second hydrogen atom from the polymer radical or by hydrogen atom combination.

O O II hv II

,,~ CH2CNHCH2CH 2,-~ - - ~ ,,~ C H 2 C N H C H C H 2,-~ - H " ,~

crosslink

O H

- - - . ~ C H 2 C N H C H ~ C H - H " + H 2

Scission involves the direct homolysis of a C-N bond with subsequent carbon monoxide and ethylene formation:

O O II hv II

,~ CH2CH2CH2CNHCH 2,~ ----~ ~ (CH2)3C" + i~HCH2 /

J - C O

"~ CI-[2CI-[2CH 2" --~ ~ CH 2" + CH2~CH" 2 etc.

Moore (1963) has investigated the photodegradation of nylon-6,6 under nitrogen and in air and compared the results with those obtained by photo- lyzing model N-alkylamides. Two general processes occurred: (a) photolysis, independent of oxygen, by radiation of wavelength less than 3000 A, and (b) photooxidation, which was subject to sensitization, under radiation having wavelengths above 3000A. The presence of water had little effect. The mechanism proposed for photolysis in the absence of air was analogous to that given above.

For photooxidation, Moore proposed that oxidative attack occurred at the carbon adjacent to the nitrogen atom, with the ultimate formation of an alkoxy radical and its subsequent decomposition (p. 81). Other reactions could also occur to a lesser extent. These processes were accelerated, but not altered, in the presence of titanium dioxide added as a delustrant to yarns.

F P . P . S .

82 ROBERT B. FOX

CO(CH2)4CONH

NH(CH2) ,CH2CHO"

, - +

CO(CH2)4CONH I

..~ NH(CH2),CH2C=---O

J . . . ~

,-~ CO(CIff2)4CONH I

,-- NH(CH2)4CH; + H C = O

CO(CH2)4CONH - . L

,,- NH(CH2) ,CH2CHOH

,~ CO(CH2)4CONH 2 + ~ NH(CH2)4CH2CHO

Ford (1957) early suggested that peroxides were involved in this photooxidation. An iodometric method for the determination of hydroperoxides has recently been described (Anton, 1965). Ford also observed a post-effect in the ultraviolet spectrum of degraded nylons. This post-effect appears to have involved impurities; recent reports have failed to note such effects.

A peroxide mechanism for the photooxidation of poly(e-caprolactam) has been given by Kroes (1963), based on the work of Boasson, Kamerbeek, Algera and Kroes (1962), who studied the products formed during the sunlamp-irradiation of yarns of this material. Initiation involves an oxygen- independent scission of the C-N bond, followed by peroxidation of the carbon-headed radical and subsequent reactions:

..~ NFICO(CFf2)4CFI200" + . .-CONHCH2(CH2) 5 .-~

--~ ... NHCO(CIff2)4CH2OOH + .-~ CONHCH(CH2) s .-~ I [

02 0 0 " I

CONHCH(CH2) s ,-~ ~ etc. ,-, NHCO(CH2)4CHO + H 2 0 I

,~ NHCO(CI-[2),(~O ~ etc.

Cellulose and Cellulose Derivatives

In the face of the immense mass of literature concerning the effect of light on cellulose and cellulose derivatives under various and sundry conditions, it is possible to arrive at only one conclusion: the pen is mightier than the sword. However, in recent years, attention has been turning toward the detailed processes of degradation taking place in cellulose and its derivatives. It has

P H O T O D E G R A D A T I O N O F H I G H P O L Y M E R S 83

long been known that the physical properties of cellulose degenerate in air during near ultraviolet irradiation, and most of the older work has been concerned with this problem. As in the case of the polyamides just discussed, photooxidation during near ultraviolet irradiation and photolysis by 2537-A radiation in the absence of oxygen are separate and distinct processes. The

H

H--C - - O H

C[5__ O H / I ",, o

~ / H \ ' /'" " \ I / "" C4 C

\ \ / \ / OH H / ",, ~o , l t / H

3 C - - - - C 2

I I H OH

[---~

f

H I

H - - C - - 0 H I

C - - - - 0 H / [ \ O

\ / H x/\, C C \?-

. / H C - - - - C

H OH

+H"

H I

H - - C - - O H I

C - - - - O H /I \ 0

\ / U \ / \ C C

/ \,, OH H / "\, 0 ~ , l I / " H

C - - - - C f 1

H O.

+H"

84 ROBERT B. FOX

effects of water and temperature, for example, are quite different under the two sets of conditions (Launer and Wilson, 1949; Egerton, Attle and Rathor, 1962; Egerton, Attle, Guirguis and Rathor, 1963).

Flynn, Wilson and Morrow (1958) 2537-A irradiated purified dry cotton cellulose sheets at 40°C in vacuum. They found that aldehyde and carboxyl groups were formed, that the degree of polymerization diminished, and obtained an initial quantum yield for hydrogen formation of 10-2 and for the sum of carbon monoxide and carbon dioxide, 10-3. The aldehyde groups and the hydrogen were at least in part due to reactions not connected with chain scission (Flynn, 1958); photolysis of alcohol groups (which are not strong absorbers of 2537-A radiation) to form carbonyl groups and hydrogen was implicated.

This and other points have been strengthened by more recent work (Flynn and Morrow, 1964a, 1964b) in which were investigated the effects of changes in the initial functional group content and of deuterium substituted for alco- holic hydrogen. At 40°C in the absence of air, the quantum yields for both scission and for the volatile products were markedly dependent upon the extent of pre-irradiation oxidation of the cellulose; carboxyl groups decreased both photochemical and thermal stability of these materials. Carbon monoxide and carbon dioxide appeared to be related to the scission process, and much of the chain scission and carboxyl group formation occurred during a post-irradiation decomposition of long-lived oxy-radicals. The concomitant formation of carbonyl groups and hydrogen involves homolysis of a C-H or an O-H bond at the C2, C3, or C6 carbon atoms in the anhydroglucose unit. There was no evidence for hydrogen abstraction by hydrogen atoms. Rather, disproportionation, possibly between the initially formed

\ CHO" + H'--[

~ / ----¢. C ~ _ _ _ O + H 2

\ :OH + H'-- /

radicals, is the dominant route to hydrogen formation. The ESR spectrum of ultraviolet-irradiated cellulose was simpler than that obtained with high energy radiation, but no identification of the radicals has been made (Florin and Wall, 1963); the spectrum was independent of the presence of oxygen, except that eventually peroxides form (Kleinert, 1964).

Considerable effort has gone into the identification of the nonvolatile fragments formed during the photolysis of cellulose in air. Beelik and Hamilton (1959) chromatographed an aqueous extract of irradiated pulp and


identified xylose, xylobi(tri-, tetra-, and penta-)ose, o-glucose, cellobiose, cellotriose, o-arabinose, 3-fl-cellobiosido-D-arabinose, and 3-fl-D-glucosido-D- arabinose. On the basis of these products and the products isolated from irradiated model compounds (Beelik and Hamilton, 1961), it was determined that the initial cleavage took place between the C1 and C2 carbon atoms. Absorp- tion of the photon was stated to have occurred at least to some extent through an "acetal chromophore" which absorbs weakly at 2660A. Other workers have obtained somewhat different products, depending upon the source of the cellulose (Gingras, Cooney, Jackson and Bayley, 1963).

Zapol'skii (1961) noted a steady increase in 3660-A excited luminescence during the irradiation of cellulose films in air and related this to the formation of carbonyl groups as luminescence centers. Since the photochemical reactions were retarded in later stages of irradiation, it was suggested that an absorbed photon could migrate to the luminescence center, there to discharge its energy by the generation of radiation or heat. A similar change in luminescence has been observed in the photodegradation of ethyl cellulose (Katibnikov, Ermolenko, Somova, Efremova and Glikman, 1960; Ermolenko, Katibnikov and Somova, 1961).

Cellulose acetate, in the form of powder, undergoes random scission in air under irradiation by a high pressure mercury source; the quantum yield was 2.5 × 10-3, independent of intensity or initial molecular weight (Jortner, 1959). In the presence ofphenyl salicylate, which acted as a filter, the rates of scission but not the quantum yield were decreased.

Irradiation (2537A) of extensively nitrated cellulose in ethyl acetate or partially nitrated cellulose in methanol under nitrogen at 25°C resulted in random scission accompanied by a post-effect on viscosity; a quantum yield of about 0.02, based on energy absorbed by the polymer, was reported for the methanol solution (Claesson, Palm and Wettermark, 1961). Methanolic solutions of cellulose nitrate with and without fl-naphthylamine were irradiated with 3020-A, 3340-A, and 3650-A light (Claesson and Wettermark, 1961). In the absence of the amine, scission occurred only with 3020-A radiation, with a quantum yield of 0.009. ~-Naphthylamine is a strong absorber at 3340 A and a sensitized degradation occurred at these wavelengths with a quantum yield of 0.0006; with 3020-A radiation, a decreasing viscosity post-effect lasting two weeks was observed.

CONCLUDING REMARKS

A relatively easy way of assessing recent progress in polymer photodegradation would be to construct a table containing quantum yields and other information of the type given in this review. Such a table is very useful and one such appeared in 1962 (Jellinek, 1962b). Simple subtraction would gauge the speed of this advancing avalanche of data.

86 ROBERT B. FOX

Perhaps a better index to progress in this field is the realization that the proportion of useful contributions to the field is on the increase. Comparison with earlier reviews might indicate this, too. But not indicated is the seemingly new appreciation now being felt by many for the need for purity in polymer work of this kind, a need for purity of the type commonly employed in photochemistry. Perhaps it is the realization that photodegradation of polymers is as much a field of photochemistry as it is of polymer chemistry, and that it is becoming less and less the province of testing and evaluation, important though this may be.

It is apparent that we are at last crossing the threshold from mere data acquisition to a knowledge of the actual acts which occur when a polymer is degraded. Having learned what took place, the next step is to learn how it happened--and then, and only then, will we be in a position to exercise real control over the degradation process.

R E F E R E N C E S

ABRAHAM, R. J., MELVILLE, H. W., OVENALL, D. W., and WHIFFEN, D. H., (1958) Trans. Faraday Soc. 54, 1133.

ACHHAMMER, B. G., REINEY, M. J. WALLs L. A., and REINHART, F. W., (1952) J. Polymer Sci. 8, 555.

ACHHAMMER, B. G., REINHART F. W., and KIIt~, G. M., (1951)./. Res. Nat. Bur. Stands. 46, 391.

ALEXANDER, A. L., NOONAN, F. M., COWLINO, J. E., and KAGAPdSE, R. E., (1959) U.S. Naval Res. Lab. Rept. 5257.

A~rroN, A., (1965) J. Appl. Polymer Sci. 9, 1631. BALAaAN, L., (1963) Chem. Pr~mysL 13, 45. BALAB.t, rq, L., (1965) Chem. PrtimysL 15, 158. BASILE, L. J., (1962) J. Chem. Phys. 36, 2204. BAXENDALE, J. H., and THOMAS, J. K., (1958) Trans. Faraday Soc. 54, 1515. BEEUK, A., and HAMILTON, J. K., (1959) DasPapier 13, 77. BEELIK, A., and HAM1L'rON, J. K., (1961)J. Org. Chem. 26, 5074. BERLIN, A. A., POPOVA, Z. V., and YANOVSKII, D. M., (1965) VysokomoL Soedin. 7, 569. BERSCH, C. F., HARVEY, M. R., and ACrrdAMMER, B. G., (1958) J. Res. Nat. Bur. Stands. 60,

481. BEVINGTON, J. C., (1959)./. Polymer Sci. 34, 680. BoASsoN, E. H., KAMERUEEK, B., ALOERA, A., and KROES, G. H., (1962) Rec. Tray. Chim. 81,

624. BOWERS, G. H., and LOVEaOY, E. R., (1962) Ind. Eng. Chem., Prod. Res. Dev. 1, 89. BROWr,aNO, JR., H. L., ACKERMANN, H. D., and PATTON, H. W., (1965) Polymer Preprints,

6, 1014. CHAPIRO, A., (1962) Radiation Chemistry of Polymeric Systems, Interscience Publ., New

York. CHARLESBY, A., (1962) Radiation Chem., Proc. Tihany Sympos., 175. CHARLESBY, A., GRACE, C. S., and PILKINGTON, F. B., (1962) Proc. Roy. Soc. A268, 205. CHARL~BY, A., and MOOR~, N., (1964) Int. J. AppL Radiation andlsotopes 15, 703. CHARLESBY, A., and THOMAS, D. K., (1962) Proc. Roy. Soc. A269, 104. CHARLESUY, A., and PARTRIDGE, R. H., (1965a)Proc. Roy. Soc. A283, 312. CHARLESBY, A., and PARTRIDGE, R. H., (1965b) Proc. Roy. Soc. A283, 329. CHEN, S., (1949) J. Phys. and Colloid Chem. 53, 486. CHotJ, C. H., and JELLINEK, H. H. G., (1964) Can. J. Chem. 42, 522.


CLAESSON, S., PALM, G., and WETTERMARK, G., (1961) Arkiv Kemi 17, 579. CLAESSON, S., and WETTER.MARK, G., (1961) Arkiv Kemi 17, 355. COTTEN, G. R., and SACKS, W., (1963)J. Polymer Sci. A1, 1345. COWLEY, P. R. E. J., and MELVILLE, H. W., (1952a)Proc. Roy. Soc. A210, 461. COWLEY, P. R. E. J., and MELVILLE, H. W., (1952b)Proc. Roy. Soc. A211, 320. DUNCALF, B., and DtrNN, A. S., (1964) J. Appl. Polymer Sci. 8, 1763. EGERTON, G. S., ATrLE, E., and RATHOR, M. A., (1962) Nature, 194, 968. EGERTON, G. S., A'ITLE, E., GUIRGUIS, F., and RATHOR, M. A., (1963) J. Soc. Dyers Colour-

ists, 79, No. 2, 49. ERMOLENKO, L N., KATIBNIKOV, M. A., and SOMOVA, A. I., (1961) Vysokomol. Soedin. 3, 30. ERSHOV, YU. A., LUKOVNIKOV, A. F., and BATURINA, A. A., (1964) Kinetica i Kataliz 5, 752. FLORIN, R. E., and WALL, L. A., (1963)J. Polymer Sci. A1, 1163. FLYNN, J. H., (1958) J. Polymer Sci. 27, 83. FLYNN, J. H., and MORROW, W. L., (1964a) J. Polymer Sci. A2, 81. FLYNN, J. H., and MORROW, W. L., (1964b) J. Polymer Sci. A2, 91. FLYNN, J. H., WILSON, W. K., and MORROW, W. L., (1958) J. Res. Nat. Bur. Stands. 60, 229. FORD, R. A., (1957) J. ColloidSci. 12, 271. Fox, R. B., ISAACS, L. G., SAALEELD, R. E., and McDoWELL, M. V., (1965) U.S. Naval Res.

Lab. Rept. 6284. Fox, R. B., ISAACS, L. G., and STOKES, S., (1963) J. Polymer Sci. A1, 1079. Fox, R. B., ISAACS, L. G., STOKES, S., and KAGARISE, R. E., (1964) J. Polymer Sci. A2, 2085. Fox, R. B., and PRICE, T. R., (1964) PolymerPreprints, 5, 390. Fox, R. B., and PRICE, T. R., (1965a) J. Polymer Sci. A3, 2303. Fox, R. B., and PRICE, T. R., (1965b) Org. Coatings and Plastics Chem. Div., Amer. Chem.

Soc., Preprints, 25, No. 2, 138. FROLOVA, M. I., EEIMOV, L. I., and CrIEKMODEEVA, I. V., (1964) Plast. Massy, No. 3, 38. FROLOVA, M. I., EFIMOV, L. I., and RIABOV, A. V., (1964) Tr.po Khim. TeknoL, 304. FROLOVA, M. I., and RIABOV, A. V., (1959) Vysokomol. Soedin. l , 1453. FROLOVA, M. I., NEVSKII, L. V., and R1ABOV, A .V., (1961) Vysokomol. Soedin. 3, 877. GARDNER, D. G., and EPSTEIN, L. M., (1961) J. Chem. Phys. 34, 1653. GINGRAS, B. A., COONEY, D., JACKSON, K. A., and BAYLEY, C. H., (1963) Textile Res. J. 33,

1000. GOLE, J., and CALVAYROC, H., 0965) Compt. Rend. 260, 163. GOLUB, M. A., and PARKER, J. A., (1965) Makromol. Chem. 85, 6. GRAFMULLER, F., and HUSEMANN, E., (1960) Makromol. Chem. 40, 161. GRASSIE, N., (1956) Chemistry of High Polymer Degradation Processes, Interscience Publ.,

New York. GRASSlE, N., and MACCALLUM, J. R., (1964) J. Polymer Sci. A2, 983. GRASSIE, N., and WEIR, N. A., (1965a) J. Appl. Polymer Sci. 9, 963. GRASSIE, N., and WEIR, N. A., (1965b) J. Appl. Polymer Sci. 9, 975. G RASSIE, N., and WEIR, N. A., (1965c)J. Appl. Polymer Sci. 9, 987. GRASSIE, N., and WEIR, N. A., (1965d)J. Appl. Polymer Sci. 9, 999. GUILLET, J. E., and NORRISH, R. G. W., (1955a) Proc. Roy. Soc. A233, 153. GUILLET, J. E., and NORRLSH, R. G. W., (1955b)Proc. Roy. Soc. A233, 172. HAHN, W., and GRAEMf0LLER, F., (1956) Makromol. Chem. 21, 121. HATCHARD, C. G., and PARKER, C. A., (1956) Proc. Roy. Soc. A235, 518. HIPPE, Z. S., JABLONSKI, H. T., and KRZYT~ANOWSKA, T. B., (1965) J. Oil and Co/our

Chemists' Assn. 48, 447. HSIEN, P.-K., HSlANG, P.-C., LIAO, J.-C., LIANG, Y.-J., WANG, H.-J., and FANG, T.-C.,

(1962) Sci. Sinica, 11, 1513. Hsu, C.-P., and RAFIKOV, S. R., 0962) Vysokomol. Soedin. 4, 1474. INAMI, A., and MORIMOTO, K., (1962) Kogyo KagakuZasshi, 65, 293. ISAACS, L. G., unpublished results. ISAACS, L. G., and Fox, R. B., (1965a) J. Appl. Polymer Sci., 9, 3489 ISAACS, L. G., and Fox, R. B., (1965b) U.S. Naval Res. Lab. Rept. 6339. JACOBS, H., and STEELE, R., (1960a) J. AppL Polymer Sci. 3, 239. JACOBS, H., and STEELE, R., (1960b) J. Appl. Polymer Sci. 3, 245.

88 ROBERT B. FOX

JELLINEK, H. H. G., (1955) Degradation of VinylPolymers, Academic Press, New York. JELLrNEK, H. H. G., (1962a) J. Polymer ScL 62, 281. JELLINEK, H. H. G., (1962b) Pure andAppL Chem. 4, 419. JELLINEK, H. H. G. (1964), J. Polymer Sci. B2, 457. JELLINEK, H. H. G., and BASTmN, I. J., (1961) Can. J. Chem. 39, 2056. JELLINEK, H. H. G., and SCHLLrETER, W. A., (1960) J. AppL Polymer Sci. 3, 206. JELLINEK, H. H. G., and WANG, I. C., (1965) Koll.-Z. 202, 1. JORTNER, J., (1959) J. Polymer Sci. 37, 199. KARYAKrN, A. V., GmSHIN, G. V., and KURYKIN, B. D., (1965) VysokomoL Soedin. 7, 389. KATmNIKOV, M. A., ERMOLENKO, I. N., SOMOVA, A. I., EFREMOVA, O. G., and GLrC, a~AN,

S. A., (1960) Vysokomol. Soedin. 2, 1805. KELLEHER, P. G., and JASSIE, L. B., (1965),/. AppL Polymer Sci. 9, 2501. ~RILOVA, E. I., MATVEEVA, E. N., LEITMAN, K. A., and FRATK_rNA, G. P., (1962) Plast.

Massy, No. 11, 3. KnULOVA, E. I., MATVEEVA, E. N., LErr~,~, K. A., and FRATraNA, G. P., (1964), Plast.

Massy, No. 3, 10. KLEn~ERT, T. N., (1964) Monatsh. 95, 387. KORDUB, N. V., and DVSmNA, R. Sh., (1964) Doklady Akad. Nauk Uz.S.S.R. 21, No. 2, 16. KROES, G. H., (1963) Rec. Tray. Chim. 82, 979. KRYSZEWSKI, M., and MUCHA, M., (1965) Bull. Acad. Polon. Sci., Ser. Sci. Chim. 13, 53. LAtrNER, H. F., and WILSON, W. K., (1949)./. Am. Chem. Soc. 71, 958. LIANG, Y.-H., WANG, H.-Y., FANG, T.-C., HSIANG, P.-C., and HSmN, P.-K., (1962) ScL

Sinica, 11,903. MACCALLUM, J. R., (1964) Polymer, 5, 213 McDoWELL, M. V., ISAACS, L. G., Fox, R. B., and SAALrELD, F. E., (1963) Rept. of NRL

Progress, Nov., 37. McGARY, JR., C. W., (1960)J. Polymer Sci. 46, 51. MAEROV, S. B., (1965)./. Polymer Sci. A3, 487. MASSON, C. M., BOEKELHEIDE, V., and NOyES, JR., W. A., (1956) Photochemical Reactions in

Technique of Organic Chemistry, A. Weissberger, ed., Interscience Publ., New York vol. 2 (2nd ed.).

MATStrOA, T., KURmARA, F., and MORt, H., (1959) Nippon Gomo Kyokaishi, 32, 12. MILrNCHUK, V. K., and PSHEZHETSKII, S. YA., (1963) Doklady Akad. Nauk, S.S.S.R. 152,

665. MIL~CHUK, V. K., and PSHEZHETSKIL S. YA., (1964) VysokomoL Soedin. 6, 1605. MINSK, L. M., SMITH, J. G., VAN DEUSEN, W. P., and WRIGHT, J. F., (1959) J. AppL Polymer

Sci. 2, 302. M6NIG, H., (1958) Naturwiss. 45, 12. Mt3NIG, H., and KRtEGEL, H., (1960a) Proc. 3rd Int. Congress on Photobiology, Copenhagen,

618. M6NIG, H., and KRIEGEL, H., (1960b)Z. Naturforsch. 15b, 333. MO~G, H., and KRmGEL, H., (1964) Biophysik, 2, 22. MOORE, R. F., (1963)Polymer, 4, 493. MoRt, M., and KUMAGAI, M., (1964) Japan J. Appl. Phys. 3, 11. NOVAK, J., (1962) Kunststoff, 52, 269. NOYES, JR., W. A., HAMMOND, G. S., and Prrrs, JR., J. N., (1963-6) Advances in Photo.

chemistry, Interscience Publ., vol. 1-4. OrrNism, S., St~GIMOTO, S., and NWrA, I., (1963) J. Chem. Phys. 39, 2647. OSBORNE, K. R., (1959)J. Polymer Sci. 38, 357. OSTER, G., (1956) J. Polymer Sci. 22, 185. OSaZR, G., (1964) J. Polymer Sci. 132, 1181. OSTER, G., OSTER, G. K., and KRYSZEWSKI, M., (1961) Nature, 191, 164. OSrER, G., OSaER, G. K., and KRYSZEWSKI, M., (1962)J. Polymer Sci. 57, 937. OS~R, G., OSaZR, G. K., and MOROSON, H., (I 959) J. Polymer Sci. 34, 671. OWENS, F. H., and Z~tr~RMAN, F. E., (1963) J. Polymer Sci. A1, 2711. OZAWA, S., and MAEKAWA, K., (1963) Kobunshi Kagaku, 20, 357. PARt, R, C. A., (1953) Proc. Roy. Soc. A220, 104.

PHOTODEGRADAT1ON OF HIGH POLYMERS 89

PETrr, J., and ZAITOUN, G., (1963) Compt. Rend. 256, 2610. PRICE, T. R., and Fox, R. B., (1966) J. Polymer Sci. IM, 77 l RABEK, J. F., 0965) J. Appl. Polymer Sci. 9, 2121. RABEK, J. F., and RABEK, T. I., (1963) J. Appk Polymer Sci. 7, $38. RAFIKOV, S. R., and Hsu, C.-P., (1961) Vysokomol. Soedin. 3, 56. RODE, V. V., YAROV, A. S., and RAFIKOV, S. R., (1964) Vysokomok Soedin. 6, 2168. SATO, Y., (1964) Kobunshi Kagaku, 21,513. SCH6NBERG, A., (1958) Pr~'parative Organische Photochemie, Springer-Verlag, Berlin. SEARLE, N. Z., and HIRT, R. C., (1962)Polymer Preprints, 3, 56*. SEELY, G. R., (1962) J. Am. Chem. Soc. 84, 4404. SHUL'rZ, A. R., (1958) J. Chem. Phys. 29, 200. SHULTZ, A. R., 0960) J. Polymer Sci. 47, 267. SHULTZ, A. R., (1961) J. Phys. Chem. 65, 967. SHULTZ, A. R., KNOLL, N., and MORNEAU, G. A., (1962) J. Polymer Sci. 62, 21 I. SHULTZ, A. R., and LEAHY, S. M., (1961) J. AppL Polymer Sci. 5, 64. SIEGEL, S., and EISENTHAL, K., (1965)J. Chem. Phys. 42, 2494. SIEGEL, S., and JUDEIKIS, H., 0965) J. Chem. Phys. 43, 343. SIEWERT, H., (1964)Z. Naturforsch. 19b, 806. SOBUE, H., TABATA, Y., and TAJIMA, Y., (1958) J. Polymer Sci. 27, 596. SOBUE, H., and TAJIMA, Y., (1959) Kogyo KagakuZasshi, 62, 1149. STEPANE~C, J., and DOLEZEL, B., (1963) Chem. Listy, 57, 818. STEPHENSON, C. V., LACEY, JR., J. C., and WILCOX, W. S., (1961) J. Polymer Sci. 55, 477. STEPHENSON, C. V., MOSES, B. C., BURKS, JR., R. E., COBURN, JR., W. C., and WILCOX, W. S.,

(1961) J. Polymer Sci. 55, 465. STEPHENSON, C. V., MOSES, B. C., and WILCOX, W. S., ( 1961) J. Polymer Sci. 55, 451. STEPHENSON, C. V., and WILCOX, W. S., (1963) J. Polymer Sci. A1, 2741. STOKES, S., and Fox, R. B., 0962)J. Polymer Sci. 56, 507. TAKAHASHI, T., and SUZUKI, K., (1964) KobunshiKagaku, 21,494. TANAKA, Y., (1964) Sen-i Gakkaishi, 20, 111. TRUDELLE, Y., and NELL, J., (1965) Compt. Rend. 260, 1950. TRYON, M., and WALL, L. A., (1958) J. Phys. Chem. 62, 697. TSUDA, M., (1964)J. Polymer Sci. A2, 2907. TSUNODA, T., and YAMAOKA, T., (1964) J. AppL Polymer Sci. 8, 1379. TURI, E., ROLDAN, L. G., RAHL, F., and OSWALD, H. J., (1964)PolymerPreprints, 5, No. 2,

558. URhZOVSKII, S. S., and POSTOEVA, M. E., (1962) Tr. Khar'kovsk. Politekhn. Inst. 39, 77. V6LKER, TH., (1961a) MakromoL Chem. 44/46~ 107. V6LKER, TH., (1961b) Osterr. Chem. Ztg. 62, 345. WALL, L. A., (1962) in Analytical Chemistry of Polymers. Part 11. Analysis of Molecular

Structure and Chemical Groups, G. M. Kline, ed., Interscience Publ., New York, Chap. VI.

WALL, L. A., HARVEY, M. R., and TRYON, M., (1956) J. Phys. Chem. 60, 1306. WALL, L. A., and FLYNN, J. H., (1962) Rubber Chem. and Technol. 35, 1157. WILKINSON, F., (1964) Advances in Photochemistry, 3, 241. WIt.SKI, H., (1959) Angew. Chem. 71,612. WIt.SKI, H., (1963) Kolloid-Z. 188, No. 1, 4. WrNKLER, D. E., (1959) J. Polymer Sci. 35, 3. WISSBRtrN, K. F., (1959),/. Am. Chem. Soc. 81, 58. YOSmDA, H., and RArCBY, B., (1964) J. Polymer ScL B2, 1155. YosraNO, T., and MAN,E, Y., (1963)J. Polymer ScL AI, 2135. ZArrotrN, G., (1962) Compt. Rend. 254, 2980. ZArrotn% G., (1964) F.A.T.I.P.E.C., Congr., 7, 337. ZAPOL'SKrL O. B., (1961) VysokomoL Soedin. 3, 376. Z~O, OL'SKn, O. B., (1965) Vysokomol. Soedin. 7, 615. ZHUZHGOV, E. L., BUBNOV, N. N., and VOEVODSKII, V. V., (1965) Kinetica iKataliz, 6, 56.

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