Photosensitized Degradation of Polymers - ACS Publications

18 Photosensitized Degradation of Polymers J. F. RABEK Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 8, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch018

Department of Polymer Technology, The Royal Institute of Technology, Technical University, Stockholm, Sweden

Photosensitization of polymers has been found to be important for a variety of purposes such as 1) Photopolymerization, 2) Photomodification (photo-grafting), 3) Photodegradation, and 4) Photostabilization (1). In commercial polymers photodegradation reactions are usually photosensitized by the presence of alien groups in the chain or by the presence of trace impurities (2,3). The first group of impurities is formed during storage and processing of polymers in the presence of air. Hydroperoxide, carbonyl and hydroxyl groups, and also unsaturated bonds, belong to the internal impurities. These groups are mainly formed during the moulding operations which require heat and pressure to shape the polymer into the required form. The thermal history of a polymer has been shown to have a marked effect on its subsequent photostability (4-8). Such groups may also be formed on the polymer surface during the exposure to sunlight. Second group of impurities consists of traces of compounds added for the synthesis of the polymers, e.g., catalysts, modifiers, emulsifiers, solvents, etc. It has been suggested that the nomenclature used for the description of photosensitized reactions should precisely be applied (9), namely: 1. "Photoinitiator" is a compound which absorbs light and is excited by it to a higher energy state having a total energy content in excess of that required to effect a homo lytic scission of some bonds in polymer molecule to form free radicals, which promote secondary reactions. 2. "Photosensitizer" is a compound which by absorption of light is transferred to excited states and then donates the energy to another compound by inter- or intramolecular energy transfer. 3. "Photosensitized reactions" are strictly speaking such reactions which are activated by photoinitiators or photosensitizers. In many cases a chemical compound may, in dependence upon the conditions of a photochemical reaction, behave either as a photoinitiator or as a photosensitizer. 255 Labana; Ultraviolet Light Induced Reactions in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

256

UV LIGHT

INDUCED

REACTIONS

IN POLYMERS

Free r a d i c a l reactions

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 8, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch018

The study of the photochemistry of inorganic and organic compounds gives valuable information on t h e i r photolysis during which free radicals are formed (10-12)· Extensive studies have especially been made i n the f i e l d of photo chemistry of a l i p h a t i c ketones,ethers and peroxides (12-14)· A l l these compounds have been found to be good photoi n i t i a t o r s which i n i t i a t e degradation and c r o s s l i n k i n g of polymers. The mechanism of these reactions seems to be simples RH + hv — Η · + Η· RR + hv — ~ Η· + ϋ· FH + Ε· Ρ· + RH wheref RH(RR)-photoinitiator,FH-polymer molecule* Free r a d i c a l s formed during the photolysis of a compound abstract hydrogen from the polymer backbone o r from side groups and form macroradicals* This simple photolysis mechanism i s however complicated bys 1*The formation of charge-transfer complexes between i n i t i a t o r s and polymer macromolecules* The absorption of charge-transfer complexes i s more intense than that of the components and i s s h i f t e d towards long wavelengths* 2.The formation free radicals i n s o l i d state,the d i f f u s i o n of free radicals into polymer matrix,the k i n e t i c s of hydrogen atom abstraction* 3 . I n the case of photodegradation i n solution the cage e f f e c t due to solvent molecules which surround the r a d i c a l pairs formed and enclose them as i n a cage (15-16)* 4*Reactions o f free radicals with oxygen and formation of peroxy radicals* Photodegradation influenced by the presence of solvents The f i n d i n g of a best method f o r introducing photoi n i t i a t o r s and photosensitizers into polymers i s a very important p r a c t i c a l problem* Two main methods are i n t h i s case appliedt a polymer f i l m i s cast from a solution with the respective photosensitizer added,or the photosensitizer i s pressed into the f i l m a t an elevated temperature* In the f i r s t method i t i s sometimes very d i f f i c u l t to remove a l l traces of solvent,which may influence the photoreactions observed* In the second method conditions of pressing (temperature of 100-200°C and pressure of 100-200 atm) may a l t e r the polymer and compound added* Many common solvents are usually considered to be i n e r t i n photochemical reactions* Only few solvents, such as alcohols and p a r a f f i n i c hydrocarbons are indeed i n e r t when i r r a d i a t e d i n the range of 200-700 nm* Other solvents,such as ketones,aromatic hydrocarbons,tetrahydrofurane,chloroform and carbon tetrachloride are i n this range photolyzed ( 3 ) .

Labana; Ultraviolet Light Induced Reactions in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

18. RABEK

Photosensitized Degradation of Polymers

257

Photo-oxidative reactions may occur i n many solvents i n the presence o f oxygen. Photochemical reactions o f several common solvents are presented below*

1.Ketones (10,11,13)

NORRISH TYPE I

RJCHCR^CRJCOR

V -

1

CR -CR


2

•»CR =CR 2

2

+CR=C

2

NORRISH TYPE II

2.Benzene (17-20) BENZENE

!wA-A-A-

9

X

—-A-A-A -A-A-A—8 The l i f e t i m e of migrating exciton i s 10 sec. During that time the exciton may " v i s i t " several molecules over a long


264

UV L I G H T INDUCED REACTIONS I N P O L Y M E R S

distance of 10*2-10^ JL Such migrating exciton may be captured by impurities or traps (e.g.physical defects i n the c r y s t a l ) and loses i t s energy which i s then transformed into the v i b r a t i o n a l energy of an atom or a molecule. The exciton energy transfer can occur i n many polymers with an adequate symmetry.


Photosensitized reactions of polymer degradation The c l a s s i f i c a t i o n of photosensitized reactions according to the polymer structure seems to be a resonable approach,but unfortunately the quantitative evidence necesary f o r such c l a s s i f i c a t i o n i s s t i l l inadequate. In practice i t i s convenient to group the s e n s i t i z e d reactions according to the photochemical reactions of the i n i t i a t o r s and s e n s i t i z e r s , e . g . Benzophenone The t r i p l e t state i s the photochemically reactive state of t h i s compound (9-12,91)»

BENZOPHENONE

L

J

I t has been found that only ketones,in which the lowest t r i p l e t state has the n, p o l y ( v i n y l alcohol) (94)» polyisoprene (95,96),polyurethane (97) and polyadenylic acid (98)· In s o l i d state benzophenone also produce an extensive c r o s s l i n k i n g of polyethylene (99-103)· I t has also been found that benzophenone and i t s derivatives caused an i n i t i a l rapid oxidation,increasing with ketone concentration (70)· Currently the r e l a t i v e importance of singlet oxygen formation i n energy transfer reaction between excited benzophenone and molecular oxygen i s discussed

(92,104): \θ

θ ] * ° » -~ Ο 3

2,3ς

ί

Ό *V' +1(


18. RABEK


265

Quinones


On exposure to UV i r r a d i a t i o n the reactions of quinones involve t h e i r carbonyl groups and the r i n g double bonds, Quinones arecapable of abstracting hydrogen atoms from various hydrogen donors. The s i n g l e t and t r i p l e t states of quinones are considered to be the i n i t i a l reactive i n t e r mediate i n the reactions

Quinones are e f f e c t i v e compounds f o r the photodegradation of polystyrene (105-107)tPolyisoprene (108 109)•polypropylene (110).polyamides (111),polyurethanes (97) and c e l l u l o s e (112). lîuring the quinone-sensitized photooxidative degradation of polystyrene f i l m and i t s solution i n benzene,an i n i t i a l rapid decrease of average molecular weight has been observed by v i s c o s i t y measurement (Pig.8) and GPC (Pig.9) (107)· t

Figure 8.

Figure 9. (Legends on following page)


266

U V L I G H T INDUCED REACTIONS I N P O L Y M E R S

Figure 8. (page 262, left) Change of viscosity number in benzene solution during uv irradiation in the presence of air and (Φ) p-quinone, (O) duroquinone, ( V ) anthraquinone, (*ψ) chloranil, and (X) without quinone. Motor ratio of styrene units to quinone = 88:1.


Figure 9. (page 262, right) Gel permeation chromatograms of (—) undegraded polymer and polystyrene degraded after 5 hr uv irradiation with (--) p-quinone, ( ) duro quinone, (" -) chloranil, (—) anthraquinone, and without quinone ( ). (A), polysty rene as film; (B), polystyrene in benzene solution. Molar ratio of styrene units to qui none = 88:1.

The reaction rates are strongly increased by quinone such as p-quinoneιduroquinone,anthraquinone,and c h l o r a n i l . l t has been suggested ( l O j ) that t h i s photosensitized degradation of polystyrene occurs by a s i n g l e t oxygen reaction which might be related to an energy transfer mechanism from excited t r i p l e t states of quinone to molecular oxygen. Peroxides They are e s p e c i a l l y important i n the process o f thermal and photodegradation of polymers. The primary photod i s s o c i a t i o n of a l k y l and a r y l peroxides takes place i n the f i r s t absorption region. The weak RO-OR* bond i s disrupted! RO-OR* + hv — - RO* + 'OR' Below 2 2 0 nm a l k y l peroxides dissociate i n another way:

sometimes very important differences of the natures of the r a d i c a l s produced i n thermal and photochemical d i s s o c i a t i o n are found. I t was shown ( 1 1 ? ) that the peroxide molecules are dissociate by heat to pairs of benzyloxy radicals,some of which may dissociate f u r t h e r to phenyl r a d i c a l s and carbon dioxide. Photochemical d i s s o c i a t i o n leads to the d i r e c t production of some phenyl r a d i c a l s (114). I t has been found that the photodegradation of several polymers such as p o l y ( v i n y l chloride)(115)> polyisoprene ( I 0 9 t 1JjS),bisphenol A polycarbonate ( 1 1 7 ) i s s e n s i t i z e d by peroxides. P o l y c y c l i c aromatic hydrocarbons They are well known as compounds which are photochemically very r e a c t i v e . The excited s i n g l e t and t r i p l e t states are the photochemically reactive states,and they also can p a r t i c i p a t e i n energy transfer reactions ( 2 , 3 , 1 0 , 1 1 ) . Some of these compounds such as anthracene,phenanthrene,pyrene added to polyethylene effect the photodegradation of polymer ( 1 1 8 ,



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1 1 9 ) · They can s e n s i t i z e the abstraction of the a l l y l i c hydrogen of unsaturated groups i n polyethylene* Photoexeited t r i p l e t state transfers i t s excess energy to the unsaturated bond to excite it,and the excited unsaturated groups release t h e i r a l l y l i c hydrogen atom,producing an a l l y l i c r a d i c a l . Hexahydropyrene s e n s i t i z e d chain s c i s s i o n of polypropylene and polyisobutylene during l i g h t i r r a d i a t i o n ( J 2 0 ) . P o l y c y c l i c hydrocarbons have a important role i n s e n s i t i z e d photo oxidation of polyisoprene ( 1 2 2 ) , p o l y s t y r e n e ( I 2 3 ) t poly(methyl methacrylate) ( 1 2 3 - 1 2 6 ) « I t i s quite probable that these reactions can also occur with p a r t i c i p a t i o n of s i n g l e t oxygen. Polymers r e a d i l y degraded by l i g h t The photosensitizing degradation may considerable contribu te to the solution of the p l a s t i c - l i t t e r problem. In order to improve the standards of our environment the amounts of p l a s t i c l i t t e r should be reduced and t h i s aim may e a s i l y be achived by the use of p l a s t i c s which would r e a d i l y degradable by sunlight. This p r a c t i c a l problem has been f u l l y reviewed by the following publications ( 3 , 1 2 7 ) . Conclusions I t has been demonstrated i n t h i s short review that the understanding of the photosensitization mechanism i n polymers may be applied f o r the improvement of t h e i r p h o t o s t a b i l i t y and f o r the development of polymers with c o n t r o l l e d l i f e t i m e .

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