Effect of naphthalene additive on the radiation degradation of polymethylmethacrylate

Radiat. Phys. Chem. Vol. 46, No. 4-6, pp. 843-846, 1995

Pergamon

0969-806X(95)00274-X

Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0969-806X/95 $9.50 + 0.00

EFFECT OF NAPHTHALENE ADDITIVE ON THE RADIATION DEGRADATION OF POLYMETHYLMETHACRYLATE O.V.

Kolninov, I.P. Shelvkhov, E.R. Klinshpont, Z.N. Lavrova*, A.N. B a r a n * , V.N. L e v i n *

Branch

of Karpov Institute of Physical Chemistry Obninsk, Kaluga reeion. 249020, Russia *Scientific-Production Center for Polymer Optical Tver, Russia

Fibres,

ABSTRACT Radiolysis a t 300K o f p o l y m e t h y l and octadeuteromethylmethacrylates containin8 naphthalene results in the formation o f the stable macroradicals and charged particles cations and anions of the naphthalene additive, trapped electrons of polymer matrix. The contribution of charsed particles in the radiation-induced optical losses is 2000 dB/km in the resion 6 0 0 - 1 0 0 0 nm a t c o n c e n t r a t i o n s of additive 0.8-1.2 M. It was established that naphthalene sensitizes the radiation deeradation of polymethyl methacrylate with formation of polyenyl macroradicals. KEYWORDS

Radiolysis,

destruction,

desradation,

additive,

polymer

fibre

INTRODUCTION

Irradiation of polymer optical materials changes optical properties increasing the absorption in transparent region of polymers. The main source of the radiation-induced optical losses of polymethyl methacrylate are radical products of chemical reactions: the terminal alkyl macroradicals, macroradicals of allyl and polyenyl types and the products of their oxidation also. Additives which provide protection of polymer a~ainst radiation destruction of macromolecules are introduced into polymer materials to increase the radiation resistance. T h e m o s t common a d d i t i v e s are low-molecular aromatic compounds. These compounds are chosen because they have low-lyin8 electronic states and are hishly resistant to ionizin8 radiations. In the present work we investisated the influence of addin~ naphthalene on the processes of destruction and desradation of polymethyl methacrylate. EXPERIMENTAL We i n v e s t i s o t e d polymethyl methacrylate (PMMA) a n d o c t a d e u t e r o m e t h y l methacrylate (PMMA-D8), t o w h i c h n a p h t h a l e n e and perdeuteronaphthalene ape added and polymer optical fibers (POF) b a s e d o n t h e m . P o l y m e r s a m p l e s , 1 . 5 cm in diameter a n d 1 cm h i g h , with polished end were irradiated by Cobalt-60 8amma-rays to a dose o f 3 5 kGy w i t h a d o s e r a t e of 2.5 Gy/s at 300 K in air. The absorption spectra were recorded with a SP-700 spectrophotometer. The absorption spectra of POF w e r e m e a s u r e d d u r i n 8 irradiation with the help of a KSVU-t2 spectral-computtn8 system at dose rate of 0.08 Gy/s. 843

0 . ~ Kolninovetal.

844 RKSULTS Visible

absorption

spectra

of

irradiated

PMMA w i t h

naphthalene

I t i s w e l l known t h a t n a p h t h a l e n e protects PMMA f r o m r a d i a t i o n inducin~ damage a n d d e c r e a s e s the number of ruptures of polymer chains. In this connection d o p i n ~ PMMA w i t h n a p h t h a l e n e should chan~e the absorption intensity of terminal alkyl macroradicals. The l a t t e r has the absorption b a n d a t 430 nm ( Z h d a n o v e t a l . , 1 9 8 3 ) . The s p e c t r a of irradiated initial PMMA a n d c o n tainin~ naphthalene a r e ~ i v e n i n F i e . 1. One m a i n a b s o r p t i o n bond of terminal alkyl macroradicals i s o b s e r v e d a t 4 2 0 nm f o r PMMA m a t r i x ( F i ~ . l a ) . At additive concentrations of 0.4 M the absorption spectrum differs sifnificantly from that of the undoped sample. The d o p e d s a m p l e w e t s a w e a k l y r e solved electronic - vibrational structure characteristic for polyenyl radicals. Thus, naphthalene at concentration less 0.4 M causes radiation inducine degradation o f PMMA w i t h f o r m a t i o n o f m a c r o r a d i c a l s of the polyenyl type. Simultaneously the formation of terminal alkyl macroradicals contributes in the same absorption spectra (Fie. lb). The similar absorption spectra were observed for the irradiated deuterated samples.

1,0

5oo A.nm F i f f . 1. Absorption spectra at concentrations 0.4 M (b),

of 0.8

Visible

POF d u r i n ~

absorption

The o b j e c t s

spectra

of study

of

PNHA M (c)

(a) and

and PNNA with naphthalene 1 . 2 M ( d ) . D o s e o f 35 kGy.

irradiation

w e r e POF w i t h a PMMA-D8 c o r e c o n t a i n i n e 0.8 and 1.2 M (N), and with poly-2,2,3,3-tetraflooropropyl methacrylate clothing. The a b s o r p t i o n spectra o f POF d u r i n ~ i r r a d i a t i o n are eiven in Fi~.2. When irradiation stops the induced char~es relax rapidly (Fif. 2c). Two a b s o r p t i o n b o n d s a t 5 6 0 nm a n d 9 2 0 nm c a n u n a m b i g u o u s a t t r i buted to the dimer cation of naphthalene N-N~(Bad~er et al., 1 9 6 5 ) . The a b sorption i n t h e r a n g e ? 5 0 - 8 5 0 nm s h o u l d b e a t t r i b u t e d to the anion of naphthalene (Jafers-Grodzinski et al:, t 9 6 5 ) . No s u c h a b s o r p t i o n is observed in PMMA m a t r i x . Only the wide absorption bond with no distinct maximum i n t h e teflon 5 0 0 - 6 0 0 nm c a n b e r e f e r r e d to trapped electrons. The k i n e t i c curves of accumulation of char~ed particles as a function of the irradiation dose and the correspondlnf optical losses i n POF w i t h n a p h t h a l e n e tend to saturate (Fir. S). Thus, the chareed particles are responsible for optical losses induced in POF i n the re~ion 600-1000 nm w i t h r e a c h e d v a l u e s o f 1 0 0 0 - 2 0 0 0 dB/km a t c o n c e n t r a t i o n s of additive 0.8-1.2 M. I r r a d i a t i o n Increases losses by a factor of 9-4, without additive and 5-10 with additive.

perdeuteronaphthalene

845

9th International Meeting on Radiation Processing

1.0

/.\

0.6

0.2

\

•

400 - -

Fie.

2.

600

800

.~, nm

S p e c t r a o f POF b a s e d on PNNA-D8 w i t h 1.2 M perdeuteronaphthalene ( a ) , by i r r a d i a t i o n a t 1 . 2 kGy ( b ) , at once after irradiation (c) and the difference of spectra b and c (d).

~ d6/krn

t

4.0

2.0

45 F i e . 3.

i

to

O p t i c a l l o s s e s i n POF b a s e d on PNNA-D8 w i t h 1 . 2 M ( b , d. g) and w i t h 0 . 8 M p e r d e u t e r o n a p h t h a l e n e (a, c, e) vs d o s e . At 780 nm - ( n , b ) , 850 am - ( c , d) and 560 am - ( e , 8 ) .

high optical l o s s e s w e r e o b s e r v e d i n t h e r e g i o n 4 0 0 - 6 0 0 nm a s a r e s u l t of formation of stable uncharged products. The p r o t e c t i v e effect of napht h a l e n e c a n b e r e m a r k e d when r a t e o f i n c r e a s e o f t h e l o s s e s , f o r e x a m p l e a t 430 am, i s c o m p a r e d i n t h e r e g i o n 0 . 6 - 1 , 2 kGy o f t h e l i n e a r d o s a g e d e p e n dence. These quantities were o b t a i n e d f o r POF w i t h o u t naphthalene (5.5 dB/km. G y ) , w i t h 0 . 8 H N ( 2 . 7 dB/km, Gy) a n d 1 . 2 N N (2 d B / k m / . G y ) .

The

DISCUSSION

The p r e s e n t participation

results allow to of charsed particles

PaNX - - ~ - - ~ e e~

e ~

+ +

,

propose the followin8 durin8 8amma-radiolysis

®

et

PNNA___~,,.PNNA~ PMNA÷_~_....PMNA~..luminescence

(X) (2) (3) (4)

reactions o f PNNA:

with

846

O.V. K o l n i n o v e t a l .

where e andS)are mobile the excited state of In

the presence ® e N~

+ + +

char~es; ~ a n d PMMA~are t r a p p e d a macromolecule.

of naphthalene N N N-

-~ ~

N* NN÷

the

following

reactions

charges;

PMMA~is

can o c c u r : (5)

(6) ~lominescence

(7)

Nhere N'and N-are the cation and anion of the additive molecule, respectively, a n d N ~ i s an e x c i t e d s t a t e o f t h e a d d i t i v e m o l e c u l e . At h i g h c o n c e n t rations naphthalene ion p a i r s a r e formed and r e c o m b i n e d f o l l o w i n g reaction (7) ( D a v i d e t a l . , 1978). The r e c o m b i n a t i o n o f an e l e c t r o n with the dimer c a t i o n (7) y i e l d s s i n f f l e t a n d t r i p l e t excimers, the latter being formed in the larger amount. In our case the reactions (1), ( 3 ) , (5) a n d (7) c a n be considered. Then we o b t a i n t h e f o l l o w i n g e x p r e s s i o n f o r a c c u m u l a t i o n o f N~:

d[N~/dt

= GoDKs[N]/[

K3[PMMA] + K s [ H I

- ~u~N~ ] ,

(8)

where GO i s t h e p r i m a r y r a d i a t i o n yield of ionization; D i s t h e dose r a t e ; K3) K5. and ~are rate constants of corresponding reactions. Assuming that the trapped charges (N +) v a n i s h d u r i n g i r r a d i a t i o n by t h e f i r s t order of the reaction (7) w i t h r e s p e c t to concentrations o f ion p a i r s and t a k i n g i n to account the f a c t t h a t [N] a n d [PMMA] a r e n o t c h a n g e d b e c a u s e o f t h e i r l a r g e e x c e s s , t h e t i m e d e p e n d e n c e o f IN +] c a n be o b t a i n e d i n i n t e g r a l form: [N~ = GoDK~[N](1 - e x p ( - ~ t ~ q t ) / ( K ~

3. [PMMA] + K f [ N ] )

(9)

According to this expression the ion concentrations tend to saturate, that corresponds to the obtained experimental results (Fig. 3). The p r o t e c t i v e effect of naphthalene at high concentrations is realized due t o t h e r e a c t i on ( 7 ) . At low a d d i t i v e concentrations the reaction (7) b e c o m e s i m p o s s i b l e . As shown by ( F i g . l ) , n a p h t h a l e n e a d d e d t o PMMA c a n s e n s i t i z e the degradation o f PMMA w i t h f o r m a t i o n o f p o l y e n y l m a c r o r a d i c a l s p r o b a b l y doe to the r e action of PMMA d e h y d r a t i o n . This reaction c a n be a s s o c i a t e d with triplet-triplet energy transfer from the highest level of the naphthalene molec u l e ( 5 . 7 eV) i n t o a t r i p l e t level, localized on t h e C = 0 b o n d o f t h e e t h e r g r o u p o r on t h e d o u b l e b o n d s f o r m e d i n t h e m a i n c h a i n d u r i n g r a d i o l y s i s of the polymer.

REFERENCES Zhdanov G.S., L.G. K h a m i d o v a and V.K.Milinchok (1983). Nature of radiation,color centers of polymethylmethacrylate in visible region of s p e c t r u m . Khim. V y s o k . E n e r g . , 17, p p . 4 7 - 4 9 . O h n i s h i S . o Y. Nakayma a n d J . ~ N i t t - a ( 1 9 6 2 ) . Mechanism of discoloration of irradiated polyvinyl chloride. J. Appl. Polymer Sci., 6, p p . 6 2 9 - 6 3 8 . B a l d e r B. a n d B. B r o c k l e h u r s t (t970). Absorption spectra of dimer cations. T r a n s . F a r a d a y s o c . , 5 6 , pp. 2 9 3 9 - 2 9 4 7 . Jagers'Grodzinski J . , M. W e ld , S. Yang a n d M. S z w a r e ( 1 9 6 5 ) . E l e c t r o n a f f i nities of aromatic hydrocarbons in tetrahydroforan Solution. J. phys. Chem., 69, pp. 6 2 8 - 6 3 5 . David C.,--A. P r o u a e n - D e m i d d e l e e r and G. Geuskens ( 1 9 7 8 ) . F o r m a t i o n and decay o f r a d i c a l ions of naphthalene and e m i s s i o n o f t h e r m o l u m i n e s c e n c e in irradiated polymethylmethacrylate. R a d i a t . P h y s . C h e m . , 11__, p p . 6 3 - 7 0 .