Investigation of oxidized polymers using a positron annihilation technique

Polymer Science U S S R. Vol 24, No 12, lap 2966-2971, 1982 Printed in Poland

0032-3950/82 $7 5 0 + . 0 0 © 1983 Pergamon Press Ltd

INVESTIGATION OF OXIDIZED POLYMERS USING A POSITRON ANNIHILATION TECHNIQUE* V A. 0~ISCHUK, V. S. PUDOV, V P SHANTAlCOVIC~Iand L. L. YAsI~A C h e m i c a l P h y s m s I n s t i t u t e , U S.S.R A c a d e m y of Sciences

(Received 15 Judy 1981) Poly-4-methyl-l-pentene samples oxidized at vamous temperature in the interv a l 120-160 ° h a v e b e e n i n v e s t i g a t e d u s i n g a p o s i t r o n t e c h n i q u e . A c t l v a t m n e n e r g m s of o x i d a t i o n a v e r a g e d o v e r t h i s t e m p e r a t u r e i n t e r v a l h a v e b e e n e v a l u a t e d for a m o r p h o u s a n d c r y s t a l l i n e regions s e p a r a t e l y . T h e a c t i v a t i o n energies o f o x i d a t i o n for t h e a m o r p h o u s a n d c r y s t a l h n e regions are 99 0 q - 4 6 a n d 183 I:E 12-4 k J / m o l e respectively, A p h a s e t r a n m t m n w a s d e t e c t e d a t 140-150 °.

A rOSIT~O~ technique is widely used by investigators studying the electron structure and micro defects in condensed matter [1]. Entering into a medium the positrons (e.g. from a *2Na radioactive source) are thermolyzed in the course of 10-12-10 -In sec A positron may in the final moment of flight form a positronium (Ps) atom (e+e - atom), picking up an electron either b y an "Ore gap" mechanism [2] or from a spur created b y the same positron on slowing down [3] Upon slowing down to thermal energy levels the possibility of positron annihilation can be practically ruled out [1]. After thermolysis the positron is annihilated, being either in the quasi-free delocalized state, or being captured b y some defect, or forming a bound state with some centre. These same annihiIation routes are equally possible for positronium. Each annihilation mechanism has its own average lifetime. Our aim m this investigation was to discover whether information on the oxidation of the amorphous and crystalline domains of semi-crystalline polymers m a y be obtained for each separately from the positron annihilation spectra. T h e s t u d y o b j e c t s were p o l y - 4 - m e t h y l - 1 - p e n t e n e s a m p l e s o x i d i z e d a t v a r i o u s t e m p e r a t u r e s in t h e i n t e r v a l f r o m 120 t o 160 °. T h e a n m h l l a t l o n s p e c t r a were o b t a i n e d a t 20 ° u s i n g t h e s t a n d a r d e q m p m e n t ( O R T E C ) w i t h a r e s o l u t m n functxon w i t h a h a l f h e i g h t t o t a l w i d t h o f 300 nsec. T h e p o s i t r o n source w a s 2~NaC1 salt d e p o s i t e d o n a m y l e r s u b s t r a t e . Some ~ 10°~) of all p o m t r o n s a n m h f l a t e in t h e source s u b s t r a t e T h e s p e c t r a were a n a l y z e d b y a c o m p u t e r [4] I n t h e t h r e e . c o m p o n e n t a n a l y s i s w i t h o u t r e s t r m t m n s b e i n g i m p o s e d t h e h f e t l m e of t h e l o n g - h v e d (third) c o m p o n e n t r e m a m e d p r a c t i c a l l y c o n s t a n t , a t a level o f 2.644-0.10 nsec, for all t h e s a m p l e s e x a m i n e d . T a b l e 1 gives t h e r e s u l t s of s p e c t r u m a n a l y s i s c a r r i e d o u t w i t h a fixed v a l u e of v a ~ 2 64 nsee. * V v s o k o m o l . soyed. A24 1~o 12, 2579-2583, 1982 2966

Investlgatmn of oxlehzed polymers

296T

T h e c o m p o n e n t whose lifetime v3-= 2.64 nsec was i n t e r p r e t e d as being the result o f pick-off annihilation* of ortho-positronium (ortho-Ps) t r a p p e d b y defects (volume 200 A a) [5] in a m o r p h o u s regions; t h e c o m p o n e n t with r2=:0"9 nsec was associated w i t h ortho-Ps annihilation in defects (volume ~ 30 A 3) in crystalhne regions. F a c t o r s contributing t o t h e c o m p o n e n t with T1 -~ 0 35 nsec are annihilation f r o m t h e states of para-Ps a n d from t h e deloealized states of t h e positron and

ortho-Ps. T h e a b o v e i n t e r p r e t a t i o n for c o m p o n e n t s with lifetimes ~t a n d % has n o w b e c o m e generally a c c e p t e d for p o l y m e r s [6]. As regards t h e c o m p o n e n t with a % lifetime t h e v i e w p o i n t held b y us is s u p p o r t e d b y the following considerations. F o r t h e c o m p o n e n t in Teflon w i t h a similar lifetime (T ~-1.2 nsec) it appears t h a t its i n t e n s i t y increases with increasing crystallinity of t h e s a m p l e ( I ~ - 9.3-4-1-5°/o f o r / ~ - 4 4 ~ / o a n d I~----14.3±1.5% for fl~-76~/o [7]). This means t h a t it is related t o t h e crystalline phase. T h e c o m p o n e n t with a lifetime o f ~ 1 nsec in the simplest molecular crystals is associated b y some a u t h o r s w i t h annihilation o f ortho-Ps in vacancies [8]. H o w e v e r , t h e r e is also t h e view t h a t t h e c o m p o n e n t with • - 1 nsec is related to positrons t r a p p e d in defects [9-11] T h e l a t t e r s t a n d p o i n t is unsatisfactory in t h e case o f our polymer. Certainly, let us say t h a t the second comp o n e n t (vz ~_0.9nsec) does relate to locMized positrons. O x i d a t i o n of t h e p o l y m e r is a c c o m p a n i e d b y chain scission, a n d b y t h e a p p e a r a n c e o f related s t r u c t u r a l defects. H o w e v e r , ~vhen t h e n u m b e r of defects increases t h e r e o u g h t t o be a higher r a t e o f positron t r a p p i n g b y defects, as well as a n increase in the i n t e n s i t y o f t h e long-lived c o m p o n e n t I2. This does n o t t a l l y the results p r e s e n t e d in Table 1 (relations of I~ t o o x i d a t i o n t i m e a t a c o n s t a n t t e m p e r a t u r e ) . Those who refer ix) a positronium n a t u r e of t h e i n t e r m e d i a t e c o m p o n e n t m a y well be c o r r e c t in t h e i r assumption.

TABLE 1. RESULTS OF POSITRON ANNIHILATIONINVESTIGATIOlqS Oxldatmn temp., °C No omdation 120

160 130 140 150

Time, mm

T~, nsec

I1, %

%, nsec

/o Is, o~

ra, nsec

I3, (~o

80 160 320 960 10 30 60 80 60 30

0.3634-0 003 0.315±0 004 0.3384-0 004 0 3554-0 004 0 3694-0.005 0 3284-0 004 0 352 4-0 004 0.3704-0 004 0.3514-0.004 0-3504-0 004 0 3584-0-004

60.64-0 6 59 64-0.6 63 84-0.6 65 3 i 0 6 68-94-1 2 61-04-0 6 62-64-0 6 69 64-0.8 59.8±0.6 62.6±0.6 61.14-0 6

0 892±0 032 0 8704-0 043 0.851 4- 0 042 0 8384-0 042 0 9144-0 042 0 8384-0 034 0 9264-0 038 1.031 4- 0.046 0-8444- 0.040 0-9044- 0 040 0-902 4- 0 045

10 9 ± 0 41 11"34-0 41 11 0 ± 0 4] 11 4±0"61 10 64-1 1[ 13 04-0 4[ 13.5+0-41

2'64 2 64 2 64 2 64 2 64 2 64 2.64 2.64 2.64 2 64 2-64

28-5~:0.3 29 l i 0 3 125 2 ± 0 2 23 3 ± 0 3 20 54-0 3 26 04-0 2 23-9!0-3 22 1±0.3 28 0±0-3 24 6 1 0 3 27 3~0-3

8 3±0"71 12.2-}- 0.41

12.8-4-0.4l 11.64-0 4l

* Anmhilahon of a Ps pomtron revolving a "foreign" electron.

2968

V.A. O~SHeHVK at a/.

To derive information from the results presented in Table 1 we started with the following reasonable assumptions. Ps formation takes place b y a spur mechanism. Posltronium is formed mainly in a quasi-free state, after which it m a y become trapped m some defect. 1. Defects formed durmg oxidation m a y lead to inhibition of Ps formation either on account of defects trapping electrons from a positron-created spur, or on account of electron and positron mobility being reduced. There is simultaneously increased probability that the Ps formed wall be trapped. Competition between these two processes m a y lead either to an increase or a reduction m the mtenmty of long-hved components, 12 and 18. 2 In amorphous parts of the polymer tunneling of Ps into larger original defects, from oxidation-reduced defects, may occur m periods of time that are much shorter than the mean lifetime of ortho-Ps Accordingly the longlived tlfird component is assigned to amorphous regions, and the intermediate component is assigned to amorphous regions, and the intermediate component to •crystalline regions. 3 Diffusion of Ps between regmns of amorphous and crystalline structure, whwh does occur in regions of the order of ~>10a/~ may be neglected. B y analogy with inhibition of the Ps yield W by electron acceptors in liquids, where relations of the type reported in [12], viz. W= Wo/(I+K[Ac]) apply where K > 0 , let us say that these relations apply equally to inhibition of the Ps yield by oxidation induced defects. Now, allowing for possible trapping of Ps b y these defects, we have

I~=

W0 ~t/()4+~t--~a), 1+ KNok

(1)

where Nok is the number of oxidation defects in crystalline ( i = 2 ) or amorphous (i=-3) phase; )~t is the rate of trapping of Ps from the quasi-free state; ~ is the almihilation rate for quasi-free positromum, and ~ the rate for positronium trapped in a defect. The number of defects in amorphous parts of polymers is large even in an initial (unoxldized) sample [13] (Nok ~--1019 cm-8), in view of which 2t)>Q--2d, and accordingly

Ia(O)/Ia(x)-~ 1 +ax,

(2)

where x is a &mensionless positive value that is proportional to the amount of oxidation-induced defects In an approximatmn of experimental values it is more natural that expression (2) should be replaced b y

]a(O)/Ia(x)=b+ax

(3)

Relations between the amount of hydroperoxide formed during oxidation ~nd the number of defects thus induced, and the oxidation time t are given b y

I n v o s h g a t l o n of oxidized polymers

2969

the exptrlcal equation [14] N=Nmax [1--exp(--at)] s. Therefore we have to replace x in expression (3) by the value of [1--exp(--at)] 2 A least squares approximation of the experimental data for Is(t) at 120 ° g~ves the following values* for paiameters forming part of expressmn (3) TABLE

2

3~

T° 120 130 140 150 160

28 28 24 25 22

32=0 4±0 6~0 0±0 1E0

Ia,

V A L U E S OF C O M P O N E N T S ~2 A N D

09 3 3

0 0552~0 0 0504±0 0 424J0 0 384~0 0 743~0

3

3

A N D OF ~V

12 0060 031 036 031 038

10 11 12 12 8

X

9±0 7±0 8±0 9±0 3±0

4 4 4 4 7

0 0 0 0 0

000±0 048~0 124±0 131±0 882±0

022 025 032 033 029

a = 0 4 1 7 1 0 007 b=0-984±0.002 ~ = ( 0 0045-Jz0 0002)l/rain With the aid of the obtained approximation we now extrapolate values ~)f Ia(t) and x(t) to the same time of oxldatmn (t-- 60 rain) at all tile temperatures examined The results of this extrapolation appear in the first two columns of Table 2, and are shown in the Figure, a A similar procedure may likewise

6 %

,%

IJ:

a

,T

[

26

I

a'8

I//

a'3 11

/ a~

g44

2//

[

1,

I

lJa

150

fYg

/50

Temperature dependence of the long llfehme components (1) and degrees of oxidation (2) for the amorphous (a) and erystallme phases (b)

be followed for I2(t). However, in crystalline regions of a similar (unoxidized) polymer the number of defects in which Ps trapping may take place is not so * The quanttty Is (0) was taken as bemg equal to I3 for the initial (unoxlchzed) polymer.

2970

V.A. ONISHCHUKe~ a/.

large that the quantity ~ I - - ~ in formula (I) compared with ~t m a y be neglected. This means that an increase in the number of oxidation-induced defects m a y lead initially to an increase in 12 (the area where 2t ~<~f--~) on account of enhanced probability of Ps trapping, after which, when the number of defects becomes large, and all the Ps atoms formed are trapped in the latter (~t>>~f--2~), it may result in a fall in 12 on account of reduced probability of Ps formation. It is cleal from the data in Table 1 that scarcely any oxidation takes place at 120 ° in crystalline regions of the polymer (constancy of 12(t) within the error limits), whereas at 160 ° the dependence of 12 on t is of the type described above (a rise followed b y a fall on extending t). Approximating at 160 ° functlon Is(t ) b y a parabola I~(t)-~ax2-t-bx-~c, (4} where x:[1--exp(--at)] ~, with I s expressed as a percentage, and Is(0 ) equal to I s for the initml sample, we obtain a:--24-111-1 b:18.3~0.5 c : 10.9=L0.4 ~ : (0.047 ~=0.002) 1/min The last two columns in Table 2 as well as the Figure b give the results of extrapolation of values of Is(t) and the corresponding values of x to the same time ( t : 6 0 min), using expression (4). Using the x(t) plot, showing the degree of oxidation vs. temperature we obtain information regarding activation energies (with respect to oxidation) for amorphous and crystalline regions of the polymer simultaneously, and likewise data on the phase transitions that take place. Thus, ff we assume that at the same oxidation time x ( T ) : A exp(--E/RT), where T is the oxidation temperature at K, and, neglecting the dependence of A on T, we find that in the amorphous regions E ~ E a - ~ 9 3 " 0 ~ 3 " 8 and in crystalline ones E ~ E a : 1 5 4 " 5 ± 1 0 " 5 kJ/mole. The activation energies and the error values were evaluated b y least squares. The fractured curve in the Figure shows the relations of x(T) obtained using the found activation energy values. I t can be seen that in the interval from 140 to 150 ° a phase transition occurs in poly-4-methyl-l-pentene. The largest defects in which positroninm could be localized are in the vicinity of interfaces between regions with ordered and disordered structures, and so it is most probable that the observed transition takes place at the crystallite surface. The activation energies cited above are average values for conformations before and after the phase transition. Given a large number of temperature points it should theoretically be possible to use the positron technique to obtain activation energy values for each of the conformations in both the amorphous and the crystalline regions. Thus we have shown that it is theoretically possible to use the anmhilation

Partially crystalline polymers

2971

technique to obtain information about reactions tal~ing place simultaneously in amorphous and crystalline parts of the polymer. It is also possible to detect phase transitions occurring simultaneously in both regions. Translated by R. J. A. I-IENDRY REFERENCES 1 R . N . WEST, Advances Phys. 22: 3, 264, 1973 2 A. ORE, Umv. Berg. Arh l~aturwlt Rekke, 9, 1949 3 0 . E . MOGENSEN, J Chem. Phys. 60: 3, 998, 1974 4 V. A. ONISHCHUK a n d V P. SHANTAROVICH, Programma obrabotkl vromennykh spektrov anmgflyatsn pohtronov (Program of Analyms of Time Spectra of Pomtron Anmhllatmn). Moscow, VNTI centre, No. 3, 85, 1980 5 M. ELDRUP, N. J. PEDERSEN and J. N. SHERWOOD, Phys. Rev. Letters, 43: 19, 1407, 1979 b W. RRANDT and I. SPIRN, Phys. Rev. 142: 1. 231, 1966 7 S . J . TAO a n d J. H. GREEN, Proc Phys Soc. 85: 545, 463, 1965 8 0 . E. MOGENSEN a n d M. J. ELDRUP, J. Glacml. 21: 85, 1978 9 S. TAO, J Appl. Phys 10: 1, 67, 1976 10 G. D e B L O N D E , S Y. CHUANG a n d D. M. HOGG, Canad. J. P h y s . 50: 14, 1619, 1972 11. S. Y. CHANG and S. TAO, J Appl Phys 11: 1,247, 1976 12. V. I. GOL'DANSKIL T. A. SOLONENKO and V. P. SHANTAROVICH, Dokl. Akad. Nauk SSSR 151: 8, 608, 19~13 13. L. G. A R A V I N , Yu. K . YESIPOV, Yu. V. Z H E R D E V , I. N. MUSAELYAN and V. P. SHANTAROVICH, Dokl. Akad. Nauk SSSR 251: 3, 900, 1980 14 V. S. PUDOV a n d M B. NEIMAN, In: Stareme 1 stabfllzatmya pohmerov (Ageing a n d Stabfllzatmn of Polymers), (Ed:ted b y A S K u z ' n n n s k n ) p. 5, Kh~mlya, Moscow, 1966

PolymerSetenceU S S R. Vol. 24, No. 12, pp. 2971-2976,1982 Printed m Poland

THE

0032-3950/82 $7.50+.00 © 1983PergamonPress L~d.

FREE INDUCTION SIGNAL FROM AMORPHOUS OF PARTIALLY CRYSTALLINE POLYMERS*

PARTS

V P. G~IOOR'EV Chuvash State Umvers:ty :m I N U l ' y a n o v

(Recewed 17 July 1981) A study has bet'n made of the asymptotw behavlour of free induct:on s:gnals m motropm sohd matter for short and long ~nncu. Tile theoretmM relations obtained ale m good agreement with the expemmentM d~ta for PE. * Vysokomol. soycd. A24: No. 12, 2584-2588, 1982.