Characterization of Highly Cross-linked Polymers - American

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Highly Cross-linked CR-39 Polycarbonate and Its Degradation by High-Energy Radiation 1

J. L. CERCENA, J. GROEGER, Α. A. MEHTA, R. PROTTAS, J. F. JOHNSON , and S. J. HUANG 1

Institute of Materials Science, University of Connecticut, Storrs, CT 06268

Highly crosslinked CR-39 polycarbonate samples were prepared from carefully p u r i f i e d monomer. The degradation of these polycarbonates by high energy radiation was studied. Results indicated CR-39 polycarbonate to be suitable material f o r dosimetry application. H i g h l y r a d i a t i o n s e n s i t i v e polymers have been used as r e s i s t s f o r production o f m i c r o e l e c t r o n i c c i r c u i t r y and as d e t e c t o r s f o r i d e n t i f i c a t i o n o f tracks of energetic nuclear p a r t i c l e s . Diethylene g l y c o l b i s ( a l l y l carbonate), CR-39, can be polymerized w i t h peroxide i n i t i a t o r s i n t o o p t i c a l l y c l e a r h i g h l y c r o s s l i n k e d polymer f i l m s . These f i l m s undergo h i g h energy r a d i a t i o n induced s c i s s i o n o f the carbon main c h a i n s , F i g . 1. Base e t c h i n g o f t h e i r r a d i a t e d films r e s u l t s i n detectable tracks of the energetic n u c l e a r p a r t i c l e s . (1-7) The p o s s i b i l i t y o f u s i n g these f i l m s as dosimeters f o r h i g h energy r a d i a t i o n has been explored. CR-39 polymer i s one o f the most s e n s i t i v e polymers p r e s e n t l y a v a i l a b l e f o r t h i s purpose, F i g . 1. Among the d i f f i c u l t i e s encountered i n u s i n g the p r e s e n t l y a v a i l a b l e CR-39 polymers f o r dosimetry are the i m p u r i t i e s present i n the commercially a v a i l a b l e monomer samples, the poor r e p r o d u c i b i l i t y o f the p o l y m e r i z a t i o n process, and t h e l a c k o f understanding o f the s t r u c t u r e o f the h i g h l y c r o s s l i n k e d polymer samples. The purposes o f our present study were t o : 1) p u r i f y the monomer; 2) develop a r e p r o d u c i b l e p o l y m e r i z a t i o n process; 3) e s t a b l i s h methods f o r t h e c h a r a c t e r i z a t i o n o f t h e h i g h l y c r o s s l i n k e d polymer samples; 4) study the e f f e c t s o f h i g h energy r a d i a t i o n on the polymer and develop a s e n s i t i v e method f o r counting the t r a c k s o f r a d i a t i o n . 1

To whom correspondence should be directed.

0097-6156/ 84/0243-0301 $06.00/ 0 © 1984 American Chemical Society Labana and Dickie; Characterization of Highly Cross-linked Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

HIGHLY CROSS-LINKED POLYMERS

Figure 1. CR-39 Polymer

Labana and Dickie; Characterization of Highly Cross-linked Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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Experimental P u r i f i c a t i o n o f CR-39 Monomer. Samples o f CR-39 monomer which were d i s s o l v e d i n chloroform were p u r i f i e d by p a s s i n g t h e s o l u t i o n through a column c o n t a i n i n g a c t i v a t e d c h a r c o a l , aluminum o x i d e , o r s i l i c a . The t r e a t e d monomer i n c h l o r o f o r m was then analyzed by Gas Chromatography/Mass Spectrometry (GCMS). To avoid t h e l a r g e s o l v e n t peak due to chloroform a second set o f samples were passed through columns w i t h o u t s o l v e n t . The samples were r u n through a c t i v a t e d c h a r c o a l , aluminum o x i d e o r s i l i c a columns and a l s o through s e q u e n t i a l combinations of these columns (see Table I ) . P r e p a r a t i o n o f CR-39 Polymer. Benzoyl peroxide was d i s s o l v e d i n CR-39 monomer a t room temperature i n an argon atmosphere. Once d i s s o l v e d , the s o l u t i o n s were poured i n t o g l a s s c a s t i n g p l a t e s and placed i n an 80°C oven i n an argon atmosphere. The samples were removed from the oven a f t e r 24 h r s . I r r a d i a t i o n o f Polymers. Polymer samples were exposed t o r e c o i l protons w i t h 1.06 MeV energy a t B a t t e l l e P a c i f i c Northwest Laboratories. C h a r a c t e r i z a t i o n o f Polymers. S w e l l i n g experiments were c a r r i e d out by p l a c i n g s m a l l c u t out s e c t i o n s o f t h e c r o s s l i n k e d polymer i n t e t r a h y d r o f u r a n f o r a p e r i o d o f 24 hours. A f t e r which time the weight o f the polymer was measured. T h i s wet weight was then s u b s t r a c t e d from t h e d r y weight (the weight o f the polymer a f t e r e x t r a c t i o n and s u f f i c i e n t d r y i n g ) t o o b t a i n the percent s w e l l i n g of the polymer. Percentage o f e x t r a c t a b l e s t e s t s c o n s i s t e d o f measuring the weight o f t h e polymer both before and a f t e r the 24 hour e x t r a c t i o n i n t e t r a h y d r o f u r a n . The d i f f e r e n c e i n weight y i e l d s t h e percentage o f e x t r a c t a b l e s . Counting o f High Energy R a d i a t i o n Tracks. A computerized image a n a l y z e r , the Cambridge Instrument model 900, was used t o count the r a d i a t i o n t r a c k s o f t h e i r r a d i a t e d polycarbonate samples. The measurements i n c l u d e t r a c k s i z e (diameter), t r a c k d e n s i t y i n each f i e l d o f view, roundness, sample area i n each f i e l d , and t o t a l area o f the sample. Samples o f polycarbonates examined i n c l u d e d those prepared i n our l a b o r a t o r y and neutron i r r a d i a t e d samples provided by Dr. G. T a r l e o f U n i v e r s i t y o f C a l i f o r n i a a t B e r k e l e y . R e s u l t s and D i s c u s s i o n Commercially a v a i l a b l e CR-39 monomer samples c o n t a i n v a r i o u s i m p u r i t i e s . The nature and t h e amounts o f these troublesome i m p u r i t i e s cause d i f f i c u l t y i n producing polycarbonate dosimeters of uniform q u a l i t y . T h e r e f o r e , o b t a i n i n g a h i g h p u r i t y CR-39 monomer sample was o f utmost importance i n s o l v i n g the problem o f


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r e p r o d u c i b i l i t y i n t h e p r e p a r a t i o n o f CR-39 polymer samples. CR-39 samples from v a r i o u s sources were examined by GC and GC-mass spectrometry. The two major i m p u r i t i e s were found t o be a l l y l d i e t h y lene g l y c o l monocarbonate and water. Methods t o remove these from commercial samples were explored. Washing w i t h aqueous HC1 d i d not remove t h e i m p u r i t i e s . D i s t i l l a t i o n over calcium hydride a t reduced pressure d i d remove moisture but n o t the hydroxy terminated a l l y l d i e t h y l e n e g l y c o l monocarbonate. Column chromatography u s i n g aluminum o x i d e , s i l i c a g e l , and a c t i v a t e d carbon was found to be an e f f e c t i v e method f o r t h e removal o f I m p u r i t i e s from commercial CR-39 monomer samples. Table I .

Table I . P u r i f i c a t i o n o f CR-39 Monomer Samples

Treatment None Aq. HC1 Distillation over CaH Al 0 Si6 Carbon 3

Note:

Polysciences

PPG

Sola Optical

444

444

4+

444-

444 44 trace trace

+ + +

44trace trace trace

Allymer 444444444 trace trace trace

444- - h i g h i m p u r i t y 0.1-1% ++ - moderate + - light t r a c e - l e s s than 10 ppm

CR-39 monomer i s g e n e r a l l y t h e r m a l l y polymerized w i t h d i i s o p r o p y l percarbonate as i n i t i a t o r i n t o c r o s s l i n k e d polymer samples w i t h a l l y l s i d e chains. The presence o f a l l y l s i d e chains can r e s u l t i n s e v e r a l u n d e s i r a b l e p r o p e r t i e s i n terms o f the use o f CR-39 polymer i n dosimetry. A l l y l groups a r e s e n s i t i v e t o o x i d a t i o n by a i r . Surface o x i d a t i o n o f polymer f i l m s can be expected and indeed has been observed by Tar lé. (4) This w i l l shorten t h e s h e l f l i f e o f the dosimeter. A l l y l groups might a l s o undergo p o l y m e r i z a t i o n caused by r a d i a t i o n . The s t r e s s due t o volume shrinkage during such p o l y m e r i z a t i o n could cause c r a c k i n g o f t h e polymer f i l m , a secondary degradation process t h a t i s u n d e s i r a b l e . D i i s o p r o p y l percarbonate i s an extremely thermally s e n s i t i v e i n i t i a t o r . P u r i f i c a t i o n o f the i n i t i a t o r i s i m p r a c t i c a l . This has c o n t r i b u t e d t o the d i f f i c u l t y i n o b t a i n i n g r e p r o d u c i b i l i t y i n the p o l y m e r i z a t i o n o f CR-39 monomer. The f a s t r a t e o f thermal decomposition o f d i i s o p r o p y l percarbonate a l s o r e s u l t s i n quick g e l formation d u r i n g t h e CR-39 p o l y m e r i z a t i o n . This prevents t h e


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complete r e a c t i o n o f the a l l y l groups and r e s u l t s i n low c r o s s l i n k i n g d e n s i t y o f the f i n a l polymer. We chose to use the more s t a b l e benzoyl peroxide and l o n g r e a c t i o n time to achieve maximum r e a c t i o n o f the a l l y l groups i n order to prepare r e p r o d u c i b l e h i g h l y c r o s s l i n k e d samples. Our best r e s u l t was obtained c a r r y i n g out the p o l y m e r i z a t i o n a t 80°C under argon w i t h 2% benzoyl peroxide as i n i t i a t o r . Increase i n the amount o f peroxide caused f a s t g e l a t i o n which r e s u l t e d i n incomplete r e a c t i o n o f the a l l y l groups - polymer samples o f l e s s e r c r o s s l i n k i n g were obtained. The degree o f c r o s s l i n k i n g was examined by s w e l l i n g i n THF and measuring the amount o f e x t r a c t a b l e m a t e r i a l s i n the polymer samples. Representative r e s u l t s are l i s t e d i n Table I I . The GPC a n a l y s i s , F i g . 2 and Table I I I showed t h a t although the samples have s i m i l a r s t r u c t u r e s the h i g h l y c r o s s l i n k e d Sample 1-2 showed the l e a s t extent o f s w e l l i n g and a l s o the l e a s t amount o f e x t r a c t ables. B e t t e r r e p r o d u c i b i l i t y was a l s o obtained with lower i n i t i a t o r c o n c e n t r a t i o n i n the p o l y m e r i z a t i o n mixture and longer r e a c t i o n time. The h i g h l y c r o s s l i n k e d CR-39 polycarbonate samples were i r r a d i a t e d with r e c o i l protons at B a t t e l l e Memorial I n s t i t u t e , P a c i f i c Northwest L a b o r a t o r i e s . Although the amount o f r a d i a t i o n caused degradation r e s u l t e d i n s i m i l a r i n c r e a s e s i n the amount o f e x t r a c t a b l e f r a c t i o n , the i n c r e a s e s o f % s w e l l i n g were q u i t e d i f f e r e n t . The h i g h l y c r o s s l i n k e d sample I-2-A showed no i n c r e a s e of % s w e l l i n g w h i l e the l e s s c r o s s l i n k e d sample I-5-A showed an i n c r e a s e o f 15.6% s w e l l i n g . The s i x f o l d i n c r e a s e i n the amount of e x t r a c t a b l e m a t e r i a l s and the n e g l i g i b l e change o f s w e l l i n g a f t e r i r r a d i a t i o n with r e c o i l protons make 1-2 the most s u i t a b l e f o r dosimetry a p p l i c a t i o n . The Cambridge Instruments Quantimet 900 Image Analyzer was used to analyze the r a d i a t i o n t r a c k s on exposed CR-39 polymer. T h i s system f e a t u r e s a computer c o n t r o l l e d stage and focus system. Automatic focus was s e l e c t e d f o r the microscope. These acces s o r i e s allow the system t o a u t o m a t i c a l l y scan samples f o r f e a t u r e s of i n t e r e s t over an average area o f 8cm χ 10cm, w h i l e keeping the

Table I I .

Sample

E f f e c t s o f R a d i a t i o n on CR-39 Polymers

% BPO a i n Polymerization 2 2 5 5

a. b.

% Extractable 1 7 15 22

% Swelling 4.6 4.7 30.2 45.8

A t 80°C, 24 h r s . A f t e r i r r a d i a t i o n with proton.


306


Table I I I . G.P.C. Peaks Present i n CR-39 Polymer Samples Sample

% Extractable

1-2

a.

1 7 15 22 After i r r a d i a t i o n with

Peaks 3,4 2,3,4 1,3,4 1,2,3,4

proton.

d e s i r e d f e a t u r e i n focus. R e s u l t s on the a n a l y s i s o f a CR-39 polymer sample i r r a d i a t e d w i t h neutrons a r e reported here. The 8cm χ 8cm sample was i r r a d i a t e d and etched by Dr. G. Taule o f the U n i v e r s i t y o f C a l i f o r n i a a t Berkeley. The measurements made on t h i s sample i n c l u d e t r a c k s i z e (diameter), t r a c k d e n s i t y i n each f i e l d o f view, roundness, sample area i n each f i e l d , and t o t a l area o f the sample. The roundness measurement i s made f o r a p p l i c a t i o n as a f e a t u r e acceptance c r i t e r i o n ; the t r a c k s f o r normal i n c i d e n c e are almost p e r f e c t l y round and i t i s t h e r e f o r e easy t o r e j e c t anomalous f e a t u r e s u s i n g t h i s parameter. T h i s i s shown by S EM p i c t u r e o f the sample, F i g . 3. F i g . 4 and 5 show the d i s t r i b u t i o n o f t r a c k s s i z e s and the t r a c k d e n s i t y as a f u n c t i o n o f f i e l d number. The r e s u l t s from t h i s sample show t h a t the d i s t r i b u t i o n o f t r a c k s i z e s i s c e r t a i n l y non-Gaussian. I t has very d e f i n i t e upper and lower bounds. The r e s o l u t i o n o f t h e instrument f o r t h i s a n a l y s i s was approximately 1 micron. The d i s t r i b u t i o n o f t r a c k d e n s i t y shows a reasonably uniform d i s t r i b u t i o n o f t r a c k d e n s i t y versus l o c a t i o n . The sample was scanned i n s e q u e n t i a l f i e l d s and the center o f the p l o t corresponds t o the center o f the sample. I t appears t h a t the dosage r a t e on t h i s sample i s not completely homogeneous but the v a r i a t i o n s are random. The a n a l y s i s of 4200 f i e l d s t o cover t h e e n t i r e sample used f o r t h i s r e p o r t took approximately 4.0 hours of machine time. The time spent on each f i e l d was about 3-4 seconds and v a r i e d as a f u n c t i o n o f the number of detected f e a t u r e s per f i e l d . Conclusions We have shown t h a t commercial CR-39 monomer samples can be p u r i f i e d by column chromatography. P o l y m e r i z a t i o n w i t h benzoyl peroxide as i n i t i a t o r gives h i g h l y c r o s s l i n k e d polycarbonate samples s u i t a b l e f o r use as h i g h energy r a d i a t i o n dosimeters. A method has been developed t o c h a r a c t e r i z e t h e polymer and t o count r a d i a t i o n t r a c t s w i t h a computer c o n t r o l l e d Quantimet image analyzer·


CERCENA ET AL.

15


FLOW TIME (min.)

307

23 FLOW RATE: 1 ral/min CHART SPEED: 2 cm/min SOLVENT: Unstab. THF

Figure 2. Gel Permeation Chromatograph of CR-39 Polymer

Figure 3. SEM p i c t u r e of neutron i r r a d i a t e d and etched CR-39 polymer.


308


UNITS

COUNT

Β:

30.00 · 31.00 ·

8*:

56. 101. M. 0. 0. «Α.

η: tr 47.00 ·

s $3:

Λ:

73. 0. 49. 76. 89. 0. 76. 110.

Ρ 50.00 u 51.00·

S §:8: " Μ η:

tr 47.00 · 70.00 · 71.00·

8:8: 76.00 « 79.00· Number -

F i g u r e 4.

Neutron t r a c k s i z e d i s t r i b u t i o n o f CR-39 polymer analyzed by Quantimet Image Analyzer.


18.


CERCENA ET AL.

309

UMTS 140.00 210.00 200.00 350.00

M

5*0.00 «30.00

85:8

II

1120.00 1190.00 1260.00 1330.00 1400.00 1470.00 1540.00 1610.00 1660.00 1750.00 1620.00 1690.00 . 1960.00 οι 2030.00 -g 2100.00 S 2170.00 ζ 2240.00 •ο 2310.00 ·- 2360.00 * 2450.00 u. 2520.00 2590.00 2660.00 2730.00

R8 2940.00 3010.00

188:8 J710.L 3760.00 3650.00 3920.00 3990.00 4060.00 4130.00 Track Density •

F i g u r e 5.

Neutron t r a c k d e n s i t y o f CR-39 polymer analyzed by Quantimet Image A n a l y z e r .


310

HIGHLY CROSS-LINKED

POLYMERS

Acknowledgments T h i s work was supported by a Department o f Energy subcontract B081293-A-U from B a t t e l l e Memorial I n s t i t u t e , P a c i f i c Northwest L a b o r a t o r i e s . The authors a r e indebted t o Dr. W i l l i a m T. B a r r y , Consultant i n M a t e r i a l s Science, f o r h e l p f u l t e c h n i c a l d i s c u s s i o n s .

Literature Cited 1. 2. 3. 4. 5. 6. 7.

Sullivan, D. O.; Price, P. B.; Kinoshita, K.; Wilson, C. G. J . Ekctrochem. Soc. 1982, 811-813. Tarié, G.; A l l e n , S. P.; Price, P. B. Nature 1981, 293, 556. Price, P. B. P h i l . Mag. 1982, 45(2), 331-346. Tarlé, G., Ninth DOE Workshop on Personnel Neutron Dosimetry, P a c i f i c Northwest Laboratory, PNL-SA-10714, 1982, pp. 74-84. Huang, S. J . ; Johnson, J. J. i b i d , 1982, pp. 85-91. Faermann, S.; Εisen, Y.; Schlesinger, T.; Ovadia, E. i b i d , 1982, pp. 92-102. G r i f f i t h , R. V.; McMahon, T. A. i b i d , 1982, pp. 103-119.

RECEIVED September 14, 1983


Characterization of Highly Cross-linked Polymers - American

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