16 13
A C NMR Study of Radiation-Induced Structural Changes in Polyethylene
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J. C. RANDALL—Phillips Petroleum Company, Bartlesville, OK 74004 F. J. ZOEPFL—Pickard, Lowe and Garrick, Inc., Washington, DC 20036 JOSEPH SILVERMAN—University of Maryland, College Park, MD 20742 Polyethylenes, irradiated to doses just short of the gel dose, have been examined for structural changes using C NMR in a study of the effects of ionizing radiation on polyethylene. Radiation gelled polyethylenes do not produce high resolution NMR resonances possibly because NMR dipolar interactions in the gel phase are not effectively averaged by the available molecular motions. One of the major structural units formed during irradiation of polyethylene is the Y type of long chain branch. Other structural entities monitored and followed versus radiation conditions were trans and cis double bonds, terminal vinyl end groups, saturated end groups, hydroperoxide groups and carbonyl groups. In addition to NMR measurements, molecular weight changes were monitored through intrinsic viscosity, gel permeation chromatography and low angle laser light scattering measurements. It was possible through the appropriate experimental conditions both before and during irradiation, to convert a linear, high density polyethylene to a medium density, exclusively long chain branched polymer, which was free of gel. Attempts to detect the Η-link in polyethylenes irradiated to doses short of the gel dose met with little success. 13
T h e d i r e c t d e t e c t i o n o f r a d i a t i o n i n d u c e d c r o s s l i n k s i n p o l y e t h y l e n e has been a major g o a l o f r a d i a t i o n c h e m i s t s for many y e a r s . It was r e c o g n i z e d as e a r l y as 1967 t h a t s o l u t i o n 1 3 c n u c l e a r m a g n e t i c resonance ( N M R ) s p e c t r o s c o p y c o u l d be used t o d e t e c t s t r u c t u r e s p r o d u c e d i n p o l y m e r s f r o m i o n i z i n g r a d i a t i o n . F i s c h e r and L a n g b e i n ( l ) r e p o r t e d the first direct detection of radiation induced crosslinks (H-links) in p o l y o x y m e t h y l e n e using 1 3 c N M R . B e n n e t t et αί.(2) used 1 3 c N M R t o detect radiation induced crosslinks in n-alkanes irradiated in vacuum in the m o l t e n s t a t e . B o v e y et ai.(3) used this t e c h n i q u e to i d e n t i f y b o t h r a d i a t i o n i n d u c e d H - l i n k s and l o n g c h a i n branches ( Y - l i n k s ) i n n - a l k a n e s
0097 6156/84/0247 0245S06.75/0 © 1984 American Chemical Society Randall; NMR and Macromolecules ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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irradiated in the molten state. Bovey et ah also determined that few, if any, H-links and long chain Y branches form in n-alkanes irradiated in the solid state. Previous investigators have had little success in obtaining resonances arising from the gel structure formed during irradiation of polyethylene. Partially gelled polyethylene can be swollen with an appropriate solvent to produce a "solution" which appears suitable for 1 3 c NMR liquid measurements. However, the results of this study demonstrate that only the mobile, soluble component produces observable resonances under these conditions. It is possible that NMR dipolar interactions in the gel phase are not effectively averaged through the available polymer molecular motions. Under such circumstances, the resonances would become so broad as not to be observed in normal high resolution liquid 1 3 c NMR spectra. The problems involved in observing resonances from gelled polyethylene were also encountered in this study. We decided, therefore, to examine polyethylene samples irradiated with absorbed doses less than the gel dose. By so doing, some of the radiation induced structural changes produced prior to gelation could be detected. It may be instructive at this point to review some of the advantages provided by the 1 3 c N M R technique for examining soluble irradiated polyethylenes: (a) Measurements at 50 MHz and 398 Κ at solution concentrations between 10 and 20 percent by weight can yield high resolution spectra displaying a sensitivity of approximately 0.5 structural units per 10,000 total carbon atoms. (b) Each separate polyethylene structural entity gives rise to a unique array of resonances; the number and relative intensities depend upon the number and types of carbon atoms associated with each particular structural unit. Thus a number of different resonances will be associated with a structural unit. Model compounds and model polymers can lead to unequivocal assignments for the various types of polyethylene carbons. (c) Each resonance intensity is directly proportional to the number of contributing carbon atoms. Thus there is only one proportionality constant for all resonances and there is no need to determine various extinction coefficients as required for infrared and ultraviolet spectral measurements. (d) Saturated end groups and both short and long chain branches can be monitored independently using the 1 3 c N M R technique. (e) Internal cis and trans double bonds and terminal vinyl groups can be monitored directly and independently with the 1 3 c N M R method. Advances in high resolution 13c NMR equipment, such as higher magnetic field strengths, improved probes, improved dynamic ranges and sophisticated dedicated computer systems have greatly increased the sensitivity of 1 3 c N M R measurements. New instruments, operating at frequencies of either 300 or 500 MHz, have extended the capability to detect long chain branching in polyethylene to as few as 1-5 branches per 100,000 carbon atoms.
Randall; NMR and Macromolecules ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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R A N D A L L ET A L .
Radiation-Induced
Changes in
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The c o n c e n t r a t i o n s of s t r u c t u r e s p r o d u c e d i n i r r a d i a t e d p o l y e t h y l e n e a r e on t h e o r d e r o f 1 p e r 10,000 c a r b o n a t o m s f o r absorbed doses o f approximately 2.0 M r a d . Although the approach of examining p o l y e t h y l e n e s i r r a d i a t e d w i t h absorbed doses less t h a n t h e g e l dose p l a c e d a p r e m i u m on s e n s i t i v i t y , we w e r e able t o d e t e c t the f i r s t d i r e c t r a d i a t i o n induced l o n g c h a i n b r a n c h e s i n h i g h d e n s i t y p o l y e t h y l e n e (4).
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Experimental Irradiation. A l l irradiations were performed at the U n i v e r s i t y of M a r y l a n d w i t h a 25,000 c u r i e c o b a l t 60 g a m m a source. T h e a b s o r b e d dose r a t e o f 1.2 M r a d per h r was d e t e r m i n e d by f e r r o u s s u l f a t e d o s i m e t r y . A l l s a m p l e s w e r e i r r a d i a t e d under s e c o n d a r y e l e c t r o n e q u i l i b r i u m c o n d i t i o n s . F o l l o w i n g i r r a d i a t i o n , a l l s a m p l e s i r r a d i a t e d i n v a c u u m w e r e annealed i n v a c u u m a t 380 Κ f o r 24 hours; this treatment reduces t h e long-lived radical concentration to undetectable levels. M a t e r i a l s . T h e n - h e x a t r i a c o n t a n e ( H T C , n-C36, M.P. 75°C) s a m p l e s w e r e obtained from A l d r i c h C h e m i c a l Company. Irradiations were performed in v a c u u m i n both t h e s o l i d a n d m o l t e n s t a t e s t o r e p r o d u c e t h e r e s u l t s r e p o r t e d by b o t h B e n n e t t et αί.(2) and B o v e y et αϊ.(3) w h e r e t h e H - l i n k was r e p o r t e d t o f o r m . T h e Η-link c h e m i c a l s h i f t d a t a f r o m t h e m o d e l a l k a n e i r r a d i a t i o n s (at 100 Mrad) c o u l d be used t o i d e n t i f y t h e Η-link a f t e r c o r r e s p o n d i n g N M R m e a s u r e m e n t s on i r r a d i a t e d p o l y e t h y l e n e s . P h i l l i p s M a r l e x 6003, a high d e n s i t y p o l y e t h y l e n e ( M = 53,000; M = 18,000) possessing p r e d o m i n a n t l y s a t u r a t e d end groups was i r r a d i a t e d t o 4.0 M r a d in v a c u u m in p e l l e t f o r m as supplied. N B S 1475 is a c o m m e r c i a l p o l y e t h y l e n e c o n t a i n i n g 111 ppm o f Irganox 1010. O t h e r e x p e r i m e n t s i n v o l v e d h e a t i n g i n v a c u u m t o 500 Κ f o r 24 h p r i o r t o i r r a d i a t i o n t o 3.0 M r a d at 500 K. T h e N B S 1475 s a m p l e r e q u i r e d a g r e a t e r a m o u n t o f i r r a d i a t i o n to r e a c h the g e l dose than d i d M a r l e x 6003. w
n
G e l F r a c t i o n Determinations. The irradiated polyethylene samples were measured f o r g e l c o n t e n t b y e x t r a c t i o n w i t h b o i l i n g x y l e n e f o r 72 h. O n l y one sample, N B S 1475 i r r a d i a t e d t o 8.0 M r a d in v a c u u m a t 300 K, showed a measurable g e l c o n t e n t . 13ç N M R M e a s u r e m e n t s . A V a r i a n X L - 2 0 0 N M R s p e c t r o m e t e r , l o c a t e d at t h e P h i l l i p s R e s e a r c h C e n t e r , was used t o make a l l o f the 1 3 c N M R m e a s u r e m e n t s a t 50.3 M H z . T h e s a m p l e s were d i s s o l v e d i n 1,2,4t r i c h l o r o b e n z e n e a t 15 w e i g h t p e r c e n t a n d m a i n t a i n e d under a n i t r o g e n a t m o s p h e r e a t 398 Κ i n the probe c a v i t y d u r i n g d a t a a c q u i s i t i o n . O t h e r c o n d i t i o n s w e r e as f o l l o w s : P u l s e angle: P u l s e delay: Acquisition time: Spectral width D a t a points/spectrum: Free Induction Decays (FID's) a c c u m u l a t e d :
90° 15.0 seconds 1.0 seconds 8000 H z 32,000 5,000 - 20,000
Randall; NMR and Macromolecules ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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D o u b l e p r e c i s i o n a r i t h m e t i c was e m p l o y e d d u r i n g d a t a a c q u i s i t i o n and the s o f t w a r e i n c l u d e d a f l o a t i n g point F o u r i e r t r a n s f o r m c a p a b i l i t y . A f a c t o r c o n t r o l l i n g the u l t i m a t e s e n s i t i v i t y a v a i l a b l e is the l i n e w i d t h s at o n e - h a l f height of the p o l y e t h y l e n e r e s o n a n c e s . It is d e s i r a b l e t o c r e a t e c o n d i t i o n s w h i c h l e a d to the most n a r r o w l i n e w i d t h s possible when e x a m i n i n g the i r r a d i a t e d p o l y e t h y l e n e s by NMR. At c o n c e n t r a t i o n s o f 15 w e i g h t p e r c e n t and at t e m p e r a t u r e s o f 398 K, l i n e w i d t h s o f 1.0 - 1.5 H z w e r e o b t a i n e d at o n e - h a l f height f o r the major p o l y e t h y l e n e resonances f o r the r e c u r r i n g e q u i v a l e n t m e t h y l e n e u n i t s , δ+δ+, at 29.98 ppm f r o m T M S as shown in T a b l e I. S u b s t a n t i a l l y l a r g e r l i n e w i d t h s at o n e - h a l f height w e r e o b t a i n e d f o r s o l u t i o n c o n c e n t r a t i o n s higher t h a n 15 w e i g h t p e r c e n t . N a r r o w resonances l e a d t o g r e a t e r peak heights, w h i c h i n t u r n l e a d to g r e a t e r s e n s i t i v i t y . A n o t h e r p r o b l e m e n c o u n t e r e d d u r i n g q u a n t i t a t i v e m e a s u r e m e n t s was w h e t h e r peak h e i g h t s c o u l d be r e l i a b l y used f o r i n t e n s i t i e s of weak resonances. T h e end group resonances w e r e o f s u f f i c i e n t s t r e n g t h t o a l l o w a c o m p a r i s o n o f peak heights t o i n t e g r a t e d areas. It was c l e a r f r o m the n a r r o w o b s e r v e d l i n e widths (
