Polymer Characterization by ESR and NMR - ACS Publications

6 Characterization of Long-Chain Branching in Polyethylenes Using High-Field Carbon-13 N M R

Downloaded by CORNELL UNIV on October 5, 2016 | http://pubs.acs.org Publication Date: November 10, 1980 | doi: 10.1021/bk-1980-0142.ch006

J. C. RANDALL Phillips Petroleum Company, Bartlesville, OK 74004 The simplest polymer molecule examined for both its dynamic and structural characteristics utilizing carbon-13 nuclear magnetic resonance has been polyethylene. At first sight, a polymer molecule which is essentially a "polymethylene" would not seem to possess enough different structural characteristics to warrant more than a casual investigation. However, there are a variety of different polyethylenes available commercially that have substantially different physical properties and, subsequently, different end use applications. The fact that catalysts are sought to produce polyethylenes with desirable properties and that post treatments may be used to alter certain physical characteristics imply that there is a direct link between physical properties and molecular structure. This link between structure and properties and the commercial importance of polyethylene have brought about a need for a thorough structural characterization. Polyethylenes produced commercially via high pressure, free radical processes have densities around 0.92 g/cc and are referred to simply as "low density" polyethylenes. It has been well established from infrared measurements that these low density polyethylenes possess appreciable quantities of ethyl and butyl branches (1-3) but it was not until C-13 NMR became available that an absolute identification, both qualitatively and quantitatively, of the short branches became possible (4-8). Long chain branching is also present in high pressure process low density polyethylenes and carbon-13 NMR was useful here also in establishing the identity and relative amounts of long versus short chain branches (9-11). Polyethylenes with densities around 0.96 g/cc are categorized as high density polyethylenes and are prepared using either titanium or chromium based catalysts. These polyethylenes are usually linear although the physical and rheological properties of some high density polyethylenes have suggested the presence of long chain branching (12J at a level one to two orders of magnitude below that found for low density polyethylenes prepared by a high pressure process. A measurement of long chain branching in 0-8412-0594-9/80/47-142-093$06.50/0 © 1980 American Chemical Society Woodward and Bovey; Polymer Characterization by ESR and NMR ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

94

POLYMER

C H A R A C T E R I Z A T I O N B Y ESR A N D N M R

h i g h d e n s i t y p o l y e t h y l e n e s has been e l u s i v e because of t h e c o n c e n t r a t i o n s i n v o l v e d {13) and can o n l y be d i r e c t l y p r o v i d e d by r e c e n t l y a v a i l a b l e , h i g h f i e l d , h i g h s e n s i t i v i t y NMR s p e c t r o m e ters. The p u r p o s e o f t h i s c h a p t e r w i l l be t o r e v i e w b r i e f l y t h e h i s t o r y o f s t r u c t u r a l s t u d i e s o f p o l y e t h y l e n e and show where t h e s e r e c e n t advances i n C-13 NMR i n s t r u m e n t a t i o n have g r e a t l y enhanced o u r knowledge about p o l y e t h y l e n e s t r u c t u r e . Among t h e most i m p o r t a n t p o l y e t h y l e n e s t r u c t u r a l c h a r a c t e r i s t i c s a r e t h e w e i g h t and number a v e r a g e m o l e c u l a r w e i g h t s , and M , and t h e m o l e c u l a r w e i g h t d i s t r i b u t i o n c h a r a c t e r i z e d by M /M . S i n c e C-13 NMR can a l s o be used t o measure a number a v e r a g e m o l e c u l a r w e i g h t , i t may be a d v a n t a g e o u s t o examine some of the molecular weight c h a r a c t e r i s t i c s of p o l y e t h y l e n e s . As shown by t h e s i z e e x c l u s i o n ( o r gel p e r m e a t i o n ) c h r o m a t o g r a p h s i n F i g u r e 1, t h e m o l e c u l a r w e i g h t d i s t r i b u t i o n s a r e g e n e r a l l y broad b u t can be s i g n i f i c a n t l y c h a r a c t e r i s t i c t o d i s t i n g u i s h among c e r t a i n types of p o l y e t h y l e n e s . For example, the f i r s t polymer i n F i g u r e 1 has a r e l a t i v e l y narrow m o l e c u l a r w e i g h t d i s t r i b u t i o n w i t h an M / M of approximately t h r e e . The second p o l y e t h y l ene has a much b r o a d e r m o l e c u l a r w e i g h t d i s t r i b u t i o n w i t h an M /M of approximately twenty. F i n a l l y , the t h i r d p o l y e t h y l ene has a d i s t i n c t l y bimodal molecular weight distribution. T h e s e s i z e e x c l u s i o n chromatograms do s e r v e as distinguishing f i n g e r p r i n t s f o r t h e m o l e c u l a r w e i g h t d i s t r i b u t i o n s w h i c h can o n l y be measured q u a n t i t a t i v e l y by t h e r a t i o , M / M . Recent improvements i n s i z e e x c l u s i o n chromatography techniques have p e r m i t t e d f a s t e r and more r e l i a b l e m o l e c u l a r w e i g h t d e t e r m i n a t i o n s (14). P o l y e t h y l e n e s p r e p a r e d w i t h a Z i e g l e r t y p e , t i t a n i u m based c a t a l y s t have p r e d o m i n a n t l y n - a l k y l o r s a t u r a t e d end groups. T h o s e p r e p a r e d w i t h chromium based c a t a l y s t s have a p r o p e n s i t y t o w a r d more o l e f i n i c end g r o u p s . As w i l l be seen l a t e r , t h e r a t i o o f o l e f i n i c t o s a t u r a t e d end groups f o r p o l y e t h y l e n e s p r e p a r e d w i t h chromium based c a t a l y s t s i s a p p r o x i m a t e l y u n i t y . The end group d i s t r i b u t i o n i s , t h e r e f o r e , a n o t h e r s t r u c t u r a l f e a t u r e o f i n t e r e s t i n p o l y e t h y l e n e s because i t can be r e l a t e d t o t h e c a t a l y s t employed and p o s s i b l y t h e e x t e n t o f l o n g c h a i n b r a n c h ing. I n f r a r e d has been a u s e f u l t e c h n i q u e f o r m e a s u r i n g t h e v a r i o u s t y p e s o f o l e f i n i c end groups ( 1_5), w h i c h may be n


w

n

w

w

n

n

w

-CH -CH=CH 2

-CH -C=CH 2

CH

"vinyl"

2

"vinylidene"

2

3

-CH -CH=CH-CH 2

n

3

"internal

cis

or

trans"

Woodward and Bovey; Polymer Characterization by ESR and NMR ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

RANDALL

Long-Chain

Branching

in

Polyethylenes


6.

MOLKCl.'LAR WKIGHT

Figure 1.

SECs of (A) NBS 1475, (B) Phillips PE 5003, and (C) Hizex 7000


95


96

P O L Y M E R CHARACTERIZATION B Y ESR A N D N M R

P r i o r t o t h e a v a i l a b i l i t y o f C-13 NMR, t h e r e was no t e c h n i q u e f o r m e a s u r i n g d i r e c t l y t h e s a t u r a t e d end group c o n c e n t r a t i o n . Now i t i s p o s s i b l e not o n l y t o measure c o n c e n t r a t i o n s o f s a t u r a t e d end g r o u p s , but a l s o t h e o l e f i n i c end g r o u p s a n d , s u b s e q u e n t l y , an end group d i s t r i b u t i o n . S h o r t c h a i n b r a n c h e s can be i n t r o d u c e d d e l i b e r a t e l y i n a c o n t r o l l e d manner i n t o p o l y e t h y l e n e s by c o p o l y m e r i z i n g e t h y l e n e with a 1-olefin. The i n t r o d u c t i o n o f 1 - o l e f i n s a l l o w s t h e d e n s i t y t o be c o n t r o l l e d and b u t e n e - 1 and h e x e n e - 1 a r e commonly used f o r t h i s purpose. Once a g a i n , as i n t h e c a s e o f h i g h pressure p r o c e s s low d e n s i t y p o l y e t h y l e n e s , C-13 NMR c a n be used t o meas u r e e t h y l and b u t y l b r a n c h c o n c e n t r a t i o n s i n d e p e n d e n t l y o f t h e s a t u r a t e d end g r o u p s . T h i s r e s u l t g i v e s C-13 NMR a d i s t i n c t a d v a n t a g e o v e r c o r r e s p o n d i n g i n f r a r e d measurements b e c a u s e t h e l a t t e r t e c h n i q u e can o n l y d e t e c t m e t h y l groups i r r e s p e c t i v e of w h e t h e r t h e methyl group b e l o n g s t o a b u t y l b r a n c h o r a c h a i n end (16). As w i l l be seen s h o r t l y , C-13 NMR a l s o has a d i s a d v a n t a g e i n b r a n c h i n g measurements b e c a u s e o n l y b r a n c h e s f i v e c a r b o n s in l e n g t h and s h o r t e r can be d i s c r i m i n a t e d i n d e p e n d e n t l y o f l o n g e r c h a i n branches ( 5 J ( 9 J . B r a n c h e s s i x c a r b o n s i n l e n g t h and l o n g e r g i v e r i s e t o t h e same C-13 NMR s p e c t r a l p a t t e r n i n d e p e n d e n t l y o f the chain length. T h i s l a c k o f d i s c r i m i n a t i o n among t h e l o n g e r s i d e - c h a i n b r a n c h e s i s not a d e t e r r i n g f a c t o r , h o w e v e r , i n t h e u s e f u l n e s s o f C-13 NMR i n a d e t e r m i n a t i o n o f l o n g c h a i n b r a n c h ing. By f a r t h e most d i f f i c u l t s t r u c t u r a l measurement a n d , as s t a t e d p r e v i o u s l y , t h e most e l u s i v e , has been l o n g c h a i n b r a n c h ing. Long c h a i n b r a n c h i n g 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 s has l o n g been c o n s i d e r e d a f a c t o r a f f e c t i n g c e r t a i n o b s e r v e d physical p r o p e r t i e s , f o r example, environmental s t r e s s c r a c k i n g , r h e o l o g i c a l p r o p e r t i e s and p r o c e s s i n g b e h a v i o r a l t h o u g h c o n c l u s i v e p r o o f has been d i f f i c u l t t o o b t a i n . In low d e n s i t y p o l y e t h y l e n e s , t h e c o n c e n t r a t i o n o f l o n g c h a i n b r a n c h e s i s such (>0.5 p e r 1,000 c a r bons) t h a t c h a r a c t e r i z a t i o n through s i z e e x c l u s i o n chromatography i n c o n j u n c t i o n w i t h e i t h e r low a n g l e l a s e r l i g h t s c a t t e r i n g o r i n t r i n s i c v i s c o s i t y measurements becomes f e a s i b l e ( 9 - 1 1 ) ( 1 3 ) ( 1 7 18). When c a r b o n - 1 3 NMR measurments have been compared t o r e s u l t s f r o m p o l y m e r s o l u t i o n p r o p e r t y m e a s u r e m e n t s , good agreement has been o b t a i n e d between l o n g c h a i n b r a n c h i n g f r o m s o l u t i o n p r o p e r t i e s w i t h the c o n c e n t r a t i o n of branches s i x carbons long and l o n g e r ( 9 J ( 1 0 J . Unfortunately, these techniques u t i l i z i n g s o l u t i o n p r o p e r t i e s do not p o s s e s s s u f f i c i e n t s e n s i t i v i t y to d e t e c t l o n g c h a i n b r a n c h i n g i n a range o f one i n t e n t h o u s a n d carbons, the l e v e l suspected in high d e n s i t y p o l y e t h y l e n e s . The a v a i l a b i l i t y o f s u p e r c o n d u c t i n g magnet s y s t e m s has made m e a s u r e ments o f l o n g c h a i n b r a n c h i n g by C-13 NMR a r e a l i t y because o f a g r e a t l y improved s e n s i t i v i t y . An enhancement by f a c t o r s between 20 t o 30 o v e r c o n v e n t i o n a l NMR s p e c t r o m e t e r s has been a c h i e v e d t h r o u g h a c o m b i n a t i o n o f h i g h e r f i e l d s t r e n g t h s , 20 mm p r o b e s , and t h e a b i l i t y t o examine p o l y m e r samples i n e s s e n t i a l l y a m e l t



6.

RANDALL

Long-Chain

Branching

in

97

Polyethylenes

state. The d a t a d i s c u s s e d i n t h i s s t u d y have been o b t a i n e d f r o m b o t h c o n v e n t i o n a l i r o n magnet s p e c t r o m e t e r s w i t h f i e l d s t r e n g t h s o f 2 3 . 5 kG and s u p e r c o n d u c t i n g magnet systems o p e r a t i n g at 47 k G . L e t us now b e g i n o u r d i s c u s s i o n of p o l y e t h y l e n e long chain b r a n c h i n g w i t h an e x a m i n a t i o n o f t h e C-13 NMR s t r u c t u r a l s e n s i t i v i t y and l a t e r t u r n t o q u a n t i t a t i v e s e n s i t i v i t y and d e t e c t i o n 1imit. The C-13 NMR s p e c t r a f r o m a homologous s e r i e s of s i x l i n e a r e t h y l e n e 1 - o l e f i n c o p o l y m e r s b e g i n n i n g w i t h 1 - p r o p e n e and e n d i n g w i t h 1 - o c t e n e a r e r e p r o d u c e d i n F i g u r e s 2 and 3 . The s i d e - c h a i n b r a n c h e s a r e , t h e r e f o r e , l i n e a r and p r o g r e s s f r o m one t o s i x c a r bons i n l e n g t h . A l s o , t h e r e s p e c t i v e 1 - o l e f i n c o n c e n t r a t i o n s a r e l e s s t h a n 3%; t h u s , o n l y i s o l a t e d b r a n c h e s are p r o d u c e d . Unique s p e c t r a l f i n g e r p r i n t s a r e o b s e r v e d f o r each b r a n c h l e n g t h . The c h e m i c a l s h i f t s , w h i c h can be p r e d i c t e d w i t h t h e G r a n t and P a u l p a r a m e t e r s (j>)(_19) a r e g i v e n i n T a b l e I f o r t h i s s e r i e s o f model e t h y l e n e - l - o l e f i n copolymers. The n o m e n c l a t u r e , used t o d e s i g n a t e t h o s e p o l y m e r backbone and s i d e - c h a i n c a r b o n s d i s c r i m i n a t e d by C-13 NMR, i s as f o l l o w s :

γ 3 α α 3 Ύ -CH -CH -CH -CH -CH-CH -CH -CH -CH -CH 2

2

2

2

2

2

2

2

2

I 5 CH ι 4 CH

2

2

ι 3

CH ι

2

CH

2

1

CH

3

2

The d i s t i n g u i s h a b l e backbone c a r b o n s a r e d e s i g n a t e d by Greek sym b o l s w h i l e the s i d e - c h a i n carbons a r e numbered consecutively s t a r t i n g w i t h t h e methyl group and e n d i n g w i t h t h e m e t h y l e n e c a r bon bonded t o t h e p o l y m e r backbone (j>). The i d e n t i t y o f each r e s o n a n c e i s i n d i c a t e d i n F i g u r e s 2 and 3 . I t s h o u l d be n o t i c e d i n F i g u r e 3 t h a t the " 6 " carbon resonance f o r the hexyl branch i s t h e same as a , t h e " 5 " c a r b o n r e s o n a n c e i s t h e same as 3, and t h e " 4 " c a r b o n r e s o n a n c e i s t h e same as γ . Resonances 1, 2 and 3 , l i k e w i s e , a r e t h e same as t h e end group r e s o n a n c e s o b s e r v e d f o r a linear polyethylene. Thus a s i x c a r b o n b r a n c h p r o d u c e s t h e same C-13 s p e c t r a l p a t t e r n as any s u b s e q u e n t b r a n c h o f g r e a t e r l e n g t h . C a r b o n - 1 3 NMR, a l o n e , t h e r e f o r e c a n n o t be used t o d i s t i n g u i s h a l i n e a r s i x c a r b o n b r a n c h f r o m a b r a n c h o f some i n t e r m e d i a t e l e n g t h or a t r u e long c h a i n b r a n c h .



P O L Y M E R CHARACTERIZATION B Y ESR A N D N M R

Figure 2. C-13 NMR spectra at 25.2 MHz of (top) an ethylene-1 -propene copolymer, (middle) an ethylene-1-butene copolymer, and (bottom) an ethylene-1pentene copolymer



RANDALL

Long-Chain

Branching

in

Polyethylenes

7 β

et β 7

«

-CH2CH2CH2CHCH2CH2CH2-

CH 6 2

Figure 3. C-13 NMR spectra at 25.2 MHz of (top) an ethylene-1-hexene co polymer, (middle) an ethylene-1 -heptene copolymer, and (bottom) an ethylene-1octene copolymer


Woodward and Bovey; Polymer Characterization by ESR and NMR ACS Symposium Series; American Chemical Society: Washington, DC, 1980. I

Hethine

33.3

39.7

37.8

38.2

38.2

38.2

Branch Length

1

2

3

4

5

6

34.6

27.3

27.3

34.6

22.8 22.8

14.1 14.1

23.4

14.1

27.3

20.3

14.6

27.3

34.6

34.4

26.7

2

11.2

20.0

1

27.3

27.5

37.6

34.1

β

α

32.2

32.8

36.8

3

34.6 27.3 30.4

5

26.9

34.2

4

34.6

6

P o l y e t h y l e n e Backbone and S i d e - C h a i n C-13 C h e m i c a l S h i f t s i n ppm from Τ MS (+0.1) as a F u n c t i o n of B r a n c h L e n g t h (γ Carbon C h e m i c a l S h i f t s , w h i c h o c c u r near 30.4 ppm, a r e not g i v e n because t h e y are o f t e n o b s c u r e d by t h e m a j o r 30 ppm r e s o n a n c e f o r t h e " n " e q u i v a l e n t , r e c u r r i n g m e t h y l e n e c a r b o n s ) . Sol v e n t : 1,2,4-trichlorobenzene. Temperature: 125°C.

TABLE



6.

Long-Chain

RANDALL

Branching

in

101

Polyethylenes

The c a p a b i l i t y f o r d i s c e r n i n g t h e l e n g t h o f s h o r t chain b r a n c h e s has made C-13 NMR a p o w e r f u l t o o l f o r c h a r a c t e r i z i n g low d e n s i t y p o l y e t h y l e n e s produced from f r e e r a d i c a l , high p r e s s u r e processes. The C-13 NMR s p e c t r u m o f such a p o l y e t h y l e n e i s shown i n Figure 4. It i s e v i d e n t t h a t the major s h o r t c h a i n branches a r e b u t y l , amyl and e t h y l . O t h e r s a r e a l s o p r e s e n t , and A x e l s o n , M a n d e l k e r n , and L e v y , i n a c o m p r e h e n s i v e s t u d y ( 6 J have c o n c l u d e d t h a t no u n i q u e s t r u c t u r e can be used t o c h a r a c t e r i z e low d e n s i t y polyethylenes. They have f o u n d n o n l i n e a r s h o r t c h a i n b r a n c h e s as w e l l as 1,3 p a i r e d e t h y l b r a n c h e s . B o v e y , S c h i l l i n g , McCracken and Wagner (9) compared t h e c o n t e n t o f b r a n c h e s s i x and l o n g e r i n low d e n s i t y p o l y e t h y l e n e s w i t h the long c h a i n branching r e s u l t s o b t a i n e d through a combination of gel permeation chromatography and i n t r i n s i c v i s c o s i t y . An o b s e r v e d good agreement l e d t o t h e c o n c l u s i o n t h a t the p r i n c i p a l short c h a i n branches c o n t a i n e d few e r t h a n s i x c a r b o n s and t h e s i x and l o n g e r b r a n c h i n g c o n t e n t c o u l d be r e l a t e d e n t i r e l y t o l o n g c h a i n b r a n c h i n g . O t h e r s have now r e p o r t e d s i m i l a r o b s e r v a t i o n s i n s t u d i e s where s o l u t i o n m e t h ods a r e combined w i t h C-13 NMR [10). However, as a r e s u l t o f t h e p o s s i b l e u n c e r t a i n t y of the branch l e n g t h s , a s s o c i a t e d with the r e s o n a n c e s f o r b r a n c h e s s i x c a r b o n s and l o n g e r , C-13 NMR s h o u l d be used i n c o n j u n c t i o n w i t h i n d e p e n d e n t methods t o e s t a b l i s h t r u e long chain branching. From t h e r e s u l t s we have seen t h u s f a r , i t i s easy t o p r e d i c t t h e C-13 NMR s p e c t r u m a n t i c i p a t e d f o r e s s e n t i a l l y l i n e a r p o l y e t h y l e n e s c o n t a i n i n g a small degree of long c h a i n b r a n c h i n g . An e x a m i n a t i o n o f a C-13 NMR s p e c t r u m f r o m a c o m p l e t e l y l i n e a r p o l y e t h y l e n e , c o n t a i n i n g b o t h t e r m i n a l o l e f i n i c and s a t u r a t e d end g r o u p s , shows t h a t o n l y f i v e r e s o n a n c e s a r e p r o d u c e d . A major r e s o n a n c e a t 30 ppm a r i s e s f r o m e q u i v a l e n t , r e c u r r i n g m e t h y l e n e c a r b o n s , d e s i g n a t e d as " n " , w h i c h a r e f o u r o r more removed from an end group o r a b r a n c h . Resonances at 1 4 . 1 , 2 2 . 9 and 3 2 . 3 ppm a r e f r o m c a r b o n s 1,2 and 3 , r e s p e c t i v e l y , f r o m t h e s a t u r a t e d , l i n e a r end g r o u p . A f i n a l r e s o n a n c e w h i c h i s o b s e r v e d at 3 3 . 9 ppm, a r i s e s f r o m an a l l y l i c c a r b o n , d e s i g n a t e d as " a " , f r o m a t e r m i n a l o l e f i n i c end g r o u p . These r e s o n a n c e s , d e p i c t e d s t r u c t u r a l l y below, are fundamental t o the s p e c t r a of a l l p o l y e t h y l enes .

CH -CH -CH -CH -CH 3

1

2

2

2

3

2

2

-(CH ) 2

"n"

n

-CH -CH=CH 2

2

"a"

An i n t r o d u c t i o n o f b r a n c h i n g , e i t h e r l o n g or s h o r t , w i l l c r e a t e a d d i t i o n a l resonances t o those d e s c r i b e d above. For long c h a i n b r a n c h e s , t h e s e w i l l be an α, β, (and sometimes γ ) and a m e t h i n e r e s o n a n c e as d e p i c t e d s t r u c t u r a l l y b e l o w :


POLYMER

CHARACTERIZATION

BY

ESR


102


AND

NMR

6.

RANDALL

Long-Chain

[Υ]

3

α

α

-CH -CH -CH -CH -CH-CH 2

2

2

2

α

CH

3

CH I CH

[Y]


CH

103

in Polyethylenes

Branching

2

3

[γ]

CH -CH -CH 2

2

2

2

2

2

2

From t h e o b s e r v e d C-13 NMR s p e c t r u m o f t h e e t h y l e n e - l - o c t e n e c o p o l y m e r ( F i g u r e 3 ) , we f i n d t h a t t h e a , 3 and m e t h i n e r e s o n a n c e s a s s o c i a t e d w i t h b r a n c h e s s i x c a r b o n s and l o n g e r o c c u r a t 3 4 . 5 6 , 2 7 . 3 2 and 3 8 . 1 7 ppm, r e s p e c t i v e l y . Thus 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 s , where l o n g c h a i n b r a n c h i n g i s e s s e n t i a l l y t h e o n l y t y p e p r e s e n t , c a r b o n - 1 3 NMR c a n be used t o e s t a b l i s h u n e q u i v o c a l l y t h e p r e s e n c e o f b r a n c h e s s i x c a r b o n s l o n g and l o n g e r . I f no comonome r has been used d u r i n g p o l y m e r i z a t i o n , i t i s v e r y l i k e l y t h a t t h e p r e s e n c e o f such r e s o n a n c e s w i l l be i n d i c a t i v e of t r u e l o n g chain branching. In any e v e n t , C-13 NMR can be used t o p i n p o i n t t h e a b s e n c e o f l o n g c h a i n b r a n c h i n g and p l a c e an upper l i m i t upon t h e l o n g c h a i n b r a n c h c o n c e n t r a t i o n whenever b r a n c h e s s i x c a r b o n s and l o n g e r a r e d e t e c t e d . The need f o r a c o m p l e m e n t a r y measurement t o C-13 NMR i n s t u d i e s o f l o n g c h a i n b r a n c h i n g s h o u l d be a p p a r e n t . I t has been p o i n t e d out t h a t a p r o m i s i n g p o s s i b i l i t y a p p e a r s t o be f l o w a c t i v a t i o n e n e r g i e s o b t a i n e d f r o m dynamic s h e a r m o d u l i as a f u n c t i o n o f t e m p e r a t u r e (2J3). F l o w a c t i v a t i o n e n e r g i e s range f r o m a p p r o x i m a t e l y 6.0 k c a l / m o l f o r l i n e a r s y s t e m s t o around 13.5 k c a l / m o l f o r systems c o n t a i n i n g e x t e n s i v e long c h a i n b r a n c h i n g . Chain en t a n g l e m e n t s a r e one of t h e f a c t o r s i n f l u e n c i n g f l o w a c t i v a t i o n e n e r g i e s , but t o t h e e x t e n t t h a t l o n g c h a i n b r a n c h e s a l s o i n f l u ence c h a i n e n t a n g l e m e n t s , t h i s t e c h n i q u e can be an i n d i c a t o r of the presence of long c h a i n b r a n c h i n g . Four p o l y e t h y l e n e s , l a b e l l e d " A " t h r o u g h "D" and s e l e c t e d f o r C-13 NMR c h a r a c t e r i z a t i o n on a b a s i s o f t h e o b s e r v e d f l o w a c t i v a t i o n e n e r g i e s , a r e d e s c r i b e d in Table II. A f i f t h p o l y m e r , c a l l e d " E " , was a l s o exam i n e d as a r e f e r e n c e p o l y m e r b e c a u s e i t was not e x p e c t e d t o c o n t a i n any s i g n i f i c a n t l o n g c h a i n b r a n c h i n g as i n d i c a t e d by i t s f l o w a c t i v a t i o n energy (see Table I I ) . C a r b o n - 1 3 NMR d a t a were o b t a i n e d a t a h i g h f i e l d (50 MHz, 47 kG) t o a c h i e v e i m p r o v e d s e n sitivity. The 50 MHz s p e c t r a o f t h e s e p o l y m e r s , A t h r o u g h E, a r e reproduced i n Figures 5 through 9. Instrumental c o n d i t i o n s nec e s s a r y f o r q u a n t i t a t i v e measurements w i l l be d i s c u s s e d l a t e r ; f o r t h e p r e s e n t , we w i l l be c o n c e r n e d w i t h t h e use o f C-13 NMR as a means f o r s i m p l y d e t e c t i n g t h e p r e s e n c e o f l o n g c h a i n b r a n c h i n g .



224,000

148,000

226,000

172,000

Β

C

D

Ε

w

159,000

M

A

Polymer

n

*These

a

6.1 k c a l / m o l

9.6 k c a l / m o l

9.3 kcal/mol

8.6 k c a l / m o l

8.0 kcal/mol

E

Catalyst

t i t a n i u m based

See

None

of

text.

Hexene-1

None

chromium based chromium based

None

Comonomer

a Series

chromium based

for

None

Catalyst

of

chromium based

and Type

II

" g " f a c t o r s a r e p r o b a b l y not v e r y a c c u r a t e .

32,400

8,500

17,900

13,600

19,100

M

M o l e c u l a r Weights, Flow A c t i v a t i o n Energies Examined f o r Long C h a i n B r a n c h i n g .

TABLE


0.7

0.6

0.5

0.8

factor*

(1+)

"g"

Polyethylenes

6.

RANDALL

Long-Chain

Branching

in Poly ethylenes

105

ο

ο

s: «χ»


go -S

0

•S ο

^

^

S*

5 δ .s

a. ?I * S 8

as ••χ ι

13

s: Q

â|



Figure 6. C-13 NMR spectrum at 50 MHz of Polymer Β (~ 75% in trichlorobenzene at 125°C, number of transients accumulated 4,900). Spectrum was provided courtesy of Nicolet Technology Corporation.


ο

> α

00

m

α -
Η δ

m 2

Ο Η

>

Ο Χ > 73

90

m

*ϋ Ο r

ON


Figure 7. C-13 NMR spectrum at 50 MHz of Polymer C PE homopolymer (~ 10% in trichlorobenzene at 125°C, number of transients accumulated 50,000). Spectrum was provided courtesy of Varian Associates.


ο

Woodward and Bovey; Polymer Characterization by ESR and NMR ACS Symposium Series; American Chemical Society: Washington, DC, 1980. /

32.18

ALLYLIC CARBON 33.88

4B 34.16

7 30.48 27.78

2S 22.84

2B 23.35 1B+1S 14.09

Figure 8. C-13 NMR spectrum at 50 MHz of Polymer D ethylene-1 -hexene copolymer (~ 10% in trichlorobenzene at 125°C, number of transients accumulated 8,743). Spectrum was provided courtesy of Varian Associates.

CH 38.16

34.56

C % 29.98

i



Figure 9. C-13 NMR spectrum at 50 MHz of Polymer Ε (~ 75% in trichlorobenzene at 125°C, number of transients accumulated 10,100). Spectrum was provided courtesy of Nicolet Technology Corporation.


ο


110

POLYMER

CHARACTERIZATION

B Y ESR A N D N M R

F o r t h o s e p o l y e t h y l e n e s where s h o r t c h a i n b r a n c h i n g i s p r e s e n t i n a d d i t i o n t o l o n g c h a i n b r a n c h i n g (see F i g u r e s 8 and 9 ) , a m o d i f i c a t i o n i n t h e n o m e n c l a t u r e scheme i s needed t o d i s t i n g u i s h i d e n t i f i a b l e s h o r t c h a i n carbon resonances from the c o r r e s p o n d i n g c a r b o n s i n c h a i n end g r o u p s . C o n s e q u e n t l y , an " S " ( f o r s a t u r a t e d end g r o u p s ) has been added t o t h e 1, 2 and 3 c a r b o n n o m e n c l a t u r e f o r chain ends. The numbers f o r c a r b o n s i n b r a n c h e s o f l e n g t h f i v e and s h o r t e r have a " B " a d d e d . In f i g u r e s 5 - 9 , t h e r e s o nances f r o m c h a i n e n d s , s i x and l o n g e r , a r e c l e a r l y i d e n t i f i e d from carbon resonances from the short s i d e - c h a i n b r a n c h e s . The 50 MHz C-13 flMR s p e c t r u m o f p o l y m e r " A " i s r e p r o d u c e d i n F i g u r e 5. I t i s n e a r l y a c l a s s i c a l r e p r e s e n t a t i o n of t h e s p e c trum anticipated for a polyethylene containing long chain branching. Only the f i v e resonances expected f o r l i n e a r p o l y e t h y l e n e s y s t e m s p l u s t h e α, β, and m e t h i n e r e s o n a n c e s f o r l o n g c h a i n branches are observed. A s i m p l e i n s p e c t i o n of the α, β i n t e n s i t i e s as compared t o t h e end g r o u p r e s o n a n c e s i n d i c a t e s t h a t f e w e r t h a n one p o l y m e r m o l e c u l e out o f t h r e e has a l o n g c h a i n branch. A s i m i l a r r e s u l t was o b t a i n e d f o r p o l y m e r " B " shown i n F i g u r e 6. B o t h p o l y m e r s " A " and " B " c o n t a i n l o n g c h a i n b r a n c h e s a n d , on an a v e r a g e , have a p p r o x i m a t e l y e q u a l numbers of both s a t u r a t e d and t e r m i n a l o l e f i n end g r o u p s . P o l y m e r " C " , shown i n F i g u r e 7 , g i v e s a more complex C-13 NMR s p e c t r u m t h a n o b s e r v e d f o r p o l y m e r s " A " and " B " because f o u r c a r b o n s i d e - c h a i n b r a n c h e s a r e p o s i t i v e l y i n d i c a t e d even t h o u g h no comonomer was added d u r i n g p o l y m e r i z a t i o n . The p r e s e n c e o f butyl branches complicates an i d e n t i f i c a t i o n o f long chain b r a n c h i n g b e c a u s e t h e α, β r e s o n a n c e s f r o m b u t y l b r a n c h e s have t h e same c h e m i c a l s h i f t s as do t h e α, β r e s o n a n c e s f r o m l o n g e r , l i n e a r branches (see T a b l e I ) . The l o n g c h a i n b r a n c h i n g f o r s y s tems c o n t a i n i n g b u t y l b r a n c h e s , t h e r e f o r e , can o n l y be d e t e r m i n e d from the d i f f e r e n c e s observed i n the r e l a t i v e i n t e n s i t i e s f o r the " 4 " c a r b o n and t h e α c a r b o n . In F i g u r e 3 , t h e α t o " 4 " c a r b o n resonance i n t e n s i t i e s are d i s t i n c t l y 2 : 1 . In F i g u r e 7 , t h e r e l a t i v e i n t e n s i t i e s f o r the α to "4" carbon resonances are 2.7:1 and l o n g c h a i n b r a n c h i n g i s , t h e r e f o r e , i n d i c a t e d . The p o l y m e r i z a t i o n c o n d i t i o n s have somehow l e d t o an i n t r o d u c t i o n of b u t y l b r a n c h e s w i t h o u t a l s o i n t r o d u c i n g amyl b r a n c h e s o r b r a n c h l e n g t h s s h o r t e r than f o u r . T h i s r e s u l t i s i n t e r e s t i n g and l e a d s t o t h e s u g g e s t i o n t h a t t h e l o n g c h a i n b r a n c h e s i n p o l y m e r " C " may not be truly "long". A d i s p r o p o r t i o n a t e number of t h e s e " l o n g c h a i n " b r a n c h e s may be i n t e r m e d i a t e i n l e n g t h . Further study i s c l e a r l y i n d i c a t e d f o r t h i s polyethylene system. W i t h o u t C-13 NMR, how e v e r , one may not have been aware t h a t s h o r t c h a i n b r a n c h e s were being introduced i n t o a polyethylene p o l y m e r i z a t i o n without add i n g comonomer. P o l y m e r " B " i s an e t h y l e n e - l - h e x e n e c o p o l y m e r by d e s i g n . A f l o w a c t i v a t i o n e n e r g y o f 9.6 k c a l / m o l s u g g e s t s t h a t l o n g c h a i n b r a n c h i n g may be p r e s e n t . The C-13 NMR s p e c t r u m , h o w e v e r , i s c o m p l i c a t e d by t h e p r e s e n c e o f b u t y l b r a n c h e s as i n t h e c a s e o f



6.

Long-Chain

RANDALL

Branching

in

111

Polyethylenes

polymer " C " . Once a g a i n , t h e d e t e c t i o n o f l o n g c h a i n b r a n c h i n g must be based on a r a t i o o f t h e o b s e r v e d i n t e n s i t i e s f o r t h e " a " c a r b o n r e s o n a n c e t o t h e " 4 " c a r b o n r e s o n a n c e , w h i c h i s 2.04 i n t h i s c a s e and w i t h i n e x p e r i m e n t a l e r r o r o f t h e NMR i n t e n s i t y mea surements. Thus l o n g c h a i n b r a n c h i n g c a n n o t be d e t e r m i n e d r e a d i l y u n l e s s one r e s o r t s t o a t e d i o u s s e t o f measurements designed t o e s t a b l i s h whether the excess i n t e n s i t y of the "a" resonance i s o u t s i d e the l i m i t s of e x p e r i m e n t a l e r r o r . P o l y m e r " E " was p r e p a r e d w i t h a Z i e g l e r t y p e of c a t a l y s t and has o n l y a s m a l l q u a n t i t y of t e r m i n a l o l e f i n end g r o u p s , as shown by t h e NMR s p e c t r u m i n F i g u r e 9 . I t was not e x p e c t e d t o have any s i g n i f i c a n t d e g r e e o f l o n g c h a i n b r a n c h i n g because t h e f l o w a c t i v a t i o n e n e r g y was 6.1 k c a l / m o l . The C-13 NMR s p e c t r u m i s c o n s i s t e n t w i t h t h e p r e s e n c e o f v e r y low q u a n t i t i e s o f l o n g c h a i n b r a n c h i n g (see " a " i n F i g u r e 9 ) . An u n e x p e c t e d r e s u l t d i d o c c u r , h o w e v e r , b e c a u s e e t h y l b r a n c h e s c o u l d be c l e a r l y i d e n t i f i e d by the chemical s h i f t s f o r the " 0 2 " , " β " , m e t h i n e and s i d e - c h a i n carbon resonances. Note i n F i g u r e 9 t h a t t h e m e t h y l r e s o n a n c e f o r t h e e t h y l b r a n c h i s not shown; t h e o t h e r s can be c l e a r l y i d e n t i f i e d in Figure 9). From a q u a n t i t a t i v e v i e w p o i n t , i t i s e v i d e n t t h a t t h e r e l a t i v e i n t e n s i t i e s of t h e r e s o n a n c e s f r o m c a r b o n s a s s o c i a t e d w i t h b r a n c h e s and end groups can be compared t o t h e i n t e n s i t y f o r t h e m a j o r m e t h y l e n e r e s o n a n c e , " n " , at 3 0 . 0 0 ppm t o d e t e r m i n e b r a n c h c o n c e n t r a t i o n s and number a v e r a g e m o l e c u l a r w e i g h t o r c a r b o n num ber. The f o l l o w i n g d e f i n i t i o n s a r e u s e f u l i n f o r m u l a t i n g t h e a p propriate algebraic relationships:

η = i n t e n s i t y of t h e m a j o r m e t h y l e n e r e s o n a n c e s = average

i n t e n s i t y f o r a s a t u r a t e d end group

a = the a l l y l i c

C^tot = t h e t o t a l

ά = 1/2

carbon

= average

carbon

ppm

intensity

(α + β) c a r b o n

M = average N+2

c a r b o n i n t e n s i t y at 3 3 . 9

a t 30 ppm

intensities

number o f l o n g c h a i n b r a n c h e s

per p o l y m e r m o l e c u l e

number o f end groups per p o l y m e r m o l e c u l e

W i t h t h e p r e v i o u s d e f i n i t i o n s , t h e p o l y m e r c a r b o n number and b e r a v e r a g e m o l e c u l a r w e i g h t a r e g i v e n by:


num

112

POLYMER

Carbon Number = C

t o t

M

Number

n

= 14 X Carbon

CHARACTERIZATION

B Y ESR A N D N M R

( N + 2 ) / ( s + a)

(1) (2)


F o r l i n e a r p o l y m e r s where " N " i s z e r o , t h e c a r b o n number and num b e r a v e r a g e m o l e c u l a r w e i g h t can be e a s i l y and r e l i a b l y d e t e r mined. F o r t h o s e p o l y m e r s c o n t a i n i n g l o n g c h a i n b r a n c h i n g , one must u t i l i z e t h e r a t i o o f t h e c a r b o n r e s o n a n c e i n t e n s i t i e s a s s o c i a t e d w i t h b r a n c h i n g t o t h e end group c a r b o n r e s o n a n c e i n t e n s i t i e s t o d e t e r m i n e " N " as f o l l o w s :

N = 2 â / ( 3 ( s + a) - â) The number o f l o n g c h a i n i s s i m i l a r l y given by:

branches

(3) per t e n t h o u s a n d

Branches/10,000 C = ((1/3 â ) / ( C

Χ 10 )) 4

t o t

carbon

atoms

(4)

E q u a t i o n 4 c a n be e a s i l y m o d i f i e d f o r t h e number o f s h o r t c h a i n b r a n c h e s ( f e w e r t h a n s i x c a r b o n s ) p e r 10,000 c a r b o n s by r e p l a c i n g 1/3 α w i t h an i n t e n s i t y a p p r o p r i a t e f o r one b r a n c h c a r b o n f r o m the short chain branch. With t h e comprehensive s t r u c t u r a l i n f o r m a t i o n a v a i l a b l e from C-13 NMR a n a l y s e s o f b o t h l o w and h i g h d e n s i t y polyethylenes, q u a n t i t a t i v e measurements become h i g h l y d e s i r a b l e . As seen f r o m e q u a t i o n s 1-4, t h e q u a n t i t a t i v e r e l a t i o n s h i p s necessary f o r a n a l y s e s o f b r a n c h i n g , m o l e c u l a r w e i g h t and end g r o u p d i s t r i b u t i o n s c a n be r e a d i l y d e r i v e d w i t h o u t r e s o r t i n g t o l i m i t i n g a s s u m p t i o n s . The a s s i g n m e n t s have been r i g o r o u s l y e s t a b l i s h e d ; t h u s t h e o n l y q u e s t i o n remaining i s the r e l i a b i l i t y of the data with respect to the observed r e l a t i v e i n t e n s i t i e s . D i f f e r e n c e s among n u c l e a r Overhauser e f f e c t s , n o n - e q u i l i b r i u m dynamic c o n d i t i o n s during d a t a g a t h e r i n g and s o f t w a r e - h a r d w a r e p r o b l e m s w i t h t h e dynamic r a n g e a r e f a c t o r s w h i c h would a d v e r s e l y a f f e c t t h e r e l a t i v e o b s e r v e d C-13 NMR s p e c t r a l i n t e n s i t i e s . I t has been d e m o n s t r a t e d by Levy and c o w o r k e r s ( 6 ) ( 2 J J t h a t n u c l e a r Overhauser e f f e c t s i n polyethylenes are f u l l and, t h e r e f o r e , a r e not a c o n s i d e r a t i o n under t h e e x p e r i m e n t a l c o n d i t i o n s (^10% s o l u t i o n s , 125°C) employed i n C-13 NMR q u a n t i t a t i v e mea s u r e m e n t s o f p o l y e t h y l e n e . B e c a u s e t h e C-13 NMR e x p e r i m e n t u t i l i z e s t h e F o u r i e r t r a n s f o r m t e c h n i q u e and f r e e i n d u c t i o n decay d a t a i s g a t h e r e d i n a t i m e dependent f r a m e w o r k , s p i n - l a t t i c e r e l a x a t i o n t i m e s , T j / s , a r e i m p o r t a n t f a c t o r s t h a t must be c o n s i d e r e d when d e s i g n i n g a p u l s e sequence f o r l o n g t e r m d a t a a v e r a g ing. G e n e r a l l y , t h e p u l s e s p a c i n g s must be f i v e t i m e s t h e l o n g e s t o b s e r v e d T i t o e n s u r e c o m p l e t e r e l a x a t i o n between r f p u l s e s (22). The m e t h y l groups i n c h a i n ends and s h o r t b r a n c h e s can have T ^ ' s r a n g i n g f r o m 3 t o 7 s e c o n d s (21) ; t h u s t h e t i m e r e -



6.

RANDALL

Long-Chain

Branching

in

113

Polyethylenes

q u i r e d t o g a t h e r t h r e e t o f i v e t h o u s a n d F I D ' s can become i n o r d i nately long. When p l a n n i n g q u a n t i t a t i v e e x p e r i m e n t s on w e l l known s y s t e m s , i t i s a d v i s a b l e t o s e l e c t t h o s e r e s o n a n c e s most f a v o r a b l e from a T j s t a n d p o i n t f o r q u a n t i t a t i v e measurements b e c a u s e i n s t r u m e n t t i m e i s e x p e n s i v e and s h o u l d be used effi ciently. Dynamic r a n g e i s an i m p o r t a n t c o n s i d e r a t i o n i n q u a n t i t a t i v e NMR measurements b e c a u s e s m a l l r e s o n a n c e s may become t r u n c a t e d w i t h r e s p e c t t o l a r g e r e s o n a n c e s (^1000:1) d u r i n g t i m e a v e r a g i n g i f a s u f f i c i e n t computer v/ord l e n g t h i s not a v a i l a b l e ( 2 3 ) . It i s i m p e r a t i v e i n measurements o f l o n g c h a i n b r a n c h i n g i n p o l y e t h y l e n e t h a t an NMR i n s t r u m e n t be e q u i p p e d w i t h e i t h e r a 16K com p u t e r s y s t e m w i t h b l o c k a v e r a g i n g o r have a 16K c o m p u t e r s y s t e m w i t h e i t h e r double p r e c i s i o n or f l o a t i n g point a r i t h m e t i c . In s t r u m e n t s a r e now a v a i l a b l e t h a t have b o t h d o u b l e p r e c i s i o n and f l o a t i n g p o i n t a r i t h m e t i c , which are a l s o v a l u a b l e i n p r e v e n t i n g unwanted t r u n c a t i o n s d u r i n g F o u r i e r t r a n s f o r m a t i o n s . With r e a s o n a b l e c a r e g i v e n t o b o t h t h e s o f t w a r e and hardware problems a s s o c i a t e d w i t h q u a n t i t a t i v e C-13 NMR o f p o l y m e r s , one f i n d s t h a t measurements o f end group d i s t r i b u t i o n s , m o l e c u l a r w e i g h t , and b r a n c h i n g i n p o l e t h y l e n e are r o u t i n e l y a v a i l a b l e . One o f t h e b e s t t e s t s f o r s a t i s f a c t o r y NMR i n s t r u m e n t a l c o n d i t i o n s d u r i n g q u a n t i t a t i v e C-13 NMR measurements i s t o examine known r e f e r e n c e s t a n d a r d s . The b e s t f o r t h i s p u r p o s e a r e NBS standards 1482 (M = 13,600, M = 1 1 , 4 0 0 ) and 1483 (M = 32,100, M = 28,900). The NBS s t a n d a r d 1483 was examined under p r e c i s e l y t h e same e x p e r i m e n t a l NMR c o n d i t i o n s as p o l y e t h y l e n e s C and Ε r e p o r t e d i n T a b l e I I . The C-13 NMR s p e c t r u m i s shown i n F i g u r e 10. I n s t r u m e n t a l c o n d i t i o n s and p e r t i n e n t i n t e n s i t y d a t a are given below: w

n

w

n

Resonance Pulse Angle: 50° Pulse Spacing: 1 second Double P r e c i s i o n A r i t h m e t i c

M

n

= 30,560

32.18 33.91 30.00 34.09

ppm ppm ppm ppm

Peak H e i g h t 4.5 1.1 6082 3.9

( C a l c u l a t e d from E q u a t i o n s

( O n l y 3S was used t o d e t e r m i n e s of the three t e r m i n a l carbons.)

because

Assignment

it

3S η αο

1 and has

2).

the

shortest

T^

As was t h e c a s e f o r p o l y m e r " E " , a s m a l l amount of e t h y l b r a n c h i n g was d e t e c t e d i n NBS 1483 as i n d i c a t e d i n F i g u r e 10 by t h e r e s o n a n c e s o b s e r v e d f o r t h e a p p r o p r i a t e m e t h i n e , a » 3 and 2B carbons. (The m e t h y l r e s o n a n c e i s not shown.) The c o n c e n t r a t i o n o f e t h y l b r a n c h e s i s 3 p e r 10,000 c a r b o n s as d e t e r m i n e d u s i n g a m o d i f i e d v e r s i o n of E q u a t i o n 4 . NBS 1483 was p r o b a b l y p r e p a r e d u s i n g a Z i e g l e r t y p e c a t a l y s t s y s t e m , as s u g g e s t e d by t h e r e l a 2



Figure 10. C-13 NMR spectrum at 50 MHz of NBS 1483 (~ 10% in trichlorobenzene at 125°C, number of transients accumulated 229,462). Spectrum was provided courtesy of Varian Associates.


6.

Long-Chain

RANDALL

Branching

in

115

Polyethylenes

t i v e l y low i n t e n s i t y of t h e 3 3 . 9 resonance f o r the a l l y l i c c a r bon. I t i s i n t e r e s t i n g t h a t a low amount o f e t h y l b r a n c h i n g was d e t e c t e d i n t h i s sample and i n p o l y m e r " E " , w h i c h was p r e p a r e d w i t h a Z i e g l e r t y p e c a t a l y s t . NBS 1482 was examined u s i n g an X L 100 s p e c t r o m e t e r at 25 MHz. I n s t r u m e n t a l c o n d i t i o n s and i n t e n s i t y data are given below: Resonance


Pulse Angle: 90° Pulse Spacing: 10 s e c o n d s Double P r e c i s i o n A r i t h m e t i c

s = 14.5 M

n

(IS

= 11,970

14.09 22.86 32.16 33.00 29.98

was not

ppm ppm ppm ppm ppm

Peak H e i g h t

Assignment

11 15 14 8 9574

IS 2S 3S "a" η

used).

( C a l c u l a t e d from E q u a t i o n s

1 and

2).

E t h y l b r a n c h i n g was not d e t e c t e d i n NBS 1482; h o w e v e r , t h e i n s t r u m e n t a l s e n s i t i v i t y was such t h a t b r a n c h i n g i n a range o f 1-5 p e r 10,000 c a r b o n s would not be o b s e r v e d . The agreement between t h e number a v e r a g e m o l e c u l a r w e i g h t s c a l c u l a t e d f r o m t h e NMR d a t a and t h a t r e p o r t e d by NBS s e r v e s t o i n d i c a t e t h e v i a b i l i t y of t h e NMR method f o r d e t e r m i n i n g number a v e r a g e m o l e c u l a r w e i g h t . The method d i s c u s s e d a b o v e , when a p p l i e d t o t h e C-13 NMR d a t a f r o m p o l y m e r s A t h r o u g h E, gave t h e r e s u l t s l i s t e d i n T a b l e I I I for N , Ν and d e g r e e o f b r a n c h i n g . n

TABLE Number A v e r a g e Chain Branching D and E.

III

M o l e c u l a r W e i g h t , Long C h a i n B r a n c h i n g , Short and End Group D i s t r i b u t i o n f o r P o l y m e r s A, B, C,

C^ Branches/ Molecule +

Polymer A B C D Ε

0.28 0.29 0.23

Branches/10,000 (C ) (C ) (C ) (Butyl) (Butyl) (Ethyl) 6 +

6 +

6 +

1.8 2.1 1.4) 5.5) 64 2.2

Mn 21,700 18,680 23,100 10,300 28,650

s/a 1/1 1/1 1/7.1 1/1 2.2/1

Number a v e r a g e m o l e c u l a r w e i g h t d a t a f r o m s i z e e x c l u s i o n c h r o m a t o g r a p h y f o r p o l y m e r s A t h r o u g h Ε has been g i v e n p r e v i o u s l y


116

POLYMER

CHARACTERIZATION

B Y ESR A N D N M R

in Table II. These r e s u l t s s h o u l d be compared t o t h e number a v e r a g e m o l e c u l a r w e i g h t s o b t a i n e d f r o m NMR i n T a b l e I I I . In s i z e e x c l u s i o n c h r o m a t o g r a p h y , b o t h l i n e a r and b r a n c h e d m o l e c u l e s a r e s e p a r a t e d a c c o r d i n g t o t h e i r r e s p e c t i v e hydrodynamic volumes, that is


n

lin lin M

=

n

br br, M

(5)

t h e p r i n c i p a l o f u n i v e r s a l c a l i b r a t i o n (_18). In t h i s p a r t i c u l a r SEC a n a l y s i s , no c o n s i d e r a t i o n was g i v e n t o t h e p o s s i b i l i t y t h a t the p o l y m e r m o l e c u l e s c o u l d be b r a n c h e d ; t h u s low m o l e c u l a r w e i g h t r e s u l t s s h o u l d be a n t i c i p a t e d because b r a n c h e d polymers have o v e r a l l s m a l l e r d i m e n s i o n s t h a n l i n e a r p o l y m e r s o f t h e same molecular weight. Lower r e s u l t s were o b t a i n e d as shown by t h e d a t a i n T a b l e II a l t h o u g h t h e i n t e r n a l t r e n d s a r e i d e n t i c a l t o t h o s e o b t a i n e d f r o m NMR. I t i s p o s s i b l e t o use t h e number a v e r age m o l e c u l a r w e i g h t s f r o m SEC and NMR t o c a l c u l a t e t h e f a m i l i a r " g " f a c t o r (18) t h r o u g h t h e r e l a t i o n s h i p ,

g

l/2

=

n

b

r

/

n

]

i

n

(6)

and e q u a t i o n 5. V a l u e s o f " g " l e s s t h a n one were o b t a i n e d a l t h o u g h t h e y a r e somewhat s m a l l e r t h a n t h e v a l u e s p r e d i c t e d by Zimm and S t o c k m a y e r (24) f o r t h e amount o f l o n g c h a i n b r a n c h i n g d e t e r m i n e d by NMR. The d i f f e r e n c e s c o u l d be e a s i l y a c c o u n t e d by t h e e r r o r (+ 10% i n t h e m o l e c u l a r w e i g h t s f r o m S E C ) . Although t h e s e " g " f a c t o r s are probably i n a c c u r a t e , they are i n the c o r r e c t d i r e c t i o n f r o m t h e NMR d a t a and f r o m SEC. P o l y m e r " E " gave t h e h i g h e s t number a v e r a g e m o l e c u l a r w e i g h t o f t h e p o l y m e r s e x a m i n e d , and f o r t h i s r e a s o n , i t p r o b a b l y gave t h e l e a s t a c c u r a t e r e s u l t f r o m b o t h NMR and SEC, w h i c h a r e p r o b ably w i t h i n experimental e r r o r . The e r r o r i n t h e NMR measurement w i l l i n c r e a s e w i t h m o l e c u l a r w e i g h t u n l e s s an e f f o r t i s made t o o b t a i n s p e c t r a w i t h t h e same s i g n a l - t o - n o i s e r a t i o f o r t h e r e s p e c t i v e resonances a s s o c i a t e d with branches. The u t i l i t y and a c c u r a c y o f t h e C-13 NMR method i n p r o v i d i n g q u a n t i t a t i v e p o l y m e r s t r u c t u r a l d a t a , however, i s g r a t i f y i n g . The C-13 NMR method has been c r i t i c i z e d i n t h e p a s t because i t i s time consuming. F i v e t o t e n t h o u s a n d t r a n s i e n t s w i t h 10-15 s e c o n d s p u l s e d e l a y s a r e u s u a l l y r e q u i r e d w i t h 1 0 - 1 5 % by w e i g h t s o l u t i o n s t o o b t a i n s i g n a l - t o - n o i s e a d e q u a t e f o r a one p a r t i n one t h o u s a n d measurement. T h i s t i m e f a c t o r can be r e d u c e d s u b s t a n t i a l l y i f one uses t w e n t y m i l l i m e t e r sample t u b e s and a s u p e r c o n d u c t i n g magnet s y s t e m and examines t h e p o l y e t h y l e n e i n a melt s t a t e . T h i s improvement p l u s t h e f a c t t h a t t h r e e m e a s u r e m e n t s , number a v e r a g e m o l e c u l a r w e i g h t , end g r o u p d i s t r i b u t i o n and d e g r e e o f b r a n c h i n g , a r e a c c o m p l i s h e d i n one make C-13 NMR a h i g h l y a t t r a c t i v e method f o r c h a r a c t e r i z i n g p o l y e t h y l e n e s . A


6. RANDALL

Long-Chain Branching in Polyethylenes

117


serious drawback is not encountered even though branches six carbons in length and longer are measured collectively. The short branches are generally less than six carbons in length and truly long chain branches tend to predominate. On occasions, there may be special exceptions for "intermediate" branch lengths, as shown by polymer "C" in this study, so independent rheological measurements should be sought as a matter of course. Nevertheless, a direct method, which possesses the required sensitivity to determine long chain branching in high density polyethylenes, is now available.

REFERENCES 1.

M. L. Miller, "The Structure of Polymers", Reinhold, New York, 1966 p. 118.

2.

A. H. Willbourn, J. Polym. Sci., 34, 569 (1959) Nottingham Symposium.

3.

E. P. Otocka, R. J. Roe, M. Y. Hellman, and P. M. Muglia, Macromolecules, 4, 507 (1971).

4.

D. E. Dorman, E. P. Otocka, and F. A. Bovey, Macromolecules, 5, 574 (1972).

5.

J. C. Randall, J. Polym. Sci., Polym. Phys. Ed., 11, 275 (1973).

6.

D. E. Axelson, G. C. Levy and L. Mandelkern, Macromolecules, 12, 41 (1979).

7.

M. E. A. Cudby and A. Bunn, Polymer, 17, 345 (1976).

8.

J. C. Randall, J. Appl. Polym. Sci., 22, 585 (1978).

9.

F. A. Bovey, F. C. Schilling, F. L. McCrackin and H. L. Wagner, Macromolecules, 9, 76 (1976).

10. G. N. Foster, Polymer Preprints, 20, 463, (1979). 11. D. E. Axelson and W. C. Knapp (in press). 12. J. P. Hogan, C. T. Levett and R. T. Werkman, S. P. E. Journal 23(11), 87 (1967).


118

POLYMER CHARACTERIZATION BY ESR AND NMR

13. G. Kraus and C. J. Stacy, J. Polym. Sci., Symposium No. 43, 329 (1973). 14. S. D. Abbott, American Laboratory, August, 1977, p. 41. 15. D. R. Rueda, F. J. Balta Calleja and A. Hidalgo, Spectrochimica Acta, 30A, 1545 (1974). 16. D. R. Rueda, F. J. Balta Calleja and A. Hidalgo, Spectrochimica Acta, 35A, 847 (1979). Downloaded by CORNELL UNIV on October 5, 2016 | http://pubs.acs.org Publication Date: November 10, 1980 | doi: 10.1021/bk-1980-0142.ch006

17. A. Barlow, L. Wild, and R. Ranganath, J. Appl. Polym. Sci., 21, 3319 (1977). 18. L. Wild, R. Ranganath and A. Barlow, J. Appl. Polym. Sci, 21, 3331 (1977). 19. D. M. Grant and E. G. Paul, J. Am. Chem. Soc., 86, 2984 (1964). 20. J. K. Hughes, submitted for publication. 21. D. E. Axelson, L. Mandelkern, and G. C. Levy, Macromole cules, 10, 557 (1977). 22. T. C. Farrar and E. D. Becker, "Pulse and Fourier Transform NMR", Academic Press, New York (1971) p. 21. 23. J. C. Randall, "Polymer Sequence Determination: Carbon-13 NMR Method", Academic Press, New York (1977) p. 100. 24. Β. H. Zimm and W. H. Stockmayer, J. Chem. Phys., 17, 130 (1949). RECEIVED June

20, 1980.


Polymer Characterization by ESR and NMR - ACS Publications

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