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STRUCTURE OF NATURAL MACROMOLECULAR
SOLIDS
C. Wert, M. Weller
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C10, supplkment au n 0 1 2 , Tome 46, dkcembre 1985 page C10-561
STRUCTURE OF NATURAL MACROMOLECULAR SOLIDS
C.A. WERT AND M. WELLERf
Department of Metallurgy and Mining Engineering and Materials Research Laboratory, University of Illinois, Urbana, IL 61801, U.S.A.
'Max-Planck-Institut fur Metallforschung, Institut
fur
Werkstoffwissenschaften, D-7000 Stuttgart, F.R.G.
Abstract
-
I n t e r n a l f r i c t i o n and d i e l e c t r i c l o s s measurements on coal, o i l shale, amber and o t h e r n a t u r a l macromolecular m a t e r i a l s show a n e l a s t i c c h a r a c t e r s i m i l a r t o t h a t o f s y n t h e t i c polymers. Macromolecular u n i t s which undergo r e o r i e n t a t i o n under e i t h e r a mechanical o r e l e c t r i c a l s t r e s s produce a c h a r a c t e r i s t i c l o s s spectrum. Thi; paper shows general features o f l o s s spectra i n coal and amber, describes t h e k i n e t i c s o f t h e r e l a x a t i o n s and proposes molecular models f o r these macromolecular m a t e r i a l s .I
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INTRODUCTIONThe s t r u c t u r e o f n a t u r a l macromolecul a r m a t e r i a l s can be examined by many techniques. One method, energy l o s s o f mechanical o r e l e c t r i c a l o r i g i n , can
e l u c i d a t e important f e a t u r e s o f c e r t a i n macromolecular u n i t s . We have r e p o r t e d such measurements on a v a r i e t y o f s o l i d s : coal, o i l shale, amber, wood, c e l l u l o s e and l i g n i n 11-41. Here we describe f u r t h e r features o f t h e k i n e t i c s of t h e r e l a x a t i o n s and use t h e observations t o v e r i f y macromolecular models p r e v i o u s l y assigned. Combined i n t e r n a l f r i c t i o n and d i e l e c t r i c l o s s techniques permit p r o b i n g o f t h e s t r u c t u r e o f s o l i d s i n a s e n s i t i v e manner. The response t o mechanical o r e l e c t r i c a l stresses produces a spectrum which can be i n t e r p r e t e d as r e o r i e n t a t i o n under s t r e s s o f p a r t i c u l a r macromolecular u n i t s . Combined use o f t h e techniques permits
i d e n t i f i c a t i o n o f molecular u n i t s which have e i t h e r mechanical o r e l e c t r i c a l dip01 a r character. This combination aids i n t h e designation o f models.
I 1
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EXPERIMENTSInternal f r i c t i o n measurements near 1Hz were c a r r i e d out i n an i n v e r t e d t o r s i o n a l pendulum. Samples about 50 x 1.5 x 1.5 mm were c u t from s o l i d coals, mainly i n t h e bedding plane. They were glued i n t o brass p i n s which f i t t e d i n t o t h e t o r s i o n a l pendulum. Measurements were f i r s t made on bituminous coal : a t y p i c a l measurement i s shown by curve "a" i n Fig. 1. Prominent peaks are seen near 130, 200 and 400K. T h i s coal had carbon content about 82% by weight. Measurements f o r a coal w i t h carbon content near 77% C i s shown by curve "b". Measurements on an a n t h r a c i t e of about 92% C i s shown by a curve "c". One sees t h a t t h e t h r e e peaks f o r these,coals occur a t about t h e same place. This study i s focused on t h e lower temperature peaks, c a l l e d t h e y - and B-peaks.
C10-562 JOURNAL DE PHYSIQUE
0
100
200
300
400
K
100
200
300
K
TEMPERATURE TEMPERATURE
Fig. 1. I n t e r n a l f r i c t i o n peaks o f 3 Fig. 2. I n t e r n a l f r i c t i o n o f coal coals a t
1Hz.
i n t h e k i l o c y c l e range. The curves i n Fig.1
show t h a t even though t h e peaks appear a t about t h e same temperature f o r t h e d i f f e r e n t coals, they d i f f e r i n peak height. We have r e p o r t e d e a r l i e r t h e v a r i a t i o n of peak h e i g h t w i t h carbon c o n c e n t r a t i o n /4/. The present study i s concerned w i t h t h e k i n e t i c s o f r e o r i e n t a t i o n of t h e u n i t s c o n t r i b u t i n g t o t h e energy loss. Some data have been r e p o r t e d e a r l i e r /I/, b u t we a m p l i f y those r e s u l t s w i t h a d d i t i o n a l measurements.The y-peak has p r e v i o u s l y been measured a t frequencies near
1Hz
and w i t h e l a s t i c a f t e r - e f f e c t measurements. We have made a d d i t i o n a l measurements near1,000Hz
u s i n g f l e x u r a l o s c i l l a t i o n s of a t h i n reed c u t from coal. This measurement i s shown i n Fig. 2, t h e curve designated "W". A broad maximum i s seen i n t h e i n t e r n a l f r i c t i o n spectrum j u s t above200K.
S i m i l a r measurements near50,000Hz
have been made by Gibala and by Ledbetter /5/. T h e i r measurements are a l s o sketched i n Fig.2
w i t h designations "6" and " L o o . A general s i m i l a r i t y e x i s t s among these measurements. We have made a t r i a l decomposition o f t h e data and propose t h a t t h e broadened peak i s a merging of t h e two peaks y and B. With t h e p o s i t i o n s f o r7000Hz
designated by t h e dashed l i n e s i n Fig.2,
an Arrhenius p l o t f o r t h e y-peak i s given i n Fig. 3. The data over8
orders of magnitude i n frequency f i t an equation T = T, exp (H/kT).H
=0.18
eV.10-ls
-
1 0 - ~
-
'C1 f 3
-
H
=0.18
eV
-
-
1,
1/T .004 .008.012
.003
.DO4
.OG5
Fig. 3. Arrhenius p l o t f o r y-peak i n coal. Fig. 4. Arrhenius p l o t f o r 6-peak-
AMBER0
-
- .004 .006 .008 . 0 1 Fig. 5. Crossover o f y- and B-peaks
f o r c o a l and amber.
150 250 350 K TEMPERATURE
F i g . 6. D i e l e c t r i c l o s s peaks o f c o a l . We have made a s i m i l a r f i t t o an A r r h e n i u s e q u a t i o n f o r t h e 8-peak ( s e e F i g . 4) and f i n d t h e f o l l o w i n g c o n s t a n t s :
It i s n o t s u r p r i s i n g t o see why t h e two s p e c t r a merge a t h i g h f r e q u e n c i e s i f t h e d a t a f o r t h e y - and B-peaks a r e p l o t t e d t o g e t h e r . One sees t h a t t h e v a l u e s o f t h e parameters i n t h e A r r h e n i u s e q u a t i o n f o r t h e t w o r e l a x a t i o n s cause them t o c r o s s a t about 50 kHz, F i g . 5.
Dielectric
loss
measurements
were made on s m a l l w a f e r s c u t f r o m c o a l and amber a tf r e q u e n c i e s between 100 and 10,000Hz. An example o f one o f t h e s e measurements i s shown i n F i g . 6 f o r a c o a l o f c o m p o s i t i o n about 79% C. One sees o n l y one d i e l e c t r i c l o s s peak, which we i d e n t i f y w i t h t h e B-peak. The peak p o s i t i o n s f o r t h e t h r e e f r e q u e n c i e s a r e p l o t t e d as t h e open c i r c l e s a l o n g t h e l i n e Fig. 4. One sees a n i c e c o r r e l a t i o n , i n d i c a t i n g t h a t t h e mechanical and d i e l e c t r i c t i m e c o n s t a n t s a r e r e l a t e d by t h e same parameter t o t h e r e o r i e n t a t i o n r a t e c o n s t a n t /6/. The presence o f a d i e l e c t r i c l o s s peak a s s o c i a t e d w i t h t h e 6 - r e l a x a t i o n means t h a t t h e
macromolecular u n i t producing. t h i s re1 a x a t i o n i s b o t h a mechanical and e l e c t r i c a l d i p o l e . The absence o f t h e y - r e l a x a t i o n i n d i e l e c t r i c l o s s shows t h a t t h e macromolecular u n i t g i v i n g r i s e t o t h i s r e l a x a t i o n has no e l e c t r i c d i p o l e moment. These f a c t s a i d i n d e s i g n a t i o n o f a macromolecular model f o r t h e r e l a x a t i o n s .
Measurements o f b o t h mechanical and e l e c t r i c a l l o s s i n s e v e r a l ambers have a l s o been measured. These measurements a r e r e p o r t e d i n a n o t h e r paper i n t h i s volume.
R e l a x a t i o n t i m e s observed f o r a l l o f t h e ambers f i t o u r k i n e t i c s curves o f F i g . 5, as w e l l . These a r e d e s i g n a t e d by t h e symbols
"
0 " and " 0"
i n F i g . 5.I 1 1
-
DISCUSSIONThe measurements d e s c r i b e d i n t h i s paper a l l o w t h e d e s i g n a t i o n o f macromolecular models t o be made f o r b o t h o f t h e l o w t e m p e r a t u r e r e l a x a t i o n s . They a r e a l s o c o n s i s t e n t w i t h e a r l i e r models made f o r c o a l by o t h e r t y p e s of measurements /7,8/. We propose t h a t t h e y - r e l a x a t i o n i s caused by t h e t w i s t i n g under a mechanical s t r e s s o f methyl ene u n i t s -CH2-CH2-CH -CH2-. Because o f a n g u l a r and s p a t i a l r e s t r i c t i o n s on m o t i o n o f t h e s e c h a ~ n s , we g e l i e v e t h a t t h e r e o r i e n t a t i o n does n o t t a k e p l a c e as a s i n g l e step, b u t occurs by a d e f e c t r u n n i n g a l o n g t h e l i n e a r chain. A v a r i e t y of d e f e c t s have been proposed f o r methylene c h a i n s i n p o l y m e n a n d we suppose t h a t such d e f e c t s may e x i s t i n c o a l a l o n g t h e s e u n i t s /9-11/. These d e f e c t s p e r m i t r e l a x a t i o n i n s y n t h e t i c polymers /12/. The model i s c o n s i s t e n t w i t h t h e absence of a
( 2 0 - 5 6 4 JOURNAL DE PHYSIQUE
The B - r e l a x a t i o n i s t h o u g h t t o be a s s o c i a t e d w i t h r e o r i e n t a t i o n under s t r e s s o f small s i d e u n i t s on main c h a i n s o r a r o m a t i c r i n g s i n t h e coal. The s m a l l v a l u e o f
T, i m p l y s t h a t i t i s caused by m o t i o n o f a small u n i t such as OH, CH OH, COOH o r o t h e r s i m p l e s i d e chain. T h i s r e l a x a t i o n e x i s t s i n many polymers an8 has been proposed t o r e s u l t f r o m r e o r i e n t a t i o n o f such m o l e c u l a r u n i t . The e x i s t e n c e o f an e l e c t r i c a l d i p o l e moment which produces d i e l e c t r i c l o s s i s c o n s i s t e n t w i t h t h i s model. S i m i l a r r e l a x a t i o n s e x i s t i n o i l shale, a s p h a l t s , wood,. c o r k and o t h e r macromolecular m a t e r i a l s . We presume t h a t t h i s model i s a l s o a p p l i c a b l e t o t h e s e s o l i d s .
The h i g h temperature peak, t h e a - r e l a x a t i o n , i s a l s o seen i n many s y n t h e t i c
polymers. It has been r e l a t e d t o a gross r e o r i e n t a t i o n o f main c h a i n s under e i t h e r e l e c t r i c a l o r mechanical s t r e s s . It has a l s o been a s s o c i a t e d w i t h t h e g l a s s - t r a n s i t i o n - t e m p e r a t u r e f o r t h e s y n t h e t i c polymers.
ACKNOWLEDGEMENTS
The t e c h n i c a l a s s i s t a n c e of Miss Hamm, M r . Henke, Miss Lakemeyer and Mr. Rahn o f t h e M a x - P l a n c k - I n s t i t u t i s g r a t e f u l l y acknowledged. Support f o r C. Wert a l s o came f r o m t h e D i v i s i o n o f M a t e r i a l s Research, DOE under C o n t r a c t DE-AC02-76ER01198. Support d u r i n g t h e i n i t i a l stages o f t h i s i n v e s t i g a t i o n a l s o came t h r o u g h t h e Humboldt- S t i f t u n g , Bonn.
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