Polymer Structure of Bituminous Coals - ACS Symposium Series

2 Polymer Structure of Bituminous Coals

Downloaded by PURDUE UNIV on May 29, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0071.ch002

JOHNW.LARSEN and JEFFREY KOVAC Department of Chemistry, University of Tennessee, Knoxville, TN 37916

1-3

Many structures have been proposed for bituminous coals. The only point of agreement is that these coals contain varying amounts of small molecules M ( W < 1000) which are extractable and that they also contain a mixture of larger molecules. While agreement is not universal, most coal chemists would accept van Krevelen's statement that "coal has a polymeric character", that it consists of macromolecules. There is no agreement about the size distribution of the macromolecules and their degree of cross linking. Consideration of the bulk, plastic properties of coals leads not only to the conclusion that coal is a cross linked macromolecular network, but also provides estimates of the number average molecular weight per cross link (Mc ) . It is convenient to consider coal structure on several levels. Following the protein chemists (at quite a distance) we will define three structural levels. This division has been done on the basis of experimental convenience but also provides a very useful conceptual framework allowing the complete structure to be treated as the sum of three nearly independent levels of description (substructures). This approach is much more useful than trying to solve the whole structure all at once. Different techniques and approaches are usually used to gain information about each of the structural levels. We find this division aids greatly in sharpening our thinking about coal structure. 4

F i r s t order s t r u c t u r e i s the s i z e d i s t r i b u t i o n of the macromolecules and molecules i n c o a l and the degree of cross l i n k i n g . The topology of the macromolecular network a l s o f a l l s i n t h i s category as does the amount and i d e n t i t y of the smaller v o l a t i l e or e x t r a c t a b l e molecules present i n the c o a l . While we know a good b i t about the smaller, e x t r a c t a b l e molecules i n c o a l i t has not been recognized that a s i g n i f i c a n t body of i n f o r m a t i o n l e a d i n g to a coherent p i c t u r e of the f i r s t order s t r u c t u r e of bituminous c o a l s e x i s t s . I n t h i s paper we attempt

0-8412-0427-6/78/47-071-036$05.00/0 ©

1978 American Chemical Society

Larsen; Organic Chemistry of Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1978.


2.

LARSEN AND KOVAC

37

Structure of Bituminous Coals

to sketch t h i s f i r s t order s t r u c t u r e . Second order s t r u c t u r e i s the chemical i d e n t i t y of the cross l i n k s and the s t r u c t u r e s of the carbon s k e l e t o n of c o a l . For example, consider the view that the macromolecules comprising bituminous c o a l s are composed of p o l y c y c l i c aromatic and hydroaromatic u n i t s l i n k e d together by methylene b r i d g e s and ether l i n k a g e s . The secondary s t r u c t u r e would then be the i d e n t i t y and q u a n t i t y of the v a r i o u s l i n k s between the aromatic and hydroaromatic u n i t s as w e l l as the average and extreme s t r u c t u r e s (carbon s k e l e t o n only) of those hydroaromatic u n i t s . Perhaps i t i s worth taking a l i t t l e time to deplore the current emphasis on "average" s t r u c t u r e s . T h i s emphasis i s understandable; average s t r u c t u r e s are so easy to d e r i v e that even a computer can do i t and they provide a convenient c r u t c h when one wants to draw a c o a l s t r u c t u r e . However, f o r most purposes, a knowledge of the extremes of the range of s t r u c t u r e s present w i l l be much more u s e f u l i n p r e d i c t i n g the chemical behavior of coals. F i n a l l y , the t h i r d order s t r u c t u r e of c o a l i s the nature and d i s t r i b u t i o n of the f u n c t i o n a l groups. This needs l i t t l e comment or explanation. Our knowledge of t h i s area i s more complete than the other two, but l a r g e holes remain. Superimposed on a l l of t h i s i s the p h y s i c a l s t r u c t u r e of c o a l , the Neavel f r u i t c a k e . — Each maceral w i l l have f i r s t , second, and t h i r d order s t r u c t u r e . Each c o a l w i l l be a d i f f e r e n t mixture of macérais. F a c t o r s such as pore s t r u c t u r e w i l l have a strong e f f e c t on observed r e a c t i v i t y . The Nature of the Macromolecular Network. In t h i s paper we are concerned only w i t h f i r s t order s t r u c t u r e . We begin by c o n s i d e r i n g the time dependent response of bituminous coals to a constant s t r e s s and show that t h i s behavior i s cons i s t e n t only w i t h bituminous c o a l being a cross l i n k e d , three dimensional macromolecule. I f a cross l i n k e d polymer network i s subjected to a s t r e s s , i t w i l l deform u n t i l i t reaches the l i m i t set by the cross links. Chains w i l l be s t r e t c h e d and s t r a i g h t e n e d as the m a t e r i a l deforms, but the presence of covalent cross l i n k s places a l i m i t on t h i s deformation. Thus, a p l o t of s t r a i n (eg. length of a piece of c o a l under tension) vs_ time w i l l i n c r e a s e then l e v e l out. Furthermore, when the s t r e s s i s removed, the polymer w i l l r e t u r n to i t s o r i g i n a l dimensions. I f the macromolecules are entangled or h e l d together by weak f o r c e s , there w i l l be no i n t e r n a l l i m i t on the flow and the deformation w i l l continue to i n c r e a s e w i t h time. Thus w i t h a polymer which i s not cross l i n k e d a constant f i n i t e value w i l l not be reached and the s t r a i n w i l l not be r e c o v e r a b l e . Very high entangled m a t e r i a l s may be e x c e p t i o n a l , but given the h i g h l y planar s t r u c t u r a l u n i t s i n c o a l t h i s seems most u n l i k e l y . There i s some data i n the l i t e r a t u r e d e s c r i b i n g the time ç 1 7


ORGANIC CHEMISTRY OF COAL


38



2.

LARSEN AND KOVAC


39

dependence of s t r e s s on s t r a i n f o r c o a l s . Much of i t i s o l d work done to study the behavior of c o a l i n v a r i o u s processing operations, such as g r i n d i n g . — The r e s u l t s o f t e n have not been completely described and the work o f t e n was done on l a r g e samples and so some of the behavior might be due t o p h y s i c a l flaws such as cracks and not be r e p r e s e n t a t i v e of the i n t r i n s i c molecular s t r u c t u r e of the c o a l . The best study i s that of Morgans and Terry and the s t r a i n - t i m e curve f o r Barnsley Hards bituminous c o a l i s shown i n F i g . 1.— Only 0.5% of the t o t a l s t r a i n was i r r e c o v e r a b l e . A C l a r a i n showed 1% unrecoverable s t r a i n . — These data a r e q u i t e c o n s i s t e n t w i t h bituminous c o a l being c o v a l e n t l y c r o s s - l i n k e d macromolecules i n which hydrogen bonding and van der Waals f o r c e s c o n t r i b u t e i n only a small way to macromolecular a s s o c i a t i o n . This s t r u c t u r e i s a l s o i n c o n s i s t e n t with pure entanglement model. However, i f there i s a mixture of c r o s s - l i n k s and entanglements, measurements such as these cannot d i s t i n g u i s h t h i s from a cross l i n k e d s t r u c t u r e . I f there a r e enough chemically bonded cross l i n k s t o complete the polymer network, the modulus w i l l be increased by entanglements and both s t r u c t u r e s i n F i g . 2 w i l l behave the same i n a l i n e a r compression or extension experiments.

Figure 2.

Entanglement and cross-linked networks

The p r i n c i p l e p o i n t here i s that bituminous c o a l s a r e c o v a l e n t l y cross l i n k e d macromolecules. Weak a s s o c i a t i v e f o r c e s such as hydrogen bonds and van der Waals i n t e r a c t i o n s cannot e x p l a i n the r e s u l t s of these experiments. Average Molecular Weight Per Cross L i n k . Having e s t a b l i s h e d that c o a l i s a cross l i n k e d macromolecule, the next step i s to determine the frequency of the cross l i n k s . There a r e two independent approaches which can be used. The f i r s t i s to u t i l i z e data from s o l v e n t s w e l l i n g of c o a l and the



40


second i s to use the moduli derived from the behavior of the polymer under s t r e s s to c a l c u l a t e the number average molecular weight per cross l i n k (M ) . I f the macromolecules i n c o a l are cross l i n k e d to form a network, i t i s p o s s i b l e to use a q u a n t i t a t i v e model of the e l a s t i c p r o p e r t i e s of a polymer network to o b t a i n an estimate of the average frequency of the cross l i n k s . For an i s o t r o p i c network only one parameter, the e l a s t i c modulus, c o n t r o l s the p h y s i c a l p r o p e r t i e s of the network. This parameter can be measured by any one of a v a r i e t y of experiments. Data are a v a i l a b l e f o r solvent s w e l l i n g and mechanical measurements of Young's modulus of bituminous c o a l s so these experiments w i l l be used as inde pendent methods of determining the e l a s t i c modulus. I t should be emphasized that the two experiments are measuring e s s e n t i a l l y the same q u a n t i t y . In t h i s paper the simplest s t a t i s t i c a l theory of polymer network e l a s t i c i t y * w i l l be used to c a l c u l a t e the number average molecular weight per cross l i n k (M ) from the two d i f f e r e n t kinds of measurements. Before p r e senting the experimental data we w i l l d e s c r i b e the s t a t i s t i c a l theory of polymer network emphasizing the p h y s i c a l model on which i t i s based and presenting the equations necessary to c a l c u l a t e M . The i n t e r e s t e d reader w i l l f i n d a l u c i d e x p o s i t i o n of the subject of polymer network e l a s t i c i t y i n The Physics of Rubber E l a s t i c i t y by L.R.G. T r e l o a r . — The s t a t i s t i c a l theory of polymer network e l a s t i c i t y assumes that the e l a s t i c r e s t o r i n g f o r c e i n a polymer network r e s u l t s from s t r e t c h i n g each chain i n the network from i t s most probable (equilibrium) conformation to a l e s s probable (stretched) conformation. Only the entropie f a c t o r s are considered, the i n t e r m o l e c u l a r f o r c e s are ignored. For the r e l a t i v e l y long chains found i n many polymers (eg. n a t u r a l rubber) i t i s safe to assume that the p r o b a b i l i t y of any chain conformation i s given by a Gaussian d i s t r i b u t i o n . — The entropy i s then given by Boltzmann's formula 1 1

1 2

S = -k In Ρ

(1)

where Ρ i s the p r o b a b i l i t y , and the f o r c e (f) i s obtained by d i f f e r e n t i a t i o n with respect to length (1), f = - | f

(2)

I t i s a l s o assumed that each chain i n the network c o n t r i b u t e s independently to the f o r c e . P h y s i c a l l y t h i s means that entangle ments between chains are not important and that i n the model chains are allowed to pass through each other as the network i s extended. T h i s i s sometimes c a l l e d the phantom chain assumption. With these assumptions one can d e r i v e s t r e s s - s t r a i n r e l a t i o n s h i p s f o r v a r i o u s experiments. For example, f o r a sample extended i n one dimension by a f a c t o r


2.

LARSEN AND KOVAC

41


χ -

s t r a i n e d length unstrained

length 2

f = G (λ - λ " ) where G i s the e l a s t i c

(3)

(shear) modulus

f

G =

W


C

In eq. (4) ρ i s the d e n s i t y , R the gas constant, Τ the absolute temperature and M the average molecular weight per cross l i n k . G appears as the s i n g l e e l a s t i c constant i n a l l s t r e s s - s t r a i n measurements. Since p, R, and Τ are a l l experimental q u a n t i t i e s any measurement of G allows a c a l c u l a t i o n of M . A convenient method of determining M i s 6y e q u i l i b r i u m solvent s w e l l i n g . I f a polymer network i s brought i n t o contact with a good s o l v e n t , the solvent w i l l be absorbed by the network u n t i l the s w e l l i n g pressure i s e x a c t l y balanced by the e l a s t i c f o r c e of the network. Using the s t a t i s t i c a l theory of polymer network e l a s t i c i t y and the Flory-Huggins theory of polymer s o l u t i o n s , the f o l l o w i n g equation can be d e r i v e d . — M

c

=

P

I 2

V

V

2

1

t-

l n

1

V

( - 2

)

V

~2

"

x

V

2

]

( 5 )

where p i s the o r i g i n a l d e n s i t y of the polymer, V the molar volume of the s o l v e n t , V the volume f r a c t i o n of the polymer at e q u i l i b r i u m , and χ the F l o r y i n t e r a c t i o n parameter r e l a t e d to the heat of t r a n s f e r of solvent from pure solvent to the pure polymer i n u n i t s of k T . — χ must be determined experimentally but given χ, then M i s easy to o b t a i n from e q u i l i b r i u m swel ling. One measurement of solvent s w e l l i n g was made by Sanada and H o n d a — who studied the s w e l l i n g of a number of Japanese c o a l s i n p y r i d i n e a f t e r e x t r a c t i n g them with p y r i d i n e . The choice of solvent i s not a good one s i n c e some of i t i s incorporated i n t o the c o a l . — Unfortunately a numerical e r r o r was made i n calculating M so the values shown i n the paper are i n c o r r e c t . The c o r r e c t values f o r the molecular weights are given i n Table 1. The r e s u l t s are numerically reasonable and behave as expected. The sudden drop with high rank coals (Yatake) i s expected, due to the approach of a more graphite l i k e , more h i g h l y cross l i n k e d s t r u c t u r e . The general increase i n M from low to high rank (excluding a n t h r a c i t e s ) can be a s c r i b e d £o the presence of l a r g e r molecules assembled together to form the macromolecules or to fewer cross l i n k s i n the higher rank c o a l s . The f i r s t explanation i s more i n tune with c u r r e n t thought. A rather d i f f e r e n t a p p l i c a t i o n of the same p r i n c i p l e i s contained i n a paper by K i r o v et a l . — They studied the e q u i l i b r i u m s w e l l i n g or three c o a l s by 17 s o l v e n t s . In t h i s way 2

2

C


42


Table I .

Average Molecular Weight per Cross Link i n Some Japanese Coals

Coal

% C

% H

M c

65.1

5.0

1190

Nakago

74.3

5.3

684

Takamatsu

79.0

5.0

832

Bibai

80.9

5.9

825

Ashcbetsu

81.1

5.5

1120

Yubari - I

84.9

6.2

1640


Odaira

Yubari - I I I

85.2

6.3

1500

Hoshima

86.6

5.6

1840

Yatake

88.7

4.4

Hongei

93.0

3.3

a)

assuming χ = 0.8


49.^

2. LARSEN AND KOVAC


43

they were able to c a l c u l a t e the best values of χ and V to f i t t h e i r data (V i s the molecular volume per cross l i n k ) ? T h e i r values of V were converted to M by m u l t i p l y i n g by an estimated d e n s i t y f o r t h e s e c o a l s of 1.3. 0

Table I I .

C

Average Molecular Weight per Cross L i n k f o r Some A u s t r a l i a n Coals

Coal

M

% H

% C


c Hebe

75.9

4.5

1530

Greta

82.4

6.2

1200

Bulli

88.2

5.1

522

The agreement between the two s e t s of r e s u l t s i s s u r p r i s i n g l y good. The drop o f f a t very high rank i s reproduced and the order of magnitude of the M values i s the same. T h i s s t r o n g l y supports the n o t i o n that these values a r e reasonable. Values of M can a l s o be c a l c u l a t e d from measurements of the mechanical p r o p e r t i e s of c o a l . From s t u d i e s c a r r i e d out by van K r e v e l e n * i t i s c l e a r that the shear modulus G i s constant a t about 1.6 χ 1 0 dyne/cm f o r c o a l s having between 82 and 92% carbon. A f t e r t h i s i t r i s e s s h a r p l y , as expected due to the r a p i d l y i n c r e a s i n g cross l i n k i n g i n the a n t h r a c i t e s and n e a r - a n t h r a c i t e s . Using t h i s value f o r G, and assuming ρ = 1.3g. cm" , M = 2 . The G values used here were d e r i v e d from sound v e l o c i t y measurements. Bangham and M a g g s — have reported values f o r Young's Modulus f o r three c o a l s measured by compression. For a cross l i n k e d polymer which undergoes no volume change on compression, the shear modulus i s 1/3 Young's modulus. - - Table I I I contains t h e i r data and the c a l c u l a t e d values f o r M . The r e s u l t s are a l s o very s m a l l . Other values f o r Young's modulus f o r bituminous c o a l s can be found i n the literature. A l l a r e s i m i l a r and g i v e very low values f o r M^. 1 7

1 8

1 0

2

3

5

LZ

Table I I I .

Values f o r M C a l c u l a t e d from the Young's Modulus of Bangham and Saggs—

Coal

Young's Modulus 2 (dyne/cm )

Welsh A n t h r a c i t e Welsh Steam Coal Northumberland House Coal The

40 χ 1 0

9

7 χ 10

9

q

20 χ 10

Shear Modulus 2 (dyne/cm ) 13 χ 1 0

9

2.3 χ 1 0

9

M

c

(g/mole)

ç 6.7 χ 10

2.5 14

4.8

2 3 10 - 10 discrepancy i n M^ between the solvent s w e l l i n g



44


and the s t r e s s - s t r a i n measurements must be i n v e s t i g a t e d . Both solvent s w e l l i n g and the s t r e s s - s t r a i n measurements give i n f o r mation about the same property of the network, the e l a s t i c modulus. This modulus i s then used to c a l c u l a t e Μ , using the same model i n both cases. This descrepancy must be due to experimental d i f f e r e n c e s and not to some f a i l u r e of the model. There must be d i f f e r e n t f a c t o r s operating i n the two types of experiments. In some polymer systems, s t r e s s induced c r y s t a l l i z a t i o n w i l l produce a l a r g e modulus and t h e r e f o r e a small value of M · Perhaps an increase i n i n t e r m o l e c u l a r i n t e r a c t i o n s due to the increased order i n u n i d i r e c t i o n a l s t r e s s i s worth i n v e s t i g a t i n g . One sure source of some of the descrepancy' i s that extracted coals were used f o r the solvent s w e l l i n g and unextracted c o a l s were used i n the s t r e s s - s t r a i n measurements. The presence of small molecules w i t h i n the polymer network w i l l have s e v e r a l e f f e c t s ; the density of the system w i l l change, and small molecules w i l l occupy volume i n the network thus h i n d e r i n g the i n t e r n a l motions of the chains. The h i n d e r i n g of the motion w i l l tend to increase the modulus of the network and hence lower the apparent molecular weight. We propose two q u a l i t a t i v e arguments to e x p l a i n part of the discrepancy. 1) Swollen network theory Consider the unextracted c o a l to be a swollen polymer network. For concreteness assume a t y p i c a l value of 30% e x t r a c t a b l e m a t e r i a l and 70% network m a t e r i a l . According to the simple theory of rubber e l a s t i c i t y the modulus of a swollen rubber i s given by

G' =

ψ

V f (6) c where V i s the volume f r a c t i o n of the network and ρ i s the density of the unswollen r u b b e r . — I f one d i d not consider the e f f e c t of s w e l l i n g than the modulus would be given by f

2

G = ψ (7) M c G (or G') i s an experimental quantity and we are i n t e r e s t e d i n the values of M obtained from d i f f e r e n t i n t e r p r e t a t i o n s of G. Hence, we equate the r i g h t hand sides of (1) and (2) pRT p'RT 1/3 M M 2 c c (8) =

f

Μ·

C _

P'VJ

/

3

£

Μ

" ρ c The r a t i o MVM represents the r a t i o of the true to the apparent M ? fy . i s always l e s s than u n i t but f o r V = 0.7, V| = 0.9 so V| can be assumed to be near one. The question 1

c

3

2


2. LARSEN AND KOVAC


then i s the change i n d e n s i t y with s w e l l i n g . I f upon e x t r a c t i n g the c o a l there i s n e g l i g i b l e volume change, then the swollen density p' w i l l be greater than the unswollen d e n s i t y p. To get an estimate, assume u n i t d e n s i t y of the swollen network, equal d e n s i t i e s of network and e x t r a c t a b l e m a t e r i a l and no change of volume upon e x t r a c t i n g . Then f o r 70% network we obtain M

1 / 3

c K0.7) M 0.7 c Hence the true molecular weight w i l l be l a r g e r than the apparent M . (2) Intermolecular O b s t r u c t i o n s . The theory here was developed by J . L. Jackson and co-workers. I t i s based on the n o t i o n that the polymer network i s excluded from p a r t of the volume a v a i l a b l e to i t . The " i n t e r m o l e c u l a r o b s t r u c t i o n s " are taken i n t o account by means of a s i t e f r a c t i o n occupied by the polymer on a l a t t i c e . A smaller s i t e f r a c t i o n represents a l a r g e r number of obstruc tions. In the improved theory Jackson obtains the f o l l o w i n g s t r e s s - s t r a i n r e l a t i o n s h i p f o r l i n e a r extension. 1


±

2

β

Ζ

7

/

2 1 9 2 2

C

This i s eq. (18) i n Jackson's p a p e r . — f i s the s i t e f r a c t i o n and M the number of segments between cross l i n k s . I t i s harder to compare the moduli here because of the form of the equation. We w i l l d e f i n e the modulus at a p a r t i c u l a r s t r a i n as

c c To o b t a i n an estimate of the e f f e c t s of the s i t e f r a c t i o n we w i l l solve eq. (5) f o r v a r i o u s values of f f o r the s p e c i a l case

pRT M

=

100

λ

=

1.1

Jackson c a l c u l a t e s f - 0.95 - 0.98 f o r t y p i c a l rubbers so we w i l l assume that i n the obstructed network f = 0.95 and w i l l c a l c u l a t e the change i n f o r v a r i o u s degrees of e x t r a c t i o n .


45

46

Table


4.

Values of M

c

f o r Various S i t e F r a c t i o n s


Site Fraction (f)

M

c

0.95

5.61

0.85

4.03

0.75

3.15

0.65

2.51

The t a b l e shows c l e a r l y that f o r a f i x e d modulus the apparent chain l e n g t h must increase as the number of o b s t r u c t i o n s i n c r e a s e s . I f we consider the e x t r a c t e d c o a l to have a s i t e f r a c t i o n of 0.7 and the unextracted c o a l to have a s i t e f r a c t i o n of 0.95 then there w i l l be a f a c t o r of two d i f f e r e n c e i n the apparent M . Both She arguments developed here are q u a l i t a t i v e but they both show that the molecular weight d e r i v e d from the simple theory f o r the unextracted c o a l i s probably smaller than the r e a l M · Using the Jackson theory one obtains at l e a s t a f a c t o r of two d i f f e r e n c e . The explanations given here are based on simple models and are meant only to be suggestive. The very l a r g e modulus observed i n l i n e a r extension experiments could a l s o be due to i n t e r m o l e c u l a r f o r c e s not accounted f o r i n the s t a t i s t i c a l theory. For example, Su and Mark and co-workers have r e c e n t l y determined that the l a r g e i n c r e a s e i n modulus at high extension of rubbers i s due to s t r e s s - i n d u c e d c r y s t a l l i z a t i o n . — A s i m i l a r phenomenon could occur i n the unextracted c o a l s . A proper r e s o l u t i o n of the discrepancy must come from a systematic study of the molecular weight of e x t r a c t e d c o a l s by both methods. Use of the S t a t i s t i c a l Model. In t h i s paper the s t a t i s t i c a l theory of polymer network e l a s t i c i t y has been used to c a l c u l a t e the average molecular weight per cross l i n k i n v a r i o u s c o a l s from experimental data on s o l vent s w e l l i n g and mechanical behavior. Although the numerical values f o r M obtained from the solvent s w e l l i n g data are q u i t e reasonable, Ehey should only be regarded as order of magnitude estimates because the assumptions u n d e r l y i n g the s t a t i s t i c a l theory are probably not f u l f i l l e d i n the c o a l s t r u c t u r e . Some of the assumptions w i l l be discussed i n the f o l l o w i n g paragraphs along with the p o s s i b i l i t y of c o r r e c t i n g them. The primary assumption of the s t a t i s t i c a l theory i s that the chains obey Gaussian s t a t i s t i c s , that i s , that the proba b i l i t y d i s t r i b u t i o n of the end-to-end d i s t a n c e (R) has the form P(R) = constant χ e



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I t i s easy to show that any chain w i l l obey Gaussian s t a t i s t i c s i f i t i s both long and f l e x i b l e . — The chains i n the c o a l network are u n l i k e l y to be e i t h e r long or f l e x i b l e . The molecu l a r weights obtained are of the order of 1000 which correspond to 10-15 aromatic r i n g s per c h a i n . A chain composed l a r g e l y of fused aromatic r i n g s i s b e t t e r c h a r a c t e r i z e d as short and stiff. A simple modified Gaussian theory i s c u r r e n t l y being developed to t r y to more a c c u r a t e l y represent the c o a l n e t w o r k . — P r e l i m i n a r y c a l c u l a t i o n s i n d i c a t e that M w i l l be l a r g e r by approximately a f a c t o r of two using the modified Gaussian model. A f u r t h e r and more d i f f i c u l t problem i s the assumption that the chains a c t independently. Entanglements and i n t e r molecular f o r c e s which tend to i n c r e a s e the modulus could w e l l be important as w e l l as "loose-end" c o r r e c t i o n s which tend to decrease the modulus. The phenomenon of s t r e s s induced c r y s t a l l i z a t i o n might a l s o be found. Developing a model to account f o r cooperative network e f f e c t s w i l l be d i f f i c u l t . In s p i t e of these drawbacks, the r e s u l t s of a p p l y i n g t h i s model to c o a l are reasonable, and at l e a s t a good beginning i n our attempts to determine the f i r s t order s t r u c t u r e of c o a l . I f one accepts the M values f o r bituminous c o a l s are of the order of 1500-1800, estimates of the number of subunits per cross l i n k can be made. Here we assume that bituminous c o a l s are composed of aromatic and hydroaromatic u n i t s l i n k e d t o gether. The Heredy-Neuworth depolymerization i s thought to cleave the a l k y l chains l i n k i n g the aromatic u n i t s . — Most of the bituminous c o a l s which have been depolymerized g i v e products having number average molecular weights between 300 and 500,-^ although a few are l a r g e r . Accepting these r e s u l t s at face value (molecular weight d i s t r i b u t i o n s have not been done), one concludes that the average cross l i n k chain contains 3-6 aromatic u n i t s . In a very i n s i g h t f u l paper, van Krevelen t r e a t e d c o a l as a cross l i n k e d polymer g e l and the e x t r a c t a b l e m a t e r i a l as the unreacted "monomer".— T h i s treatment leads to an average molecular weight value of 400 f o r the s t r u c t u r a l u n i t . T h i s i s i n e x c e l l e n t agreement with the depolymerization r e s u l t s . This h i g h l y cross l i n k e d s t r u c t u r e f o r c o a l suggests that donor s o l v e n t l i q u e f a c t i o n i n v o l v e s major degradation of the coal. I t a l s o has i m p l i c a t i o n s f o r the use of s o l u b i l i t y d i f f e r e n c e s to f o l l o w the progress of c o a l conversion ( i . e . asphaltenes, preasphaltenes, etc.) I t must be emphasized that t h i s i s the f i r s t systematic use of bulk s o l i d p r o p e r t i e s of c o a l to a determination of i t s s t r u c t u r e . The c o n c l u s i o n that c o a l i s a three dimensionally cross l i n k e d macromolecular network r e s t s on a q u a l i t a t i v e treatment of s t r a i n - t i m e curves and seems sound. The use of two d i f f e r e n t techniques, s t r e s s - s t r a i n measurements and s o l v e n t s w e l l i n g , to measure the e l a s t i c modulus of the network gives c o n f l i c t i n g r e s u l t s which are probably due to d i f f e r e n c e s i n

American Chemical Society Library 16th St. N. W. Washington, D. C. 20036 1155



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the sample pretreatment and d i f f e r e n c e s which are a r t i f a c t s of the experimental procedure. One technique was a p p l i e d to a swollen or obstructed network (whole c o a l - the e x t r a c t a b l e m a t e r i a l i s d i s s o l v e d i n and swells or o b s t r u c t s the network) and the other to an e x t r a c t e d , unswollen network. To examine t h i s s t r u c t u r a l hypothesis f u r t h e r we a r e attempting to r e s o l v e the discrepancy between the s t r e s s s t r a i n and solvent s w e l l i n g measurements and to d e r i v e a s t a t i s t i c a l mechanical theory of polymer network e l a s t i c i t y which f i t s bituminous c o a l s . This work i s i n progress.


Acknowledgement : We are g r a t e f u l to the Department of Energy (JWL), NSF (JWL), and the F a c u l t y Research Fellowship Fund of the U n i v e r s i t y of Tennessee (JK) f o r support of t h i s work. Abstract The p h y s i c a l p r o p e r t i e s of bituminous coals can be used to determine the nature of t h e i r macromolecular s t r u c t u r e . Bitu minous coals are c r o s s - l i n k e d macromolecular networks and a pre l i m i n a r y estimate of the number average molecular weight per cross l i n k is 1500-1800.

Literature Cited 1. Vahrman, Μ., Chem. in Britain, 8, 16 (1972); Fuel, 49, 5 (1970). 2. Sternberg, Η., Storch Award Address, Fuel Div. Preprints, ACS National Meeting, Sept. 1976. 3. van Krevelen, D. W., Elements of Coal Chemistry Rotterdam, 1948, p. 170 Dryden, I.G.C., Chem. and Ind., 502 (1952); Fuel, 30, 39 (1952). 4. van Krevelen, D. W., Coal Elsevier Publishing Co., New York, N.Y., 1961, p. 440. 5. Neavel, R., (Exxon-Baytown) numerous lectures and seminars. 6. Ferry, John D., Viscoelastic Properties of Polymers, John Wiley and Sons, Inc., New York, 1961. 7. van Krevelen, D. W., and Hoftyzer, P. J., Properties of Polymers, Elsevier Publishing Co., New York, 1972. 8. The most recent summary is Brown, R. L. and Hiorno, F. J., in Chemistry of Coal Utilization, Suppl. Vol., H. H. Lowry Ed., John Wiley and Sons, Inc., New York, 1963. 9. Morgons, W. Τ. Α., and Terry, Ν. B., Fuel, 37, 201 (1958). 10. Macrae, J . C. and Mitchell, A. R., Fuel, 36, 423 (1957). 11. Treloar, L. R. G., The Physics of Rubber Elasticity, Clarendon Press, Oxford, 1975. 12. Flory, P. J., Principles of Polymer Chemistry, Cornell University Press, Ithaca, N.Y., 1953.



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Coals

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13. Flory, P. J., Statistical Mechanics of Chem. Molecules, John Wiley and Sons, New York, 1969. 14. Sanada, Y., and Honda, H., Fuel, 45, 295 (1966). 15. Collins, C. J., Hagaman, E. W., and Raan, V. F., Coal Technology Quarterly Report, Oak Ridge National Laboratory, ORNL 5252, Dec. 31, 1976, p. 143. 16. Kirov, N.Y., O'Shea, J . Μ., and Sergeant, G. D., Fuel, 47, 415 (1968). 17. Schuyer, J., Dijkstra, H., and van Krevelen, D. W., Fuel, 33, 409 (1954). 18. van Krevelen, D. W., Chermin, H. A. G., and Schuyer, J., Fuel, 38, 438 (1959). 19. Bangham, D. H., and Maggs, F. A. P., Proceedings of a Conference on the Ultra-fine Structure of Coals and Cokes, BCURA, 1944, Lewis, H. K. and co., Ltd., London, p. 118. 20. Jackson, J. L., Shen, M. C., and McQuarrie, J. Chem. Phys., 44, 2388 (1966). 21. Jackson, J. L., J. Chem. Phys., 57, 5124 (1972). 22. Kovac, J., "Modified Gaussian Model of Rubber Elasticity," Macromolecules, in press. 23. Heredy, L. Α., and Neuworth, M. B., Fuel, 41, 221 (1962). 24. Larsen, J. W. and Kuemmerle, E. W., Fuel, 55, 162 (1976). 25. van Krevelen, D. W., Fuel, 47, 229 (1966). 26. Su, T. Κ. and Mark, J. E., Macromolecules, 10, 120 (1977). RECEIVED April 5, 1978


Polymer Structure of Bituminous Coals - ACS Symposium Series

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