The Effects of Hostile Environments on Coatings and Plastics

4 Thermal Analysis of Metal-Containing Polymers: Generalizations CHARLES E. CARRAHER, JR. Wright State University, Department of Chemistry, Dayton, OH 45435

The topic of thermal characterizations of inorganic and organometallic polymers was recently reviewed by the author in a comprehensive review (1). The review emphasized particular findings and included a review of phosphorus-containing polymers and catenation polymers. The current chapter will emphasize general trends with respect to both instrumentation and results for metal-containing polymers with the realization that exceptions exist for most generalizations. The u s u a l thermal techniques o f thermal g r a v i m e t r i c analys i s (TG), d i f f e r e n t i a l thermal a n a l y s i s (DT), d i f f e r e n t i a l scanning c a l o r i m e t r y (DSC), thermal mechanical a n a l y s i s (TM), t o r s i o n a l b r a i d a n a l y s i s (TB) and p y r o l y s i s gas chromatography (PGC) have been u t i l i z e d i n d e s c r i b i n g the thermal p r o p e r t i e s o f i n o r g a n i c and o r g a n o m e t a l l i c polymers. Because o f the v a r i e t y o f bonding energies present i n many m e t a l - c o n t a i n i n g polymers, " s t a b i l i t y plateaus" occur where degradation followed by TG occurs through s e v e r a l somewhat d i s t i n c t steps being seen as temperature ranges where l i t t l e or no l o s s o f weight occurs compared with s i m i l a r temperature ranges where weight l o s s i s more r a p i d . We have u t i l i z e d t h i s to advantage i n e v a l u a t i n g thermal techniques s i n c e degradation pathways may be more c l e a r l y d e f i n e d . Counter, the presence o f l o w - l y i n g f i l l e d and u n f i l l e d d - o r b i t a l s , present f o r most metals, permits a myriad o f o p t i o n a l pathways f o r thermal a c t i v a t i o n followed by e v o l u t i o n , c r o s s l i n k i n g and/or rearrangement t o occur. Elements such as phosphorus, s i l i c o n , t i n , germanium and s u l f u r form catenated polymers s i m i l a r t o carbon, but such c a t e n a t i o n does not u s u a l l y l e a d to (homo) chains g r e a t e r than t f

f f

0097-6156/ 83/0229-0025S06.00/0 © 1983 American Chemical Society

EFFECTS OF HOSTILE ENVIRONMENTS

26

10. F u r t h e r , such products might be expected to (and i n f a c t do) o f f e r poorer thermal s t a b i l i t i e s than carbon-based polymers s i n c e t h e i r bond energies are g e n e r a l l y s i g n i f i c a n t l y l e s s ( f o r i n s t a n c e the C-C bond energy f o r t y p i c a l hydrocarbons i s about 82 kcal/mol; S-S bond energy i s about 58, S i - S i i s 53 kcal/mol, P-P i s 48, and Sn-Sn i s 39 k c a l / m o l ) . The a l t e r n a t i v e o f u t i l i z i n g heteroatom back-bones i s a t t r a c t i v e s i n c e the r e s u l t a n t bonds can e x h i b i t g r e a t e r bond energies than the C-C bond ( f o r i n s t a n c e , P-N has a bond energy o f 138 k c a l / mol; Be-0 i s 124, B-0 i s 113, S i - 0 i s 108, Si-N i s 104, B-C i s 89, and P-S i s 82, e x h i b i t i n g $ - i o n i c bond characters o f ~6 f o r B-C to 55 f o r Sn-0; many o f these bonds have π-bond c o n t r i b u t i o n s ) . The o f t e n r e l a t i v e l y high i o n i c bond character can s t r o n g l y i n f l u e n c e polymer behavior through chain and atom i n t e r a c t i o n s such as π-bond o v e r l a p s , chain s t i f f n e s s , ease o f undergoing rearrangement r e a c t i o n s and a b i l i t y to undergo associative reactions involving "low-lying" d - o r b i t a l s . In t h i s chapter macromolecules whose backbone or connective bridge c o n t a i n m e t a l - c o n t a i n i n g moieties which e x h i b i t s u f f i c i e n t covalent character so as t o possess d i r e c t i o n a l bonding w i l l be c o n s i d e r e d . Thus polymers such as polysodium a c r y l a t e w i l l be omitted s i n c e the metal i s bound through l a r g e l y i o n i c f o r c e s .

Generalizations G e n e r a l i z a t i o n s are appearing as the number o f thermally r e l a t e d s t u d i e s are i n c r e a s i n g . F o l l o w i n g i s a l i s t i n g o f these g e n e r a l trends along with a b r i e f d i s c u s s i o n . The term " c l a s s i c a l polymers" r e f e r to polymers as p o l y s t y r e n e and p o l y ethylene. 1. Thermal p r o p e r t i e s are approximately independent o f molecular weight a f t e r a DP o f about t h r e e . T h i s i s unusually low i n comparison with many more c l a s s i c a l polymers where such thermal p r o p e r t i e s as Τ , Τ and thermal degradation may vary through DP»s o f 100. 2. Most thermograms show s t a b i l i t y plateaus - the r e s u l t of the presence o f s i g n i f i c a n t l y d i f f e r e n t bond energies w i t h i n the polymer backbone or s i d e c h a i n . More c l a s s i c a l polymers g e n e r a l l y do not e x h i b i t such plateaus with "complete" v o l a t i l i z a t i o n o c c u r r i n g w i t h i n a few degrees a f t e r i n t i a l weight l o s s o c c u r s . Such s t a b i l i t y plateaus may be roughly d i v i d e d i n t o two c a t e g o r i e s - k i n e t i c a l l y independent plateaus which are independent o f h e a t i n g r a t e and time and k i n e t i c a l l y dependent plateaus which are dependent on both heating r a t e and time. As a crude r u l e -those plateaus which are p a r a l l e l t o the temper ature or time a x i s are k i n e t i c a l l y independent whereas those which slope downward are k i n e t i c a l l y dependent. In more c l a s s i c a l polymers the k i n e t i c a l l y dependent behavior has been u t i l i z e d to determine such f a c t o r s as s p e c i f i c r a t e constants and r a t e s δ

m

4.

CARRAHER

27

Thermal Analysis

of v o l a t i l i z a t i o n , order o f degradation and energy o f a c t i v a t i o n , but because o f seemingly g r e a t e r complexity o f organometallic polymer t h i s has thus f a r not been undertaken. 3. Organometallic polymers g e n e r a l l y undergo a darkening of c o l o r (often to a brown to black to gray) i n the 300 to 600°C range, becoming l i g h t e r (often white) about 900 to 1200°C. The c o l o r a t i o n may r e s u l t from formation o f aromatic intermedi a t e s . The subsequent l i g h t c o l o r corresponds to the c o l o r of the metal oxide ( i n a i r ) , normally white. 4. I n f r a r e d s p e c t r a l bands broaden f o r the residues as degradation proceeds. Some new bands form—some which are sharp i n d i c a t i n g formation of p r e f e r r e d intermediate r e s i d u e s . 5. I n i t i a l degradation i s o f t e n s i m i l a r i n a i r and under i n e r t c o n d i t i o n s c o n s i s t e n t with i n i t i a l degradation being nonoxidative. Subsequent degradation i s d i s s i m i l a r and c o n s i s tent with o x i d a t i v e l y degradation o c c u r r i n g i n a i r . DSC appears to be a more p r e c i s e i n d i c a t o r than TG when comparing degradation r o u t e s . Thus many TG thermograms appear s i m i l a r i n a i r and under an i n e r t atmosphere yet the DSC thermo grams are markedly d i f f e r e n t . 6. A number o f organometallic polymers undergo i n i t i a l degradation (with or without a s s o c i a t e d weight l o s s ) at r e l a t i v e l y low temperatures (50 to 200°C range) but with r e t e n t i o n of major p o r t i o n s o f the organic p o r t i o n to g r e a t e r than 400°C. T h i s i s g e n e r a l l y described as the product's having poor to moderate low temperature s t a b i l i t i e s and moderate to good high temperature s t a b i l i t i e s . 7. The i n i t i a l thermally induced t r a n s i t i o n s o f t e n a f f e c t m a t e r i a l processing p r o p e r t i e s as Τ , Τ and s o l u b i l i t y and o f t e n i n v o l v e s rearrangement about fhe metal atom. T h i s i s d i s a p p o i n t i n g and p o i n t s to the o f t e n f r u i t l e s s attempts to o b t a i n o x i d a t i v e l y s t a b l e m a t e r i a l s through s h i e l d i n g the metal from o x i d a t i o n . A l t e r n a t e attempts might emphasize procedures for f i x i n g the geometry about the metal atom. Such " f i x i n g " , though, t y p i c a l l y r e s u l t s i n chain s t i f f e n i n g and subsequent poorer processing p r o p e r t i e s . The a f o r e i s a consequence o f a) o f t e n high i o n i c - c h a r a c t e r of the metal-associated bonds and b) presence o f l o w - l y i n g , a v a i l a b l e d - o r b i t a l s or other h y b r i d i z e d o r b i t a l s both p e r m i t t i n g ready arrangement about the metal atom. 8. Metal-containing moieties t y p i c a l l y a c t as chain s t i f f e n ing agents i n c r e a s i n g Τ and T but decreasing s o l u b i l i t y . F l e x i b i l i t y i s imparted by the hydrocarbon p o r t i o n s . 9. There does not appear to be a d i r e c t c o r r e l a t i o n between thermal, o x i d a t i v e and h y d r o l y t i c s t a b i l i t i e s , thus products must be designed to emphasize the separate d e s i r e d property. H y d r o l y t i c s t a b i l i t y i s o f t e n inherent i n many organo m e t a l l i c polymers due to the hydrophobic nature o f the organic portions as long as the chains are not "wetted". For instance 11

1 1

ffl

g


28

t i n p o l y e s t e r s are s t a b l e t o b o i l i n g water f o r a week, yet r a p i d l y degrade when d i p o l a r a p r o t i e l i q u i d s as DMSO are added. 10. Many o r g a n o m e t a l l i c polymers undergo i n i t i a l thermal degradation through s o l i d t r a n s i t i o n s without m e l t i n g . 11. While some o r g a n o m e t a l l i c polymers e x h i b i t t r u l y outstanding weight r e t e n t i o n , i t must be remembered that some times most o f the remaining residue i s i n o r g a n i c ( s p e c i f i c a l l y i n a i r -the metal oxide) thus care should be e x e r c i s e d when e v a l u a t i n g weight r e t e n t i o n data t o note how much i s t r u l y o r g a n i c . Thus one should be c o n t i n u a l l y aware o f the meaning o f weight r e t e n t i o n s , p a r t i c u l a r l y with metal-containing products which can o x i d i z e at lower temperatures l e a v i n g not a d e s i r e d moldable and/or f l e x i b l e product but r a t h e r a powdery metal o x i d e . Even i n an i n e r t atmosphere the m e t a l - c o n t a i n i n g moiety g e n e r a l l y i s one o f the l a s t t o v o l a t i l i z e . For i n s t a n c e , the S c h i f f base c o o r d i n a t i o n polymers o f 5,5'-p-phenylene-bis( m e t h y l i d y n e n i t r i l o ) d i - 8 - q u i n o l i n o l show reasonable weight r e t e n t i o n t o 400 t o 500°C with only about 15-35$ weight l o s s e s , but on c o n s i d e r i n g the metal content t h i s represents 50 t o 75$ l o s s o f the organic p o r t i o n o f the polymer (2.). In f a c t , we have found that f o r the m a j o r i t y o f condensation polymers made by our group, the metal remains and i n many cases thermal degradation can be u t i l i z e d i n determining the amount o f metal present i n the o r i g i n a l polymer Q ) . 12. Most o r g a n o m e t a l l i c polymers e x h i b i t equal to b e t t e r weight r e t e n t i o n under i n e r t c o n d i t i o n s compared t o a i r . Degradation Pathways A number o f degradation mechanisms have been i d e n t i f i e d . Some o f these mechanisms are b r i e f l y d e s c r i b e d i n t h i s s e c t i o n . Rearrangement r e a c t i o n s a r e one l a r g e c l a s s o f degradation mechanisms i n c l u d i n g r e o r g a n i z a t i o n o f groups about the metal atom and r i n g formation. Inorganic monomers tend t o form s m a l l , unstrained r i n g s as i n the case o f s i l o x a n e s which t y p i c a l l y undergo extensive rearrangements a t temperatures i n excess o f 350°C, o f t e n forming c y c l i c products which may i n v o l v e an e q u i l i b r i u m between the c y c l i c product and the polymer. Gee (_3) has t r e a t e d such e q u i l i b r i u m as f o l l o w s f o r an e q u i l i b r i u m between a short c h a i n o r c y c l i c product R, a s h o r t e r chain C and a longer chain composed o f C + R, CR. C + R Ζ CR At e q u i l i b r i u m AG = 0. For spontaneous processes AG must be negative and at constant temperature TAS > ΔΗ. The e q u i l i b rium constant i s Κ =

[1]

4.

CARRAHER

29

Thermal Analysis

and where the mixture i s mainly polymer Κ = 1/[R]

[CR] = [C]

and [2]

The e q u i l i b r i u m constant i s r e l a t e d to the standard f r e e energy change AG

0

= -RT In Κ = RT In [R]

[3]

I f [R] i s the c o n c e n t r a t i o n o f c y c l i c ringed products i n pure c y c l i c product, then AG = AG

0

- RT In [ R ]

AG = RT In [R]

the

q

ο

and

[4] [5]

In the absence o f s o l v e n t [R]/[R] = 1 - Φ, where Φ i s weight f r a c t i o n o f chain polymer i n an e q u i l i b r i u m mixture. The r e l a t i o n s h i p AG = AH - TAS reduces to -R In (1-Φ) = AS - ΔΗ/Τ

[6]

Three s i t u a t i o n s are derived from t h i s e q u a t i o n . F i r s t , i f AH i s negative (exothermic r e a c t i o n ) , the polymer concentra t i o n , [CR], decreases as temperature i n c r e a s e s . I f AS i s a l s o negative, no polymer can e x i s t above a c e i l i n g temperature. I f AS i s p o s i t i v e or zero, polymer can e x i s t at any temperature (below which other a l t e r n a t i v e r e a c t i o n s o c c u r ) . I f AH = 0, the [CR] i s independent o f temperature and i s given by -R In (1 - Φ) = AS. I f AS i s n e g a t i v e , the polymer w i l l be unstable with respect to formation o f R. I f AH i s p o s i t i v e (endothermic), [CR] w i l l i n c r e a s e as temperature i n c r e a s e s . I f AS i s p o s i t i v e , polymer w i l l not e x i s t below a lower temperature g i v e n by Τ = ΔΗ/AS. I f S i s negative, no polymer can e x i s t r e g a r d l e s s o f temperature. Thus p r e d i c t i o n s can be made with regard t o the e f f e c t of temperature on such e q u i l i b r i u m r e a c t i o n s given determined AH and AS v a l u e s . Polyphosphazenes t y p i c a l l y are o f the f i r s t type w h i l e s u l f u r and selenium are o f the l a s t type (regarding AH v a l u e s ) . The r e a c t i o n o f Be^OiOCOCH^),. with d i a c i d c h l o r i d e s (4) g i v e s low molecular weight polymers. On heating, interchange o c c u r r e d and Be^OiOCCH^)^ could be sublimed at 110-140°C at 10" t o r r pressure f o r tne product derived from a d i p y l c h l o r i d e . The use o f t e r e p h t h a l y l c h l o r i d e appears to i n c r e a s e the s t a b i l i t y o f the product such that a temperature o f 340°C was r e q u i r e d to o b t a i n sublimed b e r y l l i u m a c e t a t e . Interchange a l s o slowly occurs at room temperature. E l i m i n a t i o n can occur l e a d i n g t o the formation o f s t a b l e residues or may s i g n a l the onset o f continued, f u r t h e r dégrada-


30

11

11

t i o n . For uranyl p o l y e s t e r s O ) e l i m i n a t i o n o f i n n e r - c o r e waters at about 150°C t y p i c a l l y leads to the formation o f (2) accompanied by a change i n geometry about uranium from an o c t a hedral bipyramide to an octahedral s t r u c t u r e ( 5 ) . Residue 2 i s s t a b l e to about 450°C ( 6 ) . P y r o l y s i s o f manganese p o l y amines (3) begins with e l i m i n a t i o n o f the p y r i d i n e moiety about 250°C. T h i s i s followed by a massive breakup o f the diamine moiety and e l i m i n a t i o n o f water (6_) without generation o f a stable, i d e n t i f i a b l e , isolatable residue.

H

H'

70-250°C

11 ο

ϋ

H

v/

H

Ν'

Μη

3°8

Η Η 1 ! • N-R-N4

Λ Η

Η

E l i m i n a t i o n r e a c t i o n s l e a d i n g to f u r t h e r degradation i s by f a r the more common type o f e l i m i n a t i o n degradation pathway. Many organometallic monomeric compounds undergo thermally induced d i s p r o p o r t i o n a t i o n . D i s p r o p o r t i o n a t i o n a l s o occurs i n metal-containing polymers. Aryloxy polyaluminum oxides(4) d i s p r o p o r t i o n a t e on heating, forming aluminum oxides ( 7 ) . OAr 1 4A1

0·)

4

unzipping pathways have been c i t e d as a p o s s i b l e i n i t i a l degradation pathway (J_). Oxidation o f the metal atom t y p i c a l l y occurs at some point during degradation processes o c c u r r i n g i n a i r (J_). Thus f o r

4.

CARRAHER

Thermal Analysis

t i n polyamines, entrance o f oxygen t o the t i n atom through an a s s o c i a t i v e pathway with subsequent degradation i n v o l v i n g e v o l u t i o n o f both fragments d e r i v e d from the o r g a n i c backbone and organic groups bonded t o the t i n i s b e l i e v e d t o occur i n air O ) . R H H i l I 4Sn-N-R -N-) I R 1

R

n

°2

0 \/ • 4Sn-N-R ' / I I R H H o 2

» Sn0

o

[7 ]

2

I t i s i n t e r e s t i n g to note that there i s evidence t h a t f o r some s i t u a t i o n s o x i d a t i o n occurs away from the metal where the metal i s connected through ether" oxygens. For instance f o r a l k y l s i l i c o n e s , the a l k y l groups a r e o x i d i z e d t y p i c a l l y about 250-350°C ( 8 ) . Thus the s i t u a t i o n with regard t o the s i t e o f o r i g i n a l o x i d a t i o n i s complex and not s e t t l e d . O x i d a t i o n o f t e n r e s u l t s i n the formation o f c r o s s l i n k i n g through oxygen b r i d g e s . Such products can be f a i r l y s t a b l e . Thus p o l y p h e n y l s i l o x a n e l o s e s 8$ o f i t s weight t o 400°, an a d d i t i o n a l 40$ t o 450°C and an a d d i t i o n 8$ t o 550°C (9). Polyv i n y l s i l o x a n e l o s e s 5$ a t 250°, an a d d i t i o n a l 12$ t o 350° and an a d d i t i o n a l 5$ t o 550°C. Such s i l a n e s a r e b e l i e v e d t o undergo o x i d a t i v e degradation i n i t i a l l y through cleavage o f the organic groups with c r o s s l i n k s formed through s i l o x a n e bonds, w i t h the new s t r u c t u r e r e s i s t i n g f u r t h e r o x i d a t i o n . When the metal i s bonded through nonoxygen bonds the r e s u l t s i n a i r a r e such that i n i t i a l degradation may i n v o l v e any number o f pathways and s i t e s and i s dependent on the p a r t i c u l a r polymer. Formation o f c r o s s l i n k s , t y p i c a l l y accompanied by e l i m i n a t i o n o f a s s o c i a t e d m o i e t i e s , i s a common occurrence and serves as the b a s i s f o r the formation o f g r a p h i t e , carbon and boronc o n t a i n i n g f i b e r s . T h i s i s p a r t i c u l a r l y true when nonaromatic double and t r i p l e bonds are present such as 7 o r where aromatic i t y can be gained through bond r e l o c a t i o n . 11

CH=C=NN=CH4


32

Borazole (8) undergoes a complex polycondensation above 500° y i e l d i n g H I B

H X

N

Ν 1

I

Η

ι

Η

Η a BNH h i g h l y c r o s s l i n k e d s t r u c t u r e (11) and p o l y b u t y l t i t a n a t e a l s o decomposes t o a complex, h i g h l y c r o s s l i n k e d t i t a n i a network. These networks a r e o f t e n then heated t o s e v e r a l thousand degrees f u r t h e r developing the c r o s s - l i n k e d network and l e a d i n g t o m a t e r i a l s which e x h i b i t both outstanding thermal s t a b i l i t i e s and s t r e n g t h s . Degradation through attack o f the organometallic polymer by i m p u r i t i e s i s probably one o f the most prevalent modes o f polymer degradation, y e t a mode where l i t t l e has been r e p o r t e d . Extensive s t u d i e s c a r r i e d out on p o l y s i l o x a n e s show that the thermal p r o p e r t i e s are a l s o dependent on such items as chain l e n g t h , endgroup, and source ( i . e . , nature o f s y n t h e s i s ) . The l a t t e r i s r e l a t e d t o i n c l u s i o n o f i m p u r i t i e s , amount and d i s t r i b u t i o n o f c r y s t a l l i n i t y , e t c . The presence o f minute amounts o f i m p u r i t i e s can give misleading r e s u l t s with respect to most p h y s i c a l and chemical p r o p e r t i e s i n c l u d i n g thermal stability. The chemical and p h y s i c a l , i n c l u d i n g thermal, r e s i s t a n c e to degradation o f p o l y s i l o x a n e s i s a consequence o f both the high Si-0 bond energy (106 kcal/mol) and the r e l a t i v e l y l a r g e amount o f i o n i c character w i t h i n the Si-0 moiety. The i o n i c character o f the Si-0 bond a l s o f a c i l i t a t e s acid-and base-cata l y z e d rearrangement and/or degradation r e a c t i o n s . Thus h i g h l y p u r i f i e d polymethyl s i l o x a n e s a r e s t a b l e t o 350-400°C where s i l o x a n e bond interchange t o form c y c l i c products occurs ( 1 2 ) . Yet i n the presence o f s u l f u r i c acd, s i l i c o n e rubber degrades at room temperature ( 13) « Thus the presence o f t r a c e amounts o f n u c l e o p h i l i c or e l e c t r o p h i l i c agents can lead t o a dramatic decrease i n the thermal s t a b i l i t y o f organometallic polymers. T h i s becomes i n c r e a s i n g l y important when i t i s remembered that many polymeri zations employ such reagents as c a t a l y s t i c agents and even l e s s recognized i s the presence o f such i m p u r i t i e s i n the mono mers themselves. One o f the most s e r i o u s problems f a c i n g polymer s c i e n t i s t s i s the e l i m i n a t i o n o f t r a c e amounts o f water. T h i s i s p a r t i c u l a r l y true with metal c o n t a i n i n g polymers which a r e p a r t i c u l a r l y s u s c e p t i b l e to h y d r o l y s i s . F o r many organometallic polymers, s t a b i l i t y t o water r e s u l t s from the hydrophobic nature o f the

4. CARRAHER

33

Thermal Analysis

organic p o r t i o n s . As p r e v i o u s l y noted, r a p i d h y d r o l y s i s occurs when the polymers are wetted by a d d i t i o n o f l i q u i d s which are compatible with the e l e c t r o n i c nature o f the polymer c h a i n . The a c t i v a t i o n energy f o r p e n e t r a t i o n o f the "organic s h i e l d " by water i s not l a r g e and t y p i c a l l y achieved below 200°C f o r most organometallic polymers and such degradation may be respons i b l e f o r the small amount o f degradation o c c u r r i n g below 200°C observed f o r many organometallic polymers. Counter t o t h i s , we have observed that the thermal s t a b i l i t i e s o f many condensat i o n organometallic polymers i s independent o f the mode o f s y n t h e s i s ( i . e . employing aqueous present and f r e e r e a c t i o n s y n t h e t i c systems). Even so, we have not r i g o r o u s l y kept the polymer from the a i r so that water may be a t t r a c t e d t o the s u r f a c e o f the polymers. T h i s area should be studied f u r t h e r . Thus there e x i s t s a v a r i e t y o f i n i t i a l degradation pathways. F u r t h e r , i t i s p o s s i b l e t h a t s e v e r a l o f these can occur almost simultaneously. More in-depth s t u d i e s should be h e l p f u l i n i d e n t i f y i n g the l e a s t s t a b l e s i t e ( s ) a l l o w i n g m o d i f i c a t i o n o f these polymers at that s i t e . Thus metal p o l y e s t e r s derived from 1,1'-diearboxyferrocene g e n e r a l l y degrade i n a i r at about 250°C. The degradat i o n begins with a s p l i t t i n g out o f C 0 and the ferrocene moiety. The degradation i s r a p i d , p o s s i b l y e x p l o s i v e , with as great as 70$ weight l o s s o c c u r r i n g over a 10°C range (14). The analogous polyethers t y p i c a l l y are s t a b l e t o about 300°C where a more g e n t l e degradation o c c u r s . Thus replacement o f the e s t e r group, which r e a d i l y forms COp, f o r an ether grouping allowed f o r the s y n t h e s i s o f more thermally s t a b l e polymers. 2

Attempts t o Enhance Thermal S t a b i l i t i e s As p r e v i o u s l y noted, s i l o x a n e s undergo extensive rearrangements a t temperatures i n excess o f 350°C, o f t e n forming c y c l i c products. Because o f the s i g n i f i c a n t mechanical, chemical, and e l e c t r i c a l p r o p e r t i e s o f f e r e d by s i l o x a n e s and modified s i l o x a n e s , great e f f o r t i s s t i l l d i r e c t e d towards i n c r e a s i n g the use temperature o f such products. T h i s work i s c l o s e l y a s s o c i a t e d with s o p h i s t i c a t e d thermal a n a l y s i s systems and i s o f t e n aimed at preventing the depolymerization (reversion) reaction. One approach often employed f o r preventing r e v e r s i o n i s m o d i f i c a t i o n o f the s i l o x a n e . Several recent attempts are described below.

34


Reversion and r i n g formation have been p a r t i a l l y overcome through placement o f c h a i n - s t i f f e n i n g u n i t s i n the s i l i c o n e backbone. Thus l i n e a r D^-m-carborane-siloxanes (11-14) with one t o three t r i f l u o r o p r o p y l moieties per repeating u n i t ) e x h i b i t b e t t e r thermal and o x i d a t i v e s t a b i l i t y than s i l i c o n e s and f l u o r o s i l i c o n e s ( 15). I n i t i a l degradation occurs i n a i r about 300 to 350°C, almost 100° above that t y p i c a l l y experienced f o r s i l o x a n e s and f l u o r o s i l i c o n e s . The carborane-siloxanes e x h i b i t Τ 's from -50 t o 0°C (15). The carborane moiety a l s o acts t§ i n h i b i t formation o f six-membered r i n g s because o f i t s s i z e .

CH

0

CH I CH CH CH ι 3 1 3 I 4SI—CB i—O-Si—0-4 10 C - S I I I CH. CH. CH. 0 2

0

0

0 2

CH CH CH I3 I3 I3 - 4 S i - •CB, Η C - S i — 0 - S i 0 - 4 C H 10 10 ι j 3 l CH. CH. CH 0

0

Λ

Λ

A

o

3

Ucarsil F

U c a r s i l Me,

11

F

12

?3

3

F

? 3

H

Si—CB H I CH. 1 Q

1 Q

? 3 CSi—O-Si—04-

CH, Ï 2 CSi^Si-O4 H

-fSi—CB

i

0

H

i

0

I

CH.

u c a r s i l F, 13

?3

CH.

CH

CH.

3

CH.

U c a r s i l F. 14

Thermal a n a l y s i s o f a number o f l i n e a r polycarboranesiloxane copolymers and block polymers (such as 15-17) i n c l u d i n g TG, DT, TB, and TM was made by R o l l e r and G i l l h a m (J_6). A number of p h y s i c a l t r a n s i t i o n s were reported i n c l u d i n g c r y s t a l l i z a t i o n temperature (10 to 230°C), g l a s s t r a n s i t i o n temperature (-108 to 25°C), g l a s s y - s t a t e t r a n s i t i o n temperature (-145 t o -90°C), and m e l t i n g temperature (40 t o 260°C). T h i s i s one o f the most complete s i n g l e papers on the thermal a n a l y s i s o f i n o r g a n i c polymers regarding the types o f thermal p r o p e r t i e s c i t e d . A number o f r e s u l t s were found t h a t may have otherwise gone unnoticed i n b r i e f e r s t u d i e s (16). The c y c l i z a t i o n mechanism o f degradation which occurs f o r polydimethylsiloxane i s

4.

CARRAHER

Thermal Analysis

35

not o p e r a t i v e f o r polyearboranesiloxanes. At high temperatures the polyearboranesiloxanes s t i f f e n , p o s s i b l y due to c r o s s l i n k i n g . In argon the polyearboranesiloxanes g e n e r a l l y showed an i n c r e a s e i n thermal s t a b i l i t y o f about 50°C over that o f polydimethyls i l o x a n e with many being s t a b l e at 500 to 550°C. CH. CH. I 3 ι 3 - f S i — C - B H, C - S i - 0 - 4 I 10 10 J Λn

CH

CH CH. CH CH. CHL ι 3 ι 3 | 3 t 3 | 3 -{O-Si-0-Si-C-B H, - C - S i — 0 — S i — 0 — S i - 4 1 I 10 10 ι j j 0

n

3

C H

0

Λn

^

CH

3

CH

n

CH

3

CH

3

J5

CH

3

16

CH. I 3

-fÇi—0-4 CH

3

Jl Another u t i l i z e d approach f o r preventing r e v e r s i o n i n v o l v e s c r o s s l i n k i n g o f the s i l o x a n e s . For i n s t a n c e , a number o f methyl s i l i c o n e rubbers were c r o s s l i n k e d , (about 1 c r o s s l i n k per 3,000 to 20,000 u n i t s (10-21) f o r c r o s s l i n k e d products mainly composed of dimethylsiloxane; R e f s . 17 and 1J3). DSC and TM was u t i l i z e d to evaluate the thermal p r o p e r t i e s o f the products. TM was u t i l i z e d to determine c r o s s l i n k d e n s i t y along with t y p i c a l modulus p r o p e r t i e s . Both enhancement and decrease i n thermal p r o p e r t i e s were observed depending on the mode and c o n d i t i o n s of c r o s s l i n k i n g . CH. ι 3 -fSi—0-4

H

Λ 18

0

H j and -(-Si—0-4

0

i I HLC-Si—CH —CH —Si—CH 0

2

0

2

4-

2

+

Η

CH

' 3 2

19

20

C H

3 -4Si-CB CH

3

C H

l 0

H

1 0

3 C>4Si-^^ CH

3

21 Another approach to combat r e v e r s i o n i s the placement of the s i l o x a n e or modified s i l o x a n e on an " i n e r t " support

3

3


36

which can a s s i s t i n maintaining the o r i g i n a l nonreverted s t r u c t u r e . Thus F i n c h (19) reported the formation o f a new chromato graphic phase s t a b l e t o 500°C based on a polycarboranesiloxane s t a t i o n a r y phase. The system was capable o f e f f i c i e n t separa t i o n s from 50 to 500°C. In f a c t , TG's o f the polycarborane s i l o x a n e phase showed complete weight r e t e n t i o n t o about 700°C. The meta-carborane was u t i l i z e d i n t h i s study. S i l o x a n e - l a d d e r polymers have been known f o r some time. A polymer derived from p h e n y l t r i c h l o r o s i l a n e (2), w h i l e i n f u s i b l e , i s s o l u b l e i n s o l v e n t s such as tetrahydrofuran and benzene from which f i l m s can be cast (20,2_p. TG thermograms i n a i r show no weight l o s s t o 525° C but an upper l i m i t f o r u s e f u l a p p l i c a t i o n i s probably near 300° C (21).

\

\

Several researchers have s u c c e s s f u l l y employed the a d d i t i o n of a n t i o x i d a n t s t o improve thermal s t a b i l i t y . Thus N a n n e l l i , Gillman and Black (22) found that the degradation o f 23 proceeds i n a i r through o x i d a t i o n and cleavage o f the organic s i d e groups with a degradation i n c e p t i o n temperature around 200°C. The gas chromatograms o f the v o l a t i l e decomposition products o f Cr[0P(CH" )(C H )0] 0P(C H ) 0 showed a t l e a s t 14 products of which 2-octane, a c e t i c a c i d , and η-butyric a c i d are among the most abundant. Decomposition does occur at 200°C f o r long exposure times with a 13$ weight l o s s f o r an 18-hour exposure time. A d d i t i o n o f a n t i o x i d a n t s such as d i s t e a r y l t h i o d i p r o p i o nate i n c r e a s e s the l o n g e v i t y o f the polymer f i l m s f l e x i b i l i t y at 200°C. 3

6

5

2

6

ο

ρ

23

4.

CARRAHER

Thermal Analysis

37

D i f f e r e n t i a t i o n o f Depolymerization

Mechanisms

MacCallum (25) r e l a t e d the change i n number average mole c u l a r weight, DP, with depolymerization mechanism d i v i d i n g the pathways i n t o the groups c h a r a c t e r i z e d by the nature o f the i n i t i a t i o n reaction-random s c i s s i o n or end group. These groups can be f u r t h e r d i v i d e d i n t o i n i t i a l degradation followed by a) p a r t i a l unzipping and b) complete u n z i p p i n g . Polymer degradation can be described i n terms o f the weight of polymer, W, at time t , number o f molecules, N, at time t , and DP as f o l l o w s where m i s the molecular weight o f each mer unit. W = N m DP

[8]

D i f f e r e n t i a t i o n with respect to time g i v e s

m

-1 dW dt =

dDP

N

St

— +

D

P

dN dt

Γ η Ί [ 9 ]

For random i n i t i a t i o n followed by incomplete unzipping and assuming that depolymerization i s f i r s t - o r d e r with respect to sample weight g i v e s dN/dt = kW/m

and

[10]

-dW/dt = kWZ

[11]

where Ζ i s the average unzipping l e n g t h , i . e . number o f mers separated from the c h a i n . Combination o f [8]-[11] followed by i n t e g r a t i o n g i v e s s

DP/DP 0

DP

ο

( J f \C DP

C

ο

)= f (1-C)/(C+f) + Z/

[12]

where f = Z/DP"

and C, the f r a c t i o n a l conversion, i s (W -W)/W . ο ο ο For random i n i t i a l depolymerization followed by complete unzipping, Ζ can be r e l a t e d to DP through a parameter, B, t h a t i s r e l a t e d to the p o l y d i s p e r s i t y o f the polymer sample. Ζ = DP Β Combining [10], [11] and DP/DP = ( 1 - C ) Q

[13] [13] i n t o [9] with i n t e g r a t i o n g i v e s ( B

"

1 ) / B

[14]

38


For chain-end

i n i t i a t i o n followed by p a r t i a l unzipping

-DN/dt = 0

[15]

and from [ 9 ] , [11] and [15] with i n t e g r a t i o n DP/DP = 1-C ο

[16]

For i n i t i a l end-group depolymerization followed by complete unzipping -dN/dt = kN

[17]

and combining [ 8 ] , [ 9 ] , [11] and [17] followed by i n t e g r a t i o n yields DP/DP

ο

=1

[18]

In a s i t u a t i o n where i n t r a m o l e c u l a r "loops" o f constant s i z e a r e formed and r a p i d l y removed (as v o l a t i l e s under reduced or ambient pressure) equations [11] and [15] apply and can be incoporated i n t o [9] g i v i n g [16] a f t e r i n t e g r a t i o n . Thus f o r end-group depolymerization, DP i s a l i n e a r f u n c t i o n of time f o r p a r t i a l unzipping and independent o f time f o r complete unzipping. F o r random bond breakage p a r t i a l unzipping i s a complicated l o g a r i t h m i c f u n c t i o n o f DP r e l a t e d t o r e a c t i o n extent whereas complete unzipping i s r e l a t e d t o a l i n e a r f u n c t i o n o f 1/DP with time. The a f o r e s i t u a t i o n can be p i c t u r e d as shown i n F i g u r e 1. L i n e A, corresponding t o i n i t i a l end-group depolymerization followed by complete unzipping shows no change i n molecular weight s i n c e the i n i t i a t i o n o f depolymerization i s the r a t e determining s t e p ; i . e . chain lengths o f remaining chains remain unchanged. Counter, i f end-group depolymerization i s followed by incomplete unzipping, C, average chain length w i l l decrease slowly, the change i n average chain l e n g t h r e l a t e d t o the s i z e of segment removed, i n c l u d i n g l o o p s . A curve such as Β r e s u l t s where random s i s s i o n i s followed by complete unzipping s i n c e there i s a r e l a t i v e l y slow decrease i n molecular weight with conversion. Random depolymerization followed by incomplete unzipping r e s u l t s i n a r a p i d decrease i n molecular weight with conversion and i s p i c t o r i a l i z e d as D. Both Β and D a r e members of a number o f s i m i l a r curves, the exact shape being dependent on the exact depolymerization parameters. These curves can be computer-generated f o r v a r y i n g s i t u a t i o n s . Degradation may i n v o l v e more than one pathway. The a f o r e approach can be extended t o consider these cases and o t h e r s . Thus f o r degradation o c c u r r i n g through formation o f v o l a t i l e loops as w e l l as through an i n t e r n a l , i n t e r m o l e c u l a r process,

4.

CARRAHER

Thermal Analysis

0

F r a c t i o n o f Depolymerization

1

Figure 1. Idealized plots relating molecular weight ratio to depolymerization mechanisms. Key: A, end-group depolymerizationfollowed by complete unzipping; B, random scission followed by complete unzipping; C, end-group depolymerization followed by incomplete unzipping; and D, random depolymerization followed by incomplete unzipping. equations [8] and i s given by -dW/dt

[9] s t i l l h o l d . s kmZW =

f

kW

The r a t e o f weight l o s s [19]

The r a t e o f change o f molecular p o p u l a t i o n i n the condensed phase depends on the r e l a t i v e magnitudes o f the r a t e o f loop formation forming v o l a t i l e and n o n v o l a t i l e oligomers, e t c . These changes can be assumed to be a weak f u n c t i o n o f time as f o l l o w s where A ( t ) d e s c r i b e s the time dependence o f the p a r t i c u l a r systems. dN/dt = A ( t )

[20]

T h i s f u n c t i o n can be parametized i n a number o f ways i n c l u d i n g i n v o l v i n g i n i t i a l population, polymer l i f e t i m e and other s e a l i n g parameters. Models combining growth and decay through v a r i o u s routes can be made and computer p l o t s made with ensuing curve f i t t i n g . Thus there e x i s t s s e v e r a l ways to determine the mode o f depolymerization with most r e q u i r i n g the products to be s o l u b l e p e r m i t t i n g molecular weight to be determined. Further these approaches are best s u i t e d to s i t u a t i o n s where depolymerization occurs through a s i n g l e mechanistic pathway. Thus f o r polymers as s i l o x a n e s the a f o r e techniques should be r e a d i l y a p p l i c a b l e but f o r many metal-containing polymers degradation proceeds through the formation o f i n s o l u b l e r e s i d u e s .

40


Coupled Instrumentation Employing MS PY-GC-MS and GC-MS have been e x t e n s i v e l y u t i l i z e d i n both t r a d i t i o n a l product i d e n t i f i c a t i o n and i n s e l e c t s t u d i e s t o a s s i s t i n the d e s c r i p t i o n o f the thermal degradation o f the m a t e r i a l s . These techniques t y p i c a l l y do not a l l o w a ready, s t r a i g h t f o r w a r d i n t e r p r e t a t i o n o f degradation sequences due to c o n s i d e r a b l e combination o f evolved fragments on the GC columns. In an e f f o r t t o overcome problems o f combination, we f i r s t constructed a TG-MS assembly where the evolved s p e c i e s from thermodegradation were swept, by a helium purge gas, i n a s t r a i g h t l i n e where the d i s t a n c e between the TG sample compartment (boat) and the MS i o n i z a t i o n source was about three f e e t (23). While numerous i o n fragments were found, i t was s u r p r i s i n g that c e r t a i n species s t i l l underwent combination. T h i s was found f o r phenyl groups d e r i v e d from a number o f d i f f e r e n t samples. The amount o f biphenyl formed was i n great excess with that c a l c u l a t e d u s i n g simple gas c o l l i s i o n theory. Presumably combination i s o c c u r r i n g w i t h i n the s o l i d sample p r i o r t o e v o l u t i o n o f that fragment. Formation o f biphenyl occurred even when the samples were ground t o a f i n e powder. The TG-MS assembly c o n s i s t e d o f a double-focusing DuPont 21-491 Mass spectrometer coupled through a s i n g l e - s t a g e g l a s s j e t separator t o a DuPont 951 Thermal G r a v i m e t r i c Analyzer which was attached t o a DuPont 990 Thermal Analyzer Console. The MS was equipped with a Hewlitt-Packard, HP-2216C computer having 24K core memory and a d i s c - o r i e n t e d data system s p e c i a l l y developed f o r the DuPont 21-491 Mass Spectrometer. The MS system can be c o n t r o l l e d by the computer system, which i n c l u d e s a dual 2.5M byte d i s c d r i v e , a Hewlett-Packard Cathode Ray Tube t e r m i n a l , a T e k t r o n i x storage scope ( f o r d i s p l a y ) d r i v e n by a dual 12 b i t d i g i t a l - t o - a n a l o g (D/A) converter, and Versatec p r i n t e r / p i o t t e r . Data was acquired u s i n g a 14 b i t analog-tod i g i t a l (A/D) converter (13 plus s i g n ) . T h i s system can operate and process data a t r a t e s up t o 8 KHz. A thermocouple, i n t e r f a c e d with the Hewlett-Packard 2116C computer, was employed t o sense the temperature during the course o f the thermogravimetric a n a l y s i s . T h i s probe was attached to the TG i n order t o monitor the a c t u a l temperature o f the sample during each r u n . During sample a n a l y s i s , the TG-jet separator connection was wrapped with heating tape and maintained at a temperature o f 125°C while the j e t separator oven was maintained a t 135°C. The MS-jet separator t r a n s f e r l i n e was a l s o wrapped with heat tape and maintained a t 165°C while t h e source temperature was h e l d a t 230°C. The assembly and c a l i b r a t i o n procedures are d e s c r i b e d i n d e t a i l i n reference 23. Depending on the h e a t i n g r a t e , up t o s e v e r a l hundred complete s p e c t r a are obtained as a f u n c t i o n

4.

CARRAHER

Thermal Analysis

41

o f temperature with the data a v a i l a b l e as a f u n c t i o n o f absolute i o n abundance and normalized i o n abundance f o r each i o n such that p l o t s o f i o n i n t e n s i t y as a f u n c t i o n o f temperature can be made f o r a l l mass numbers. A number o f m e t a l - c o n t a i n i n g polymers were evaluated employ i n g the TG-MS assembly. Organometallic polymers were chosen f o r s e v e r a l reasons. F i r s t , s i n c e degradation o f these compounds t y p i c a l l y occur by d i f f e r e n t routes i n a i r and i n i n e r t e n v i r o n ments, the two environments can be c l e a r l y , e a s i l y d i f f e r e n t i a t e d . Second, some o f the organometallic polymers e x h i b i t good high temperature s t a b i l i t y , l o s i n g l e s s than 20% o f t h e i r i n i t i a l weight t o the 800-1200°C range. I d e n t i f i c a t i o n o f the degrada t i o n products from the polymer would be u s e f u l i n b e t t e r under standing t h e i r good s t a b i l i t y , and i n d e s i g n i n g and s y n t h e s i z i n g s t i l l more s t a b l e polymers. A t h i r d reason f o r using organo m e t a l l i c polymers i n a s s e s s i n g the TG-MS i s t h a t the degradation o f these m a t e r i a l s occurs i n a stepwise manner over a wide temperature range ( o f t e n over a range i n excess o f 600 C°) with i n t e r s p e r s e d " s t a b i l i t y p l a t e a u s " . Continuous monitoring o f the evolved chemical products, as i s p o s s i b l e with the TGMS d e s c r i b e d , i s u s e f u l i n understanding these t r a n s i t i o n s i n the degradation sequence. F i n a l l y , the organometallic polymers are good candidates f o r study by TG-MS because they are d i f f i c u l t to c h a r a c t e r i z e by other techniques. Conventional C, Η, Ν elemental a n a l y s i s o f t e n y i e l d s poor r e s u l t s , and s i n c e many o f these polymers are only s p a r i n g l y s o l u b l e i n most s o l v e n t s , c h a r a c t e r i z a t i o n by NMR o r ESR i s not p r a c t i c a l . Methods such as those described h e r e i n are t h e r e f o r e needed f o r c h a r a c t e r i z i n g the s t r u c t u r e s o f such o r g a n o m e t a l l i c polymer m a t e r i a l s . A number o f t i t a n i u m p o l y e t h e r s d e r i v e d from hydroquinone d e r i v a t i v e s were evaluated employing the TG-MS assembly and i n f r a r e d spectroscopy (24). Two general trends were found. F i r s t , the i n i t i a l l y evolved degradation product was c y c l o p e n t a diene t y p i c a l l y appearing about 100°C. Second, t h i s was f o l l o w e d by e v o l u t i o n o f the hydroquinone moiety along with the a s s o c i a t e d oxygens. There was great v a r i a t i o n with respect t o the extent and r a t e o f degradation o f the hydroquinone-containing moiety from the s e v e r a l polymers s t u d i e d , and the q u a n t i t i e s o f organic components i n the f i n a l r e s i d u e s from degradation a l s o v a r i e d markedly.

24

42


The afore suggests that the l i m i t i n g thermally s t a b l e moiety i s the cyelopentadiene m o i e t i e s and that c o n s t r u c t i o n of thermally s t a b l e t i t a n i u m - c o n t a i n i n g polymers should avoid the CppTi moiety. kz a heating r a t e o f 20 CVmin, the c y c l e time f o r sample i s i n excess o f one hour making such determinations commercially costly. More r e c e n t l y we have developed a s i m i l a r assembly except employing a programmable PY i n place o f the TG (JJO. Thus TG are obtained on samples to determine appropriate temperatures f o r MS to be o b t a i n e d . B r i e f l y , uranyl p o l y e s t e r s show three somewhat d i s t i n c t s t a b i l i t y p l a t e a u s . The PY was programmed to heat the samples to the i n c e p t i o n o f the s t a b i l i t y plateaus plus 50 to 100C°. The MS obtained at 250 to 300°C i n d i c a t e d e v o l u t i o n o f water accounting f o r a 5 to 10$ weight l o s s . At 450°C mass fragments derived from C 0 were found. At 900°C mass fragments derived from the v i n y l group were found. 2

H H I I -4C—c-4 H

I 25

H

H

The c y c l e can be accomplished w i t h i n 15 mins. making i t commercially more a c c e p t a b l e . Summary Few d e f i n i t i v e thermal s t u d i e s o f metal-containing polymers (with the exception o f s i l i c o n - c o n t a i n i n g polymers) e x i s t i n the l i t e r a t u r e . Thermal a n a l y s i s i s t y p i c a l l y done as a matter o f p r e l i m i n a r y t e s t i n g on new polymers. F u r t h e r , most o f the s t u d i e s were done p r i o r to the advent o f the automatic, programmed thermal a n a l y s i s instruments.

4.

CARRAHER

Thermal Analysis

43

The lack o f d e f i n i t i v e s t u d i e s i s due to a mixture o f reasons i n c l u d i n g 1) wide v a r i e t y of polymers; 2) newness o f i n t e r e s t i n the area; 3) wide v a r i e t y o f a p p l i c a t i o n s (both p o t e n t i a l and a c t u a l ) o f i n o r g a n i c and organometallic polymers not r e q u i r i n g thermal s t a b i l i t y or thermal a n a l y s i s (uses as anchored metal c a t a l y s i s , c o n t r o l r e l e a s e agents, e l e c t r i c a l and photochemical a p p l i c a t i o n s , s p e c i a l i t y adhesives); 4) i n s u f f i c i e n t d e s c r i p t i o n , i d e n t i f i c a t i o n , o f the products; 5) wider v a r i e t y o f degradation routes and other thermal behavior i n comparison to organic polymers; and 6) many products were synthe s i z e d and b r i e f l y c h a r a c t e r i z e d before the advent o f modern thermal i n s t r u m e n t a t i o n . The thermal techniques that have already been u t i l i z e d on more c l a s s i c a l polymers are g e n e r a l l y d i r e c t l y a p p l i c a b l e to organometallic and i n o r g a n i c polymers. The thermal a n a l y s i s of i n o r g a n i c and organometallic polymers i s both more and l e s s d i f f i c u l t i n comparison to s t u d i e s performed on organic polymers. On the negative s i d e there are 1) a much wider v a r i e t y o f products with many polymers showing a great dependence o f the thermal p r o p e r t i e s on the presence and nature o f i m p u r i t i e s ; 2) general l a c k o f d e f i n i t i v e thermal s t u d i e s and experience upon which to b u i l d ; and 3) added d i f f i c u l t y a s s o c i a t e d with d e f i n i n g thermal responses. On the p o s i t i v e s i d e the wider v a r i e t y o f bond energies present i n organometallic and i n o r g a n i c polymers can allow ( f o r c e r t a i n products) a c l e a r e r i s o l a t i o n and d e f i n i t i o n o f p a r t i c u l a r thermal t r a n s i t i o n s , such as Τ »s and degrada t i o n pathways. F u r t h e r , the wider v a r i e t y o f thermal response permits i n c r e a s e d p o t e n t i a l u s e f u l n e s s o f such compounds showing unusual ( e i t h e r i n extent, range, or a c t u a l thermal response) thermal responses. Recommendations There i s a need both f o r s t u d i e s i n v o l v i n g a wide v a r i e t y of products and in-depth s t u d i e s o f s e v e r a l o f these systems. The "survey" s t u d i e s can i n d i c a t e p o t e n t i a l l y i n t e r e s t i n g p o l y mers such as products showing unusual and/or p o t e n t i a l l y u s e f u l thermal responses. The c o r r e l a t i o n o f bonding group, metal atom, and f i n e and gross polymer s t r u c t u r e with such f a c t o r s as degradation pathway, bonding energies and inherent s t a b i l i t i e s should begin. There i s a need to evaluate, by newer thermal a n a l y s i s instrumentation and techniques, polymers p r e v i o u s l y only b r i e f l y c h a r a c t e r i z e d , emphasizing those products which show p o t e n t i a l i n d u s t r i a l a p p l i c a t i o n or other m e r i t o r i o u s p r o p e r t y . Such products should be w e l l c h a r a c t e r i z e d with regard to such f a c t o r s as c h a i n l e n g t h , molecular weight d i s t r i b u t i o n , endgroup, p u r i t y , nature and amount o f i m p u r i t i e s , and a c t u a l morphological s t r u c ture o f the polymer.

44


The i n c r e a s e d a p p l i c a t i o n o f modern thermal techniques and companion techniques must occur i f the thermal c h a r a c t e r i z a t i o n o f o r g a n o m e t a l l i c and i n o r g a n i c polymers i s t o progress. A number o f companion techniques have been shown t o be u s e f u l and merit f u r t h e r use. These i n c l u d e NMR, IR, MS, GC-MS, AA, X-ray microscopy, and GPC. ESR has not been a p p r e c i a b l y u t i l i z e d as a companion technique but may be considered, p a r t i c u l a r l y with polymers c o n t a i n i n g E S R - s e n s i t i v e metals. G e l permeation chromatography (GPC) should be considered as a companion t o o l f o r thermal degradation e v a l u a t i o n s where depolymerization i s suspected. There i s a need f o r a number o f well-chosen thermal a n a l y s i s s t u d i e s o f both the v o l a t i l e products by TGMS, e t c . ( f o r both chemical and t o x i c o l o g i c a l purposes) and the r e s i d u e . Such s t u d i e s should occur throughout the s t u d i e d temperature range r a t h e r than j u s t a t the room and f i n a l temperatures . Acknowledgment The author thanks M a r t e l Z e l d i n f o r h i s help i n p r e p a r i n g the s e c t i o n on d i f f e r e n t i a t i o n o f depolymerization mechanisms.

Literature Cited

1.

C. Carraher, J. Macromol. Sci.-Chemistry, A17(8), 1293 (1982). 2. E. Horowitz, M. Tryon, R. Christensen and T.P. Perros, J. Appl. Polym. Sci., 9, 2321 (1915). 3. G. Gee, Chem. Soc. (London) Spec. Publ. No. 15, 65 (1961). 4. C.S. Marvel and M.M. Martin, J. Amer. Chem.Soc.,619 (1958). 5. C. Carraher and J. Schroeder, Polymer P.,19(2),619 (1978). 6. C. Carraher, V. Foster, H. Molloy, M. Taylor, T, Tiernan and J. Schroeder, J. Macromol. Sci.-Chem., A16(1), 231 (1981). 7. R. Brotherton, Conference on High Temperature Polymer & Fluid Research, A50-TDR-62-372 (August 1962), p. 389. 8. K. Andrianov, Soc. Chem. Ind. (London) Monograph No. 13, 75 (1961). 9. K. Andrianov, Metalorganic Polymers, Interscience, NY, 1965. 10. E. Neuse, H. Rosenberg and R. Carlen, Macromolecules, 1, 424 (1968). 11. A.W. Laubengayer, P.C. Moews and R.F. Porter, J. Amer. Chem.Soc.,83, 1337 (1961). 12. C.W. Lewis, J. Polymer Sci., 33, 153 (1958). 13· A.V. Tobolsky, Inorganic Polymers, Academic Press, N.Y., 1962, pp. 10-27.

4.

14. 15.

16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

CARRAHER

Thermal Analysis

C. Carraher and J . Schroeder, Unpublished r e s u l t s . E. P e t e r s , D. Stewart, J . Bohan, R. M o f f i t t , C. Beard, G. Kwiatkowski and E. Hedayn, J . Polymer S c i . , 15, 973 (1977). M.B. R o l l e r and J.K. G i l l h a m , Polym. Eng. S c i . , 14, 567 (1974). E.M. Barrall, M. Flander and J.A. Logan, Thermochim. Actn, 5, 415 (1973). P.J. James, E.M. B a r r a l l , B. Dawson and J.A. Logan, J . Macromol. Sci.-Chem., A8, 135 (1974). R.W. F i n c h , Analabs Res. Notes, 1 0 ( 3 ) , 1 (1970). J.F. Brown, L. Vogt, A. Karchman, J . Eustance, K. K i s e r and K. Krantz, J . Amer. Chem. Soc., 82, 6194 (1960). J.F. Brown, J . Polymer S c i . , C1, 83 (1963). P. N a n n e l l i , H. G i l l m a n and B. Block, J . Polymer Sci., A-1, 9, 3027 (1971). C. Carraher, H. Molloy, T. T i e r n a n , M. T a y l o r and J . Schroeder, J . Macromol. Sci.-Chem., A16(1), 195 (1981). C. Carraher, H. Molloy, T. T i e r n a n , S. T s u j i , M. T a y l o r and S. C o l d i r o n , Unpublished r e s u l t s . J.R. MacCallum, European Polymer J . , 2, 413 (1966).

RECEIVED January 20, 1983

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