Thermogravimetric Analysis-Infrared Spectroscopy - Advances in

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BP

2

Thermogravimetric Analysis-Infrared Spectroscopy A Technique To Probe the Thermal Degradation of Polymers Charles A. Wilkie and Martin L. Mittleman 1

2

Department of Chemistry, Marquette University, Milwaukee, WI 53233 American Research, 4440Warrensville Center Road, Cleveland, OH 44128 1

The coupling of an infrared spectrometer to equipment for thermogravimetric analysis can help to develop a mechanistic understanding of the degradation mechanism of a polymer in the presence or absence of an additive. Because the additive can have an important effect upon the applications of the polymer, this technique permits correlation of its end uses with structural changes that may occur as a result of polymer-additive interaction. In this paper, we examine the effect of two additives, perfluorinated ionomer (Nafion-H) and manganese(II) chloride, on the thermal degradation of poly(methyl methacrylate) and propose mechanisms to account for the volatile products that evolve from these systems during thermal degradation.

JALDDITIVES

CAN

HAVE

A N IMPORTANT

EFFECT

O N T H E PROPERTIES

o f poly

mers; they m a y b e used as plasticizers, antioxidants, light stabilizers, a n d flame

retardants. E a c h o f these additives has some important effect o n the

potential e n d uses o f the p o l y m e r . Because these additives c a n m a r k e d l y affect the properties o f the p o l y m e r , they must also affect its structure i n some way. T h e particular additives o f interest here are flame retardants. F l a m e retardant additives m a y f u n c t i o n i n the vapor phase o r i n the c o n densed phase. V a p o r phase retardants generally are b e l i e v e d to generate 0065-2393/93/0236-0677$06.00/0 © 1993 American Chemical Society

In Structure-Property Relations in Polymers; Urban, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

678

STRUCTURE-PROPERTY RELATIONS IN POLYMERS

radicals that c o m b i n e w i t h the radicals o f the flame, thus r e m o v i n g t h e m f r o m the combustion zone; any c h e m i c a l reactions occur only after t h e r m a l degradation a n d hence do not effect the structure o f the polymer. I n the condensed phase, some c h e m i c a l reactions do occur between the additive and the p o l y m e r i c substrate. These reactions can alter the structure, and hence the properties, o f the substrate

(J).

T h e t h e r m a l degradation o f p o l y (methyl methaerylate)

(PMMA)

has

b e e n studied b y a great many workers. T w o recent series o f papers have Downloaded by UCSF LIB CKM RSCS MGMT on November 21, 2014 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0236.ch028

focused n e w attention o n this process.

Kashiwagi a n d co-workers

(2-5)

i m p l i c a t e d weak links i n the p o l y m e r as the p r i n c i p a l sites f r o m w h i c h degradation may occur. T h i s research group showed that P M M A p r e p a r e d b y a radical process degrades i n three distinct steps, whereas anionically poly m e r i z e d material degrades i n only a single step. This single step for the anionically p r e p a r e d p o l y m e r is an end-chain scission process, w h i c h is typical o f many polymers. T h e highest temperature degradation step for the radically p r e p a r e d p o l y m e r occurs at the same temperature as that observed for the anionically p r e p a r e d p o l y m e r a n d is ascribed to the same process. T h e other two steps are b e l i e v e d to be the result o f the cleavage o f weak links i n the p o l y m e r chain; these are specifically described as head-to-head linkages a n d unsaturated e n d groups (5). M a n r i n g ( 6 - 9 ) has postulated that the weak links are less important than previously thought a n d that degradation is b e g u n b y the cleavage o f the carbomethoxy group f r o m the m a i n chain o f the P M M A . M o n o m e r is the p r i n c i p a l product that is observed w h e n P M M A is thermally degraded. Trace quantities o f other products, notably C 0 , C O , C H , a n d C H O H , are also 2

4

3

observed, but these products are truly present i n very small amounts a n d are not easily seen. T h e initial step i n the M a n r i n g degradation pathway is always the cleavage o f the carbomethoxy group f r o m the p o l y m e r chain w i t h the formation o f a carbomethoxy radical a n d a radical along the m a i n chain. T h e carbomethoxy radical may degrade to produce C 0

2

a n d a m e t h y l radical or

C O a n d a methoxy radical. T h e m e t h y l a n d methoxy radicals can abstract hydrogens f r o m the p o l y m e r a n d y i e l d methane a n d methanol, respectively. T h e m a i n chain radical w i l l degrade to give m o n o m e r . T h e t h e r m a l degradation o f P M M A i n the presence o f additives has b e e n extensively studied recently b y the M c N e i l l group i n Scotland a n d o u r group at M a r q u e t t e University. T h e M c N e i l l group has examined the reaction o f P M M A w i t h silver acetate, a m m o n i u m polyphosphate, a n d zinc and cobalt bromides. T h e group at M a r q u e t t e University has examined reactions

of

P M M A w i t h r e d phosphorus a n d W i l k i n s o n ' s catalyst. M c N e i l l a n d co-workers have examined the effect o f silver acetate (JO), degradation by the use o f t h e r m a l volatilization analysis ( T V A ) . W h e n silver acetate is used as an additive w i t h P M M A , significant destabifization o f the


28.

WILKIE & M i T T L E M A N

TGA-IR

679

Spectroscopy

p o l y m e r is seen, presumably caused b y the diffusion o f acetoxy radicals into the P M M A chain, w h i c h initiates chain scission. A similar process is observed i n a study o f the degradation o f P M M A i n the presence o f P V C . It is postulated that the chlorine atoms p r o d u c e d b y P V C degradation initiate r a p i d degradation o f the P M M A chain (14). It m a y b e c o n c l u d e d that the presence o f radicals is deleterious to p o l y m e r stability because they c a n initiate chain scission. I n a b l e n d o f P M M A a n d a m m o n i u m polyphosphate ( I I ) , the major Downloaded by UCSF LIB CKM RSCS MGMT on November 21, 2014 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0236.ch028

product o f t h e r m a l degradation is m o n o m e r , b u t significant amounts o f other products, notably methanol, C O , C 0 , d i m e t h y l ether, a n d char are also 2

p r o d u c e d . It was suggested that the degradation o f a m m o n i u m polyphosphate produces the strong acid, polyphosphoric acid, a n d that this catalyzes the hydrolysis o f ester groups o n the P M M A to give some charring. T h e combination o f zinc b r o m i d e a n d P M M A ( 1 2 , 13) seems to signifi cantly retard depolymerization o f the p o l y m e r . T h e initial step appears to b e the coordination o f zinc to the carbonyl carbon o f the P M M A . T h i s zinc complex c a n lose m e t h y l halide a n d ultimately f o r m a zinc salt. I n this laboratory w e have b e e n c o n c e r n e d w i t h developing a mechanistic understanding o f the reactions o f P M M A w i t h various additives. T h e b e l i e f is that i f one c a n predict, i n detail, h o w a p o l y m e r a n d a variety o f additives w i l l chemically interact, t h e n a n additive to p e r f o r m a specific function c a n b e designed. Initially w e investigated the reaction o f P M M A a n d r e d phospho rus. T h i s investigation was motivated b y reports that indicated some efficacy for r e d phosphorus as a flame retardant

f o r oxygenated polymers ( 1 5 ) .

H o w e v e r , there were n o reports that delineated the course o f the reaction. O u r investigation showed that r e d phosphorus attacks the carbonyl moiety o f the P M M A w i t h the formation o f m e t h y l methoxy p h o s p h o n i u m ions a n d an intramolecular anhydride ( 1 6 , 17). Because attack occurs at a carbonyl site, retardant.

A s such, w e chose to use W i l k i n s o n ' s catalyst,

ClRh(PPh ) . 3

3

Reaction proceeds between P M M A a n d W i l k i n s o n ' s catalyst b y an oxidative insertion o f the r h o d i u m species into a c a r b o n - o x y g e n b o n d o f the polymer; b o t h intra- a n d intermolecular anhydrides are f o r m e d a n d extensive formation occurs.

char

It is significant that the l i m i t i n g oxygen index ( L O I )

increases b y 6 points w h e n the r h o d i u m c o m p o u n d is physically c o m b i n e d w i t h P M M A (18,

19).

T h e r e is a significant difference between degradation carried out i n a static system, such as a sealed tube, a n d a dynamic system, such as thermogravimetric analysis. I n a sealed tube reaction, the degradation products are contained a n d may undergo further reaction, whereas i n a dynamic system the products are swept out o f the system as q u i c k l y as they are f o r m e d so that further reaction is prevented. T h e degradation o f P M M A i n a sealed tube leads to significant quantities o f volatile products. W h e n m o n o m e r alone is


680



heated u n d e r identical conditions, smaller quantities o f these volatile p r o d ucts are obtained, w h i c h indicates that these products arise not only f r o m degradation o f p o l y m e r , but also f r o m some degradation o f m o n o m e r . In this chapter w e w i l l summarize o u r recent studies o n the interaction o f Nafions a n d manganese(II) chloride w i t h P M M A a n d offer a n e w interpreta tion o f the manganese(II) chloride reaction. A l t h o u g h these studies have u t i l i z e d b o t h sealed tube reactions and thermogravimetric analysis ( T G A ) c o u p l e d to infrared ( I R ) spectroscopy, the focus o f this paper is o n the development o f a mechanistic understanding of the reaction b y T G A - I R . Because the sealed tube reaction is a static system a n d the T G A - I R experi ment is a dynamic system, it is not too surprising that the results are somewhat different i n the details; however, they still provide the same overall conclusion. T G A - I R is a valuable technique for investigating the mechanistic aspects o f the reaction between a p o l y m e r a n d its additives because it provides b o t h t e m p o r a l a n d temperature resolution o f the thermal degrada tion processes.

Experimental Oetails T G A - I R was p e r f o r m e d using a thermogravimetric analyzer (supplied by O m n i t h e r m Corporation) c o u p l e d to a F o u r i e r transform i n f r a r e d spectrome ter ( D i g i l a b F T S - 6 0 ) . p r o v i d e d elsewhere

Specific details o f this integrated system have b e e n (20,

21).

O f significance is the fact that the T G A

interface a n d slave processor are b o t h controled by a single data station, system runs u n d e r the d i r e c t i o n o f a single macro. TGA

sample sizes for this study ranged f r o m 20 to 25 m g a n d the

samples w e r e heated at 20 ° C / m i n u n d e r an inert gas purge o f 50 c m / m i n . 3

Gases evolved f r o m the heated sample were transferred to an I R gas c e l l via a glass transfer l i n e heated to 210 °C. T h i s line h a d an overall length o f 47 c m and an i n n e r diameter o f 0.2 c m . T h e stainless steel gas c e l l h a d a 10-cm path length a n d 0.6-cm i n n e r diameter; its temperature was maintained at 235 °C. Spectroscopic data were collected using the F o u r i e r transform i n f r a r e d spectrometer e q u i p p e d w i t h a K B r b e a m splitter and a w i d e - b a n d l i q u i d nitrogen-cooled m e r c u r y - c a d m i u m telluride ( M C T ) detector. Spectra w e r e collected at 8 - c m

- 1

resolution, coadding 16 scans p e r spectrum. T h i s re

sulted i n a t e m p o r a l resolution of 4 s, more than sufficient for the gradual gas evolution rates characteristic

o f most T G A profiles. T h e problems

and

possibilities for quantification o f evolved gases using the spectral i n f o r m a t i o n f r o m T G A - I R have b e e n reported (22).

T h e present w o r k is c o n c e r n e d only

w i t h qualitative identification.


28.

TGA-IR


Spectroscopy

681

Results and Discussion


Degradation of Perfluorinated Ionomer (Naflon-H). T h e structure of N a f i o n - H consists o f a poly(tetrafluoroethylene) ( P T F E ) back bone w i t h pendant sulfonic acid groups. B e f o r e the t h e r m a l degradation of Nafions can be understood, it is important to have some insight into the t h e r m a l degradation process for P T F E . P T F E is one of the most thermally stable linear polymers. Its t h e r m a l stability is attributed to the h i g h C ~ F b o n d strength a n d the shielding effect of the very electronegative fluorines. T h e t h e r m a l degradation o f P T F E commences at about 450 °C a n d is believed to p r o c e e d b y r a n d o m chain scission w i t h the formation of difluorocarbene. T h i s reactive carbene leads to the observed m o n o m e l i c tetrafluoroethylene ( T F E ) a n d oligomeric products (23-25). T h e r e is little degradation of N a f i o n - H i n a T G A experiment (26) b e l o w 280 °C i n an inert atmosphere ( F i g u r e l a ) . A small 5 % weight loss occurs b e l o w this temperature a n d the only gases that are detected are H 0 , S 0 , poly(arylene sulfonate) to t h e r m a l degradation i n v a c u u m a n d observed that S 0 was evolved b y cleavage o f the C —S b o n d ; the m a x i m u m evolution occurred between 250 and 350 °C. 2

2

2

A T G A weight loss of 7 % occurs between 280 a n d 335 °C. T h e evolution of S 0 and C 0 increases throughout this region whereas that of water decreases. S i F (1026 c m ) , C O , H F , substituted carbonyl fluorides (1957 and 1928 c m ) , a n d absorbances i n the C — F stretching region also appear over this temperature range. A n I R spectrum o f the gases evolved at 367 °C is shown i n F i g u r e 2. O f particular note are the bands just b e l o w 2000 c m attributable to carbonyl fluorides. S i F is not a p r i m a r y product of the reaction, but rather it arises f r o m the attack o f evolved H F o n glass. T h u s w h e n S i F is observed, the formation of H F is indicated a n d the actual product w i l l be identified as H F . A t the highest temperatures, 3 5 5 - 5 6 0 °C, dramatically at 365 °C a n d are no longer of consequence. T h e major absorbances i n this temperature region are due to H F , carbonyl tion is presented i n Scheme I. 2

2

4

- 1

- 1

- 1

4

4

T h e C —S b o n d is initially b r o k e n , w h i c h produces a C F radical, S 0 a n d a hydroxyl radical (eq 1). A n alternative explanation is that this occurs i n two steps: the initial formation o f an S O H radical, f o l l o w e d b y cleavage to f o r m S 0 a n d the O H r a d i c a l I n either case, the fluorocarbon radical can then lose two difluorocarbenes (eqs 2 a n d 3), w h i c h produces an oxygen-based radical. This radical can subsequendy lose a substituted carbonyl fluoride (eq 4) a n d carbonyl fluoride ( e q 5). T h e r e m a i n i n g P T F E - l i k e backbone w i l l then degrade to tetrafluoroethylene m o n o m e r a n d oligomers. 2

s

2


2

fluorid

682


(a)


100.00

yj

50.00

60.00

120.00

180.00

240.00 300.00

340.00

420.00 480.00

540.00 620.00

(b) 100.00 PMMA

alone

\

\ ω

50.00 \

f ι ι

»

\• \l1/

\V t I

60.00

120.00

180.00 240.00 300.00

340.00

420.00 480.00

540.00 620.00

Figure 1. TGA curve for Nafion-H (a), PMMA (b), and a blend of the two (c). These curves were obtained under an inert atmosphere at a scan rate of 20 °C.


28.


TGA-IR

Spectroscopy

683


(c)

60.00

120.00

180.00

240.00 300.00

340.00

420.00 480.00

540.00 620.00

T E M P E R A T U R E (C)

Figure 1. Continued Verification o f this mechanism comes f r o m an examination o f the degra dation o f the potassium salt o f the sulfonic acid, N a f i o n - K , p r e p a r e d b y soaking a sample o f N a f i o n - H i n aqueous K O H . Because the t h e r m a l stability of amine arenesulfonates is greater than that o f the corresponding sulfonic acids (28), cleaving the C —S b o n d s h o u l d b e more difficult f o r N a f i o n - K than i t is f o r N a f i o n - H . T h i s hypothesis is supported b y the fact that w h e n N a f i o n - K is subjected to T G A - I R investigation, there is n o weight loss u n t i l 390 °C, some 100 °C higher than that observed f o r N a f i o n - H . S 0 is not observed; the only products are H F a n d fluorocarbon oligomers that w o u l d be expected f r o m P T F E . 2

Interaction of Nafion-H and P M M A . T G A - I R studies o f a b l e n d prepared b y casting a P M M A film onto a N a f i o n - H film reveal a significant difference between the i n d i v i d u a l components a n d the b l e n d (26). T h e T G A of N a f i o n - H is shown as F i g u r e l a , P M M A is shown i n F i g u r e l b , a n d the b l e n d is shown i n F i g u r e l c . B o t h P M M A a n d N a f i o n - H have completely volatilized at 500 °C, whereas the b l e n d has about 1 0 % residue r e m a i n i n g at 600 °C. T h e T G A curve o f the b l e n d indicates that degradation occurs i n three stages. I n the first stage, f r o m 120 to 265 °C, 1 1 % o f the sample is volatilized. T h e gases evolved d u r i n g this stage are water, w h i c h is retained b y the N a f i o n , a n d c h l o r o f o r m , the solvent f r o m w h i c h the P M M A is cast onto


684


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Thermogravimetric Analysis-Infrared Spectroscopy - Advances in

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