Thermal properties of polyacrylic acid

1848

Y . P . K~A~OV e~ a/.

4. V. A. KARGIN, T. I. SOGOLOVA and T. IL SHAPOSHNIKOVA, Dokh AN SSSR, 180: 901, 1968 5. H. SCHONHORN, Maeromolecules 1: 145, 1968 6. J. R. COLLIER and L. M. NEAL, Polymer Engng Sci. 9; 182, 1969 7. A. YIM and L. E. ST. PIERRE, d. Polymer Sci. BS; 241, 1970 8. D. R. FrrCHMUM and S. NEWMAN, J Polymer Sci. 8: A-2, 1545, 1970 9. A. SIEGMANN and P. H. GEIL, Macromolec. Sei B4: 239, 1970 10. Yu. M. MALINSKII, N. M. TITOVA and M. G. ARESBrFDZE, ¥ysokomoh soyed. BI4; 485, 1972 (Not translated ,in Polymer Sci. U.S.S.R.) 11. Yu. M. MALINSK~ M. G. ARES~I~ZE and N. F. BAKEYEV, I)okh AN SSSR 208: 1142, 1973 12. R. L. CORMIA, F. P. PRICE and D. TURNBULL, J. Chem. Phys. 37, 1888; 1962 13. Spravoehnik khimika (Chemist's Handbook). Goskl~imi~dat, 1951 14. M. A. AVRAMI, J. Chem. Phys. 7; 1103, 1939; 8: 212, 1940; 9: 177, 1941 15. M. G. ARESI~BZE, Dissertation, 1973

THERMAL PROPERTIES

OF POLYACRYLIC ACID*

V. P. KA~NOV, V. A. DUBNITSKAYAand S. N. K~AR'KOV All-Union Scientific Research Institute for ~an~-made Fibres (R6ceiv~/ 29 ~lpr/1 1974) Dynamic thermogrammetry, mass spectrometry and I R spectroscopy were used to study thermal decomposition of polyacryhe acid (PAA) and polymethacrylie amds (PMAA) over a wide range of temperature (20-500°). Oonslderable dJfferences were detected m the heat stablhty of the acids examined, Heating PMAA results m the intermediate formation of polymethacryllc anhydride, whereas PAA only undergoes decarbexylatlon. Mathematical analysis of experimental thermogravmaetrlc curves was used to calculate apparent activation energaes of cyclodehydratlon of PMA_A, decomposition of the polymethacrylic anhydride formed and decarboxylatlon of PAA, these values were found to be 21, 40 and 194-2 kcal/mole, respectively. POLYACRYLIC acids are widely u s e d for sizing a n d m o d i f y i n g s y n t h e t i c fibres [1-3], including h e a t r e s i s t a n t fibres. T h e successful use o f p o l y acids in t h e s e fields d e p e n d s to a large e x t e n t on t h e i r t h e r m a l p r o p e r t i e s . P r o b l e m s of t h e r m a l b r e a k d o w n o f p o l y a c r y l i c (PAA) a n d p o l y m e t h a c r y l i e acids (PMAA) h a v e b e e n r e p e a t e d l y d e a l t w i t h in t h e l i t e r a t u r e [4-10]. H o w e v e r , t h e n a t u r e of t h e i r p y r o l y s i s u p t o n o w r e m a i n s d e b a t a b l e a n d i n f o r m a t i o n a v a i l a b l e is conflicting [7, 9-11]. A n a t t e m p t h a s o n l y r e c e n t l y b e e n m a d e t o s t u d y t h e r m a l p r o p e r t i e s o f P A A a n d P M A A using t h e s a m e m e t h o d s a n d u n d e r i d e n t i c a l conditions [12]. * Vysokomol. soyed. A17: No. 7, 1604-1609, 1975.

Thermal properties of polyacryhc acid

1849

Thermal properties of PAA and PMAA were therefore examined over t h e temperature range of 20-500 ° and main trends of decomposition processes determined. Dynamic thermogravimetry was the main method of investigation which, together with mass spectrometric analysis of volatile products of decomposition and I R spectroscopic analysis of the solid residue, is very informative and enables individual reactions of decomposition and the temperature range of these reactions to be clearly established. In addition, in kinetic investigations a sole thermogravimetrie (TG) curve can in m a n y cases replace numerous isothermal curves Appropriate mathematical analysis of thermogravimetric results enables us in these cases to evaluate the energy parameters of the process.

10 3

02 I 0

f ZOO

I

I 4OO 7;,*C

3600

2000

120g v, cm "~

FIG 2

:FIG 1

FIG. 1. DTA (a) and TG (b) curves of mltlal PMAA (1) and samples after heating (2) to

200° for 6 hr. FIG. 2. IR spectra of the mltlal PMAA (1) and PM.AAsamples continuously decomposed m air for 30 mln at temperatures, °C: 2--100; 3--150; 4-250; 5--300. Figure 1 shows TG curves of the breakdown of PMAA and DTA curves, which reflect the enthalpy variation of the system. An analysis of these curves suggests the presence of at least two different reactions which correspond to two endothermic m a x i m u m values on the DTA curve and two degrees on the curve of gravimetric loss. The endothermie maximum of low intensity (70-150 °) with a m a x i m u m of about 125 ° is no doubt due to the elimination of solvent in polymerization of toluene and the equilibrium moisture. During decomposition of PMAA over the temperature range of 180-260 °, to which the first strong endothermic m a x i m u m corresponds on the DTA curve, cyclodehydration is the

1850

V.P. K~ov

e~ a/.

decisive trend of breakdown to form six membered anhydride structures CHs I

CHz I

I

.CHs r,]

-H,o

I

C00H

ti\

C00H

0 C

CHa I

(D

/1~ x-0 C 0

Reaction (1) is confirmed b y the appearance in I R spectra of partially decomposed samples of absorption bands at 1820, 1780 and 1025 cm -t (Fig. 2, curves 3 and 4), corresponding to anhydride groups. It should be noted that cyclization takes place fairly intensively and is practically complete at 250 °, whereby the anhydride groups formed are very stable and do not noticeably decompose up to 300 ° (Fig. 2, curve 5). A similar description of breakdown is also fully confirmed b y mass spectrometric analytical results of volatfles (Table 1), which indicate that water separated during cyclization is the main product of breakdown at 208 ° . TABLE

1. C O M P O S I T I O N OF V O L A T I L E P R O D U C T S OF "£~t~: B R E A K D O W N OF P ~ A ~ L aL_T VARIOUS TEMPERA.TURES

T, o(]

160 208 325 400

Composition of v o l a t i l e products, %

WlIlln~

rn~n 20 20 10 7

H~O 9

73 76 3.9

COl 1 5.1 23 69

monomer 5 0.4 0-4

toluene 27 21 0.3 0-I

unidentified products 58* Traces Traces 27*

* Allphatic aldehydes and alcohols 'f Unsaturated hydrocarbons.

Polyanhydride obtained as a result of dehydration of PMAA has a fairly high ( ~ 2 1 0 ° ) softening point, which is an satisfactory agreement with results obtained b y Bresler et al. [13], who synthesized a linear high molecular weight polyanhydride of methacrylic acid (PMAAn) b y cyclopolymerization of methacrylic anhydride with Tso~ 200 °. Furthermore, PMAAn is a highly heat resistant polymer, intensive decomposition takes place at temperatures exceeding 350 ° (Fig. 1). The absence from the DTA curve for P M A A of an endothermie maxim u m at 230 ° is evidence of the completion of cyclization under the conditions described. B y mathematical analysis of thermogravimetric results it was proposed [14-16] to evaluate kinetic parameters of main reactions of thermal decomposition of PMAA. The apparent activation energy of eyelodehydration of PMAA in the temperature range of 180-250 ° was 21 kcal/mole with first order process kinetics, which is in satisfactory agreement with the value of 19.5 kcal/mole [5] b u t at the same time differs considerably from results of Grant and Grassi [4] ( 3 7 + 3

Thermal propertms of polyacryhc acid

I851

keal/mole) and Geuskens et al. [6] (42q-1 kcal/mole) Decomposition of PMAAn at 370-430 ° not studied previously ~s of first order kinetics The apparent activation energy calculated by the authors and derived by analysing thermograms for PMAA and PMAAn is 40 kcal/mole for this reaction. The following sections of TG curve can be isolated using the results described which correspond to main stages of thermal decomposition of PMAA Stage I (75-175 °) General weight loss is ~5°/o and is determined b y the loss of solvent and equilibrium moisture (~1.5~o) with a slight proportion of depolymerization The isolation of noticeable (~3~o) amounts of aliphatie aldehydes and alcohols ~s very interesting and has not been mentioned previously under these conditions. Stage I I (180-260 °) Section corresponding to the first endothermlc maxim u m on the DTA curve of PMAA General weight loss ( N 1 2 ~ ) is mainly determined by the separation of water during cychzation (theoretical amount 10.5~/o) and b y the ehminatlon of solvent residues and partial, deeper conversions which are due to the liberation of C02 probably b y the reaction CHa I

CI-h I

CH3 -CO~

.~ --CH~--C--CH2--C--~ ~

CH3

I

I

~ --CH2--C--CIt2--C-- N

\c/ U

(2)

0 Stage I I I (260-350°). In this interval weight losses are very slight (~4~/o) and are due to the partial breakdown of PMAA formed in the previous stage. Judging b y the appearance of a band in I R spectra at about 1150 em -1 (Fig. 2, curves 4 and 5), which is due to absorption of the keto-group [17], breakdown in this range of temperature is determined by reaction (2). The accuracy of this assumption is confirmed b y mass spectrometric results (Table 1), which indicate that at 400 ° the amount of C09. isolated during breakdown, suddenly increases. Stage I V (375-475°). Section corresponding to the second endothermic maxim u m on the DTA curve. The reaction is accompanied b y considerable (up to 60%) weight loss of the polymer and considerable conversions which are difficult to measure. However, the apparent activation energy calculated in the temperature range of 370-430 ° makes sense since weight loss in this interval is mainly due to reaction (2) Further decomposition of the polymer is due [18] to the liberation of hydrocarbons and CO It should be noted that as shown by results in Table l, the proportions of depolymerization in stages I I - I V is not significant, which conflicts with results obtained b y authors of an earlier paper [5], who observed considerable amounts of monomer at temperatures up to 300 °. We also established that the polymethaerylic anhydride formed during dehydration of PMAA retains solubility in D M F on heating the samples up to 300 ° (although it becomes insoluble in water at 200°). This is evidence of the formation of almost only intramolecular

V. P. KAna.~ov ~ al.

1852

anhydride structures as a result of the separation of a water molecule from adjacent carboxyl groups. The t y p e of decomposition of PAA under similar conditions of breakdown differs considerably from that described for PMAA. Thermogravimetric results (Fig. 3) clearly indicate the presence of three endothermic maxima superimposed on each other in the range of 180-450 ° . Most scientists who have previously examined thermal breakdown of PAA [7-12] noted that cyclization takes place to some extent at different temperatures of decomposition and attribute to it an important rSle when describing the decomposition of poly acid. However,

1

I

b

0"8

\

~ 0.0

I

0

I

200

I~G. 3

I

I

400 T,°G

I

3600

,

,

i

2000

~

l

=12 IDDI

I

800

V , Crr/"t

FIG. 4

l~m. 3. DTA (a) and TG (b) curves of samples of initial PAA (1) and after heating to 200° for 6 hr (2). FIG. 4. IR spectra of the mltlal PAA (1) and PAA samples decomposed m stages m air for 30 rmn at temperatures, °C: 2--100; 3-150; 4--200; 5--250; 6--300. under the conditions of our experiment, we were unable to detect the formation of anhydride rings in proportions which determine the direction of decomposition. This conclusion m a y be drawn using I R spectra of partially decomposed samples, in view of the absence from the spectra of typical absorption of anhydride groups ((Fig. 4) and from mass spectrometric results (Table 2), which indicate that carbon dioxide is the main gaseous product of pyrolysis at temperatures higher than 150 ° . The presence in volatile products at low temperatures of small amounts of water and significant amounts of toluene, as with the decomposition of PMAA, is due to elimination of solvent and equilibrium moisture. According to I R spectroscopic results (absence of absorption of cyclic ketone

Thermal propertms of polyacryhc acid

1863

at 1780 cm -1) and due to the absence of water from pyrolytic products, decomposition b y the processes previously proposed [10] is very doubtful.

--CHz--CH--CHz--CH

1

--H,0 . . . . .

I

COOH

CH~--CH--CH2--CH~ ~-

-co,

\

COOH

\

c

/

/

(3)

o Since almost exclusively products of thermal decomposition are carbon dioxide, this suggests that the first two reactions of decomposition of PAA are decarboxylation, characterized b y two endothermic maxima on the DTA curve in the TABLE 2 COMPOSITION OF VOLATILE BREAKDOWN FRODUCTS OF P A A AT DIFFERENT TEMI~ERATURES (PYROLYSIS IN STAGES)* Time,

T, °C

I Composltmn of volatile products, %

ram

l

20 20 10

150 208 326

H~O

[

62 Traces

3O

CO~ 178 765 97 0

I

toluene

86 0 13 5 Traces

* No acryhc acid was found in the volatile products

temperature range of 180-350 ° with maxima at 260 and 290 °. These reactions are detected at 180 ° and determine the direction of decomposition up to 350 °. At the upper temperature limit gravimetrm losses correspond to separation of ~ 50% of carboxyl groups of the polymer --CH2--CH--CH2--CH-- ~

I COOH

i C00H

- c o , ~ ~CH2--CH2--CHz--CH-- --~

I

(4)

C00H

Reaction (4) is confirmed b y a weakening of the absorption band intensity of carboxyl groups in the region of 2500-3500 cm -1 (Fig. 4, curves 5 and 6). The existence of two endothermic maxima confirms that the energetics of carboxylation change on eliminating a given amount of carboxyl groups. Preheating of PAA samples to 200 ° for 6 hr, in contrast to PMAA, has practically no effect on the heat stability of the polymer, although it causes some changes in the type of decomposition {Fig 4), to which changes are related in the position and intensity of endothermic maxima over the temperature range studied. This fact m a y be evidence of a very low rate of decomposition of PAA at temperatures of up to 200 °. The apparent activation energy of decarboxylation at temperatures of 180-260 ° was found to be 1 9 ~ 2 kcal/mole. The order of the reaction is fractional and is between 0.5 and 1.0. It should be noted that the evaluation of

1854

V . P . KAB~OV ~ aJ.

b r e a k d o w n kinetics o f P A A involves considerable a s s u m p t i o n s a n d t h e a c c u r a c y o f d e t e r m i n i n g e n e r g y p a r a m e t e r s is lower t h a n in t h e case of P M A A . A c c o r d i n g t o results in t h e l i t e r a t u r e [7], t h e a c t i v a t i o n e n e r g y of d e c a r b o x y l a t i o n of P A A is 27 kcal/mole. T h e r e a c t i o n in t h e t e m p e r a t u r e r a n g e of 400-500 °, to w h i c h a t h i r d e n d o t h e r m i c p e a k c o r r e s p o n d s on t h e D T A c u r v e involves considerable conversions, a q u a n t i t a t i v e e v a l u a t i o n of w h i c h p r e s e n t s considerable difficulties. Tt should also be n o t e d t h a t t h e r e is no d e p o l y m e r i z a t i o n o v e r t h e entire t e m p e r a t u r e range. This f a c t is n o t u n e x p e c t e d a n d confirms t h e general condition [19] concerning t h e p r e f e r e n t i a l d e p o l y m e r i z a t i o n of c o m p o u n d s h a v i n g a quaternary carbon atom. PAA and PM_~A samples were obtained by radical polymerization m toluene. Polymer molecular weights were 13,000 and 14,000 for PAA and PM_AA, respectively. A thermogrammetrlc study was m ~ e nslng a Pauhk-Pauhk-Erdel denvatograph (Hungary) at temperatures of 20-500 ° at a rate of increasing temperature of 5 deg/rmn using samples of 100-200 rag. I R spectra of partially broken down samples were recorded m the form of pellets v~th KBr using a UR-10 speetrophotometer at 700-4000 cm-L Mass spectroraetrlc analysis of thermal breakdown was carried out m a MU-1305 mass spectrometer with a modified inlet system. Kinetic reaction parameters were calculated by at least two methods premously proposed [14-16]. Calculated values were vemfied m respect of the accuracy of agreement of the experunental and theoretical curve using the Tables pubhshed by Doyle [16]. The authors are grateful to G. G. Sltmkova for prowdmg the polyacryhe acid samples and V. P. Akseneva for carrying out the mass spectrometric analysis. Tra~lat~d by E. S E ~ REFERENCES 1. U.S A. Patent 3487039, 1965

2. U.S.A. Patent 2702796, 1955 3. U.S.A. Patent 3672975, 1970 4. D. H. GRANT and N. GRASSI, Polymer 1: 125, 1960 5. V. Z. POGORELKO, N. A. KARETNIKOVA and A. V. RABOV, Trudy po khimii i khi~meh. tekhnologli, (Studies on the Chermstry and Chemical Technology of Polymers). 1, Gorky, 1972 6. G. GEUSKENS, E. HELLINCKX and C. DAVID, Europ° Polymer J. 7: 561, 1971 7. A. EISENBERG, T. YOKOYAMA and E. SAMBALIDO, J. Polymer Sei. 7; A - l : 1717, 1969 8. J. LLERAS and S. COMBET, J. chim. phys. et phys.-chim, biol. 69: 1626, 1972 9. S. COMBET and J. LLERAS, Compt. rend. C270: 1280, 1970 10. N. A. KARETNIKOV, V. Z. POGORELKO and A. V. RYABOV, Trudy po khimii i khimich. tekhnologii (Studies on the Chemistry and Chemical Technology of Polymers). 1, Gorky, 1972 11. M. S. MCGAUCH and S. KOTTLE, J. Polymer ScL BS: 817, 1967 12. F. X. ROUX, R. AUDEBERT and C. QUIVORON, Europ. Polymer J. 9: 815, 1973 13. C. E. BRESLER, M. M. KOTON, A. T. OS'MINSKAYA, A. G. POPOV and M. N. SAVIT$KAYA, Vysokomol. soyed. 1: 1070, 1959 (Translated in Polymer Sci. U.S.S.R. 1: 3, 393, 1960)

Substantial revermble deformahons m glassy polymers

1855

14. V. S. PAPKOV and G. L. SLONIMSKII, Vysokomol. soyod. 8: 80, 1966 (Translated m Polymer ScL U.S S R. 8: 1, 80, 1966) 15. J. R. MCCALLUMand J. TANNER, Europ. Polymer J. 6: 1033, 1970 16. C. D. DOYLE, J. Appl Polymer Scl. 5: 285, 1961 17. K. NAKANISI, Infrakrasnyye spektry 1 stroyemye orgamchesklkh soyedmomi (IR Spectra and Structure of Organic Compounds). Izd. "Mlr", 1965 18 L. S. MCNEIL, Europ. Polymer J. 6: 373, 1970 19 N. GRASSI, KhLmlya protsessov destruktsn pohmerov (Chermstry of the Breakdown of Polymers) Izd. inostr, h t , 1959

S U B S T A N T I A L R E V E R S I B L E D E F O R M A T I O N S IN GLASSY POLYMERS * A. L. VOLYNSKII and N. F. BAK~rEV M. V. Lomonosov State University, Moscow (Received 11 June 1974)

A study was made of processes taking place :n glassy polymers during deformation m adsorphon-actlve media. It was shown that glassy polymers are able to undergo considerable non-entropy type reversible deformation. A mechamsm m proposed for the effect flk REPORT has been made [1] about the occurrence of considerable reversible deformations in amorphous glassy polymers. This effect m a y be observed in those cases when the polymer is subjected to uniaxial elongation in adsorptionactive media Figure 1 shows evaluahons of revermble deformation for several polymers in the amorphous state The Figure shows t h a t contraction after deformation of the polymer m an adsorption-active medium (n-propanol) reaches 80-99%, i.e. deformation is practically reversible During deformation of P E T P and polycarbonate (PC) in air'neck' formation occurs and reversible deformation under these conditions is ~ 10°//o (Fig. la and b, curves 2). These polymers only show signs of the contraction described when the medium is removed. Polymers (PS and PMMA) undergoing brittle decomposition in air with small deformations also show considerable contraction in the wet state (Fig lc and d, curves 2), whereas during drying contraction reaches practically 100% (curves 1, Fig. lc and d). Experimental results obtained for several objects showed for the first time the ability of polymers to undergo considerable (for P E T P up to 200%), practically reversible deformations below glass temperature. This paper seeks to explain the mechanism of this effect. * Vysokomol. soyed AI7: No. 7, 1610-1617, 1975