Structure, stability and degradation of organosilicon aramids

Polymer Degradation and Stability 13 (1985) 327-336

Structure, Stability and Degradation of Organosilicon Aramids J. Y. J a d h a v * Division of Polymer Chemistry, National Chemical Laboratory, Pune 411 008, India (Received: 10 June, 1985)

ABSTRACT The thermal degradation of twelve structurally related high molecular weight silicon-containing aramids has been studied by dynamic thermogravimetrie analysis. The analyses were carried out in air over a temperature range of 25-900°C with a heating rate of lO°C/min. Kinetic parameters of decomposition like activation energy and preexponential factor were determined from original thermograms. The thermal stability of aramids is a function of structure and molecular weight. The activation energy of decomposition and the pre-exponential factor depend upon the molecular weight of the polyamide. The chemical stability and degradation of aramid films are discussed.

INTRODUCTION Wholly aromatic polyamides (aramids) such as poly(1,3-phenyleneisophthalamide), its post-chlorinated form and poly(1,4-phenyleneterephthalamide), commercially known as Nomex ®, Durette ® and Kevlar ®, respectively, have been utilized successfully in the manufacture of the interiors of wide-bodied aircraft and protective clothing. They have *Present address: Polymer Science and Engineering Department, University of Massachusetts, Amherst MA 01003, USA. 327 Polymer Degradation and Stability 0141-3910/85/$03"30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain

328

J. Y. Jadhav

applications also in high-temperature fibers and films because of their outstanding thermomechanical properties. The thermal degradation of these macromolecules has been the subject of many investigations.I-6 The rate and mode of degradation depend on the temperature, heating medium and structure of the polymer. The thermal degradation in aramids principally depends upon the chemical structure of the repeating unit and upon the orientation of the aromatic ring with the amide function, for example, A < B < C < D: CO-_HN@N

CO--

H--CO-@ --HN-@NH--CO~ B

A

C

--HN@-N

H--CO-~CO--

D This investigation was undertaken with a view to correlating the polymer structure of silicon-containing aramids with the thermal and chemical degradation. EXPERIMENTAL Polymer preparation

The preparation of aramids from silicon-containing acid dichlorides and aromatic diamines via low-temperature interfacial condensation was reported recently by Ghatge and Jadhav. 7 Viscosity measurements

The inherent viscosities of aramids were determined in 0-5 ~o concentration in dimethylacetamide at 30°C using an Ubelohde viscometer.

Structure, stability and degradation oJ organosilicon aramids

329

Film casting Aramid films were cast from 10 ~o solutions in dimethylacetamide using the doctor blade technique. To ensure complete removal of solvent, films were heated overnight at 100°C and for 3 h at 200°C in vacuum.

Thermogravimetric analysis Thermogravimetry (TG), derivative thermogravimetry (DTG) and differential thermal analysis (DTA) were carried out simultaneously on a Mom-Budapest derivatograph type OD-102 described by Paulik et al.8 The analyses were made in air at a heating rate of 10 °C/min.

Chemical stability The chemical resistance of aramid films was measured by immersing films in different solvents for 7 days at room temperature.

RESULTS A N D DISCUSSION The structure of the repeating unit in aramids, inherent viscosities and important thermal characteristics are summarized in Table 1. The thermal characteristic values such as the temperature at which 10~o weight loss occurs (T~o), the temperature of maximum decomposition rate (Tmax) and the integral procedural decomposition temperature 9 (IPDT) indicate that the thermal resistance of these macromolecules depends upon the structure of the repeating unit. The thermal analytical plots of weight loss ( ~ ) against temperature are given in Figs 1 to 4, which show that the degradation of aramids starts between 300 and 400 °C. The overall nature of the curves in Figs 1 to 4 is more or less identical and indicates a similar type of thermal degradation. The temperature of maximum decomposition rate is derived from D T G curves. In DTA, no endothermic transition associated with glass transition temperature or melting temperature was observed but an exothermic peak was observed in every case. Several expressions have been reported ~o - 12 for the determination of activation energy of decomposition but none takes into account variation in sample size and heating rate. Dharwadkar and Karakhanwala ~3 have

J. Y. Jadhav

330

< t~

0

~

~,~

.~.

.~.

I

'

~

j

Z

~

~

e

Z

Z

Z

Z

, I

z

Z

Z

Z

o_-~ o__~ o=~

o=~

o=~

Z

o_-~

e~

I

I

I

I

I

I

Structure, stability and degradation of organosilicon aramids

331 cO

e~ 0

o

o

o

~

2.

ooo

o

o

o

o

o

-5 ..o

I~"

I

I

z

I

I

z E

z

z f0 ..~ ©

E 0

z

z

I

z

I

z

I

z

I

E E

z

I

I

= . = '~ 0

I

I

I

I

I

I

-=

~0

.~

332

J. Y. Jadhav

k~

+ o

CD

I--

o

o,1

I

0 0

I

0 ~0

I

I

0 0 ~0 ~" % "SSOl ~qS!~M

I

0

f..,)

0

0

.a

8

+

o° ' ud ©

<~ e"

8 I

I 0

% t~Ol

I 0

~.qf!m/.A

I 0

o

.~

Structur e, stability and degradation of organosilicon aramids

333

I0

\ i

+

ou

O E O~

~-~_

I

I

0 0

0 CO

I

I

0 0 ',.D. ~I" % ~SSOl ~ 6 ! a M

I

0 O~

0

\

+ "v

,f ~E 0

oE

8 I

0 0

I

O GO

I

0 ',O

I

0 ~I"

I

0 c~

0

Q-

O~

334

J. Y. Jadhav

modified the original expression of Horowitz and Metzger 12 to overcome certain drawbacks in the determination of the activation energy of decomposition by dynamic thermogravimetry. Their modified equation is as follows: E In In (1 - w) -1 --.,,~,~17t X

100

iT--Tf-- × 0 + C

where w is the weight loss at a particular temperature; E is the activation energy of decomposition; Ti is the initial decomposition temperature; TI is the final decomposition temperature; 0 = T-Tm~ x, where T is the temperature under consideration and Tmax is the temperature of the maximum decomposition rate; C is a constant; and R is the gas constant. The activation energy of decomposition derived by this method depends a great deal on the accuracy with which Ti, Tf and Tmax are determined. The activation energies of decomposition of these aramids obtained by this method vary between 126 and 58 kJ/mole. The pre-exponential factor is calculated using the following equation: lnART2max-

ERn

E RTm,x

where E is the activation energy of decomposition, R is the gas constant, Tmax is the temperature of maximum decomposition rate, R n is the heating rate and A is the pre-exponential factor. The values of activation energy of decomposition and frequency factor are given in Table 2. The frequency factor varies between 6.48 × 102 and 1.01 x 10 a. All the aramids showed similar behavior with solvents and typical observations are summarized in Table 3. Aramid film swells in methyl ethyl ketone, tetrahydrofuran and hexafluoroisopropanol, and the sample appears like a rubber after 7 days at room temperature. Aramids are practically unaffected by common solvents, as well as less common solvents like formic acid, trichloroethylene, chlorobenzene and 1,2,4trichlorobenzene. These aramids have better resistance to dilute alkali and dilute acid, but in concentrated alkali degradation starts. Aramid films are soluble in concentrated sulfuric acid and in polar solvents such as dimethylformamide, dimethylacetamide, dimethylsulfoxide and N-methyl pyrrolidone.

TABLE 2 Activation Energies of Decomposition of Organosilicon Aramids

4ramid

Temperature range (°C)

E (k J/mole)

A B C D E F G H I J K L

450-650 450-700 450-650 440-700 425-625 400~,70 450-600 425-595 430-610 420 610 440-625 400-510

87-17 115.28 103.73 96-34 126.33 118.96 58.98 78.24 98.84 91.45 96-02 113.28

A

9.9 × 6.43 x 6-75 x 2.85 x 4.18 x 1.01 x 6'46 × 9.74 x 1.77 x 7.61 × 1.71 x 1.64 x

104 106 105 l0 s 107 108 102 103 105 104 l0 S 107

E, activation energy of decomposition. A, pre-exponential factor. TABLE 3 Chemical Stability of Organosilicon Aramids ~

Serial number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Solvent Benzene Ethyl alcohol Chloroform Methyl ethyl ketone Formic acid Glacial acetic acid Hexafluoroisopropanol Trichloroethylene Chlorobenzene 1,2,4-Trichlorobenzene 1,4-Dioxane Tetrahydrofuran N, N-Dimethylformamide 5 5~ NaOH 5 % H2SO 4 Conc. NaOH Conc. HzSO 4

Effect aJter 7 days None None None Swells None None Swells None None None None Swells Dissolves None None Degrades Dissolves

a Tests were carried out by keeping the film sample in various solvents at room temperature for 7 days.

336

J. Y. Jadhav ACKNOWLEDGEMENT

The author is indebted to the Council of Scientific and Industrial Research, New Delhi, India, for the award of a post-doctoral fellowship.

REFERENCES 1. Anon., High temperature resistance and thermal degradation of polymers, Soc. Chem. Ind. (London), Monograph 13 (1961). 2. O. B. Edger and R. Hill, J. Polym. Sci., 8, 1 (1952). 3. E. L. Whittbecker. Paper presented in symposium of Am. Chem. Soc., Minnesota, June (1953). 4. H. Batzer, Makromol. Chem., 10, 13 (1953). 5. R.A. Diene-Hart, B. J. C. Moore and W. W. Wright, J. Polym. Sci., B-2,369 (1964). 6. C. W. Bunn, J. Polym. Sci., 16, 223 (1955). 7. N. D. Ghatge and J. Y. Jadhav, J. Polym. Sci., Polym. Chem. Edn, 22, 1565 (1984). 8. F. Paulik, J. Paulik and L. Erdey, Talanta, 13, 1405 (1966). 9. C. D. Doyle, Anal. Chem., 33, 77 (1961). 10. A. W. Coats and J. P. Redfern, Nature, 201, 68 (1964). 11. L. Reich, J. Polym. Sci., Polym. Lett. Edn, 3, 231 (1965). 12. H. H. Horowitz and G. Metzger, Anal. Chem., 35, 1464 (1963). 13. S.R. Dharwadkar and M. D. Karakhanwala, Thermal analysis in inorganic materials and physical chemistry, Vol. 1 (R. F. Schwenker and P. D. Garn (Eds)), Academic Press, New York (1969).