Crosslinking of fusible polyetherimides during thermal treatment

Polymer Science U.S.S.R. Vol. 30, No. 11, pp. 2600-2605, 1988 Printed in Poland

0033-3950!88 $10.00+.OO C 1990 Ptn'glmon Press pic

CROSSLINKING OF FUSIBLE POLYETHERIMIDES DURING THERMAL TREATMENT* M. M. KOTON, S. YA. FRENKEL', YU. N. PANOV, L. S. BOLOTNIKOVA,V. M. SVETLICHNYI,L. A. SHIBAYEV, S. G. KULICmg.mN, YE. Y~. I~trPNOVA, A. S. P~UTOV and I. L. USHAKOVA Institute of Macromolecular Compounds, U.S.S.R. Academy of Sciences (Received 22 June 1987)

Viscosity, the two components of the complex modulus of elasticity in shear, and the concentration of paramagnetic centres were measured at 350°C for three polyetherimides differing in the structure of the diamine component. After a distinct inductio~ period the melt viscosity begins to rise sharply. The time dependences of elasticity of the melt and of the concentration of paramagnetic centres obey the first-order kinetic equation. The composition of volatile reaction products evolved from the individual polymers during their thermal treatment provides evidence that free radicals are formed not only at both ends but along the entire chain length as well. Thermal treatment leads to the formation of a three-dimensional network. The polyetherimide containing in the diamine component both sulphide and $ulphone linkages displays the highest thermal stability. MANY investigators have studied recently the properties o f linear polyetherimides containing different aromatic groups (such as diphenyl oxide, diphenyl sulphone, diphenylpropane) connected by simple ether linkages [1-6]. These polymers, called simple polyetherimides (SPEI), are of considerable interest s i n c e - i n contrast to the common polyimides- they can be processed from the melt; moreover, heating to high temperatures can improve their mechanical properties [7]. Diphenyl sulphone or diphenyl oxide units can be incorporated either to the diamine [1, 2, 6] or the dianhydride monomer; in both instances the required fusibility is retained and mechanical properties are satisfactory. In this paper we investigate structural rearrangements that take place during thermal treatment of three SPEI's differing in the chemical nature of the diamine component. The knowledge of reasons underlying such changes is prerequisite for the selection of appropriate conditions (temperature and duration) during the'thermal treatment or the processing of the material to the final product. The three SPErs were prepared by the known two-stage process [8]; their chemical structure and formulae are given below: poly[4,4'-bis(4-N-phenyl)diphenylsulphonimide-l,3-bis(3,4-dicarboxy-

phenoxybenzene] (SPEI-I), * Vysokomol. soyed. A30: No. 11, 2425-2430, 1988. 2600

Crosslinking of fusible polyetherimides CO - - N/

0

\,d\/

0

2601

CO

\/%,/ \//\/

CO

~. _/2---~_o_,,f-~,_soo S---%, o /2--~ CO

poly [4,4'-bis(4-N-phenoxy)diphenyloxymide-1,3-bis(3,4-dicar boxyphenoxy)benzene] (SPEI-II) C()

0

()

CO

/r]

.-o- I

CO

CO

poly [4,4-bis(4-N-phenylthio)diphenylsulphonimide-1,3-bis(3,4-carboxyph enoxy)benzene (SPEI-III) ('O

\

()

O

k/ CO

CO

,,/\

/ CO

'

\ j

-

,__1

The individual polyamic acids were prepared and their subsequent imidization was carried out according to [2], which also lists the characteristics of the starting compounds. The molecular weight of the polymer estimated from viscosity and light scattering data was 5 x 104 [6]. Rheological characteristics of polymer melts (the components G' and G" of the complex modulus in shear, measured at small amplitudes in the region of linear viscoelasticity over the frequency interval 10-2~
2602

M . M . KOTON et aL

The rheological studies which provided the time dependences of r/, G', and G" enabled us to estimate the characteristic time of gelation t*; several evaluation methods w~re used to enhance the reliability of obtained results. Figure 1 depicts the time variation of viscosity measured at a constant shear rate y = 3 x 10 -2 see-1 (the optimum experimental temperature whereat the time variation is sufficiently rapid but the sample does not show any sign of thermal degradation). The onset of gelation t* is defined by the point of intersection of the tangents to the curve t/(t).

'Z/go 5

4

8O

1 u.j

40

~

3

~g

20

6O

4O

Time, rain

&O

Time, rain

FIo. 1 Fio. 2 Fie. 1. Time dependence of viscosity of SPEI-I at 350°C. Fie. 2. Components of complex shear modulus of elasticity as a function of time. SPEI-I, 350"C: 1 - G', 2 - G". Cz-~RAerm~aac names( ~ ) or 6ELATZONAT 3500C Polymer SPEI-I SPEI-II SPEI-III

t~ 35 36 47

t~ 42 45 65

t~--t~ 7 9 18

t~ 35 35 46

t,T 15 46 40

The end of gelation denoted as t* is more clearly seen on the semilogarithmic plot of log ~/(t) (r/goes to infinity). The rate of gelation can be qualitatively characterized by the difference t*2- t*. The data collected in the Table demonstrate that samples SPEI-I and SPEI-II behave similarly, whilst SPEI-III displays a higher thermal stability. Another method which can be used to characterize gelation is based on the determination of the time t* whereat G ' = G" [10]; an example is shown in Fig. 2. Both approaches yield comparable results (Table 1). Let us now turn attention to the data obtained with the torsional pendulum yielding directly the mechanical loss factor tan ~ (Fig. 3); the position of the maximum defines the characteristic gelation time t*. It is apparent that the order of individual polymers remains the same, although the time t* is somewhat smaller than the characteristic

Crosslinking of fusible polyetherimides

2603

times determined by the other procedures, probably because of the method employed in sample preparation (moulding at elevated temperatures). Thus, the changes of theological characteristics, induced by thermal treatment lead to a loss of the ability of the material to undergo reversible deformation. The initial slow rise of viscosity during the early stages of thermal treatment is followed by a period of rapid increase. Such variation in r/is typical for systems which exhibit a relaxation transition due to structural rearrangements, in particulaI to the formation of crosslinked regions [11]. The appearance of a three-dimensional network is evidenced also by the fact that the system begins to display thixotropic behaviour characteristic of filled polymers-see the hysteresis of the flow curve.

tono~ 02-

1.0i I

2

05

0./

i I

2O

Time, min

6O

I 600

z00 Time, rain

FIG. 3 :FIG.4 Pro. 3. Time dependence of the loss tangent of SPEI melts at 350°C. Here and in Fig. 5: 1 - SPEI-I, 2 - SPEI-II, 3 - SPEI-III. Fxo. 4. Time dependence of the extent of curing, expressed as ,8= 1-exp (-kt); SPEI melts: I SPEI-I, kl = 4"6 x 10- 3, 2 - SPEI-II, k2 = 3.8 x 10- 3, .3- SPEI-III, k3 = 2.5 x 10- 3 mill- 1. The time variation of the modulus is also of interest. The parameter fl = ( G ' - G'o)/ /(G'~ - G'o) can measure the extent ot proceeding changes- G~ ( G ' ) is the initial (final) value of G'. Figure 4 shows/~ as a function of time; the curves follow an exponential course typical for first-order chemical reactions, fl=l-exp(-k0, where k is an apparent rate constant which describes the time variation of the modulus. The curves in Fig. 4 show that the rate of the process diminishes from SPEI-I to SPEI-II. This result can be explained by assuming the reaction of SPEI to proceed by a radical mechanism; the rate-limiting step is the thermally-induced formation of radicals: it is known that the rate of such processes usually obeys the first-order kinetic law. The character of paramagnetic centres as measured by EPR and their time dependence also confirm that the reaction is governed by a radical mechanism: the shape of the curve which describes the time dependence of CPMS is similar to the function

(t) fFig. 5).

2604

M . M . KOTON e t

aL

Data on viscosity, modulus, and CPMS all confirmed that crosslinking accompanies heating of SPEI; moreover, the diffusion coefficient of D M F A (e.g. in SPEI-I) decreased from 10 -a to 10 -1° cm2.sec -1. The experimental results which speak in favour of crosslinking in heated SPEI samples aze corroborated by mass spectrometry. Composition of the main gaseous products (H20, CO, CO2, SO2, and the hydroquinone fragment C6H402), evolved from SPEI-I heated/n v a c u o to 230--470°C is charactered in Fig. 6. One may reasonably assume that this picture is a result of several superimposed processes. .el.un. -

1

q

# o

3

3

5 2

2

t,,

BO

180 Time ~rain

FIO. 5

300

qO0 T °

FIO. 6

FIO. 5. Concentration of paramagnetic centres in SPEI melts as a function of time; 350"C. Flo. 6. Mass-thermogram of SPEI-I: I - H 2 0 (M=18), 2 - C O (M=28), 3 - C O 2 (M=44), 4 SO2 ( M = 68), 5 - O -- C6H4- O ( M = 108).

Evolution of CO2 and H 2 0 is very intensive between 230 and 360 °C. Owing to the increased mobility of SPEI macromoleculcs, acylation of the terminal amino groups by anhydrides probably sets in, and can be accompanied by additional condensation and decarboxylation of amic groups, as evidenced by the concurrent increase of CO2 and H 2 0 in the volatile products. Such process must naturally lead to an increase of molecular weight and, on the other hand, to the formation of macromolccules caIryiug free radical centres along their whole length. Decomposition at elevated temperatures of the terminal anhydride groups represents another reason for the increased amount of evolved CO2. As the amount of released CO remains constant over this temperature interval, one may assume that free radicals of the type - C 6 H a - C O ' , formed by decomposition of anhydride groups, participate in recombination reactions which lead both to an increase of molecular weight and to crosslinking of SPEI chains. It is also noteworthy that crosslinking can result from reamidation reactions between amidc end groups and imide rings. Degradation processes in the main polymer chain start at 320°C and lead to increased concentration of radicals and intensive crosslinking. The bonds C~H4-SOz-C6H,~ apapparently represent the weakest link in SPEI-I, as evidenced by the evolution of SOn which sets in already at 320°C. The C 6 H 4 - O - C ~ H 4 groups are somewhat stronger.

Crosslinking of fusible polyetherimides

2605

Substantial decomposition of imide groups begins at 360°C, as demonstrated by the rapidly rising yield of CO2 and CO. SPEI-II which contains only oxyphenylene groups behaves similarly as SPEI-I. SPEI-III is thermally more stable: evolution of the main mass of volatiles begins above 370°C, and the crosslinking process is slowed down correspondingly. The presence of sulphur linkages in the macromolecule apparently raises the polymer resistance to heat-induced degradation. The authors express their gratitude to L. M. Kalyuzhnaya for the evaluation of diffusion coefficients. Translated by M. KUBfN REFERENCES 1. G. L. BRODE, K. KAWAKAMI, G. T. KWIATKOWSKI and A. W. BEDWIN, J. Polym. Sci. Polym. Chem. Ed. 12: 575, 1974 2. M. M. KOTON, V. M. SVETLICHNYI, V. V. KUDRYAVTSEV, V. Ye. SMIRNOVA, T. A. MARICHEVA, Ye. P. ALEKSANDROVA, G. S. MIRONOV, V. A. USTINOV and Yu. A. MOSKVICHEV, Vysokomol. soyed. A22: 1058, 1980 (Translated in Polymer Sci. U.S.S.R. 22: 5, 1163, 1980) 3. H. D. BURKS and T. L. StCLAIR, J. Appl. Polym. Sci. 29: 1027, 1984 4. D. E. FLORYAN and J. W. SERFATY, Mod. Plast. Ind. 12: 38, 1982 5. O. B. JONES and H. N. CASSEY, U.S. Pat. 3926913 6. A. V. SIDOROVICH, O. V. KALLISTOV, V. V. KUDRYAVTSEV, V. K. LAVRENT'EV, V. M. SVETLICHNYI, I. G. SILINSKAYA, Ye. P. ALEKSANDROVA and M. M. KOTON, Vysokomol. soyed. B25: 563, 1983 (Not translated in Polymer Sci. U.S.S.R.) 7. M. M. KOTON, L. S. BOLOT~KOVA, V. M. SVETLICHNYI, I. F. DAVYDOVA, B. A. KISELEV, V. V. KUDRYAVTSEV, S. S. MNATSAKANOV, Yu. N. PANOV, B. A. PEROV and S. Ya. FRENKEL', Plast. massy, 4, 11, 1986 8. M. I. BESSONOV, M. M. KOTON, V. V. KUDRYAVTSEV and L. A. LAIUS, Poliimidy-klass termostoikikh polimerov (Polyimides--A Class of Thermally Stable Polymers). 328 pp., Leningrad, 1983 9. G. V. VINOGRADOV, A. Ya. MALKIN, Ye. P. PLOTNIKOVA, A. A. KONSTANTINOV, A. K. KULANOV, V. M. BOGOMOLOV, A. A. SHAKHRAI and B. A. ROC_~V, Vysokomol. soyed. A20: 226, 1978 (Translated in Polymer Sci. U.S.S.R. 20: 1, 262, 1978) 10. H. H. WINTER and F. CHAMBON, J. Rheol. 30: 367, 1986 11. A. Ya. MALKIN and S. G. KULICHIKHIN, Reologia v protsessakh obrazovania i prevrashchenia polimerov (Rheology in Processes of Polymer Formation and Transformation). 240 pp., Moscow, 1985