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Study of the softening of certain polyarimides and of structural-chemical changes due to their heating
Study of softening of certain polyarimides
1627
REFERENCES 1. N. M. SEIDOV, New Synthetic Rubber Based on Ethylene and Propylene (in Russian). Azerb. State Univ., 1966 2. N. M. SEIDOV, Ts. M. NAIBERG and M. A. DAL1N, Dokl. AN Azerb.SSR 22: 34, 1966 3. N. V. SOKOLOVA, V. A. ORESTOVA and N. A. NIKOLAYEVA, Zh. analytich, khimii 14: 472, 1959 4. G. NATTA, J. Polymer Sci. 8A: 1, 1965 5. O. FREDRIKSEN, Makromolek. Chem. 100: 231, 1967 6. R. V. DZHAGATSPANYAN, V. I. KOLBASOV and S. B. BARDENSHTEIN, Vysokomol. soyed. 7: 1959, 1965 (Translated in Polymer Sci. U.S.S.R. 7: 11, 2150, 1965) 7. R. V. DZHAGATSPANYAN, B. M. KOROLEV, V. I. ZETKIN and M. V. SHI8HglNA, Vysokomol. soyed. 8: 125, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 1, 134, 1966) 8. S. A. ADYLOV, I. F. LESHCHEVA, D. E. IL'INA, M. V. SHISHKINA and B. A. KRENTSEL, Neftekhimiya 1: 82, 1963 9. R. ALLIROT, Rev. gen. cautchouc 40: 42, 1955 10. A. P. LIZOGUB, Spectral Analysis in Org. Chemistry (in Russian). Puhl. by "Tekhnika", 1964 11. Documentation of Molecular Spectra, Catalog of Infrared Data, Butterworths, 1960 12. R. B. BARNES, U. LIDDEL and V. Z. WILT,IAMS, Industr. and Engng. Chem., Anal. Ed. 15: 676, 1943 13. The Use of Spectroscopy, ed. by W. WEST, 1959 14. E. I. AMBROSE, Proc. Roy. Soc. 199: 183, 1949 15. L. BELLAMY, I R Spectra of Complex Molecules, 1963 16. I. P. LUONGO, Analyt. Chem. 33: 1816, 1961 17. Ch. LYANG, Latest Successes in Study of Polymers, ed. by Ki, publ. by "Mir", 196 ;
STUDY OF THE SOFTENING OF CERTAIN POLYARIMIDES AND OF STRUCTURAL-CHEMICAL CHANGES DUE TO THEIR HEATING* A. V. SIDOROVICII a n d YE. V. KUVSIIINSKII High-molecular Compounds Institute, U.S.S.R. Academy of Sciences
(Received 24 July 1967) POLYARIMIDES are cyclo-chain polymers of a new type, and study of the synthesis of various polyarimides has been developing in recent years. While all polyarimides have good heat resistance there may be great differences in their mechanical properties. For example, poly[N,N'(p,p'-oxydiphenylene)pyromeliitimide](PM) -
N/CO--~-CO"
N
/~--~" O--//--%
* Vysokomol. soyed. AIO: No. 6, 1401-1407, 1968.
- -
1628
A . V . SIDO~OVIC~ and YE. V. KUVSH~SKU
remains solid right up to 400 °. At this temperature its normal elastic modulus E is ~ 10,000 kg/cm 2, and its elongation at break, 150-200%. On the other hand poly[N,N'(:p,p'-oxydiphenylene)m,m'-oxydiphenylene imide] (DPH)
[
N CO //
0
__CO
N
//--~--0
-© <>l.
has a well defined softening point and at 0>270 ° is in the highly-elastic state, E ~ 10 kg/cm 2. At elevated temperatures structural-chemical changes taking place in the polyarimides influence the mechanical properties of these substances. The nature of these processes is as yet far from clear. In order to elucidate ehanges in the phase-aggregate state of the imides the two products indicated above were studied by the authors using the dilatometric, thermomeehanical, thermographio and X-ray methods. Polyarimides are obtained in two stages: in the first polyamido acids are obtained; the second stage, t h a t of imidization through thermal processing at 150-400 °, converts the polyacids into polyimides. The chemical structure of the substances investigated is shown below, where (a) a n d (d) are the initial polyamido acid forms, and (b), (c) and (f) are the final polyimide forms. D P H differs from PM owing to the presence of two benzene rings linked b y oxygen in t h e dianhydride component
HO--CO'~F~_O__ ~_CO__OH
DPH[
• --NH--co/%2 |
co
t
[
_~./
COX" z,,
]
~
--
~ ~ \
~ _ / _
. --
o
- ~~_- ~
(a)
- j'.
z,. /CO
"[
"( ~o-(~" y
",co/~J
!
%N--J-%--. O--d./~-%-.| ~,2~,co / ~ \=/ ].
(b)
--N\co/%/ CO
(c)
PM
HO--CO,,~/CO--OH
]
--N--CO ~ / ' % / ~ C O - - N t t ~ O ~
J,,.
(d)
1629
Study of softening of certain polyarimides
/co.
- /co (e)
L
\co/%Aco
~
~
\=
J~
/ CO
CO
CO
(f) CO\ ~ / CO CO
In the initial state all the products investigated were polymido acids in the form of fi ms 0.05-0-1 mm thick obtained by evaporating dimethylformamide solutions of diaminodiphenyl ether and equimolecular amounts of the corresponding dianhydrides (hydroxyphenyltetracarboxylie and pyromellitic dianhydrides in the ease of DPH and PM respectively). The films used for dilatometrie and thermographic investigation were cut into rectangular strips with lengths l0 40-50 ram, thickness 0.05-0.1 mm and width 2-3 ram. Freely suspended samples were used for the dilatometric measurements; for the thermomechanical curves a small pendulum P was attached to the samples. Rises in temperature at the rate of 2 deg/min were controlled by a programming device; changes in the dimensions Al of the samples were recorded by a cathetometer. The curve of linear thermal expansion was determined from the dilatometrie measurements. Using the thermomechanical curves we 1 ,~l
P
calculated the curves of pliability D = ; ~0 ( a = ~
is the stress on the true cross section
S of the sample) which is a quantitative characteristic of the temperature dependence of polymer deformability [1]. The usual DTA method of thermographic investigaton was used [2]. Two capsules were placed in a block heated at a constant rate of 5 deg/min, one capsule containing the substance under investigation in the form of pieces of film, and the other containing A1208 in powder form for reference. The junctions of the differential thermocouple were placed in the capsules. The difference in the capsule temperatures was plotted as a function of the temperature of the block by a double-coordinated self-recording device. Figure 1 shows curves 1, 2 o f linear t h e r m a l expansion for D P H . There is a well defined b r e a k on the curve for the sample p r e p a r e d from the film p r e h e a t e d a t 290 ° for 30 m i n (curve 1). The b r e a k corresponds to t h e softening p o i n t (glass t r a n s i t i o n t e m p e r a t u r e ) for t h e D P H imide 0 s = 2 5 0 °. W h e n t h e same sample undergoes repea¢ed (15 times) h e a t i n g u p to 350 ° a n d cooling d o w n to 20 ° the expansion curve is s m o o t h (curve 2). I n s t e a d of t h e b r e a k there is a s m o o t h inflexion. Changes o f this kind on dilation curves are characteristic for those t y p e s o f three-dimensional p o l y m e r s where h e a t i n g results in greater crosslink d e n s i t y [3]. The fact t h a t gradual "sewing u p " begins in D P H with h e a t i n g is also shown b y the t h e r m o m e c h a n i c a l measurements. Curve 3 in Fig. 1 r6presents change in the pliability o f t h e p o l y a m i d o acid. I t s softening p o i n t (calculated from t h e s t a r t o f t h e steep b r a n c h of the curve) is 115 °. A f t e r imidization the
1630
A. V. SIDOROVICH and YE. V. KUVSK~S~CIZ
softening point for D P H rises. Accordingly the inflexion for the sample heated at 290 ° for 30 min (see curve 4) is displaced 150 ° to the right; the onset of softening occurs at 0,=250°; curve 5 is for the same sample, b u t after only 10 cycles of heating and cooling (within limits of 20-350°); curve 6 is for the same sample after 15 heating and cooling cycles. Curves 3, 5, 6 show that already after the initial thermal processing 0~ reaches its maximum, and further thermal treatment has little effect on thermal expansion. At the same time its pliability in the high-elastic state continues to decrease. I f the former effect is due to the completion of imidization the latter m a y be the result of a process related to a relatively small increase in the number of crosslinks, plainly a secondary process. I t is assumed that crosslinking in polyimides is the result of transimidization; the rupturing of imide rings and the formation of radicals in a single molecule and the attachment of these to radicals now belonging to neighbouring macromoleeules (Fig. lc, f) [4]. D,On ~/I~.,0,I0-2
6 l/lo ~I0-~ 6
3
'3
0
2C"
~
2 50
150
250
350
FIG. 1. Curves of linear thermal expansion for DPH: /--immediately after imidization (heating at 290°, 30 rain); 2--same sample after 15 heating-cooling cycles from 20 to 350°. Temperature curve of pliability D(a----3 kg/cm)2; 3--initial polyamido acid; 4--polyimido (after repeated heating at 290°, 30 min); 5--after 10 heating-cooling cycles from 20 to 350°: 6--ditto 3, but after 15 heating-cooling cycles. X-ray analysis showed that D P H is an amorphous polymer in the initial state and remains so after much cyclic heating. All the experimental data therefore indicate that D P H in polyimide form is an amorphous three-dimensional "crosslinked" polymer, 0~----250°. With repeated heating up to 300-350 ° the crosslink density of the polymer gradually increases, b u t nevertheless remains quite small: in the high-elastic state the elastic modulus E remains within limits of E~< 100 kg/cm 2. In contrast to the vulcanizates [5] the temperature curves of pliability for D P H (Fig. 2) have no sections where the pliability remains constant or decreases with increasing temperature. The pliability constantly increases. This peculiarity is understandable only if we assume that the crosslinks in D P H are labile (at
0>250°). The thermomechanical curves for PM were plotted under a stress of a
1631
Study of softening of certain polyarimides =
3 kg/cm 2 (the same stress as for D P H ) (Fig. 2a). The first section of the curve
(up t o 125 ° ) shows thermal expansion. I n the region of 125-220 ° there is con-
siderable (5%) contraction of the film; at higher temperatures the film lengthens again, the degree of elongation over 100 ° being 1.5-2%. This is exactly in accordance with the thermal expansion of the sample. With PM therefore, in contrast to D P H , there is no softening as such: sections on the thermomechanical curve above 220 ° and below 125 ° show only thermal expansion. In the intermediate region the sample contracts: this m a y be due to various factors, including processes of polycondensation (imidization), structurization (transimidization) and crystallization.
CI
5.01
~
D,cmZ/kg" I0-4
-
£'I0-2
0
-50 t
I
100
I
200
I
300 O,°C
250
i
300
I
250
O, °C z400
Fro. 2. a--Therrnomechanical curves for PM ( a = 3 kg/cm2): 1--polyamido acid (initial
heating); 2--repeated beating of the same sample from 20 to 350°; 3--after 5 heating-cooling cycles, from 20 to 350 °. b - Temperature curves of pliability D with different tensile stresses, a: 2--50; 3--75; 4--100; 5--150 kg/cm2; / - - c u r v e of linear thermal expansion.
As is shown b y several authors [6] it is precisely at 125-220 ° that the imidization of PM is rapid. I t is therefore possible to interpret curve 1 in Fig. 2a as follows. On heating the polyamido acid form of PM at the rate of 2 deg/min the imidization begins at 125 °, and transimidization (structurization) apparently takes place simultaneously. In PM (in contrast to D P H ) these processes take place so rapidly and with such thoroughness that the softening of the polyamido acid cannot be observed. On completion of both processes at 220 ° we are already dealing with a very rigid polymer. Changes in the properties of PM due to crosslinking must be more far-reaching than in the case of D P H . I f the factors governing the structurization of PM are the same as for D P H , then with the same volume concentration of crosslinks the chain of PM will in fact be more rigid, i.e. more impeded and less mobile, than that of D P H , owing to the absence of oxygen in the dianhydride component of PM. Curves 2 and 3 show that this conclusion is correct: curve 2 was obtained using the same sample as for curve 1, while curve 3 was obtained after 5 heating-
1632
A.V.
SIDOROVICK and Y~.. V. KUVSHINSKIX
cooling cycles from 20 to 350 °. Curve 2 consists of two linear sections with a break at 250 ° evidently due to the glass temperature of the PM imide. There is no break anywhere on curve 3; the continuation of crosslinking during repeated heating eliminates "liquid" high-elastic properties and the elastic modulus of the polymer remains high at E~> 104 kg/cm =. The lengthening of the sample caused b y heating does not exceed 2% per 100°: it is due solely to thermal expansion. With low stresses, therefore, no softening would be detectable for PM. Thermomechanical measurements were also taken under much higher tensile stress. Figure 2b shows the pliability curves for samples of the PM imide under tension stresses of a = 5 0 , 75, 100, 150 kg/cm2. * I t will be seen (curve 2) t h a t there is softening under high stresses at temperatures above 350 ° (but not a t 250 ° or below, as is understandable in view of the data referred to above). This m a y be taken as indicating that the softening caused b y high stresses is not connected with the transition of PM to the high-elastic state, b u t is the result of other factors, e.g. melting of the crystalline phase of the polymer which always occurs at temperatures above the glass temperature [7]. On the other hand the high-temperature instability of crosslinks formed through imidization could be involved. To solve this problem thermographic measurements were taken for PM" Figure 3 shows the thermogram for the amido acid (curve 1); it will be seen that over the range 80-110 ° there is heat absorption (the low "hump") due to the removal of solvent radicals. At 145 ° there is a peak due to heat absorption (!)
°°fi u~
100 fl
#0
80
120
160
200
2~0 280 8,°C
FIG. 3. Thermograms ofPM: 1--polyamido acid; 2,3--polyimide, heating and cooling. caused b y the removal of water in the imidization of PM. These effects are no longer observed in the case of the sample heated at 400 °. The heating (curve 2) and cooling (curve 3) are characterized b y monotonic curves. X-ray analysis also shows no alteration in the phase composition of PM. The amorphous struc* The imidization was effected by heating for 30 min at 400°.
Study of softening of certain polyarimldes
1633
ture of the unheated (polyamido acid form) and heated (polyimido acid form) is shown b y D e b y e crystallograms. We conclude, therefore, that only processes characteristic of amorphous polymers, such as the formation of an unstable network and consequently increased pliability, are able to take place in PM at temperatures above 350% D, cmZ/kg,10-z 61
zl-
2
0
100
I
I
200
300
I
O, °C 000
FIG. 4. Tne~omeeha~ieal curves of PM polyLmide: ]--h~_itial film; 2 - - ~ m after heating at 250° for 2000 hr. F i g u r e 4 shows the thermomechanical curves for PM films heated for at 1 hr 400 ° (curve 1) and additionally heated at 250 ° for 2000 hr (curve 2). During the heating at 250 ° the polymer became more rigid owing to crosslinking of the macromoleculcs. The effects are therefore the same as those observed for D P H . In our experiments there was evidently no crystallization of PM (or at any rate it was so inconsiderable as not to be detectable b y the X-ray and thermographic methods). This is no w a y indicates that PM is not obtainable in the crystalline state [8]. The conditions of PM crystallization are still far from clear. CONCLUSIONS
(1) I t has been shown that I ) P H is an amorphous polymer with a softening point of 250% The structurization of poly N,N'(p,p'-oxyphenylene)m,m'-oxydiphenylene imide (DPH) does not result in its complete hardening, and consequently even after repeated cyclic heating there is a fairly clear transition from the glass-like to the high-elastic state. The crosslinks in D P H are fairly unstable at 0 > 2 5 0 °. (2) I t has been shown that the polyimide N,N'(p,p'-oxydiphenylene)pyromellitimide (PM) has a glass transition temperature of 250 °. The greater rigidity of thermally processed PM masks thermal softening in the high-elastic state, and
1634
B. I. S~zm~ and Yv. I. VASmENOX
consequently P M remains fairly h a r d right u p to 350 °. A b o v e this t e m p e r a t u r o deformations develop t h r o u g h labilization of t h e imide r~etwork. W i t h p r o l o n g e d heating a t 0 = 2 5 0 - 3 5 0 ° s t r u c t u r i z a t i o n in P M as also in D P H , gradually increases.
Translated by R. J. A. HEN'DRY REFERENCES
1. A.V. SIDOROVICH and Ye. V. KUVSHINSKII, Vysokomol. soyed. 3: 1698, 1961; Zavodsk. lab. 25: 1124, 1959 2. A. V. SIDOROVICH, Theses at XVI Conf. on high-melee, compounds, Riga, 1966 3. K. UEBERREITER and G. KANIG, J. Chem. Phys. 18: 399, 1950; I. A. MAIGEL'DINOV, Trans. VII Conf. on high melee, compounds, publ. by AN SSSR, 196, 1952; S. K. ZAKHAROV and Ye. V. KUVSHINSKII, Processing of Plastics, p. 283, publ. by "Khimiya", 1966 4. A. G. BOLDYREV, N. A. ADROVA, M. I. BESSON0V, M. M. KOTON, Ye. V. KUVSHINSKII, A. P. RUDAKOV and F. 8. FLORINSKII, Dokl. AN SSSR, 163: 1143, 1965 5. A. V. SIDOROVICH and Ye. V. KUVSHINSKII, Theses of reports at VII (1960) and VIII (1961) Confs. of Inst. of High molec. Compounds, AN SSSR, Leningrad 6. A. P. RUDAKOV, M. I. BESSONOV, M. M. KOTON, Ye. I. POKROVSKII and Ye. F. FEDOROVA, Dokl. AN SSSR 161: 617, 1965 7. A. V. SIDOROVICH and Ye. V. KUVSHINSKII, Vysokomol. soyed. 2: 778, 1960 (Not translated in Polymer Sci. U.S.S.R.) 8. C. E. SROOG, A. L. ENDREY, 8. V. ABRAMO, C. E. BERR, W. M. EDWARDS and K. L. OLIVER, J. Polymer Sci. 3A: 1373, 1965
THE EFFECT OF COMPLEX FORMATION ON THE ELECTRICAL CONDUCTIVITY OF POLYVINYLPYRIDINE SOLUTIONS" B. I. SAZHn~I a n d ¥1I. I. VASlLEI~IOK Addition Polymers Research Istitute, Leningrad
(Received 15 July 1967) SEVERAL a u t h o r s have r e p o r t e d on the f o r m a t i o n of complexes b e t w e e n polymers a n d low-molecular substances in solution [1-8]. H o w e v e r these investigations did n o t include joint s t u d y of this process b y means of t h e electronic spectra a n d electrical c o n d u c t i v i t y . Complexes o f high-molecular compounds are generally o b t a i n e d b y conducting the reactions in solutions, a n d the a b s o r p t i o n spectra are also investigated in the latter. On t h e o t h e r h a n d films a n d pellets press m o u l d e d f r o m p o w d e r are used as samples for s t u d y i n g eleetrophysical behaviour. No strict comparison o f t h e a b s o r p t i o n spectra a n d electrical c o n d u c t i v i t y (a) is possible in these * Vysokomol. soyed. A10: No. 6, 1408-1413, 1968.
1627
REFERENCES 1. N. M. SEIDOV, New Synthetic Rubber Based on Ethylene and Propylene (in Russian). Azerb. State Univ., 1966 2. N. M. SEIDOV, Ts. M. NAIBERG and M. A. DAL1N, Dokl. AN Azerb.SSR 22: 34, 1966 3. N. V. SOKOLOVA, V. A. ORESTOVA and N. A. NIKOLAYEVA, Zh. analytich, khimii 14: 472, 1959 4. G. NATTA, J. Polymer Sci. 8A: 1, 1965 5. O. FREDRIKSEN, Makromolek. Chem. 100: 231, 1967 6. R. V. DZHAGATSPANYAN, V. I. KOLBASOV and S. B. BARDENSHTEIN, Vysokomol. soyed. 7: 1959, 1965 (Translated in Polymer Sci. U.S.S.R. 7: 11, 2150, 1965) 7. R. V. DZHAGATSPANYAN, B. M. KOROLEV, V. I. ZETKIN and M. V. SHI8HglNA, Vysokomol. soyed. 8: 125, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 1, 134, 1966) 8. S. A. ADYLOV, I. F. LESHCHEVA, D. E. IL'INA, M. V. SHISHKINA and B. A. KRENTSEL, Neftekhimiya 1: 82, 1963 9. R. ALLIROT, Rev. gen. cautchouc 40: 42, 1955 10. A. P. LIZOGUB, Spectral Analysis in Org. Chemistry (in Russian). Puhl. by "Tekhnika", 1964 11. Documentation of Molecular Spectra, Catalog of Infrared Data, Butterworths, 1960 12. R. B. BARNES, U. LIDDEL and V. Z. WILT,IAMS, Industr. and Engng. Chem., Anal. Ed. 15: 676, 1943 13. The Use of Spectroscopy, ed. by W. WEST, 1959 14. E. I. AMBROSE, Proc. Roy. Soc. 199: 183, 1949 15. L. BELLAMY, I R Spectra of Complex Molecules, 1963 16. I. P. LUONGO, Analyt. Chem. 33: 1816, 1961 17. Ch. LYANG, Latest Successes in Study of Polymers, ed. by Ki, publ. by "Mir", 196 ;
STUDY OF THE SOFTENING OF CERTAIN POLYARIMIDES AND OF STRUCTURAL-CHEMICAL CHANGES DUE TO THEIR HEATING* A. V. SIDOROVICII a n d YE. V. KUVSIIINSKII High-molecular Compounds Institute, U.S.S.R. Academy of Sciences
(Received 24 July 1967) POLYARIMIDES are cyclo-chain polymers of a new type, and study of the synthesis of various polyarimides has been developing in recent years. While all polyarimides have good heat resistance there may be great differences in their mechanical properties. For example, poly[N,N'(p,p'-oxydiphenylene)pyromeliitimide](PM) -
N/CO--~-CO"
N
/~--~" O--//--%
* Vysokomol. soyed. AIO: No. 6, 1401-1407, 1968.
- -
1628
A . V . SIDO~OVIC~ and YE. V. KUVSH~SKU
remains solid right up to 400 °. At this temperature its normal elastic modulus E is ~ 10,000 kg/cm 2, and its elongation at break, 150-200%. On the other hand poly[N,N'(:p,p'-oxydiphenylene)m,m'-oxydiphenylene imide] (DPH)
[
N CO //
0
__CO
N
//--~--0
-© <>l.
has a well defined softening point and at 0>270 ° is in the highly-elastic state, E ~ 10 kg/cm 2. At elevated temperatures structural-chemical changes taking place in the polyarimides influence the mechanical properties of these substances. The nature of these processes is as yet far from clear. In order to elucidate ehanges in the phase-aggregate state of the imides the two products indicated above were studied by the authors using the dilatometric, thermomeehanical, thermographio and X-ray methods. Polyarimides are obtained in two stages: in the first polyamido acids are obtained; the second stage, t h a t of imidization through thermal processing at 150-400 °, converts the polyacids into polyimides. The chemical structure of the substances investigated is shown below, where (a) a n d (d) are the initial polyamido acid forms, and (b), (c) and (f) are the final polyimide forms. D P H differs from PM owing to the presence of two benzene rings linked b y oxygen in t h e dianhydride component
HO--CO'~F~_O__ ~_CO__OH
DPH[
• --NH--co/%2 |
co
t
[
_~./
COX" z,,
]
~
--
~ ~ \
~ _ / _
. --
o
- ~~_- ~
(a)
- j'.
z,. /CO
"[
"( ~o-(~" y
",co/~J
!
%N--J-%--. O--d./~-%-.| ~,2~,co / ~ \=/ ].
(b)
--N\co/%/ CO
(c)
PM
HO--CO,,~/CO--OH
]
--N--CO ~ / ' % / ~ C O - - N t t ~ O ~
J,,.
(d)
1629
Study of softening of certain polyarimides
/co.
- /co (e)
L
\co/%Aco
~
~
\=
J~
/ CO
CO
CO
(f) CO\ ~ / CO CO
In the initial state all the products investigated were polymido acids in the form of fi ms 0.05-0-1 mm thick obtained by evaporating dimethylformamide solutions of diaminodiphenyl ether and equimolecular amounts of the corresponding dianhydrides (hydroxyphenyltetracarboxylie and pyromellitic dianhydrides in the ease of DPH and PM respectively). The films used for dilatometrie and thermographic investigation were cut into rectangular strips with lengths l0 40-50 ram, thickness 0.05-0.1 mm and width 2-3 ram. Freely suspended samples were used for the dilatometric measurements; for the thermomechanical curves a small pendulum P was attached to the samples. Rises in temperature at the rate of 2 deg/min were controlled by a programming device; changes in the dimensions Al of the samples were recorded by a cathetometer. The curve of linear thermal expansion was determined from the dilatometrie measurements. Using the thermomechanical curves we 1 ,~l
P
calculated the curves of pliability D = ; ~0 ( a = ~
is the stress on the true cross section
S of the sample) which is a quantitative characteristic of the temperature dependence of polymer deformability [1]. The usual DTA method of thermographic investigaton was used [2]. Two capsules were placed in a block heated at a constant rate of 5 deg/min, one capsule containing the substance under investigation in the form of pieces of film, and the other containing A1208 in powder form for reference. The junctions of the differential thermocouple were placed in the capsules. The difference in the capsule temperatures was plotted as a function of the temperature of the block by a double-coordinated self-recording device. Figure 1 shows curves 1, 2 o f linear t h e r m a l expansion for D P H . There is a well defined b r e a k on the curve for the sample p r e p a r e d from the film p r e h e a t e d a t 290 ° for 30 m i n (curve 1). The b r e a k corresponds to t h e softening p o i n t (glass t r a n s i t i o n t e m p e r a t u r e ) for t h e D P H imide 0 s = 2 5 0 °. W h e n t h e same sample undergoes repea¢ed (15 times) h e a t i n g u p to 350 ° a n d cooling d o w n to 20 ° the expansion curve is s m o o t h (curve 2). I n s t e a d of t h e b r e a k there is a s m o o t h inflexion. Changes o f this kind on dilation curves are characteristic for those t y p e s o f three-dimensional p o l y m e r s where h e a t i n g results in greater crosslink d e n s i t y [3]. The fact t h a t gradual "sewing u p " begins in D P H with h e a t i n g is also shown b y the t h e r m o m e c h a n i c a l measurements. Curve 3 in Fig. 1 r6presents change in the pliability o f t h e p o l y a m i d o acid. I t s softening p o i n t (calculated from t h e s t a r t o f t h e steep b r a n c h of the curve) is 115 °. A f t e r imidization the
1630
A. V. SIDOROVICH and YE. V. KUVSK~S~CIZ
softening point for D P H rises. Accordingly the inflexion for the sample heated at 290 ° for 30 min (see curve 4) is displaced 150 ° to the right; the onset of softening occurs at 0,=250°; curve 5 is for the same sample, b u t after only 10 cycles of heating and cooling (within limits of 20-350°); curve 6 is for the same sample after 15 heating and cooling cycles. Curves 3, 5, 6 show that already after the initial thermal processing 0~ reaches its maximum, and further thermal treatment has little effect on thermal expansion. At the same time its pliability in the high-elastic state continues to decrease. I f the former effect is due to the completion of imidization the latter m a y be the result of a process related to a relatively small increase in the number of crosslinks, plainly a secondary process. I t is assumed that crosslinking in polyimides is the result of transimidization; the rupturing of imide rings and the formation of radicals in a single molecule and the attachment of these to radicals now belonging to neighbouring macromoleeules (Fig. lc, f) [4]. D,On ~/I~.,0,I0-2
6 l/lo ~I0-~ 6
3
'3
0
2C"
~
2 50
150
250
350
FIG. 1. Curves of linear thermal expansion for DPH: /--immediately after imidization (heating at 290°, 30 rain); 2--same sample after 15 heating-cooling cycles from 20 to 350°. Temperature curve of pliability D(a----3 kg/cm)2; 3--initial polyamido acid; 4--polyimido (after repeated heating at 290°, 30 min); 5--after 10 heating-cooling cycles from 20 to 350°: 6--ditto 3, but after 15 heating-cooling cycles. X-ray analysis showed that D P H is an amorphous polymer in the initial state and remains so after much cyclic heating. All the experimental data therefore indicate that D P H in polyimide form is an amorphous three-dimensional "crosslinked" polymer, 0~----250°. With repeated heating up to 300-350 ° the crosslink density of the polymer gradually increases, b u t nevertheless remains quite small: in the high-elastic state the elastic modulus E remains within limits of E~< 100 kg/cm 2. In contrast to the vulcanizates [5] the temperature curves of pliability for D P H (Fig. 2) have no sections where the pliability remains constant or decreases with increasing temperature. The pliability constantly increases. This peculiarity is understandable only if we assume that the crosslinks in D P H are labile (at
0>250°). The thermomechanical curves for PM were plotted under a stress of a
1631
Study of softening of certain polyarimides =
3 kg/cm 2 (the same stress as for D P H ) (Fig. 2a). The first section of the curve
(up t o 125 ° ) shows thermal expansion. I n the region of 125-220 ° there is con-
siderable (5%) contraction of the film; at higher temperatures the film lengthens again, the degree of elongation over 100 ° being 1.5-2%. This is exactly in accordance with the thermal expansion of the sample. With PM therefore, in contrast to D P H , there is no softening as such: sections on the thermomechanical curve above 220 ° and below 125 ° show only thermal expansion. In the intermediate region the sample contracts: this m a y be due to various factors, including processes of polycondensation (imidization), structurization (transimidization) and crystallization.
CI
5.01
~
D,cmZ/kg" I0-4
-
£'I0-2
0
-50 t
I
100
I
200
I
300 O,°C
250
i
300
I
250
O, °C z400
Fro. 2. a--Therrnomechanical curves for PM ( a = 3 kg/cm2): 1--polyamido acid (initial
heating); 2--repeated beating of the same sample from 20 to 350°; 3--after 5 heating-cooling cycles, from 20 to 350 °. b - Temperature curves of pliability D with different tensile stresses, a: 2--50; 3--75; 4--100; 5--150 kg/cm2; / - - c u r v e of linear thermal expansion.
As is shown b y several authors [6] it is precisely at 125-220 ° that the imidization of PM is rapid. I t is therefore possible to interpret curve 1 in Fig. 2a as follows. On heating the polyamido acid form of PM at the rate of 2 deg/min the imidization begins at 125 °, and transimidization (structurization) apparently takes place simultaneously. In PM (in contrast to D P H ) these processes take place so rapidly and with such thoroughness that the softening of the polyamido acid cannot be observed. On completion of both processes at 220 ° we are already dealing with a very rigid polymer. Changes in the properties of PM due to crosslinking must be more far-reaching than in the case of D P H . I f the factors governing the structurization of PM are the same as for D P H , then with the same volume concentration of crosslinks the chain of PM will in fact be more rigid, i.e. more impeded and less mobile, than that of D P H , owing to the absence of oxygen in the dianhydride component of PM. Curves 2 and 3 show that this conclusion is correct: curve 2 was obtained using the same sample as for curve 1, while curve 3 was obtained after 5 heating-
1632
A.V.
SIDOROVICK and Y~.. V. KUVSHINSKIX
cooling cycles from 20 to 350 °. Curve 2 consists of two linear sections with a break at 250 ° evidently due to the glass temperature of the PM imide. There is no break anywhere on curve 3; the continuation of crosslinking during repeated heating eliminates "liquid" high-elastic properties and the elastic modulus of the polymer remains high at E~> 104 kg/cm =. The lengthening of the sample caused b y heating does not exceed 2% per 100°: it is due solely to thermal expansion. With low stresses, therefore, no softening would be detectable for PM. Thermomechanical measurements were also taken under much higher tensile stress. Figure 2b shows the pliability curves for samples of the PM imide under tension stresses of a = 5 0 , 75, 100, 150 kg/cm2. * I t will be seen (curve 2) t h a t there is softening under high stresses at temperatures above 350 ° (but not a t 250 ° or below, as is understandable in view of the data referred to above). This m a y be taken as indicating that the softening caused b y high stresses is not connected with the transition of PM to the high-elastic state, b u t is the result of other factors, e.g. melting of the crystalline phase of the polymer which always occurs at temperatures above the glass temperature [7]. On the other hand the high-temperature instability of crosslinks formed through imidization could be involved. To solve this problem thermographic measurements were taken for PM" Figure 3 shows the thermogram for the amido acid (curve 1); it will be seen that over the range 80-110 ° there is heat absorption (the low "hump") due to the removal of solvent radicals. At 145 ° there is a peak due to heat absorption (!)
°°fi u~
100 fl
#0
80
120
160
200
2~0 280 8,°C
FIG. 3. Thermograms ofPM: 1--polyamido acid; 2,3--polyimide, heating and cooling. caused b y the removal of water in the imidization of PM. These effects are no longer observed in the case of the sample heated at 400 °. The heating (curve 2) and cooling (curve 3) are characterized b y monotonic curves. X-ray analysis also shows no alteration in the phase composition of PM. The amorphous struc* The imidization was effected by heating for 30 min at 400°.
Study of softening of certain polyarimldes
1633
ture of the unheated (polyamido acid form) and heated (polyimido acid form) is shown b y D e b y e crystallograms. We conclude, therefore, that only processes characteristic of amorphous polymers, such as the formation of an unstable network and consequently increased pliability, are able to take place in PM at temperatures above 350% D, cmZ/kg,10-z 61
zl-
2
0
100
I
I
200
300
I
O, °C 000
FIG. 4. Tne~omeeha~ieal curves of PM polyLmide: ]--h~_itial film; 2 - - ~ m after heating at 250° for 2000 hr. F i g u r e 4 shows the thermomechanical curves for PM films heated for at 1 hr 400 ° (curve 1) and additionally heated at 250 ° for 2000 hr (curve 2). During the heating at 250 ° the polymer became more rigid owing to crosslinking of the macromoleculcs. The effects are therefore the same as those observed for D P H . In our experiments there was evidently no crystallization of PM (or at any rate it was so inconsiderable as not to be detectable b y the X-ray and thermographic methods). This is no w a y indicates that PM is not obtainable in the crystalline state [8]. The conditions of PM crystallization are still far from clear. CONCLUSIONS
(1) I t has been shown that I ) P H is an amorphous polymer with a softening point of 250% The structurization of poly N,N'(p,p'-oxyphenylene)m,m'-oxydiphenylene imide (DPH) does not result in its complete hardening, and consequently even after repeated cyclic heating there is a fairly clear transition from the glass-like to the high-elastic state. The crosslinks in D P H are fairly unstable at 0 > 2 5 0 °. (2) I t has been shown that the polyimide N,N'(p,p'-oxydiphenylene)pyromellitimide (PM) has a glass transition temperature of 250 °. The greater rigidity of thermally processed PM masks thermal softening in the high-elastic state, and
1634
B. I. S~zm~ and Yv. I. VASmENOX
consequently P M remains fairly h a r d right u p to 350 °. A b o v e this t e m p e r a t u r o deformations develop t h r o u g h labilization of t h e imide r~etwork. W i t h p r o l o n g e d heating a t 0 = 2 5 0 - 3 5 0 ° s t r u c t u r i z a t i o n in P M as also in D P H , gradually increases.
Translated by R. J. A. HEN'DRY REFERENCES
1. A.V. SIDOROVICH and Ye. V. KUVSHINSKII, Vysokomol. soyed. 3: 1698, 1961; Zavodsk. lab. 25: 1124, 1959 2. A. V. SIDOROVICH, Theses at XVI Conf. on high-melee, compounds, Riga, 1966 3. K. UEBERREITER and G. KANIG, J. Chem. Phys. 18: 399, 1950; I. A. MAIGEL'DINOV, Trans. VII Conf. on high melee, compounds, publ. by AN SSSR, 196, 1952; S. K. ZAKHAROV and Ye. V. KUVSHINSKII, Processing of Plastics, p. 283, publ. by "Khimiya", 1966 4. A. G. BOLDYREV, N. A. ADROVA, M. I. BESSON0V, M. M. KOTON, Ye. V. KUVSHINSKII, A. P. RUDAKOV and F. 8. FLORINSKII, Dokl. AN SSSR, 163: 1143, 1965 5. A. V. SIDOROVICH and Ye. V. KUVSHINSKII, Theses of reports at VII (1960) and VIII (1961) Confs. of Inst. of High molec. Compounds, AN SSSR, Leningrad 6. A. P. RUDAKOV, M. I. BESSONOV, M. M. KOTON, Ye. I. POKROVSKII and Ye. F. FEDOROVA, Dokl. AN SSSR 161: 617, 1965 7. A. V. SIDOROVICH and Ye. V. KUVSHINSKII, Vysokomol. soyed. 2: 778, 1960 (Not translated in Polymer Sci. U.S.S.R.) 8. C. E. SROOG, A. L. ENDREY, 8. V. ABRAMO, C. E. BERR, W. M. EDWARDS and K. L. OLIVER, J. Polymer Sci. 3A: 1373, 1965
THE EFFECT OF COMPLEX FORMATION ON THE ELECTRICAL CONDUCTIVITY OF POLYVINYLPYRIDINE SOLUTIONS" B. I. SAZHn~I a n d ¥1I. I. VASlLEI~IOK Addition Polymers Research Istitute, Leningrad
(Received 15 July 1967) SEVERAL a u t h o r s have r e p o r t e d on the f o r m a t i o n of complexes b e t w e e n polymers a n d low-molecular substances in solution [1-8]. H o w e v e r these investigations did n o t include joint s t u d y of this process b y means of t h e electronic spectra a n d electrical c o n d u c t i v i t y . Complexes o f high-molecular compounds are generally o b t a i n e d b y conducting the reactions in solutions, a n d the a b s o r p t i o n spectra are also investigated in the latter. On t h e o t h e r h a n d films a n d pellets press m o u l d e d f r o m p o w d e r are used as samples for s t u d y i n g eleetrophysical behaviour. No strict comparison o f t h e a b s o r p t i o n spectra a n d electrical c o n d u c t i v i t y (a) is possible in these * Vysokomol. soyed. A10: No. 6, 1408-1413, 1968.