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Changes in supermolecular structure of polyamic acid during thermal cyclization
Polymer Science U.S.S.R. Vol. 28, No. 11, pp. 2642-2646, 1986 Printed in Poland
0032-3950[86 $10.00 + .00 © 1987 Pergamon Journals Ltd.
CHANGES IN SUPERMOLECULAR STRUCTURE OF POLYAMIC ACID DURING THERMAL CYCLIZATION* V. M. STARTSEV, N. F. CHUGUNOVA, V. V. MATVEYEV and A. YE. CHALYKH Institute of Physical Chemistry, U.S.S.R. Academy of Sciences (Received 3 April 1985)
Transmission electron microscopy and laser optical microscopy have been employed to investigate changes in structure and morphology of polyamic acid films during stepwise thermal cyclization. The principal structural changes in the form~ition of a polyimide involve primary associates existing in the polyamic acid solution. A hierarchic structure is assumed to exist in both polyamic acid and polyimide. FOR MANY years the elucidation o f supermolecular structure existing in polyamic acids (PAA) and in polyimides (PA) represents an important and urgent task. Thus, little is known about correlation between changes in some physical and mechanical characteristics o f films and coatings in PAA and, on the other hand, the extent o f imidization; e.g., the temperature dependences of adhesion b~tween PI and an aluminium sheet used as substrate as well as the strain-at-break of P[ films themselves, measured in the temperature interval 293 to 403 K, were found to go through an extreme [1]. The authors assumed this behaviour to be connected with structural changes in bulk or at the PAA/substrate interphase. Further, since the formation o f PI films starts from a PAA solution, concentration effects might play an important role in that the structure of both the intermediate and the final product depends on the initial concentration of the solution. Finally, some results [2-4] obtained for crystalline and oriented PI (mostly PI fibres) refer to fracture surfaces and ultrathin sections, so that the information is difficult to interpret properly. In this paper we investigate supermolecular structure o f PAA synthesized by reacting dianhydride o f 3,Y,4,4'-benzophenonetetracarboxylic acid with diaminodiphenyl ether in dimethylformamide (DMFA), and also o f the corresponding PI formed by thermal cyclization between 293 and 503 K. Films (about 40 am thick) were cast from a I0% solution of PAA in DMFA on an aluminium foil resting on a glass support. Thermal cyclization in air was carried out in a stepwise manner: 72 hr at 293 K, 1 hr at 353 K, 2 hr at 383 K, 3 hr at 403 K, I hr at 423 K, 3 hr at 473 K, and 1 hr at 503 K. The extent of cyclization was estimated from IR spectra by measuring the intensity of the band at 720 cm -1 [5]. Maximum cyclization (by convention defined as 100K) was achieved after heating the specimens to 503 K. After each step of the thermal treatment the solid film, delaminated * Vysokomol. soyed. A28: No. 11, 2378-2381, 1986.
2642
Changes in supermolecular structure of polyamic acid
2643
from the substrate, was etched (from the surface originally in contact with air) in a high-frequency oxygen plasma and fixed following the method described in [6]. Replicas of the surface were analysed with the electron microscope EM-301. Microdiffraction patterns proved that amorphousstructure persisted in the films in all stages of the thermal treatment. To elucidate a possible role of phase transitions in the transformation PAA--rPI, films heated to 293,353, and 383 K were dissolved in D M F A to a concentration of 1% and the structure of these quasiequilibrium solutions was also investigated. For the last two temperatures the extent of cyclization was found to be 4 and 31%, respectively. Films heated to higher temperatures were insoluble in D M F A at room temperature. Thin films (3 to 5/zm) were cast from these dilute solutions onto a freshly split mica and dried at ambient temperature. The investigation of 1% solutions proved to be useful also for revealing the effect of concentration and of the size factor on the dimensions of structural elements observed.
The structure of a film (around 40 am thick), prepared from a 10% solution of PAA and dried at room temperature to a residual DMFA content of 25 ~, is shown in Fig. la, it consists of large (0.8 to 1"0 am) associates of asymmetric shape, each composed of a large number of smaller elements. A slow evaporation of the solvent from a thick film obviously favours aggregatign of small structural elements into bulky associates held together by physical interactions. The size, shape, and distribution of these elements are apparent from the inset which shows the structure of a 1% solution of PAA after drying at room temperature (magnification is the same in all micrographs).
FIG. 1. Electron micrographs of the surface of PAA and PI films in various stages of thermal treatment: after 293 (a), 353 (b), 383 K (c).
2644
V.M. STARTSlVet al.
Figure la clearly demonstrates that the starting PAA solution is an associated singlephase liquid, and the structure of the PAA film forms the basis for structural organization formed during cyclization. When a thick film was heated to 353 K (4 ~o cyclization), its structure became less coarse and heterogeneous, consisting of asymmetric associates (Fig. lb). The decreased size of associates (compared to those apparent in Fig. la) is due to destruction of the weakest intra- and intermolecular bonds, brought about by increased mobility of macromolecules. The increased packing density is a consequence of the smaller residual content of DMFA. A weakly pronounced granular structure and almost complete absence of inter-grain volume are typical features observed in the inset to Fig. lb, which shows the structure of a 1% solution prepared from the film in question. The extent of cyclization reached 30 to 32 % after thermal treatment at 383 K. The imidization apparently proceeds inside the associates, as seen from the fact that their anisometry is more pronounced although the overall morphology remains practically the same (Fig. lc). The structure of a film prepared by dissolving the latter specimen in DMFA to a 1 Yo solution is apparent from the inset to Fig. lc" the structural elements are small with sharp contours, and the size of grains is almost the same as in the thin film dried at ambient temperature. This demonstrates the absence of phase transitions in the polymer during the thermal treatment and leads us to assume that imidization proceeds inside the primary associates. Globularization of associates is apparently due to partial cyclization which promotes intermolecular interactions inside the associates. Such associates only swell in DMFA and in the thin films the solvent is removed rapidly mostly from the interjaeent volume. C H A N G E S I N THI~ SIZE OF MICRO- A N D MACROSTRUCTURAL ELEMENTS ( L ) , OF ADHESION STRENGTH, A N D OF DEFORMATION CHARACTERISTICS D U R I N G THERMAL C Y C L I Z A T I O N OF
Temperature, K PAA 293 353 383 403 423 473 503
thick films from 10~ solutions 0"9 0"2 0"15 0-14 0-13 0.12 0.11
PAA
L x 106, m thin films from 1 solutions
according to
230 150 50 100
40 17
0"67 1"34
15"9 19"9
35
0'61 0"69 0"45 0"37
8"4 8"3
Fig. 2
Resistance to delamination Strain-at-break ,4, kg/cm m
35
m
10"1 9"9
Structure of films that underwent the above temperature regime remained practically unchanged, only the size of the asymmetric associates somewhat decreased, their packing density increased, and the density fluctuations on the surface became less
Changes in supexmolocularstructuro of polyamic acid
2645
prono unced (the relief was less apparent). The films were insoluble in DMFA and in sulphuric acid owing to strongly increased intermolecular interactions in bulk. Table 1 collects the data from Fig. 1, which characterize changes in the size of structural elements in thin (3 to 5 am) and thick (40 am) films, observed in various stages of the thermal treatment. The mean size of structural elements in thick films decreased by a factor of 6 when the temperature was raised from 293 to 353 K (i.e., to the onset of cyclization), while in the temperature interval between 383 and 503 K (100~o cyclization) the size of the structural elements remained practically the same.
FIG. 2. Macrostructure of films heated to 293 (a), 353 (b), and 403 K (c). The size of structural elements in thin films cast from 1 Yo solutions prepared by dissolving the material of the corresponding thick specimen showed a minimum for films heated to 353 K. Another level of structural organization is apparent in PAA and PI: a macrostructure in the bulk, consisting of microstructural elements. Figure 2 shows photomicrographs obtained by laser transmission optical microscopy (He-Ne laser, 2= 632 nm) of films (40/tm) heated to 293, 353, and 383 K, respectively. It is apparent that the size of elements is almost the same in films prepared from materials treated at 293 and at 383 K, but much smaller when the treatment was stopped at 353 K (cf. Table 1). We may thus conclude by stating that the structure of PAA and PI must be considered as hierarchic. The highest level (submicrostructure) exists in thin films at small solution concentrations. The second level (microstructure) is of a more or less intermediate character, consists of elements of the highest level, and appears in more concentrated PAA solutions and in thicker films. Finally, a macrostructure (structural network) is formed from elements of the intermediate level in thick films. With regard to correlation of physico-mechanical characteristics with structural organization of films made of PAA (and PI prepared by thermal cyclization), we note a good agreement of the trends in the size of structural elements on the first and third hierarchical levels and in adhesion and deformation properties of films and coatings [1]. Translated by M. KuBfN
2646
N . V . D'X'AKONOVAet aL
REFERENCES 1. V. M. STARTSEV, N. F. CHUGUNOVA, A. F. CHALYKH, L. P. KAZANSKII and A. Ye. RUBTSOV, Kolloid zh. 46: 961, 1984 2. M. I. BESSONOV, M. M. KOTON, V. V. KUDRYAVTSEV and L. A. LAIUS, Poliimidyklass termostoikikh polimerov (Polyimides - A Class of Thermally Stable Polymers). p. 205, Nauka, L., 1983 3. T. Ye. POGODINA and A. V. SIDOROVICH, Vysokomol. soyed. A26; 974, 1984 (Translated in Polymer Sci. U.S.S.R. 26: 5, 1086, 1984) 4. V. K. LAVRENT'EV and A. V. SIDOROVICH, Vysokomol. soyed. B26: 3, 1984 (Not translated in Polymer Sci. U.S.S.R.) 5. M. I. TSAPOVETSKII, L. A. LAIUS, M. I. BESSONOV and M. M. KOTON, Dokl. Akad. Hank SSSR 240: 132, 1978 6. M. R. KISELEV, E. I. YEVKO and V. M. LUK'YANOVICH, Zavod. Lab. 32: 201, 1966
PolymerScienceU.S.S.R.Vol. 28, No. 11, pp. 2646-2653,1986 Printedin Poland
0032-3950/86$10.00+ .00 O 1987PergamonJournalsLtd.
INITIAL STAGES OF STRUCTURIZATION IN DILUTE SOLUTIONS OF POLYAMIC ACIDS AND POLYIMIDES* N. V. D'YAKONOVA, N. V. MIKHA1LOVA, V. P. SKLIZKOVA, I. A. BARANOVSKAYA, YU. G. BAKLAGINA, V. V. KUDRYAVTSEV, A. V. SIDOROVICH, V. YE. ESKIN a n d M. M. KOTON Institute of Macromolecular Compounds, U.S.S.R. Academy of Sciences
(Received 14 April 1985) By means of IR spectroscopy, light scattering, and X-ray diffraction, strong intermolecular interactions have been found to exist in dilute solutions (at concentrations c~>0"5 ~.) of polyamic acids and of soluble polyimides in amidic solvents.
IT HAS been already established [1] that the concentration dependence of intensity of light scattered by dilute solutions of polyamic acids in amidic solvents (dimethylformamide-DMFAanddimethylacetamide-DMAA) shows some specific features at concentrations above 2 %. As an example, the quantity c H / I e for poly[(4,4'-oxydiphenylene) pyromellitamic acid] (PMAC) with Mw= 5 × 103, dissolved in DMAA, is plotted against the polymer concentration c in Fig. la (H is an optical constant and Ie is the excess scattered intensity); curve I corresponds to a solution prepared directly from the reaction mixture, curve 2 to a solution prepared from the polymer purified by precipitation [2]. * Vysokomol. soyed. A ~ : No. I1, 2382-2387, 1986.
0032-3950[86 $10.00 + .00 © 1987 Pergamon Journals Ltd.
CHANGES IN SUPERMOLECULAR STRUCTURE OF POLYAMIC ACID DURING THERMAL CYCLIZATION* V. M. STARTSEV, N. F. CHUGUNOVA, V. V. MATVEYEV and A. YE. CHALYKH Institute of Physical Chemistry, U.S.S.R. Academy of Sciences (Received 3 April 1985)
Transmission electron microscopy and laser optical microscopy have been employed to investigate changes in structure and morphology of polyamic acid films during stepwise thermal cyclization. The principal structural changes in the form~ition of a polyimide involve primary associates existing in the polyamic acid solution. A hierarchic structure is assumed to exist in both polyamic acid and polyimide. FOR MANY years the elucidation o f supermolecular structure existing in polyamic acids (PAA) and in polyimides (PA) represents an important and urgent task. Thus, little is known about correlation between changes in some physical and mechanical characteristics o f films and coatings in PAA and, on the other hand, the extent o f imidization; e.g., the temperature dependences of adhesion b~tween PI and an aluminium sheet used as substrate as well as the strain-at-break of P[ films themselves, measured in the temperature interval 293 to 403 K, were found to go through an extreme [1]. The authors assumed this behaviour to be connected with structural changes in bulk or at the PAA/substrate interphase. Further, since the formation o f PI films starts from a PAA solution, concentration effects might play an important role in that the structure of both the intermediate and the final product depends on the initial concentration of the solution. Finally, some results [2-4] obtained for crystalline and oriented PI (mostly PI fibres) refer to fracture surfaces and ultrathin sections, so that the information is difficult to interpret properly. In this paper we investigate supermolecular structure o f PAA synthesized by reacting dianhydride o f 3,Y,4,4'-benzophenonetetracarboxylic acid with diaminodiphenyl ether in dimethylformamide (DMFA), and also o f the corresponding PI formed by thermal cyclization between 293 and 503 K. Films (about 40 am thick) were cast from a I0% solution of PAA in DMFA on an aluminium foil resting on a glass support. Thermal cyclization in air was carried out in a stepwise manner: 72 hr at 293 K, 1 hr at 353 K, 2 hr at 383 K, 3 hr at 403 K, I hr at 423 K, 3 hr at 473 K, and 1 hr at 503 K. The extent of cyclization was estimated from IR spectra by measuring the intensity of the band at 720 cm -1 [5]. Maximum cyclization (by convention defined as 100K) was achieved after heating the specimens to 503 K. After each step of the thermal treatment the solid film, delaminated * Vysokomol. soyed. A28: No. 11, 2378-2381, 1986.
2642
Changes in supermolecular structure of polyamic acid
2643
from the substrate, was etched (from the surface originally in contact with air) in a high-frequency oxygen plasma and fixed following the method described in [6]. Replicas of the surface were analysed with the electron microscope EM-301. Microdiffraction patterns proved that amorphousstructure persisted in the films in all stages of the thermal treatment. To elucidate a possible role of phase transitions in the transformation PAA--rPI, films heated to 293,353, and 383 K were dissolved in D M F A to a concentration of 1% and the structure of these quasiequilibrium solutions was also investigated. For the last two temperatures the extent of cyclization was found to be 4 and 31%, respectively. Films heated to higher temperatures were insoluble in D M F A at room temperature. Thin films (3 to 5/zm) were cast from these dilute solutions onto a freshly split mica and dried at ambient temperature. The investigation of 1% solutions proved to be useful also for revealing the effect of concentration and of the size factor on the dimensions of structural elements observed.
The structure of a film (around 40 am thick), prepared from a 10% solution of PAA and dried at room temperature to a residual DMFA content of 25 ~, is shown in Fig. la, it consists of large (0.8 to 1"0 am) associates of asymmetric shape, each composed of a large number of smaller elements. A slow evaporation of the solvent from a thick film obviously favours aggregatign of small structural elements into bulky associates held together by physical interactions. The size, shape, and distribution of these elements are apparent from the inset which shows the structure of a 1% solution of PAA after drying at room temperature (magnification is the same in all micrographs).
FIG. 1. Electron micrographs of the surface of PAA and PI films in various stages of thermal treatment: after 293 (a), 353 (b), 383 K (c).
2644
V.M. STARTSlVet al.
Figure la clearly demonstrates that the starting PAA solution is an associated singlephase liquid, and the structure of the PAA film forms the basis for structural organization formed during cyclization. When a thick film was heated to 353 K (4 ~o cyclization), its structure became less coarse and heterogeneous, consisting of asymmetric associates (Fig. lb). The decreased size of associates (compared to those apparent in Fig. la) is due to destruction of the weakest intra- and intermolecular bonds, brought about by increased mobility of macromolecules. The increased packing density is a consequence of the smaller residual content of DMFA. A weakly pronounced granular structure and almost complete absence of inter-grain volume are typical features observed in the inset to Fig. lb, which shows the structure of a 1% solution prepared from the film in question. The extent of cyclization reached 30 to 32 % after thermal treatment at 383 K. The imidization apparently proceeds inside the associates, as seen from the fact that their anisometry is more pronounced although the overall morphology remains practically the same (Fig. lc). The structure of a film prepared by dissolving the latter specimen in DMFA to a 1 Yo solution is apparent from the inset to Fig. lc" the structural elements are small with sharp contours, and the size of grains is almost the same as in the thin film dried at ambient temperature. This demonstrates the absence of phase transitions in the polymer during the thermal treatment and leads us to assume that imidization proceeds inside the primary associates. Globularization of associates is apparently due to partial cyclization which promotes intermolecular interactions inside the associates. Such associates only swell in DMFA and in the thin films the solvent is removed rapidly mostly from the interjaeent volume. C H A N G E S I N THI~ SIZE OF MICRO- A N D MACROSTRUCTURAL ELEMENTS ( L ) , OF ADHESION STRENGTH, A N D OF DEFORMATION CHARACTERISTICS D U R I N G THERMAL C Y C L I Z A T I O N OF
Temperature, K PAA 293 353 383 403 423 473 503
thick films from 10~ solutions 0"9 0"2 0"15 0-14 0-13 0.12 0.11
PAA
L x 106, m thin films from 1 solutions
according to
230 150 50 100
40 17
0"67 1"34
15"9 19"9
35
0'61 0"69 0"45 0"37
8"4 8"3
Fig. 2
Resistance to delamination Strain-at-break ,4, kg/cm m
35
m
10"1 9"9
Structure of films that underwent the above temperature regime remained practically unchanged, only the size of the asymmetric associates somewhat decreased, their packing density increased, and the density fluctuations on the surface became less
Changes in supexmolocularstructuro of polyamic acid
2645
prono unced (the relief was less apparent). The films were insoluble in DMFA and in sulphuric acid owing to strongly increased intermolecular interactions in bulk. Table 1 collects the data from Fig. 1, which characterize changes in the size of structural elements in thin (3 to 5 am) and thick (40 am) films, observed in various stages of the thermal treatment. The mean size of structural elements in thick films decreased by a factor of 6 when the temperature was raised from 293 to 353 K (i.e., to the onset of cyclization), while in the temperature interval between 383 and 503 K (100~o cyclization) the size of the structural elements remained practically the same.
FIG. 2. Macrostructure of films heated to 293 (a), 353 (b), and 403 K (c). The size of structural elements in thin films cast from 1 Yo solutions prepared by dissolving the material of the corresponding thick specimen showed a minimum for films heated to 353 K. Another level of structural organization is apparent in PAA and PI: a macrostructure in the bulk, consisting of microstructural elements. Figure 2 shows photomicrographs obtained by laser transmission optical microscopy (He-Ne laser, 2= 632 nm) of films (40/tm) heated to 293, 353, and 383 K, respectively. It is apparent that the size of elements is almost the same in films prepared from materials treated at 293 and at 383 K, but much smaller when the treatment was stopped at 353 K (cf. Table 1). We may thus conclude by stating that the structure of PAA and PI must be considered as hierarchic. The highest level (submicrostructure) exists in thin films at small solution concentrations. The second level (microstructure) is of a more or less intermediate character, consists of elements of the highest level, and appears in more concentrated PAA solutions and in thicker films. Finally, a macrostructure (structural network) is formed from elements of the intermediate level in thick films. With regard to correlation of physico-mechanical characteristics with structural organization of films made of PAA (and PI prepared by thermal cyclization), we note a good agreement of the trends in the size of structural elements on the first and third hierarchical levels and in adhesion and deformation properties of films and coatings [1]. Translated by M. KuBfN
2646
N . V . D'X'AKONOVAet aL
REFERENCES 1. V. M. STARTSEV, N. F. CHUGUNOVA, A. F. CHALYKH, L. P. KAZANSKII and A. Ye. RUBTSOV, Kolloid zh. 46: 961, 1984 2. M. I. BESSONOV, M. M. KOTON, V. V. KUDRYAVTSEV and L. A. LAIUS, Poliimidyklass termostoikikh polimerov (Polyimides - A Class of Thermally Stable Polymers). p. 205, Nauka, L., 1983 3. T. Ye. POGODINA and A. V. SIDOROVICH, Vysokomol. soyed. A26; 974, 1984 (Translated in Polymer Sci. U.S.S.R. 26: 5, 1086, 1984) 4. V. K. LAVRENT'EV and A. V. SIDOROVICH, Vysokomol. soyed. B26: 3, 1984 (Not translated in Polymer Sci. U.S.S.R.) 5. M. I. TSAPOVETSKII, L. A. LAIUS, M. I. BESSONOV and M. M. KOTON, Dokl. Akad. Hank SSSR 240: 132, 1978 6. M. R. KISELEV, E. I. YEVKO and V. M. LUK'YANOVICH, Zavod. Lab. 32: 201, 1966
PolymerScienceU.S.S.R.Vol. 28, No. 11, pp. 2646-2653,1986 Printedin Poland
0032-3950/86$10.00+ .00 O 1987PergamonJournalsLtd.
INITIAL STAGES OF STRUCTURIZATION IN DILUTE SOLUTIONS OF POLYAMIC ACIDS AND POLYIMIDES* N. V. D'YAKONOVA, N. V. MIKHA1LOVA, V. P. SKLIZKOVA, I. A. BARANOVSKAYA, YU. G. BAKLAGINA, V. V. KUDRYAVTSEV, A. V. SIDOROVICH, V. YE. ESKIN a n d M. M. KOTON Institute of Macromolecular Compounds, U.S.S.R. Academy of Sciences
(Received 14 April 1985) By means of IR spectroscopy, light scattering, and X-ray diffraction, strong intermolecular interactions have been found to exist in dilute solutions (at concentrations c~>0"5 ~.) of polyamic acids and of soluble polyimides in amidic solvents.
IT HAS been already established [1] that the concentration dependence of intensity of light scattered by dilute solutions of polyamic acids in amidic solvents (dimethylformamide-DMFAanddimethylacetamide-DMAA) shows some specific features at concentrations above 2 %. As an example, the quantity c H / I e for poly[(4,4'-oxydiphenylene) pyromellitamic acid] (PMAC) with Mw= 5 × 103, dissolved in DMAA, is plotted against the polymer concentration c in Fig. la (H is an optical constant and Ie is the excess scattered intensity); curve I corresponds to a solution prepared directly from the reaction mixture, curve 2 to a solution prepared from the polymer purified by precipitation [2]. * Vysokomol. soyed. A ~ : No. I1, 2382-2387, 1986.