Formation of morphological structures in ladder type polyheteroarylenes synthesized by solid-phase polycondensation

Formatiou of morphological structures in ladder type polyheteroarylenes

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REFERENCES

1. P. CHATELAIN, Bull. Soc. Franc. Mineral. et Cristallogr. 66: 105, 1943 2. V. S. GREBNEVA, V. L. KHODZHAYEVA, M. V. SHISI~HN'A, A. A. SITNOV, Yu. B.

AMERIK and I. I. KONSTANTIN'OV, Advances in Liquid Crystal Research and Applications, (Ed. Lanes Bats) o. 959. Pergamon Press; Oxford-Academiai Kiado, Budapest, 1980 3. N. A. PLATE and V. P. SItIBAYEV, Grebneobraznye polimery i zhidkie kristally (Comblike Polymers and Liquid Crystals). p. 285, Khimiya, Moscow, 1980

Po]ylner 8cie~LceU.S.S.[C.Vok 25, No. 3, pp. 727-734, 1983 ]'ril~ted in Poland

0032-3950/83 $10.004-.00 1984 PergamonPress Ltd.

FORMATION OF MORPHOLOGICAL STRUCTURES IN LADDER TYPE POLYHETEROARYLENES SYNTHESIZED BY SOLID-PHASE POLYCONDENSATION* V. V. KORSHAK, G. L. BERESTNEVA, Z. PEL'TSBAUER, A. YE. CHES~OKOVA and N . SH. ORMOTSADZE A. N. Nesuaeyanov Institute of Hetero-organie Compounds, U.S.S.R. Academy of Sciences Institute of Maeromolecular Chemistry, Czechoslovak Academy of Sciences, Prague (Received 25 November 1981)

Formation of morphological structures was examined in polyhe~eroarylenes, formed by the thermal interaction of initial low-molecular weight compounds. Optical and electron microscopy were used to study two typical cases of polycondensation taking place below and above the melting point of initial monomer crystals. The morphology of the polymer obtained is predetermined to a large extent by conditions of synthesis.

POLY)IEI¢ synthesis b y solid-phase p o l y c o n d e n s a t i o n is of considerable ?~heoretical a n d u n d o u b t e d practical interest d u e to the possible f o r m a t i o n o f s o m e t y p e s of p o l y m e r m a t e r i a l directly d u r i n g fabrication. A l t h o u g h s y n t h e s i s o f a n u m b e r o f p o l y h e t e r o a r y l e n e s b y this m e t h o d has been described p r e v i o u s l y [1-3], t h e r e is no i n f o r m a t i o n available a t t h e p r e s e n t t i m e concerning f o r m a t i o n m e c h a n i s m s o f i n t e r m e d i a t e p o l y h e t e r o a r y l e n e ( P H A ) a n d t h e t y p e of m o r p h o l o g i c a l f o r m a t i o n , * Vysokomol. soyed. A25: No. 3, 617-623, 1983.

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according to process conditions. At the same time, in view of the infusibility a n d limited solubility of most PH A , which hinder processing of these polymers~ evaluation of the prospects offered by this method of synthesis is pressing. Im p o r t a n t advantages are also the comparative simplicity of design, carrying o u t synthesis without using expensive solvents, absence of toxic waste, which largely predetermines the solution of a n u m b e r of ecological problems. Therefore, considerable importance is attached to the scientific basis of conditions of carrying out these reactions, resulting in the creation in the polymer of morphological structures t h a t ensure optimum deformation and strength and, also, thermal properties. This paper is aimed at investigating the formation of morphological structures in ladder t y p e P H A - - p o l y n a p h t h o y l e n e benzimidazoles (PNBI) and polyphenylquinoxaline (PPQ) synthesized by solid-phase polycondensation. Polymers were prepared by thermal interaction of 3,3,4,4'-tetraaminodiphenyl ether (TADPE) (m.p. 150-151°) with 1,4,5,8-naphtholene tetracarboxylic acid dianhydride (NTCDA) (m.p. 440°) or with 1,4-bis-(phenylglyoxalyl)benzenc (PGB)(m.p. 125--126°)Melting points of the low molecular weight substances corresponded to results in the literature [4, 5]. An equimolecular mixture of initial compounds was pulverized and shifted through a sieve with a pore diameter of 100 p. Synthesis was carried out under isothermal conditions in the temperature range of 100-300°, and under conditions of gradually increasing temperature. To remove the low molecular weight product of polycondensation (water), the reaction was carried out in inert gas heated to reaction temperature (rate of supply of argon 30 ml/min). In order to prevent the adhesion of the polymer formed to reactor walls, the glass surface was treated with a special water-repellent liquid. Regularities of solidphase polycyclocondensation were examined by gas chromatography by methods previously developed [6]. The chemical structure of the polymers obtained was identified by elemental analysis and IR spectroscopy. IR spectra were obtained using a UR-20 infrared spectrometer and KBr pellets. The intrinsic viscosity of 0.5% solutions at 25° was 0-20"6 dl/g for PPQ (m-cresol being the solvent) and 0.1-0.2 dl/g for PNBI (concentrated HISO4). The formation of morphological structures during synthesis was examined by light and electron microscopy. A Reichert (Austria) optical microscope was used for the study, which was provided with a heating platform operated both under isothermal conditions a n d by programmed increase of temperature. A special recording attachment was incorporated in the thermostatic control system, which enabled quantitative evaluation to be made of the relative intensity of birefringence. Electron-microscope photographs were obtain¢~l with the help of a scanning electron microscope (JSM-35) made in Japan; a special thermal attachment was used in a number of cases, which ensured that reactions could be carried out directly in the electron-microscope column. To understand the mechanism of thermal polyeyclocondensation without a solvent and develop methods for obtaining materials with this method, it is i m p o r t a n t to study mechanisms of structure formation of polymers during synthesis. I t was advisable to deal with two most typical eases of polycondensation in particular: t em pe r at ur e of polycondensation below the melting point o f initial monomers, i.e. in a " t r u l y " solid phase and temperature of polycondensa-

F o r m a t i o n of morphological structures in ladder t y p e polyheteroarylenes

729,

tion above the melting point of initial monomers, but below the softening point of the polymer formed. It was shown previously that the formation of macromolecules takes place at temperatures much lower than the melting point of initial compounds [7], and the process takes place at fairly high r~tes. A prelimluary study was therefore made of the formation of polymer phase using light microscopy. When heating initial monomers using the thermal stage of an optical microscope and their mechanical mixtures taken in equimoleeular ratios of components under isothermal conditions and with programmed increase of temperature it was,

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FIO. I. Dependence of the relative intensity of birefringenoe on the temperature of individu~r raw materials of P G B (1) and T A D P E (2) (a), equimolecular mechanical mixtures of initial monomers of NTCDA a n d T A D P E (1), P G B and T A D P E (2) (b), mixtures of initial substances with varying structural prehistory (e): 1 - - T A D P E crystals are placed on the surface o f P G B spherulite; 2--surfaces of P G B and T A D P E spherulites are combined; 3 - - t h e end section of PGB spherulite is placed on the surface of T A D P E spherulite. Tile rate of heating the sample is 3 deg/min.

~30

V. ¥'. KORSHAK et al.

Fro. 2. Electron-microscope photographs reflecting the formation of the polymer phase in thermal interaction of initial products below melting point: initial mixture of NTCDA a n d T A D P E (a) after carrying out the reactoin for 0.5 (b), 2 rain (c) and a mixture of P G B I T A D P E I (d) for 3 (e) and 5 rain (f). Arrows show contact planes (c) and fibrils (e).

Formation of morphological str~lctures in ladder type polyheteroarylenes

731

found that in a wide range of temperature the relative intensity I of birefl'ingence of individual substances was unchanged and decreased practically to zero (complete darkening) at the melting point of the substance (Fig. la). At the same time with every case of mechanical lnixtures of initial substances, when carrying out the reaction at a temperature lower t h a n the melting pellet of initial substances, an unusual increase in I is observed (Fig. lb). Since under these conditions ilktense chemical interaction takes place between the initial substances, resulting in the formation of the polymer, it was assumed that the effect observed is due tt~ the formation of an anisotropic polymer structure. It was later established that, the value of I depends on the degree of dispersion, degree of agitation and the mutual position of crystals of initial compounds (Fig. lc). It should be noted finally, t h a t on reaching the melting point of most highly fusible components, the optical anisotropy of the polymer phase formed always disappeared. This suggests that structure formation of the polymer phase depends considerably on process temperature. The formation of the polymer phase was therefore studied in a wide range of temperature using scanning electron microscopy. Figure 2a-c shows photomicrographs reflecting the formation of the PNB [ polymer phase, Fig. 2d-f, the formation of PPQ. It can be seen that crystals have clear faces, even through damaged (Fig. 2a). Brief heating of the mixture, to 110 ° for 0-5 min, was accompanied by blurring and "fusion" of boundaries of these crystals (Fig. 2b). A more detailed study of the surface at various gradients enabled us to detect the formation and propagation of fibres--structural elements of the polymer formed with cross-sectional dimensions of 100-300 A. It follows from an analysis of microphotographs in Fig. 2c, d that a new phase is formed in contact planes (shown by arrows) and this to some extent is due to crystal damage. A further increase in the duration of heating to 2-5 min enabled us to observe an intense increase of fibrillar structural elements in some directions (in Fig. 2e shown by arrows) and a subsequent aggregation of individual fibrilla~" elements int~ larger fibrils (Fig. 2f). Figure 3 shows a typical structure observed at final stages of formation of PPQ and P N B I (according to results of kinetic investigations [7], under these conditions the reaction is completed in 10-15 min). Further heat treatment of the reaction mass (to 60 min) did not result in a marked change of temperature. Therefore, as seen. from microphotographs shown, solid-phase polycondonsation carried o u t a~ luw temperatures, is characterized by the formation of a new polymer phase only in places of surface contact of initial substances, therefore, the processes examined may, apparently, be grouped to topochemical processes. The polymer phase formed is characterized by the presence of fibrillar structure and:its elements--fibrils are situated within the range of each local reaction zone with a h i g h degree of mutual order. It m a y be assumed that this type of arrangement of fibrils is due to the st~ric position of functional groups in crystals of the initial compounds. As a result, solid-phase polyeondensation at low reaction temperatures seems to take place on a semi-finished m a t r i x - - a crystalline

732

V. V. KORSH~X e$ al.

lattice of low molecular weight substances. Furthermore, it was established that at final stages of the process the reaction mass is heterogeneous, essentially consisting of the polymer formed and unreacted initial substances. In the reactions examined the yield of the polymer formed clearly depends on the degree of dispersion and mixing of initial substances and increases from 5 to 70% with an

FIG. 3. Electron-microscope photographs of P N B I (a) and P P Q (b) end structure obtained at a temperature below the melting point of initial monomers,

increase in the degree of agitation and dispersion. This type of dependence is due to the fact that with an increase in the degree of dispersion, the number of vacant places increases for the formation of nuclei of the new phase. Since the degree and intensity of interactions in solid-phase polycondensation depends not so much on the rate of polycondensation itself as on conditions of transfer of initial reagents into the reaction zone and on the number of vacant points for the formation of nuclei of the new phase, it may be assumed that the effective rate of the process and the degree of polycondensation will be limited by the slowest stage, the stage of transfer of materials in this case.

FIo. 4. Electron-microscope photographs of the structure of P P Q (a) and P N B I (b) obtained at a temperature close to the melting point of initial substances.

Formation of morphological structures in la4der type polyheteroarylencs

733

The structure of polymers obtained at a react~ion temperature close ¢~) the melting point of initial compounds consists of clear volumetric formations which retain, to a certain degree, interfaces (Fig. 4). I t should be noted t h a t polymer yield under these conditions _s close to the quantitative yield which is, apparently, achieved as a result of higher rate of self-diffusion of the initial substances in tile region of ~he melting point.

FIG. 5. Electron-microscope photographs reflecting the formation of morphological structures of the polymer in thermal interaction of PGB and TADPE according to reaction temperature: 200 (a), 250 (b) and 300° (c-e).

734

v. V. KORSHAKet al.

On further increasing reaction temperature, MW increases due to the intensification of interaction of reactive end groups [7]. Since the rate of formation of semi-finished oligomer products below the melting point of initial substances is fairly high, when carrying out the reaction at a temperature higher t h a n their melting point, the structure of the products formed ia the initial non-isothermal period of the reaction--when a given reaction temperature is achieved, has a decisive role in the formation of morphological structures. In proportion to the increase in reaction temperature (Fig. 5a-c) sharp boundaries separating individual volumetric formations are smoothed. In order to study in more detail morphological structures of polymers obtained up to 300 °, their surfaces were studied at different temperature gradients with considerable enlargement (Fig. 5d, e). I t follows from a study of ~hese pho~micrographs t h a t on increasing reaction temperature, the ordered arrangement of fibrils is retained. Therefore, these experimental investigations enabled us to observe for the first time the phenomenon of formation of supermolecular structural elements of ladder-type P H A formed by the interaction of initial low-molecular weight substances in the solid phase. REFERENCES

1. 2. 3. 4.

H. VOGEL and C. S. MARVEL, J. Polymer Sei. 50: 154, 511, 1961 F. DAWANS and C. S. MARVEL, J. Polymer Sci. A3: 10, 3549, 1965 P. M. HERGENROTHER ~nd H. LEVINE, J. Polymer Sei. A5: 6, 1453, 1967 Monomery dlya polikondensatsii (Monomers for Polycondensation). (Ed. V. V. Korshak), Mir, Moscow, 1976 5. J. SCHMITT, P. COMOY, $. BOITKRD and M. SUQUET, Bull. See. chim., 636, 1956 6. V. V. KORSHAK, G. L. BERESTNEVA, A. N. LOMTEVA, L. V. POSTNI]KOVA, Yu. Ye.

DOROSHENKO and Yu. B.: ZIMIN, Vysokomol. soyed. $,20: 3, 710, 1978 (Translated in Polymer Sei. U.S.S.R. 20: 3,' 954, 1978) 7. V. V. KORSHAK, G. L. BERESTNEVA, A. Ye. CHESNOKOVA, N. Sh. ORMOTSADZE, D. V, BIBILEISHVILI and Z. TSEL'TSBAUER (book): Tez. dokl. VIII Mezhdunar. mikrosimpoziuma po polikondensatsil (Prec. of Papers read at the VIII International Mierosymposium on Polyeondensation). p. 89, Alma-Ata, 1981