Process for producing polyimides

Abstract

A solvothermal synthesis process for polyimides is provided. The process uses solution polymerization of monomers in an appropriate solvent, by mixing the solvent and the monomers and heating the mixture under pressure at temperatures exceeding the respective boiling point at normal pressure. The process produces essentially completely crystalline polyimides by a) mixing and heating the solvent and the monomers by either (i) heating the solvent up to solvothermal conditions and subsequently adding the monomers to initiate the reaction, or (ii) mixing the monomers with the solvent and heating the mixture up to solvothermal conditions within a period of 5 min, the reaction temperature TR being held below the solid-state polymerization temperature TP of the monomers during the polymerization; and b) carrying out the solution polymerization until essentially complete conversion is achieved.

Claims

1. A solvothermal synthesis process for polyimides using solution polymerization of monomers in an appropriate solvent by mixing the solvent and the monomers and heating the mixture under pressure at temperatures exceeding the respective boiling point at normal pressure, wherein crystalline polyimides are produced by: a) mixing and heating the solvent and the monomers by either: a1) heating the solvent up to solvothermal conditions and subsequently adding the monomers to initiate the reaction, or a2) mixing the monomers with the solvent and heating the mixture up to solvothermal conditions within a period of 5 min, the reaction temperature TR being held below the solid-state polymerization temperature TP of the monomers during the polymerization; and b) carrying out the solution polymerization until essentially complete conversion is achieved.

2. The process according to claim 1, wherein said mixture of monomers and solvent in step a2) is heated up to solvothermal conditions within 3 min.

3. The process according to claim 2, wherein said mixture of monomers and solvent in step a2) is heated using microwave radiation.

4. The process according to claim 1, wherein, in an additional step preceding step a), stoichiometric salts are formed from the monomers, having a molar ratio between diamine and di-anhydride of 1:1.

5. The process according to claim 1, wherein the reaction temperature TR is kept at least 5 C. below the solid-state polymerization temperature TP of the monomers.

6. The process according to claim 1, wherein water, one or several alcohols, or a mixture of water and alcohol(s) is used as the solvent.

7. The process according to claim 1, wherein an aromatic diamine and/or an aromatic tetracarboxylic di-anhydride is/are used as monomeric component(s).

8. The process according to claim 7, wherein a stoichiometric salt of an aromatic di- or triamine and an aromatic tetracarboxylic di-anhydride is used as the monomer.

9. The process according to claim 1, wherein essentially completely crystalline polyimides are prepared.

10. The process according to claim 2, wherein said mixture of monomers and solvent in step a2) is heated up to solvothermal conditions within 2 min.

11. The process according to claim 10, wherein said mixture of monomers and solvent in step a2) is heated up to solvothermal conditions within 1 min.

12. The process according to claim 5, wherein the reaction temperature TR is kept at least 10 C. below the solid-state polymerization temperature TP of the monomers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is described below referring to specific exemplary embodiments and the appending drawings which show the following.

(2) FIG. 1 shows a TGA curve of the monomeric mixture according Brunel et al. (supra).

(3) FIG. 2 shows the XRD pattern of the polyimide obtained in example 1 of the invention.

(4) FIG. 3 shows a SEM image of the polyimide obtained in example 1 of the invention.

(5) FIG. 4 shows the XRD pattern of the polyimide obtained in example 3 of the invention.

(6) FIG. 5 shows a SEM image of the polyimide obtained in example 3 of the invention.

(7) FIG. 6 shows the XRD pattern of the polyimide obtained in example 4 of the invention.

(8) FIG. 7 shows the XRD pattern of the polyimide obtained in example 5 of the invention.

(9) FIG. 8 shows the XRD pattern of the polyimide obtained in example 6 of the invention.

(10) FIG. 9 shows the XRD pattern of the polyimide obtained in example 8 of the invention.

(11) FIG. 10 shows the XRD pattern of the polyimide obtained in example 9 of the invention.

(12) FIG. 11 shows the XRD pattern of the polyimide obtained in example 10 of the invention.

(13) FIG. 12 shows the XRD pattern of the polyimide obtained in example 11 of the invention.

EXAMPLES

(14) All the reactants used for the hydrothermal synthesis of polyimides described below were obtained from commercial sources and used without any further purification. Thermogravimetric analyses were carried out using a Netzsch TG 209 Analyzer, and IR spectroscopies were carried out on a Bruker Tensor 27. X-ray powder diffractograms were recorded using a PANalytical X'Pert Pro multi-purpose diffractometer, and scanning electron microscopies were carried out using a Quanta 200F FEI.

(15) Abbreviations

(16) HT: hydrothermal XRD: X-ray diffractometry IR: infra-red spectrometry TGA: thermogravimetric analysis SEM: scanning electron microscopy PDA: p-phenylenediamine, 1,4-diaminobenzene PMA: pyromellitic acid, benzene-1,2,4,5-tetracarboxylic acid PMDA: pyromellitic di-anhydride, benzene-1,2,4,5-tetracarboxylic di-anhydride PPPDI: poly(p-phenylene pyromellitic diimide) BTA: benzophenone-3,3,4,4-tetracarboxylic acid BTDA: benzophenone-3,3,4,4-tetracarboxylic di-anhydride PPBTDI: poly(p-phenylene benzophenone tetracarboxylic diimide) Bz: benzidine, 4,4-diaminobiphenyl PBBTDI: poly(p-biphenylene benzophenone tetracarboxylic diimide) TAPB 1,3,5-tris(4-aminophenyl)benzene PBTPPDI poly(benzoltri(p-phenylene)pyromellitic diimide) X.sub.cr: degree of crystallinity

Example 1Preparation of poly(p-phenylenepyromellitic diimide), PPPDI

(17) ##STR00004##
a) Preparation of the Monomeric Salt [H.sub.2PDA.sup.2+PMA.sup.2]

(18) Under an inert atmosphere, 0.327 g PMDA were added to a three-necked flask, equipped with a reflux condenser, and dissolved in 15 mL of distilled water. The solution was heated to 80 C. and 0.162 g PDA were added while stirring, resulting in the immediate precipitation of the monomeric salt as a white powder. Stirring was continued for 2 h; whereafter the salt was filtered off and dried in vacuo. TGA analysis of the dried monomeric salt resulted in a solid-state polymerization temperature T.sub.P of 205 C.

(19) b) HT Polymerization

(20) The monomeric salt was dispersed in 15 mL of distilled water, introduced into a non-stirred autoclave and heated up to HT conditions within 4.5 min and then further heated to 200 C. After 1 h at this reaction temperature, the autoclave was quickly cooled to room temperature, and the PPPDI which had been formed was filtered off, washed with distilled water, and dried overnight in vacuo at 40 C.

(21) The PPPDI was orange and completely imidated, as determined using FT-ATR-IR (1783 cm.sup.1 (CO imide); 1709 cm.sup.1 (CO imide); 1365 cm.sup.1 (CN)), showing no discernable oscillations of the monomers or the monomeric salt. Using powder XRD, complete crystallinity of the product which existed in the form of two solid crystalline phases, i.e. without any amorphous portions, was determined. The degree of crystallinity X.sub.cr thus amounted to >99%. FIG. 2 shows the XRD pattern of the obtained polyimide. SEM showed that the obtained PPPDI had a very homogeneous, regular morphology, which is further evidence of the extremely high degree of crystallinity. FIG. 3 shows SEM images of the polyimide.

Example 2Larger-scale Preparation of PPPDI

(22) ##STR00005##

(23) The method of Example 1 was essentially repeated, with the exception that the monomeric salt was formed from 8.72 g PMDA and 4.33 g PDA in 400 mL of distilled water. In a stirred reactor in an autoclave, this monomeric salt (T.sub.P 205 C.) was then heated to HT conditions within 4 min and subsequently also heated to 200 C., and the product was isolated and dried in the same way as in Example 1. The purity and crystallinity of this PPPDI, as determined by IR and XRD, corresponded to those of the product from Example 1: X.sub.cr>99%.

(24) Without wishing to be bound by theory, it is assumed that, in addition to the low water solubility of the monomers, the high rgidity of the obtained polyimide is responsible for the high degree of crystallinity of the obtained PPPDI, as mesomeric effects result in a largely planar arrangement of the repeating units of the polymer molecule.

Example 3Preparation of Poly(p-phenylenebenzophenone tetracarboxylic diimide), PPBTDI

(25) ##STR00006##

(26) In a manner analogous to the method described in Example 1, 0.48 g (1.5 mmol) BTDA in 15 mL dist. water and 0.11 g PDA were converted into the monomeric salt [H.sub.2PDA.sup.2+BTA.sup.2] while stirring, which was done at room temperature, though. The T.sub.P of this monomeric salt was 149 C., as determined by TGA, which monomeric salt was subsequently introduced into an autoclave together with 15 mL of water and heated up to HT conditions within 5 min and finally to 140 C. without stirring and then polycondensed for 12 h to obtain the polyimide PPBTI.

(27) IR of the brownish crystals showed complete imidation (1781 cm.sup.1 (CO imide); 1717 cm.sup.1 (CO imide); 1378 cm.sup.1 (CN)), as there were no discernable oscillations caused by monomers or the monomeric salt. Crystallinity was examined by powder XRD. FIG. 4 shows the XRD pattern of PPBTDI, a Gauss curve corresponding approximately to the proportion of amorphous structures being laid under the curve of the crystalline peaks. From the areas under the two curves a degree of crystallinity X.sub.cr of approx. 62% was calculated. Nevertheless, the SEM image of the polyimide presented in FIG. 5 shows that the morphology the obtained PPBTDI is highly regular.

Example 4Preparation of poly(p-biphenylenbenzophenone tetracarboxylic diimide), PBBTDI

(28) ##STR00007##

(29) In a manner analogous to the method described in Example 3, 0.48 g (1.5 mmol) BTDA in 15 mL dist. Wasser and 0.22 g Bz were converted into the monomeric salt [H.sub.2Bz.sup.2+BTA.sup.2] while stirring. The T.sub.P of this monomeric salt was 172 C., as determined by TGA, which monomeric salt was subsequently introduced into a non-stirred autoclave together with 15 mL of water and heated up to HT conditions within 4.5 min and finally to 160 C. and then polycondensed for 12 h to obtain the polyimide PPBBTDI.

(30) IR of the brownish crystals showed complete imidation (1786 cm.sup.1 (CO imide); 1709 cm.sup.1 (CO imide); 1389 cm.sup.1 (CN)), as there were no discernable oscillations caused by monomers or the monomeric salt. Crystallinity was examined by powder XRD. FIG. 6 shows the XRD pattern of PBBTDI, the degree of crystallinity X.sub.cr of approx. 61% being again calculated from the areas below the curve of the crystalline peaks and the Gauss curve laid under it to approximately account for amorphous portions.

(31) Without wishing to be bound by theory, it is assumed that the significantly lower crystallinity of the PPBTDI from Example 3 and the PBBTDI from Example 4 as compared to the PPPDI from the Examples 1 and 2 is due to the higher water solubility of benzophenone tetracarboxylic acid, BTA.

Example 5Preparation of PPBTDI using Microwave Radiation

(32) ##STR00008##

(33) Example 3 was essentially repeated, with the exception that the heating was done by microwave irradiation, so that the hydrothermal conditions were already obtained after less than 2 min and the polymerization reaction was essentially completed after only 1 h.

(34) IR also showed complete imidation in this case, and FIG. 7 shows the powder XRD pattern of the obtained dried PPBTDI. From the areas under the curve of the crystalline peaks and the Gauss curve laid under it, the degree of crystallinity X.sub.cr was calculated to be approx. 93%, which is 31 percentage points higher than the 62% of the product obtained in Example 3. The significantly faster heating process using microwaves (2 min in Example 5 instead of 4 min in Example 3) thus resulted in a considerable increase in crystallinity by 50%, as apparently an even lower proportion of the monomeric salt dissolved before HT conditions were established.

Example 6Preparation PBBTDI using Microwave Radiation

(35) ##STR00009##

(36) Example 4 was essentially repeated, with the exception that the heating was done by microwave irradiation, so that the hydrothermal conditions were already obtained after less than 2 min and the polymerization reaction was essentially completed after only 1 h.

(37) IR also showed complete imidation in this case, and FIG. 8 shows the powder XRD pattern of the obtained dried PPBTDI. From the areas under the curve of the crystalline peaks and the Gauss curve laid under it, the degree of crystallinity X.sub.cr was calculated to be approx. 80%, which is 19 percentage points higher than the 62% of the product obtained in Example 4. The significantly faster heating process using microwaves (2 min in Example 6 instead of 4.5 min in Example 4) thus resulted in a considerable increase in crystallinity by approx. 30%, as apparently an even lower proportion of the monomeric salt dissolved before HT conditions were established.

Example 7Preparation of PPPDI in Ethanol

(38) ##STR00010##

(39) Example 2 was essentially repeated, with the exception that the monomeric salt was suspended in 400 mL of ethanol instead of using water for polymerization. The reaction (after heating up to HT conditions within 4.5 min and finally to 200 C.) and the subsequent work-up were also carried out in a manner analogous to Example 2.

(40) IR was also applied in this case to verify complete imidation, the powder XRD pattern corresponding almost exactly to that from Example 1 (see FIG. 2).

(41) It was thus possible to prove that the polycondensation of PDA and PMDA into a highly crystalline polyimide is also possible in another protic solvent than water, yielding the same excellent successful results.

Example 8Preparation of Crosslinked Polyimide poly(benzenetri(p-phenylene)pyromellitic diimide), PBTPPDI, Using Microwave Radiation

(42) ##STR00011##

(43) The procedure described in Example 3 was essentially repeated, with the exception that 0.06 g (0.3 mmol) PMDA and 0.07 g (0.2 mmol) TAPB were converted into the monomeric salt [(H.sub.3TAPB.sup.3+).sub.2(PMA.sup.2).sub.3], the T.sub.P of which was determined to be 152 C. using TGA, the salt being subsequently heated using microwaves in a non-stirred autoclav and in 15 mL of water to reach HT conditions within 2 min and finally heated at 140 C. and thereafter polycondensed for 12 h to obtain the polyimide PBTPPDI.

(44) IR of the dried brown crystals again showed complete imidation (1785 cm.sup.1 (CO imide); 1723 cm.sup.1 (CO imide); 1390 cm.sup.1 (CN)), as there were no discernable oscillations caused by the monomers or the monomer salt. Crystallinity was examined by means of powder XRD. FIG. 9 shows the XRD pattern of PBTPPDI, in which no amorphous portions can be found, indicating a degree of crystallinity X.sub.cr of >99%.

Example 9Preparation of PPPDI by Injecting the Monomers into a Separately Heated Solvent

(45) ##STR00012##

(46) The monomeric salt was produced as described in Example 1 a), the charge being 10 times higher, however. The thus obtained salt was dispersed in 100 mL of dist. water, the dispersion was introduced into a high-pressure steel pipette which was connected to a 1 L steel reactor, separated from the reaction chamber which contained 400 mL of dist. water by a valve. The device was placed in an autoclave, and the water in the reaction chamber was heated at 200 C. under the respective autogenous pressure. When the reaction temperature was reached, the valve was opened and the monomeric dispersion was injected into the pre-heated solvent by means of inert gas pressure in less than 30 s. The reaction mixture was then stirred for 1 h at 200 C., whereafter the conversion was found to have been completed, and the product was isolated and dried as described in Example 1.

(47) Purity and crystallinity of this PPPDI were determined by means of IR and XRD and corresponded to that of the product from Example 1 (FIG. 10 showing the respective XRD pattern). There were no discernable oscillations caused by the monomer or the monomeric salt and no amorphous portions, indicating a degree of crystallinity X.sub.cr of 100%.

Example 10Preparation of PPBTDI by Injecting the Monomers into a Separately Heated Solvent

(48) ##STR00013##

(49) In a manner analogous to the procedure described in Example 9, the reaction of PDA with BTDA was carried out using a charge 10 times higher than in Examples 3 and 5, with the exception that the reaction mixture was stirred for 4 h at 200 C. after injecting the monomeric dispersion to guarantee complete conversion.

(50) The IR and XRD peaks of the thus obtained PPBTDI corresponded to those from Examples 3 and 5; in the present case, however, there were practically no amorphous portions, which indicates a X.sub.cr>99%. The corresponding XRD pattern is shown in FIG. 11. The product obtained after separately heating the solvent was thus significantly purer than that obtained in Example 5, as practically none of the monomers had dissolved.

Example 11Preparation of PBBTDI by Injecting the Monomers into a Separately Heated Solvent

(51) ##STR00014##

(52) In a manner analogous to the procedure described in Example 9, the reaction of Bz with BTDA was carried out using using a charge 10 times higher than in Examples 4 and 6. However, the reaction mixture, was stirred for 4 h at 200 C. after injecting the monomeric dispersion to guarantee complete conversion.

(53) The IR and XRD peaks of the thus obtained PPBTDI correspond to those from Examples 4 and 6; in the present case, however, even fewer amorphous portions were discernible, which indicated a X.sub.cr>90%. The corresponding XRD pattern is shown in FIG. 12. The product obtained after separately heating the solvent was thus significantly purer than that obtained in Example 6, as practically none of the monomers had dissolved, resulting in an increase of the product's crystallinity by 10 percentage points as compared to the product form Example 6.

(54) In summary, the results of the Examples above, which are listed in Table 1 below, are proof of the excellent crystallinity of polyimides prepared according to the present invention, which may even be further increased by a higher heating rate using microwave radiation and by separately pre-heating the solvent. For structures having a very low water solubility, it may be sufficient to heat them using a heating bath or circulating air in order to obtain excellent degrees of crystallinity.

(55) TABLE-US-00001 TABLE 1 Time T.sub.P until monomeric T.sub.R HT X.sub.cr Ex. Polyimide salt ( C.) ( C.) MW HS (min) Solvent (%) 1 PPPDI 205 200 no no <4.5 H.sub.2O >99 2 PPPDI 205 200 no no <4 H.sub.2O >99 3 PPBTDI 149 140 no no <5 H.sub.2O 62 4 PBBTDI 172 160 no no <4.5 H.sub.2O 61 5 PPBTDI 149 140 yes no <2 H.sub.2O 93 6 PBBTDI 172 160 yes no <2 H.sub.2O 80 7 PPPDI 205 200 no no <4.5 EtOH >99 8 PBTPPDI 152 140 yes no <2 H.sub.2O >99 9 PPPDI 205 200 no yes <0.5 H.sub.2O >99 10 PPBTDI 149 200 no yes <0.5 H.sub.2O >99 11 PBBTDI 172 200 no yes <0.5 H.sub.2O >90 T.sub.P: solid-state polymerization temperature T.sub.R: reaction temperature MW: microwaves HS: solvent is heated separately

(56) The advantages of preferred embodiments of the inventive method become particularly evident when comparing Examples 1, 2, and 9 for producing PPPDI, Examples 3, 5, and 10 for producing PPBTDI, and Examples 4, 6, and 11 for producing PBBTDI. A high heating rate achieved by microwaves significantly improves the crystallinity of the thus obtained products when compared to those obtained using common heating procedures. Crystallinity may even be further improved by separately heating the solvent to solvothermal conditions and only then adding the monomeric salt, as in this way practically none of the monomers will dissolve before reaching polymerization temperature.

(57) This means that, in the latter case, the reaction temperature does not have to be kept below the solid-state polymerization temperature T.sub.P of the monomers, although this may still be preferred in certain cases.

(58) The invention thus provides an improved method for preparing polyimides by solvothermal synthesis, which yields products showing a significantly higher crystallinity than used to be achievable according to prior art.