Polyamic acid synthesis

Table 1. Typical Rate Constants for Polyamic Acid Synthesis ...

As solutions of polyamic acids are difficult to be store for long periods because of their low stability [8], procedures using derivatives of polyamic acids, such as polyamic acid alkyl esters [9,10], polyamic acid trimethylsilyl esters 111, 12] and polyamic acid amides [13], have been investigated. Although the solutions of these derivatives are more stable than those of polyamic acids, synthesis of polyimides via these polyamic acid derivatives is not attractive for large-scale mass production and industrial applications because more steps are involved, and the cost is high. 

From a mechanistic point of view, the polyimide synthesis via the polyamic acid is not a convenient method of preparing the block copolyimide for two main... 

The reaction sequence for the synthesis of polyimides is shown in Scheme I. The imide ring is formed via a thermally initiated condensation reaction from a polyamic acid, a soluble precursor polymer synthesized from... 

As mentioned before, the first generation of fully aromatic homopolyimides, could be used in a few applications because they had to be applied in the form of soluble polyamic acids, and this limited the materials to be transformed almost exclusively into films or coatings [2,10]. They all had to be synthesized by a two-step method, as exemplified for an aromatic polyimide from pyromellitic dianhydride in Scheme 1. The method involves the synthesis of a soluble polyamic acid, which, after shaping, can be converted to the related polyimide by a thermal or a chemical treatment. Abundant literature is available on the methods and the mechanisms involved in the synthesis of these polymers [3,4,11-13]. 

Synthesis of Polymers. Polyamic acid solutions were prepared by condensation of the aromatic anhydride and amine in N,N-dimethylacetamide (DMAc). Polyimide modified electrodes were made by casting or spin coating the precursor polyamic acid solution onto stainless steel or platinum substrates. Imidization was achieved by either heating the films to 400°C for 60 min or through a chemical dehydration process involving immersion in a 1 1 mixture of acetic anhydride and pyridine (6). BTDA-DAPI films were made by casting from a DMAc solution and heating to 100 C. 

The traditional approach used in poly(imide-siloxane) synthesis is the reaction of aminopropyl-terminated dimethylsiloxane oligomers with aromatic dianhydrides and additional diamines (9-13). Typically, subambient temperatures and dipolar aprotic solvents are used. The resulting high-molecular-weight polyamic acid solution can be heated to effect imidization and solvent evaporation. This procedure is analogous to the synthetic method used to prepare conventional polyimides for films and coatings. 

Synthesis of Siloxane-Polyimide Elastoplastics. In a typical polymerization, a 5-L, three-neck, round-bottom flask equipped with an overhead mechanical stirrer, a Dean-Stark trap with condenser and a nitrogen inlet, and a thermometer was charged with 484.00 g (0.2406 mol) of D2o-DiSiAn, 41.61 g (0.431 mol) of mPD, 19.52 g (3 wt %) of 2-hydroxypyridine, and 2 L of o-dichlorobenzene. The mixture was warmed to 100 °C for 1 h to dissolve the monomers and the catalyst. The polyamic acids precipitated and then redissolved when the mixture was warmed to 150 °C for 2 h. To the oligomer solution was added 99.13 g of BPADA dissolved in 200 mL of o-dichlorobenzene. The mixture was maintained at 150 °C for an additional 2-h period to ensure incorporation of the dianhydride and then warmed to reflux. After approximately 100 mL of a solvent-water mixture had been removed, the solution was maintained at 180 °C for 40 h. The mixture was cooled to room temperature and diluted with 1 L of methylene chloride. Polymer was isolated from the solution by a slow addition of the polymer solution to 4 L of methanol. The resulting slurry was filtered, and the polymer was redissolved in 4 L of methylene chloride, extracted three times with 2 N aqueous HCl to remove catalyst, washed with water, dried with magnesium sulfate, reprecipitated into methanol as before, filtered, and dried in vacuo at 100 °C to obtain 522 g (85%) of a rubbery material with an IV of 0.50 dL/g. IR, NMR, and Si NMR spectroscopic analysis indicated the absence of amic acid functionalities that could be present if imidization is incomplete. 

Vapour Deposition Polymerisation. This is a little studied approach but one that offers significant potential for the fabrication of very thin films and for elaborate multilayer structures. A commercial process has been developed by the Ulvac Corporation in Japan to coat magnetic relay switches with an insulating polyimide layer. A polyamic acid is sythesised by co-deposition of two reactive monomers and is then thermally imidised. The same approach can be used for the condensation polymerisation of poly(azomethine)s, ° poly(ox-adiazoles) and poly(quinoxalines) all of which have been used in LED structures. This approach to polymer synthesis is ripe for further development. 

Upilex Type R by UBE Ind. is produced from BPDA and ODA. It is based on a unique combination of the new monomer synthesis described in the preceding section and one step high temperature solution polymerization in a phenolic solvent [42]. High quality films and fibers can be produced from the solution because a water forming reaction is not involved [43]. The polymers produced by such a process have a completely imidized structure and provide for superior properties than polymers prepared by solid state imidization of polyamic acids. For example long term oxidative and hydrolytic stabilities and retention of electrical properties are substantially better. 

Synthesis and Characterization of the f-Butyl Ester of the Oxydianiline—Pyromellitic Dianhydride Polyamic Acid... 

Figure 1. Synthesis and cure of the polyimide and precursor polyamic acid from the condensation of pyromellitic dianhydride (PMDA) and 4,4 -diaminodiphenylether (DAPE).

Some of the problems seen with the commercially available polyimides such as limited shelf life,gelation and high ionic contamination are traceable to the raw materials themselves. A zone refining technique has been perfected for use with organic materials and these precursors have been used to synthesize ultrapure polyamic acids for IC device applications. The key feature of the synthesis is the use of solid ingots of the dianhydrides. Materials prepared by this technique show low metallic impurities and have been shown to be excellent film formers for a variety of applications. In particular a polyimide derived from PMDA-ODA has been used to passivate magnetic bubble devices. IR techniques coupled with electrical measurements have been used to optimize the cure conditions and a simple resist process has been defined to passivate these devices. Device performance compares well with conventional inorganic insulators. 

For this end applications variety of commercially available polyamic acid compositions are available and were examined. An in-house synthesized polyamic acid system appeared to have the best combination of all the desired features. The method of synthesis and the fabrication processes involved in the passivation application will be described in this paper. 

In the case of the solids,initial experiments showed that the conventional purification techniques such as recrystallization from solutions do not offer sufficiently pure materials. For example,PMDA samples, after several recrystallizations and vacuum sublimations appear colorless,but show high levels of ionic contaminations when analyzed for inorganic ions. To eliminate this problem,the technique of zone refining as used in semiconductor materials purifications has been used to purify the starting materials. This technique has been applied for a number of organic materials in our laboratory. PMDA for example,when subjected to a simple zone refining process ( as few as 25 zones), shows removal of impurities as high as 3% even if preceded by recrystallization and sublimation. In syntheses of polyamic acids,a variation as small as 3% in stoichiometry can cause considerable variation in the final batch-to-batch synthesis. 

All the solvents used in the synthesis of the polyamic acids were obtained from Burdick and Jackson laboratories. All were "Distilled in Glass" quality.usually with a specified upper limit for the water content. Most of the solvents contained less than 0.009% water. No further purification of the solvents were attempted. 

Synthesis of some polyamic acid compositions are given below ... 

This paper will report on the preliminary work on the preparation of a photosensitive polyamic acid which has advantages over previously reported materials (2) in the ease of synthesis and in some properties of final imide film. 

Whang and Wu [3] have described the liquid crystalline state of polyimide precursors and shown that certain polyamic acids derived from pyromellitic anhydride exhibit lyotropic behaviour. Liquid crystal phases have also been observed by Wenzel et al. [4] in polyimides derived from pyromellitic anhydride and 2,5-di-n-alkoxy-1,4-phenyl ene diisocyanate. Dezern [5] has disclosed a synthesis for linear polyamide-imides derived from benzophenone dianhydride but the occurrence or otherwise of mesophases is not mentioned. 

Amide bands are not present in cyclic imides, but are typical for the intermediate of the Kapton synthesis, the polyamic acid. All of the above mentioned increasing bands reveal changes of the peak areas very similar to the... 

SYNTHESIS Aromatic polyetherimides are usually prepared from (a) bisphenoxide salts and aromatic dinitrobisimides via nucleophilic nitro-displacement reactions(b) two-step polycondensation of aromatic diamines and ether-dianhydrides in a polar aprotic solvent, followed by thermalor chemical cyclodehydration of the polyamic acid precursors and (c) one-step, high temperature solution polymerization of aromatic diamines and ether-dianhydrides in a phenolic solvent, removing water of condensation azeotropically. - Certain polyetherimides can also be s)mthesized via direct melt polymerization/ ... 

In the case of the synthesis of polyimide, the polymerization solvent used for polyamic acid (a precursor of polyimide) is usually dimethyl acetoamide (DMAC). We found that clay ion-exchanged dodecyl ammonium ions could be homogeneously dispersed in DMAC. A solution of this organophilic clay and DMAC was added to a DMAC solution of polyamic acid. The film was cast from a homogeneous mixture of clay and polyamic acid, and was heated at 300 °C to achieve the desired polyimide clay nanocomposite film. Its permeability to water decreased to 50% upon addition of 2.0 wt% clay [13]. It was confirmed that its permeability to carbon dioxide also decreased by half [14]. 

Polymer Synthesis. Basic structures of polyamic acids (PAA), polyamic acid esters (PAE), copolymers of PAA and PAE, and polyamic acid esters with pendant carboxylic acid (PAE-COOH) used for this study are described in Figure 1. After dissolving a diamine into N-methylpyrrolidone, polyamic acids (PAA) were synthesized by adding tetracarboxylic dianhydrides slowly for about Ihr into the ice-cooled diamine solution as described in the literarure [7]. 

K. Yamanaka, M. Jikei, M. Kakimoto, Synthesis of hyperbranched aromatic polyimides via polyamic acid methyl ester precursor. Macromolecules, 33, 1111-1114 (2000). 

Concurrent with the development of PBI as a high temperature adhesive, work was also performed on polyimides (PI). The early synthesis of PI utilized the method of Edwards (7) and Endrey (8) where a soluble processable polyamic acid was formed from the stoichiometric reaction of an aromatic diamine with an aromatic tetracarboxylie acid dianhydride (eq. 2). The polyamic acid was subsequently cyclodehydrated to the PI. In adhesive work, this ring closure was accomplished thermally. 

SYNTHESIS Poly(pyromellitimide-l,4-diphenyl ether) is generally prepared from polycondensation of pyromellitic dianhydride and 4,4 -oxydianiline followed by either thermal or chemical (in the presence of acetic anhydride and triethylamine) cyclodehydration of the polyamic acid precursor. 

Fig. 7. Synthesis and cyclodehydration of PMDA-ODA polyamic acid to yield PMDA-ODA polyimide.

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