Polyamic acid chemical imidization

The thermal imidization of a polyamic acid film (PMDA-ODA or BPDA-ODA) obtained by casting an NMP solution leads to an amorphous polyimide. Two different teams have shown that a polyamic acid solutions in NMP heated at 200°C for a short time (20 min) gives polyimide particles fully cyclized and highly crystalline, as shown by X-ray diffraction and solid 13C NMR spectroscopy.151152 The chemical imidization of the same solution gives only amorphous particles. The difference between the cyclization of a solution and a casted film in the same solvent is intriguing. In the case of the solution, the temperature and the heating time are lower than in the case of the casted film as a consequence, a less organized structure would be expected for the particle. 

Imides - Polyimides (PI) have been conventionally prepared by the chemical or thermal cyclodehydration of polyamic acids formed from the solution reaction of aromatic tetracarboxylic dianhydrides and aromatic diamines. The early PI were insoluble and relatively intractable. The polyamic acid was the processable intermediate. However, the polyamic acid precursor has two major shortcomings, hydrolytic instability and the evolution of volatiles during the thermal conversion to PI. In addition, residual solvent was left in adhesive tapes and prepregs to obtain tack, drape and flow. During the fabrication of components, the evolution of volatiles caused processing problems and led to porosity in the part. As work progressed on PI, other synthetic routes were investigated (e.g. reaction of esters of aromatic tetracarboxylic acids with diamines... 

Conditions for low temperature solution polymerizations of pyromellitic dianhydride (PMDA) have been developed for a wide variety of aromatic 1,4-phenylene [54, 55] and 4,4 -biphenylene [56-58] diamine monomers in a number of aprotic solvents to give high molecular weight prepolymers referred to as polyamic acids. Since the imidized structures are insoluble, they must be processed in the form of their polyamic acids which are subsequently imidized thermally or by chemical dehydrating agents. Although this procedure is acceptable for thin film or fibers, the fabrication of thick parts is complicated by the water of imidization. 

Starting materials and solvents were purchased from Aldrich Chemical Co. acetonitrile (ACN), N,N-dimethylformamide (DMF), and N-methyl-2-pyrrolidone (NMP) were obtained anhydrous in Sure/Seal bottles and used as received. The polyamic acid of PMDA-ODA (2545 Pyralin) was supplied by DuPont. The soluble polyimide XU-218, derived from 3,3, 4,4 -benzophenone tetracarboxylic dianhydride (BTDA) and diamino-1,1,3-trimethyl-3-phenylindan isomers (DAPI) was purchased from Ciba-Geigy Corp. The acetylene terminated imide oligomer powder (Thermid MC-600) derived from BTDA, aminophenylacetylene, and 1,3-bis (2-aminophenoxy) benzene (APB) was obtained from National Starch and Chemical Company. Kapton Type II (PMDA-ODA) films were obtained from DuPont Co., Apical polyimide films were obtained from Allied Corp., and Upilex Type-S and Type-R polyimide films derived from 3,3, 4,4 -biphenyl tetracarboxylic dianhydride (BPDA) plus p-phenylenediamine (PDA) and ODA, respectively were obtained from ICI Americas Inc. 

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. 

Only one carbonyl component appears in the NR-055X spectrum after the 300 C cure treatment. Its 3.8 eV chemical shift is consistent with imide. The F binding energy and chemical shift values were identical to those observed for the polyamic acid. The most intense C Is component peak characteristics showed little change with temperature spanning 85 -400 C. 

The surface chemical structure of several thin polyimide films formed by curing of polyamic acid resins was studied using X-ray photoelectron spectroscopy (ESCA or XPS). The surface modifications of one of the polymer systems after exposure to KOH, after exposure to temperature and humidity, after exposure to boiling water, and after exposure to O2 and 02/CF plasmas were also evaluated. The results showed imide bond formation for all cured polyimide systems. It was found that (a) K on the surface of the polyamic acid alters the "normal" imidization process, (b) cured polyimide surfaces are not invarient after T H and boiling water exposures, and (c) extensive modifications of cured polyimide surfaces occur after exposures to plasma environments. Very complex surfaces for these polymer films were illustrated by the C Is, 0 Is, N Is and F Is line characteristics. 

The polyimide powder of 6F + 3,3 -ODA was likewise tested for its solubility limit in DMAc. Uie imide powder was prepared by chemically imidizing the 6F + 3,3 -ODA polyamic acid with pyridine/acetic anhydride, precipitating in distilled water, thoroughly drying at 60°C and heating for 2 hours at 200 C. 

The curing behavior of PMDA-ODA (Figure 1) has been analyzed previously The overall process is characterized by various physical and chemical steps Decomplexation of the complex formed between NMP and polyamic acid 2 3 6 plasticization of the material by the decomplexed NMP n 12 evaporation of the solvent cycloimidization accompanied by re-formation of anhydride and amine, a side reaction which leads to chain scission 2 vitrification caused by solvent evaporation and imidization and finally molecular ordering of the polyimide near and above the glass transition n 13 14. The particular features of all these processes are heating rate dependent (e.g. compare Figure l.a and l.b) 2 n. 

The samples which were cured chemically with acetic anhydride and pyridine, and then heated to remove solvent, appeared to be incompletely imidized, and yielded lower molecular weights than thermally cured samples from the same polyamic acid. In addition, these samples produced a bright red solution in sulfuric acid, in contrast to the orange-gold color observed in solutions from thermally cured samples. The red color disappeared within 24 hours, when the molecular weights were determined. This color was also observed by Wallach (4), who carried out viscosity measurements on sulfuric acid solutions of chemically cured PMDA/DAPE polyimides. Wallach observed a slow decrease in the dilute solution viscosity with time over a period of hours from the initial preparation of the solution. We have not observed any decrease in viscosity for 24 hours for solutions prepared from thermally cured samples. Polyimide samples which have been cured chemically have been shown to contain a small percentage of isoimide, which is then converted to the more stable imide at higher temperatures (8-9). The observed red color in sulfuric acid solutions may be because protonation of... 

Hyperbranched polyimides can result due to the self-polycondensation reactions of AB2-, A2- and Bs-types. The preparation of hyperbranched polyimides involves chemical imidization of polyamic acid ester synthesized from AB2-monomers, which are carboxylic dianhydrides containing an ether bond and a diamine [6,19,76]. Polyamic acid in combination with a condensation agent is used because it is difficult to separate the synthesized polymer from AB2-type monomers. 

For example, it is possible to prepare hyperbranched polyimides from 3,5-dimethoxyphenol and 4-nitrophthalonitrile in the presence of diphenyl(2,3-dihydro-2-thioxo-3-benzoxazolyl) phosphonate (DBOP) as a condensation agent at room temperature. Hyperbranched polyimide was obtained through thermal or chemical imidization of the precursor (polyamic acid) (Scheme 1.3) [19]. The obtained hyperbranched polyimide had a relatively great molecular mass (A/ ) of about 190000gmor but low inhinsic viscosity of 0.30 dLg . Therefore, it had a compact configuration and the lack of entanglement of polymer chains. The polymer obtained via chemical imidization was soluble in apiotic polar solvents such as tetrahydrofuran (THF), while the polymer from thermal imidization was insoluble in any solvents. 

In the second step, polyamic acid is cyclo-dehy-drated at elevated temperatures (thermal imidization) or in the presence of a cyclizing agent (chemical imidization). Advantages of this method over one-step polymerization are the use of less toxic solvents and direct processing of soluble polyamic acids to form the final polyimide products in the form of films or fibers by thermal imidization. However, the storage instability of polyamic acid intermediates and the control of thermal imidization are still important issues [28]. A detail description of the thermal and chemical imidization of poly(amic acid) is given below. 

Chemical imidization is not widely used because it employs additional reagents. However, it has the advantage of low temperature imidization and can he nsed to directly form fine polyimide molding powder (16,50). A t5 ical chemical imidization reaction employs a 20-30% solids polyamic acid in an amide solvent with a slight molar excess of acetic anhydride and a molar equivalent of a triamine (triethyl amine, pyridine, or -picoline) (51-54). The percent conversion for chemical imidization is a function of polyimide solnhiUty. If the polymer crystallizes and/or precipitates from the reaction medium, imidization will he incomplete (16,50). Those systems that remain soluble must imdergo thermal treatment to convert any isoimide, and remove residual solvent. The mechanistic routes of chemical imidization are shown in Figure 4, and involve the use of a triamine... 

Fig. 4. Reaction Scheme for chemical imidization via polyamic acid.

One system that takes advantage of both the chemical and thermal imidiza-tion is the gel casting of PMDA-ODA. This system starts as polyamic acid in NMP or DMAc to which is added /3-picoline and acetic anhydride. This mixture starts to imidize and forms a gel as it is coated onto a rotating heated drum. The swollen gel film has mechanical integrity that allows it to be stripped off the drum and gripped by a tenter frame. Final conversion and solvent removal is achieved thermally (infrared and convection) while the film is undergoing mechanical orientation (57). 

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