Polyamic acid solution

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. 

In general polyimides derived from BTDA + p,p -DABP yielded rather poor quality, very brittle films which were due in part to the low viscosity of the resulting polyamic acid solution. 

The diamine (0.009 mol) was dissolved in 35 mL of DMAC contained in a 100-mL, 3-necked flask equipped with a thermometer, a magnetic stirring bar, a N2 inlet, and a CaCl2 drying tube. BTDA (0.018 mol) was then added in one portion. The temperature of the mixture quickly rose to 35 to 38°C and then slowly returned to ambient (26-28°C). After the mixture was stirred for 3 h under N2, 0.018 mol of APA was added and the stirring continued for an additional 3 h. The polyamic acid solution was stored at 0°C. 

General Inidization Procedures. Procedure 1. Acetic anhydride (25 mL) was heated to reflux in a 3-necked flask, fitted with a condenser, a N2 inlet, and a dropping funnel. The polyamic acid solution (10 mL) was then added dropwise over a period of 0.5 h. After the addition was complete, the mixture was heated at reflux for an... 

When amino silane primed wafers were baked at 400°C followed by applying polyamic acid, silane had lost its effectiveness as an adhesion promoter. Results are shown in Table VII. On the other hand when polyamic acid solution was spun onto a wafer that was first primed with silane then coated with polyamic acid solution and baked at 400°C, excellent adhesion was retained. 

Influence of Water on Heat Resistance. It is known that the viscosity of polyimide prepolymer (polyamic acid solution) is reduced by the absorption of a-small amount of water(2). This is because the absorbed water reduces the molecular weight by hydrolysis. Therefore, it seems probable that absorbed water would reduce the heat resistance of cured PIQ film. For that reason, the influence of water contained in the starting materials on the heat resistance of cured PIQ film was investigated. 

Next we consider the dispersion polymerization by polyaddition. In a typical method to prepare polyimide particles, polyamic acid solution is first obtained by coupling of pyromellitic dianhydride and oxy-dianiline, and then by heating the solution. The condensation reaction on heating causes crystallization of polyimide in a spherical form (Fig. 11.2.5, left) (33). However, on the contrary to this conventional method, polyamic acid microspheres could be obtained by dispersion polymerization if an appropriate medium is chosen (34). When a solvent that has a solubility parameter around 17 Mpa is used, submicrometer-sized monodisperse polyamic acid parti-... 

If the two different polyamic acid solutions were mixed and then cured, the glass transition temperatures of the cured polyimides are different from those corresponding to separately cured polyimides. These results are interpreted as transamidization which gives random co-polymer(s). C. Feger in Polymeric Materials for Electronic Packaging and Interconnections J.H. Lupinski and R.S. Moore Eds American Chemical Society Washington, DC, 1989 p 114. 

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. 

CHARACTERIZATION. Melting points were determined on an E. I. DuPont Series 99 Thermal Analyzer at 20°C/min. Inherent viscosities of polyamic acid solutions were obtained at a concentration of 0.5% (w/w) in DMAc at 35°C. Glass transition temperatures (T ) of the fully cured polymer films were measured by thermomechanical analysis (TMA) on a DuPont 943 Analyzer in air at 5°C/min. Films fully-cured at 300°C were tested for solubility at 3-5% (w/w) solids concentration in DMAc,N,N-dimethylformamide (DMF), and chloroform (CHCl-j). Solubilities at room temperature were noted after periods of 3 hours, 1 day and 5 days. Refractive indices of 1 mil thick films were obtained at ambient temperature by the Becke line method (11) using a polarizing microscope and standard immersion liquids obtained from R. P. Cargille Labs. 

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. 

The anhydride calibration curves were prepared by adding various amounts of the appropriate anhydride to polyamic acid solutions containing known concentrations of polymer. The films were spun onto NaCl plates or silicon wafers and cured at 2000 C for 1 h. 

Films containing the diamic acid additives were prepared by dissolving 3-15 weight percent (based on resin solids) of the additives into the polyamic acid solutions, then casting and curing as above. 

The polyimide polymers were formed by an established procedure (17) involving the addition of stoichiometric quantities of benzophenone-tetracarboxylic dianhydride (sublimed) to the diamine dissolved in anhydrous dimethylacetamide. The polyamic acid solution was cast on Teflon, warmed (80° to 90°C) to drive off solvent, then gradually heated to a temperature of 180°C. The polyimide forms rapidly at 120 to 150°C. 

Equilibrium Molecular Parameters. Polyelectrolyte effects, evidenced by increased solution viscosity and reduced scattering intensity at very low concentrations, were observed for polyamic acid solutions in NMP which had not been redistilled over P2O5. We attribute this effect to the abstraction of protons from the amic acid by amine impurities in the NMP. This is discussed in more detail in an earlier study (6). Measurements reported by other workers in other amide solvents such as dimethylacetamide, have been made with LiBr added to suppress the polyelectrolyte affect (3). Measurements described here were made on solutions in NMP which had been redistilled over P2O5 > and no polyelectrolyte effects were observed. 

The author thanks E.W. Maby and S.D. Senturia for their helpful comments and discussion and DuPont de Nemours and Company for providing the polyamic acid solutions. 

From Jenekhe s data (12) on what appears to be the same polyamic acid solution as that used in (6), the newtonian Region (p - 1.4 Pa s) persists out to a shear rate of about 300s". This indicates the flow is always in the newtonian region for the experiments reported in (6). It is, of course, difficult to estimate the boundary between newtonian and non-newtonian effects in this flow since as the solids content of the resist increases, non-newtonian behavior may be expected to set in at a lower shear rate. However, from equation (17), the shear rate experienced by the fluid decreases as well due to the increased viscosity. In order to resolve this question, more data is needed on the variation of the rheological behavior with solids content. 

Heating the polyamic acid solution under reduced pressure to obtain a PI resin,... 

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