Formation from polyamic acids

Whereas the proton transfer does not effect the stochiometry of the final PI when water is eliminated in the imidization reaction (fig. 3F), addition of an excess ODA molecule to polyamic acid could lead to the imine type crosslink formation schematically shown in figure 3G. This would lead to a deficiency of carbonyl oxygen atoms for vapor deposited polyimide and is consistent with our analysis. Mack et al. [16] proposed imine crosslink formation from their Raman spectroscopic studies for vapor deposited polyimides with excess ODA. In accordance with this model we attribute the low binding energy shoulder in the polyimide Nls line (figure 4c) to double bonded nitrogen species. However, the model gives no explanation for the carbonyl deficiency found in spin deposited polyamic acid and polyimide. In this case no excess of ODA is observed and only a very weak shoulder has been reported for the Nls line [4,11]. 

Formation of the Polyamic Acid Ester Derived From the para-di-r-butyl Ester of Pyromellitic Acid and PDA in NMP. 

The Tg of the cured undoped polyamic acid (polyimide) is determined by a differential scanning calorimetry (DSC) to be 251 °C. However, the Tg of the cured polyamic acid/ASD (polyimide/ASD) was not observed up to 275 °C from the DSC, perhaps due to the suppression of the glass transition by the formation of an inorganic matrix network. The a transition of the polyimide/ASD (peak temperature at 298 °C) was found to occur at higher temperature than that of the undoped polyimide (peak temperature at 288 °C) in dynamic... 

In two-step polymerization, initially the polyamic acid is formed from an equimolar mixture of dianhydride and diamine in a polar aprotic solvent such as A,A-dimethylacetamide (DMAc) or A,A-dimeth-ylformamide (DMF). The reaction pathway for the formation of poly(amic acid) involving intermediates is presented in Scheme 3.2. The reaction mechanism involves nucleophilic attack of the amino group to the electrophilic carbonyl carbon of the anhydride group. This opens the anhydride ring to form an amic acid group. Formation of the poly(amic acid) is an equilibration reaction in which the forward reaction... 

The conversion of polyamic acids into polyimides - imidization or dehydrocyclization - consists of the intramolecular formation of water from the polyamic acid to form a cyclic polyimide. It can be carried out in two ways, thermally and chemically. 

Most polyimides are difficult to process because of their insolubility and high melting temperatures, so polyimides are synthesized by a two-step procedure via a soluble polyamic acid, precursor of the polyimide (Eq. 1) [4-6]. In the first step, ringopening polyaddition of a diamine to a tetracarboxylic dianhydride is carried out in an amide type solvents, for example, N-methyl-2-pyrrolidinone (NMP) and N,N -dimethylacetamide (DMAc), at room temperature, leading to formation of a polyamic acid solution. After processing from the polyamic acid solution, the thermal conversion of the polyamic acid to the polyimide is performed by heating a at about 300°C or by chemical treatment with a mixture of carboxylic dianhydride and tertiary amine in the second step 17]. 

Another triamine monomer (l,3,5-tris(4-aminophenoxy)benzene, TAPE) has been used as cross-linking agent by Yin et aL [114,115] (Fig. 25). Typically, anhydride-terminated sulfonated oligomers are prepared from BAPBDS and NTDA in m-cresol at 180 °C for 20 h. After adding some triamine monomer, in a second step, the reaction medium is kept at moderate temperature (50 °C), resulting in a polyamic acid intermediate. By a thermal treatment at high temperature or in the presence of an acetic an-hydride/pyridine mixture, complete imidization is performed during film formation. 

Although polyimides can have an open-chain structure, cyclic polyimides have found more use because they are more thermally stable than open-chain polyimides. The polyether-imide prepared from pyromellitic dianhydride and 4,4 -diaminodiphenyl ether has found use in high temperature coatings, adhesives, and structural plastics. The reaction involves rapid formation of the polyamide, called polyamic acid, followed by a high-temperature ring closure step. 

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