Linear polyimide precursors

Polymers discussed in the previous section consist of linear polyimide precursors (polyamic acids) that are transformed into polyimides during the processing stage. 

Table 10 Examples of polyimide structural adhesives including the main families of linear polyimide precursors as well as oligomers terminated by ethylenic double bonds...

Table 9.10 Examples of Polyimide Structural Adhesives Including the Main Families of Linear Polyimide Precursors as Well as Oligomers Terminated by Ethylenic Double Bonds and Acetylenic Groups...

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. 

NR 150 A2 Linear fluorinated polyimide precursor 8 (Fig. 9) -i- A1 Adhesive paste... 

Polyimides (PI) exhibit very good chemical, mechanical and dielectric stability at temperatures from -150 to 250 C. These rigid polymers with a high glass transition temperature are mostly used in (micro)electronics, aircraft industry, space exploration and as polymeric separation membranes. Linear polyimides (LPI) are traditionally prepared by the two-step polymerization. The polyimideprecursor, polyamic acid (PAA) (the most often a solution in N-methyl-2-pyrrolidone), is prepared from an aromatic dianhydride and an aromatic diamine. This precursor is transformed into a polyimide using thermal or chemical imidization (Figure 2) [5]. 

Polymers discussed in the previous section consist of linear poiyimide precursors (polyamic acids) that are transformed into polyimides during the processing stage. This means that water molecules evolve within the adhesive layer and must be removed to avoid the formation of large voided areas. To circumvent this drawback, research was focused on the synthesis of fully cyclized heterocyclic polymers, which are intrinsically thermoplastic, even though the Tg can be extremely high. Polyphenylquinoxalines, poly... 

Wang et al. [135] prepared a novel ABB monomer (1-14, Scheme 8), namely 4-[4-(2,4-diaminophenoxy)phenoxy] phfhalic acid 2-methyl ester, which was polymerized to form the precursor polyamic acid monomethyl ester. The direct polycondensation of the ABB monomer was carried out to form polyamic acid monomethyl ester as a precursor and had a M of 12,000 Da. Chemical imidization in the presence of acetic anhydride and pyridine gave hb-PIs with low DB. The DB of the precursor, as determined by the H NMR spectra, was only 7%. They ascribed the low DB to the differences in the reactivities of the amino groups. End modification reactiOTis were accomplished with acetyl chloride, benzoyl chloride, and phthalic anhydride to form end-capped polyimides. The end-group-modified polyimides were soluble in polar aprotic solvents such as DMSO, DMF, and NMP. TGA measurements showed Ta,s% in the range of 400-520°C, and Tg of 200-258°C. The hb-PIs showed film-forming ability, but they were more brittle than analogous linear polymers. 

---