Synthesis and blend behavior of high performance homo‐ and segmented thermoplastic polyimides

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Abstract
New synthetic methodology was developed as part of an effort to increase the processibility of high Tg polyimide homo− and copolymers, suitable as matrix resins and structural adhesives. Molecular weight and end group control together with solution imidization techniques were successfully employed to convert a variety of poly(amic acid) intermediates to fully cyclized polyimides. The solution imidization was conducted in N‐methylpyrrolidone (NMP) with o‐dichlorobenzene used as the azeotroping agent at 165–190°C. This technique has produced products which are more soluble than polyimides prepared previously by bulk thermal cyclization of poly(amic acids) at temperature of 300°C. They are also more stable than “chemically” imidized materials. In addition, incorporation of the monofunctional reagent phthalic anhydride provides nonreactive phthalimide end groups and controlled molecular weight. This latter feature significantly further improved the melt and solution processibility of the resulting polyimides. In this study thermoplastic, fully cyclized polyimides of 10 000, 20 000, and 30 000 M̄n were prepared which displayed glass transition temperatures ranging from 260–353°C, with the highest Tg observed with phthalimide capped polyimide systems derived from 6F‐dianhydride and p‐phenylene diamine. Tough, transparent films were prepared from polymers of 20 000 and 30 000 g/mol by casting from NMP solution or by compression molding at 50–70°C above the glass transition temperature. For purposes of molecular weight assessment, t‐butyl phthalic anhydride was used as the end blocker. This permitted 400 M‐Hertz proton NMR to be used for assessing the concentration of end groups. Comparison of the 18 aliphatic protons at the end of the chain allowed M̄n values to be determined, which agreed well with theory.A series of poly(arylene ether ketone)/aromatic polyimide blends were investigated to determine the influence of structural variation and composition on miscibility. As an extension to the PEEKTM/UltemTM blend system, which has been reported to be miscible over all proportions, this study examined how structural variations in both the poly(arylene ether ether ketone) and the polyimide portions affect miscibility. In particular, replacement of the hydroquinone fraction in PEEKTM with bisphenol A or sulfonyl diphenol produced an amorphous polymer which was no longer miscible with UltemTM. Polyimide structures modified by employing 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride (BTDA) and 4,4′‐[1,4‐phenylene‐bis‐(1‐methyl ethylidene)] bisaniline (Bis P) diamine to obtain higher glass transition temperatures were also investigated. This system afforded homogeneous blends with PEEKTM when the (Bis P) diamine was utilized in the synthesis of the polyimide. Furthermore, up to 50 mole percent of hexafluoro‐bis‐dianhydride (6FDA) could be substituted for BTDA without loss of miscibility. However, when the more polar 3,3′‐diaminodiphenylsulfone diamine was employed, immiscible blends resulted. An additional important variant has been to incorporate polyimide siloxane segmented copolymers into the PEEKTM blend system. The polyimide segment can be designed to be miscible whereas the siloxane portion is homogeneously dispersed into a second phase which, in fact, enriches the surface behavior quite dramatically in siloxane content. The latter could be of some importance in allowing for atomic oxygen resistance and possibly improved flame resistance behavior.