Transimidization in thermoplastic polyimides

In order to improve the processibility in thermoplastic polyimides, a new m ethod, termed the "reactive plasticizer" approach, has been proposed. This method uses a small amount (5 similar to 15 mol%) of a less activated, weak nucleophilic diamine co-monomer as a "reactive plasticizer" to obtain copo lyimide resins which possess relatively low viscosity at low temperatures a nd can be readily processed through the autoclave cycle at low pressures. D uring a high temperature treatment, the reactive plasticizers join the reac tion to form high molecular weight copolyimides, and the preferred material properties are thus achieved. The most effective reactive plasticizer is a romatic heterocyclic diamines, such as 2, 6-diaminopyridine diamine (DAP), and the transimidization involved with a reactive plasticizer has been prop osed to play a major role for the success of this approach. In order to und erstand the transimidization mechanism, three steps have been taken in this research: first, a copolyimide system of 50% of DAP and 50% 1, 4-bis[4-ami nophenoxy]benzene diamine (DODA) with 100% of 2, 2'-bis[4-(3, 4-dicarboxyph enoxy)phenyl] propane dianhydride (Ultem(R) DA) is prepared. Second, severa l specifically designed polyimide mixture systems were used, and they consi st of two homopolyimides: one is Ultem(R) DA-DODA, and the other is Ultem(R ) DA-DAP. The third step is to investigate two mixture systems in which Ult em(R) DA-DODA is mixed with DAP monomer solution and Ultem(R) DA-DAP is mix ed with DODA monomer solution. For all systems, with increasing degree of t ransimidization upon heat-treatment, the chain structures of the mixtures a nd their thermal and dynamic mechanical transition behaviors are investigat ed via one-dimensional and two-dimensional nuclear magnetic resonance, diff erential scanning calorimetry, and dynamic mechanical analysis experiments. Experimental results indicate that in the mixture of two homopolyimides, t ransimidization takes place much mote efficiently in solution than in the m elt. For the two mixtures with monomer solutions, the transimidization can only be found in the mixture of Ultem(R) DAP/DODA system in p-chlorophenol, and this process does not effectively occur in the Ultem(R) DODA/DAP syste m in p-chlorophenol, indicating that free DODA can efficiently attack the i mide linkage of Ultem(R) DA-DAP. The final product resulting from the trans imidization in the mixtures is a random copolyimide with a major population of DAP as end groups for the copolymer system.