TRANSIMIDIZATION IN THERMOPLASTIC POLYIMIDES*

In order to improve the processibility in thermoplastic polyimides, a new method, termed the"reactive plasticizer" approach, has been proposed. This method uses a small amount (5～15 mol%) of a lessactivated, weak nucleophilic diamine co-monomer as a "reactive plasticizer" to obtain copolyimide resinswhich possess relatively low viscosity at low temperatures and can be readily processed through the autoclavecycle at low pressures. During a high temperature treatment, the reactive plasticizers join the reaction to form high molecular weight copolyimides,and the preferred material properties are thus achieved.The most effective reactive plasticizer is aromatic heterocyclic diamines,such as 2,6-diaminopyridine diamine (DAP),and the transimidization involved with a reactive plasticizer has been proposed to play a major role for the success of this approach.In order to understand 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-aminophenoxy]benzene diamine (DODA) with 100% of 2,2'-bis[4-(3,4-dicarboxyphenoxy)phenyl] propane dianhydride (Ultem DA) is prepared.Second,several specifically designed polyimide mixture systems were used,and they consist of two homopolyimides:one is Ultem DA-DODA,and the other is Ultem DA-DAP.The third step is to investigate two mixture systems in which Ultem DA-DODA is mixed with DAP monomer solution and Ultem DA-DAP is mixed with DODA monomer solution.For all systems,with increasing degree of transimidization upon heat-treatment,the chain structures and their thermal and dynamic mechanical transition behaviors are investigated via one-dimensional and two-dimensional nuclear magnetic resonance,differential scanning calorimetry,and dynamic mechanical analysis experiments.Experimental results indicate that in the mixture of two homopolyimides,transimidization takes place much more effciently in solution than in the melt.For the two mixtures with monomer solutions,the transimidization can only be found in the mixture of Ultem DAP/DODA system in p-chlorophenol,and this process does not effectively occur in the Ultem DODA/DAP system in p-chlorophenol,indicating that free DODA can efficiently attack the imide linkage of Ultem DA-DAP.The final product resulting form the transimidization in the mixtures is a random copolyimide with a major population of DAP as end groups for the copolymer system.