TRANSIMIDIZATION IN THERMOPLASTIC POLYIMIDES

摘要

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 formhigh molecular weight copolyimides, and the preferred material properties are thus achieved. The mosteffective 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 thesuccess of this approach. In order to understand the transimidization mechanism, three steps have been takenin this research: first, a copolyimide system of 50% of DAP and 50% 1, 4 -bis [4-aminophenoxy] benzenediamine （DODA） with 100% of 2, 2’-bis[4-（3, 4-dicarboxyphenoxy）phenyl] propane dianhydride (UltemRDA) is prepared. Second, several specifically designed polyimide mixture systems were used, and they consistof two homopolyimides: one is UltemR DA-DODA, and the other is UltemR DA-DAP. The third step is toinvestigate two mixture systems in which UltemR DA-DODA is mixed with DAP monomer solution andUltemR DA-DAP is mixed with DODA monomer solution. For all systems, with increasing degree oftransimidization upon heat-treatment, the chain structures of the mixtures and their thermal and dynamicmechanical transition behaviors are investigated via one-dimensional and two-dimensional nuclear magneticresonance, differentialscanning calorimetry, and dynamic mechanical analysis experiments. Experimentalresults indicate that in the mixture of two homopolyimides, transimidization takes place much moreefficiently in solution than in the melt. For the two mixtures with monomer solutions, the transimidizationcan only be found in the mixture of UltemR DAP/DODA system in p-chlorophenol, and this process does noteffectively occur in the UltemR DODA/DAP system in p-chlorophenol, indicating that free DODA canefficiently attack the imide linkage of UltemR DA-DAP. The final product resulting from the transimidizationin the mixtures is a random copolyimide with a major population of DAP as end groups for the copolymersystem.