Preparation of Meltable Aromatic Polyimides and Their Adhesive Properties

A series of molecular-weight-controlled aromatic polyimides based on 4,4'-oxydiphthalic anhydride (ODPA), 1,4-bis(4-amino-2-trifluoromethylphenoxy) benzene (6FAPB) and 3,3'-diaminodiphenylsulfone (3,3'-DDS) were synthesized in the presence of phthalic anhydride (PA) as an end-capping agent. The effect of molecular weight on the solubility, melt viscosity, thermal and mechanical properties of the polyimides was investigated. Experimental results demonstrated that the polyimides exhibit good solubility in most polar aprotic solvents and in some common organic solvents, such as DMSO and THF. Homogeneous and stable polyimide solutions with solid contents as high as 40-45 wt% were prepared. High-quality polyimide films were obtained by casting the polyimide solutions onto glass plates and baking them at a relatively low temperature. The polyimide films exhibited outstanding thermal and mechanical properties. The rheological behavior of the polyimides depends on their molecular weight. The adhesive properties of polyimide films bonded to stainless steel at different temperatures were evaluated by the lap shear strength (LSS) test. The polyimides with moderate molecular weight exhibited better adhesive properties. The LSS of polyimide films at ambient and elevated temperatures increased with increasing bonding temperature, which is attributed to the better flow and wetting of the polymer melts during the bonding process.