Adhesive strength of nano-size particles filled thermoplastic polyimides. Part-I: Multi-walled carbon nano-tubes (MWNT)–polyimide composite films

In this study, adhesive properties of thermoplastic polyimides (PI) filled with multi wall carbon nano-tubes (MWNT) are investigated. The PIs studied are ODPA/3, 4′-ODA/PA (amorphous) and BPDA/3, 4′-ODA/PA (semi-crystalline). The PIs are prepared via a two-step method involving poly(amide acid) (PAA) stage followed by thermal or chemical imidization to obtain polyimide in film or powder form. The MWNT are mixed in low wt% (0.1–1.5) during synthesis at PAA stage through mechanical agitation and sonication. PI-MWNT film composites were characterized using DMTA and tensile testing to determine viscoelastic behavior and mechanical properties. Adhesive strength and adhesive energy of bonded samples, prepared with PI-MWNT films and powder composites, were determined using single lap joints test and wedge test methods, respectively. The fractured bonded surfaces were examined by scanning electron microscope to determine failure patterns. The results showed that viscoelastic behavior of PI–MWNT composite films changes from liquid-like to solid-like by increasing MWNT content. Elastic modulus and strength at break of PI-MWNT composite films were found to increase with increase in MWNT wt%. However, elongation at break and breaking energy of PI–MWNT composite films and lap shear strength and adhesive energy of bonded samples were found to initially increase with increase in MWNT wt%, but then after a critical value decreases. The SEM analysis showed that bonded joints prepared with PI–MWNT composite films containing high wt% of MWNT failed in adhesive mode showing poor wetting of adherend surfaces.