Adhesive strength of nano-size particles filled thermoplastic polyimides. Part-II: Aluminum nitride (AlN) nano-powder–polyimide composite films

Adhesive properties of thermoplastic polyimides (PI) filled with aluminum nitride (AlN) nano-powder are investigated. Two PIs, one amorphous (oxydiphthalic anhydride (ODPA)/3,4′-oxydianiline (3,4′-ODA)/phthalic anhydride (PA)) and the other a semi-crystalline (3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA)/3,4′-ODA/PA) are synthesized via two-step method involving formation of poly(amide acid) (PAA) followed by thermal or chemical imidization to obtain PI in film or powder forms, respectively. AlN in low wt% (up to 1.5%) is added at PAA stage and mixed by mechanical agitation and sonication. Viscoelastic behavior and mechanical properties of PI films are determined using DMTA and tensile testing. Adhesive properties are determined by measuring lap shear strength (LSS) of single lap joints prepared with films and powder oligomides. Scanning electron microscopy (SEM) of fractured surfaces is carried out to determine failure modes. The result shows that with increasing AlN wt%, increase in elastic modulus and tensile strength of AlN–PI composite films is observed, whereas strain at break decreases. The viscoelastic behavior of the AlN–PI composite films also changes from liquid-like to solid-like with increasing AlN wt%. LSS measurements show that with increase in AlN wt% the LSS increased initially but after a critical value was observed to decrease. SEM analysis concluded that this decrease in LSS was attributed to poor melt behavior of PIs containing higher wt% of AlN. Measurement of LSS in elevated temperature environment showed that retention of LSS in joints prepared with AlN–PI composite films was better as compared to joints prepared with neat PIs.