Surface Hardness of Pristine and Modified Polyaniline Films

The surface microhardness of conventional thermoplastics lie between 0.1 and 0.6 GPa. The as-cast emeraldine (EM-25) base film of polyaniline (PANi) exhibits a surface microhardness of about 1 GPa. The hardness is increased to about 4 GPa in the highly cross-linked EM (EM-150) film. This hardness value is further enhanced to about 6 GPa after 1 cycle of acid-base treatment. The hardness of the EM-150 film can also be enhanced to about 8 GPa through protonation or through reduction to the leucoemeraldine state. Surface modification of the EM-150 films via graft copolymerization with acrylic acid or styrenesulfonic acid readily gives rise to hard-surfaced PANi films having microhardness values approaching 20 GPa. This surface hardness is comparable to many of those reported for the very hard-surfaced conventional polymers from high-energy ion-beam bombardments. The surface hardness of the surface-modified EM-150 film arises from the additional covalent bonding of the graft chains on the highly cross-linked EM-150 film surface and the charge-transfer interaction between the graft and the substrate chains.