Effect of Interface and Depth on Hardness and Viscoelastic Modulus in Polycarbonate and Polyester Films

Hardness and modulus of a polymer is known to depend on its structure, molecular weight and number of segments between entanglements. Nano-indentation is used increasingly as a powerful tool to determine hardness and visco-elastic modulus of polymer surfaces linear, cross-linked or composites. Hysitron Nanoindenter was used in our investigation of contact deformation of surfaces of polyester and polycarbonate supported on an aluminum substrate. Bar coatings of polymer films were made from solutions and dried all at 110C for half an hour. The coatings were subjected to indentation including sinusoidal deformation at various contact depths and hardness as well as modulus is computed. The results show that hardness of surface of polycarbonate on polyester is higher than hardness of surface of polycarbonate, which in turn is higher than polyester surface. It appears that diffusion of polyester chains into compatible polycarbonate chains results in higher modulus of the surface than the modulus of either of the two polymers. Hardness and modulus of polycarbonate is found to decrease with contact depth and reaches a plateau around 600nm while that of polyester keeps on decreasing. Differences in contact deformation including the one arising from dynamic deformation between polycarbonate from that of polyester is analyzed in terms of morphology, molecular weight and entanglements.