We show how atomic force microscopy techniques based on contact resonance (CR) can be used to measure the viscoelastic loss tangent tan δ of polymeric materials. Absolute values of tan δ do not involve intermediate calculation of loss modulus and storage modulus and do not require calibration measurements or assumptions about tip shape. We present the methods physical concepts and explore its range of applicability with sensitivity maps for typical experimental parameters. In addition, CR experiments were performed on four homogeneous polymer samples (polystyrene, high-density polyethyene, and two commercial photostress polymers) with storage modulus E′ from approximately 1 GPa to 4 GPa and tan δ in the range from approximately 0.02 to 0.2. Results compare favorably to those obtained by microscale dynamic nanoindentation and macroscale dynamic mechanical analysis, validating viscoelastic CR as an accurate technique with high spatial resolution. Our results advance the state of the art in nanomechanical measurements of polymers and biomaterials.
Pub Type: Journals