The interphase in polymers is the local region (1- 100 nm) of material in a heterogeneous system perturbed by a nearby interface. Recent developments allow atomic force microscopy (AFM) to directly characterize properties of the interphase at very small scales. However, it remains a challenge to generate quantitative measurements of local mechanical properties, particularly on polymers and soft materials, requiring a detailed understanding of AFM operation and the material characteristics.

In multiphase materials, local changes in the sample modulus and tip-sample interactions impact the acquired force curves. Secondary effects, such as the ‚Äòsubstrate effect‚Äô, require careful treatment to ensure that measured property gradients are due to changes in polymer structure. Simulations and experiments have demonstrated that AFM can probe the interphase through detailed understanding of indentation. We present two recent advances in AFM nanomechanics: error reduction via choice of cantilever operational settings and removing stress field artifacts near substrates via indentation at multiple depths. 

The first area recognizes that cantilever calibration is a major source of error in common cases of AFM modulus measurement. We analyzed the error sensitivity of indentation models and found operating conditions that cause the systematic error due to spring constant calibration error to vanish when the ratio of cantilever deflection to indentation depth is maintained at a specific (‚Äúmagic‚Äù) value.

The second area addresses model inhomogenous samples via experiment and simulation and addresses stress field artifacts when indenting near substrates or embedded particles. Simulations of indentations into a rubber interphase demonstrated that structural effects of the particle in an idealized system can be estimated and removed. We developed a ‚Äòmaster curve‚Äô that approximates the substrate effect and applied it to AFM indentations of rubber ‚Äì carbon nanocomposites, demonstrating that substrate effects are well predicted by the ‚Äòmaster curve‚Äô and can be differentiated from bound rubber.

Progress in high resolution nanomechanical AFM for exploration of nanoscale structure and properties in soft materials.

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