We present an atomic force microscopy (AFM) method termed ‘Dynamic Scanning Indentation (DSI)’ which uses cyclic indentations for rapid, direct measurement of the viscoelastic mechanical properties of soft rubbers and rubber composites at the nanoscale. DSI is a nano-DMA technique which produces master curves across 5 decades of frequency and between to -35 to 35°C in temperature range depending on the AFM used. Quantitative agreement between the viscoelastic measurements (E’, E’’ and tan δ) made with DSI and bulk DMA data is presented. In addition, DSI is shown to produce quantitatively meaningful spatial maps of viscoelastic properties in rubber composites and blends in a matter of minutes using commercial AFMs.<br/>In heterogenous materials, the impact of structural effects on an AFM indentation is of particular concern when indenting near a filler-polymer interface. The chemical and molecular interactions between nanoscale fillers and a polymer can cause local stiffening in the polymer (termed the ‘interphase’) which plays an important role in the strength of polymer nanocomposites. To understand the impact of a material’s viscoelastic state on the mechanical property measurements made by DSI as well as other AFM indentation techniques, we conduct a set of complementary simulations of AFM indentations at increasing temperature in highly confined and unconfined films via finite element analysis (FEA) and coarse-grained molecular dynamics (CG-MD). We find that that the measured interphase length is dependent on the incompressibility and viscous dissipation of the polymer as a result of continuum level behaviour and convolutes the measurement of the interphase. Additionally, we demonstrate that the interphase length scale increases with temperature in highly confined systems but decreases if the polymer is unconfined.

Dynamic scanning indentation for polymer interphases: Unlocking nanoscale viscoelastic measurements across time and space

2019 Spring National Meeting

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