An
atomic force microscope (AFM) based fast dynamic scanning indentation
(DSI) nano-DMA method, which relies only on the commonly available
capabilities of commercial AFMs to provide quantitatively accurate
high-resolution (∼15 nm) spatial maps of local viscoelastic
mechanical properties (E′, E″, and tan ϕ) in heterogeneous soft adhesive material
systems, is described. The versatility of the DSI approach is demonstrated
by successfully employing it on three industry-leading commercial
AFMs/modules (Asylum’s Cypher ES and MFP-3D Infinity AFMs with
the FastForceMapping module, and Bruker’s Dimension Icon AFM
with the PeakForce QNM module). Frequency sweep thermorheological
DSI experiments were performed to generate quantitatively accurate
nano-DMA master curves spanning an unprecedented frequency range of
5 decades. Quantitative agreement between DSI nano-DMA and bulk DMA
measurements is demonstrated for two different homogeneous elastomers
(styrene butadiene rubber, SBR, and synthetic natural rubber, SNR).
The capability of the DSI methodology in acquiring quantitatively
accurate viscoelastic property maps of heterogeneous soft solids was
validated through experiments on an SBR-SNR blend sample. Experimental
factors affecting DSI data quality (e.g., shift factor and AFM tip
size) are also discussed.
Stiffness gradients in geometrically confined polymers as measured by nanoindentation are influenced by opposing roles of the polymers viscoelastic state and the degree of confinement.
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