Insights into Nano-Scale Physical and Mechanical Properties of Epoxy/Boehmite Nanocomposite Using Different AFM Modes

Abstract

Understanding the interaction between nanoparticles and the matrix and the properties of interphase is crucial to predict the macroscopic properties of a nanocomposite system. Here, we investigate the interaction between boehmite nanoparticles (BNPs) and epoxy using different atomic force microscopy (AFM) approaches. We demonstrate benefits of using multifrequency intermodulation AFM (ImAFM) to obtain information about conservative, dissipative and van der Waals tip-surface forces and probing local properties of nanoparticles, matrix and the interphase. We utilize scanning kelvin probe microscopy (SKPM) to probe surface potential as a tool to visualize material contrast with a physical parameter, which is independent from the mechanics of the surface. Combining the information from ImAFM stiffness and SKPM surface potential results in a precise characterization of interfacial region, demonstrating that the interphase is softer than epoxy and boehmite nanoparticles. Further, we investigated the effect of boehmite nanoparticles on the bulk properties of epoxy matrix. ImAFM stiffness maps revealed the significant stiffening effect of boehmite nanoparticles on anhydride-cured epoxy matrix. The energy dissipation of epoxy matrix locally measured by ImAFM shows a considerable increase compared to that of neat epoxy. These measurements suggest a substantial alteration of epoxy structure induced by the presence of boehmite.

Keywords: atomic force microscopy; boehmite; epoxy nanocomposites; intermodulation; interphase; nanomechanical properties.

Figure A1

Free oscillation (a) and oscillation in intermittent contact (b) of a cantilever with ω₀ = 299.6 kHz on a polymer surface.

Figure A2

Tapping mode topography (top) and SKPM surface potential map (bottom) of a 10 µm × 6 µm scan area of EP/BNP5.

Figure A3

The effect of topography changes on ImAFM force measurement. (a) histogram of ImAFM stiffness and (b) work of attractive force W_attr versus the histogram first derivative of height obtained from tapping mode.

Figure A4

(a) Surface potential; (b) work of attractive forces W_attr and (c) the stiffness map of a selected area from Figure 2 scans. (d) The corresponding topography of the scanned area where the domains of particle, interphase and pure matrix are distinguished by traces with dark blue, light blue and green color, respectively. (e) 2D histogram of stiffness vs. potential. (f) typical ADFS curves related to particles (dark blue), interphase (light blue) and pure matrix(green). White pixels in (b,c) indicate error pixels.

Figure A5

T-SEM micrograph of a 100 nm thick microtome cut of the EP/BNPT5.

Figure 1

Reconstructed conservative F_I (a) and dissipative F_Q (b) forces on a polymer substrate. the red and green lines present the approach and retract curves, respectively.

Figure 2

(a) The 3-dimensional tapping mode topography; (b) Surface potential; (c) Work of attractive forces W_attr and (d) stiffness maps of epoxy/boehmite nanocomposite with 5 wt % nanoparticles. The scan sizes in all images are 860 nm × 860 nm. White pixels in W_attr show error.

Figure 3

(a) 9AFM tapping mode topography with the selected region of analysis marked with a square box; (b) surface potential map and histogram and (c) amplitude-dependence force spectroscopy (ADFS) stiffness map and histogram of the selected area; (d) ADFS curves related to three points shown with circle markers on the maps with the approximation of the location of boehmite nanoparticles (BNPs) (dark blue), epoxy matrix (green) and interphase (light blue).

Figure 4

Two-dimensional histogram of stiffness vs. surface potential of the selected area (shown in Figure 3) of the scanned surface of EP/BNP 5. The dashed lines are used to help the eyes to distinguish between three different regions of the histogram.

Figure 5

Tapping mode topography of 350 nm × 350 nm scan area of neat epoxy (a) and epoxy with 15 wt % BNPs (EP/BNP15) (b).

Figure 6

The comparison of the histograms of work of attractive forces W_attr in neat epoxy and EP/BNP15. The left-side inset image is W_attr map of neat epoxy and the right-side is W_attr map of EP/BNP15.

Figure 7

Comparison of stiffness histograms of neat epoxy and EP/BNP15. The left-side inset image related to stiffness map of neat epoxy and the right-side to EP/BNP15.

Figure 8

Comparison of energy dissipation histograms in neat epoxy and EP/BNP15. The top inset image related to energy dissipation map of neat epoxy and the bottom image to EP/BNP15.