Local Piezoelectric Response of Polymer/Ceramic Nanocomposite Fibers

Abstract

Effective converse piezoelectric coefficient (d_33,eff) mapping of poly(vinylidene fluoride) (PVDF) nanofibers with ceramic BaTiO₃ nanoparticle inclusions obtained by electrospinning was carried out by piezoresponse force microscopy (PFM) in a peculiar dynamic mode, namely constant-excitation frequency-modulation (CE-FM), particularly suitable for the analysis of compliant materials. Mapping of single nanocomposite fibers was carried out to demonstrate the ability of CE-FM-PFM to investigate the nanostructure of semicrystalline polymers well above their glass transition temperature, such as PVDF, by revealing the distribution of piezoelectric activity of the nanofiber, as well as of the embedded nanoparticles employed. A decreased piezoelectric activity at the nanoparticle site compared to the polymeric fiber was found. This evidence can be rationalized in terms of a tradeoff between the dielectric constants and piezoelectric coefficients of the component materials, as well as on the mutual orientation of polar axes.

Keywords: electrospun composite nanofiber; piezoelectric coefficient; piezoresponse force microscopy.

Figure 1

FE-SEM micrographs of the electrospun PVDF nanofiber mesh with dispersed BaTiO₃ nanoparticles. Images were acquired at 15 kV and different magnifications: (A) 2000× (scale bar 50 µm), and (B) 10,000× (scale bar 10 µm).

Figure 2

(A) Topography and CE-FM-PFM amplitude (B) and phase (C) scan of a portion of electrospun PVDF/BaTiO₃ composite nanofiber, transferred on a doped silicon substrate. Bands on the flanks of the nanofiber in (B,C) should be disregarded, since they are caused by a spurious tip shape effect on the sloped region. Nanodomains show up on the fiber upper surface, where PFM phase is inverted. (D) Line profile corresponding to the horizontal stroke in (A–C), whose length is 500 nm. Data corresponding to the sloped region of the nanofiber have been excluded from the plot for clarity. Image size is 785 nm × 785 nm. The same color bar used in image (A) also defines the values of image (B) (0/100 pm/V) and (C) (−180°/+180°).

Figure 3

Topography (A), CE-FM-PFM amplitude (B) and phase (C) scan of an electrospun PVDF/BaTiO₃ composite nanofiber deposited on doped silicon, where an embedded BaTiO₃ nanoparticle is present (NP), indicated by the arrow. Nanodomains appear at the center of the fiber upper surface, where the PFM phase is inverted. The nanoparticle shows lower piezoresponse, while its border shows a higher signal. Topography (D), CE-FM-PFM amplitude (E) and phase (F) of the same nanofiber portion after poling by application of a DC potential to a region indicated by the rectangle in (B,E). Phase inversion is obtained after poling on the PVDF nanofiber, as well as on the nanoparticle and part of the nanodomains. The scan size is 1.28 μm × 0.5 μm. The same color bar of the image of Figure 2A defines the scale for the values of topography images (A,D) (0/250 nm), PFM amplitude (B,E) (0/50 pm/V) and phase (C,F) (−180°/+180°).

Figure 4

(A) Topography and (B) PFM amplitude scan of an electrospun PVDF/BaTiO₃ composite nanofiber as deposited on the rotating electrospinning aluminum substrate, on which a BaTiO₃ nanoparticle is also visible, indicated by the arrow. (C) Line profiles corresponding to the green line in (A,B), crossing the nanoparticle. (D) Line profiles corresponding to the purple line in (A,B), not crossing the nanoparticle. The applied potential V_ac was 1 V_RMS here; therefore, the ΔA value corresponds to the piezoelectric coefficient.

Figure 5

Dependence of the overall displacement for 1 V electric potential (d_33,eff) from the relative thickness h_c/h of the two layers (crystal and polymer) considered in the simple picture leading to Equation (2). Black curves (case 1): ε_c = 1000, ε_p = 10, d_c = 100 pm/V, d_p = 20 pm/V; red curves (case 2): ε_c = 50 [40], ε_p = 10, d_c = 27 pm/V [41], d_p = 20 pm/V. Solid curves: case of same orientation of the polar axes of crystal and polymer; dashed curves: case of opposite orientations.