Piezoelectric Yield of Single Electrospun Poly(acrylonitrile) Ultrafine Fibers Studied by Piezoresponse Force Microscopy and Numerical Simulations

Abstract

Quantitative converse piezoelectric coefficient (d₃₃) mapping of polymer ultrafine fibers of poly(acrylonitrile) (PAN), as well as of poly(vinylidene fluoride) (PVDF) as a reference material, obtained by rotating electrospinning, was carried out by piezoresponse force microscopy in the constant-excitation frequency-modulation mode (CE-FM-PFM). PFM mapping of single fibers reveals their piezoelectric activity and provides information on its distribution along the fiber length. Uniform behavior is typically observed on a length scale of a few micrometers. In some cases, variations with sinusoidal dependence along the fiber are reported, compatibly with a possible twisting around the fiber axis. The observed features of the piezoelectric yield have motivated numerical simulations of the surface displacement in a piezoelectric ultrafine fiber concerned by the electric field generated by biasing of the PFM probe. Uniform alignment of the piezoelectric axis along the fiber would comply with the uniform but strongly variable values observed, and sinusoidal variations were occasionally found on the fibers laying on the conductive substrate. Furthermore, in the latter case, numerical simulations show that the piezoelectric tensor's shear terms should be carefully considered in estimations since they may provide a remarkably different contribution to the overall deformation profile.

Keywords: electrospinning; piezoelectricity; piezoresponse force microscopy; poly(acrylonitrile); poly(vinylidene fluoride).

Figure 1

(A) Molecular structure of PAN, (B) examples of illustrations of its possible syndiotactic zigzag structure and (C) of its possible isotactic 3¹-helical structure [6]. (B,C) were produced by ChemSketch software (freeware version 2023 1.2).

Figure 2

Geometry adopted for the performed numerical simulations, as described in the text.

Figure 3

(A) SEM micrographs of electrospun PAN meshes collected at different rotation speeds. (B) Related diameter distributions. (C) Box plots of the fiber diameter distributions obtained at different rotation speeds and associated mean and standard deviations. (D) Directionality histogram and mesh parameters collected at 3000 rpm, obtained with the FFT directionality analysis function of ImageJ software.

Figure 4

ATR-FTIR spectra of (A) pristine PAN powder; (B–D) PAN ultrafine fiber samples electrospun at different collector velocities: (B) 5 rpm, (C) 1500 rpm, (D) 3000 rpm; and (E) wide-range IR spectra for all samples (vertical axes were shifted for better graphical representation).

Figure 5

(A) Topography, (B) corresponding effective d₃₃ map of two PAN fibers from the same mesh (obtained at 3000 rpm collector speed).

Figure 6

(A) Topography and (B) corresponding effective d₃₃ map of three overlapped fibers of different kinds: (1) PAN @ 3000 rpm, (2) PAN @ 5 rpm, and (3) PVDF fiber used as a reference. PFM signals are lower in the overlapping region, where fibers are farther away from the conductive substrate.

Figure 7

Effective piezoelectric coefficient d_33,eff of several fibers of different types: PAN @ 5 rpm (PAN1), PAN @ 1500 rpm (PAN2), PAN @ 3000 rpm (PAN3), PVDF fibers used as a reference (PVDF), as compared to background signal (Background), indicative of typical measurement noise. Data for session #4 correspond to the image of Figure 6, while those for session #8 correspond to the image of Figure 5.

Figure 8

(A) Topography (left) and corresponding effective d₃₃ map (right) of an electrospun PVDF fiber deposited on gold. (B) Profile of d₃₃ along the fiber length (symbols) and sinusoidal fit (solid line).

Figure 9

Simulated piezoresponse as a function of the polar axis direction, β, for two different fiber diameters and two different forms of the piezoelectric coupling matrix (see text).

Figure 10

Simulated Z-displacement field due to piezoelectric effect. In the zoomed inset at the bottom, displacement has been amplified by a factor of 200 in the Z direction for illustration purposes.