2.4.1. Surface Topography

The structure and morphology of the P(VDF-HFP) film and fibers were determined by scanning electron microscopy (SEM, FEI Quanta 400, Netherlands). All samples were sputter-coated with gold prior to the SEM imaging. The average fiber diameter and porosity of each sample was analyzed by ImageJ software (National Institutes of Health, 1.46, Madison, WI, USA).

#### 2.4.2. Crystalline Structure and Phase Investigation

The crystalline structure in all samples were examined using an X-ray diffractometer (XRD; XPert MPD, Philips, Netherlands) in the 2θ range from 5◦ to 90◦ at the scan rate 0.05◦ s<sup>−</sup><sup>1</sup> using Cu-Kα radiation (wavelength 0.154 nm) under a voltage of 40 kV. The crystallinity Xc can be estimated as follows [27]:

$$\mathcal{X}\_{\rm c} = \frac{\Sigma \mathcal{A}\_{\rm cr}}{\Sigma \mathcal{A}\_{\rm cr} + \Sigma \mathcal{A}\_{\rm amr}} \times 100\% \tag{1}$$

where ΣAamr and ΣAcr are the total integral areas of amorphous halo and crystalline diffraction peaks, respectively. The α and β-phase contents were elucidated from IR spectra obtained with a Fourier transform infrared spectrometer (FTIR-8400S, Shimadzu, Tokyo, Japan). The absorbance data for all

samples covered the wavenumber range 400–1000 cm<sup>−</sup><sup>1</sup> with a resolution of 4 cm<sup>−</sup>1. The fraction of β-phase, F(β) in films or fibers, was calculated using the Lambert–Beer law [10]:

$$\mathbf{F}(\boldsymbol{\beta}) = \frac{\mathbf{A}\_{\beta}}{\left(\frac{\mathbf{K}\_{\beta}}{\mathbf{K}\_{\alpha}}\right)\mathbf{A}\_{\alpha} + \mathbf{A}\_{\beta}} = \frac{\mathbf{A}\_{\beta}}{1.26\mathbf{A}\_{\alpha} + \mathbf{A}\_{\beta}}\tag{2}$$

where Aα and <sup>A</sup>β are the absorbance at 764 and 840 cm<sup>−</sup>1, respectively. Kα = 6.1 × 10<sup>4</sup> cm<sup>2</sup> mol−<sup>1</sup> and <sup>K</sup>β = 7.7 × 10<sup>4</sup> cm<sup>2</sup> mol−<sup>1</sup> are the absorption coefficients at 764 and 840 cm<sup>−</sup>1, respectively.

The absolute β fraction (%β)is obtained from F(β) and Xc, as in [28]:

$$\mathbb{V}\mathbb{V}\mathbb{A} = \mathrm{F}(\mathbb{A}) \times \mathbb{X}\_{\mathbb{C}} \tag{3}$$
