Phase Inversion in PVDF Films with Enhanced Piezoresponse Through Spin-Coating and Quenching

Abstract

In the present work, poly(vinylidene fluoride) (PVDF) films were produced by spin-coating, and applying different conditions of quenching, in order to investigate the dominant mechanism of the β-phase formation. The influence of the polymer/solvent mass ratio of the solution, the rotational speed of the spin-coater and the crystallization temperature of the film on both the β-phase content and the piezoelectric coefficient (d₃₃) were investigated. This study demonstrates that the highest values of d₃₃ are obtained when thinner films, produced with a lower concentration of polymer in the solvent (i.e., 20 wt.%), go through quenching in water, at room temperature. Whereas, in the case of higher polymer concentration (i.e., 30 wt.%), the best value of d₃₃ (~30 pm/V) was obtained through quenching in liquid nitrogen, at the temperature of 77 K. We believe that in the former case, phase inversion is mainly originated by electrostatic interaction of PVDF with the polar molecules of water, due to the low viscosity of the polymer solution. On the contrary, in the latter case, due to higher viscosity of the solution, mechanical stretching induced on the polymer during spin-coating deposition is the main factor inducing self-alignment of the β-phase. These findings open up a new way to realize highly efficient devices for energy harvesting and wearable sensors.

Keywords: piezoelectric effect; piezoresponse force microscopy (PFM); polyvinylidene fluoride (PVDF); quenching.

Figure 1

Viscosity curves of PVDF/DMF (N,N dimethylformamide) solutions with different polymer concentrations.

Figure 2

FE-SEM low-magnification (a,c,e,g,i,m) and high-magnification (b,d,f,h,l,n) micrographs of PVDF films with polymer content of 20 wt.%., produced at spin-coater rotation speed of 7500 rpm (samples A, B, C) or 2500 rpm (samples D, E, F), and quenched at 303 K in water (samples A, D) or at 253 K in water and glycerol (samples B, E) or at 77 K in liquid nitrogen (samples C, F).

Figure 3

FE-SEM low-magnification (a,c,e,g,i,m) and high-magnification (b,d,f,h,l,n) micrographs of PVDF films with polymer content of 30 wt.%., produced at spin-coater rotation speed of 7500 rpm (samples G, H, I) or 2500 rpm (samples L, M, N), and quenched at 303 K in water (samples G, L) or at 253 K in water and glycerol (samples H, M) or at 77 K in liquid nitrogen (samples I, N).

Figure 4

FT-IR spectra of 12 PVDF solutions.

Figure 5

Morphological maps and Piezoresponse Force Microscopy (PFM) signals at V_ac = 10 V and at 15 kHz for PVDF at 20 wt.% produced using different temperatures and spinning speeds: (a,b) Sample A, (c,d) sample B, (e,f) sample C, (g,h) sample D, (i,l) sample E, (m,n) sample F.

Figure 6

Morphological maps and PFM signals at V_ac = 10 V and at 15 kHz for PVDF at 30 wt.% produced at different temperatures and spinning speeds: (a,b) Sample G, (c,d) sample H, (e,f) sample I, (g,h) sample L, (i,l) sample M, (m,n) sample N.

Figure 7

The average amplitude of the vertical displacement, measured through PFM as a function of the applied voltage $V_{ac}$: For the PVDF samples at 20 wt.% at different temperatures and spinning speeds (a) and for the PVDF samples at 30 wt.% at different temperature and spinning speed (b).

Figure 8

Results of the averaged d₃₃ as a function of $F\left(\mathsf{\beta }\right)$ of all produced samples, with standard deviation.

Figure 9

β-phase relative volume fraction and maximum measured piezoresponse coefficient of all produced samples.