Micro and nanofilms of poly(vinylidene fluoride) with controlled thickness, morphology and electroactive crystalline phase for sensor and actuator applications

Abstract

Poly(vinylidene fluoride), PVDF, thin films have been processed by spin-coating with controlled thickness, morphology and crystalline phases. The influence of the polymer/solvent mass ratio of the solution, the rotational speed of the spin-coater and the temperature of crystallization of the films on the properties of the material has been investigated. It is shown that high-quality films with controlled thicknesses from 300 nm to 4.5 μm and with a controlled amount of electroactive crystalline phases can be obtained in a single deposition step, which allows tailoring the material characteristics for specific applications.

Exclusive self-aligned β-phase PVDF films with abnormal piezoelectric coefficient prepared via phase inversion

“…First principle calculations by Bystrov et al have shown that interaction between molecular chains of PVDF leads to the orientation of each molecular dipole moment along one direction parallel to the chain plane. [35] It is noteworthy that although β-phase crystals do not have preferential orientation in PVDF films formed by phase inversion 15 or by drying in air; 12 the PVDF membranes obtained may still exhibit a certain degree of piezoelectric effect as shown by our results in Figure 3h. However, this aspect has been largely ignore in the literature with the porous PVDF films formed by phase inversion usually considered unsuitable for poling and electroactive applications.…”

“…[5] PVDF shows four significant crystalline phases α, β, δ and γ; [2,6] with the electroactive β-phase being utilized most frequently for the development of sensors, actuators [7,8] and microgenerators. [4,9,10] Owing to its importance, the formation of electroactive β crystalline phase has been intensively investigated through various routes, including melt casting, [15] solution deposition, [11] spin coating [12] and phase inversion. [13,14] While, the films or membranes formed by 2 melting/crystallisation are dominated by α-phase, [15] those obtained by spin-coating and further dried at temperature between 30-60 ºC are largely dominated by β-phase crystals.…”

“…[13,14] While, the films or membranes formed by 2 melting/crystallisation are dominated by α-phase, [15] those obtained by spin-coating and further dried at temperature between 30-60 ºC are largely dominated by β-phase crystals. [12] For the phase inversion technique, PVDF films are formed by quenching the casting films into a nonsolvent bath to induce a series of liquid-solid and liquid-liquid phase separation events. [5,13,14] The microstructure and crystalline phase of the polymer films in phase-inversion can be controlled by adjusting paramteres like composition, [5,12] type of solvent, [16,17] quenching temperature etc.…”

“…[12] For the phase inversion technique, PVDF films are formed by quenching the casting films into a nonsolvent bath to induce a series of liquid-solid and liquid-liquid phase separation events. [5,13,14] The microstructure and crystalline phase of the polymer films in phase-inversion can be controlled by adjusting paramteres like composition, [5,12] type of solvent, [16,17] quenching temperature etc. [18,19] Although PVDF films formed by this technique can have high β-phase contents, they are mostly porous and not suitable for electroactive applications.…”

“…Moreover, the β-phase crystals are randomly oriented in these PVDF films and show no specific directional preference. Polarization under high electric field, [6,20] by Corona poling, [12] or by mechanical stretching [3,6,15] is subsequently carried out to obtain electroactive PVDF films. These operations need to be done at elevated temperatures (>80 o C), requiring high voltage equipment and films with dense structure and high β-phase content.…”