**1. Introduction**

Polyvinylidene difluoride (PVDF), consisting of (–CF2–CH2–)n, with a carbon chain and hydrogen and fluorine atom on the two sides of carbon, respectively, is not a new synthe‐

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sized material, which has been found in 50 years ago. In the beginning, PVDF was studied for its high dielectric permittivity and for the diversity of crystalline-phase types. About 10 or 15 years later, piezoelectricity and ferroelectricity properties were found in PVDF materials [1–6].

As it is known, PVDF was found in possession of four or more crystalline-phase types. It consists of the α-phase, β-phase, γ-phase, and σ-phase. Phase types are related to various molecular configurations. In these different phase types, the β-phase is of polar form and the α-phase is of antipolar form; the molecular configuration is shown in Fig. 1. The antipolar form α-phase PVDF can be transformed to the β-phase polar form by rapid cooling from the melting or stretching along the carbon chain [1,2,5].

**Figure 1.** The typical molecular configuration of PVDF (a) α-phase, (b) β-phase

The other two types of PVDF phases derive from the β-phase and α-phase. Recently, it has been proved that the σ-phase PVDF is also a polar form. When the poly(trifluoroethylene) is added into the PVDF, a new type of copolymer P(VDF-TrFE), consisting of –((–CF2–CH2)*x*–(– CF2–CHF–)1-*x*)*n*– chains, is achieved. The copolymers with 50 % or less trifluoroethylene (TrFE) are ferroelectric, though with reduced polarization and transition temperatures, because some of the hydrogen atoms are replaced with fluorine, reducing the net dipole moment polymer chains [5].The β-phase P(VDF-TrFE) with ferroelectricity can be obtained easily, so the copolymer has been studied widely and popularly.

The terpolymer derived from the P(VDF-TrFE) is the ferroelectric relaxor polymer P(VDF-TrFE-CFE), which adds chlorofluoroethylene into the P(VDF-TrFE) copolymer. PVDF-based polymer films possess many special properties: dielectric, ferroelectric, piezoelectric, pyro‐ electric properties and so on. Based on these properties, these films can be used as transducers, ferroelectric memory, gate of transistor, and uncooled infrared sensor. In the following paragraph, some special properties and potential applications of PVDF-based polymer films will be introduced.

### **2. Method** preparing method is spin coating; this method is widely used for preparing the films with thickness larger than 30 nm. It is very difficult to get the ultrathin PVDF-based films based on

films;

sized material, which has been found in 50 years ago. In the beginning, PVDF was studied for its high dielectric permittivity and for the diversity of crystalline-phase types. About 10 or 15 years later, piezoelectricity and ferroelectricity properties were found in PVDF materials [1–6].

As it is known, PVDF was found in possession of four or more crystalline-phase types. It consists of the α-phase, β-phase, γ-phase, and σ-phase. Phase types are related to various molecular configurations. In these different phase types, the β-phase is of polar form and the α-phase is of antipolar form; the molecular configuration is shown in Fig. 1. The antipolar form α-phase PVDF can be transformed to the β-phase polar form by rapid cooling from the melting

The other two types of PVDF phases derive from the β-phase and α-phase. Recently, it has been proved that the σ-phase PVDF is also a polar form. When the poly(trifluoroethylene) is added into the PVDF, a new type of copolymer P(VDF-TrFE), consisting of –((–CF2–CH2)*x*–(– CF2–CHF–)1-*x*)*n*– chains, is achieved. The copolymers with 50 % or less trifluoroethylene (TrFE) are ferroelectric, though with reduced polarization and transition temperatures, because some of the hydrogen atoms are replaced with fluorine, reducing the net dipole moment polymer chains [5].The β-phase P(VDF-TrFE) with ferroelectricity can be obtained easily, so the

The terpolymer derived from the P(VDF-TrFE) is the ferroelectric relaxor polymer P(VDF-TrFE-CFE), which adds chlorofluoroethylene into the P(VDF-TrFE) copolymer. PVDF-based polymer films possess many special properties: dielectric, ferroelectric, piezoelectric, pyro‐ electric properties and so on. Based on these properties, these films can be used as transducers, ferroelectric memory, gate of transistor, and uncooled infrared sensor. In the following paragraph, some special properties and potential applications of PVDF-based polymer films

or stretching along the carbon chain [1,2,5].

152 Ferroelectric Materials – Synthesis and Characterization

**Figure 1.** The typical molecular configuration of PVDF (a) α-phase, (b) β-phase

copolymer has been studied widely and popularly.

will be introduced.

There are several methods for preparing the PVDF-based films, for instance, spin coating, wire bar method, and Langmuir–Blodgett (LB) technique [7–9]. The mostly used film preparing method is spin coating, which is widely used for preparing the films with thickness larger than 30 nm. It is very difficult to get the ultrathin PVDF-based films based on spin coating and other normal methods. The LB method can be used to prepare the ultrathin two-dimensional ferroelectrics. In this chapter, the films of PVDF-based films mostly fabricated using LB technology. In 1995, S. P. Palto, L. M. Blinov, and V. M. Fridkin et al. began studies of ultrathin ferroelectric LB films of P(VDF-TrFE) copolymers with trifluoroethylene, P(VDF-TrFE) [9]. In 2007, J.L. Wang et al. prepared the ultrathin P(VDF-TrFE-CFE) terpolymer LB films [8]. Recently, we achieved the ultrathin PVDF homopolymer films with good ferroelectric performance. The detail of preparing the PVDF-based polymer films by LB technique is described as follows. The typical characteristic of the LB method is one monolayer (ML) at a time by repeatedly dipping a substrate into a liquid subphase coated with a monolayer of the desired polymer. The PVDF-based polymer should be firstly dissolved in dimethylformamide to form a dilute solution 0.01–0.05 wt%. The liquid subphase is ultrapure water with 18 MΩ/m. After the PVDF-based polymer solution is dropped into the water, about 30 min later, the PVDF-based polymer molecule chain will be floating on the surface of the water (as shown in Fig. 2a). In this case, the films are not uniform. The next step is pressing the bar of the LB technique until the surface pressure is up to 5 mN/m or less (as described in Fig. 2b). In this processing, if the surface is too large, the film on the water surface will collapse. The final step is shown in Fig. 2c, dipping the substrate horizontally on the surface of the water covered with monolayer PVDF molecule. Then raising the substrate slowly, one monolayer of PVDF ultrathin film is achieved. Repeating this process, the different thicknesses of PVDF-based film will be produced. spin coating and other normal methods. The LB method can be used to prepare the ultrathin two-dimensional ferroelectrics. In this chapter, the films of PVDF-based films mostly fabricated using LB technology. In 1995, S. P. Palto, L. M. Blinov, and V. M. Fridkin et al. began studies of ultrathin ferroelectric LB films of P(VDF-TrFE) copolymers with trifluoroethylene, P(VDF-TrFE) [9]. In 2007, J.L. Wang et al. prepared the ultrathin P(VDF-TrFE-CFE) terpolymer LB films [8]. Recently, we achieved the ultrathin PVDF homopolymer films with good ferroelectric performance. The detail of preparing the PVDF-based polymer films by LB technique is described as follows. The typical characteristic of the LB method is one monolayer (ML) at a time by repeatedly dipping a substrate into a liquid subphase coated with a monolayer of the desired polymer. The PVDF-based polymer should be firstly dissolved in dimethylformamide to form a dilute solution 0.01–0.05 wt%. The liquid subphase is ultrapure water with 18 MΩ/m. After the PVDF-based polymer solution is dropped into the water, about 30 min later, the PVDF-based polymer molecule chain will be floating on the surface of the water (as shown in Fig. 2a). In this case, the films are not uniform. The next step is pressing the bar of the LB technique until the surface pressure is up to 5 mN/m or less (as described in Fig. 2b). In this processing, if the surface is too large, the film on the water surface will collapse. The final step is shown in Fig. 2c, dipping the substrate horizontally on the surface of the water covered with monolayer PVDF molecule. Then raising the substrate slowly, one monolayer of PVDF ultrathin film is achieved. Repeating

this process, the different thicknesses of PVDF-based film will be produced.

Figure 2. The horizontal transfer of LB technique: (a) and (b) the processing of the LB **Figure 2.** The horizontal transfer of LB technique: (a) and (b) the processing of the LB films; (c) transferring the PVDFbased film to the substrate

PVDF-based polymer films possess many special properties, for example, dielectric, ferroelec‐ tric, piezoelectric, and pyroelectric properties and so on. Based on these properties, the films can be used for preparing the transducers, ferroelectric memory, gate of transistor, and uncooled infrared sensor. In the following paragraph, some special properties and applications will be introduced.
