A popular method for creating PVDF structures is the technique of electrospinning. Electrospun plastic sheets have applications in membranes for batteries and electronics.1–5 However, this method leads to opaque structures. Another method of forming piezoelectric PVDF films is by spin-coating the polymer onto a substrate in the presence of a high electric field.1 This creates thin PVDF films that are clear and thus allow optical access to the nanoparticles embedded within to activate potential optical properties. This method of producing piezoelectric PVDF devices is cost-effective and relatively straightforward. The key is to overcome the need for a high electric field utilized to induce a ferroelectric polymer phase (500-700 kV/cm) and to consistently create a ferroelectric film with a high degree of transparency.10
Past works have shown that mechanical stress, high annealing temperature, and high electric fields all contribute to encouraging polarized phase formation and transparency during film production.10–12 Taking these parameters into account, we provide a detailed documentation of a straightforward method for producing transparent PVDF films with and without AuNPs under poled and unpoled conditions. We create multiple films under each condition to facilitate statistical assessment of the consistency of this method and analyze the effect AuNPs and poling have on the final phase composition of the films.
PVDF Film Production
The process for producing bare PVDF thin film is summarized in Figure 1. First, 2 mL of acetone and .25 g of PVDF (in powder form) are mixed in a beaker at 85 ℃ for ten minutes with a magnetic stirrer. The purpose of the acetone is to suspend and disperse the PVDF powder uniformly in a liquid mixture. This mixture is white and cloudy because PVDF does not dissolve in acetone as shown in Figure 2a. After ten minutes, 1 mL of dimethyl sulfoxide (DMSO) is introduced to the mixture to dissolve the PVDF. The solution is stirred at a temperature of 85 ℃ for 45 minutes to completely boil off the acetone, leaving only the dissolved PVDF in DMSO. The solution will gradually go from a white and cloudy mixture to a clear and viscous solution during the 45 minutes of mixing as shown in Figure 2b.
After 45 minutes, the solution is ready to be cast. A drop of the solution is placed on a 1cm x 1cm silicon substrate in the spin-coater. The spin-coater chuck is grounded. The solution is spun for 25 seconds at 4,000 rpm. To electrically pole the film, a potential is applied via an aluminum plate placed above the ground plate. We pole our films using either a weaker field at 1.31 kV/cm or a stronger field of 3.25 kV/cm. After spinning, the PVDF coated silicon substrate is transferred to a hot plate and baked for 75 minutes at 85 ℃. After baking, the PVDF film is dry enough to be peeled off and stored for XRD measurements.
PVDF-Au Film Production
The 60 nm AuNPs from Sigma Aldrich are suspended in an aqueous solution as shown in Figure 3b. The reddish color is due to the wavelength selective enhanced reflection as a result of the surface plasmon polariton resonance. We find that adding the AuNPs directly to the PVDF and acetone mixture along with the aqueous solution leads to failure in transparent film production. The main reason is that while water cannot dissolve PVDF, PVDF is miscible in DMSO which dissolves PVDF. This combination leads to the formation of a gelatinous substance which is neither transparent nor spreadable into a thin film. To solve this problem, we modify the previous procedure to eliminate the water while keeping the AuNPs.
The process for producing AuNP embedded PVDF thin film is summarized in Figure 3. We start with 1.5 mL of AuNP solution. This amount can vary depending on the desired final concentration of AuNP. The nanoparticle solution is placed in a centrifuge to separate the AuNPs from the water. After centrifuging, most of the water is removed using a pipet while leaving the AuNPs at the bottom of the test tube. The AuNPs are immediately resuspended in 4 mL of acetone with a vortex mixer to avoid permanent aggregation of the AuNPs which can destroy its plasmonic property.
The 4 mL acetone/AuNP solution is mixed with 0.25 g of PVDF powder in a beaker at 105 ℃ for 10 minutes. This increased temperature, in contrast to 85 ℃ in the bare PVDF film production, ensures that any remaining water is boiled off from the mixture. After 10 minutes, 1 mL of DMSO is introduced to the mixture. The solution is stirred continuously at 105 ℃ for another 50 minutes. As the acetone (and any remaining water) is boiled off, the PVDF dissolves into the DMSO. The solution will go from an opaque, pale pink mixture to a clear, viscous red-pink hue shown in Figure 3c. The fact that this coloration remains is a sign that the plasmonic property of the AuNPs is maintained. At this point, the solution is viscous enough to be cast. The spin coating step is the same as that in the bare film production. After the spin-coating step, the AuNP PVDF film with substrate is placed on a hot plate for 75 minutes at 85 ℃ and peeled off afterwards to be characterized in XRD. Examples of PVDF films produced without and with AuNPs are presented in Figure 4a and 4b, respectively.