Films of poly(vinylidene fluoride) copolymerized with trifluoroethylene made by spin coating have similar properties but a broader, less intense Curie peak at higher temperature than solvent-cast films.
The discovery in 1969 by Kawai1 of the strong piezoelectric activity of poly(vinylidene fluoride) (PVDF) and its copolymers—in particular the copolymers P(VDF-TrFE) of vinylidene difluoride (VDF) and trifluoroethylene (TrFE)—opened a large field of application for acoustic and piezoelectric devices (such as radar, sonar, ultrasound, and acoustic imaging). Its high piezoelectric performance, obtained without mechanical stretching during processing, makes P(VDF-TrFE) currently the polymer of choice for ferroelectric thin-film devices.2Ultrasonication is well known as an effective way to disperse inorganic phases homogeneously in suspension and prepare polymer composites with micro-/nanofillers, but it can degrade polymers. The morphology, and electromechanical and ferroelectric properties of PVDF and P(VDF-TrFE) strongly depend on the sample processing conditions.3, 4 Despite this, no study of ultrasonic degradation has been carried out on the PVDF family, although it is necessary for optimizing performance. We studied the effect of different processing conditions such as ultrasonication, annealing, and film thickness on the morphology, and thermomechanical and piezoelectric properties of thin films of P(VDF-TrFE).We prepared freestanding films and films on substrates by solvent casting and spin coating, respectively. We first dissolved pellets of VDF and TrFE copolymer (70/30mol%) in methylethylketone (MEK) by stirring at 80°C to a concentration of 14wt%. To obtain solvent-cast films of varying thickness, we cast the polymer solutions on glass and air-dried them. We subsequently peeled off and dried the films before annealing at 140°C for 60min to achieve high-ferroelectric-phase crystallinity. We prepared spin-coated films on glass using a Süss Microtech Delta 6 RC spin coater. The solutions were spin-coated for 30s at speeds of 1000–2500rpm with a total acceleration time of 1s. We then dried the resulting films to evaporate the MEK before annealing at 140°C for 60min on the glass substrate. We studied the films’ thermal properties with a TA Instruments Q200 differential scanning calorimeter. The samples were analyzed between −85 and 200°C with heating/cooling rates equal to 10°C min−1 under nitrogen flow.The results show that the Curie transition of the ferroelectric polymer films (when the spontaneous electric polarization of the material changes to induced electric polarization) was influenced by the preparation methods, although there was very little difference in the melting transition: see Figure 1. The first heating thermograms show an endothermic peak at 150°C attributed to the melting of the copolymer crystalline phase. The enthalpies of fusion and degrees of crystallinity of the two types of films are very close, although the melting peak of the solvent-cast films is slightly sharper. However, the Curie peak of the spin-coated films is broader, less intense, and shifted to higher temperature compared to the solvent-cast films. Melting the films at 200°C for 5min erased the effect of the preparation method so that the cooling and second heating thermograms of the films became identical.Figure 1.Differential scanning calorimetry heating thermograms of vinylidene (VDF)-TrFE and trifluoroethylene copolymer—P(VDF-TrFE)—films prepared by spin coating (3μm) and by solvent casting (25μm) dried at 80°C for 60min and annealed at 140°C for 60min. TCurie: Curie transition temperature. Tm: Melting temperature.We attribute the difference to a less uniform distribution of TrFE comonomer units within the crystalline and amorphous regions of the spin-coated films. Indeed, constraints induced by spin coating are expected to reduce the chain mobility, and as the rate of solvent evaporation is higher, TrFE co-monomers might be less able to disperse homogenously. However, the preparation methods had no effect on the films’ piezoelectric and mechanical properties. There was also no significant effect on the films’ piezoelectric or mechanical properties from the short ultrasonication, drying before annealing, or when the film was thicker than about 100nm, which is the critical value of crystallization of P(VDF-TrFE).Surprisingly, the ultrasonication had a clear impact on the relaxations at high temperature of the polymer chains: see Figure 2. The peak corresponding to α relaxation (the glass transition) shifted to lower temperature with a maximum at 80°C. Beyond this peak, the phase angle (tan δ), representing the ratio of the viscous to the elastic energy of the sonicated films, surprisingly continued to decrease instead of starting to increase as it approaches the melting temperature. This remains to be fully understood. In addition, this study indicates that a short annealing by 10min at 140°C was enough to obtain well-crystallized films, which is interesting for industry where an hour or more of annealing is usually used.Figure 2.Plots of the storage modulus E′and the phase angle tan δ against temperature. The plots reveal changes in α relaxation (the glass transition) as a result of sonication.In summary, we have prepared films of P(VDF-TrFE) by spin coating and solvent casting. There was very little difference between the melting transitions of the two films, but the Curie transition of the spin-coated film was broader, less intense, and shifted to higher temperature. Ultrasonication shifted the α relaxation to lower temperature, and tan δ of the sonicated films decreased with temperature as it approached the melting temperature, which is surprising and not understood. Our next step is to explore how various nanofillers embedded in the polymer affect the film properties. Indeed, we expect P(VDF-TrFE) nanocomposites to have very promising mechanical, optical, and ferroelectric properties.5–7AuthorsVan Son NguyenInstitut Jean Lamour UMR CNRS Université de LorraineDidier RouxelInstitut Jean Lamour UMR CNRS Université de LorraineMatthias MeierInstitut Jean Lamour UMR CNRS Université de LorraineBrice VincentInstitut Jean Lamour UMR CNRS Université de LorraineAbdesselam DahounInstitut Jean Lamour UMR CNRS Université de LorraineFabrice Domingues Dos SantosPiezotech SASSabu ThomasSchool of Chemical Sciences Mahatma Gandhi UniversityReferencesH. Kawai, The piezoelectricity of poly(vinylidene fluoride), Jpn. J. Appl. Phys. 8 (975-976), 1969. S. Chen, K. Yao, F. E. H. Tay and L. L. S. Chew, Comparative investigation of the structure and properties of ferroelectric poly(vinylidene fluoride) and poly(vinylidene fluoride-trifluoroethylene) thin films crystallized on substrates, J. Appl. Polym. Sci. 116, pp. 3331-3337, 2010. Q. M. Zhang, V. Bharti and G. Kavarnos, Poly(vinylidene fluoride) (PVDF) and its copolymers, Encyclopedia of Smart Materials, pp. 807-825, John Wiley & Sons, New York, 2002. D. Mao, B. E. Gnade and M. A. Quevedo-Lopez, Ferroelectric properties and polarization switching kinetic of poly(vinylidene fluoride-trifluoroethylene) copolymer, Ferroelectrics—Physical Effects, pp. 78-100, Mickaël Lallart, 2011. V. S. Nguyen, D. Rouxel and B. Vincent, Influence of cluster size and surface functionalization of ZnO nanoparticles on the morphology, thermomechanical, and piezoelectric properties of P(VDF-TrFE) nanocomposite films, Appl. Surf. Sci. 279, pp. 204-211, 2013. V. S. Nguyen, L. Badie and E. Lamouroux, Nanocomposite piezoelectric films of P(VDF-TrFE)/LiNbO3, J. Appl. Polym. Sci. 129 (1), pp. 391-396, 2013. R. Hadji, V. S. Nguyen, B. Vincent, D. Rouxel and F. Bauer, Preparation and characterization of P(VDF-TrFE)/Al2O3 nanocomposite, IEEE Trans. Ultrason., Ferroelectr., Freq. Control 59 (1), pp. 163-167, 2012. DOI: 10.2417/spepro.005078