Highly oriented poly(vinylidene fluoride-co-trifluoroethylene) ultrathin films with improved ferroelectricity

We fabricated poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE) 75/25 mol% Langmuir-Blodgett (LB) nanofilms by the assistance of amphiphilic poly(N-dodecylacrylamide) (pDDA) nanosheets. The nanosheet is formed based on a well-organized hydrogen bonding network among pDDA amide groups at the air-water interface. By the introduction of the pDDA nanosheet, the film stability of the P(VDF-TrFE) Langmuir film was greatly improved with a twofold increase in the collapse surface pressure to 57 mN m^-1 of pure P(VDF-TrFE). Then, the P(VDF-TrFE) Langmuir film was endowed a good transfer ability with a unity transfer ratio irrespective of its non-amphiphilicity. Absorbance of the amide group of pDDA in UV-vis spectra shows a good linear relation with the film thickness. The result indicates that the multilayered film takes a uniform layered structure. The β-crystal content in as-prepared LB nanofilms with no post-treatment is up to 80%, one of the highest values ever reported. The monolayer thickness was determined as 3.5 nm by AFM measurements. The good film properties make the as-prepared P(VDF-TrFE)/pDDA LB ultrathin films (18 nm) available for ferroelectricity measurement using macroscopic methods such as the typical Sawyer-Tower circuit, which is usually challenging for other ultrathin films. The measurements demonstrate improved ferroelectricity, with a high remanent polarization value of 5.0 μC cm^-2 at 10 Hz.