Thermally Induced Cooperative Molecular Reorientation and Nanoscale Polarization Switching Behaviors of Ultrathin Poly(vinylidene fluoride-trifluoroethylene) Films

Ultrathin films of the ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] have recently attracted intensive research interest due to their potential applications in emerging organic devices. As special geometry confinement systems, many aspects about their processing, microstructure, and performance are far from being well understood. Here, the cooperative molecular orientation, macroscopic ferroelectric properties, and nanoscale polarization switching behaviors of thermally crystallized ultrathin P(VDF-TrFE) films were investigated. With increasing annealing temperature, the films showed a distinct granule toward layered needle-network (LNN) morphology transition with deteriorated ferroelectricity at a critical point (T-cr) around 140 degrees C. Accompanying this is that the polymer backbone first lay more parallel relative to the substrate, and then exactly at T-cr it showed an abrupt standing-up reorientation. Interestingly, the polarization axis simultaneously showed just opposite orientation and reorientation. Nanoscale polarization switching characterization by using piezoresponse force microscopy and local ferroelectric hysteresis loops revealed a varied molecular orientation in the same needle grain and a polarization reversal constraint effect by the inhomogeneous LNN structure. On the basis of these observations, a tilted-chain lamellae structural model was proposed for the LNN film. The lying down of the polarization axis and the polarization reversal constrain effect well explain the inferior performance of the LNN film despite its higher crystallinity than that of the granular film. The,results may shed some light on the understanding of the intercorrelation among the thermal crystallization, microstructure, and macroscopic performance of ultrathin polymer films.