The structure of ferroelectric polyvinylidene fluoride/trifluoroethylene (PVDF/TrFE) copolymer Langmuir -Blodgett films

Langmuir-Blodgett (LB) films made from polyvinylidene fluoride (PVDF) and its copolymers with trifluoroethylene (PVDF/TrFE) were studied. Molecular conformation and optical properties were investigated by using Variable Angle Spectroscopic Ellipsometry (VASE) systems. Vibrational mode orientations were determined by changing the angle between the transition dipoles and the optical electric field. The well-known structural phase transition from paraelectric (100°C) to ferroelectric (25°C) phases was evident in changes to the infrared spectra. Experimental dispersion curves in the spectral range from infrared to ultraviolet were obtained through VASE data analysis, which also determined the film thickness. Crystallization and morphology of the LB films were studied by using atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray diffraction. Highly crystalline films can be obtained by annealing the LB films in the temperature range 120–135°C. Close to the melting point (150°C), annealing induced the chain out-of-plane folding and the formation of larger-size lamellae for thicker LB films (up to 100-ML). Uniform mesas were formed for films less than 5-ML under above annealing conditions. The melting and mesas formation results indicated that the chain mobility depends on the annealing temperature, film thickness and substrate adhesion. Crystal structure was studied in greater detail using transmission electron microscopy. Electron diffraction patterns recorded at multiple crystal tilt angles provided the 3D information about the crystal structure. Several distinct crystal structures were found in the copolymers. For the 70/30 copolymer, we found a new structure with a monoclinic 3D unit cell. This structure appears to be a variation of the “standard” orthorhombic structure of the ferroelectric phase, with an 8-monomer period along chain. Another observed structure is possibly composed of a mixed TTTG conformation (also with a monoclinic 3D unit cell). For the 50/50 and 80/20 copolymers, similar ferroelectric phase structures were observed and some crystals appeared to have mostly trans-gauche conformation, as indicated by their relatively large lattice parameters