Coupling various in-situ methods to reveal the peculiar semi-crystalline morphology of melt-processed fluorinated copolymers

Abstract

Poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-co-TrFE)) copolymer and poly(vinylidene fluoride-ter-trifluoroethylene-ter-chlorotrifluoroethylene) P(VDF-ter-TrFE-ter-CTFE) terpolymer thin films are usually produced by solvent-casting followed by annealing to enhance their electroactive properties in electronic devices. In this study, we investigate the effect of a different thermal treatment, namely crystallization from the melt, on the crystalline structure and morphology of P(VDF-ter-TrFE-ter-CTFE) terpolymers with varying CTFE content (ranging from 0 to 8.3 mol %) using a combination of different in-situ techniques. First, in-situ Wide-Angle X-Ray Scattering (WAXS) shows the different crystal-crystal transitions during cooling for all the samples. For the terpolymers, two sharp Bragg peaks ((110) and (200) family of planes) of the low temperature crystalline phase can be detected. Their sharpness reflects a high lateral extension of the crystalline domains. Moreover, coupled morphological studies by in-situ Small-Angle X-Ray Scattering (SAXS) and Atomic Force Microscopy (AFM) show that the crystallization starts with the formation of very extended semi-crystalline domains with a sub-micrometric thickness that contributes to the SAXS signal. For significant amounts of CTFE, crystallization is spread out, leading to the appearance of increasingly finer lamellar stacks during cooling, filling the spaces left unoccupied during crystallization at higher temperatures. This contrasts with the very rapid crystallization observed in the polymer with 0 mol % of CTFE. Finally, the Curie transition is clearly visible in the AFM mechanical images, providing an original method for detecting the structural transitions occurring in these polymers.