Morphology of Ferroelectric/Conjugated Polymer Phase-Separated Blends Used in Nonvolatile Resistive Memories. Direct Evidence for a Diffuse Interface

The morphology of blends made of a ferroelectric polymer, poly(vinylidene fluoride-co-trifluoroethylene), and a cross-linkable conjugated polymer, an oxetane-functionalized poly(9,9-dioctylfluorene-alt-bithiophene), is investigated. Making use of the insolubility of the conjugated polymer, we were able to selectively remove the ferroelectric copolymer and analyze the remaining material. Atomic force microscopy studies show a lateral phase separation, consistent with columns of the conjugated polymer surrounded by a matrix of the ferroelectric. X-ray photoelectron spectroscopy showed that even after a very long washing time (120 h), there was still ferroelectric polymer on the remaining film, which we conclude to correspond to ferroelectric chains trapped within the conjugated polymer domains. This fact suggests the existence of a diffuse interface between the two polymer domains. Resistive memories were fabricated with this blend, though exhibiting limited nonvolatility. This behavior is discussed in terms of the presence of a wetting layer and the use of poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonic acid) as electrode.