Microscopic study of polydopamine modified BaTiO3/poly(vinylidene fluoride-trifluoroethylene) nanocomposite films

Abstract

Poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] has been intensively studied because of its good piezoelectric properties and at the same time being flexible and bio-compatible, making it a strong candidate for biomedical application. Doping with inorganic piezoceramic nanoparticles will enhance its ferro-/piezoelectricity. Further enhancements in ferro-/piezoelectricity of P(VDF-TrFE) are expected from surface modification of the nanoparticles through improving the bonding between the nanoparticles and polymer matrix. In this paper, a piezoresponse force microscopy (PFM) study on P(VDF-TrFE) doped with polydopamine modified BaTiO3 (BT-Pdop) nanoparticles is introduced. The microscopic study is complicated by the fine domain structures. As ferro-/piezoelectricity is directly related with domain size, instead of a domain-wise study, a statistical correlation (domain size) analysis was carried out for this composite. Since PFM measurement is based on the converse piezoelectric effect (the first harmonic response of the sample surface upon the applied electric stimulation), topographical effects should be taken into account in analyzing the related piezoresponse images. Hence, the correlation lengths of both topographical and piezoresponse images on the same area were studied. The doping dependence of the correlation length of BT-Pdop/P(VDF-TrFE) behaves differently for piezoresponse (polarization) and topography (grain) images. In particular, the correlation lengths of piezoresponse and topography images are in different ranges: 23-29 nm for the former and 55-66 nm for the latter; also, the composition dependences are different. Accordingly, the piezocorrelation reflects the material's real piezoresponse. It can also be deduced that the macroscopic polarization behavior in Pdop-BT/P(VDF-TrFE) results from extrinsic contributions.