Confinement in oriented mesopores induces piezoelectric behavior of polymeric nanowires

We report on the preparation and the piezoelectric properties of ultrathin polymeric nanowires in the oriented pores of mesoporous silica, which are embedded in the channels of a supporting anodic alumina membrane. Poly(vinylidene difluoride) [PVDF] and its copolymer, poly(vinylidene difluoride trifluoroethylene) [PVTF], were both confined to two types of columnar silica mesopores of ∼5 and 10 nm in diameter. The extreme spatial confinement induces a preferential orientation of the crystalline domains of the polymer into a ferroelectric phase, leading to ultrahigh-aspect-ratio nanowires distributed throughout the templating host, and having up to 60 μm in length, comparable to the thickness of the hosting alumina. The resulting distributed array of piezoelectric nanowires are isolated from each other by a dielectric matrix, facilitating the handling and electrical contacting. We show, for the first time, that a remarkable piezo-response, in the absence of any poling or stretching, is obtained upon nanoconfinement on the PVDF polymer, which, in contrast, does not show any polarization when in bulk or film form without poling. The piezoelectric behavior was assessed by a piezo evaluation system (PES) and we visualized polar nanowire bundles via piezoresponse force microscopy (PFM). This "nano-structuration" represents a powerful approach, holding promise for applications for nanoactuators or bioinspired ciliated sensors with high sensitivity and resolution.