Spontaneously Piezo- and Pyroelectric Polyacrylonitrile Brushes Prepared via Surface-Initiated Polymerization

Surface-initiated polymerizations generate thin films ("brushes") consisting of polymer chains that are anchored with one chain end to a solid surface. As they are conducted from initiator-modified solid substrates, these polymerizations allow for unidirectional chain growth and enforce a stretched chain conformation. Using acrylonitrile as an example of a monomer with a dipolar side-chain functional group, this study finds that surface-initiated polymerization not only impacts the alignment and conformation of the polymer main chain but can also influence the orientation of side-chain functional groups. Surface-initiated polymerization of acrylonitrile is found to result in polyacrylonitrile (PAN) brushes that display spontaneous pyro- and piezoelectric behavior. As these properties are not observed in spin-cast PAN films, this indicates that surface-initiated growth of PAN enforces an overall parallel orientation of the dipolar nitrile side-chain functional groups. This is supported by polarized FTIR spectroscopy and NEXAFS experiments, which suggest an overall nonisotropic orientation of nitrile groups in the PAN brush, whereas the nitrile groups in the spin-cast PAN film are more isotropically oriented. Pyro- and piezoelectric polymers are attractive for use in sensors, actuators, and energy-harvesting devices but typically require electrical poling, mechanical stretching, or electrospinning to promote the alignment of molecular dipoles and enhance piezo- and pyroelectric properties. The ability to prepare thin polymer films that display spontaneous pyro- and piezoelectric behavior is significant as it renders these postprocessing steps unnecessary.