Electron donor-acceptor hybrid bonding polymers exhibit symmetry breaking and ferroelectricity.

Abstract

Hybrid bonding polymers chemically integrate supramolecular assemblies and covalent macromolecules and can therefore exhibit synergistic functionalities that can impact mechanical properties, catalytic activity, and robotic behavior, among others. In these materials, structural units are connected through both covalent and noncovalent bonds, and this hybrid bonding nature can lead to alterations in nanoscale morphology and internal ordering relative to purely supramolecular or purely covalent polymer systems. We report here on a hybrid bonding polymer in which an ionizable covalent polymer was partially functionalized with electron donor-acceptor dyads and co-assembled with similar dyad small molecules that could form supramolecular polymers on their own. Characterization with UV/Vis spectroscopy revealed charge-transfer interactions among the donor-acceptor dyads in the covalent polymer, the hybrids, and the supramolecular polymer, and wide-angle X-ray scattering indicated a gradual loss of internal order as more covalent polymer was incorporated into the hybrid structures. Interestingly, second harmonic generation confocal microscopy revealed the formation of noncentrosymmetric structures in the hybrid materials but not in the supramolecular polymer. Furthermore, measurements of polarization as a function of applied electric field yielded hysteresis loops characteristic of ferroelectric materials for the hybrid materials, but not for the pure covalent polymer or the pure supramolecular polymer. We found evidence of symmetry breaking and ferroelectricity in the hybrid assemblies only when the covalent polymer was moderately ionized, which suggested that electrostatic interactions played a key role in breaking the centrosymmetry of the donor-acceptor assemblies. Coarse-grained simulations further indicated that electrostatic repulsions between charged groups on the covalent polymers and the donor-acceptor dyad small molecules led to symmetry-breaking defects in these materials and that this effect was most prevalent at moderate degrees of covalent polymer ionization. Such defect-induced ferroelectricity has not been previously reported in other supramolecular ferroelectrics, and further investigation of this symmetry-breaking mechanism could enable simpler fabrication of flexible, lightweight ferroelectric materials.