Dielectric and piezoelectric responses of nylon-7: A first-principles study

We have systematically investigated dielectric and piezoelectric responses of single polymeric thread and bulk crystal of nylon-7 using first-principles density functional theory based calculations based on pseudopotentials, plane-wave basis and a generalized gradient approximation (GGA) of exchange correlation energy of electrons. We estimate the strength of inter-chain interaction through the binding energies of the crystal and the thread. Vibrational spectrum of the bulk structure determined here is in good agreement with experimentally available IR and Raman spectra. We demonstrate that large dielectric and piezoelectric responses of nylon-7 arise primarily from the electronic contribution and contribution of phonons is relatively smaller. Our results show that the dielectric response is dependent on the specifics of structural order, suggesting a way of interpreting measured dielectric response of polymers in terms of their ordering.