Defects in crystalline PVDF: a density functional theory-density functional tight binding study

We present a comparative density functional theory (DFT) and density functional tight binding (DFTB) study of structures, energetics, vibrational properties as well as electronic structures of the four crystalline phases of polyvinylidene fluoride (PVDF) with different types of defects. For pure phases, the relative energies of PVDF strands (i.e. absent van der Walls bonding) agree well between DFT (using a GGA or a hybrid functional) and DFTB. For crystals, DFTB needs to be calibrated due to deficiencies in the treatment of vdW interactions. Defect formation energies were computed in large-scale DFTB simulations. For single chain vacancies, they are 0.41, 0.59, 0.08 and 0.40 eV per monomer removed in α, β, γ, and δ PVDF, respectively. The energy required to form double vacancies is 0.38, 0.52, 0.33 and 0.39 eV per monomer removed, respectively, i.e. the effect is nearly additive except in the γ phase. Interstitial defects were found to be unstable and convert into vacancies. The relatively high defect formation energies (vs. kT at room temperature) imply that phase purity is feasible in PVDF. Vibrational contributions affect the relative phase energies by up to 0.1 eV but do not significantly affect the relative phase stability.