1:30 PM - 3:30 PM
▲ [15p-P4-9] Defects in Crystalline PVDF: a Density Functional Theory – Density Functional Tight Binding Study
Keywords:PVDF, polymer, defect
Polyvinylidene fluoride (PVDF) is a functional polymer which consists of strands of alternating covalently bound CH2 and CF2 units. Different conformations that the strands assume give rise to different phases. The lowest energy phases of PVDF are alpha, beta, gamma, and delta. The difference in conformation of strands of different phases leads to different crystal structures. PVDF is polar and stands out by a large induced dipole which is responsible for its useful ferroelectric and piezoelectric properties. These properties strongly depend on the PVDF’s phase. For example, the beta phase in which the dipoles of the monomer units can be aligned in the same direction exhibits a strong ferroelectric response, while the alpha phase does not. In real life PVDF applications, different phases may co-exist and are defects will be present. It is therefore important to be able to model different phases of PVDF, pure as well as defected.
We present a comparative Density Functional Theory (DFT) and (large scale) Density Functional Tight Binding (DFTB) study of structures, energetics, vibrational properties as well as electronic structures of the four crystalline phases of PVDF with different types of defects. For pure phases, relative energies of PVDF strands (i.e. absent vdW bonding) agree well between DFT (using a GGA or a hybrid functional) and DFTB. For crystals, DFTB need to be calibrated due to deficiencies in 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 alpha, beta, gamma, and delta 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 additive. 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 relative phase energies by up to 0.1 eV but do not significantly affect relative phase stability.
We present a comparative Density Functional Theory (DFT) and (large scale) Density Functional Tight Binding (DFTB) study of structures, energetics, vibrational properties as well as electronic structures of the four crystalline phases of PVDF with different types of defects. For pure phases, relative energies of PVDF strands (i.e. absent vdW bonding) agree well between DFT (using a GGA or a hybrid functional) and DFTB. For crystals, DFTB need to be calibrated due to deficiencies in 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 alpha, beta, gamma, and delta 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 additive. 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 relative phase energies by up to 0.1 eV but do not significantly affect relative phase stability.