Poly(vinylidene fluoride) under finite electric displacement fields: a removable α → δ reorientation barrier and a persistent first-order δ → β conformational change
Abstract
The α, δ, and β phases of PVDF are interconverted by electrical poling and mechanical drawing, yet the thermodynamics under a controlled electrical boundary condition has not been established from first principles. We evaluate the zero-temperature free energy at fixed electric displacement D from van der Waals density-functional calculations along two constrained coordinates in a common 24-atom cell: a tilt of the gauche C--C bonds that maps antipolar α onto polar δ (cell change <1%), and a backbone torsion that straightens TG+TG- chains into all-trans β (chain-repeat strain 10.6%). As D increases from 0 to $0.30C m^{-2}$, the lowest homogeneous free energy switches from α to δ at Dc1=0.046~C\,m−2 and from δ to β at Dc2=0.157~C\,m−2. At T=0 with domain-wall energy neglected, a δ multidomain state lies $14.3$~meV per cell below homogeneous α at every D. A Γ-point harmonic estimate suggests that vibrations reverse this ordering at room temperature. δ and β coexist for 0.11<D<0.21~C\,m−2 at E*=0.69~GV\,m−1, independent of the screening constant. The tilt barrier falls from $0.66$~eV per cell at D=0 to $17$~meV at D=0.30~C\,m−2 (below kBT at $300$~K), while the torsion barrier remains $0.66$~eV at Dc2 and $0.34$~eV at D=0.30~C\,m−2, so δ→β stays first order. Much of the difference traces to the path effective charge (0.28e for the tilt, 0.18e for the torsion). Scaling shows the tilt barrier would survive with the torsion charge; the smaller torsion charge accounts for about half the barrier it retains. The landscape accounts for why a field produces δ and why β is reached by drawing, and suggests that raising that charge could let a field alone produce β.
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BibTeX
@misc{qiu2026polyvinylidene,
title = {{Poly(vinylidene fluoride) under finite electric displacement fields: a removable $α\!\to\!δ$ reorientation barrier and a persistent first-order $δ\!\to\!β$ conformational change}},
author = {Shaohui Qiu and Jacob C. Dean and Rajagopalan Thiruvengadathan and Ramanujam Brahmadesam Thoopul Srinivasa Raghava and Yiming Su},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.31926},
eprint = {2610.31926},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.31926},
}
RIS
TY - UNPB
TI - Poly(vinylidene fluoride) under finite electric displacement fields: a removable α → δ reorientation barrier and a persistent first-order δ → β conformational change
AU - Shaohui Qiu
AU - Jacob C. Dean
AU - Rajagopalan Thiruvengadathan
AU - Ramanujam Brahmadesam Thoopul Srinivasa Raghava
AU - Yiming Su
PY - 2026
DA - 2026/10/26
PB - arXiv
DO - 10.48550/arxiv.2610.31926
UR - https://arxiv.org/abs/2610.31926
AB - The α, δ, and β phases of PVDF are interconverted by electrical poling and mechanical drawing, yet the thermodynamics under a controlled electrical boundary condition has not been established from first principles. We evaluate the zero-temperature free energy at fixed electric displacement D from van der Waals density-functional calculations along two constrained coordinates in a common 24-atom cell: a tilt of the gauche C--C bonds that maps antipolar α onto polar δ (cell change <1%), and a backbone torsion that straightens TG^+TG^- chains into all-trans β (chain-repeat strain 10.6%). As D increases from 0 to $0.30C m^{-2}$, the lowest homogeneous free energy switches from α to δ at D_c1=0.046~C\,m^−2 and from δ to β at D_c2=0.157~C\,m^−2. At T=0 with domain-wall energy neglected, a δ multidomain state lies $14.3$~meV per cell below homogeneous α at every D. A Γ-point harmonic estimate suggests that vibrations reverse this ordering at room temperature. δ and β coexist for 0.11<D<0.21~C\,m^−2 at E^*=0.69~GV\,m^−1, independent of the screening constant. The tilt barrier falls from $0.66$~eV per cell at D=0 to $17$~meV at D=0.30~C\,m^−2 (below k_BT at $300$~K), while the torsion barrier remains $0.66$~eV at D_c2 and $0.34$~eV at D=0.30~C\,m^−2, so δ→β stays first order. Much of the difference traces to the path effective charge (0.28e for the tilt, 0.18e for the torsion). Scaling shows the tilt barrier would survive with the torsion charge; the smaller torsion charge accounts for about half the barrier it retains. The landscape accounts for why a field produces δ and why β is reached by drawing, and suggests that raising that charge could let a field alone produce β.
ER -