A density functional theory study of poly(vinylidene difluoride) crystalline phases

Abstract

The structure and energetics of crystalline phases of poly(vinylidene difluoride) were investigated using density functional theory (DFT). In particular, we present data on non-polar analogues of the β and γ phases that have not yet been experimentally characterised. The results from a variety of exchange and correlation functionals were compared and it was found that van der Waals (vdW) interactions have an important effect on the calculated crystal structures. While the relative phase energies were not strongly dependent on the functional, the cohesive energies were significantly affected.

Hard and soft materials: putting consistent van der Waals density functionals to work

“…The energy landscapes are given relative to the energy of the optimal crystal structure (marked by the star in right panel). The black triangle and square are the relaxed results using vdW-DF and vdW-DF2, respectively [149], while the black cross shows relaxed result for PBE0 [145]. The red cross shows the results from x-ray diffraction on a sample drawn at 323 K [133] while the red square is x-ray diffraction from reference [134].…”

“…The red cross shows the results from x-ray diffraction on a sample drawn at 323 K [133] while the red square is x-ray diffraction from reference [134]. b Reference [149]; dispersion correction 'D2' from reference [156].…”

“…For predictions of the PVDF polarization, we follow the ideas presented by Johnsson and co-workers [147,149] while using the QUANTUM ESPRESSO implementation of the modern theory of polarization [89][90][91][92][93][94]. That is, we use a modified cell with one PVDF chain rotated 180 • , to establish nonpolar forms for both the γ and β phases and proceed with calculations that trace the effects of electron displacements in the bandstructure description of the polymer crystals [94].…”

Hard and soft materials: Putting consistent van der Waals density functionals to work

“…Hard and soft matter come in different crystal forms, as well as meta-stable variants, and there is often a need for a concerted theory-experiment analysis to resolve and understand phase stability. This is true, for example, even in simpler (compact-unit-cell and nonmagnetic) perovskites [52,53,[74][75][76][77][78][79][80][81][82][83][84][85][86][87][88][89][90][91] and in both nonpolar and ferroelectric polymers [92][93][94][95][96][97][98][99][100][101][102][103][104][105]. The structural transformations may be driven by temperature, electric fields, or strain; There can also be a release from meta-stable states that may have been locked in under synthesis or produced by usage [103].…”

“…First-principle calculations (that also accurately determine stress) may help in the analysis with volume- constrained variable-cell calculations and by determination of the phonon spectra, as well as calculations of magnetic, elastic, polarization, and strain response [87,90,91,103,[105][106][107][108][109][110][111][112][113][114][115]. For a given XC functional, we can rely on the Born-Oppenheimer (BO) approximation to determine what we call the native structure.…”