Density functional theory studies on PVDF/ionic liquid composite systems

Abstract

Density functional theory calculations with and without dispersion correction were performed to describe the effect of the addition of ionic liquids (ILs) to polyvinylidene fluoride (PVDF) molecules. All the calculations were carried out for four monomer units of $$\upalpha $$ - and $$\upbeta $$ -PVDF and 1-n-alkyl 3-methylimidazolium tetrafluoroborate [ $$\hbox {C}_{\mathrm{n}}\hbox {MIM}$$ ] [ $$\hbox {BF}_{4}$$ ] (n = 2, 4, 6, 8, 10) ionic liquids. Dispersion correction is found to be essential to describe ion pair (within the IL molecule) interaction and polymer–ionic liquid interaction. Frontier orbitals (HOMO, LUMO) compositions and energies were obtained for individual PVDF molecules, ionic liquids and ionic liquid added polymer complexes to demonstrate the variation in different chemical parameters like hardness, softness, chemical potential, electronegativity, the electron affinity of the systems. Mulliken and atomic dipole moment corrected Hirschfeld population analyses were carried out to provide a quantitative analysis of partial atomic charge distribution. Molecular electrostatic potential plots, mapped on to total electron density surfaces, are provided to depict the reactive parts of the molecules under study. Natural bond orbital analysis was also carried out to quantify the extent of electron delocalization caused by PVDF–IL anion and PVDF–IL cation weak interactions. Synopsis: Dispersion-corrected DFT studies were performed to show why the addition of ionic liquids (IL) help in polar $$\upbeta $$ -PVDF crystallization. Analyses were carried out for 1-n-alkyl-3-methylimidazolium-tetrafluoroborate ([ $$\hbox {C}_{\mathrm{n}}\hbox {MIM}$$ ][ $$\hbox {BF}_{4}$$ ], n = 2,4,6,8,10) ILs and four monomer units of $$\upalpha $$ - and $$\upbeta $$ -PVDF using 6-311+G(d,p)/B3LYP-D. Obtained results suggest ILs show better interaction with $$\upbeta $$ -PVDF than $$\upalpha $$ -PVDF.