Saeid Arabnejad

Intermolecular interactions and high dielectric energy storage density in poly(vinylidene fluoride-hexafluoropropylene)/poly(vinylidene fluoride) blend thin films

Homogeneous poly(vinylidene fluoride-hexafluoropropylene) and poly (vinylidene fluoride) (P(VDF-HFP)/PVDF) blend films were prepared via a chemical solution approach, followed by quenching, annealing, and hot pressing. The intermolecular interactions of the blends were investigated through atomic simulation. Higher melting temperature, higher crystallinity, larger elastic modulus, and improved breakdown strength (>850 MV/m) were observed in the optimized polymer blends, in comparison with either of the two constituent polymers, PVDF or P(VDF-HFP). In addition, the P(VDF-HFP)/PVDF blend film also showed larger dielectric constant. As a result, an extremely high energy density of 30.1 J/cm3 was achieved in P(VDF-HFP)/PVDF (50:50 by weight) blend films.

Poly(vinylidene fluoride-co-hexafluoropropylene)-graft-poly(dopamine methacrylamide) copolymers: A nonlinear dielectric material for high energy density storage

A nonlinear dielectric poly(vinylidene fluoride-co-hexafluoropropylene)-graft-poly(dopamine methacrylamide) [P(VDF-HFP)-g-PDMA] graft copolymer with ultra-high energy density of 33 J/cm3 was obtained by thermally initiated radical graft polymerization. It was observed that the dielectric constant of the graft copolymer films was 63% higher than that of P(VDF-HFP), with a large dielectric breakdown strength (>850 MV/m). Theoretical analyses and experimental measurements showed that the significant improvement in the electric polarization was attributed to the introduction of the highly polarizable hydroxyl groups in the PDMA side chains, and the large breakdown strength arose from the strong adhesion bonding of the catechol-containing graft copolymer to the metal electrode.

Shear-induced conformation change in α-crystalline nylon6

A study of shear deformation of α-crystalline nylon6 is undertaken, using dispersion-corrected density functional theory. The shear stress-strain relationship and shear strength for interlayer shear deformation are computed. A conformation change induced by shear is identified along twinning deformation, whereby the conformation of chains, specifically the location of non-H-bonded hydrogen atoms, changes continuously. This paves a way for the modulation of properties of this group of materials by small shear deformation, if the non-H-bonded hydrogens are chemically substituted to form non-equivalent conformations when deformed.

Influence of the aggregate state on band structure and optical properties of C60 computed with different methods

C60 and C60 based molecules are efficient acceptors and electron transport layers for planar perovskite solar cells. While properties of these molecules are well studied by ab initio methods, those of solid C60, specifically its optical absorption properties, are not. We present a combined density functional theory-Density Functional Tight Binding (DFTB) study of the effect of solid state packing on the band structure and optical absorption of C60. The valence and conduction band edge energies of solid C60 differ on the order of 0.1 eV from single molecule frontier orbital energies. We show that calculations of optical properties using linear response time dependent-DFT(B) or the imaginary part of the dielectric constant (dipole approximation) can result in unrealistically large redshifts in the presence of intermolecular interactions compared to available experimental data. We show that optical spectra computed from the frequency-dependent real polarizability can better reproduce the effect of C60 aggregation on optical absorption, specifically with a generalized gradient approximation functional, and may be more suited to study effects of molecular aggregation.

Defects in alpha and gamma crystalline nylon6: A computational study

We present a comparative Density Functional Tight Binding study of structures, energetics, and vibrational properties of α and γ crystalline phases of nylon6 with different types of defects: single and double chain vacancies and interstitials. The defect formation energies are: for a single vacancy 0.66 and 0.64 kcal/mol per monomer, and for an interstitial strand 1.35 and 2.45 kcal/mol per monomer in the α and γ phases, respectively. The presence of defects does not materially influence the relative stability of the two phases, within the accuracy of the method. The inclusion of phononic contributions has a negligible effect. The calculations show that even if it were possible to synthesize the pure phases of nylon6, the defects will be easily induced at room temperature, because vacancy formation energies in both phases are of the order of kT at room temperature. The formation of interstitial defects, on the contrary, requires the energy equivalent to multiple kT values and is much less likely; it is also much less probable in the γ phase than in α. The vibration spectra do not show significant sensitivity to the presence of these defects.