Computational chemistry & molecular modeling
Fan Zhang; Jieqi Wang; Yangyang Zhao; Xuelian He
Abstract
The effect of long chain content (XL ) on the static crystallization and tensile deformation mechanisms of bimodal HDPE/UHMWPE was investigated by molecular dynamics simulations. The crystallization of HDPE/ UHMWPE undergoes three stages: nucleation, rapid growth of lamellar crystals, and stabilization. ...
Read More
The effect of long chain content (XL ) on the static crystallization and tensile deformation mechanisms of bimodal HDPE/UHMWPE was investigated by molecular dynamics simulations. The crystallization of HDPE/ UHMWPE undergoes three stages: nucleation, rapid growth of lamellar crystals, and stabilization. The increase of XL leads to the formation of more nucleation sites, which promotes nucleation, but at the same time leads to an increase of entanglement sites, which is not conducive to the movement of the long chains to the growth front to fold and form lamellar crystals. Tensile deformation is performed on the crystallized models and the systems exhibit three stages: elastic deformation, plastic deformation and stress hardening. During deformation, the increase of XL improves the orientation nucleation and crystallinity (Xc), but when XL exceeds 4 wt.%, the entanglement effect becomes more pronounced, leading to a decrease in Xc. The effect of temperature is also taken into account: at low temperatures, a suitable range (2-4 wt.%) exists to optimize the mechanical properties of the material. At high temperatures, there is almost no stress-hardening phenomenon, but the addition of long chains has an impeding effect on the melting of the lamellar crystals, and when XL is greater than 8 wt.%, stress-induced melting is more likely to occur, accelerating the melting of the crystals.
Computational chemistry & molecular modeling
Parisa Alamdari; Farhad Sharif; Saeedeh Mazinani
Abstract
The effect of structural defects on graphene interaction with other molecules is of high interest. In this study, the interaction of ethylene molecules with pristine graphene (PG) and defective graphenes including single (SVG) and double (DVG) vacancies, were investigated using dispersion-corrected periodic ...
Read More
The effect of structural defects on graphene interaction with other molecules is of high interest. In this study, the interaction of ethylene molecules with pristine graphene (PG) and defective graphenes including single (SVG) and double (DVG) vacancies, were investigated using dispersion-corrected periodic density functional theory (DFT). We used various pairs of pseudopotentials and dispersion-corrected methods to calculate the exchange-correlation energies and long-range energies, respectively. We conducted the calculations in the ethylene-graphene equilibrium distance where vdW interaction as a long-range interaction was dominant. Both adsorption and deformation energies were calculated to examine the possibility of ethylene chemisorption. It was found that there is a critical distance from the graphene surface, where the nature of adsorption of adsorbate molecule changes from physisorption to the possible chemisorption depending on the energetical costly distortion induced in adsorbate molecule. In the case of ethylene adsorption on the graphene structures studied here, the mentioned critical distances follow the order SVG < DVG < PG. However, in the range of vdW domination and in comparison with PG, ethylene interacts more with SVG due to the presence of a dangling bond and interacts less with DVG due to the presence of a hole. Furthermore, the interactions of ethylene with reconstructed trivacancy were studied. Moreover, all possible orientations for ethylene adsorption on graphene structures were considered and energetically compared. All calculations were done on fully optimized reconstructed geometries of vacancies with structural characteristics, i.e., reconstruction length and formation energies comparable to those reported in the literature.
Computational chemistry & molecular modeling
Naeimeh bahri-Laleh; Laura Falivene; Luigi Cavallo
Abstract
In this study we have tested the ability of a standard DFT computational protocol to reproduce the experimentally obtained stereoselectivity of 26 different C2-symmetric zirconocene catalysts active in propylene polymerization. The catalysts were chosen for their relevance in metallocene catalyzed ...
Read More
In this study we have tested the ability of a standard DFT computational protocol to reproduce the experimentally obtained stereoselectivity of 26 different C2-symmetric zirconocene catalysts active in propylene polymerization. The catalysts were chosen for their relevance in metallocene catalyzed polymerization of propylene. To this end, primary insertion of both si- and re-propylene enantiofaces into the Zr-CH2-CH(CH3)2 bond was considered to simulate the growing chains step. The energy difference between these two transition states, ΔEre-si, was taken as a measure of the stereoselectivity (pentad: mmmm%) of different catalysts. The results clearly indicated that there was a good agreement between ΔEre-si and the mmmm% values, so that greater ΔEre-si could correspond to higher mmmm%. A model was fitted to the experimentally obtained mmmm% against theoretical ΔEre-si. The coefficient of determination (R2) of the resultant plot was 0.9793, which indicated a good accuracy of the model. Finally, to quantify the steric role of the studied ligands in the observed stereoselectivity, the analysis of the buried volume (VBur) and of the steric maps was performed for two representative complexes. The images revealed that a greater asymmetric localization of the %VBur around the metal center led to a higher mmmm% in the resultant polymer.
