Shin-Pon Ju

Miscibility of graphene and poly(methyl methacrylate) (PMMA): molecular dynamics and dissipative particle dynamics simulations

Nanomaterial–polymer nanocomposite materials have attracted much interest due to their advantageous polymer properties and ability to improve the properties of nanomaterials. The distribution of nanomaterials in polymer materials strongly affects the enhancement of these properties. If the nanomaterials aggregate, this enhancement diminishes. In this study, a dissipative particle dynamics (DPD) simulation is used to investigate the structure of graphene–poly(methyl methacrylate) (PMMA) with different volume fractions (1 : 9, 1 : 14.29, 1 : 24 and 1 : 49) of graphene. As the immiscibility of graphene in the PMMA polymer induces phase separation, the repulsive parameters between PMMA and graphene were modified to represent different degrees of functionalization to find what repulsive parameter value will cause functionalized graphene to disperse in the PMMA polymer. We found that when the volume fraction of graphene is higher, the graphene needs to be functionalized more to achieve dispersion. In addition, different concentrations of functionalized graphene (30%, 50%, 70%, 90% and 95%) are studied, with results showing that if the graphene is more compact, a higher concentration of functionalized graphene is needed to disperse the graphene in the PMMA polymer system.

Observation of the amorphous zinc oxide recrystalline process by molecular dynamics simulation

The detailed structural variations of amorphous zinc oxide (ZnO) as well as wurtzite (B4) and zinc blende (B3) crystal structures during the temperature elevation process were observed by molecular dynamics simulation. The amorphous ZnO structure was first predicted through the simulated-annealing basin-hopping algorithm with the criterion to search for the least stable structure. The density and X-ray diffraction profiles of amorphous ZnO of the structure were in agreement with previous reports. The local structural transformation among different local structures and the recrystalline process of amorphous ZnO at higher temperatures are observed and can explain the structural transformation and recrystalline mechanism in a corresponding experiment [Bruncko et al., Thin Solid Films 520, 866-870 (2011)].

The Diffusion Behaviors of Hydrogen Atoms Inside an Octahedral Pd Nanowire

In our previous study, we have proven two ultrathin Pd nanowires can prevent from Pd embrittlement. In this study, we further demonstrate the material properties of a larger Pd nanowire. Both classical potential function and density functional theory calculation were used. The results show once the larger Pd nanowire has interior Pd atoms, hydrogen atoms can stably stay within the nanowire. Consequently, the hydrogen embrittlement could happen with a higher probability than two ultrathin Pd nanowires reported in our previous study.

Effect of Au nanotube size on molecular behavior of water/ethanol mixtures

The molecular behavior of water/ethanol mixtures of different weight fractions inside Au nanotubes of different radii were investigated by molecular dynamics simulations. Three different weight fractions of water/ethanol (25/75, 50/50, and 75/25) and radii of Au nanotubes (13, 22, and 31.1 A) were considered in order to understand the effects of Au nanotube size and water/ethanol fraction on the structural and dynamical behaviors of the water and ethanol molecules. The density profiles show two shell arrangements inside the Au nanotubes because water molecules prefer to adsorb onto the wall of the Au nanotube. According to the density distribution, the space inside the Au nanotubes can be divided into three regions, those of contact, transition, and bulk regions, in order from the interior wall surface to the nanotube center. The bulk region has a lower local weight fraction compared to the system water/ethanol weight fraction. Meanwhile, the local water/ethanol weight fraction in the contact region is higher than that of the system. When the system water/ethanol weight fraction becomes higher, the local water/ethanol weight fraction also becomes higher. In the transition and bulk regions, diffusion coefficients for water and ethanol molecules become higher due to the weak interaction with Au atoms. The values of diffusion coefficients for water molecules in the contact regions are much lower than for those in other regions and are similar for different system water/ethanol weight fractions due to the strong interaction with Au atoms. When the radius of the Au nanotube becomes larger, the values of local weight fraction inside the larger radius Au nanotube become higher than those inside small radius Au nanotubes because the ratio of water number to the nanotube inner surface area becomes higher. In addition, water inside a larger radius Au nanotube has a shorter water–water hydrogen bond lifetime (H-bond) in the contact region because the smaller curvature causes a weaker interaction with Au atoms.