Waterlike anomalies in hard core–soft shell nanoparticles using an effective potential approach: Pinned vs adsorbed polymers

Abstract

In this work, a two dimensional system of polymer-grafted nanoparticles is analyzed using large-scale Langevin dynamics simulations. Effective core-softened potentials were obtained for two cases: one where the polymers are free to rotate around the nanoparticle core and a second where the polymers are fixed, with a 45° angle between them. The use of effective core-softened potentials allows us to explore the complete system phase space. In this way, the PT, Tρ, and Pρ phase diagrams for each potential were obtained, with all fluid and solid phases. The phase boundaries were defined analyzing the specific heat at constant pressure, system mean square displacement, radial distribution function, and discontinuities in the density–pressure phase diagram. Also, due to the competition in the system, we have observed the presence of waterlike anomalies, such as the temperature of maximum density (TMD)—in addition with a tendency of the TMD to move to lower temperatures (negative slope)—and the diffusion anomaly. Different morphologies (stripes, honeycomb, and amorphous) for each nanoparticle were observed. We observed that for the fixed polymer case, the waterlike anomalies are originated from the competition between the potential characteristic length scales, while for the free to rotate case, the anomalies arise due to a smaller region of stability in the phase diagram, and no competition between the scales was observed.