Bhuwan Poudel

PhD Student
Member of the GRK2516
Group: Prof. Kurt Kremer, MPI-P Physics
Contact: E-mail

Research Project Area A: Nanoparticles – Theory
Ordering of nanoparticles on switchable soft surfaces: Computer simulation studies

Intentionally introducing nanoparticles (NPs) into a polymer brush creates hybrid materials whose applications can be expanded from information storage devices to sensors [1,2]. Knowing the spatial arrangement of NPs in a brush is particularly important because their local ordering determines their applications. This project aims to study the organization of NPs on/in polymer brushes. The ordering of NPs can be realized by tuning the polymer-NP enthalpic interaction[2], brush density [3], or the size of NPs [4]. On the other hand, one can modulate the conformation of polymers in a brush by perturbing the environment via external stimuli such as temperature, solvents, salt types, and so on. With the alteration of brush conformation, the effective interaction between NPs can be tuned [5] and eventually their assembly.
The monomer-NP attraction does not always mean that NPs infiltrate the brush. In this context, a delicate balance between entropy and enthalpy plays an important role. In some previous studies, it has been pointed out that the interactions between NPs and a brush can lead to interesting pattern formation of NPs [1,6]. In order to better understand the complex interplay between NPs and polymer brushes, a series of molecular dynamics simulations are carried out by varying NP coverage and their strength of attraction with brush monomers. The polymers in the brush are modeled using coarse-grained bead-spring model [7] with weak bending stiffness, and the NPs are treated as smooth spheres of diameter seven times larger than a bead size. Our approach here is as follows: we systematically vary the strength of monomer-NP attraction and the NP coverage. Our preliminary observation shows a strong dependence of monomer-NP interaction on arrangement of NPs. For a dense brush, the appropriate combination of monomer-NP interaction results in the adsorption of NPs on top of the brush. On the other hand, for a strong monomer-NP attraction and for all coverage, NPs are engulfed by the brush, where they tend to aggregate.

Figure 1. Schematic illustration of the project. NPs are adsorbed onto a polymer brush, where they may aggregate on top of the brush (2D assembly) or embed inside the brush (3D assembly), depending upon brush properties, the size of the NPs, or the interaction between the brush and the NPs. Changes in factors such as solvent, temperature, and others cause a brush to change its conformation, which is thought to affect the NPs’ ordering.

Figure 2. Snapshots showing the brush/NP hybrid system at equilibrium. For a fixed coverage, increasing monomer-NP attraction results in the aggregation of NPs from their dispersed states. The light orange spheres represent the brush monomers and the blue spheres represent the NPs. The degree of polymerization, grafting density, and size of a NP, respectively, are N = 80, σg = 0.245σ−2, and dNP = 7σ.

[1] S. Cheng, M. J. Stevens, and G. S. Grest, J. Chem. Phys. 147, 224901 (2017).
[2] S. Diamanti, S. Arifuzzaman, J. Genzer, and R. A. Vaia, ACS Nano 3, 807 (2009).
[3] S. Christau, T. M¨oller, Z. Yenice, J. Genzer, and R. von Klitzing, Langmuir 30, 13033 (2014).
[4] J. Yaneva, D. Dimitrov, A. Milchev, and K. Binder, J. Colloid Interface Sci. 336, 51 (2009).
[5] Z. Lian, S. Qi, J. Zhou, and F. Schmid, J. Phys. Chem. B 119, 4099 (2015).
[6] O. Guskova, S. Pal, and C. Seidel, EPL 88, 38006 (2009).
[7] K. Kremer and G. S. Grest, J. Chem. Phys. 92, 5057 (1990).