Anomalous diffusion of polystyrene from an attractive substrate based on all-atom simulation

Abstract

The diffusion of polystyrene (PS) polymer chains from a hydroxy (-OH)-terminated Si surface with different grafting densities φG is studied based on all-atom simulation. Our particular attention is paid to the impact of the attractive substrate on the diffusive and configurational properties of PS. Our simulation results uncover a very novel and unexpected modification to polymer diffusion with the increment of φG, namely, the diffusion is slowed down most significantly from a substrate with moderate grafting densities, while in lower or full grafting cases, the diffusive dynamics is even facilitated rather than retarded. The underlying mechanism is investigated in terms of energy and conformational change in detail. Surprisingly, we obtain a consistent scenario for diffusion. Under moderate grafting densities, the energy required to be overcome for diffusion is relatively large. In addition, PS chains are more likely to be in a stretched configuration subject to a slower relaxation. These facts can account for the hindered diffusion. While under lower or full grafting densities, the energy required for diffusion becomes even smaller than the ungrafted situation. Also, PS chains prefer a shrinking configuration undergoing faster relaxation. Consequently, the diffusion of PS is reasonably promoted.

Impact of surface diffusion on transport through porous materials

“…P02, P = 6), the effect of the number of OH groups per nm 2 is less pronounced; this indicates that the surface diffusion mechanisms depend strongly on the number of OH groups per nm 2 but also on the size of the molecule. Zhang et al [35] studied by means of simulation the diffusion of polystyrene on silica surfaces grafted with different densities of hydroxyl groups. These authors showed that, at low or full grafting, diffusion is facilitated compared to moderate grafting density because the activation energy required to induce diffusion is smaller and polystyrene chain prefer a shrinking configuration leading to faster diffusion.…”

Novel Diffusion Mechanism of Polymers Pinned to an Attractive Impurity

“…Nevertheless, advances in how polymers and long molecules in general diffuse at interfaces have been consistently accomplished [ 3 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 16 , 17 , 20 , 21 , 22 , 23 , 24 ]. For example, for polymers of N monomers (or subunits) diffusing on hard surfaces, the diffusion coefficient scales as and is usually denominated by Rouse regime or Langevin dynamics [ 6 , 14 , 25 , 26 ].…”

Water Dissolved in a Variety of Polymers Studied by Molecular Dynamics Simulation and a Theory of Solutions

“…Actually, the dissolution free energy and diffusion coefficient reflect sensitively the intermolecular interaction between the permeant and polymer and the mode of aggregation of the polymer medium (crystalline vs amorphous, for example). The atomic-level description is thus desired for the permeant and polymer toward systematic development of a separation membrane. − …”

“…The degree of polymerization is up to ∼10 3 in atomistic simulations, − and the typical time for sampling is less than a microsecond. Although the (local) dynamics of a polymer system and the solubilities and diffusivities of water and gas are captured at these scales of length and time, ,,,,,,,,− ,, a question arises as to the scale determining the interaction of a permeant in polymer. The purpose of the present work is to address this question by pursuing the structural unit of a polymer that is responsible for the free energy of dissolution.…”