(495c) Molecular Simulations of Compressibility of Co-Polymer Brushes

Responsive polymer brushes are potential candidates for a variety of applications in molecular sensing and selective-gating in nano-structured materials. Responsive brush-like structures are also found in biology, in the cases of cytoskeletons of neurons and nuclear-pore complexes. In this work, we investigate the compressibility of end-tethered co-polymer brushes, composed of hydrophilic and hydrophobic segments, in solution using Langevin dynamcis simulations. Primarily, the simulations address the influence of three major parameters on brush compressibility, namely, the grafting density, the fraction of hydrophobic groups in a co-polymer, and the relative distributions of hydrophobic and hydrophilic groups along the polymer contour. We consider the following distributions of segments for any given fraction of hydrophobic groups, periodically spaced segments, randomly distributed segments, and block co-polymer arragement, with respective hydrophilic and hydrophobic blocks. The equilibrium heights of these brushes show a strong dependence on both fraction of hydrophobic groups and their distributions along the polymer contour. The first observation is that the brush height decreases in a sigmoidal manner for both periodic and random distributions with increase in hydrophobic fraction. Moreover, the equilibrium height values are nearly the same for both periodic and random distributions. Further, we study the mechanical response of these brushes when the brushes are compressed in a reversible manner with a non-interacting wall. Again, we find that the bulk moduli of the co-polymer brushes show a strong dependence on the fraction and distribution of hydrophobic groups. The bulk modulus increases monotonically with hydrophobic fraction. However, the bulk modulus variation shows two distinct regimes with respect to increase in hydrophobic fraction, a low slope regime for smaller hydrophobic fractions, and a high slope regime for larger hydrophobic fractions. We discuss these results with respect to decreased segmental diffusivity and an increase in inter-segment interactions in the co-polymer brush.