Surface enrichment driven by polymer topology

We report a molecular dynamics simulation study of free-standing films of a blend of linear and cyclic polymer chains. We find that the composition of linear chains at the interface is enhanced relative to their bulk value for short chains but is depleted for long chains. Our findings are in agreement with recent experimental evidence reported for blends of short linear and cyclic polystyrene chains and highlight the genuine surface behavior in the short chain-length regime where theoretical predictions are more difficult. We highlight surface enrichment at low-energy surfaces as the result of competition between different entropic and enthalpic contributions to the interfacial free energy of the system.

Figure 1

Local composition of linear polymers as a function of distance ($z^{*}$) from the interface. Data are scaled by the bulk composition $c_0=0.7$.

Figure 2

(a) Parallel component of the radius of gyration $R_g^P$ as a function of of the geometric center of mass of individual chains in the $z$ direction for linear (full lines) and cyclic (dashed lines) polymers. Pair of curves starting at $z^{*}=0.5$ and 2.5 are for $N_b=10$ and 100, respectively. (b) Same as left panel but for the transverse component $R_g^T$.

Figure 3

End (a) and middle (b) number of beads of linear polymers within a slab of size $\sigma$ (and normalized to their bulk value) for $N_b=10$ and 100 as a function of $z^{*}$.

Figure 4

(a) Sketch of a configuration for a linear polymer with chain ends attached to the interface and middle bead close to the interface. (b) Same as in panel (a) for the case in which the middle bead is far away from the interface.

Figure 5

Energy per bead as a function of $z^{*}$.