Thermodynamic Insights into the Translocation of a Star Polymer Through a Nanopore

The thermodynamics of voltage-driven translocation of a homogeneous star polymer through a nanopore is studied using a bead-spring model. The star polymer is modelled as f identical linear chains with one of their ends connected to a central bead. A single reaction coordinate based on the count of beads entering the nanopore is used for describing the process. The free energy landscape of the process is computed using the enhanced sampling method of metadynamics. A comparison of the free energy landscapes for three polymer architectures of identical molecular weights, viz. a linear polymer, a three-arm star polymer and a four-arm star polymer, reveals interesting insights into their mechanisms of translocation.

The free energy landscape of all three polymer architectures shows a prominent free energy barrier as the polymer fills the nanopore, which is attributed to the loss of entropy of the polymer segments inside the nanopore. The free energy landscape of a three-arm star polymer also exhibits a larger second barrier when central bead of the star polymer enters the nanopore. Additional barriers are present in the energy landscape of a four-arm star polymer. The magnitude of all these free energy barriers is found to be stronger for narrower nanopores. The mean first passage time estimated from the computed free energy using Fokker-Planck formalism is found to be in good agreement with the mean translocation time obtained using Langevin dynamics simulation.