Idealized Hydrogen Bonding Yields Negative χ in Simulated Polymer Blends

Abstract

The Flory–Huggins interaction parameter χ quantifies the excess mixing free energy for polymer blends. Without specific attractive forces, χ is typically positive, and polymers tend to demix. However, polymers with hydrogen bond donor moieties attract polymers with acceptors, resulting in a negative χ that favors mixing. Here, we investigate a bead-spring model of hydrogen bonding polymers, in which chains have charged pendant side beads and oppositely charged backbone beads, for an overall neutral chain. We study equimolar blends of two species of chains, with identical architecture but reversed charge. To compute the mixing free energy of these blends, we employ thermodynamic integration or “morphing” with respect to the background dielectric constant, which conveniently weakens Coulomb interactions, ultimately rendering the chains neutral and identical. We examine how χ depends on side bead charge and spacing, and find attractive χ values for sufficiently large side bead charges, provided that the side beads are not too closely spaced.