Impact of Composition and Placement of Hydrogen-Bonding Groups along Polymer Chains on Blend Phase Behavior: Coarse-Grained Molecular Dynamics Simulation Study

Abstract

In this paper, we study symmetric polymer blends comprised of two polymer chemistries, one containing hydrogen-bonding (H-bonding) acceptor groups and another containing H-bonding donor groups to predict the blend morphology (i.e., two-phase, ordered/lamellar, disordered, disordered microphase-separated, and bicontinuous microemulsion or BμE) for varying compositions (i.e., fraction of monomers containing hydrogen-bonding groups along the polymer chain) and placements of hydrogen-bonding groups along the polymer chains. We use molecular dynamics (MD) simulations with a previously developed coarse-grained (CG) model that captures relevant macromolecular length and time scales and both the attractive directional interactions between H-bonding acceptor and donor groups and isotropic polymer–polymer interactions. We first validate our CG MD simulation approach by reproducing the published theoretical phase diagram for end-associating polymer chains at varying H-bonding strengths vs polymer segregation strengths. We also show that with increasing H-bonding strength, end-associating blends with short-chain lengths transition from two-phase to BμE or from disordered blends to BμE depending on the polymer segregation strength and finally to disordered microphase morphologies. End-associating blends with longer-chain lengths transition from two-phase to ordered lamellar phase at high polymer segregation strengths and from two-phase to disordered microphase-separated state at low polymer segregation strengths. Next, we study blends with the center placement of a single H-bonding group in each polymer chain as well as random and regular placements of multiple H-bonding groups per polymer chain. Regardless of the number and placement of H-bonding groups, with increasing H-bonding strength, the fraction of associated H-bonding groups increases with the system transitioning from blends of unassociated polymers to a mixture of associated copolymers and unassociated polymers and finally to a melt of fully associated supramolecular copolymers. At intermediate strengths of H-bonding, we observe BμE morphologies in all systems with end, center, random, and regular placements of H-bonding group(s). At high strengths of H-bonding, the blend morphology is disordered microphase-separated with domain sizes being smallest for the center placement, followed by the end, regular, and then random placements. We find that this variation in the placement of H-bonding groups leads to a greater change in domain sizes than with variation in the strength of the isotropic polymer–polymer interaction at constant H-bonding attraction. These trends in disordered microphase domain sizes with varying compositions and placements of H-bonding groups are linked to the supramolecular copolymer architecture formed upon the association of the two homopolymer chemistries. The polymers with the center placement of H-bonding groups form miktoarm star copolymers upon association, which show smaller domain sizes compared to diblock copolymers formed by polymers with end placement at the same molecular weight; in contrast, the polymers with random and regular placements of multiple H-bonding groups form nonlinear copolymer architectures with dispersity in block length leading to larger domain sizes. Overall, our work establishes design rules for incorporating H-bonding functional groups along polymer chains to achieve precisely tuned morphology and control over the disordered microphase domain sizes.

Topics & Concepts