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 hydrogenbonding (H-bonding) acceptor groups and another containing Hbonding donor groups to predict the blend morphology (i.e., twophase, ordered/lamellar, disordered, disordered microphase-separated, and bicontinuous microemulsion or BμE) for varying compositions (i.e., fraction of monomers containing hydrogenbonding 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 Hbonding 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 twophase 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 Hbonding, 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 Hbonding 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 p...

Electrostatic Interactions Control the Nanostructure of Conjugated Polyelectrolyte–Polymeric Ionic Liquid Blends

“…We hypothesize that the microemulsion morphology of these CPCs is likely cocontinuous, rather than droplet or multilayer vesicle, due to the compositional symmetry of the blends and as suggested by simulation snapshots (Figure S6). An analogous morphology was predicted for polymer blends compatibilized by hydrogen bonding in a recent study, 57 where simulations predict a bicontinuous microemulsion structure for systems with intermediate hydrogen-bonding strength regardless of the placement of hydrogen-bonding sites along the polymer backbones.…”

“…Besides the Berk model, 55 which utilizes the generalized theory of spinodal decomposition to account for microemulsion scattering, the TS model is commonly used for systems that take on a microemulsion-like structure. 53,56,57 Geometric approaches that consider the emulsion to be a distorted lamellar structure 58 also result in a functional form of the correlation function similar to that derived by Teubner and Strey. The TS equation takes the following form…”

“…To quantitatively investigate the morphology of the CPCs in the aforementioned locally structured states, we fit their scattering traces to the Teubner–Strey (TS) model, which is based on a Landau expansion of a blend free energy. Besides the Berk model, which utilizes the generalized theory of spinodal decomposition to account for microemulsion scattering, the TS model is commonly used for systems that take on a microemulsion-like structure. ,, Geometric approaches that consider the emulsion to be a distorted lamellar structure also result in a functional form of the correlation function similar to that derived by Teubner and Strey. The TS equation takes the following form where a 2 , c 1 , and c 2 are the fitting parameters.…”

Phase Behavior and Morphology of Blends Containing Associating Polymers: Insights from Liquid-State Theory and Molecular Simulations

“…To overcome these limitations of CG models with isotropic potentials, past computational studies , have also employed orientation-dependent anisotropic potentials to describe interactions between beads participating in directional associations; however, these models suffer from increased computational complexity, making it prohibitive to run large-scale simulations to study the structure in associating polymers. Balancing computational intensity and the need to capture directionality, we − and others , have used CG isotropic potentials whereby the associating beads participating in directional interactions are modeled as smaller beads partially embedded within backbone polymer beads, and by optimizing the size, placement, and isotropic interaction strength, we modeled effectively directional interaction between the associating beads. In our past work, we used such a CG model capable of mimicking directional interactions using isotropic potentials to study blends containing two types of polymers that interacted with each other via directionally associating functional groups along the chains.…”

“…Balancing computational intensity and the need to capture directionality, we − and others , have used CG isotropic potentials whereby the associating beads participating in directional interactions are modeled as smaller beads partially embedded within backbone polymer beads, and by optimizing the size, placement, and isotropic interaction strength, we modeled effectively directional interaction between the associating beads. In our past work, we used such a CG model capable of mimicking directional interactions using isotropic potentials to study blends containing two types of polymers that interacted with each other via directionally associating functional groups along the chains. Using MD simulations, we showed the effect of various design parameters such as the polymer segregation strength, association strength, and placement and composition of associating segments along polymer chains on the blend morphology.…”