Phase Behavior and Morphology of Blends Containing Associating Polymers: Insights from Liquid-State Theory and Molecular Simulations
A Kulshreshtha and A Jayaraman, MACROMOLECULES, 55, 9297-9311 (2022).
In this work, we study a symmetric blend of two linear polymers containing associating functional groups, which upon association form supramolecular copolymers with linear or nonlinear architectures. We use a coarse-grained (CG) model where each polymer backbone is represented as a chain of isotropically interacting unassociating and associating CG beads, with each CG bead representing a Kuhn segment. We first use polymer reference interaction site model (PRISM) theory to map out the blend phase behavior (i.e., two-phase, disordered/disordered microphase, or microphase-separated morphologies) as a function of association strengths (i.e., pair-wise attraction strength between associating CG beads) and polymer segregation strengths (i.e., effective interactions between unassociating CG beads). PRISM theory provides the liquid-state structure and length scales of concentration fluctuations but does not converge to a numerical solution when the blend undergoes microphase separation at high association strengths. At those higher association strengths where PRISM theory fails to converge, we perform molecular dynamics (MD) simulations to obtain the microphase domain sizes and information about the molecular packing around associating beads. We conduct this combined PRISM theory-MD simulation study for varying placements and compositions of associating beads (i.e., end versus center placement of one associating bead per chain and random versus regular placement of multiple associating beads along chains). We find similar trends in concentration fluctuation length scales and microphase-separated domain sizes in these blends with associating groups interacting via isotropic attraction to those interacting with directional attraction. Our past work focused on the directional attraction between associating groups had established that disordered microphase domain sizes are linked to the dispersity in arm length and branching in the associated copolymer architectures, which are affected by the placement and composition of associating groups along the polymer chains; these results are also seen with isotropic interactions. The one key difference between the directional association and isotropic association is that the former enforces a "monovalent" (one-one) interaction between associating groups, while isotropic association leads to multivalency (multiple beads associating together). This difference in the valency of associating groups does not change the dispersity in arm length in the associated copolymer between directional versus isotropic nature of association; however, isotropic interactions lead to larger dispersity in branching than directional interactions. The larger dispersity in branching leads to morphological differences in particular for the end associating polymer blends that upon association form miktoarm stars with isotropic association and linear diblocks with directional association. Despite these differences in dispersity in branching arising from the nature of association, we find the same trends, namely, smaller domain sizes for end and center placements of single associating group and larger domain sizes for the regular and random placements of multiple associating groups.
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