Solvent as a Tool to Tune the Association of Structured Ionic Block Co-Polymers: Neutron Scattering and Computational Study
Date of Award
Doctor of Philosophy (PhD)
Gary S Grest
Rhett C Smith
The work probes the behavior of associating polymers including their assembly in different environments, using neutron scattering techniques coupled with molecular dynamics (MD) simulations. Polymers interact with their surroundings through van der Waals forces and through stronger association groups such as ionizable groups and p-p stacking, as well as specific chemical binding, where assembly depends on the strength of the interactions of the associating groups as well as the interactions of the polymers with their solvent environment. The current effort centers on understanding the assembly of structured polymers that consists of multiple blocks or components, each with their distinct interactions with solvents. The main body of the work focuses on the assembly of a multi- functional ionic polymers of the form ABCBA in which the center block is a sulfonated polystyrene (C) that enables transport tethered to, B, a polyethylene propylene (PEP) block, terminated by A, a t-butyl polystyrene (t-BPS) block. These polymers find broad uses in transport-controlled applications such as clean energy, separation membranes, and biotechnology. The aggregation of this polymer is driven by segregation of the ionizable block from the rest of the polymer as well as the interactions of each bock with solvents. The first part introduces experimental studies of assembly of this polymer as the solvent polarity is changed, followed by atomistic MD simulations insight into the assembly process. A more general insight into the assembly process is obtained by coarse grained MD The structural SANS studies have shown that the polymer forms core-shell aggregates with the ionic blocks in the core of the micelles in non-polar solvents such as cyclohexane. These micelles become gradually elongated with the addition of propanol to a propanol fraction of about 0.4. This change in shape of the micelles is driven by increasing of core-corona interfacial energy while collapsing the non-polar segments. Further increase in propanol results in reentrance to spherical micelles but with a smaller number of polymer molecules and significantly higher portion of solvent in the core. Solvent tuning of assembly to pentablock copolymer was further probed by fully atomistic MD simulation in cyclohexane, THF and propanol, solvents with different polarity. We find that the structure of the assemblies is driven by the different binding affinities of the solvents with polar and non-polar segments as well as the ionic fraction. Cyclohexane predominantly resides in the non-polar segments that are swollen, while the ionic blocks remain segregated in the micellar core. In contrast to cyclohexane, propanol and THF, which have an affinity towards both the ionic and non-ionic segments, swell the ionic blocks. With increasing sulfonation, the ionic blocks form a more stable spherical ionic core with cyclohexane associating around the core while THF and propanol penetrate into the core. To further understand the interactions of this structured block co polymer interactions with solvents, a thin polymer film in contact with solvent films were prepared, and the solvents were followed as they propagated across the interfaces, using MD simulations. We observed that exposure of water to pentablock copolymer membrane
decreases the interfacial width, exposing more ionizable groups whereas the interfacial width for the film in contact propanol and THF increases and is dominated by hydrophobic blocks. Water molecules associate predominantly with the ionic blocks while propanol and THF reside in both the ionic and non-ionic segments.In order to understand the effects of associating groups in a more general way, coarse grained MD simulations of association were carried out. The polymer chains are modeled by a bead-spring model and the associating groups are incorporated in the form of associating beads with a stronger interaction strength between them than between the non-associating beads. The structure and dynamics of linear and star polymer melts was followed as a function of the interaction strength of the associating beads. The results show that addition of even a small number of associating groups has dramatic effects on the mobility and viscoelastic response of polymer melts. The associating group aggregate forming a polymer network. With increasing interaction strength between the associating beads, the mobility of the chains decreases. Blends of chains with and without associating groups macroscopically phase separation even for relatively weak interaction between the associating beads. To the last part of the work was focused on understanding the effects of associating groups in soft nanoparticles. For this purpose, we synthesized polyparaphenylene ethylene (PPE) with biotin groups attached to the side chains with the ultimate goal of understanding the effect of associating groups on structure and dynamics of biocompatible soft nanoparticles. The last chapter describes the synthesis of biotin substituted PPEs, where the effect of biotin groups on assembly of PPE will be carried out in the future.
Senanayake, Manjula, "Solvent as a Tool to Tune the Association of Structured Ionic Block Co-Polymers: Neutron Scattering and Computational Study" (2020). All Dissertations. 2739.