The clustering mechanism, micellization kinetics, and various dynamical aspects and diffusional properties of asymmetric and symmetric poly(styrene)-block-poly(acrylic-acid) polyelectrolyte-neutral block copolymer chains in salt-free aqueous solution were investigated as a function of the copolymer composition (XPS) for un-ionized (acidic, f = 0) and ionized (charged, f = 1) PAA block by molecular dynamics simulations. The mechanism of the formation of the micelle was tracked using the population of unimer and clusters across the simulation trajectory, which confirmed that the asymmetric copolymer micelle follows a combined approach of unimer insertion and cluster fusion mechanism, while the symmetric micelle follows unimer insertion method exclusively. Micelle formation takes a longer time for copolymers having charged (ionized) PAA block (f &gt; 0) and relatively short PS block (XPS &lt; 0.5) due to the presence of a greater number of hydrophobic groups, in qualitative agreement with observations from DPD simulation study of model uncharged copolymer micelles in literature. Conformational dynamics studied using relaxation of backbone dihedral angle and radius-of-gyration, specifically for core and corona blocks, shows a slower relaxation of the hydrophobic insoluble PS blocks as compared to the soluble PAA blocks. The interaction dynamics of PS-b-PAA copolymer micelles studied via the relaxation times of PAA-PAA inter-chain hydrogen bonds (τHB,PP) and PAA-water inter-molecular hydrogen bonds (τHB,PW) show an increase of τHB,PP and a decrease of τHB,PW , with increase in XPS. The relaxation of PAA-water h-bonds is slower at f = 1 than at f = 0, due to the stronger affinity of ionized PAA units to water molecules. The diffusivity of the micelle cluster decreases non-linearly (exponentially) with increase in XPS in agreement with results available in literature from coarse-grained and DPD simulations of model star polymer and copolymer micelles, as well as PFG-NMR measurements of PEO-b-PCL and POE-b-PDMS micelles in water.

Dynamics and mechanism of polyelectrolyte-neutral block copolymer micellization in aqueous solution by atomistic MD simulations

2019 Fall National Meeting

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