The micellization kinetics, dynamics 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. This is the first such study on the aggregation of synthetic polyelectrolyte block copolymer in aqueous solution carried out by atomistic MD simulation in explicit solvent. 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, which is in qualitative agreement with observations of the DPD simulation study on model copolymer micelles available 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. With respect to the fraction of the PS block (XPS) the conformational relaxation time increases linearly for PS and is invariant for PAA. 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 exponentially with increase in XPS ,which is in agreement with results in the literature available for 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 explicit atomistic MD simulations

256th National Meeting (2018)

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