Mesoscopic study of salt-responsive polymeric micelles: structural inversion mechanisms via sequential addition of inorganic salts

Abstract

The structural inversion mechanisms of salt-responsive polymeric micelles formed by poly(N-(morpholino)ethyl methacrylate)-b-poly(4-(2-sulfoethyl)-1-(4-vinylbenzyl) pyridinium betaine) (PMEMA-b-PSVBP) diblock copolymer in different saline environments were explored from a mesoscopic point of view using dissipative particle dynamics (DPD) simulations and coarse-grained models. The results of mesoscopic simulations reveal that the PMEMA-b-PSVBP copolymer can generate stable spherical micelles with a specific structural conformation of the PSVBP-core and the PMEMA-corona in a purely aqueous environment at room temperature. The structural inversion of these polymeric micelles takes place via the sequential addition of inorganic salts (NaBr → Na₂SO₄, Na₂SO₄ → NaBr and a salt mixture of NaBr–Na₂SO₄) in the aqueous environment. Three structural inversion mechanisms were explored by means of mesoscopic simulations: (i) a structural inversion mechanism via micellar dissociation (NaBr → Na₂SO₄), (ii) a structural inversion mechanism via simultaneous ascent and immersion of polymeric segments (Na₂SO₄ → NaBr) and (iii) a structural inversion mechanism by means of a purely intermicellar fusion (a salt mixture of NaBr–Na₂SO₄). The transitory stages of each structural inversion mechanism are described and analyzed in this document. The structural inversion mechanisms explored in this work are dependent on the concentration and specific order in which the inorganic salts are added into the aqueous environment. The results obtained from these mesoscopic simulations can contribute to elucidating the structural inversion mechanisms of salt-responsive polymeric micelles and are consistent with available experimental outcomes.