Morphological transitions in polymer vesicles upon bilayer swelling with small hydrophobic molecules in water

Abstract

We show that when unilamellar polymer vesicles dispersed in water made from a blockcopolymer, in this case poly((ethylene oxide)45-block-(methyl methacrylate)164), poly((ethylene oxide)45-block-(methyl methacrylate)170), or poly(n-butyl methacrylate)81-block-(2-(dimethylamino)ethyl methacrylate)20, are exposed to small hydrophobic molecules, here methyl methacrylate as well as n-butyl methacrylate, they can undergo morphological transitions. Upon swelling, the polymersomes lose their original simple bilayer morphology and transform into more complex coil-like and patchy colloidal structures, as investigated experimentally by cryogenic electron microscopy (cryo-EM). Dissipative particle dynamics (DPD) simulations on a model flat bilayer indeed show that transitions can occur upon bilayer swelling, which is accompanied by a change in the mechanical bilayer properties. The transition involves the formation of water pockets in the interior regions of the bilayer. Co-existence of the various morphologies in the experiments suggests an activation barrier towards morphological changes and a possibility of multiple meta-stable states. The latter indeed is supported by the existence of multiple minima in the surface tension as a function of bilayer area, as found in the simulations

Shape Transformations of Vesicles Self-Assembled from Amphiphilic Hyperbranched Multiarm Copolymers via Simulation

“…Generally, liposomes self-assembled from phospholipids and polymersomes self-assembled from amphiphilic block copolymers are conventional model vesicles. Similar to human red blood cells, vesicles have shown a rich variety of shapes or shape changes in both experimental and theoretical simulation studies. ,− For example, Michalet et al observed equilibrium shapes of topological genus-2 (shapes with two holes) liposomes, which were found to be in agreement with theoretical predictions on the basis of a minimization of the elastic curvature energy for fluid membranes . Markvoort et al studied the liposome deformations on osmotic deflation using coarse-grained molecular dynamics simulations, and prolate ellipsoid, pear-shaped, and budded vesicles were formed by changing the pH or ion concentration .…”