Kinetics and Mechanism of the Rod-to-Vesicle Transition of Block Copolymer Aggregates in Dilute Solution

Abstract

The kinetics and mechanism of the rod-to-vesicle transition in aggregates of polystyrene310-b-poly(acrylic acid)52 diblock copolymers in dioxane/water mixtures are explored. The transition is induced by a jump in the water content from a point at which the morphologies are under equilibrium control. The transition mechanism is monitored by transmission electron microscopy (TEM). The transition intermediates are trapped by quenching solution samples to liquid nitrogen temperature and then preserving the aggregate morphologies using the freeze-drying technique. It is shown that the morphological transition goes through a lamellar intermediate state. The corresponding kinetic data are obtained from turbidity measurements. The analysis of the plot of turbidity versus time suggests two consecutive first order steps. The first step appears to be the flattening of short rods in favor of irregularly shaped or circular lamellae, which are the intermediate morphologies observed using TEM. The second step involves the closing of the lamellae to vesicles. Since each of the three species, i.e., rods, lamellae, and vesicles, makes a distinct contribution to the turbidity, the changes in the contribution of each species to the turbidity as a function of time in the transition process can be calculated quantitatively. The kinetic analysis shows that the relaxation times (τ 1 and τ 2) of the morphological transition are influenced by the initial water content and the polymer concentration. The relaxation times increase with increasing initial water content, while the reciprocals of the relaxation times approach zero when the initial water content reaches the upper boundary of the rod stability region. An increase in the polymer concentration leads to an increase in the relaxation times. The size of the jump in the water content has a little effect on the kinetics.

Recent trends in the tuning of polymersomes’ membrane properties

“…For PS-b-PAA a rod-to-vesicle transition was also observed by increasing the water content. It was proposed that those experimental results support mechanism 1 to explain vesicle formation [21].…”

“…It was also shown that this approach can reproduce morphological transitions (micelles to cylinders to vesicles) by changing the interaction Flory-Huggin's parameter χ in the simulation, which reflects the change in solvent quality during the experiment [17]. The transition states created by a change in polarity of the solvent have been experimentally quenched and observed by TEM for polystyrene-block-poly(acrylic acid) block copolymer [21] and poly(ethylene oxide)-b-poly (3-(trimethoxysilyl) propyl methacrylate) (PEO-b-PTMSPMA) [22]. In both cases the addition of a selective solvent into polymer solution led to vesicular selfassembly.…”

Chemically induced supramolecular reorganization of triblock copolymer assemblies: Trapping of intermediate states via a shell-crosslinking methodology

“…The equilibrium between micellar assemblies and individual polymer chains involves a delicate balance of supramolecular polymerpolymer and polymer-solvent interactions, which provides a lever of enthalpic and entropic control (13) for the directed evolution of micellar morphologies with changes in the solution conditions. Studies to determine the kinetics of block copolymer micellization (14)(15)(16) and the kinetics and mechanisms for transformation of assembly morphologies (17)(18)(19) have become active areas of research. For a given block copolymer composition, the introduction of ions and alteration of the solution pH (20)(21)(22), modification of the solvent composition (23)(24)(25), and changes in the polymer concentration (26,27) have been found to effect reorganization of block copolymer supramolecular assemblies.…”

Macroporous materials by self-assembly of linear oligo(phenylsilsesquioxanes).

“…The literature reports evidence similar phenomena, leading to the formation of spherical structures with a random-coil backbone conformation, for oligomers with large aromatic [58,59], dendritic [60] or amphiphilic [61,62] side groups. Analogously, rigid rodflexible coil block copolymers can self-assemble in selective solvents into specific nanostructures with morphology governed by the geometric disparity between the rod and coil segments and anisotropic interactions between rod blocks (formation of liquid crystalline or crystalline structures) [63,64]. The process described herein is not typical, since microparticles are formed by rigid molecules with relatively small side substituents.…”