Vesicles Formed from a Poly(ethylene oxide)−Poly(propylene oxide)−Poly(ethylene oxide) Triblock Copolymer in Dilute Aqueous Solution

Abstract

Micelle formation of block copolymers in selective solvents (organic solvent or water) has been studied for many years.1-3 The morphology of polymeric micelles has been the subject of more recent investigations both theoretically and experimentally.4-8 Diblock copolymers with a long soluble block compared to the nonsoluble block may self-assemble into micelles of a starlike structure while copolymers with a short soluble block often form so-called crew-cut micelles.3,9 One class of water-soluble block copolymers that has attracted great interest in the literature is the triblock copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), often abbreviated as PEOPPO-PEO or EOa-POb-EOa. These copolymers exist in a wide range of different compositions and display a rich phase behavior in water depending on their relative block lengths.10,11 It is well established that the more hydrophilic copolymers associate into micelles, composed of a PPO core and a PEO corona, with a spherical or rodlike shape depending on temperature.7,12-14 The present work focuses on the investigation of one of the most hydrophobic PEO-PPO-PEO copolymers (L121) in dilute water solutions. New interesting structures found in the L121/water system are presented and visualized by the cryogenic transmission electron microscopy (cryo-TEM) technique. For the first time, to our knowledge, unilamellar vesicles are shown to exist in a triblock copolymer system of this type. Vesicles of branched PEO-PPO block copolymers in water have only been predicted as a metastable state in earlier model calculations.15 There are only a few reports in the literature on vesicle formation of diblock and multiblock copolymers in dilute solutions of selective solvents.16-21 Vesicles were one among several different crew-cut aggregates found in polystyrene-poly(acrylic acid) (PS-PAA) and PS-PEO diblock copolymer aqueous systems.9 These vesicular aggregates were prepared by a dialysis method. Both copolymers form so-called glassy vesicles, which are preserved in water since the water-insoluble PS block is in a glassy state at ambient temperatures. However, a recent study on a polybutadiene-PAA (PBD-PAA) copolymer/water system has concluded that the vesicular morphology is not controlled by the glass temperature of the nonsoluble block.22 Other publications have reported the existence of multilamellar block copolymer vesicles in a homopolymer-rich matrix as a selective solvent and in a concentrated ternary block copolymer system exposed to a shear flow.23-27 This communication presents a different and new preparation procedure of block copolymer vesicles than described in earlier work. The method uses an extrusion method often utilized for the preparation of single-tailed and double-tailed surfactant (or lipid) vesicles, during which the dispersed bilayers close into a spherical structure with a water interior.28 The formation of unilamellar surfactant vesicles normally requires the input of energy by extrusion, ultrasonication, or dialysis, although it has been suggested that they may form spontaneously in some mixed cationic and anionic surfactant systems.29 The PEO-PPO-PEO triblock copolymer, denoted Pluronic L121, was a gift from BASF corporation and used without further purification. It has a nominal molecular weight of 4400 g/mol, and the average composition is EO5PO68EO5, which corresponds to 10 wt % of PEO. L121 copolymers do not form micelles. At low concentrations and temperatures, unimers (individual copolymer chains) coexist with larger unimer aggregates (an L1 phase). The hydrodynamic radii (RH) of the unimers and the aggregates, obtained using dynamic light scattering (DLS) at 10 °C, are 1.9 and 44 nm, respectively. Figure 1 displays one part of the binary phase diagram of L121 in water. Because of the short hydrophilic PEO chains, and therefore the small headgroup area, the copolymers can easily pack into a lamellar phase (LR) which appears at higher copolymer concentrations (>55 wt %). The lamellar phase boundaries, as preliminary determined by small-angle X-ray scattering (SAXS), are also indicated in the figure. At low concentrations and at ambient temperatures, the system phase separates, and the phase separation curve determined by visual inspection is shown in Figure 1. The two-phase region is an equilibrium between the lamellar phase and the L1 phase. The L121 block copolymer vesicles were prepared at 25 °C from dilute block copolymer solutions in this L1/LR two-phase regime by repeated extrusion of the * To whom correspondence should be addressed. E-mail address: Karin.Schillen@fkem1.lu.se. Figure 1. Schematic phase diagram of L121 in water. The vesicles were prepared in the two-phase region (2φ) at 25 °C from aqueous L121 solutions with concentrations of 0.2, 0.4, 0.6, and 1.2 wt %. 6885 Macromolecules 1999, 32, 6885-6888