Spontaneous Generation of Multilamellar Vesicles from Ethylene Oxide/Butylene Oxide Diblock Copolymers

Abstract

Vesicular structures are promising materials for encapsulation applications, and vesicles composed from block copolymers are attractive in this regard because of their stability and tunability. However, vesicle formation from copolymers generally requires specialized procedures. This article demonstrates that ethylene oxide/butylene oxide diblock copolymers spontaneously form micrometer-sized, multilamellar vesicles ("onions") over a broad range of concentrations upon simple mixing with water. These structures likely form due to the relatively large volume-to-length ratio of the butylene oxide hydrophobe, and vesicle formation also depends on the length of the ethylene oxide headgroup. Structures were characterized using plane-polarized light microscopy, dynamic light scattering, and cryogenic scanning electron microscopy. The multilamellar vesicles survive sonication and moderate shear. Vesicle size can be reduced by extrusion through a porous membrane, but final diameter is independent of shear rate and membrane pore size.

Preparation of block copolymer vesicles in solution

“…Harris et al 74 synthesized a series of poly(butylene oxide)-block-poly(ethylene oxide) copolymers ranging in length from 10 to 12 units for butylene oxide and from 5 to 18 units for ethylene oxide. They obtained multilamellar vesicles at copolymer concentrations as low as 0.05 wt % and as high as 20 wt %.…”

“…The vesicles were also found to be resistant to sonication and moderate shear. 74 Recently, they formed 120 -175 nm vesicles from sulfonated butylene oxide oligomers ranging in length from 4 to 17 units for butylene oxide. 75 The vesicles were stable for at least 5 months at room temperature and were stable for 9 days at 100°C.…”

Scattering Measurements for High Throughput Materials Science Research

“…[4] Details of the synthesis and characterization techniques can be found in the literature. [3] A 20% solution of the block copolymer in deionized water was prepared, mechanically agitated for 30 s, and allowed to stand for a minimum of 24 h before examination. On inspection, the sample appeared opaque at room temperature.…”

“…Harris et al [3] established that the vesicle phase of this copolymer is stable at room temperature up to concentrations of 20%; hence, we study this system by SAXS/WAXS over the whole concentration window from 0 to 20%, incorporating 25 samples within this range.…”

Selecting analytical tools for characterization of polymersomes in aqueous solution

“…In the case of visualization methods, Small-angle x-ray and neutron scattering (SAXS and SANS) that were used in the study, the SDL is measured from own experiments. 49,63,64 Uncertainty in true size Transmission electron microscopy Cryo-TEM 0.1 Size, lamellarity, morphology Structure preservation Freezing artifacts 49,65 Uncertainty in bilayer thickness Scanning electron miscroscopy Freeze fracture Cryo-SEM 2 Size, lamellarity, morphology 3D appearance Freezing artifacts 66,67 Scanning electron microscopy Environmental SEM 30 Size, lamellarity, morphology, concentration Native environment Poor resolution 40,60,68 Electromagnetic manipulation methods Scanning probe microscopy 3D information High sensitivity to vibration Scanning probe microscopy Atomic force microscopy 1 Size, topology, elastic properties Sensitivity Shape alteration upon attachment 37,40,54,60,69 Scanning force microscopy Amphiphile adsorption on cantilever Scanning probe microscopy Scannig tunneling microscopy 0.1 Size, topology No mechanical contact to sample Cantilever tip condition crucial 70,71 Nuclear magnetic resonance P 31 -nuclear magnetic resonance Lamellarity High accuracy Signal decrease due to convenient buffer [72][73][74] Electron paramagnetic resonance Encapsulation, bilayer flexibility, 40,75,76 Electron spin resonance bilayer polarity, lamellarity Specific to unpaired electrons Signal decrease due to water Laser doppler electrophoresis Zeta potential, surface charge potential Fast Calibration required frequently 77,78 Optical 54,86 ever care has to be taken when interpreting polydisperse samples, which are the case with most polymers and most preparation methods. Small-angle X-ray 33 or neutron scattering (SAXS or SANS; resolution limit of both: 0.5 nm) 34 provide detailed information about the polymersome bilayer 35 , but due to the need for access to large scale radiation facilities, their use for routine measurem...…”