Size Control and Fractionation of Ionic Liquid Filled
Polymersomes with Glassy and Rubbery Bilayer Membranes
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Abstract
We demonstrate control over the size
of ionic liquid (IL) filled
polymeric vesicles (polymersomes) by three distinct methods: mechanical
extrusion, cosolvent-based processing in an IL, and fractionation
of polymersomes in a biphasic system of IL and water. For the representative
ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)
imide ([EMIM][TFSI])), the size and dispersity of polymersomes formed
from 1,2-polybutadiene-<i>b</i>-poly(ethylene oxide) (PB–PEO)
and polystyrene-<i>b</i>-poly(ethylene oxide) (PS–PEO)
diblock copolymers were shown to be sensitive to assembly conditions.
During mechanical extrusion through a polycarbonate membrane, the
relatively larger polymersomes were broken up and reorganized into
vesicles with mean size comparable to the membrane pore (100 nm radius);
the distribution width also decreased significantly after only a few
passes. Other routes were studied using the solvent-switch or cosolvent
(CS) method, whereby the initial content of the cosolvent and the
PEO block length of PS–PEO were systemically changed. The nonvolatility
of the ionic liquid directly led to the desired concentration of polymersomes
in the ionic liquid using a single step, without the dialysis conventionally
used in aqueous systems, and the mean vesicle size depended on the
amount of cosolvent employed. Finally, selective phase transfer of
PS–PEO polymersomes based on size was used to extract larger
polymersomes from the IL to the aqueous phase via interfacial tension
controlled phase transfer. The interfacial tension between the PS
membrane and the aqueous phase was varied with the concentration of
sodium chloride (NaCl) in the aqueous phase; then the larger polymersomes
were selectively separated to the aqueous phase due to differences
in shielding of the hydrophobic core (PS) coverage by the hydrophilic
corona brush (PEO). This novel fractionation is a simple separation
process without any special apparatus and can help to prepare monodisperse
polymersomes and also separate unwanted morphologies (in this case,
worm-like micelles)