Jintao Zhu

Spontaneous Generation of Amphiphilic Block Copolymer Micelles with Multiple Morphologies through Interfacial Instabilities

We introduce a method for the formation of block copolymer micelles through interfacial instabilities of emulsion droplets. Amphiphilic polystyrene-block-poly(ethylene oxide) (PS-PEO) copolymers are first dissolved in chloroform; this solution is then emulsified in water and chloroform is extracted by evaporation. As the droplets shrink, the organic solvent/water interface becomes unstable, spontaneously generating a new interface and leading to dispersion of the copolymer as micellar aggregates in the aqueous phase. Depending on the composition of the copolymer, spherical or cylindrical micelles are formed, and the method is shown to be general to polymers with several different hydrophobic blocks: poly(1,4-butadiene), poly(-caprolactone), and poly(methyl methacrylate). Using this method, hydrophobic species dissolved or suspended in the organic phase along with the amphiphilic copolymer can be incorporated into the resulting micelles. For example, addition of PS homopolymer, or a PS-PEO copolymer of different composition and molecular weight, allows the diameter and morphology of wormlike micelles to be tuned, while addition of hydrophobically coated iron oxide nanoparticles enables the preparation of magnetically loaded spherical and wormlike micelles.

Tuning the assembly of amphiphilic block copolymers through instabilities of solvent/water interfaces in the presence of aqueous surfactants

The influence of the water-soluble surfactant sodium dodecyl sulfate (SDS) on the self-assembly of amphiphilic block copolymers through hydrodynamic instabilities of organic solvent/water interfaces was studied. Micropipette aspiration measurements performed on evaporating chloroform droplets containing polystyrene-b-poly(ethylene oxide) (PS-PEO) revealed that interfacial instabilities were correlated to an approach of the organic/water interfacial tension to zero. The addition of SDS to the aqueous phase lowered the interfacial tension, thereby facilitating the onset of instability at lower concentrations of PS-PEO within the droplets. Further, increased amounts of SDS led to qualitatively different mechanisms of interfacial instability and correspondingly different morphologies of the resulting copolymer/surfactant assemblies. Similar, though less pronounced, effects were obtained using poly(vinyl alcohol) (PVOH) as a water-soluble surfactant, reflecting its more weakly concentration-dependent surface activity. These results enabled a single PS-PEO or polybutadiene-PEO block copolymer of fixed composition to be processed into aggregates that could be easily tuned from multi-vesicular particles to wormlike micelles and to spherical micelles.

Interfacial tension of evaporating emulsion droplets containing amphiphilic block copolymers: effects of solvent and polymer composition.

Evaporating droplets of volatile organic solvent containing amphiphilic block copolymers may undergo hydrodynamic instabilities that lead to dispersal of copolymer micelles into the surrounding aqueous phase. As for related phenomena in reactive polymer blends and oil/water/surfactant systems, this process has been ascribed to a nearly vanishing or transiently negative interfacial tension between the water and solvent phases induced by adsorption of copolymer to the interface. In this report, we investigate the influence of the choice of organic solvent and polymer composition for a series of polystyrene-b-poly(ethylene oxide) (PS-PEO) diblock copolymers, by in situ micropipette tensiometry on evaporating emulsion drops. These measurements suggest that the sensitivity to the organic solvent chosen reflects both differences in the bare solvent/water interfacial tension as well as the propensity of the copolymer to aggregate within the organic phase. While instabilities coincident with an approach of the interfacial tension nearly to zero were observed only for copolymers with PEO content greater than 15 wt.%, beyond this point the interfacial behavior and critical concentration needed to trigger surface instability were found to depend only weakly on copolymer composition.