Swollen Vesicles and Multiple Emulsions from Block Copolymers

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Entry by Zin Lin, AP 225

General Information

Authors: Ani T. Nikova, Vernita D. Gordon, Galder Cristobal, Maria Ruela Talingting, David C. Bell, Cara Evans, Mathieu Joanicot, Joseph A. Zasadzinski, and David A. Weitz

Title: Swollen Vesicles and Multiple Emulsions from Block Copolymers

Keywords: Micelles, emulsification, amphiphilic, hydrophobic, vesicle, polymers

Summary and Discussion

The paper reports on the formation of novel vesicle morphologies by engineered composition of amphiphilic block copolymers and hydrophobic homopolymers.

Dry thin films of 50-50 diblock copolymer PBA-PAA (chemical name: poly(butyl acrylate)-b-poly(acrylic acid) ) mixed with various proportions of homopolymer hPBA are rehydrated by deionized water under bubbling nitrogen. The paper reports on the observation of the following structures:

Fig 1. Swollen micelles with "stuffed" hydrophobic homopolymers. Fig taken from Ref [1]

(1) a high density of "stuffed" swollen micelles. The authors suggest that most of the homopolymers mixed into the diblock copolymer get emulsified into the cores of diblock micelles. In order to verify their surmise, the authors made the samples with tagged hPBA, passed them through a series of membrane filters with smaller and smaller pores and measured sample fluorescence after each filtering pass. Most of the fluorescence occur in structures that pass through 100-800 nm pores, in good agreement with the TEM-imaged sizes of "swollen" micelles (Fig 1).

(2) "multiple" emulsions: micron-sized objects containing several droplets of water within a larger sphere of hydrophobic polymer (hPBA) with interfaces stabilized by the amphiphilic diblock (PBA-PAA). This internal multi-droplet structure is best imaged by encapsulated fluorescent water/dye (Fig 2).

Fig 2. Multiple emulsions with internal water/dye droplets seen by fluorescence. Fig taken from Ref [1]

(3) homopolymer hPBA incorporated into membrane bilayers. This can be observed in vesicles whose membranes flouresce with tagged hPBA (Fig 3). The authors also suggest that one might be able to engineer membrane thickness through controlled content of the homopolymer.

Fig 3. Homopolymers incorporated into membrane seen by fluorescence. Fig taken from Ref [1]

Micellization, vesicular formation and emulsification of amphiphilic molecules in aqueous mixtures is one of the most interesting yet far from established areas of soft-matter physics. The observations reported in the paper are yet another evidence of intriguing complexity manifested by these systems. Even more intriguing is the engineering possibilities opened up by the authors' work.

References

[1] Ani T. Nikova et al. Swollen Vesicles and Multiple Emulsions from Block Copolymers, Macromolecules 2004, 37, 2215-2218.