Session N29: Block Copolymers II
8:00 AM–10:48 AM, Wednesday, March 23, 2005
LACC Room: 504
Sponsoring Unit:
DPOLY
Chair: Thomas Epps, NIST
Abstract ID: BAPS.2005.MAR.N29.2
Abstract: N29.00002 : Visualizing worm micelle dynamics and phase transitions of a charged diblock copolymer in water
8:12 AM–8:24 AM
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Abstract 
Authors:
Dennis Discher
Yan Geng
Assemblies of block copolymer amphiphiles are sometimes viewed as glassy, frozen, or static colloids, especially in strongly segregating solutions. Here we visualize by fluorescence microscopy and AFM the dynamics and transitions of single cylindrical micelles and vesicles composed of a charged diblock copolymer in water. In mapping the salt- and pH-dependent phase diagrams of a near-symmetric diblock of polyacrylic acid--polybutadiene, low pH and high salt (NaCl, CaCl)$_{2}$ neutralize and screen the charged corona sufficiently to foster membrane formation and generate vesicles. Decreased salt and neutral pH increases intra-coronal repulsion and drives a transition to multi-branched cylinders and highly stable, but fluid and flexible worm micelles. Ca$^{2+}$ both stiffens cylinders and stabilizes them relative to spheres. Further increase of intra-coronal repulsion generates spherical micelles by fragmentation and pinch-off at the ends of worms. Both transition kinetics and phase diagrams indicate divalent cation is about 5-10 fold more effective than monovalent in stabilizing all non-spherical morphologies.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2005.MAR.N29.2