Ordering of urchin-like charged copolymer micelles: Electrostatic, packing and polyelectrolyte correlations

We report the results of small-angle neutron scattering (SANS) studies on aqueous solutions of spherical polyelectrolyte micelles formed by association of charged-neutral diblock copolymers. The neutral moieties are found to self-assemble into small dense spheres (cores of the micelles) whose sizes are independent of the polymer concentration c. In the dilute regime, c<c ^*, where c^* is the overlap concentration of the micelles, the conformation of the charged groups, which form the corona of the micelles, is found to be extended. A liquid-like order is observed over a wide concentration range spanning from the dilute regime to the concentrated regime. For c>c ^*, polyelectrolyte correlations appear at smaller spatial scales and coexist with the liquid-like order. These results suggest that for dense brushes, above c^*, the rod-like statistics of the charged chains begin to disappear due to contraction of corona arms or by interpenetration of coronae. For less dense brushes, the charged chains are found to be extended up to concentrations far above c^*, before the progressive development of polyelectrolyte correlations. Received 8 October 1999     

Photolabeling studies of polymer–cell surface interaction. Evaluation of 3-[(4-azidophenyl)dithio]propionic acid as a polymer-bound photolabel

3-[(4-Azidophenyl)dithio]propionic acid ( 1a ) was prepared in four steps from 4,4′-diamino-diphenyldisulfide. Attachment of 1a to poly[(3-hydroxypropyl)oxirane] was accomplished under very mild conditions via an acid-catalyzed caarbodiimide coupling. Photolysis of polymer-bound 1a with an electronic flash unit proceeded without detectable disulfide bond cleavage. Mild reduction of the disulfide bond of an analogue of 1a which carried no azido group confirmed that 1a should be useful in photolabeling studies of polymer–cell surface interaction.     

Template-directed olefin cross metathesis

[reaction: see text] A template containing two secondary dialkylammonium ion recognition sites for encirclement by olefin-bearing dibenzo[24]crown-8 derivatives has been used to promote olefin cross metatheses with ruthenium-alkylidene catalysts. For monoolefin monomers, the rates of metatheses and yields of the dimers are both amplified in the presence of the template. Likewise, for a diolefin monomer, the yield of the dimer is enhanced in the presence of the template under conditions where higher oligomers are not formed.     

New SFA techniques for studying surface forces and thin film patterns induced by electric fields

We describe two ways to measure normal and/or lateral forces between two surfaces in a surface forces apparatus (SFA) while an electric field is applied between the surfaces. The first method involves depositing thin conductive layers on the exposed substrate (usually mica) sheets; the second involves using the optically reflecting silver layers on the back surfaces of the sheets as the electrodes. Two types of experiments were performed using these new techniques: (1) measuring the effects of an electric field on the rheology of an approximately 40-microm-thick film of zeolite particles suspended in silicone oil and (2) a dynamic study of electric field-induced pattern formation of a thin polymer film. In the first study, under an electric field of strength approximately 106 V/m the shear force or effective viscosity of the colloid suspension was found to be two orders of magnitude higher than in the absence of the field, when the expected bulk value was measured. In the dynamic study, the initially uniform film transformed into a 2-D honeycombed network of depressed cells bounded by elevated ridges that grew slowly with time in a way consistent with previously derived theories. The new techniques should be applicable to studies of other systems and interactions, such as double-layer forces, micro- and nanoelectrorheology, electric field-induced ordering of particles, and the effects of electric fields on adhesion, friction, and lubrication.