Aggregates composed of branched polymers of the type PEnPEPm (n, m ) 1, 2 with m + n ) 3,4), called miktoarm stars, in the selective solvent decane were investigated by small-angle neutron scattering using the contrast variation technique. The PE (polyethylene) arms were about 75% deuterated and of fixed molecular weight (7300), while the PEP [poly(ethylenepropylene)] chains were completely protonated and had molecular weights of 4900, 9100, and 15 700. The crystallization following the segregation of PE in decane drives the assembly process. As in the case of diblock copolymers PE-PEP, the miktoarm stars form lamellar structures with a flat dense crystalline PE core and a soft corona of PEP hairs sticking out on both sides of the core. Laterally, the aggregates are largely extended and modeled as thin disks. The densities of both the core and the corona were represented by step functions convoluted with a Gaussian each. The structural parameters (the thickness of the core and the extension of the hairs) were extracted using a model fitting. With increasing the PEP molecular weight, the average extension of the PEP chains in the corona increases and the thickness of the core decreases except for the symmetric architecture PE2 PEP2. The changes are different from those expected from an equivalent increase of molecular weights in a diblock. At room temperature almost all the polymer is found in the micelles. Thus, a thermodynamic description is based on the free energy of a single micelle. It yields scaling relations dependent on the star architecture. While the model worked well for the diblocks, it does not describe the core thickness variation correctly for the miktoarm systems.

No abstract

Two routes for the synthesis of star block copolymers having exactly two arms of polyisoprene (PI) and two of polybutadiene (PB) are described. The first route starts by condensing one molecule of silicon tetrachloride with two of poly(isoprenyl)lithium which has been end-capped by a few units of styrene. The second route starts by linking two poly(isoprenyllithium) chains with one molecule of silicon tetrachloride at a suitably reduced temperature. Both routes are completed by reaction of the two-armed intermediate (PI)2SiCl2 with an excess of poly(butadienyllithium). The resulting (PI)2Si(PB)2 star polymer was purified by fractionation. The reaction steps were monitored by size exclusion chromatography and the products characterized by low-angle laser light scattering, osmometry, and NMR.

The synthesis of well-defined, near monodisperse, nonlinear block copolymers of the A,B (3-miktoarm star copolymers), A3B (Cmiktoarm star copolymers), (AB),(BA), (inversed 4-miktoarm star block copolymers), and A3BA, (bridged miktoarm star copolymers) types is described. A is polyisoprene (PI) and B is polystyrene (PS). The synthetic approach involves the reaction of methyltrichlorosilane or tetrachlorosilane with monofunctional or difunctional macroanions of B under conditions unfavorable to chain coupling or linking, followed by addition of the monofunctional macroanion A. Characterization was carried out by size exclusion chromatography, low-angle laser light scattering, laser differential refractometry, membrane and vapor pressure osmometry, and NMR and UV spectroscopy. Microphase separation was studied by transmission electron microscopy and small-angle x-ray scattering. Comparison with the corresponding linear block copolymers showed that the macromolecular architecture not only strongly affects the morphological domain borders but can introduce new morphologies as well.

Star‐branched poly(methyl methacrylate)s (PMMA) were synthesized by linking ‘living’ arms (produced by anionic polymerzation) with ethylene glycol dimethacrylate. Stars having arm molecular weights of 10000 and 40000 and between 4.9 and 18.7 branches were produced. The polymers were characterized using light scattering, size exclusion chromatography, and viscometry. It was found that well‐defined PMMA stars were obtained only at the higher (40000) arm molecular weight. The stars prepared using the lower molecular weight (c. 10000) arms contained very high molecular weight gel components.