INFLUENCE OF POLYMER ARCHITECTURE ON THE FORMATION OF MICELLES OF MIKTOARM STAR COPOLYMERS POLYETHYLENE POLY(ETHYLENEPROPYLENE) IN THE SELECTIVE SOLVENT DECANE/

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 molecul ar weights of 4900, 9100, and 15 700. The crystallization following th e segregation of PE in decane drives the assembly process. As in the c ase of diblock copolymers PE-PEP, the miktoarm stars form lamellar str uctures with a flat dense crystalline PE core and a soft corona of PEP hairs sticking out on both sides of the core. Laterally, the aggregat es are largely extended and modeled as thin disks. The densities of bo th the core and the corona were represented by step functions convolut ed with a Gaussian each. The structural parameters (the thickness of t he core and the extension of the hairs) were extracted using a model f itting. With increasing the PEP molecular weight, the average extensio n of the PEP chains in the corona increases and the thickness of the c ore decreases except for the symmetric architecture PE2 PEP2. The chan ges are different from those expected from an equivalent increase of m olecular weights in a diblock. At room temperature almost all the poly mer is found in the micelles. Thus, a thermodynamic description is bas ed, on the free energy of a single micelle. It yields scaling relation s dependent on the star architecture. While the model worked well for the diblocks, it does not describe the core thickness variation correc tly for the miktoarm systems.