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 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.