Poly(ethylene-alt-propylene)-poly(ethylene oxide) diblock copolymer micelles:: a colloidal model system with tunable softness

Kinetically frozen micelles formed by the amphiphilic diblock copolymer poly(ethylene-alt-propylene)-poly(ethylene oxide) (PEP-PEO) are proposed as a new model system for soft colloids. In this context soft is used with a twofold meaning: the intraparticle softness, i.e. the molecular architecture of an individual micelle, as well as the interparticle softness, i.e. the effective potential for micellar interactions. Both contributions can be precisely adjusted from hard sphere-like to ultrasoft (star-like) by changing the diblock copolymer composition and/or interfacial tension, as shown by small angle neutron scattering in combination with contrast variation techniques. Depending on the degree of softness, PEP-PEO micellar solutions respond variably to the application of external shear fields. In particular, in the star-like regime solutions are already extremely sensitive at low shear rates. Therefore these micelles are an excellent starting point for a comprehensive study on the relation between softness and non-equilibrium phase behaviour in colloidal systems.