Micellization of model graft copolymers of the H and π type in dilute solution

The dilute solution properties of model graft copolymers of the H, S₂IS₂, and π type, (S,I)I-(I,S), have been studied in common good (THF) and selective (n-decane and ethyl acetate) solvents. The molecules have two trifunctional branch points and are composed of polyisoprene (PI) bridges and polystyrene (PS) or polyisoprene branches. Three different methods were employed, low-angle laser light scattering, dynamic light scattering, and viscometry, in order to extract information about the aggregation number, the size, and the shape of the micelles formed in selective solvents for the PI backbones (n-decane) and for the PS branches (ethyl acetate). Both graft copolymers form multimolecular micelles in n-decane. Their hydrodynamic behavior resembles that of hard spheres. In the case of the H copolymer in ethyl acetate, large, loosely bound aggregates are formed above the cmc which are transformed to more compact micelles at higher concentrations. For the π copolymer, regions of existence of unimolecular and compact multimolecular micelles were observed, together with a sharp and relatively high critical micelle concentration. The behavior is in agreement with the closed association model. In both cases, the aggregation numbers were lower than the ones observed for diblock and triblock copolymers in similar solvents. The influence of macromolecular architecture on the micellization behavior of these model compounds is discussed in view of the additional constraints and parameters imposed by the macromolecular topology.