Micellar Morphological Changes Promoted by Cyclization of PS-b-PI Copolymer: DLS and AFM Experiments

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Abstract

We study the effect of the cyclization of a linear poly(styrene-b-isoprene) PS−PI copolymer on the morphology of the micelles formed in heptane (a selective solvent for PI). The linear and the cyclic copolymers have exactly the same degree of polymerization, i.e. 290 for PS and 110 for PI, i.e., volume fraction ΦPS = 0.78. We use dynamic light scattering (DLS) and atomic force microscopy (AFM) techniques to characterize the micellar morphology made from copolymer molecules, focusing on the effect of cyclization. We also report the concentration effect on the micelle size and shape made from the linear and cyclic copolymers. Our results show that the micelles made from linear PS−PI adopt a “classical” spherical shape and keep the same morphology parameters over the range of investigated concentrations. In contrast, the micelles arising from cyclic copolymer chains adopt a giant wormlike shape. In this later case, both the size and the shape evolve when the concentration is increased, starting from small individual “sunflower-like” micelles, at low concentrations, to giant wormlike micelles at higher concentrations. Our results suggest also that these giant wormlike micelles made from cyclic copolymer chains result from the self-assembly of “sunflower-like” elementary micelles.

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