A new morphology of crew-cut aggregates prepared from highly asymmetric triblock copolymers of 5-(N,N-diethylamino)isoprene and styrene in dilute solution is reported. After quaternization of the polar block, using dimethyl sulfate, the copolymers consist of a long block of polystyrene (PS) with short poly[5-(N,N,N-diethylmethylammonium)isoprene] (PAI) blocks at both chain ends. The aggregates were prepared by first dissolving the copolymers in a common solvent for both blocks and then adding water to induce the segregation of the PS chains. 1,4-Dioxane, THF, or a DMF/THF mixture was used as the common solvent in the preparation of these structures. The bowl-shaped aggregates are essentially highly polydisperse spheres, containing an asymmetrically placed single void space, which has broken through the surface. The continuous phase is composed of an assembly of reverse micelles (PAI core and PS corona) with hydrophilic PAI chains surrounding the structure at the polymer/aqueous solution interface. It is believed that the formation of the bowl-shaped morphology is under kinetic control and does not represent an equilibrium state. A possible mechanism for the formation of this aggregate is proposed, based on two other previously reported crew-cut morphologies from diblock copolymers. This study illustrates the importance of the preparative conditions on the self-assembly of nonequilibrium aggregates from amphiphilic block copolymers.

A combination of dynamic (DLS) and static (SLS) light scattering measurements was employed to study the self-assembly behavior of a new series of triblock copolymers bearing poly[5-(N,N-diethylamino isoprene)] (PAI) short outer blocks and polystyrene (PS) as the major middle block. Previously, it was verified that PAI outer blocks can be quaternized leading the formation of crew-cut aggregates in water (Riegel, I. C.; Eisenberg, A.; Petzhold, C. L.; Samios, D. Langmuir 2002, 18, 3358). Herein, we focus on the copolymer's ability in the nonquaternized version to undergo self-aggregation in dimethylformamide (DMF), a selective solvent for the middle block. Light scattering measurements showed that formation of well-defined flowerlike micelles is likely to occur. Aggregates with a relatively narrow distribution, small average size, and number of aggregation ranging from 21 to 39 chains/micelle were experimentally observed. The results also suggested that approximately 5-6 polymeric units per each short outer block are needed to induce aggregation. The middle block length governs the size of the micelles and influences the number of aggregation of the resultant particles as well. Furthermore, when the polystyrene middle block was particularly long (degree of polymerization DP > 600), dynamic and static light scattering measurements suggested the formation of bridged micelles in an open structure in concentrations as low as 15 mg mL-1.

We report the characterization through SAXS measurements of micelles produced from a new series of block copolymers: one diblock and four triblock copolymers bearing short poly[5-(N,N-diethylamino)isoprene] and long polystyrene blocks. Micellar aggregates produced in DMF (selective solvent for polystyrene) from the same set of samples were previously successfully characterized through light scattering measurements. The X-ray scattering profiles of starlike (from the diblock copolymer sample) and flowerlike micelles (from the triblock copolymers samples) could be fitted using the spherical copolymer micelle model proposed by Pedersen and Gerstenberg (Macromolecules 1996, 29, 1363.) where in the case of flowerlike micelles, the particles were understood as formed by hypothetical diblock copolymers having half of the true polymeric molar mass. Using the spherical copolymer micelle model, it could be possible to attest the unswollen nature of the micellar cores. The total micellar size suggested thus that the chains forming the corona are extended which is mainly related to a small core surface area per corona chain entering the core (Ac/n), which also induced a small number of aggregation (N(agg)) of all self-assembled particles. The total micellar size fits well with our previous light scattering measurements.