Self-assembly star-shaped block

Recently, we have also prepared nanosized polymersomes through self-assembly of star-shaped PEG-b-PLLA block copolymers (eight-arm PEG-b-PLLA) using a film hydration technique [233]. The polymersomes can encapsulate FITC-labeled Dex, as model of a water-soluble macromolecular (bug, into the hydrophilic interior space. The eight-arm PEG-b-PLLA polymersomes showed relatively high stability compared to that of polymersomes of linear PEG-b-PLLA copolymers with the equal volume fraction. Furthermore, we have developed a novel type of polymersome of amphiphilic polyrotaxane (PRX) composed of PLLA-b-PEG-b-PLLA triblock copolymer and a-cyclodextrin (a-CD) [234]. These polymersomes possess unique structures the surface is covered by PRX structures with multiple a-CDs threaded onto the PEG chain. Since the a-CDs are not covalently bound to the PEG chain, they can slide and rotate along the PEG chain, which forms the outer shell of the polymersomes [235,236]. Thus, the polymersomes could be a novel functional biomedical nanomaterial having a dynamic surface. 

A special case of a miktoarm star copolymer with many arms are so-called Janus Micelles, which are formed by cross-linking the short middle block of a triblock terpolymer in the microphase separated bulk state, in which the center block self-assembles in spherical [ 189,190] or cylindrical domains [191]. By this procedure the two different outer blocks are oriented to the two opposite hemicoronas around the center block domain and subsequent dissolution leads to amphiphilic particles (Figure 14). While spherical Janus Micelles form superstructures in solution, the cylindrically shaped Janus Micelles seem to have a lower tendency of self-aggregation to higher superstructures. 

Figure 4.2 Macromolecular topologies of amphiphilic polymers capable of forming self-assembled networks (a) linear block copolymers, (b) star-shaped copolymers, (c) graft copolymers.

Abraham S, Ha CK, Kim I (2006) Self-assembly of star-shaped polystyrene-block-polypeptide copolymers synthesized by the combination of atom transfer radical polymerization and ring-opening hving polymerization of a-amino acid-N-carboxyanhydrides. J Polym Sci A Polym Chem 44 2774—2783... 

Dai X-H, Dong C-M (2008) Synthesis, self-assembly and recognition properties of biomi-metic star-shaped poly(epsilon-caprolactone)-b-glycopolymer block copolymers. J Polym Sci A Polym Chem 46(3) 817-829... 

When the copolymer is star shaped such as the ABC miktoarm star copolymer in which there are three different blocks (A-C blocks) with a common joint junction, the richer phase stmc-ture can be found [34,35]. When the interaction and length are the same, a symmetrical honeycomb structure can be found (Fig. 15.5a, only phases of blocks A and B are shown) and each block self-assembles into cylinder phase. The junction points are inhomogeneously distributed over the intermaterial dividing surface around cylinders formed by the respective blocks A-C. The region I shown in Figure 15.5a is the interface of different domains, which contains the mixtures of blocks A-C and star junctions. 

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