Stabilization Star-block copolymers

When soft colloids (such as stars, block copolymer micelles, or particles sterically stabilized with grafted chains) are suspended in solvents of intermediate quality at high concentrations, an increase of temperature leads to an increase of their effective volume fraction, which in turn can yield vitrification [26,190,199]. A representative example is depicted in Fig. 23a for a star with nominal / = 128 and... 

Three arm amphiphilic star-block copolymers of isobutyl vinyl ether (IBVE) and 2-hydroxyethyl vinyl ethers were also prepared by Higashimura et al. [23]. The synthetic strategy involved the sequential living cationic polymerization of IBVE and 2-acetoxyethyl vinyl ether, initiated by the trifunctional initiator system composed of tris(trifluoroacetate) and ethylaluminum dichloride, with excess of 1,4-dioxane as a carbocation-stabilizing Lewis base (Scheme 8). 

Antioxidants are usually employed to prevent oxidation of the double bond in either of the polydienes. Specialty grades are available where the polydiene has been hydrogenated, which gives much better chemical stability and uv resistance. Multiarm and star-block copolymers are some examples of specialty copolymers that can be used to modify the performance of block copolymer adhesives (19). 

Serrano, E., Zubeldia, A. et al. Effect of different thermal treatments on the self-assembled nanostructures of a styrene-butadiene-styrene star block copolymer. Polymer Degradation and Stability, 83 (2004), p. 495-507... 

Millan MD, Taranekar P, Waenkaew P, Advincula RC (2005) Formation of gold nanoparticles stabilized by a star block copolymer and simultaneous polymerization of a dithiophenylpyrrole monomer. Polymer Preprints 46 652-653... 

There are additional factors that may reduce functionality which are specific to the various polymerization processes and the particular chemistries used for end group transformation. These are mentioned in the following sections. This section also details methods for removing dormant chain ends from polymers formed by NMP, ATRP and RAFT. This is sometimes necessary since the dormant chain-end often constitutes a weak link that can lead to impaired thermal or photochemical stability (Sections 8.2.1 and 8.2.2). Block copolymers, which may be considered as a form of end-functional polymer, and the use of end-functional polymers in the synthesis of block copolymers are considered in Section 9.8. The use of end functional polymers in forming star and graft polymers is dealt with in Sections 9.9.2 and 9.10.3 respectively. 

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. 

Meier MAR, FUali M, Gohy J-F, Schubert US (2006) Star-shaped block copolymer stabilized palladium nanoparticles for effident catalytic Heck cross-coupling reactions. J Mater Chem 16 3001-3006... 

The criterion of stability (R) is the constant limit value of the emulsified volume percentage at 20°C (32, 32.). It appears that 7/3 water toluene emulsions are more efficiently stabilized by star-shaped block copolymers whereas linear block copolymers give better results for 3/7 water toluene emulsions the stability of the l/l water toluene emulsions seems to be insensitive to the molecular architecture of PTBS/PO block copolymers. 

Recently, a versatile class of poly(ethylene propylene)/poly(ethylene oxide) block copolymer micelles were introduced they were stable due to a combination of high block incompatibility, kinetically frozen core, and high interfacial tension between core and solvent [53, 58]. Moreover, by using a co-solvent of varying composition, the aggregation number was controlled and soft spheres from star-like to micelle-like could be obtained. Another way is core stabilization via chemical crosslinking, say by UV radiation [59-64]. 

Tire morphology of a micelle is primarily determined by the composition of the copolymer and the incompatibility between the blocks and the solvent.. Symmetric block copolymers produce micelles in which the core and the corona have comparable volume, leading to colloidal particles akin to the sterically stabilized particles described above. By contrast, very asymmetric copolymers form star-like particles... 

---