Diblock copolymer molecular architecture

The purpose of this contribution is to show how the design of a new dianionic species can lead to interesting advances in block copolymerization and especially to original PA(P0)2 starshaped block copolymers. PA is a hydrophobic block polystyrene, polytertiarybutylstyrene (PTBS) or polyisoprene (Pi). The surface activity of this novel and well mastered molecular architecture is considered and compared as far as possible with the behavior of the corresponding PA-PO diblock copolymers. 

One can have the same type of situation in a blend of two mutually immiscible polymers (e.g., polymethylbutene [PMB], polyethylbutene [PEB]). When mixed, such homopolymers form coarse blends that are nonequilibrium structures (i.e., only kinetically stable, although the time scale for phase separation is extremely large). If we add the corresponding (PEB-PMB) diblock copolymer (i.e., a polymer that has a chain of PEB attached to a chain of PMB) to the mixture, we can produce a rich variety of microstructures of colloidal dimensions. Theoretical predictions show that cylindrical, lamellar, and bicontinuous microstructures can be achieved by manipulating the molecular architecture of block copolymer additives. 

Figure 21.5 Molecular architectures and models of (a) AB diblock copolymer, (b) extended amphiphilic dendron, and (c) ABC triblock copolymer 28 (Reprinted with permission from B. K. Cho et al., Chem. Mater. 2007, 19, 3611 -3614. Copyright 2007 American Chemical Society.)...

These local results suggest that the presence of h3rperbolic interfaces in diblock copolymer assemblies is expected for a range of molecular architectures. For these structures to be realisable in space, the curvatures must be compatible with global dimensions (e.g. the radii of curvature must allow a tiling of space without overlap or voids). 

Rgure 8.5 Representative polymer-polymer phase behaviour with different molecular architectures. Microphase separation (a) results when thermodynamically incompatible linear homopolymers are mixed. The covalent bond between blocks in a diblock copolymer leads to microphase segregation (c). A mixed architecture of linear homopolymers and the corresponding diblock copolymer produces a surfactant-like stabilized intermediate-scale phase separation (b). 

Diblock and triblock copolymers are usually synthesized anionically, which inherently limits the possible blocks and homopolymers. However, the anionic route gives very good control over molecular weight, molecular architecture and polydispersity and is ideally suited for a systematic study of the effect of connector molecules. Furthermore, the problems frequently encountered during the anionic synthesis have been ironed out for many common polymers and the synthesis, although tedious, can be considered a routine procedure. 

Presented polymer mixtures are composed of amorphous macromolecules with different molecular architecture homopolymers and random copolymers, with different segments distributed statistically along the chain, form partly miscible isotopic and isomeric model binary blends. The mixing of incompatible polymers is enforced by two different polymers covalently bonded forming diblock copolymers. Here only homopolymers admixed by copolymers are considered. The diblock copolymer melts have been described recently in a separate review by Krausch [17]. 

An additional factor driving the segregation exists whenever the anchoring moieties change interactions at the interface. The related reduction A in brush chain free energy may depend on the molecular architecture of diblocks (i.e.,the size of blocks and of copolymer) as was recognized by our recent studies [254] described in Sect. 4.2.3. The results of these studies are analyzed with mean field and self consistent mean field approaches, both yielding identical adsorption parameters. 

Compared to low molecular weight amphiphiles, the size of polymeric amphi-philes allows for much more diverse arrangements of the hydrophilic and hydrophobic segments, as exemplified in Fig. 1. Accordingly, micellar polymers are characterized by versatile molecular architectures, giving rise to distinct subgroups. Diblock-copolymers with a clear separation of the hydrophilic... 

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