Phase in block copolymers

The association of block copolymers in a selective solvent into micelles was the subject of the previous chapter. In this chapter, ordered phases in semidilute and concentrated block copolymer solutions, which often consist of ordered arrays of micelles, are considered. In a semidilute or concentrated block copolymer solution, as the concentration is increased, chains begin to overlap, and this can lead to the formation of a liquid crystalline phase such as a cubic phase of spherical micelles, a hexagonal phase of rod-like micelles or a lamellar phase. These ordered structures are associated with gel phases. Gels do not flow under their own weight, i.e. they have a finite yield stress. This contrasts with micellar solutions (sols) (discussed in Chapter 3) which flow readily due to a liquid-like organization of micelles. The ordered phases in block copolymer solutions are lyotropic liquid crystal phases that are analogous to those formed by low-molecular-weight surfactants. 

The physics of the glass transition in block copolymers are essentially the same as those of homopolymers, and little experimental attention has been devoted to this aspect. Ordered phases in block copolymer melts can be vitrified by cooling below the glass transition temperature of a glassy block, and indeed this is often the method for preparing samples for transmission electron... 

Thus, these mesostructures are predominantly lamellar, and identified as conventional (parabolic) lamellar phases, although they may in fact be hyperbolic. Indeed, unless v/al is exactly unity, a planar interface (lamellar mesophase) incurs a bending energy cost hyperbolic sponge monolayers or bilayers or mesh monolayer mesophases are favoured if v/al differs from unity. It is likely then that many "lamellar"" phases in fact adopt a hyperbolic geometry. Careful neutron-scattering studies of a lamellar phase have revealed the presence of a large number of hyperbolic "defects" (pores within the bilayers) in one case [16]. (An example of this mis-identification of hyperbolic phases in block copolymers is discussed in section 4.10.)... 

FIGURE 5.2 Surface area effect on the broadening of the glass transition temperature of polystyrene nanospheres and lamellar phases in block copolymers. Open squares are for lamellar phases, and filled triangles are for nanospheres. (Modified figure based on original from Gaur and Wunderlich [1980].)... 

The simplest are diblock copolymers, where two different polymeric chains are bound together and with an increase of block number, tri- or multiblocks with a variety of structures can be obtained [31,32]. Most block copolymers used today are prepared by living anionic polymerization, which is a feasible method to prepare block copolymers with controlled architecture. Different polymers do not mix well due to thermodynamic reasons [33], especially if their molecular mass is sufficiently high, they have a strong tendency to form separated phases. In block copolymers, this phase separation can occur only intermolecularly (micro- or nanophase separation) [34]. Those block copolymers... 

Bates, F.S. (2005) Network phases in block copolymer melts. MRS Bulletin, 30,525-532. 

The situation is different when a covalent link can be established between the phases. In block copolymers the various blocks are also nonmiscible, but the dyad linking the two blocks ensures the cohesion of the system and the phase dispersion. 

LeiblerL., Theory of microphase separation in block copolymers. Macromolecules, 13, 1602, 1980. Eoerster S., Khandpur A.K., Zhao J., Bates E.S., Hamley I.W., Ryan A.J., and Bras W. Complex phase behavior of polyisoprene-polystyrene diblock copolymers near the order-disorder transition. Macromolecules, 21, 6922, 1994. 

Various types of power law relaxation have been observed experimentally or predicted from models of molecular motion. Each of them is defined in its specific time window and for specific molecular structure and composition. Examples are dynamically induced glass transition [90,161], phase separated block copolymers [162,163], polymer melts with highly entangled linear molecules of uniform length [61,62], and many others. A comprehensive review on power law relaxation has been recently given by Winter [164],... 

Phase ordering in block copolymers can be described by the same dynamic equation as in the case of homopolymer blends [Eqs. (53)—(55)] with the LG... 

Molecular architecture modifies the phase behavior of block copolymers. In block copolymers, macrophase separation is prevented by the connectivity of the polymer chains. The transition from a homogeneous melt to a heteroge-... 

The more recently developed cryo-TEM technique has started to be used with increasing frequency for block copolymer micelle characterization in aqueous solution, as illustrated by the reports of Esselink and coworkers [49], Lam et al. [50], and Talmon et al. [51]. It has the advantage that it allows for direct observation of micelles in a glassy water phase and accordingly determines the characteristic dimensions of both the core and swollen corona provided that a sufficient electronic contrast is observed between these two domains. Very recent studies on core-shell structure in block copolymer micelles as visualized by the cryo-TEM technique have been reported by Talmon et al. [52] and Forster and coworkers [53]. In a very recent investigation, cryo-TEM was used to characterize aqueous micelles from metallosupramolecular copolymers (see Sect. 7.5 for further details) containing PS and PEO blocks. The results were compared to the covalent PS-PEO counterpart [54]. Figure 5 shows a typical cryo-TEM picture of both types of micelles. 

In addition to the previously mentioned driving forces that determine the bulk state phase behavior of block copolymers, two additional factors play a role in block copolymer thin films the surface/interface energies as well as the interplay between the film thickness t and the natural period, Lo, of the bulk microphase-separated structures [14,41,42], Due to these two additional factors, a very sophisticated picture has emerged from the various theoretical and experimental efforts that have been made in order to describe... 

The minor phase of block copolymer microdomains are selectively removed by either chemical or physical degradation, thereby generating a defined porosity in crosslinked polymer bulk matrices or thin films... 

These concentration fluctuations are pivotal to the phase transitions in block copolymer melts and are dynamic in nature. They lead to a renormahzation of the relevant interaction parameters and are thought to be responsible for the induction of the first-order nature of the phase transition [264,265]. Such fluctuations are better studied in dynamic experiments. Thus, one can observe an increasing interest in diblock copolymer dynamics. These dynamic properties are being analysed through experimental, theoretical [266,267] and computer simulation approaches [268,269] with the aim of determining the main featirres of diblock copolymer dynamics in comparison to homopolymer dynamics. There are three main issues ... 

The benefits of utihzing combinatorial methods for investigating polymer properties have been outlined recently [19,166,167]. Polymer gradient brush assemblies are expected to play an active role in further combinatorial material effort. Possible areas of interest include (but are not hmited to) study of phase behavior (stability) in hquid [168] and polymer blend [169] systems, morphological transitions in block copolymers [170,171], cell culturing [58,172], and others. 

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