Microphase segregation separation

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). 

The phase diagram in Fig. 15 is a prediction of thermodynamic equilibrium however, the process path of a sample intuitively affects the observed phase. For example, if end-functionalized polymers are miscible in the melt, then microphase separation could occur upon cooling. However, if macroscopic phase separation occurs first, then pseudo block copolymers can only form at the interface of a phase-separated mixture, further blocking the molecular transport required for structure formation. If both free and dimerized homopolymer are present during phase separation or microphase segregation, then the length scale of phase segregation/... 

Figure 5.2 AFM image of the microphase segregated SMPF. The phase image indicates the microphase separation of the polymer with lighter hard segment phase regions and darker soft segment phase regions. Source [10] Reproduced with permission from the Royal Society...

PMMA or PS with P (S-b-MMA) The microphase separation was observed only when low M- homopolymers were used. Blends of a polymer C having strong specific interactirais with A did not show any M limit for the microphase segregation Lowenhaupt et al. 1994... 

A. Macrophase separation into two layers, which commonly arises when two homopolymers are mixed. B. Microphase segregation can arise with diblock copolymers. Here, striated layers appear. C. Mixing homopolymers with the corresponding diblock copolymer gives intermediate-scale phase separation. A surfactant-like mixture occurs. Figure modified from Bates, F. S. "Polymer-Polymer Phase Behavior." Science, 251, 898 (1991). 

Similar to the branches in copolymer stars and miktoarms, the grafted chains in brushes can be of different chemical compositions. Brown et al. [223] studied the microphase separation of grafted mixtures of homopolymer chains composed of immiscible A and B units and also [224] of diblock AB copolymers. In the former case, the brushes expand laterally and then experience lateral microphase separation. In the latter case, however, monomers segregate vertically to the surface forming a three layer structure. 

F-BDAF Tg for various blend compositions, see Fig. 14. The microphase-separated morphology further manifests itself in the self-adhesion behavior of polyimide films derived from such mixtures. For mixture containing at least 25 wt% of the flexible component, peel tests of polyimide bilayer samples prepared by solution casting, bulk failure of the test specimens was observed. Since the flexible component contained fluorine, the samples could be examined by X-ray photoelectron spectroscopy to determine the surface composition. At only 10% loading, the flexible component comprised 100% of the top 75 A of the sample. The surface segregation of the flexible component is believed to be responsible for the adhesion improvements. 

In a blend of immiscible homopolymers, macrophase separation is favoured on decreasing the temperature in a blend with an upper critical solution temperature (UCST) or on increasing the temperature in a blend with a lower critical solution temperature (LCST). Addition of a block copolymer leads to competition between this macrophase separation and microphase separation of the copolymer. From a practical viewpoint, addition of a block copolymer can be used to suppress phase separation or to compatibilize the homopolymers. Indeed, this is one of the main applications of block copolymers. The compatibilization results from the reduction of interfacial tension that accompanies the segregation of block copolymers to the interface. From a more fundamental viewpoint, the competing effects of macrophase and microphase separation lead to a rich critical phenomenology. In addition to the ordinary critical points of macrophase separation, tricritical points exist where critical lines for the ternary system meet. A Lifshitz point is defined along the line of critical transitions, at the crossover between regimes of macrophase separation and microphase separation. This critical behaviour is discussed in more depth in Chapter 6. 

The phase diagram for weakly segregated diblocks was first computed within the Landau mean field approximation by Leibler (1980). Because it has proved to be one of the most influential theories for microphase separation in block copolymers, an outline of its essential features is given here.The reader is referred to the original paper by Leibler (1980) for a complete account of the theory. 

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