Macrophase separation

Fig.49 Composition distribution dependence of reduced domain spacing, D/D0, of PS- -P2VP with single microphase-separated structure. Do = 60.8 nm is domain spacing of parent copolymer with Mn = 125 kg/mol. Hatched region macrophase separation. From [160], Copyright 2003 American Chemical Society...

The effect of blending an AB diblock copolymer with an A-type homopolymer has been the subject of many research activities. On a theoretical basis the subject was investigated e.g. by Whitmore and Noolandi [172] and Mat-sen [173]. If a diblock exhibiting lamellae morphology is blended with a homopolymer of high molecular weight, macrophase separation between the... 

In general, the less the asymmetry in the composition of the low molecular weight PS-b-PI, the larger molecular weight of the other component is required to allow formation of cylinders. This trend leads, however, to a certain ratio in which the system macrophase separates before cylinders are formed. Therefore, the formation of cylinders tends to occur in a range in which the miscibility between the chains abruptly changes. 

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 formation of such structures was attributed to short-range attractive forces arising from transient fluctuations in the periphery of the corona, leading at times to attractive interactions between micelles. These aggregated states, i.e., strings and networks, were observed whenever the added salt concentration was increased. Further increase of salt concentration eventually led to very large networks and macrophase separation into a dilute micellar phase and a concentrated gel phase [15]. 

In order to resolve these challenges, it is essential to account for chain connectivity, hydrodynamic interactions, electrostatic interactions, and distribution of counterions and their dynamics. It is possible to identify three distinct scenarios (a) polyelectrolyte solutions with high concentrations of added salt, (b) dilute polyelectrolyte solutions without added salt, and (c) polyelectrolyte solutions above overlap concentration and without added salt. If the salt concentration is high and if there is no macrophase separation, the polyelectrolyte solution behaves as a solution of neutral polymers in a good solvent, due to the screening of electrostatic interaction. Therefore for scenario... 

In binary blends of A homopolymer and AB diblock copolymer, the interplay between microphase separation and macrophase separation is controlled mainly by the relative length of the chains, in addition to the composition of the mixture. Homopolymers shorter than the corresponding block tend to be solubilized within the corresponding domain of a microphase-separated structure. As the homopolymer molecular weight increases to approach that of the corresponding... 

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. 

Turbidometry is simply the quantitative measurement of light transmission in turbid solutions, and is employed to locate the cloud point (i.e. onset of macrophase separation) in block copolymer solutions, as discussed in Chapter 3. 

In blends of two or more homopolymers with block copolymer, there is an interplay between macrophase separation of the homopolymers and microphase separation of the block copolymer. Which effect predominates depends on the relative lengths of the polymers, and on the composition of the blend. 

Binary blends of block copolymers can also macrophase separate if the mismatch in molecular weights is sufficient. In the other limit, blends of diblocks of... 

Lowenhaupt and Hellmann (1991) have determined whether microphase separation or macrophase separation occurs in blends of a PS-PMMA diblock with PMMA homopolymer with a < 1 and a > 1 using TEM. They found that the transition between purely microphase separation and macrophase separation occurs for a lower diblock content for blends with a smaller a, as supported by calculations of the instability limit using the random phase approximation. Blends with a < 1.4 were always initially microphase separated, although in a blend with a - 1.4 this was followed by macrophase separation. However, the macrophase-separated structure took the form of aggregates of micelles (see Fig. 6.1), suggesting a nucleation and growth mechanism for the secondary... 

Fig. 6.2 Representative micrographs showing macrophase separation (Lowenhaupt and Hellmann 1991) (a) and (b) are bicontinuous structure, typical of those for spinodal decomposition (c) and (d) show discrete domains, consistent with a nucleation and growth process of macrophase separation. The diblock details are as Fig. 6.1, the homopolymer has A/w = 161 kg mol-1. Temperatures and volume fraction of copolymer are indicated.

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