Block copolymer/homopolymer ternary blends

This chapter is organized as follows. Section 6.2 is concerned with experiments on binary block copolymer/homopolymer blends, Section 6.3 deals with experiments on ternary blends containing a block copolymer and in Section 6.4 experiments on binary blends of block copolymers are reviewed. Theory for the corresponding type of blend is discussed successively in Sections 6.5 to 6.7. Finally, experiments on thin films are discussed in Section 6.8, separately from the work on bulk blends, in keeping with earlier chapters. 

Table 6.2 Experimental studies of ternary block copolymer/homopolymer blends. Adapted and extended from Roe and Rigby (1987)...

Banaszak M, Whitmore MD. Mean field theory of the phase behavior of ternary block copolymer-homopolymer blends. Macromolecules 1992 25 249-260. 

A number of classes of polymer blends containing block copolymers have been studied. Namely, binary blends of a block copolymer with a homopolymer, ternary mixtures of a block copolymer with two homopolymers and blends of two block copolymers. Experimental and theoretical studies of all these mixtures are the subject of Chapter 6. 

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. 

One of the most important applications of block copolymers is as compatibi-lizers of otherwise immiscible homopolymers. This compatibilization results from the reduction of interfacial tension due to segregation of copolymer to the interface between homopolymers. Experiments and theory concerned with the understanding of the thermodynamics of these ternary blends are discussed in this chapter. 

There is an extensive literature on studies on ternary blends of a block copolymer with two homopolymers, largely focussed on the compatibilization of homopolymers by block copolymers. A cross-section of the literature is listed in Table 6.2. 

Fig. 6.37 Interfacial volume fraction profiles calculated for a ternary blend of a PS-PB block copolymer with PS and PB homopolymers in a good solvent (Noolandi and Hong 1982). The diblock has N — 600 and f = The homopolymers have infinite molecular weight. The solid lines are the volume fractions of homopolymer (A = PS) (B = PB), the dashed lines indicate the volume fractions of PS and PB blocks of the diblock. The dots correspond to the total volume fractions of the A and B components and the position is measured in units of a segment length a = 6.95 A.

The phase behaviour of blends of homopolymers containing block copolymers is governed by a competition between macrophase separation of the homopolymer and microphase separation of the block copolymers. The former occurs at a wavenumber q = 0, whereas the latter is characterized by q + 0. The locus of critical transitions at q, the so-called X line, is divided into q = 0 and q + 0 branches by the (isotropic) Lifshitz point. The Lifshitz point can be described using a simple Landau-Ginzburg free-energy functional for a scalar order parameter rp(r), which for ternary blends containing block copolymers is the total volume fraction of, say, A monomers. The free energy density can be written (Selke 1992)... 

Fig. 6.42 Schematic of the critical line as a function of ip - 0A -+ n, jj = 0A

Fig. 17a,b. Change of normalized contact number nAB between segments of homopolymer A and B in the ternary blends containing a attractive (A05f5) b repulsive block copolymers (R1 lf5) of different chain lengths with time [71]... 

Zhou, N., Lodge, T.P. and Bates, RS. (2006) Influence of conformational asymmetry on the phase behavior of ternary homopolymer/block copolymer blends around the bicontinuous microemulsion channel. /. Phys. Chem. B, 110, 3979-3989. 

Noolandi et al. developed a theory for the interfacial region in three-component polymeric systems comprising di-block copolymer. There are two aspects to consider the phase separation in block copolymer upon addition of one or two homopolymers, and the modification of the A/B blend properties upon addition of a block copolymer (either A-B or X-Y type). The second aspect is more pertinent for the polymer blend technology. In particular, the ternary blends comprising two homopolymers and a copolymer, either A/B/A-B, or A/B/X-Y are of industrial interest. 

Stoykovich et al. ° formed nonregular patterns by combining ternary blends of PS-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers and PMMA homopolymers on chemically nanopattemed substrates. The ternary... 

The use of more complex systems such as ternary blends allows the functionalization of the surfaces with varies chemical functionalities. For instance a PS matrix was mixed with two block copolymers, a hydrophobic (PS-b-P5FS) and an amphiphilic polystyrene-b-poly[poly(ethylene glycol) methyl ether methacrylate] (PS-6-P(PEGMA)) copolymer [96], The chemical distribution of the resultant surface pattern implies an enrichment of the holes in the amphiphilic copolymer with an external surface mainly functionalized in the fluorinated copolymer with low surface energy (Scheme lO.lg). Other ternary blends combining incompatible copolymers and homopolymers have been reported leading to more complex topographies and chemical distributions [148],... 

Hillmeyer, M.A., Maurer, W.W., Lodge, T.R, Bates, F.S., and Almdal, K., Model bicon-tinuous microemulsions in ternary homopolymer/block copolymer blends, 7. Phys. Chem. B, 103, 4814, 1999. 

M.A. Hillmyer, K. Ahndal, T.P. Lodge, W.W. Maurer, F.S. Bates, Model bicontinuous microemulsions in ternary homopolymer/block copolymer blends. Journal of Physical Chemistry B 103 (1999)4814-4824, http //dx.doi.org/10.1021/jp990089z. 

Thermodynamically stable, bicontinuous microemulsions have recently been shown to be obtainable in symmetric ternary blends of two homopolymers and a diblock copolymer by formulating alloys with compositions near mean-field isotropic Lifshitz points. In the present paper, it is argued that practical apphcation of this design criterion could require use of homopolymers of unequal molec.wts. and block copolymers of different structure. The existence of, and explicit location of, mean-field isotropic Lifshitz points in ternary blends with homopolymer molec.wt. asymmetry and either AB diblock or ABA triblock copolymer structures were demonstrated. These calculations significantly expanded the parameter space for observing bicontinuous miCToemulsions and allowed for more flexibility in tailoring melt rheological properties and solid-state mechanical properties. 29 refs. 

Muller, M., Pike, D.Q., and dePablo, J.J. (2011) Monte-Carlo simulation of ternary blends of block copolymers and homopolymers. J. Chem. Phys., 135,... 

The effect of sequence distribution on the miscibility of blends of random copolymers is not embodied in the mean field approach discussed previously in this section. Balazs et al. described a parameter, 6, which relates to the binary sequence distribution 6 = 0 for a block copolymer, 0.5 for a random copolymer and 1 for a purely alternating copolymer) [210]. This approach was applied to copolymer/homopolymer blends [210], copolymer-copolymer blends [211] and ternary blend of two homopolymers and a copolymer [212]. It was shown... 

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