Polymer blends phase segregation

Figure B3.6.5. Phase diagram of a ternary polymer blend consisting of two homopolymers, A and B, and a synnnetric AB diblock copolymer as calculated by self-consistent field theory. All species have the same chain length A and the figure displays a cut tlirough the phase prism at%N= 11 (which corresponds to weak segregation). The phase diagram contains two homopolymer-rich phases A and B, a synnnetric lamellar phase L and asynnnetric lamellar phases, which are rich in the A component or rich in the B component ig, respectively. From Janert and Schick [68].

The data of Zink et al. (1998) illustrate the measurement by NRA of near-surface composition profiles in isotopically labelled polymer blends. If a mixture of polymers is adjacent to a phase interface (e.g. a solid or an air surface), often one of the components is preferentially attracted to the surface and will segregate to it, and this phenomenon will influence the tribological behaviour the interface (lubrication, wear and adhesion). 

In this chapter we have discussed the thermodynamic formation of blends and their behavior. Both miscible and immiscible blends can be created to provide a balance of physical properties based on the individual polymers. The appropriate choice of the blend components can create polymeric materials with excellent properties. On the down side, their manufacture can be rather tricky due to rheological and thermodynamic considerations. In addition, they can experience issues with stability after manufacture due to phase segregation and phase growth. Despite these complications, they offer polymer engineers and material scientists a broad array of materials to meet many demanding application needs. 

The only real sources of difference between the additive and comonomer systems would be the potential lack of homogeneity of the polymer blends, which may result either through incomplete blending or segregation by phase separation. Since such nonhomogeneity could lead to irreproducibility in the burning trials, this would be detected by the flammability testing. 

Compared to bulk polymer mixtures, the interfacial behaviour of polymer blends is essentially different [341]. The demixing process in thin films is strongly affected by the thin film confinement and the interfacial interactions of the blend components with the confining phases (e.g., substrate and air). Even in the one-phase region of the phase diagram, preferential segregation of the components at one of the interfaces leads to a certain composition profile as a function of the distance from the free surfaces and the substrate plane [342,343]. In the... 

The role of the miscibility of semi-IPN components on the mechanical properties has been discussed. The linear bisnadimide was a benzhydrol bisnadimide (Fig. 33). Three polyimides prepared from the same diamine and three different dianhydrides (Fig. 37) were used as linear components. The blends were cured up to 300 °C in a similar fashion to the bisnadimide alone. The results for the blend containing 20% by weight of linear polymers are summarized in Table 9. The non-miscible character of the components gives a phase segregation leading to the best toughness [121]. 

Rather than worry about whether a copolymer of two different monomers is heterogeneous or not, why not just make homopolymers of each and mix them together (make a polymer blend) The answer is easy Most polymers do not like other polymers, and, on mixing, each will tend to segregate into a phase that excludes any other polymers (will phase separate). These different phases tend not to stick to each other very well, resulting in a polymer sample with poor physical properties. We will discover the practical consequences of this in Chapter 7 when we discuss the properties of certain block copolymers called thermoplastic elastomers and in Chapter 9 when we discuss the recycling of plastics. 

Section 3.1 considers the segregation from binary polymer blends towards external interface of a thin film described in a semi-infinite mixture approach. We relate the segregation with wetting phenomena. The role of a vapor and a gas in a classic formulation of this problem is played by two coexisting polymer phases. 

A mean field theory has recently been developed to describe polymer blend confined in a thin film (Sect. 3.2.1). This theory includes both surface fields exerted by two external interfaces bounding thin film. A clear picture of this situation is obtained within a Cahn plot, topologically equivalent to the profile s phase portrait d( >/dz vs < >. It predicts two equilibrium morphologies for blends with separated coexisting phases a bilayer structure for antisymmetric surfaces (each attracting different blend component, Fig. 32) and two-dimensional domains for symmetric surfaces (Fig. 31), both observed [94,114,115,117] experimentally. Four finite size effects are predicted by the theory and observed in pioneer experiments [92,121,130,172,220] (see Sect. 3.2.2) focused on (i) surface segregation (ii) the shape of an intrinsic bilayer profile (iii) coexistence conditions (iv) interfacial width. The size effects (i)-(iii) are closely related, while (i) and (ii) are expected to occur for film thickness D smaller than 6-10 times the value of the intrinsic (mean field) interfacial width w. This cross-over D/w ratio is an approximate evaluation, as the exact value depends strongly on the... 

At the time of this writing, the most immediate application for the surfaces discussed in this section is to provide models for viscous isotropic phase liquid crystals and certain phase-segregated polymer blends and block copolymers. In this subsection, we show how to calculate diffraction peak intensities from a class of model structures based on these surfaces. The method applies to scattering-density profiles (electron densities for x-ray scattering) determined by ... 

Segregated polymer brushes were fabricated utilizing phase-separation phenomena in ultrathin polymer blend films (PS-COOH/PMMA) and the combination of grafting-to and grafting-from methods. 

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