Compatibilization Compatibilizer practical

The properties of polymer materials can e greatly extended by blending two or more homopolymers together. Blends may be classified as compatible or incompatible - although this does depend on the dimensions being considered. Compatibility is influenced by the molecular weight of the homopolymers and is enhanced in practice by incorporation of block copolymers and other compatibilizers. The effects of radiation on blends depend on the degree of compatibility and the extent of inter-molecular interaction (physically and chemically) between the different types of homopolymers. 

As noted above, PBT forms well-compatibilized blends with SAN. Usually an ABS resin is used to improve practical impact. ABS resins impart to the blend less shrink and better dimensional stability. The ABS is present as a separate... 

Block copolymers are widely used industrially. In the solid and rubbery states they are used as thermoplastic elastomers, with applications such as impact modification, compatibilization and pressure-sensitive adhesion. In solution, their surfactant properties are exploited in foams, oil additives, solubilizers, thickeners and dispersion agents to name a few. Particularly useful reviews of applications of block copolymers in the solid state are contained in the two books edited by Goodman (1982,1985) and the review article by Riess etal. (1985). The applications of block copolymers in solution have been summarized by Schmolka (1991) and Nace (1996). This book is concerned with the physics underlying the practical applications of block copolymers. Both structural and dynamical properties are considered for melts, solids, dilute solutions and concentrated solutions. The book is organized such that each of these states is considered in a separate chapter. 

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. 

Given the morphological complexity of AB diblock and ABA triblock copolymers, it might be expected that the phase behaviour of ABC triblocks would be even more rich, and indeed this has been confirmed by recent experiments from a number of groups. From a practical viewpoint, ABC triblocks can also act as compatibilizers in blends of A and C homopolymers (Auschra and Stadler 1993). In addition to the composition of the copolymer, an important driving force for structure formation in these polymers is the relative strength of incompatibilities between the components, and this has been explored by synthesis of chemically distinct materials. 

Since they act as surfactants, copolymers are added in only small amounts, typically from a thousandth parts to a few hundredth parts. Theoretically, Leibler [30] showed that only 2% of a diblock copolymer may thermodynamically stabilize an 80%/20% incompatible blend with an optimum morphology (submicronic droplets). However, in practice kinetic control and micelle formation interfere in this best-case scenario. To a some extent, compatibilization increases with copolymer concentration [8,31,32], Beyond a critical concentration (critical micellar concentration cmc) little or no improvement is observed (moreover, for high amounts, the copolymer can act as a plasticizer). Copolymer molecular weight influence is similar to that of the concentration effect. For example, in a PS/PDMS system [8,31,32], when the copolymer molecular weight increases, domain size decreases to a certain extent. Hu et al. [31] correlated their experimental results with theoretical prediction of the Leibler s brush theory [30]. Leibler distinguishes two regimes to characterize the behaviour of the copolymer at the interface... 

The importance of these block copolymers is realized when they are used as minor components with normally immiscible homopolymers (each having the composition of one of the blocks). In the absence of the block copolymer (compatibilizer), the resulting two-phase system might not be of practical usefulness. The often high interfacial tension between the two phases results in poor dispersion of the minor phase in the other, continuous phase. On processing, this will lead to macroscopic separation, so the result is a crumbly, useless material. The presence of a minor third component, the block copolymer, can enhance adhesion between the two components and so stabilize the morphology of the system. This process is discussed in more detail later. 

The concept of the solubility parameter (Section 1.3.1) leads to the conclusion that the ideal block-copolymer compatibilizer would have components that were identical to the two phases that were to be stabilized. Ideally, the chain length of each block would also match that of the corresponding phase, so ensuring total interpenetration of the copolymer block into each homopolymer. However, it has been demonstrated (Boimer and Hope, 1993) that this is not required and practical considerations dominate, such as... 

In practice, salting-out electrolytes make water even more incompatible with oil. The result is a decrease in the Winsor transition temperatures and an increase in the supertricritical character. The amount of amphiphile necessary to compatibilize water and oil generally increases in the presence of a salting-out electrolyte. All these tendencies are reversed with a salting-in electrolyte. Figure 3.22 illustrates the effects of different electrolytes on a C4E2-C13 acetate-water system. 

It is also not the purpose of this chapter to summarize examples of compatible polymer blends formed in a solution step involving dissolution of the polymer components. In some cases such blends are only pseudo-stable , since they may not have been processed above the Tg of one or both of the components. Also, mixing in solution followed by devolatilization is rarely economical for practice in industry, particularly since many commercially important compatibilized polymer blends comprise at least one semicrystalline component e.g., PA) which is poorly soluble in common solvents. There are included in the Tables a small number of examples of solution blended polymer blends when these complement similar examples prepared by melt processing. 

Reactive Compatibilization has been discussed in earlier reviews [Brown and Orlando, 1988 Tzoganakis, 1989 Brown, 1992 Liu and Baker, 1992a]. As practiced commercially. Reactive Compatibilization is a continuous extruder process with material residence time usually 1 -5 min. Such a process permits large scale preparation of a polymer blend as needed ( Just-In-Time inventory control). 

Simple blends of ABS and PA are highly immiscible and hence are of little practical value. Compatibilization of ABS with polyamide was accomplished by several methods, most of which involving structural modification of ABS. In one approach, ABS was modified by copolymerization with acrylamide, during the preparation of ABS by the standard emulsion polymerization. The introduction of polar acrylamide units on the... 

The mechanical properties and end-use performance of a blend have been improved by compatibilization. From a practical point of view, a blend is often considered to be compatible if a certain set of mechanical properties is achieved. 

Practically all plastics are compounded with other products (additives, fillers, reinforcements, etc.) to provide many different properties and/ or processing capabilities. It includes mechanical mixing/blending. They do not normally depend on chemical bonds, but do often require special compatibilizers. Mechanical compoimding is extensively used worldwide. 

Table 1 lists examples of compatible, partially incompatible and incompatible polymer blends. When creating a polymer blend, one must always keep in mind that the blend will most probably be remelted in subsequent processing or shaping processes. For example, a rapidly cooled system that was frozen as a homogenous mixture can split into separate phases, due to coalescence, when reheated. For all practical purposes, such a blend would not be acceptable for processing. To avoid this problem, compatibilizers which are macromolecules used to ensure compatibility in the boundary layers between the two phases, are common. 

The dynamic behavior of polymer blends under low strain has been theoretically treated from the perspective of microrheology. Table 2.3 lists a summary of this approach. These models well describe the experimental data within the range of stresses and concentrations where neither drop-breakup nor coalescence takes place. The two latter models yield similar predictions as that of Palierne. The last entry in the Table 2.3 is an empirical modification of Palieme s model by replacement of the volume fraction of dispersed phase by its efiective quantity (Eq. (2.24)), which extends the applicability of the relation up to 0 < 0.449. However, at these high concentrations the drop-drop interactions absent in the Palierne model must complicate the flow and coalescence is expected. The practical solution to the latter problem is compatibilization, but the presence of the third component in blends has not been treated theoretically. 

Xanthos, M. (2009) Chapter 20 compatibilizers (theory and practice), in Mixing and Compounding of Polymers, 2nd edn (ed. I. Manas-Zloczower), Carl Hanser Verlag, Munich, Germany. 

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