Compatibilization concepts

In this chapter, the basic thermodynamic considerations regarding polymer miscibility win first be described, followed by a discussion of the phase behavior in heterogeneous polymer hlends, and of the properties of the interfaces and interphases involved. The morphology of polymer blends and its development during processing will then he outlined, after which details of the most common compatibilization concepts will be summarized. However, before discussing these topics in detail, it is important to define the terms used ... 

Gaylord NG. Compatibilization concepts in polymer applications. Adv Chem 1975 142 76-84. 

Caulfield, D.F., Koutsky, J.A. and Quillen, D.J. (1993). CeUulose/polypropylene composites the use of AKD and ASA sizes as compatibilizers. In Wood Fiber/Polymer Composites Fundamental Concepts, Processes, and Material Options, Wolcott, M.P. (Ed.). Forest Products Society, pp. 128-134. 

By combining the concepts of copolymer homogeneity, matching refractive indices, and partial compatibilization via grafting, impact resistant polyblend systems can be produced from numerous monomer combinations that approach optical clarity. 

Over the past decade or so, these remarkable achievements by nature have been recognized by the polymer science community. This has led to an increased interest in the use of biological concepts to synthesize polymers or to control the structure and properties of synthetic polymers. Of particular interest are peptide hybrid polymers. Combining peptide and synthetic polymer segments into a single macromolecule offers interesting possibilities to synergize the properties of the individual components and to compatibilize bio- and synthetic systems. 

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... 

Extrapolation of the concept of the compatibility of polymers with different structures but similar solubility parameters to the construction of effective compatibilizing agents leads to the use of block and graft copolymers having segments with suitable solubility parameters to compatibilize polymers which differ both in structure and solubility parameter. Thus, an AB block or graft copolymer compatibilizes polymers A and C when C has a solubility parameter similar to that of B, polymers D and B when D has a solubility parameter similar to that of A, and polymers C and D when the solubility parameter of C is similar to that of B, and D has a solubility parameter similar to that of A. 

PC/ABS blend is an injection moldable material widely used for automotive parts. These blends, developed in 1960, can be prepared by simple meltmixing of PC with ABS, without any compatibilizer. In this early stage of polymer blends technology, the phase diagram of polymer/polymer mixture, the spinodal decomposition, and the effect of the flow fields were not seriously considered. Nowadays, using these concepts, one can describe the processes that are behind the simple melt-mixing . 

Specifically, PVC blends with polyethylene, polypropylene, or polystyrene could offer significant potential. PVC offers rigidity combined with flammability resistance. In essence, PVC offers the promise to be the lowest cost method to flame retard these polymers. The processing temperatures for the polyolefins and polystyrene are within the critical range for PVC. In fact, addition of the polyolefins to PVC should enhance its ability to be extruded and injected molded. PVC has been utilized in blends with functional styrenics (ABS and styrene-maleic anhydride co-and terpolymers) as well as PMMA offering the key advantage of improved flame resistance. Reactive extrusion concepts applied to PVC blends with polyolefins and polystyrene appear to be a facile method for compatibilization should the proper chemical modifications be found. He et al. [1997] noted the use of solid-state chlorinated polyethylene as a compatibilizer for PVC/LLDPE blends with a significant improvement in mechanical properties. A recent treatise [Datta and Lohse,... 

A key paper involving the experimental interfacial aspects of polymer blends discussed the blends of more than two components wherein a polymeric constituent will concentrate at the interface between two of the blend constituents [Hobbs et al., 1988]. Employing the concepts of interfacial relationships, it was shown that a ternary component can concentrate at the interface between the other constituents and allow for compatibilization of dissimilar and incompatible components. As an example, it was shown that in the ternary blend of PMMA/PC/PBT, PC encapsulates PMMA as a dispersed phase in a matrix of PBT. PC, which exhibits partial miscibility with PMMA and PBT thus compatibil-izes PMMA/PBT blends. 

Another approach is to incorporate compatibilizers or interfacial agents into the blend to improve adhesion between phases. The concept of compatibilization of polymers is described in a later section. 

In the following, the influence of the degradation on structure and properties of the polymeric materials will be reviewed, considering in particular the thermomechanical degradation. As for the blends, the basic concepts regarding the behavior of polyphasic systems and compatibilization will be presented. 

Compatibilization by the achievement of thermodynamic miscibility is a concept that has been exploited in only a handful of situations to produce a commercial blend. The miscibility between polymers is determined by a balance of enthalpy and entropic contributions to the free energy of mixing. While the entropy of small molecules is high enough to ensure miscibility, the entropy of polymers is almost zero, which means that the entropy will determine the miscibility. " ... 

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