Enthalpically Driven Miscible Systems

This section covers the polymeric systems that, in spite of the dissimilar chemical structures of the blend partners, show miscibility. This is because AH takes in these cases values generally negative, ensuring the miscibility of the system. This situation is made possible via two mechanisms (i) specific interactions and (ii) if at least one component of the blend is a copolymer, the so-called intramolecular repulsion effect or copolymer repulsion effect.  

Experimentally, there are many miscible polymer pairs in which at least one of the components is a random copolymer but specific interactions are not present [40]. This phenomenon is attributed to the so-called intramolecular repulsion effect. Within the familiar Flory-Huggins description, and in the case of a random copolymer A Bi blended with a homopolymer C, three interaction parameters, Xab,Xac and Xbc are required to describe the enthalpy of mixing. Using a mean field theory, the mixture can be described in terms of one parameter XeS given by  

Assuming high-molecular-weight polymers, miscibility requires this parameter to be either zero or negative. As the equation shows, this can be accomplished even if all parameters are positive (i.e., systems without specific interactions), provided that Xab is sufficiently large hence, the phrase intramolecular repulsion  

Revision of Polymer Blends Based on Biod radable Polyesters 

In this section, the structure, preparation, miscibility and properties of different polymer blends based on biodegradable polyesters are reviewed. Polylactide-based blends are first revisited and discussed, and the miscibility behaviors of other commercially available biopolymers of great interest such as poly(E-caprolactone) (PCL), poly(3-hydroxybutyrate) (PHB), poly(p-dioxanone) (PPDO) and polyglyco-lide (PGA) are briefly reviewed. Finally, Appendix 2.A provides a brief outline of the investigations used to develop the miscibility study presented here. In this recap, it is possible to find the chemical structures of the polymers and their solubility parameters, and also some brief comments summarizing the research studies. 

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