Intermolecular interactions, miscibility, blended polymer thermodynamics

In a fundamental sense, the miscibility, adhesion, interfacial energies, and morphology developed are all thermodynamically interrelated in a complex way to the interaction forces between the polymers. Miscibility of a polymer blend containing two polymers depends on the mutual solubility of the polymeric components. The blend is termed compatible when the solubility parameter of the two components are close to each other and show a single-phase transition temperature. However, most polymer pairs tend to be immiscible due to differences in their viscoelastic properties, surface-tensions, and intermolecular interactions. According to the terminology, the polymer pairs are incompatible and show separate glass transitions. For many purposes, miscibility in polymer blends is neither required nor de-... 

The thermodynamics of blends of polymers of various flexibility and rigid molecules has been thoroughly treated from the theoretical point of view by Flory and Abe [126], who analyzed the phase equilibria as a function of various parameters, such as molecular structure, molecular mass, temperature, and concentration. The theoretical data indicate that systems constituted by rigid and flexible molecules are mainly heterophasic with hmited miscibility effects between the components. In general, it has been observed that the phase behavior of polymer/LC blends is affected by the chemical structure and concentration of the polymer, as well as by the intermolecular interactions with the LC component [127,128]. Studies on the thermodynamics and kinetics of phase transitions in polymer/LC blends have been reported for a few systems. These studies are of considerable interest due to the possibility of varying the stability range of the mesophase, as a function of composition, or even obtaining the formation of induced mesophases [129]. 

In real systems, nonrandom mixing effects, potentially caused by local polymer architecture and interchain forces, can have profound consequences on how intermolecular attractive potentials influence miscibility. Such nonideal effects can lead to large corrections, of both excess entropic and enthalpic origin, to the mean-field Flory-Huggins theory. As discussed in Section IV, for flexible chain blends of prime experimental interest the excess entropic contribution seems very small. Thus, attractive interactions, or enthalpy of mixing effects, are expected to often play a dominant role in determining blend miscibility. In this section we examine these enthalpic effects within the context of thermodynamic pertubation theory for atomistic, semiflexible, and Gaussian thread models. In addition, the validity of a Hildebrand-like molecular solubility parameter approach based on pure component properties is examined. 

There are several factors that are either not included or assumed to be unimportant in the Stockmayer-Kennedy theory. Hydrodynamic interactions between two chains in a block copolymer are not included in the theory. This can be a serious omission, especially when dealing with low-molecular-weight diblock copolymers, such as the SI diblock copolymers considered above. Intermolecular (thermodynamic) interactions between chemically dissimilar chains are also neglected. Inclusion of such interactions can be important, especially when the chemical affinity between two chemically dissimilar chains is rather poor, such as the case in the SI diblock copolymers considered here. In Chapter 7, which discusses the rheology of miscible polymer blends, we pointed out the importance of the segmental interaction parameter in the prediction of the linear... 

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