Miscibility Criterion in Polymer Blends and its Determination

Valeria Arrighi,John M.C. Cowie, Steven Fuhrmann, and Abdelsallam Youssef 

The mixing of two or more polymers is an established method used to attain new property combinations that overcomes the need to synthesize new structures with the desired characteristics [1], The development of any polymer blend system, including selection and performance, crucially depends on our ability to predict and control phase behavior as well as blend morphology. To achieve this, understanding of the parameters that govern polymer-polymer miscibility is vital. 

The term miscibility is used to describe a mixture containing two or more components able to form a one-phase (solid or liquid) system that is homogeneous down to the molecular level [2]. Thermodynamics offers an unambiguous way to predict whether a binary mixture is miscible or immiscible and provides a quantitative determination of the polymer-polymer interaction energies. This is important since the interaction between polymer-polymer pairs affects the physical and mechanical properties of miscible systems and, for immiscible ones, it determines the nature and the width of the interface between the two polymers. 

While the above definition of miscibility is unequivocal as it corresponds to a precise thermodynamic description of the system, in practical terms the experimental determination of polymer blend miscibility is not free from ambiguities. This is because the experimental determination of homogeneity in a blend is dependent on the experimental probe and, consequently, while a system may appear to be one-phase if examined at sufficiently large length scales, it may not be truly miscible at the molecular level. 

One of the primary tasks of the investigations carried out in the past few decades has been the control of the structure and properties of multicomponent systems. These are interrelated and therefore the control and design of the structures that develop in multicomponent systems is fundamental to produce novel properties. Understanding of phase separation and spinodal decomposition is fundamental and this involves knowledge of the thermodynamics of the system, phase transitions and critical phenomena and a general evaluation of the degree of miscibility of the polymeric blend components. 

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