ABSTRACT
I. OVERVIEW A. Introduction Polymer blends can be placed into two widely used categories: miscible and im miscible (1-4). Miscible blends involve thermodynamic solubility and are char acterized by the presence o f one phase and a single glass transition temperature. Immiscible blends are phase separated, exhibiting the glass transition tempera tures and/or melting temperatures o f each blend component. The latter blends are o f great interest because they can combine some o f the important characteristics o f each constitutive component. The im m iscib ility problem o f most polymer blends is caused not only by differences in the chemical structure o f thermody namically immiscible components, but also by differences in the phase behavior o f these polymers. Two polymers or copolymers can be blended to give a misci ble polymer blend only when certain temperature and composition conditions are met. The focus in research on the m iscibility o f polymers is usually the determi nation o f the Flory interaction parameter x-From the classical Huggins-Flory re lation the free energy o f mixing can be expressed as follows:
(1)
where R, T, V, 4>i, and x 12 are, respectively, the gas constant, temperature, (mo lar) volume o f the system, the volume fraction o f component / = 1, 2 , and the polymer-polymer interaction parameter. Equation (1) states that for the m iscibil ity o f the polymer blend system X12 < 0 is required (the condition o f AGm < 0 for the m iscibility o f polymer blends can exist only for negative values o f the binary polymer-polymer interaction parameter). But in fact for most mixtures, the en thalpy o f m ixing AH m is positive. The m iscib ility o f polymers is determined largely by the value o f x, which is the dominating parameter for the equilibriumphase diagram o f the blend. The magnitude o f x depends on the reference volume V, and thus one cannot compare the values from one blend with those from another unless the reference volumes are the same. As a result, the cohesive (interaction) energy density B was applied as a parameter to describe the m iscibility o f a blend B = X12RT/V. The advantage o f the cohesive energy density approach is that it can easily be compared between blends, and it can be estimated from solubility pa rameters by using the equation: B — {81 — 82)2, where 8, is the solubility param eter o f component /.
