chapter  18
26 Pages

Triblock Copolymers for Toughness Modification of Immiscible Engineering Polymer Blends

ByVolker Altstädt, Miroslawa El Fray, Thorsten Kirschnick, Harald Ott

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:


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 /.