Modifiers Initially Miscible in Thermoset Precursors

Modifier miscibility plays an important role in this preparation. On the one hand, modifiers must be miscible with the reactive system on the other hand, they must phase-separate during cure. Final morphologies are influenced by the phase-separation conditions. 

The state of miscibility of any mixture is governed by the Gibbs free energy of mixing, AG, which may be described by the lattice theory of Flory-Huggins (Flory, 1953), as follows  

A unit cell is defined with a molar volume Vref. This reference volume may be selected as the molar volume of the thermoset precursor(s), Vxs, or as the molar volume of the constitutive repeating unit of the modifier, Vm. R is the gas constant and )ts and are the volume fractions of the thermoset precursor(s) and of the modifier i-mer, respectively )i = S )M,i- 

In principle, the interaction parameter x must be considered as a function of temperature, T, composition, and the average degree of polymerization of the modifier component. If / decreases with temperature, then 

The interaction parameter x can also be estimated by the use of solubility parameters (8) x is proportional to (8m — 8xs) - But this approach has a considerable error (A8 in the range of 0.4 MPa ), and considers only the excess free enthalpy. For these reasons it is better to determine the miscibility window experimentally. 

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