Christian Wohlfarth
Enthalpies of solution or mixing, expressed as the enthalpy change per unit mass of polymer, are given in the table at infinite dilution, i.e., a very small amount of polymer and a large excess of solvent were mixed isothermally to form a homogeneous solution. By thermodynamics, ΔsolHB∞ or ΔMHB∞ are obtained from the following derivatives:
ΔsolHB∞ = limmB→0(∂Δsolh/∂mB)P,T,mj≠B (1)
ΔMHB ∞ = limmB→0(∂ΔMh/∂mB)P,T,mj≠B (2)
with a unit of J g-1. Thus, they are the partial specific enthalpies of solution or mixing of the polymer B at infinite dilution where ∆solh or ∆Mh is the extensive enthalpy of the solution or mixing process.
The state of the polymer before dissolution can significantly affect the enthalpy of solution. The dissolving of a semicrystalline polymer requires an additional amount of heat associated with the disordering of crystalline regions. Consequently, its enthalpy of solution is usually positive and depends on the degree of crystallinity of the given polymer sample. An amorphous polymer below its glass transition temperature, Tg (see the Tg-table of this section), often dissolves with the release of heat. The enthalpy of solution of a glassy polymer is additionally dependent to some extent on the thermal history of the glass-forming process. An amorphous polymer above Tg can show endothermic or exothermic dissolution behavior depending on the nature of the solvent and the interaction energies involved as is the case for any enthalpy of mixing. This enthalpy of mixing is then independent of any crystalline or glassy aspects of the polymer. It can be obtained without difficulties for liquid/molten polymers mixed with a solvent. Therefore, the enthalpies given in the table are either enthalpies of solution or enthalpies of mixing, depending on the state of the polymer.
The enthalpies depend on temperature and molar mass. The necessary molar mass information for a system is provided in the table (if available) by the corresponding number average, Mn, mass average, Mw, or viscosity average, Mη, values of the polymer as given in the original sources. Outside the oligomer range, specific enthalpies of solution or mixing do not remarkably depend on molar mass, however. More enthalpy data of polymer-solvent systems can be found in Ref. 106.
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