  Theoretical considerations based on chain connectivity and conformational
variability of polymers have lead to an uncomplicated relation for the
dependence of the Flory-Huggins interaction parameter, chi, on the volume
fraction of the polymer, phi, and on its number of segments, N. The validity of
this expression is being tested extensively by means of vapor pressure
measurements and inverse gas chromatography (complemented by osmotic and light
scattering data from literature) for solutions of poly(dimethylsiloxane) in the
thermodynamically vastly different solvents n-octane (n-C8), toluene (TL), and
methylethylketone (MEK) over the entire range of composition for at least six
different molecular masses of the polymer. The new approach is capable to model
the measured chi (phi, N) very well, irrespective of the thermodynamic quality
of the solvent, in contrast to traditional expressions, which are often
restricted to good solvents but fail for bad ones and vice versa. At constant
polymer concentration the chi values result lowest for n-C8 (best solvent) and
highest for MEK (theta solvent); the data for TL fall between. The influences
of N depend strongly on the thermodynamic quality of the solvent and are not
restricted to dilute solutions. For good solvents chi increases with rising N.
The effect is most pronounced for n-C8, where the different curves for chi
(phi) fan out considerably. The influences of N become less distinct for TL,
and for MEK they vanish at the (endothermal) theta temperature. For worse than
theta conditions, the chi values of the long chains become less than that of
short ones. This change in the sign of N-influences is in agreement with the
present concept of conformational relaxation.
