  This is the first of two parts investigating the Flory-Huggins interaction
parameter chi as a function of composition and chain length. Part I encompasses
experimental and theoretical work. The former comprises the synthesis of
poly(dimethylsiloxane)s with different molar mass and the measurements of their
second osmotic virial coefficients in solvents of diverse quality as a function
of M via light scattering and osmotic pressures. The theoretical analysis is
performed by subdividing the dilution process into two clearly separable
states. It yields a simple expression for the range of pair interaction
containing three parameters. The parameter alpha measures the effect of contact
formation between solvent molecules and polymer segments at fixed chain
conformation, whereas the parameter zeta quantifies the contributions of the
conformational changes taking place in response to dilution; zeta becomes zero
for theta conditions. &#61472;The influences of M are exclusively contained in
the parameter lambda&#61486;&#61472; The new relation is capable of describing
hitherto incomprehensible experimental findings, like a diminution of chi with
rising M. The evaluation of experimental information for different systems
according to the established equation displays the existence of a linear
interrelation between zeta and alpha. Part II of this investigation generalizes
the present approach to solutions of arbitrary composition and discusses the
physical meaning of the parameters in more detail.
