Schneider, A. and Wolf, B. A.

"Specific features of the interfacial tension in the case of phase separated solutions of random copolymers "

**Abstract:** Phase diagrams (cloud point curves, critical points, tie
lines for constant critical composition) and interfacial tensions as a function
of temperature were measured for solutions of two random copolymers: poly(dimethylsiloxane-ran-methylphenylsiloxane)
[I] and poly(styrene-ran-acrylonitrile) [II]. Acetone and anisole served as
solvents for I and toluene for II; all solutions exhibit UCSTs between 300 and
310 K. The phase separation behaviour can be well modeled if one accounts for
the molecular and chemical non-uniformities of the random copolymers used in
this study. The interfacial tensions s differ most markedly from that of comparable
homopolymer solutions in their correlation s = st tm, where t=(Tc-T)/Tc. For
all three systems st results considerably less and the critical exponent m varies
widely from 0.68 to 2.18 (in contrast to the normal case where m is on the order
of 1.3 to 1.5). Both observations are explained in terms of the capability of
copolymers to minimize the interfacial energy by suitably arranging their different
monomeric units. Model calculations were performed in terms of the energy required
to transfer molecules from one phase to the other, assembling the average polymer/solvent
interaction parameter from the three binary interaction parameters gij, required
to describe copolymer solutions. These results demonstrate that the experimentally
observed particularities of copolymers are more likely dominated by dissimilarities
in the concentration dependence of gij than by unlike temperature dependencies.
Particularities in the correlations of the length of the tie line with t and
s, respectively, are also discussed.

preprint number: 201