called theupper critical solution temperature(UCST). Nevertheless, some polymer
solutions exhibit the critical point for phase separation upon temperature rise. The
latter is called thelower critical solution temperature(LCST), as illustrated in
Fig.9.3.
With further consideration of the mixing entropy in the interaction parameter
w¼AþB
T
(9.16)
the LCST phenomenon occurs whenA>0 andB<0. There might be various
mechanisms for the LCST phenomenon. One is the stable hydrogen-bonding
interactions formed between solute and solvent, and the phase separation occurs
at high temperatures that favor the strong thermal motions to suppress these specific
interactions (the lattice-fluid theory, see Sect.8.3.2). Another more general mecha-
nism is the higher compressibility of the solvent with the increase of temperature
near its boiling point. Phase separation occurs due to the entropy constraint to
Fig. 9.2 Illustration of phase
diagrams for the sequential
precipitation of various
molecular weight fractions
Mwith the increase of the
interaction parameterw
Fig. 9.3Illustration of LCST
and UCST phase diagrams
170 9 Polymer Phase Separation