Handbook of Solvents - George Wypych - ChemTech - Ventech!

5.4 Mixed solvents - polymer solubility 269
Figure 5.4.1. Phase separation temperature (T p), as a
function of solvent mixture composition, determined on
the system PMMA-acetonitrile + chlorobutane, at several
polymer concentrations, x(x 10 2 /g cm -3 ): (a): c = Δ,
12.2; O, 9.17; � 7.14; Δ, 4.36: �, 2.13. (b):c=O,9.47;
�, 8.22; Δ, 5.89; �, 3.69; Δ, 2.17; �, 0.442. From
Fernandez-Pierola and Horta. 65 (Copyright by H�thig &
Wepf Verlag (Basel), used with permission).
( dT dx)
c /
0
⎛ V2
⎜
χ23 −χ13 − χ
⎝
V1
=
( −dχ
/ dT)
13
12
⎞
⎟
⎠
The cosolvency phenomenon was discovered
in 1920’s experimentally for cellulose
nitrate solution systems. 61 Thereafter
cosolvency has been observed for numerous
polymer/mixed solvent systems. Polystyrene
(PS) and polymethylmethacrylate
(PMMA) are undoubtedly the most studied
polymeric solutes in mixed solvents. 62,63
Horta et al. 64 have developed a theoretical
expression to calculate a coefficient
expressing quantitatively the cosolvent
power of a mixture (dT c/dx) 0, where T c is
the critical temperature of the system and x
is the mole fraction of liquid 2 in the solvent
mixture, and subscript zero means x→0.
This derivative expresses the initial slope of
the critical line as a function of solvent
composition (Figure 5.4.1). 65 Large negative
values of (dT c/dx) are the characteristic
feature of the powerful cosolvent systems
reported. 65 The theoretical expression developed
for (dT c/dx) 0 has been written in
terms of the interaction parameters χ i for
the binary systems:
[5.4.1]
where:
Vi the volume fraction of polymer and solvent, respectively
All the magnitudes on the right hand side of this equation are to be evaluated at the
critical temperature corresponding to x=0.
The expression provides a criterion to predict whether or not the mixed solvent is expected
to be a cosolvent of the polymer. When Tc1, is a UCST (as is the case in these phase
separation studies), -dχ13/dT > 0 and (dTc/dx) 0 has the same sign as the numerator of the
equation. Choosing solvent 1 such that Tc2 0,atTc1, the numerator in the equation [5.4.1] is
negative (cosolvent system) if the unfavorable interaction between the two liquids is large
enough to compensate for their different affinity towards the polymer. The equation proposed
gives a more detailed criterion for cosolvency than the simple criterion of G E >0.The
information needed to predict (dTc/dx) 0 from equation [5.4.1] includes the binary interaction
parameters of the polymer in each one of the two solvents as a function of temperature,
and χ12(G E ) for the mixed solvent too. Table 5.4.1 summarizes results reported by Horta et
al. 64 for some cosolvents of polymethylmethacrylate (PMMA). 65