Handbook of Solvents - George Wypych - ChemTech - Ventech!

5.4 Mixed solvents - polymer solubility 277
5.4.5 POLYMER STRUCTURE OF THE POLYMER DEPENDENCE OF
PREFERENTIAL ADSORPTION. POLYMER MOLECULAR WEIGHT AND
TACTICITY DEPENDENCE OF PREFERENTIAL ADSORPTION
There are some important structural aspects of the polymer which are necessary to take into
account in the analysis of the polymer behavior in mixture solvents, such as its polarity,
chemical structure, microtacticity, molecular weight. The analysis of these properties
shows that they are determinant factors in preferential adsorption phenomena involved. It
has been pointed out 73 that the effect of tacticity, and particularly the molecular weight, is a
complex problem. In the case of poly(2-vinylpyridine), when the polar solvent is preferentially
adsorbed, preferential solvation is independent of molecular weight; but when the
non-polar solvent is adsorbed, there is a dependence on the molecular weight. 62
The reported experimental evidence 109-111,67,73,77 seems to show that the coefficient of
preferential adsorption λ, for a given polymer in a mixed solvent of fixed composition depends
on molecular weight of the polymer sample. It is important to remember, however,
that this dependence of λ on M has not been always detected. Particularly for molecular
weights lower than a certain value. 100,111,91
According to Dondos and Benoit, 109 Read, 104 and Hertz and Strazielle, 112 the dependence
of preferential adsorption coefficient λ, with molecular weight or with segment density
can be expressed empirically as:
or
λ= λ∞
+
−
AM 12 /
λ= λ + η
∞
−
K[] 1
[5.4.4]
[5.4.5]
where:
λ∞ the value of λ extrapolated to M→∞
[η] the intrinsic viscosity
A and K constants
It is interesting to note that the variation of λ with M is more pronounced. A in equation
[5.4.4] is larger in mixtures which are poor solvents close to θ-conditions than in mixtures
with excluded volume. 109,111,112
Apparently, there will not be exhaustive results either with the chemical structure of
the polymer on the preferential adsorption, 113 or the influence of the tacticity on the preferential
adsorption. In the last years, investigations regarding the effect of ortho-substituents
in polymers with aromatic bulky side groups on the preferential adsorption and viscometric
behavior have been reported for poly(phenyl methacrylate) and its dimethyl and diisopropyl
ortho derivatives in tetrahydrofuran/water. 114 Figure 5.4.5 from ref. 114 shows the λvalues for
three polymers in THF/water.
The λ values diminish when the volume of the side groups increases and there is a
strong water adsorption. The behavior reported 114 indicates that the cosolvent effect decreases
or disappears when the preferential adsorption is very small or is not observed. 114
In another publication, 102 the systems studied were a series of poly(alkyl methacrylates)
including the methyl (PMMA), ethyl (PEMA), isobutyl (PiBMA) and cyclohexyl
(PCHMA) substituents in the mixture solvent 1,4-dioxane/methanol. The experimental