Chemistry Reference
In-Depth Information
given polymer. This occurs if one nonsolvent
value is higher and the other is
lower than the solubility parameter of the solute. The solubility parameter of the
mixture
δ
δ
m
is usually approximated from
δ
m
5
δ
A
φ
A
1
δ
B
φ
B
(5-15)
where the
's are volume fractions.
It is believed that the temperature dependence of
ϕ
can be neglected over the
range normally encountered in industrial practice. Most
δ
tabulated solubility
parameters refer to 25
C.
Solubility can be expected if
δ
1
δ
2
is less than about 2(cal cm
2
3
)
1/2
[4
(MPa)
1/2
] and there are no strong polar or hydrogen-bonding interactions in either
the polymer or solvent. Crystalline polymers, however, will be swollen or soft-
ened by solvents with matching solubility parameters but will generally not dis-
solve at temperatures much below their crystal melting points.
Table 5.3
lists solubility parameters for some common polymers and solvents.
The units of
are in (energy/volume)
1/2
and those tabulated, in cal
1/2
cm
2
3/2
, are
called hildebrands. The use of the geometric mean expedient to calculate w
12
in
Eq. (5-6)
in effect assumes that the cohesion of molecules of both species of the
mixture is entirely due to dispersion forces, as mentioned. To allow for the influ-
ence of hydrogen-bonding interactions, it has been found useful to characterize
solvents qualitatively as poorly, moderately, or strongly hydrogen-bonded. The
solvents listed in
Table 5.3
are grouped according to this scheme. Mutual solubil-
ity may not be achieved even if
δ
δ
1
Cδ
2
when the two ingredients of the mixture
have different tendencies for hydrogen bond formation.
The practice of matching solubility parameters and hydrogen-bonding ten-
dency involves some serious theoretical problems, but it is useful if used with
caution. For example, polystyrene, which is classed as poorly hydrogen-bonded
and has a
value of 18.4 (MPa)
1/2
, is highly soluble in the poorly hydrogen-
bonded solvents benzene and chloroform, both of which have matching solubility
parameters. The polymer can be dissolved in methyl ethyl ketone (
δ
19.0
medium hydrogen bonding), but the latter is not nearly as good a solvent as either
of the first pair. (The intrinsic viscosity of a polystyrene of given molecular
weight is higher in chloroform or benzene than in methyl ethyl ketone.) On the
other hand, poly(methyl methacrylate) has practically the same
δ
5
as polystyrene
but is classed as medium hydrogen-bonded. The two polymers are regarded as
incompatible when both have high molecular weights, but benzene and chloro-
form do not seem to be weaker solvents than methyl ethyl ketone for poly(methyl
methacrylate.)
Some of the problems noted here probably reflect the use of an oversimplified
view of hydrogen bonding, in general. However, any attempt to correct this defi-
ciency will most likely complicate the predictive method without a commensurate
gain in practical utility. Improvements on the simple solubility parameter
approach are summarized in
Section 5.2.3
.
δ
