Handbook of Solvents - George Wypych - ChemTech - Ventech!

12.1 Rheological properties, aggregation, permeability 695
Precipitation value, g
40
35
30
25
20
15
10
propionic acid
2-propanol
water
5
15 20 25 30 35 40 45 50 55
Temperature, o C
Figure 12.1.24. Precipitation value for PEI/NMP system
with different non-solvents vs. temperature. [Data
from D Wang, Li K, W K Teo, J. Appl. Polym. Sci., 71,
No.11, 1789-96 (1999).]
Skin layer thickness,μm
0.2
0 2 4 6 8 10
polymer/solvent/non-solvent system. One practices in use is the addition of non-solvents to
the casting solution. 10 The mixture should be designed such that homogeneous casting is
still possible but the thermodynamic condition approaches phase separation. Because of
cooling and/or evaporation of one of the solvents, demixing occurs before material enters
coagulation bath. 10
Figures 12.1.22 and 12.1.23 explain technical principles behind formation of efficient
and selective membrane. Figure 12.1.22 shows a micrograph of hollow PEI fiber produced
from N-methyl-2-pyrrolidone, NMP, which has thin surface layer and uniform pores and
Figure 12.1.23 shows the same fiber obtained from a solution in dimethylformamide, DMF,
which has a thick surface layer and less uniform pores. 9 The effect depends on the interaction
of polar and non-polar components. The compatibility of components was estimated
based on their Hansen’s solubility parameter difference. The compatibility increases as the
solubility parameter difference decreases. 9 Adjusting temperature is another method of control
because the Hansen’s solubility parameter decreases as the temperature increases. A
procedure was developed to determine precipitation values by titration with non-solvent to
a cloud point. 9 Use of this procedure aids in selecting a suitable non-solvent for a given
polymer/solvent system. Figure 12.1.24 shows the results from this method. 9 Successful in
membrane production by either non-solvent inversion or thermally-induced phase separation
requires careful analysis of the compatibilities between polymer and solvent, polymer
and non-solvent, and solvent and non-solvent. Also the processing regime, which includes
temperature control, removal of volatile components, uniformity of solvent replacement
must be carefully controlled.
Efforts must be made to select solvents of low toxicity and to minimize solvent consumption.
An older method of producing polyetherimide membranes involved the use of a
mixture of two solvents: dichloromethane (very volatile) and 1,1,2-trichloroethane together
1.6
1.4
1.2
1
0.8
0.6
0.4
Evaporation time, min
Figure 12.1.25. Skin thickness vs. evaporation time of
formation of asymmetric membrane from polysulfone.
[Data from A Yamasaki, R K Tyagi, A E Fouda,
T Matsura, K Jonasson, J. Appl. Polym. Sci., 71, No.9,
1367-74 (1999).]