Handbook of Solvents - George Wypych - ChemTech - Ventech!

348 George Wypych
Diffusion distance, mm
3.5
3
2.5
2
1.5
1
0.5
0
0 2000 4000 6000 8000 10000
Time, min
Figure 7.1.13. Diffusion distance of 1,4-dioxane
into PVC vs. time. [Adapted, by permission, from
M Ercken, P Adriensens, D Vanderzande, J Gelan,
Macromolecules, 28, 8541-8547, (1995).]
Equilibrium degree of swelling
25
2 3 4 5 6 7 8
tems, etc. Maximum swelling is obtained at pH = 7. At this point there is complete dissociation
of the acidic groups present in the hydrogel. The behavior of hydrogel can be modelled
using Brannon-Peppas equation. 22
Figure 7.1.15 shows the 1,1,2,2-tetrachoroethylene, TCE, uptake by amorphous
poly(ether ether ketone), PEEK, as a function of time. 24,25 The swelling behavior of PEEK in
this solvent is very unusual - the sample mass is increased by 165% which is about 3 times
more than with any other solvent. In addition, the solvent uptake by PEEK results in a
change in optical properties of the solution from clear to opaque. A clear solution is typical
of amorphous PEEK and the opaque solution of crystalline PEEK. It was previously suggested
by Fokes and Tischler 26 that polymethylmethacrylate forms weak complexes with
various acid species in solution. This may also explain the unusual swelling caused by
TCE. Because of the presence of C=O, C-O-C, and aromatic groups, PEEK acts as organic
base. TCE is an electron acceptor due to electron-deficient atoms in the molecule. 24 This interaction
may explain the strong affinity of solvent and polymer. Figure 7.1.16 illustrates
the effect of crystallization. Below 250 o C, the carbonyl frequency decreases. But a more
rapid decrease begins below 140 o C which is glass transition temperature. Above 250 o C, the
carbonyl frequency increases rapidly. Above the glass transition temperature there is rapid
crystallization process which continues until the polymer starts to melt at 250 o C then it
gradually reverts to its original amorphous structure. The presence of solvent aids in the
crystallization process.
7.1.3 DRYING
Solvent removal can be accomplished by one of three means: deswelling, drying or changes
in the material’s solubility. The deswelling process, which involves the crystallization of
solvent in the surrounding gel, was discussed in the previous section. Here attention is focused
on drying process. The changes due to material solubility are discussed in Chapter 12.
40
35
30
pH
Figure 7.1.14. Effect of pH on the equilibrium swelling of
hydrogel. [Data from M Sen, A Yakar, O Guven, Polymer,
40, No.11, 2969-74 (1999).]