Handbook of Solvents - George Wypych - ChemTech - Ventech!

7.1 Introduction to diffusion, swelling, and drying 347
Degree of swelling
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 20 40 60 80 100
2 [ ( 1 ) ]
n RT
Π= Aϕ
=− ln − ϕ + ϕ+ χϕ
[7.1.12]
v
1
Time, min
i-octane
toluene
Figure 7.1.11. Swelling kinetics of EVA in toluene and
i-octane. [Data from H J Mencer, Z Gomzi, Eur. Polym.
J., 30, 1, 33-36, (1994).]
Degree of swelling
where:
Π osmotic pressure
A coefficient
ϕ volume fraction of polymer in solution
n = 2.25 for good solvent and =3forΘsolvent
v1 molar volume of solvent
χ Flory-Huggins, polymer-solvent interaction parameter
The above relationship is used in to study swelling by measuring shear modulus. 20
Figure 7.1.11 shows swelling kinetic curves for two solvents. Toluene has a solubility
parameter of 18.2 and i-octane of 15.6. The degree to which a polymer swells is determined
by many factors, including the chemical structures of polymer and solvent, the molecular
mass and chain flexibility of the polymer, packing density, the presence and density of
crosslinks, temperature, and pressure. In the example presented in Figure 7.1.11 the solubility
parameter has a strong influence on swelling kinetics. 17 The effect of temperature on
swelling kinetics is shown in Figure 7.1.12. Increasing temperature increases swelling rate.
During the initial stages of the swelling process the rate of swelling grows very rapidly and
then levels off towards the swelling equilibrium.
In Figure 7.1.13, the diffusion distance is almost linear with time as predicted by equation
[7.1.11]. The coefficient n of the equation was 0.91 meaning that the swelling process
was relaxation rate controlled.
Figure 7.1.14 shows the relationship between hydrogel swelling and pH. Hydrogels
are particularly interesting because their swelling properties are controlled by the conditions
around them (e.g. pH). 21-23 Because they undergo controllable volume changes, they
find applications in separation processes, drug delivery systems, immobilized enzyme sys-
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
298K
303K
308K
0 20 40 60 80 100 120
Time, min
Figure 7.1.12. Swelling kinetics of EVA in
tetrahydrofuran at different temperatures. [Adapted, by
permission, from H J Mencer, Z Gomzi, Eur. Polym. J.,
30, 1, 33-36, (1994).]