Biomedical Engineering Reference
In-Depth Information
uptake. The known successful fabrication of the sulfonated ion-exchange membranes
was based upon grinding ion-exchange resins to microscale powders and mixing
them with hydrophobic thermoplastic materials including polyethylene or polyviny-
lidinefluoride and sheeting them at elevated temperatures.
Recently, inclusion of a hydrophobic polymer at the formulation stage became
common and known as the paste method (Davis et al., 1997). In such a method, a fine
powder of hydrophobic polymer is mixed into paste with the liquid phase monomers.
The initiator and a plasticizer typically control the final product. The basic chemistry
is shown in figure 3.1, where the copolymerization is carried out with a predetermined
ratio of styrene and divinylbenzene and, occasionally, with ethylstyrene. Also, the
random intertwining of the polymer chains is illustrated in the three-dimensional
manner. The fixed anionic groups are typically sulfonated in nature. The effective
sulfonation process can be carried out under highly concentrated sulfuric acid at
elevated temperature for a long time. As the reaction proceeds, swelling and evolution
of heat are produced so as to require precautions in order to prevent material weakening.
Although the chemical processes to make ion-exchange materials are well tai-
lored, the remaining challenges are how to fabricate them into membrane format. A
popular method known today is to incorporate hydrophobic thermoplastic materials,
such as polyvinylidinefluoride or polyethylene, and press role them out as a sheet
form at elevated temperatures. Such a method can provide a good mechanical
strength and chemical stability. However, the drawback is to lower the electric
conductivity of the membrane products. Heterogeneous fabrication techniques were
also often incorporated to address such a problem.
Another interesting technique is to modify the membrane surfaces to improve
ion selectivity by engineering them to effectively distinguish monovalent and divalent
cations for specific applications. This can be done by immobilizing the positively
CH CH
2
CH CH
2
CH CH
2
CH CH
2
CH CH
2
CH
2
CH
2
SO
3
Polymer chain
Crosslink
FIGURE 3.1
Styrene/divinylbenzene-based ion-exchange material (top) and
its structural
representation (bottom).
