Page 156 - Chiral Separation Techniques

134 5 Membranes in Chiral Separations

a) b)

Fig. 5-4. (a) Separation of d,l-phenylalanine by an amino acid immobilized in the pores of a polysul-
fone ultrafiltration membrane. (b) Effect of volume flux on the separation factor, Jv = volume flux,
T = 37 °C [32].

5.2.3 Molecular Imprinted Polymers

In addition to polymers typically used for chromatographic purposes, molecular
imprinted polymers, i.e. polymers having enantiospecific cavities in the bulk phase,
can be used as a basis for chiral membranes. In the past two decades, a novel tech-
nique of introducing molecular recognition sites into polymeric materials by molec-
ular imprinting has been developed [42, 43]. Molecularly imprinted polymers
(MIPs) find among others application as stationary phases in chromatography, sen-
sors, membranes, and catalysts. The preparation procedure of a MIP involves the for-
mation of a template– monomer complex. During the subsequent polymerization
with crosslinking agents, the geometry of the self-assembled template–monomer
complex is captured in the polymer matrix. After removal of the templates, the cav-
ities of the MIP possess a shape and an arrangement corresponding to the functional
groups of the template (Fig. 5-5).
Recently, an in-depth review on molecular imprinted membranes has been pub-
lished by Piletsky et al. [4]. Four preparation strategies for MIP membranes can be
distinguished: (i) in-situ polymerization by bulk crosslinking; (ii) preparation by dry
phase inversion with a casting/solvent evaporation process [45–51]; (iii) preparation
by wet phase inversion with a casting/immersion precipitation [52–54]; and (iv) sur-
face imprinting.
With regard to MIP membranes, the polymer morphology directly affects perme-
ability and selectivity of the membrane. The interactions of the template with poly-
mer domains may cause conformational reorganization of the MIP network struc-
ture. It is suggested [44] that chiral separation by MIP membranes occurs by sieving
as well as by selective transport through the template-specific cavities of the poly-
mer barrier (Fig. 5-6). The former mechanism is provided by the polymer structure
micropores, which are formed around template molecules during polymerization.
The latter mechanism results from the ligand-functional groups interactions inside
the MIP.