Chemistry Reference
In-Depth Information
Figure 2.4 Temperature responsive micellar formation in PNIPAM-based polymers.
Thus, the PEO segment actually becomes hydrophobic at higher temperatures. This
temperature-dependent change converts the amphiphilic block copolymer to a
water-insoluble hydrophobic polymer (Topp et al. 1997; Chung et al. 2000). The
temperature at which the polymer exhibits this transition is called its lower critical
solution temperature (LCST). In addition to PEO, substituted poly(N-isopropyl acryl-
amide) (PNIPAM; Chart 2.1) exhibits temperature sensitivity, where the LCST can be
tuned by varying the alkyl functionality. The guest encapsulation combined with the
temperature-sensitive precipitation of the polymers has been exploited to sequester
and separate guest molecules from aqueous solutions (Fig. 2.4).
Despite the interesting combination of temperature sensitivity and micelle forming
behavior of nonionic micelles, they have the disadvantage of being less stable
mechanically. The lack of mechanical stability is mainly due to the fact that the
repeat units in PEO or PNIPAM interact with water through hydrogen bonding,
but they are otherwise hydrocarbon based. Therefore, the difference in compatibility
between the hydrophilic and hydrophobic blocks is not significant enough. It is
because of this that we often find block copolymer micelles that are based on ionic
repeat units as the hydrophilic segments to be more stable.
In addition to the hydrophobic interaction mentioned above to encapsulate guest
molecules, other types of nonspecific interactions have also been explored to
enhance binding. For example, block copolymer micelles based on PEO as hydrophi-
lic segments and poly(b-benzyl
L
-aspartate) as hydrophobic blocks have used to
encapsulate doxorubicin. The encapsulation efficiency of doxorubicin, an aromatic
anticancer drug molecule, has been found to be significantly higher. This observation
has been attributed to the p-p interaction between the anthracycline moiety of
doxorubicin and the benzyl group of poly(b-benzyl
L
-aspartate) (Cammas-Marion
et al. 1999).
Similar to micellar assemblies in water, reverse micelles have also been utilized
to bring about nonspecific binding interactions in organic solvents. Akiyoshi et al.
(2002) have synthesized an amphiphilic block copolymer containing PEO and
an amylase chain as receptor for methyl orange (MO; Chart 2.2). Amylases are
insoluble and methoxy-PEO (MPEO) is soluble in chloroform. Hence, an MPEO-
amylase block copolymer forms reverse micelles in chloroform. Akiyoshi et al. esta-
blished the capability of the buried receptors to extract the complementary analyte by
studying the ultraviolet visible (UV-vis) spectra. A solution of polymer was shaken
