the absence of the porogen [51]. More recently, by the use of core segments, which
contain repeat units that incorporate thymine groups, nanospheres with very high
affinity for adenine (which was used as the porogen during the micelle preparation) were prepared. The porous nanospheres generated after removal of the adenine porogen exhibited very high affinity and uptake rates for adenine, paving the
way for possible utilization of such specifically imprinted nanospheres as microextractors [52].
15.6.2
Shell-Crosslinked Systems
As depicted in Figure 15.12, shell-crosslinked micellar systems are formed by
crosslinking the solvent-soluble block, which forms the shell of the micellar aggregates, by using appropriate chemical strategies. Unlike the core-crosslinked
micelles, these systems offer several advantages in terms of both structural and
functional control: (i) the core of these structures being un-crosslinked exhibits
higher mobility as well as greater swellability and thereby should possess greater
host-uptake capability; (ii) control of the thickness as well as the extent of crosslinking of the shell-layer can be readily achieved by variation of the block copolymer structure, which in turn can control functional parameters, like permeability
and maximum core-swellability; (iii) by selection of the core segment that can be
chemically degraded, its role can be reduced to that of a scaffold for the generation
of empty crosslinked polymeric shells i.e. nanocapsules.
The early efforts to generate shell-crosslinked systems utilized a diblock copolymer, poly(styrene)-block-poly(4-vinyl pyridine), abbreviated PS-b-P4VP, wherein the
P4VP block was partially quarternized using 4-chloromethyl styrene, which functioned as the potentially crosslinkable site [53]. Micellar aggregates containing a PS
core and a P4VP shell were formed in aqueous-THF and the shell was crosslinked
by a free-radical-initiated polymerization (using an appropriate water-soluble
photo-initiator) of the styrene units present in the shell-region (Figure 15.14).
AFM and dynamic light scattering (DLS) studies clearly demonstrated that the
size of such crosslinked micelles is readily controlled by the relative sizes of the
two blocks; R h by DLS was shown to increase from 7 to 10 to 15 nm, when
the styrene:vinyl pyridine ratio in the diblock copolymer was increased from 1:2 to
1:1.2 to 1.9:1, respectively [53].
In an effort to develop alternative strategies that permit greater control over
some critical molecular properties of the crosslinked shell, such as the crosslink
density and shell composition, Wooley and coworkers utilized poly(styrene)-blockpoly(acrylic acid), PS-b-PAA diblock copolymers [54]. These diblock copolymers
formed micelles containing a PS core and PAA shell; the shell was then crosslinked using diamines, such as diamino(oligoethylene oxide), via an amidation reaction, via a process similar to that shown in Figure 15.16, see next section. By
varying the length and the relative amount of the oligoethylene oxide segment in
the crosslinker, the crosslink density and hence the permeability characteristics of
the shell layer are readily modulated. In the context of the utilization of these
15.6 Nanostructures Based on Lyotropic Mesophases 497
contain repeat units that incorporate thymine groups, nanospheres with very high
affinity for adenine (which was used as the porogen during the micelle preparation) were prepared. The porous nanospheres generated after removal of the adenine porogen exhibited very high affinity and uptake rates for adenine, paving the
way for possible utilization of such specifically imprinted nanospheres as microextractors [52].
15.6.2
Shell-Crosslinked Systems
As depicted in Figure 15.12, shell-crosslinked micellar systems are formed by
crosslinking the solvent-soluble block, which forms the shell of the micellar aggregates, by using appropriate chemical strategies. Unlike the core-crosslinked
micelles, these systems offer several advantages in terms of both structural and
functional control: (i) the core of these structures being un-crosslinked exhibits
higher mobility as well as greater swellability and thereby should possess greater
host-uptake capability; (ii) control of the thickness as well as the extent of crosslinking of the shell-layer can be readily achieved by variation of the block copolymer structure, which in turn can control functional parameters, like permeability
and maximum core-swellability; (iii) by selection of the core segment that can be
chemically degraded, its role can be reduced to that of a scaffold for the generation
of empty crosslinked polymeric shells i.e. nanocapsules.
The early efforts to generate shell-crosslinked systems utilized a diblock copolymer, poly(styrene)-block-poly(4-vinyl pyridine), abbreviated PS-b-P4VP, wherein the
P4VP block was partially quarternized using 4-chloromethyl styrene, which functioned as the potentially crosslinkable site [53]. Micellar aggregates containing a PS
core and a P4VP shell were formed in aqueous-THF and the shell was crosslinked
by a free-radical-initiated polymerization (using an appropriate water-soluble
photo-initiator) of the styrene units present in the shell-region (Figure 15.14).
AFM and dynamic light scattering (DLS) studies clearly demonstrated that the
size of such crosslinked micelles is readily controlled by the relative sizes of the
two blocks; R h by DLS was shown to increase from 7 to 10 to 15 nm, when
the styrene:vinyl pyridine ratio in the diblock copolymer was increased from 1:2 to
1:1.2 to 1.9:1, respectively [53].
In an effort to develop alternative strategies that permit greater control over
some critical molecular properties of the crosslinked shell, such as the crosslink
density and shell composition, Wooley and coworkers utilized poly(styrene)-blockpoly(acrylic acid), PS-b-PAA diblock copolymers [54]. These diblock copolymers
formed micelles containing a PS core and PAA shell; the shell was then crosslinked using diamines, such as diamino(oligoethylene oxide), via an amidation reaction, via a process similar to that shown in Figure 15.16, see next section. By
varying the length and the relative amount of the oligoethylene oxide segment in
the crosslinker, the crosslink density and hence the permeability characteristics of
the shell layer are readily modulated. In the context of the utilization of these
15.6 Nanostructures Based on Lyotropic Mesophases 497