of swelling, thereby generating nanocages of larger dimension. Nanocages utilizing other hydrolytically degradable core segments such as poly(caprolactone), PCL,
have also been prepared by using diblock copolymers containing a PCL block and
a poly(acrylic acid) block, which was crosslinked by using a diamine via an amidation process. The biodegradable PCL core of the parent nanospheres could also
serve as a potential reservoir for controlled drug release applications [60].
Apart from the approach described above, wherein solid core–shell aggregates
are shell-crosslinked followed by core degradation to generate hollow liquid-filled
nanocapsules, hollow structures can also be directly generated from certain amphiphilic diblock copolymers of modest molecular weights (@4000). Poly(ethylene
oxide)-block-poly(ethylene) (PEO 40 -b-PE 37 ) spontaneously organizes into a variety of
structures, such as spherical and rod-like micelles and bilayer vesicles [61]. These
vesicular structures, with typical bilayer thickness of about 8 nm, are much more
robust than those formed from simple low molecular weight surfactants. Release
of entrapped guest species was shown to be much slower in these systems compared to the standard liposomes, confirming the relatively lower segment mobility
of the bilayer structure. Further experiments with poly(ethylene oxide)-block-poly(butadiene) (PEO 26 -b-PB 46 ) permitted free-radical initiated crosslinking via polymerization of the olefinic sites in the butadiene segment, to permanently lock-in
the vesicular membrane capsule [62]. Elegant experiments were done to probe the
efficacy of the crosslinking process, and these clearly demonstrated a dramatic increase, by several orders of magnitude, in both the elastic modulus and the rupture
strength of the bilayer membrane upon crosslinking. Utilizing a mixture of the
saturated diblock copolymer (PEO 40 -b-PE 37 ) and PEO 26 -b-PB 46 in various ratios,
the crosslinking density, and thereby the elastic modulus and rupture strength of
the membrane, were fine-tuned [62].
Another interesting and novel vesicular architecture was recently reported by
Shen and Eisenberg using PS-b-PAA, wherein equi-spaced concentric bilayers
build-up an onion-like vesicular structure, with overall diameters exceeding 1 mm
[63]. As many as 6 concentric bilayers, of thickness @22 nm and inter-layer spacing
of about 60 nm, were clearly visible in some cases (Figure 15.17). Similar multilayer vesicular structures with no space between the layers, so-called solid oniontype vesicles, were formed when quaternized PS-b-P4VP block copolymers were
used. In contrast to the regular liquid-filled vesicles this type of layered vesicle,
which has liquid chambers between the layers, could find some potential use in
controlled-release applications as the overall diffusion rates will be governed by the
number of bilayer shells that the encapsulated species will need to cross before
reaching the bulk phase. It must be added that although these vesicles are not stabilized by polymerization or crosslinking, their stability is significantly greater than
those formed by simple surfactants, due to the polymeric nature of the diblock copolymer amphiphile.
It is evident from the discussions so far that block copolymer-based strategies to
construct structurally fixed nanosize objects provide ample opportunities to modulate the final size, shape and properties of the nanostructures. Much of the finetuning is done at the stage of the design and synthesis of the diblock copolymers.
15.6 Nanostructures Based on Lyotropic Mesophases 501
have also been prepared by using diblock copolymers containing a PCL block and
a poly(acrylic acid) block, which was crosslinked by using a diamine via an amidation process. The biodegradable PCL core of the parent nanospheres could also
serve as a potential reservoir for controlled drug release applications [60].
Apart from the approach described above, wherein solid core–shell aggregates
are shell-crosslinked followed by core degradation to generate hollow liquid-filled
nanocapsules, hollow structures can also be directly generated from certain amphiphilic diblock copolymers of modest molecular weights (@4000). Poly(ethylene
oxide)-block-poly(ethylene) (PEO 40 -b-PE 37 ) spontaneously organizes into a variety of
structures, such as spherical and rod-like micelles and bilayer vesicles [61]. These
vesicular structures, with typical bilayer thickness of about 8 nm, are much more
robust than those formed from simple low molecular weight surfactants. Release
of entrapped guest species was shown to be much slower in these systems compared to the standard liposomes, confirming the relatively lower segment mobility
of the bilayer structure. Further experiments with poly(ethylene oxide)-block-poly(butadiene) (PEO 26 -b-PB 46 ) permitted free-radical initiated crosslinking via polymerization of the olefinic sites in the butadiene segment, to permanently lock-in
the vesicular membrane capsule [62]. Elegant experiments were done to probe the
efficacy of the crosslinking process, and these clearly demonstrated a dramatic increase, by several orders of magnitude, in both the elastic modulus and the rupture
strength of the bilayer membrane upon crosslinking. Utilizing a mixture of the
saturated diblock copolymer (PEO 40 -b-PE 37 ) and PEO 26 -b-PB 46 in various ratios,
the crosslinking density, and thereby the elastic modulus and rupture strength of
the membrane, were fine-tuned [62].
Another interesting and novel vesicular architecture was recently reported by
Shen and Eisenberg using PS-b-PAA, wherein equi-spaced concentric bilayers
build-up an onion-like vesicular structure, with overall diameters exceeding 1 mm
[63]. As many as 6 concentric bilayers, of thickness @22 nm and inter-layer spacing
of about 60 nm, were clearly visible in some cases (Figure 15.17). Similar multilayer vesicular structures with no space between the layers, so-called solid oniontype vesicles, were formed when quaternized PS-b-P4VP block copolymers were
used. In contrast to the regular liquid-filled vesicles this type of layered vesicle,
which has liquid chambers between the layers, could find some potential use in
controlled-release applications as the overall diffusion rates will be governed by the
number of bilayer shells that the encapsulated species will need to cross before
reaching the bulk phase. It must be added that although these vesicles are not stabilized by polymerization or crosslinking, their stability is significantly greater than
those formed by simple surfactants, due to the polymeric nature of the diblock copolymer amphiphile.
It is evident from the discussions so far that block copolymer-based strategies to
construct structurally fixed nanosize objects provide ample opportunities to modulate the final size, shape and properties of the nanostructures. Much of the finetuning is done at the stage of the design and synthesis of the diblock copolymers.
15.6 Nanostructures Based on Lyotropic Mesophases 501