general, diblock copolymers with a short soluble block and a long insoluble block
tend to form crew-cut micelles, vesicles, cylinders or donuts, while when the relative block lengths are just reversed spherical micellar aggregates (star micelles) are
formed [48]. In lyotropic systems, one other tunable parameter is the nature of
the solvent, increasing the concentration of the block-selective solvent relative to
the mutual solvent, which dissolves both the blocks, favors the formation of vesicles and cylinders. Thus, lyotropic systems offer a greater degree of control than
bulk systems and, furthermore, are more adaptable for larger scale preparation
of nanosize polymeric objects. Additionally, shell-crosslinked systems are more
readily achieved using lyotropic systems, but this would require the use of triblock
copolymers, if it were to be achieved based on bulk phase-separated morphologies.
15.6.1
Core-Crosslinked Systems
Early efforts to freeze the spherical aggregates of block copolymers in solution
by core-crosslinking often led to the formation of insoluble products due to
micelle fusion during the crosslinking process. Using diblock copolymers of
poly(polystyrene)-block-poly(2-cinnamoylethyl methacrylate), PS-b-PCEMA, Liu and
coworkers prepared core-crosslinked hairy nanospheres, via the photo-crosslinking
of the PCEMA core [49]. More interestingly, they also prepared water soluble
nanospheres using poly(2-cinnamoylethyl methacrylate)-block-poly(acrylic acid),
PCEMA-b-PAA [50]. Spherical micelles having a PCEMA core and a PAA shell
were formed by dissolution of the diblock copolymer in DMF–water mixtures, and
photo-crosslinking of the core generated water dispersible nanospheres which were
shown to be stable even in the presence of DMSO, a mutual solvent for both the
blocks, confirming the enhanced stability derived from the crosslinking process.
Typical hydrodynamic radii R h , of these nanospheres ranged from ca. 40 to 60 nm.
The lightly crosslinked hydrophobic cores are readily swollen in suitable media and
can act as scavengers of organic compounds from water. Their studies demonstrated that while the uptake capacity for organic compounds from water was limited in the unswollen state, the use of a small amount of an organic co-solvent, like
DMSO or acetone, led to a drastic increase in the uptake capacity [50]. The potential for these nanospheres to take up organic guest species into the hydrophobic
cores, makes them potentially useful candidates for water clean-up operations (oil
spill) as well as for drug delivery applications.
With the object of increasing the uptake capacity of nanospheres, specifically
designed diblock copolymers, that contain one block made up of a random copolymer of CEMA and 2-octanoylethyl methacrylate, OEMA, and the other of PAA,
were used [51]. To increase the porosity of the crosslinked core, the spherical
micelles were prepared in the presence of the homopolymer, POEMA that codissolved in the hydrophobic core during micellization (Figure 15.13). Photocrosslinking of the core, followed by extraction of the porogen, POEMA, using an
organic solvent generated the porous nanospheres, whose perylene uptake capacity
was shown to be significantly higher than the identical nanospheres prepared in
15.6 Nanostructures Based on Lyotropic Mesophases 495
tend to form crew-cut micelles, vesicles, cylinders or donuts, while when the relative block lengths are just reversed spherical micellar aggregates (star micelles) are
formed [48]. In lyotropic systems, one other tunable parameter is the nature of
the solvent, increasing the concentration of the block-selective solvent relative to
the mutual solvent, which dissolves both the blocks, favors the formation of vesicles and cylinders. Thus, lyotropic systems offer a greater degree of control than
bulk systems and, furthermore, are more adaptable for larger scale preparation
of nanosize polymeric objects. Additionally, shell-crosslinked systems are more
readily achieved using lyotropic systems, but this would require the use of triblock
copolymers, if it were to be achieved based on bulk phase-separated morphologies.
15.6.1
Core-Crosslinked Systems
Early efforts to freeze the spherical aggregates of block copolymers in solution
by core-crosslinking often led to the formation of insoluble products due to
micelle fusion during the crosslinking process. Using diblock copolymers of
poly(polystyrene)-block-poly(2-cinnamoylethyl methacrylate), PS-b-PCEMA, Liu and
coworkers prepared core-crosslinked hairy nanospheres, via the photo-crosslinking
of the PCEMA core [49]. More interestingly, they also prepared water soluble
nanospheres using poly(2-cinnamoylethyl methacrylate)-block-poly(acrylic acid),
PCEMA-b-PAA [50]. Spherical micelles having a PCEMA core and a PAA shell
were formed by dissolution of the diblock copolymer in DMF–water mixtures, and
photo-crosslinking of the core generated water dispersible nanospheres which were
shown to be stable even in the presence of DMSO, a mutual solvent for both the
blocks, confirming the enhanced stability derived from the crosslinking process.
Typical hydrodynamic radii R h , of these nanospheres ranged from ca. 40 to 60 nm.
The lightly crosslinked hydrophobic cores are readily swollen in suitable media and
can act as scavengers of organic compounds from water. Their studies demonstrated that while the uptake capacity for organic compounds from water was limited in the unswollen state, the use of a small amount of an organic co-solvent, like
DMSO or acetone, led to a drastic increase in the uptake capacity [50]. The potential for these nanospheres to take up organic guest species into the hydrophobic
cores, makes them potentially useful candidates for water clean-up operations (oil
spill) as well as for drug delivery applications.
With the object of increasing the uptake capacity of nanospheres, specifically
designed diblock copolymers, that contain one block made up of a random copolymer of CEMA and 2-octanoylethyl methacrylate, OEMA, and the other of PAA,
were used [51]. To increase the porosity of the crosslinked core, the spherical
micelles were prepared in the presence of the homopolymer, POEMA that codissolved in the hydrophobic core during micellization (Figure 15.13). Photocrosslinking of the core, followed by extraction of the porogen, POEMA, using an
organic solvent generated the porous nanospheres, whose perylene uptake capacity
was shown to be significantly higher than the identical nanospheres prepared in
15.6 Nanostructures Based on Lyotropic Mesophases 495