light scattering, the authors proposed that the core contains bilayer molecules that are
stabilized by intramolecular hydrogen bonding and π-π interactions. The uniformity
in diameter of the PEG corona, although the core is anisotropic, implies that PEG
coils near the core stretch in order to fill the space and the ones away from the core
are more coil like. Shorter coils (DP
coil
= 45) could not form the shell around the
aggregates and resulted in the formation of rod-like micelles.
Using a variety of hydrophobic and hydrophilic polypeptide chemistries ((poly
(γ-benzyl-L-glutamate) (PBLG), poly(γ- L-glutamic acid) (PLGA), poly(Z-LLysine) (PZLL), poly(N-trifluoroacetyl-L-Lysine) (PNTLL), and poly
(hydroxyethyl-L-glytamine) (PHLG)) as rod blocks and PS, PB, and polysarcosine
as coil block, Gallot et al. investigated the phase behavior of these RCBCPs both in
solution and melt (Douy and Gallot 1982; Gallot 1996). Seven different phase
structures were reported in these systems based on the orientation of the rods
although Sm and N phases were predominant. All the BCP systems demonstrated
the remarkable preference of the L phases in RCBCPs. In L forming RCBCP sample,
the α-helical rods were arranged perpendicular to the IMDS and possess hexagonal
in-plane symmetry. Based on layer thickness and extended chain length of the
helices, it was observed that the helices generally adopted a folded chain conformation (Fig. 6a) (Douy and Gallot 1982). Folded conformation imparts more interfacial
area for the coil block and is hence preferred. In general, interdigitation, tilting, and
folding of the rods provide more interfacial area to the rod blocks thereby reducing
the coil stretching penalty. Hexagonal-in-lamellar morphologies were also observed
in rod-coil-rod triblock samples where PB and polysarcosine formed the coil block,
and PBLG, PNTLL, and PHLG formed the rod block. However, in these samples,
within the Sm layers the helical rods were tilted at an angle with respect to the layer
normal and the tilt angle increased as f
coil increased (Fig. 6b). These tilted rods were,
however, still arranged in a hexagonal lattice with the same lattice constant although
the thickness of the rod layer decreased as the tilt angle increased. Theoretical
calculations of Semenov et al. showed predominantly N, SmA, and SmC structures
in their model RCBCP system (Semenov 1986). Halperin’s theoretical prediction
showed tilting of the Sm layers with an increase in the f
coil in order to offer more
interfacial area to the coil to reduce the coil stretching penalty and is in agreement
with the experimental observation of Gallot et al (Halperin 1989, 1990). While the
influence of f
coil on the overall morphology was investigated by Gallot and
coworkers, Losik et al. reported the influence of solvent (dimethyl formamide,
DMF) on the conformation of the polypeptide (Losik et al. 2004). In polypeptidebased RCBCPs with PBLG and PZLL as the rod block, with constant f
coil
(PS DP = 52) and DP PBLG = 104 and PZLL = 111, they observed hexagonalin-L hierarchical structures. Within the rod blocks, PZLL adopted a fully stretched
helical conformation whereas PBLG formed helices that were twice folded. This
folding effect was attributed to the ability of the solvent (DMF) to penetrate the
PBLG and soften the helical backbone by reducing the hydrogen bond interactions.
This also led to a decrease in the order of the hexagonal lattice in PBLG rods. PDI of
the system also influences the BCP structures. Schlaad et al. also investigated the effect
of variation in PDI on the interface using PZZL based RCBCPs (Schlaad et al. 2004).
7 Structure and Assembly of Liquid Crystalline Block Copolymers
185
stabilized by intramolecular hydrogen bonding and π-π interactions. The uniformity
in diameter of the PEG corona, although the core is anisotropic, implies that PEG
coils near the core stretch in order to fill the space and the ones away from the core
are more coil like. Shorter coils (DP
coil
= 45) could not form the shell around the
aggregates and resulted in the formation of rod-like micelles.
Using a variety of hydrophobic and hydrophilic polypeptide chemistries ((poly
(γ-benzyl-L-glutamate) (PBLG), poly(γ- L-glutamic acid) (PLGA), poly(Z-LLysine) (PZLL), poly(N-trifluoroacetyl-L-Lysine) (PNTLL), and poly
(hydroxyethyl-L-glytamine) (PHLG)) as rod blocks and PS, PB, and polysarcosine
as coil block, Gallot et al. investigated the phase behavior of these RCBCPs both in
solution and melt (Douy and Gallot 1982; Gallot 1996). Seven different phase
structures were reported in these systems based on the orientation of the rods
although Sm and N phases were predominant. All the BCP systems demonstrated
the remarkable preference of the L phases in RCBCPs. In L forming RCBCP sample,
the α-helical rods were arranged perpendicular to the IMDS and possess hexagonal
in-plane symmetry. Based on layer thickness and extended chain length of the
helices, it was observed that the helices generally adopted a folded chain conformation (Fig. 6a) (Douy and Gallot 1982). Folded conformation imparts more interfacial
area for the coil block and is hence preferred. In general, interdigitation, tilting, and
folding of the rods provide more interfacial area to the rod blocks thereby reducing
the coil stretching penalty. Hexagonal-in-lamellar morphologies were also observed
in rod-coil-rod triblock samples where PB and polysarcosine formed the coil block,
and PBLG, PNTLL, and PHLG formed the rod block. However, in these samples,
within the Sm layers the helical rods were tilted at an angle with respect to the layer
normal and the tilt angle increased as f
coil increased (Fig. 6b). These tilted rods were,
however, still arranged in a hexagonal lattice with the same lattice constant although
the thickness of the rod layer decreased as the tilt angle increased. Theoretical
calculations of Semenov et al. showed predominantly N, SmA, and SmC structures
in their model RCBCP system (Semenov 1986). Halperin’s theoretical prediction
showed tilting of the Sm layers with an increase in the f
coil in order to offer more
interfacial area to the coil to reduce the coil stretching penalty and is in agreement
with the experimental observation of Gallot et al (Halperin 1989, 1990). While the
influence of f
coil on the overall morphology was investigated by Gallot and
coworkers, Losik et al. reported the influence of solvent (dimethyl formamide,
DMF) on the conformation of the polypeptide (Losik et al. 2004). In polypeptidebased RCBCPs with PBLG and PZLL as the rod block, with constant f
coil
(PS DP = 52) and DP PBLG = 104 and PZLL = 111, they observed hexagonalin-L hierarchical structures. Within the rod blocks, PZLL adopted a fully stretched
helical conformation whereas PBLG formed helices that were twice folded. This
folding effect was attributed to the ability of the solvent (DMF) to penetrate the
PBLG and soften the helical backbone by reducing the hydrogen bond interactions.
This also led to a decrease in the order of the hexagonal lattice in PBLG rods. PDI of
the system also influences the BCP structures. Schlaad et al. also investigated the effect
of variation in PDI on the interface using PZZL based RCBCPs (Schlaad et al. 2004).
7 Structure and Assembly of Liquid Crystalline Block Copolymers
185