evaporation. The densitometer scan of the SAXD pattern of the film (Fig. 4b,
curve a) shows a well-defined maximum at a spacing of 114.5 Æ 1.4 A ˚ , as well
as the corresponding second-order reflection at 57.0 Æ 0.3 A ˚ . The coincidence of
the helix length and the observed layer spacing strongly suggests smectic-like
ordering in the cast solid film. In contrast, a solid film of polydisperse PBLG with
comparable molecular weight (DP % 98, PDI ¼ 1.2) yields no small-angle
reflections in a similar diffraction experiment (Fig. 4b, curve b). Shown in Fig. 4c
is the illustration for the smectic LC structures. As can be seen, the helical rods
arrange in layers of thickness 114.5 A ˚ , and the distance between two PBLG rods is
12.5 A ˚ , which corresponds to the diameter of PBLG rods.
The polydispersity of polymer is the main reason for the lack of a smectic LC
phase of chemically synthetic polypeptides. However, when a compact and bulky
group is capped on polypeptide ends, the situation can be rather different. During
self-assembly in concentrated solutions, such modified polypeptides can selfassemble into a smectic LC phase [48, 79]. For example, Winnik et al. found that
dendron-helical PBLG (DHP; Fig. 5a) copolymers can form smectic phases in
CHCl 3 [48]. The dendritic block prevents the DHP copolymers from aligning into
a nematic phase. As shown in Fig. 5b, the concentrated solution (>40 wt%) of the
DHP copolymers in THF shows a birefringent texture under the POM observation.
SAXS experiments on the dried solid from the concentrated THF solution show two
diffraction peaks at 7.2 and 3.3 nm, which suggests that the DHP exhibits smecticlike order in concentrated solution. The proposed two-dimensional layer structure
of DHP that results from an antiparallel stacking of DHP is presented in the inset of
Fig. 5b. The result suggests that smectic ordering of the DHP copolymers in
solution is feasible with a dendritic molecule as a non-random coil block, even
though the PBLG block is polydisperse in chain length. However, it should be noted
that due to the polydispersity of PBLG blocks and the flexible nature of the
peripheral alkyl chains of the dendron block, the LC phase of DHP is not well
ordered and thus it is proper to call the LC structure a “smectic-like” phase.
The success in constructing highly ordered smectic phase in solution by using
modified polydisperse polypeptide homopolymers rather than monodisperse
0.5
1.0
1.5
2.0
2θ (deg.)
Relative intensity
114 Å
57 Å
12.5 Å
114.5 Å
a
b
c
Fig. 4 (a) POM image of PBLG in CHCl 3 /TFA. (b) Densitometer scans of the SAXD patterns of
films prepared from solutions of monodisperse PBLG (curve a) and polydisperse PBLG (curve b).
(c) Smectic structure of PBLG, showing the origin of the 12.5 A ˚ and 114.5 A ˚ reflections. Reprinted
from [49], by permission from Macmillan Publishers Ltd: Nature, copyright (1997)
Ordering of Polypeptides in Liquid Crystals, Gels and Micelles
167
curve a) shows a well-defined maximum at a spacing of 114.5 Æ 1.4 A ˚ , as well
as the corresponding second-order reflection at 57.0 Æ 0.3 A ˚ . The coincidence of
the helix length and the observed layer spacing strongly suggests smectic-like
ordering in the cast solid film. In contrast, a solid film of polydisperse PBLG with
comparable molecular weight (DP % 98, PDI ¼ 1.2) yields no small-angle
reflections in a similar diffraction experiment (Fig. 4b, curve b). Shown in Fig. 4c
is the illustration for the smectic LC structures. As can be seen, the helical rods
arrange in layers of thickness 114.5 A ˚ , and the distance between two PBLG rods is
12.5 A ˚ , which corresponds to the diameter of PBLG rods.
The polydispersity of polymer is the main reason for the lack of a smectic LC
phase of chemically synthetic polypeptides. However, when a compact and bulky
group is capped on polypeptide ends, the situation can be rather different. During
self-assembly in concentrated solutions, such modified polypeptides can selfassemble into a smectic LC phase [48, 79]. For example, Winnik et al. found that
dendron-helical PBLG (DHP; Fig. 5a) copolymers can form smectic phases in
CHCl 3 [48]. The dendritic block prevents the DHP copolymers from aligning into
a nematic phase. As shown in Fig. 5b, the concentrated solution (>40 wt%) of the
DHP copolymers in THF shows a birefringent texture under the POM observation.
SAXS experiments on the dried solid from the concentrated THF solution show two
diffraction peaks at 7.2 and 3.3 nm, which suggests that the DHP exhibits smecticlike order in concentrated solution. The proposed two-dimensional layer structure
of DHP that results from an antiparallel stacking of DHP is presented in the inset of
Fig. 5b. The result suggests that smectic ordering of the DHP copolymers in
solution is feasible with a dendritic molecule as a non-random coil block, even
though the PBLG block is polydisperse in chain length. However, it should be noted
that due to the polydispersity of PBLG blocks and the flexible nature of the
peripheral alkyl chains of the dendron block, the LC phase of DHP is not well
ordered and thus it is proper to call the LC structure a “smectic-like” phase.
The success in constructing highly ordered smectic phase in solution by using
modified polydisperse polypeptide homopolymers rather than monodisperse
0.5
1.0
1.5
2.0
2θ (deg.)
Relative intensity
114 Å
57 Å
12.5 Å
114.5 Å
a
b
c
Fig. 4 (a) POM image of PBLG in CHCl 3 /TFA. (b) Densitometer scans of the SAXD patterns of
films prepared from solutions of monodisperse PBLG (curve a) and polydisperse PBLG (curve b).
(c) Smectic structure of PBLG, showing the origin of the 12.5 A ˚ and 114.5 A ˚ reflections. Reprinted
from [49], by permission from Macmillan Publishers Ltd: Nature, copyright (1997)
Ordering of Polypeptides in Liquid Crystals, Gels and Micelles
167