Chemistry Reference
In-Depth Information
α
-sheet (Fig. 7). The block length required to allow such a transi-
tion varies from
-helix to
β
∼
40 repeat units for the PS
10
-
b
-PBLG
n
diblock oligomers to
∼
80
α
-amino acids for the PS
10
-
b
-PZLys
n
series, reflecting the lower
α
-helix-
forming propensity of Z-Lys in comparison with Bn-Glu.
In contrast to peptide diblock copolymers based on general polymers,
Deming and Pochan et al. reported on unique copolymers that are completely
peptidic [64]. Leucine (L), racemic copolymers of l-andd-leucine (racL),
or a random copolymer of L and valine (V) blocks with the inherent sec-
ondary structures of random coil (racL or LV) or rigid rod (L), were attached
to PBLG molecules. In PBLG diblock copolymers with relatively small ad-
ditional blocks, cholesteric liquid crystalline ordering was observed in bulk
films. However, depending on the kinetics of film formation and the amount
of non-PBLG block, the nanostructure or microstructure were able to be con-
trolled. These purely peptidic block molecules can provide the opportunity to
pattern materials with peptidic functionalities by taking advantage of block
copolymer phase behavior and liquid crystal ordering.
Very recently, the self-assembly of poly(
-benzyl-l-glutamate)-
b
-poly(l-
lysine) rod-coil copolypeptide via ionic complexation was reported by Ikkala,
Hadjichristidis and coworkers [65]. Complexation between the anionic sur-
factants dodecyl benzenesulfonic acid and the cationic poly(l-lysine) chains
occurs via proton transfer from the acid group to the base, resulting in elec-
trostatically bonded comb-like structures, and fluid-like liquid crystalline
structures at room temperature due to efficient plasticization of dodecyl ben-
zenesulfonic acid.
γ
3
Triblock Rod-Coil System
3.1
ABC Coil-Rod-Coil Triblock Copolymers
If a chemically distinct hydrophobic chain is attached to the opposite ends of
rod segment, segregation of incompatible chain ends takes place and leads to
an ordered phase composed of three distinct sub-layers [66]. Coil-rod-coil
ABC triblock molecules give rise to the formation of self-assembled struc-
tures with higher interfacial areas in comparison with AB diblock molecules.
In contrast to that of diblock molecules based on a PEO coil, which show
isotropic or smectic phase depending on the coil length, the ABC triblock
molecule exhibits a hexagonal columnar mesophase [67]. Molecule (
9
)with
22 ethylene oxide (EO) repeating units, for example, exhibits hexagonal
columnar mesophase which, in turn, undergoes transformation into a dis-
crete spherical micellar structure in which rod segments are packed into
a discrete bilayer lamellar structure that is encapsulated with PEO coils
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