BULK AND SOLUTION PROPERTIES OF PEPTIDE-POLYMER CONJUGATES - Biologically-Responsive Hybrid Biomaterials - page 10

Self-assembly of these diblock copolymers in a β-sheet type fashion results
in the lamellar structures observed by SAXS. For PS 10 - b -PBLG 20 a peculiar and
previously unprecedented structure was found. This structure, which consisted
of hexagonally packed diblock copolymer molecules that are organized in a
hexagonal superlattice, has been referred to as the double hexagonal or
hexagonal-in-hexagonal morphology.
Apart from several unconventional solid-state nanoscale structures, another
factor that distinguishes the phase diagrams in Figure 5 from those of the
majority of conventional, conformationally isotropic block copolymers is the
influence of temperature. For a number of diblock copolymers, increasing the
temperature above 200 °C results in a change from a hexagonal-in-hexagonal
(PS 10 - b -PBLG 20 ) or hexagonal (PS 10 - b -PBLG 40 , PS 10 - b -PZLLys 80 ) to a lamellar
morphology. FTIR spectroscopy experiments suggested that these morphological
transitions are induced by an increase in the fraction of peptide blocks that have a
Ȳ -strand conformation. Interestingly, the self-assembly of the amphiphilic form
of these diblock copolymers, namely polystyrene- block -poly(glutamic acid)
(PS- b -PGA) revealed that the high immiscibility between PS and PGA blocks
(conformational asymmetry) drove all of these rod-coil block copolymers to self-
assemble in a hexagonal in lamellar (HL) morphology in the nanometer size
range, regardless of their composition and architecture [25].
Polyether-based diblock copolymers
Poly(ethylene glycol)-polypeptide block copolymers are of particular interest,
from both a structural and a functional point of view. In addition to microphase
separation and the tendency of the peptide blocks towards aggregation,
crystallization of poly(ethylene glycol) (PEG) introduces an additional factor that
can influence the structure formation of these hybrid block copolymers. Ma et al.
[26] have investigated the solid-state structure and properties of three
poly(ethylene glycol)- block -poly( L -Alanine) (PEG- b -PLAla) copolymers that
were prepared from a PEG macroinitiator with M n = 2 kg/mol. The diblock
copolymers contained 39.8, 49.6 and 65.5 mol % alanine. From FTIR spectra and
differential scanning calorimetry (DSC) measurements, the authors proposed a
microphase-separated bulk structure.
AB diblock and ABA triblock copolymers composed of PEG as the A block
and random coil segments of poly( D , L -valine- co - D , L -leucine) as the B block(s)
were investigated by Cho, Kim and Jung [27]. DSC experiments revealed PEG
crystallization and showed that the PEG melting temperature was decreased