BULK AND SOLUTION PROPERTIES OF PEPTIDE-POLYMER CONJUGATES - Biologically-Responsive Hybrid Biomaterials - page 19

a) f rod < 0.26 b) f rod > 0.4

d~10 nm

Fig. 7. Highly schematic model of the phase state in the PBLG- b -PEG- b -PBLG triblock
copolymers. (a) Phase state corresponding to low peptide volume fractions depicting a microphase-
separated copolymer consisting of all the peptide and PEG secondary structures. (b) Phase state
corresponding to f rod > 0.4 depicting phase mixing resulting in the appearance of only one
( ŋ -helical) secondary structure. Adapted from [58].

amorphous PEG segments. The presence of a microphase-separated structure was
confirmed by TEM micrographs of RuO 4 -stained thin films.
Akashi et al. [61] reported on the solid-state nanoscale structure of ABA type
triblock copolymers composed of a central PEG block flanked by two poly(β-
benzyl L -aspartate) (PBLAsp) blocks. The molecular weight of the central PEG
block was 11 or 20 kg/mol and the degrees of polymerization of the peptide
blocks ranged from 12 to 32. WAXS and POM studies on CH 2 Cl 2 -cast films
showed PEG crystallization in all samples. DSC experiments showed that the
melting temperature of the crystalline PEG domains decreased linearly with
increasing PBLAsp content, reflecting the strong influence of the peptide
segments on PEG crystallization. More interestingly, the authors found that
heating the as-cast films above 333 K and cooling down to 303 K converted a
certain fraction of the α-helical PBLAsp chains into β-strands and was
accompanied by a decrease in PEG crystallinity. On a macroscopic level, this led
to increased strength and elasticity of the films.
Cho et al. [62] have studied triblock copolymers composed of a middle block
of poly(propylene glycol) (PPG) with a molecular weight of 2,000 flanked by
two PBLG segments. Three triblock copolymer samples were investigated with
PPG contents of 17.0, 26.0 and 60.0 mol %, respectively. According to infrared
spectra and WAXS that were recorded from CHCl 3 -cast films, the PBLG blocks
possessed an α-helical secondary structure. No further details on the solid-state
nanoscale structure and the possibility of microphase separation were reported.