BULK AND SOLUTION PROPERTIES OF PEPTIDE-POLYMER CONJUGATES - Biologically-Responsive Hybrid Biomaterials - page 18

While the data reported by Cho et al. described the structure and
organization of thin solvent cast films of PBLG- b -PEG- b -PBLG, Floudas and
coworkers have extensively studied the bulk nanoscale organization of these
materials [58]. To this end, a series of triblock copolymers with PBLG volume
fractions ( f PBLG ) ranging from 0.07-0.89 was investigated using SAXS/WAXS,
polarizing optical microscopy (POM), DSC and FT-IR spectroscopy. For triblock
copolymers with f PBLG ≤ 0.25, PEG crystallization was observed, but with
significant undercooling. Triblock copolymers with f PBLG ≥ 0.43 did not show
PEG crystallization. SAXS experiments, which were carried out at 373 K, i.e.
above the melting point of PEG, also revealed a different behaviour for triblock
copolymers with small and large PBLG volume fractions. For triblock
copolymers with f PBLG ≥ 0.43 only a weakly phase separated structure was found,
whereas for samples with f PBLG ≤ 0.25 the SAXS data clearly indicated a
microphase-separated structure. WAXS patterns showed that in the microphase-
separated state the PEG phase was semi-crystalline and the peptide phase
consisted of hexagonally ordered assemblies of PBLG α-helices that coexisted
with β-sheet structures. For triblock copolymers with f PBLG ≥ 0.43, PEG is
amorphous and interspersed with aggregates of α-helical PBLG segments and
unordered peptide chains (Figure 7).
Additional insight into the solid-state nanoscale organization of the triblock
copolymers just discussed could be obtained by combining SAXS/WAXS with
different microscopic techniques (TEM and AFM) [59]. A “broken lamellar”
morphology was observed in the TEM micrographs of PBLG 58 - b -PEG 90 - b -
PBLG 58 ( f PBLG = 0.58). Annealing converted this metastable structure into a non-
uniform microphase-separated pattern, which was proposed to consist of “puck-
like” PEG domains in a PBLG matrix. For PBLG 105 - b -PEG 90 - b -PBLG 105 ( f PBLG =
0.67), a lamellar morphology was found in the as-cast film, which was
transformed into a “broken lamellar” structure upon annealing. Based on the
results, a morphology map was constructed.
Cho et al. [60] reported on the solid-state structure of a series of PZLLys- b -
PEG- b -PZLLys composed of a PEG block with M n = 2 kg/mol and PZLLys
contents of 25.2, 49.9 and 83.0 mol % (= 68, 86 and 98 vol %). Infrared spectra
of CHCl 3 -cast films were in agreement with a helical secondary structure of the
peptide blocks. DSC experiments provided a first hint for the existence of a
microphase-separated structure and revealed two T g 's for all samples. The higher
T g was very close to that of the PZLLys homopolymer and the lower T g
approximately 20 °C higher than that of PEG homopolymer. A PEG melting
transition was not observed. These results were interpreted in terms of a
microphase-separated structure with hard, crystalline PZLLys domains and soft,