Biomedical Engineering Reference
In-Depth Information
compared to that of PEG homopolymer. TEM micrographs suggested a lamellar
microphase-separated structure for one of the triblock copolymer samples.
Polyester-based diblock copolymers
One of the first studies focusing on the solid-state properties of polypeptide-
polyester synthetic hybrid block copolymers was reported by Jérôme
et al.
[28]
Differential scanning calorimetry (DSC) experiments on a poly(ε-
caprolactone)
50
-
block
-poly(γ-benzyl
L
-glutamate)
40
(PCL
50
-
b
-PBLG
40
) diblock
copolymer revealed two endotherms. The first endotherm was found at 60 °C and
is due to the melting of the PCL. The second endotherm which was located at
110 °C, was, mistakenly, interpreted as the melting transition of PBLG. This
transition, however, is not a melting transition, but instead reflects the
conformational transition of the PBLG helix from a 7/2 to an 18/5 helical
structure. Although no further structural investigations were carried out, the
observation of two separate endotherms occurring at temperatures identical to the
transitions found for the respective homopolymers is a first indication for the
existence of a microphase-separated structure. Similar results were reported by
Chen
et al.
[29] who investigated the thermal properties of a series of PCL-
b
-
PBLG copolymers composed of PCL blocks containing 13-51 repeat units
peptide segments with and 22-52 amino acid repeat units.
Caillol
et al.
[30] have studied the solid-state structure and properties of a
series of poly(
L
-lactide)-
block
-poly(
ȳ
-benzyl
L
-glutamate) (PLLac-
b
-PBLG)
copolymers. The PLLac block in these copolymers contained 10-40 repeat units
and the peptide segments were composed of 20-100 repeat units. DSC
thermograms of the block copolymers revealed three transitions corresponding to
the glass transition temperature (
T
g
) of PLLac (~50 °C), the 7/2 to 18/5 helix
transition of PBLG (~100 °C) and the melting temperature of PLLac (~160 °C),
respectively. This observation already provided a first hint towards a
microphase-separated bulk morphology. WAXS experiments, which were
performed at 100 °C, indicated the existence of hexagonally ordered assemblies
of α-helical PBLG chains. With decreasing glutamate content, the peaks
corresponding to this hexagonal organization decreased in intensity and another
scattering peak appeared, which was ascribed to a lamellar assembly of PBLG
chains with a β-strand secondary structure. Increasing the temperature to 200 °C
resulted in the melting of PLLac, but also led to a decrease in intensity of the
diffraction peaks corresponding to the hexagonally ordered α-helical PBLG
segments and an increase in the fraction of PBLG segments that are ordered in a
lamellar β-strand fashion.
Search WWH ::
Custom Search