2.3. Measurements
The number-average molecular weight (Mn) and molecular
weight distribution (MWD) of the PLLA and PDLA were determined
by Gel Permeation Chromatography (GPC) with a Waters e2695
separations module equipped with PLgel 10 mm mixed B 2 columns
operating at 40 C and employing a refractive index detector.
Tetrahydrofuran was used as the solvent at a flow rate of 1.0 mL/
min.
The specific optical rotation of the PLLA and PDLA was determined
in chloroform at a concentration of 1 g/dL at 25 C using a
Bellingham and Stanley Polarimeter ADP220 at wavelength of
589 nm. The specific optical rotation of polylactide was used to
calculate the L enantiomer content [14,15].
The thermal transition properties of the PLLA, PDLA and scPLAs
were determined with a Perkin-Elmer Pyris Diamond differential
scanning calorimeter (DSC) under a nitrogen flow. For DSC, samples
of 3e5 mg in weight were heated at 10 C/min over a temperature
range of 0 to 250 C (the 1st heating scan). Then, the samples were
quenched to 0 C according to the DSC instrument's own default
cooling mode before heating from 0 to 250 C (the 2nd heating
scan). The homocrystallinity (Xhc) and stereocomplex crystallinity
(Xsc) of the polylactides were determined from the heats of melting
of the homocrystallites (DHhc) and stereocomplex crystallites
(DHsc) using equations (1) and (2), respectively.
Xhc ¼ ðDHhc=93 J=gÞ 100% (1)
Xsc ¼ ðDHsc=142 J=gÞ 100% (2)
where the DHhc and DHsc were the heats of melting of homocrystallites
and stereocomplex crystallites, respectively, that were
obtained from the DSC method. The heats of melting for 100%
crystallinity of the homocrystallinity and stereocomplex crystallinity
were 93 J/g and 142 J/g, respectively [16].
The wide angle X-ray diffraction (XRD) spectra of the scPLAs
were recorded with a Bruker D8 Advance wide-angle X-ray
diffractometer at 25 C using CuKa radiation at 40 kV and 40 mA.
For XRD, the scanning angle range of 2q ¼ 5e30 at a scan speed of
3/min was used to determine the homocrystalline and stereocomplex
crystalline structures of the polylactides.
The melt flow index (MFI) of the scPLAs was measured using a
Tinius Olsen MP1200 melt flow indexer. The temperature of the
melt was kept at a uniform 235 C and a 5.0 kg load was applied to
extrude the molten scPLAs. A 100 g rod was used as a plunger. The
MFI was averaged from at least five determinations.