Page 145 - Biodegradable Polyesters

5.6 Stereocomplex Crystals of Poly(L-lactide)/Poly(D-lactide) 123

All curves shown in Figure 5.13 are characterized by the presence of two max-
ima, as is typical for PLA. Variation of the molar mass and optical purity of the
macromolecules affects the temperature-position of the two maxima as well as the
′
relative rates of growth of α-and α -spherulites, which permits an easy identifica-
tion of the temperature range where each crystal modification prevails. A higher
stereoregularity/lower D-lactic acid content shifts both maxima toward higher
′
temperatures. Also the maximum-growth-rate ratio between α-and α -spherulites
is affected by the optical purity of the monomer, as at very low D-lactic acid content
′
the low temperature maximum, where growth of the α -modification predomi-
nates, is more pronounced [15].

5.6
Stereocomplex Crystals of Poly(L-lactide)/Poly(D-lactide)

The low crystallization rate has led to thorough analysis of the crystallization
kinetics of PLA over the years, in efforts to improve its crystallization rate. One
of the most effective nucleating agents for PLA is the stereocomplex formed
upon mixing PLLA and PDLA [80]. The most successful stereocomplex is the one
formed at a PLLA/PDLA 50/50 blend ratio, which has a melting temperature of
∘ ∘
230 C, 50 C higher than that of PLLA or PDLA. The overall crystallization rate
of the stereocomplex is higher than that of pure PLLA or PDLA, owing to faster
nucleation and higher growth rate of stereocomplex spherulites, which sizably
fastens the phase transition of PLA [81].
The PLLA/PDLA stereocomplex, which is another crystal modification of PLA,
was first discovered by Ikada and coworkers [80]. Its structure and physical prop-
erties have been studied using a number of different techniques, including infrared
spectroscopy [82], optical microscopy [83], calorimetry [84], and X-ray diffraction
[85]. Recent reviews by Tsuji and Fukushima et al. summarize the main properties
of the stereocomplex [86, 87].
Two structural modes have been proposed for the crystals of PLA stereocom-
plex. Okihara et al. [88] suggested a triclinic cell (space group P1) with the lattice
∘
∘
dimensions a = b = 0.916nm, c = 0.87nm, = = 109.2 and = 109.8 ,
packed by two chains per unit cell. A later study by Brizzolara et al. [85]
confirmed these results. On the other hand, Cartier et al. [89] proposed a
larger trigonal cell involving six chains per unit cell, with lattice parameters
∘
∘
a = b = 1.498nm, c = 0.87nm, = = 90 , = 120 ,and R3c symmetry,
supported by Sawai et al. [90]. On the basis of real-time infrared spectroscopy
analysis, it was suggested that specific hydrogen bonds between the PLLA and
PDLA chains in the stereocomplex crystal are the driving force for racemic
nucleation of the stereocomplex [82, 91].
Besides blending equimolar amounts of PLLA and PDLA, a number of different
PLA architectures, able to form a stereocomplex, have been synthesized, including