PLA stereo-complex by co-crystallization

PLA stereo-complexes are obtained by co-crystallization of PLLA and PDLA in bulk or in organic solvents such as chloroform or acetonitrile. It has been shown that the minimal degree of polymerization (DP) of PLLA or PDLA oligomers for crystallization is 11, with corresponding melting temperature (Tm) at 59 °C and melting enthalpy (AH i) of 39 J g. In contrast, in blends of PLLA and PDLA oligomers, crystallization or stereo-complexation is 

Different procedures are used for co-crystallization or stereo-complexation in bulk from melt (l) crystallization at a fixed temperature directly from the melt (2) crystallization at a fixed temperature quenching from the melt (3) crystallization during cooling down from the melt and (4) crystallization during heating of a quenched melt/ ° 

Together with stereo-complexation, homo-crystallites only composed of either PLLA or PDLA can also be formed. The formation of stereo-complex crystallites and/or homo-crystallites depends on parameters, such as the blending ratio, ° the molar mass and optical purity, the cocrystallization conditions, such as the temperature and time, the nature of solvents, etc. In general, equimolar blending or low molar mass tends to favour exclusive formation of stereo-complex without formation of homo-crystallites. The blending mode and nature of solvents also strongly affect stereo-complexation. 

Moreover, stereo-complexation can occur in block copolymer mixtures provided that the copolymers contain l-LA and d-LA sequences of sufficient lengths. PLA multiblock stereo-copolymers can easily form a stereo-complex as mentioned above. Examples also include stereo-complexation between bloek copolymers of L-lactide or o-lactide with s-caprolactone, between L-lactide-rich PLA and D-lactide-rich PLA, between copolymers of L-lactide or D-lactide with glycolide and between block copolymers of L-lactide or D-lactide with PEG.  

Interestingly, PLA stereo-complex can also be obtained during in vitro degradation of poly(DL-lactide), which is an intrinsically amorphous polymer. In fact, poly(DL-lactide) contains isotactic l-LA and d-LA se- 

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