PLLA/PDLA stereocomplex crystals

The PLLA/PDLA stereocomplex, which is another crystal modification of PLA, was first discovered by Ikada and coworkers [80]. Its structure and physical properties 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 stereo complex [86, 87]. 

Crystalline structure and molecular dynamics in alpha and alpha crystals of poly(L-lactide) (PLLA) and PLLA/poly(D-lactide) (PDLA) stereocomplex crystals have been investigated by solid-state C CPMAS NMR spectroscopy. The crystal forms of polylactide (PLA) have different line shapes, and resonance splittings in solid-state NMR spectra due to the crystallographically inequivalent sites within crystal unit cell. 

It has been observed that a 1 1 mixture of pure PLLA with pure PDLA yields a stereocomplex of the two polymers during crystallization or polymerization. The PLA stereocomplex consists of racemic crystalline strucmres in which l-PLA and d-PLA chains are packed side by side, with a 1 1 ratio of l d monomer units [4, 32, 33]. While the melting temperature of a- and P-crystalline forms of PLA falls in the range 170-180°C, the 7) of PLA stereocomplex is between 220 and 230°C [33]. The high of PLLA/PDLA stereocomplex makes it a difficult material for processing however, it is interesting to note that the comparison between PLLA/PDLA equimolar blends and the starting materials shows mechanical properties that are markedly improved. 

PDLA or PLLA when solely stereocomplexation takes place [107], However, spherulites with complicated morphologies are evident when stereocomplexation and homo-crystallization occur simultaneously [104,107,108]. Maillard and Prud homme observed mirror-image stereocomplex crystalline assemblies for PLLA- and PDLA-rich PLLA/PDLA mixtures [109]. As observed for crystalline polymers, PLLA spherulites become smaller with decreasing temperature and time [110,111]. 

Figure 5.14 (a) WAXS profiles of a stretched PLLA/PDLA blend recorded upon heating. a, a, and denote the a -, a-fbrm homo-crystals, and stereocomplex crystals, respectively. The strong peaks around 20 = 16.5° and 19.0° are the characteristic reflections of the a - and a-form homo-crystals of PLA. The three peaks around 20 = 11.1°, 20.0°, and 23.2°... 

A high voltage in electro-spinning suppressed the formation of a-crystals, and helped to form PLLA/PDLA fibers with predominately stereocomplex crystals. ... 

A polymeric implantable device from a PLLA/PDLA blend such as a stent with improved fracture toughness is disclosed. A stereocomplex nucleating agent (a combination of PLLA/PDLA) results in an increase in nucleation density and reduced crystal size, which increases fracture toughness of the formed device. ... 

In 1990, Yui et al. [24] revealed that stereocomplex with little he crystallization can be formed easily from diblock-type sb-PLA (PLLA-/)-PDLA) because of the easy interaction of the two enantiomeric block sequences. 

Figure 2.8 (a) PDLA (top) and PLLA (bottom) chains in parallel orientation, as located in the stereocomplex crystal. The model has been built by bringing the chains until they fit in the experimental dimensions of the unit cell. Hydrogen-bonding angles for the interactions... 

Figure 8.7 Crystal structure of PLA stereocomplex [33]. The lines between PLLA and PDLA chains were added to original figure. (Reproduced with permission from ref [33]. Copyright 1991, Taylor Francis.)...

Poly(L-lactic acid) is reported to crystallize into lozenge-Uke [37,112-114] and hexagonal-like [37, 114] single crystals in dilute solutions. In contrast, single crystals of the stereocomplex of PLLA and PDLA have a peculiar triangular shape when they are formed in p-xylene at solution concentrations as low as 0.04% [33, 115]. 

MaUlard, D. and Prud homme, R.E. (2010) Difference bet ween crystals obtained in PLLA-rich or PDLA-rich stereocomplex mixture. Macromolecules, 43, 4006-4010. 

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