Stereocomplex crystal structure

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.)...

Okihara, T, Tsuji, M., Kawaguchi, A. et al. (1991) Crystal structure of stereocomplex of poly(L-lactide) and poly(D-lactide). Journal of Macromolecular Science Physics, B30, 119-140. 

Abstract Stereocomplexes have been formed by mixing the two isotactic poly(a-methyl-a-cthyl-p-propriolactones) (PMEPL) of opposite chirality. This leads to an insoluble complex exhibiting a melting transition which is 40 C above that of the initial isotactic components. Structural differences between these samples have been determined by nuclear magnetic resonance spectroscopy and electron diffraction. It was found by NMR that the stereocomplex crystallizes in a 2 helical conformation whereas the corresponding isotactic chains exhibit a helical or extended chain conformation depending upon the method of sample preparation. Electron diffraction confirms these measurements with the determination of a,b and c dimensions of the orthorombic unit cells. 

In the present paper, differences between isotactic PMEPL and the stereocomplex are examined by solid state nuclear magnetic resonance (NMR) spectroscopy. These studies reveal new polymorphic behaviour of the isotactic polymer and differences in crystal structure which depend on tacticity. Crystal structures of the various polymorphs were also determined by electron and x-ray diffraction studies. 

PLA crystalline structures. Unit cell parameters for non-blended PLLA and stereocomplex crystals. 

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. 

These had the same crystallographic form as complexes isolated from melt crystallization and from concentrated solutions. The stereocomplex is a 2 1 molar ratio of syndiotactic isotactic. Since the melting point also varied with crystallization temperature, lamella and single-crystal structure have been suggested for the dilute solution complex. 

On the basis of the crystal structure and unit cell parameters, the crystal density q c (g/cm ) of the stereocomplex crystal can be calculated from Equation 5.1 ... 

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]. 

Triangular polymer single crystals stereocomplexes, twins, and frustrated structures. Macromolecules,... 

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