Poly stereocomplex crystals

Tsuji, H., Wada, T., Sakamoto, Y. and Sugiura, Y. (2010) Stereocomplex crystallization and spherulite growth behavior of poly(L-lactide)- -poly(D-lactide) stereodiblock copolymers. Polymer, 51, 4937-4947. 

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. 

Stereocomplex Crystals of Poly(L-lactide)/Poly(o-lactide)... 

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. 

Xu H, Wu D, Yang X, Xie L, Hakkarainen M (2015) Thermostable and impermeable nano-barrier walls constructed by poly (lactic acid) stereocomplex crystal decorated graphene oxide nanosheets. Macromolecules 48(7) 2127-2137... 

The tacticity of PLA influences the physical properties of the polymer, including the degree of crystallinity which impacts both thermo-mechanical performance and degradation properties. Heterotactic PLA is amorphous, whereas isotactic PLA (poly(AA-lactide) or poly (55-lac tide)) is crystalline with a melting point of 170-180°C [26]. The co-crystallization of poly (RR-lactide) and poly(55-lactide) results in the formation of a stereocomplex of PLA, which actually shows an elevated, and highly desirable, melting point at 220-230°C. Another interesting possibility is the formation of stereoblock PLA, by polymerization of rac-lactide, which can show enhanced properties compared to isotactic PLA and is more easily prepared than stereocomplex PLA [21]. 

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]. 

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. 

Tsuji, H., Takai, H. and Saha, S.K. (2006) Isothermal and non-isothermal crystallization behavior of poly(L-lactic acid) Effects of stereocomplex as nucleating agent. Polymer, 47, 3826-3837. 

Narita, 1., Katagiri, M. and Tsuji, H. (2011) Highly enhanced nucleating effect of melt-recrystaUized stereocomplex crystallites on poly(L-lactic acid) crystallization. Macromolecular Materials and Engineering, 296, 887-893. 

Fig. 8 Crystallization of stereocomplex PLA by blending poly(D-lactic acid) with poly(L-lactic acid) to form nucleating sites (Tsuji H. 2005)...

The latter form can be prepared at a high draw ratio and a high drawing temperature [28]. The 7-form is formed by epitaxial crystallization [29]. It has been observed that a blend with equivalent poly(L-lactide) PLLA and poly(D-lactide) PDLA contents gives stereo-complexation (racemic crystallite) of both polymers. This stereocomplex has higher mechanical properties than those of both PLAs, and a higher melting temperature of 230°C. The literature reports different density data [4] for PLA, with most values for the crystalline polymer around 1.29 compared with 1.25 for the amorphous material. 

Fujita, M., Sawayanagi, T., Abe, H., Tanaka, T., Iwata, T., Ito, K. et al. (2008) Stereocomplex formation through reorganization of poly(L-lactic acid) and poly(D-lactic acid) crystals. Macromolecules, 41, 2852-2858. 

At the same time, the crystallization of the excess poly-R is observed only at enantiomeric excesses between 63% and 100%. At other compositions, the crystallization of the excess poly-R is hindered by the crystallization of the stereocomplex. Indeed, the crystallization of the stereocomplex occurs first, leaving an excess of uncrystallized Isotactic pol)nner which is dispersed in a rigid matrix of crystalline material. The crystallization of the excess pol3nner is then impeded when the excess is lower than 63% at... 

Similarly, as shown in Figure 6, the morphology of poly-R/poly-S binary mixtures of PMEPL is controlled by the formation of the stereocomplex at high temperatures, followed by the crystallization of the excess polymer at lower temperatures. It is believed that the excess polymer is trapped between the spherulite lamellae of the stereocomplex, and at their boundaries. Large spherulites are then observed for the racemate sample, and for all binary mixtures in which the stereocomplex can be formed, but very small spherulites are found for pure isotactic chains. 

Urayama, H., Kanamori, T., Fukushima, K., Kimura, Y., 2003. Controlled crystal nucleation in the melt crystallization of poly (L-lactide) and poly(L-lactide)/poly(D-lactide) stereocomplex. Polymer 44, 5635—5641. 

---