Polylactide physical properties

Pluta, M., Galeski, A., Alexandre, M., Paul, M. A., and Dubois, P. (2002). Polylactide/montmorillonite nanocomposites and microcomposites prepared by melt blending Structure and some physical properties. J A lPoI rrhSd., 86,1497-1506. 

Over the past several decades, polylactide - i.e. poly(lactic acid) (PLA) - and its copolymers have attracted significant attention in environmental, biomedical, and pharmaceutical applications as well as alternatives to petro-based polymers [1-18], Plant-derived carbohydrates such as glucose, which is derived from corn, are most frequently used as raw materials of PLA. Among their applications as alternatives to petro-based polymers, packaging applications are the primary ones. Poly(lactic acid)s can be synthesized either by direct polycondensation of lactic acid (lUPAC name 2-hydroxypropanoic acid) or by ring-opening polymerization (ROP) of lactide (LA) (lUPAC name 3,6-dimethyl-l,4-dioxane-2,5-dione). Lactic acid is optically active and has two enantiomeric forms, that is, L- and D- (S- and R-). Lactide is a cyclic dimer of lactic acid that has three possible stereoisomers (i) L-lactide (LLA), which is composed of two L-lactic acids, (ii) D-lactide (DLA), which is composed of two D-lactic acids, and (iii) meso-lactide (MLA), which is composed of an L-lactic acid and a D-lactic acid. Due to the two enantiomeric forms of lactic acids, their homopolymers are stereoisomeric and their crystallizability, physical properties, and processability depend on their tacticity, optical purity, and molecular weight the latter two are dominant factors. 

Tsuji, H. and Ikada, Y. (1999) Physical properties of polylactides, in Current Trends in Polymer Science, vol. 4 (eds K.L. DeVries et al.). Editorial advisory board. Research Trends, Trivandrum (India), pp. 27-46. 

Polylactide (PLA), a biodegradable aliphatie polyester, can efficiently be produced by the ROP of LA using metal-alkoxides, whereas enzymatic ROP of LA has had limited success. Laetide is a hiorenewable monomer derived from corn, sugar beets and wheat, and ean he obtained as either l- or d-LA single enantiomers, racemic mixture (rac-LA) or meso (meso-LA). The PLA microstructures (isotactic, syndiotactic, heterotactic and atactic) depend on both the type of LAs used and the catalysts selected, and influence the PLA s physical properties (Scheme 6.1)." ... 

The properties of PLA, as indeed those of other polymers, depend on its molecular characteristics, as well as on the presence of ordered structures, such as crystalline thickness, crystallinity, spherulite size, morphology and degree of chain orientation. The physical properties of polylactide are related to the enantiomeric purity of the lactic acid stereo-copolymers. Homo-PLA is a linear macromolecule with a molecular architecture that is determined by its stereochemical composition. PLA can be produced in a totally amorphous or with up to 40 per cent crystalline. PLA resins containing more than 93 per cent of L-lactic acid are semi-crystalline, but, when it contains 50-93 per cent of it, it is entirely amorphous. Both meso- and D-lactides induce twists in the very regular PLLA architecture. Macromolecular imperfections are responsible for the decrease in both the rate and the extent of PLLA crystallization. In practise, most PLAs are made up of L-and D,L-lactide copolymers, since the reaction media often contain some meso-lactide iir turities. 

Computer simulations of polymer-chain growth can be compared to experimental data in order to propose or support models of polymerization, as has been done for polylactide [126]. Physical properties, such as the dielectric constant of poly(N-vinyl carbazole), can often be correlated with structural features such as tacticity [127]. 

As discussed above, PLAs have physical properties useful in fibers, packaging, and other applications traditionally dominated by petroleum-based resins. Although the general literature on polylactides is extensive, only a few... 

The doublet structure of the methyl signal at 17 ppm is caused by the stereochemistry. Polylactides, not yet described in any pharmacopoeias, are rather new biodegradable polyesters derived from the chiral lactic acid and used, e. g., in drug delivery systems. The stereochemistry of the polymer is important to the physical and chemical behavior, especially the polymer properties. Pure tactic polymerization can be differentiated from atactic or mixed polymers by simple comparison of the C NMR spectra (Figure 3-10) [4]. 

Pluta, M., Murariu, M., Ferreira, A.D.S. et al. (2007) Polylactide compositions. 11. Correlation between morphology and main properties of PLA/calcium sulfate composites. Journal of Polymer Science Part B Polymer Physics, 45, 2770-2780. 

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