**2. Physical and chemical properties of PLA**

L-lactic acid and D-lactic acid, the two isomers of lactic acid, are shown in Scheme 1. Pure Llactic acid or D-lactic acid, or mixtures of both components are needed for the synthesis of PLA.

Scheme 1. The stereoisomers of lactic acid.

The homopolymer of LA is a white powder at room temperature with *T*g and *T*m values of about 55°C and 175°C, respectively. High molecular weight PLA is a colorless, glossy, rigid thermoplastic material with properties similar to polystyrene. The two isomers of LA can produce four distinct materials: Poly(*D*-lactic acid) (PDLA), a crystalline material with a regular chain structure; poly(*L*-lactic acid) (PLLA), which is hemicrystalline, and likewise with a regular chain structure; poly(*D*,*L*-lactic acid) (PDLLA) which is amorphous; and *meso*-PLA, obtained by the polymerization of *meso*-lactide. PDLA, PLLA and PDLLA are soluble in common solvents including benzene, chloroform, dioxane, etc. and degrade by simple hydrolysis of the ester bond even in the absence of a hydrolase. PLA has a degradation half-life in the environment ranging from 6 months to 2 years, depending on the size and shape of the article, its isomer ratio, and the temperature. The tensile properties of PLA can vary widely depending on whether it is annealed or oriented, or its degree of crystallinity (Garlotta et al., 2001). Some of the physical and chemical properties of PLA are summarized in Table 1.


Table 1. Selected physical and chemical properties of PLA.
