Due to the chiral nature of lactic acid, several distinct forms of polylactide exist: poly-L-lactide (PLLA) is the product resulting from polymerization of L,L-lactide (also known as L-lactide). PLLA has a crystallinity of around 37%, a glass transition temperature 60–65 °C, a melting temperature 173–178 °C and a tensile modulus 2.7–16 GPa.[8][9] Heat-resistant PLA can withstand temperatures of 110 °C.[10] PLA is soluble in chlorinated solvents, hot benzene, tetrahydrofuran, and dioxane.[11]
PLA has similar mechanical properties to PETE polymer, but has a significantly lower maximum continuous use temperature.[12]
Polylactic acid can be processed like most thermoplastics into fiber (for example, using conventional melt spinning processes) and film. The melting temperature of PLLA can be increased by 40–50 °C and its heat deflection temperature can be increased from approximately 60 °C to up to 190 °C by physically blending the polymer with PDLA (poly-D-lactide). PDLA and PLLA form a highly regular stereocomplex with increased crystallinity. The temperature stability is maximised when a 1:1 blend is used, but even at lower concentrations of 3–10% of PDLA, there is still a substantial improvement. In the latter case, PDLA acts as a nucleating agent, thereby increasing the crystallization rate. Biodegradation of PDLA is slower than for PLA due to the higher crystallinity of PDLA.
There is also poly(L-lactide-co-D,L-lactide) (PLDLLA) – used as PLDLLA/TCP scaffolds for bone engineering.[13]