THERMAL-PROPERTIES AND PHYSICAL AGING OF POLY(L-LACTIC ACID)

Poly (L-lactic acid) (PLLA) was investigated by differential scanning calorimetry and dynamic mechanical and dielectric spectroscopies. In the dynamic mechanical and dielectric spectra no secondary relaxations were observed below the glass-to-rubber transition (70-degrees-C at 3 Hz). The thermal history strongly affects the physical properties of PLLA, inducing changes of crystalline:amorphous ratio as well as large physical ageing effects on the glassy amorphous phase. This latter phenomenon is clearly observed as a consequence of room-temperature storage and develops at a faster rate as the ageing temperature T(a) approaches T(g). Ageing experiments carried out at the same undercooling (DELTA-T = T(g) - T(a)) on PLLA samples of different molecular weight (M(v)BAR = 5300, 20 000, 691 000) show that a decrease in molecular weight increases the magnitude of the enthalpy relaxation at the glass transition.