Abstract
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°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 ( ΔT = T g - T a) on PLLA samples of different molecular weight ( M ̄ v = 5300, 20 000, 691 000 ) show that a decrease in molecular weight increases the magnitude of the enthalpy relaxation at the glass transition.
Published Version
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