Enthalpy Relaxation and Embrittlement of Poly(l-lactide) during Physical Aging

The enthalpy relaxation behavior of poly(l-lactide) (PLLA) below glass transition temperature (Tg) and its effects on the mechanical properties were studied using differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), and tensile test. Both the enthalpy loss (δH) and peak endothermic temperature (Tp) in the glass transition region show a linear increase with the logarithm of aging time (ta). On the basis of DSC results, the kinetics of the enthalpy relaxation process were analyzed, and the kinetic parameters of PLLA were obtained: relaxation rate of enthalpy at 40 °C βH = 1.77 J g-1 per decade, and apparent activation energy Δh* = 1107 kJ mol-1. Besides, analysis on the kinetic of enthalpy relaxation for poly(dl-lactide) (PDLLA) shows that the Δh* value of PDLLA is similar to that of PLLA. Moreover, it was found that the strength and modulus of PLLA increase gradually; on the contrary, the toughness reduces dramatically during the physical aging. The yield strength and tensile modulus increased respectively 18.8% and 23.1% after being aged at 40 °C for 1008 h, as compared to the unaged sample. Also, the fracture strain of PLLA decreased from more than 300% to about 6% with aging at 25 °C for 24 h. Moreover, the fractographic examination evaluated by scanning electron microscopy showed that the fracture of PLLA changed from the "ductile" to "inductile" manner during physical aging. The drop of ductility in physical aging is correlated to the rearrangement of polymer chains from disordered to more ordered morphology.