Physical aging and glass transition of the rigid amorphous fraction in poly(l-lactic acid)

The semicrystalline state of polymers implies the formation of a noncrystalline interphase beside lamellar crystals known as the rigid amorphous fraction (RAF). This devitrifies on heating at temperatures much higher than the one typical of the bulk-amorphous fraction. Its glass transition and physical aging in the glassy state have thus far remained elusive. Here, we study the RAF glassy dynamics in poly(L-lactic acid) (PLLA) applying recently developed aging thermal protocols based on fast scanning calorimetry (FSC). Specifically, semicrystalline samples are aged in different conditions between the glass transition of mobile amorphous fraction and the crystals’ melting temperature (TM). A distinct endothermic peak at temperatures above that of aging develops with time. We provide compelling evidence that in the initial part of aging the origin of this peak is due to the enthalpy recovery of glassy RAF. At longer aging times, the aging peak is at least partly associated with secondary crystallization. Isochronous experiments at different aging temperatures enable to obtain a fair estimate of the RAF glass transition temperature, whose upper limit is about 135 °C. The proposed method holds the promise of gaining new insights into the elusive glassy dynamics of the RAF of semicrystalline polymers.