Physical Aging and Glass Transition of the Rigid Amorphous Fraction in Poly(L-lactic acid)

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