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Amorphous phase dynamics in Poly(lactic acid) (PLA) with different crystallinity degrees have been investigated from the vitreous state to the glass transition by means of two calorimetric methods. Temperature Modulated Differential Scanning Calorimetry was used to characterize the heat capacity signals and the average cooperativity length at the glass transition in non‐aged materials. Standard DSC was used to study the physical aging. It is shown that amorphous and fully crystallized PLA exhibit different relaxation parameters. For semicrystalline PLA with an intermediate degree of crystallinity, the peaks of the enthalpy of recovery and the out‐of‐phase heat capacity component are bimodal. The bimodality of the peaks is attributed to the relaxations of the inter‐spherulitic and intra‐spherulitic amorphous phases, respectively. Thus, in partially crystallized PLA, the non‐crystalline fraction of the material could be divided in three fractions, namely the Rigid Amorphous Fraction, the inter‐spherulitic Mobile Amorphous Phase (MAP), and the intra‐spherulitic MAP. Each of them exhibits a distinct molecular mobility. POLYM. ENG. SCI., 54:1144–1150, 2014. © 2013 Society of Plastics Engineers

The thermal behavior of a polymeric material during a cooling ramp was simulated by means of the bond fluctuation model. By introducing both an intramolecular and an intermolecular potential, if the cooling rate is fast enough, the glass transition occurs, and the states attained at low temperatures can be characterized as disordered glasses. The evolution of the resulting amorphous systems was then studied during isothermal periods both for systems starting as an amorphous liquid and as an amorphous glass. The results show that after a very long annealing time at temperatures above the glass transition, an excess of energy loss appears in the system when compared to the usual glass theory. The Monte Carlo method was used to simulate the physical aging phenomena at long time scales.