Physical Aging and Glass Transition Dynamics of Semicrystalline Poly(lactic acid) for 3D-Printing Applications Investigated by Fast Scanning Calorimetry

Abstract

Physical aging and the glass transition dynamics of semicrystalline poly­(lactic acid) (PLA), designed for three-dimensional (3D)-printing applications, are investigated by fast scanning calorimetry for identifying the effect of crystallinity on the relaxation behavior of the amorphous phase. PLA was crystallized by annealing the isotropic melt at 363 K for different times between 100 and 1000 s, yielding a two-phase structure composed of a single amorphous phase and up to 40% crystals. Then, these samples were physically aged at temperatures between 270 and 310 K, that is, at temperatures about 60 to 20 K lower than the glass transition temperature T g, respectively. The presence of crystals slows down the relaxation rate of the amorphous glass at temperatures slightly lower than T g, indicating a confinement effect. In agreement with this observation, the Vogel temperature of the amorphous phase increases by almost 10 K, when the crystallinity is increased from 0 to 40%. For semicrystalline PLA, the glass transition dynamics is different after cooling the system faster or slower than about 100 K/s, being in line with the former observation of nonisothermal secondary crystallization and possible formation of a rigid amorphous fraction on slow cooling.