Positron annihilation lifetime spectroscopy (PALS) was conducted to follow the evolution of free volume during crystallization of PLLA at 100 °C. A finite lifetime distribution of three components was used to fit the PALS spectra. The analysis of the longest lifetime component (τ3) indicates that the free volume distribution evolves during crystallization by increasing the number of holes yet decreasing their size; moreover, the free volume fraction increased during crystallization. Following the evolution of the shortest components a correlation was found with crystalline and amorphous phase contents present in PLLA. The first component (τ1) was assigned to positron annihilation in occupied zones of the crystalline phase whereas the second component (τ2) was attributed to annihilation by different amorphous arrangements. A model for the supramolecular arrangement of PLLA chains was devised in terms of free volume enlargement for annealed samples. According to this model transformations occur in mobile amorphous phase (MAP) and rigid amorphous phase (RAP) with PLLA chains evolving from folded or coil conformations in the as-quenched samples containing uniquely MAP to opener (more extended) conformations in samples containing larger RAP and crystalline fractions. The proposed model provides a rational for the understanding of some unexpected effects associated with free volume that have been observed in several semicrystalline polymer systems, i.e., the lowering of density during crystallization (dedensification), the acceleration of polymer chains dynamics around the Tg (dynamic fragility) due to a rigid amorphous phase confined by crystallites, and also the gas permeability behavior in terms of solubility and diffusion coefficients.