Polylactic acid, called PLA, is currently one of the most used bioplastics in the field of food packaging, despite ample global market domination by petrochemicals. As one of the major challenges to produce high performance PLA packaging at a large scale lies in the improvement of its gas barrier properties, the tailoring of the PLA microstructure from thermal crystallization [1,2], drawing [3] has been strongly investigated in the recent years. New strategies are actually studied to obtained better effects. One of them consists in confinement of the polymer at the molecular scale using the layer-multiplying co-extrusion process combined eventually with annealing processes to create nanometric thickness layers. Confinement is known to act on the crystalline phase [4] in order to obtain singular crystalline high barrier structure but acts also on the amorphous phase. PLLA, an isomeric form of PLA had not been studied following this type of analyze. In this work, we propose to design multi-layer coextruded PLLA/PS in order to analyze the macromolecular mobility in PLLA at the glass transition thanks to the cooperative rearranging region (CRR) concept when it is confined by polystyrene. The investigations were carried with the help of modulated temperature differential scanning calorimetry (MT-DSC) on both amorphous and semi-crystalline materials. We studied the amorphous phase of PLLA considering the three-phase model which includes besides the crystalline phase two fractions of the amorphous phase, the mobile amorphous fraction (MAF) and the rigid amorphous fraction (RAF). Results showed that the crystallization of PLA under confinement impacted largely its capacity to génerate RAF.