Polylactide (PLA) is biodegradable and biobased glassy polyester with satisfying mechanical properties. One of its main applications is food packaging and catering. To increase the spectrum of applicability, the barrier properties of PLA need to be improved. The impact of polymer microstructure and dynamics of amorphous fractions on the barrier properties of glassy and semicrystalline polymers is not completely understood and quantified today.In that aim, different types of PLA microstructure were created in the aim to confine the amorphous phase between crystalline lamellae or between layers of auxiliary polymers by multi-nanolayer co-extrusion. The use of those two confinement techniques allowed the engineering of the interfaces between amorphous PLA and rigid surfaces, affording changes in coupling between both phases (1,2).We could show that the coupling of the amorphous phase with the crystalline phase governed the barrier properties. Decoupling of phases allows increasing oxygen barrier properties by a factor of 6. The same result was obtained, when PLA was crystallized in a confined space between auxiliary polymer layers. The confined crystallization induced also decoupling of crystalline and amorphous phases and increased oxygen barrier properties. These results help to formulate recommendations for the processing of PLA in the aim to increase its barrier properties.