The development of biopolymers for food-packaging applications implies ecofriendly character to specific industrial requirements as low-cost and good mechanical, thermal and barrier properties. Polylactide (PLA) has exhibited promising applicability in the past decades due to its performance in renewability, melt processing and mechanical properties [1]. As one of the major challenges to consider producing 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 [2,3] and drawing [4] has been strongly investigated in the recent years as the polymer blends route by blending PLA with biobased or biodegradable polymers as for example PHB, PCL, PBS, PBSA, etc ... . New strategies are currently studied to obtained stronger effects. One of them consists in the geometrical confinement of the polymers at the molecular scale using the original layer-multiplying co-extrusion process to create nanometric thickness layers. This technology is environmentally friendly and health secured and has already proved its efficiency to improve the gas barrier properties for other polymers [5] by the way of an improved crystallization. In this work we propose to design multi-layer coextruded based PLA films to analyse the ability of PLA and PBS and PBSA to crystallise under confinement and its effect on the final gas barrier properties. It has been shown that geometrical confinement acts differently on the formation of a rigid amorphous fraction (RAF) for both PLA and PBSA by creating or slowing its formation. Finally it was shown that PLA exhibits better gas barrier properties by slowing the less impermeable RAF formation while PBSA exhibits also better gas barrier properties by increasing its more impermeable RAF phase.(1) Auras, R.; Harte, B.; Selke, S. Macromol. Biosci. 2004, 4, 835–864.(2) Guinault, A.; Sollogoub, C.; Ducruet, V.; Domenek, S. Eur. Polym. J. 2012, 48, 779–788.(3) Cocca, M.; Lorenzo, M. L. D.; Malinconico, M.; Frezza, V. Eur. Polym. J. 2011, 47, 1073–1080.(4) Delpouve, N.; Stoclet, G.; Saiter, A.; Dargent, E.; Marais, S. J. Phys. Chem. B 2012, 116, 4615–4625.(5) Carr, J. M.; Mackey, M.; Flandin, L.; Hiltner, A.; Baer, E. Polymer 2013, 54, 1679–1690.