Understanding block copolymer (BCP) self-assembly under confi nement via conventional melt extrusion is advantageous as technology moves towards thin fi lm applications. Previous research has revealed that confi ning elastomeric BCPs in thin fi lms with polystyrene (PS) via microlayering results in an increase in ductility with decreasing layer thickness and that the interfacial region dramatically infl uences the elastic modulus. This contribution investigated the role of interfacial width and layer thickness on deformation mechanics by comparing poly(methyl methacrylate) (PMMA) and PS as wetting layers in confi ned multilayers. The interfacial region was found to be crucial to tailoring the mechanical response of composite materials.group has shown that thin fi lms of confi ned polymers exhibit unexpected physical properties, such as barrier, [ 4 ] thermal, [ 5 ] and mechanical [ 6 ] when compared to their bulk counterparts. [ 7 ] One of the most signifi cant contributions, thus far, has been utilizing forced assembly to confi ne the crystallization of semicrystalline polymers. [ 3 ] The confi ned crystallization of poly(ethylene oxide) (PEO) via microlayer co-extrusion demonstrated that in thick layers, large spherulites developed, while the PEO chains crystallized into in-plane, single lamellae in thinner layers due to the constraint of the confi ning layer. This confi ned crystallization behavior resulted in a reduction in O 2 permeability by two orders of magnitude through the thickness of the multilayer fi lm. [ 8 ] Further research revealed that the nature of the confi ning layer alters the crystal lamellae size in these PEO layered composites, which further infl uences the oxygen permeability of the multilayer fi lms. [ 9 , 10 ] Similar research utilizing microlayering technology investigated the effect of layer thickness on the glass transition temperature ( T g ) of two immiscible polymers and revealed a critical layer thickness at which the fi lm becomes completely "interphase region," resulting in a merging of the two material T g s into a single T g for the composite. [ 5 , 11 ] The understanding of structure-property relationships of