The true stress−strain characterization of the tensile yield as a function of temperature and cross-head speed is reported for three ethylene−butene copolymers having different crystal weight fractions in the range 0.66−0.35. Evidence is given of two plastic processes operating competitively depending on the experimental conditions. Crystal shear and crystal block sliding are suggested on the basis of the concept of the mosaic block structure of the crystalline lamellae. Small-angle and wide-angle diffraction measurements together with infrared measurements support this conclusion. Two theoretical models of plastic deformation borrowed from solid state physics are proposed to account for our findings. These are a homogeneous crystal slip involving thermal nucleation of dislocations and a heterogeneous crystal slip operating through the defective block boundaries. The competitive action of the two processes which relies on the requirement of the lower energy-consuming pathway of deformation is discussed in terms of a combination between thermal activation and strain-hardening. The proposed models provide an explanation for the major influence of the crystal thickness on the temperature of occurrence of the homogeneous slip. The role of the amorphous phase is discussed in relation to the strain-hardening accompanying the two processes.

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Multilayer coextrusion processing was applied to produce 2049-layer film of poly(butylene succinate-co-butylene adipate) (PBSA) confined against poly(lactic acid) (PLA) using forced assembly, where the PBSA layer thickness was about 60 nm. This unique technology allowed to process semicrystalline PBSA as confined polymer and amorphous PLA as confining polymer in a continuous manner. The continuity of PBSA layers within the 80/20 wt % PLA/PBSA layered films was clearly evidenced by atomic force microscopy (AFM). Similar thermal events to the reference films were revealed by thermal studies; indicating no diffusion of polymers during the melt-processing. Mechanical properties were measured for the multilayer film and the obtained results were those expected considering the fraction of each polymer, revealing the absence of delamination in the PLA/PBSA multinanolayer film. The confinement effect induced by PLA led to a slight orientation of the crystals, an increase of the rigid amorphous fraction (RAF) in PBSA with a densification of this fraction without changing film crystallinity. These structural changes allowed to strongly improve the water vapor and gas barrier properties of the PBSA layer into the multilayer film up to two decades in the case of CO gas. By confining the PBSA structure in very thin and continuous layers, it was then possible to improve the barrier performances of a biodegradable system and the resulting barrier properties were successfully correlated to the effect of confinement on the microstructure and the chain segment mobility of the amorphous phase. Such investigation on these multinanolayers of PLA/PBSA with the aim of evidencing relationships between microstructure implying RAF and barrier performances has never been performed yet. Besides, gas and water permeation results have shown that the barrier improvement obtained from the multilayer was mainly due to the reduction of solubility linked to the reduction of the free volume while the tortuosity effect, as usually expected, was not really observed. This work brings new insights in the field of physicochemical behaviors of new multilayer films made of biodegradable polyesters but also in interfacial processes due to the confinement effect induced in these multinanolayer structures obtained by the forced assembly coextrusion. This original coextrusion process was a very advantageous technique to produce eco-friendly materials with functional properties without the help of tie layer, additives, solvents, surface treatments, or inorganic fillers.

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