Morphology, thermal behavior and mechanical properties of binary blends of compatible biosourced polymers: Polylactide/polyamide11

Blends of entirely biosourced polymers, namely polylactide (PLA) and polyamide11 (PA11), have been melt-compounded by twin-screw extrusion without the use of any compatibilizing agent. The crystallization and melting behavior, the morphology and mechanical properties of the melt-compounded binary blends have been investigated over the whole composition range. Albeit immiscibility prevails in all blends, the micronic and sub-micronic dispersion of the minor phase reveals a self-compatibilization behavior of the PLA/PA11 system as directly evidenced via scanning electron microscopy. For PLA compositions below 50%, PLA appears to be dispersed as globules in the PA11 matrix. With increasing PLA content beyond 50%, the blends exhibit co-continuous intertwined phases, then thread-like PA11 phase dispersed in a PLA matrix. For PLA composition greater than 80%, PA11 displays globular dispersion. In the case of PLA-rich blends, the crystallization of PA11 from the melt displayed a contribution of “homogeneous nucleation” corroborating the high degree of dispersion of the minor phase. However, the earlier crystallization of PA11 upon cooling did not promote that of PLA suggesting low interfacial free energy at the boundaries of the phase-separated domains, i.e. roughly neutral interactions between the unlike species. The non-symmetric structural behavior of the blends over the whole composition range was found to influence the mechanical properties. If the elastic modulus of the blends roughly obeys an additive mixture law at room temperature (RT), this is not the case above the Tg of both PLA and PA11. More particularly in the PLA-rich range, the thread-like dispersion of the stiff PA11 component plays the role of in situ fibrillar reinforcement of the soft PLA matrix. The strain at break and the yield stress also do not obey a simple mixture law, both at RT and above Tg. Perspectives of morphological and mechanical improvements of PLA/PA11 blends are discussed.