Annealing effect on the shape memory properties of polylactic acid (PLA)/thermoplastic polyurethane (TPU) bio-based blends

PLA and TPU were melt-blended to form shape memory bio-based blends with or without post-annealing effect. To the authors’ best knowledge, this is the first work to discuss the annealing effect on the PLA-based SMP blends. Annealed TPU showed regularly fractured surfaces unlike the macro-phase segregated domains for non-annealed TPU. After 3 h-annealing treatment, spherulites were observed for PLA, but not for TPU. The crystallinity of PLA increased, close to 3-fold increment, for annealed blends in comparison with non-annealed blends. The shape memory behaviors of PLA/TPU blends predeformed under three different predeformation temperatures (25, 80, 120 °C) were investigated. The annealing effect was helpful in enhancing the shape fixing ratio of the PLA/TPU (60/40) blend at high predeformation temperature of 120 °C in comparison with 25 °C. However, the suitable selection of the optimum predeformation temperature at 80 °C outweighed the annealing effect to attain the high shape fixing ratio, even in the case of non-annealed blends. The annealing effect often increased the perfection of crystal domains/interfaces and the larger crystal sizes, which would be detrimental to the molecular extensibility. The overall annealing effect on the shape recovery ratios were quite effective for both PLA/TPU blends of 80/20 and 60/40 without sacrificing the shape fixing ratios at the optimum predeformation temperature of 80 °C, attributing to the increased crystallinity of PLA and homogenized phase domains of TPU. Particularly, the annealing treatment did significantly increase the recovery ratio of the blends, more than 2-fold increment, especially for PLA/TPU (60/40) blend. At both lower or higher predeformation temperatures, the stress concentration between the increased crystalline domains and amorphous regions tended to dominate the annealing effect, leading to a negative contribution to the shape recovery processes.