The challenge of hitherto elaborating a feasible pathway to overcome the conflicts between strength and toughness of polylactide (PLA) still remains among academia and industry. In the current work, a unique hierarchal structure of flexible poly(butylene adipate-co-terephthalate) (PBAT) in situ nanofibrils integrating with abundant PLA shish-kebabs as a strong building block was disclosed and expresses its capability to conquer this dilemma. Substantially simultaneous enhancement on tensile strength, impact strength, and elongation at break could be achieved up to 91.2 MPa, 14.9 KJ/m, and 15.7%, respectively, compared with pure PLA (61.5 MPa, 4.3 KJ/m, and 6.2%). Through investigating the phase (and crystalline) morphology and molecular chain behavior in the PLA/PBAT system, the formation mechanism of this structure facilitated by a coupling effect of PBAT flexible phase and shear flow was definitely elucidated. The dispersed phase of PBAT would be more inclined to existing as a fibrillar form within the PLA matrix benefiting from low interfacial tension. Interestingly, this phase morphology with large specific surface area changes the crystallization behavior of PLA significantly, once introducing an intense shear flow (∼10 s), in situ shear-formed nanofibrils of PBAT would show strong coupling effect with shear flow on PLA crystallization: they can not only induce abundant shish-kebabs of PLA at its interfaces, which possesses lengthened shish and more densely arranged kebabs, but also further retard the relaxation of PLA chains through hysteretic relaxation of its PBAT phase, which can effectively prevent the collapse of established shish. Of immense significance is this particular hierarchical-architecture composed by flexible nanofibers (PBAT) and rigid shish-kebabs (PLA), which provides significant guidance for the simultaneous reinforcement and toughness of polymer materials.
The inhomogeneity of the cooling
rate and shear rate during polymer
processing such as injection molding usually leads to the harvest
of a layered structure, which is often difficult to tune. By introducing
oscillation shear flow during the packing stage of an injection molding
cycle, special injection-molded polylactide (PLA) parts with different
thicknesses and crystallinity of skin layers were fabricated via controlling
shear durations and oscillation frequencies, as revealed by microbeam
wide-angle X-ray diffraction results. For the part of the 2000 μm
thick layer with 50% crystallinity, the heat distortion temperature
and Vicat softening temperature reach 96.6 and 159.3 °C, respectively.
Moreover, the Young’s modulus rises remarkably with the increase
of thickness and crystallinity. Significantly, the maximal shear rate
appeared 850 μm from the surface, about 1068 s–1 (up to 103 s–1), which plays a crucial
role for the formation of oriented crystalline morphology.
The simultaneous reinforcement and toughening of poly(butylene succinate) was achieved via an oriented hierarchical structure induced by oscillation shear stress and promoted by poly(l-lactide).
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