Durably ductile polylactide monofilaments via stress-induced mesophase formation

Significant progress has been made in achieving lasting ductility for polylactide (PLA), yet the structural features resisting physical aging-induced embrittlement remain unclear. In this work, PLA monofilaments with simultaneous enhancement in strength and ductility have been prepared despite physical aging. This persistent ductility was achieved by controlling oriented mesophase formation. Specifically, molecular orientation was induced during hot-drawing, whereas abundant mesophase was developed only at draw ratios ≥3.0 and grew with annealing time ( t a ) at 40 °C (< T g ). Compared to the brittle behavior of as-spun monofilament, the drawn monofilament with the medium draw ratio (DR-2.0) displayed distinct yielding and strain-hardening, suggesting a brittle-to-ductile transition. Due to the scarcely developed mesophase, this transition could be ascribed to the enhanced orientation. However, such kind of monofilament (DR-2.0) embrittled quickly with increasing t a , primarily governed by the growth of the fraction of rigid amorphous phase ( X RAF ). In comparison, the persistent ductility was achieved for the drawn monofilaments with high draw ratios (DR-3.0 & 3.8). Unlike the embrittled samples, these samples showed invariant X RAF with increasing t a . This invariant X RAF enabled transverse contraction, preventing crazing and ensuring ductility against the usual physical aging-induced embrittlement .