Microstructural Evolution and Mechanical Property Enhancement of Poly(Lactic Acid) via Confined Stretching: Effects of Commercial Grade and Die Geometry

Three commercial poly(lactic acid) (PLA) grades (2003D, 4032D, and 6100D) were subjected to confined stretching through circular and rectangular converging dies at a draw ratio of 4°C and 70°C. A controlled three‐grade × two‐die matrix, multiscale characterization, and qualitative finite element simulations were used to compare their structural and mechanical responses. Die geometry differentiated the resulting mechanical‐property profiles rather than producing a uniform reinforcement effect. Rectangular‐die 6100D and 4032D showed the highest tensile and flexural strengths, 113.9 ± 0.7 MPa and 180.2 ± 7.8 MPa, respectively. Circular‐die specimens had higher mean elongation at break and impact strength than their rectangular‐die counterparts, with maxima of 53.5% ± 4.5% and 177.5 ± 7.0 kJ·m −2 . Across the matrix, the largest shape‐matched gains were approximately sevenfold in elongation and 23‐fold in impact strength. SEM, XRD, DSC, and DMA identified grade‐ and geometry‐dependent differences in fibrillar‐looking fracture morphology, crystallization response, and thermomechanical relaxation. Together, the data establish condition‐specific processing–structure–property correlations. Molecular orientation is considered a plausible mediator, but it was not measured directly. Commercial PLA grade and die geometry therefore provide an empirical basis for selecting the strength–toughness profile without additional modifiers under the conditions examined.