Super enhancement of rheological properties of amorphous PLA through generation of a fiberlike oriented crystal network

Blends containing 85 wt. % of an amorphous polylactide with 15 wt. % of three different semicrystalline PLA (cPLA) grades with different crystallizabilty were separately blended via a twin-screw extruder below the melting temperature of the cPLAs. The extrudates were either directly pelletized or pelletized after being drawn at a draw ratio of 10. The small amplitude oscillatory shear behavior of the samples revealed that while the rheological properties of the undrawn samples were enhanced, those of the drawn samples were much more dramatically increased. In undrawn samples, the enhancements were due to the presence of unmelted crystal clusters, which could form a solid network structure in the blend. The much more pronounced increases in drawn samples, however, were due to the transformation of the crystal clusters into the fiberlike oriented crystal network, which formed a stronger solid network. This reinforcing behavior in both undrawn and drawn samples was even more pronounced when cPLA with a higher degree of crystallinity and a higher melting temperature was used. In drawn samples, the stress growth experiments confirmed the formation of such oriented crystal structure during which the primary overshoot caused by the crystal network structure could be formed again after molecular relaxation. This was while, in undrawn samples, stress overshoots were barely visible.