Quantification of the Effect of Crystalline Microfibrillar Superstructures on the Superior Impact Strength of Neat Polylactide Sheets by Controlling Prestretching Temperatures above the Glass Transition Temperature

Abstract

The fabrication of high-strength and super-tough polylactide (PLA) products without the addition of any foreign toughening agents remains a significant challenge, particularly in building up the quantitative relationship between crystalline microfibrillar superstructures and impact toughness. In this work, an exceptional combination of both high notched Izod impact strength and tensile strength for neat PLA sheets has been achieved by prestretching PLA in the temperature range from 60 to 80 °C, slightly above the glass transition temperature of neat PLA. Different from our previous study that primarily focused on the qualitative function of fibrillar superstructures consisting of shish-kebab microstructures, this work makes an effort to systematically establish a quantitative correlation between the crystalline fibrosis parameters, namely crystallinity (xc), the degree of orientation for microfibrils (<P2>), and the microfibrillar aspect ratio (lf/D), and the notched Izod impact strength. Comprehensive characterizations, including polarized optical microscopy, scanning electron microscopy, wide-angle X-ray diffraction, small-angle X-ray scattering, small-angle light scattering, and differential scanning calorimetry, reveal that the formation of highly oriented crystalline microfibrillar superstructures, composed of interlocked shish-kebab microstructures and oriented PLA amorphous chains, governs the toughening behavior. A unified crystalline fibrosis parameter (xc<P2>lf/D) is proposed, which exhibits a linear correlation with the notched Izod impact strength across all prestretching temperatures, demonstrating its temperature-dependent predictive capability. This finding provides a quantitative framework for understanding the toughening mechanism for prestretched neat PLA bulk materials and offers a new pathway for designing self-reinforced polymer materials.