Nano-structuring for strengthening semi-crystalline polymers

Crystalline grain refinement is a highly universal technique for increasing the strength of metals and alloys, and recent challenges have explored the possible application of such fine-structuring effects to various types of materials. Recently, a remarkable strengthening phenomenon has been found for a semi-crystalline polyethylene with a crystalline/amorphous lamellar structure, whose tensile strength is increased by an order of magnitude after applying the pre-heat-elongation (PHE) process. Here we provide a series of unambiguous evidences that such toughening is indeed due to nano-structuring and not to an increase in crystallinity, as is commonly used for semicrystalline polymers. Electron microscopy observations as well as X-ray diffraction experiments clearly show that the PHE process introduces a remarkable ultrafine-mosaic (~10 nm) into the crystalline lamellae. Ultralarge-scale molecular dynamics simulations provide further insights at the molecular level that chain spreading leads to the development of nanocrystalline mosaics, where the amorphous tie-chains effectively bridge across the multiple nano-lamellae, thereby strengthening the polyethylene. We show that a crystalline-size effect on strength can be well described by the Hall-Petch type relationship of size-inverse dependence, a well-known empirical rule for strengthening metals/alloys, providing a novel strength-ductility tailoring method based on higher-order structure control (i.e., crystalline-size control) of semi-crystalline polymers. The present work reports a novel and unique strategy for strengthening semi-crystalline polymers; nano-structuring by introducing ultra-fine mosaics into the crystalline lamellae indeed increases the strength. This is inspired by the well-known empirical strengthening principle for metals/alloys, the so-called Hall-Petch relation, which has clearly shown that simply having smaller crystalline grains makes metals/alloys stronger. In contrast to the increasing-crystallinity strategy that has been used so far for semi-crystalline polymers, the present nano-structuring effects provide an alternative strength-ductility tailoring method based on higher-order structure control (i.e. crystalline-size control) that can be achieved simply by a heat elongation process. This study explores how to make polymers stronger for use in things like cars. Traditionally, metals are strong because of a principle called the Hall-Petch relation, which says smaller grains make metals stronger. The team used a process called PHE on polyethylene to test this. They heated and stretched the plastic, then examined its structure using techniques like electron microscopy and x-ray diffractometry. They found that the process made the polymer’s crystalline parts smaller and more organized, which increased its strength to about 170 MPa. The study concluded that this method effectively strengthens polymers by creating more tie-chains, which help distribute stress. This research could lead to stronger, lightweight materials for various applications in the future. This summary was initially drafted using artificial intelligence, then revised and fact-checked by the author.