Modeling and Mechanistic Study of Polyethylene Chain Cleavage during Ball Milling

Area of Science:

• Polymer Chemistry
• Materials Science
• Chemical Engineering

Background:

• Polyethylene (PE) and polypropylene are major global polymers.
• Mechanochemical conversion via ball milling shows promise for chemical recycling.
• Limited understanding exists on factors influencing polyolefin conversion during milling.

Purpose of the Study:

• Investigate the mechanochemical degradation of PE during ball milling.
• Determine the influence of crystallinity, polymerization degree, entanglement, and temperature on conversion.
• Elucidate the role of air and milling media on chain cleavage.

Main Methods:

• Controlled chain growth polymerization to synthesize PE with defined characteristics.
• Ball milling of PE under various conditions: cryogenic, room temperature (RT), with/without air, steel/zirconia spheres.
• Analysis of molecular weight distributions using a statistical chain cleavage model.
• Nuclear Magnetic Resonance (NMR) spectroscopy to analyze chemical changes.

Main Results:

• Chain cleavage probability follows a statistical Gaussian distribution, favoring central chain locations.
• Entanglements significantly promote chain cleavage, while initial molar mass, crystallinity, and brittleness showed no measurable influence.
• Cryogenic milling accelerated molar mass decrease due to suppressed radical recombination.
• Air exposure, especially with steel spheres, increased permanent scissions by up to 2.6 times, suppressing recombination.

Conclusions:

• Ball milling of PE is governed by chain entanglement and air-mediated reactions.
• Cryogenic conditions and air exposure impact degradation pathways by influencing radical recombination.
• Findings provide fundamental insights for optimizing mechanochemical polymer recycling processes.