Modeling and Mechanistic Study of Polyethylene Chain Cleavage during Ball Milling

Abstract

Mechanochemical conversion of polyethylene (PE) and polypropylene was shown to produce monomers and is thus interesting for chemical polymer recycling. As these polymers make up more than 50% of the worldwide polymer production, studying their conversion during ball milling is especially relevant. However, fundamental knowledge on the effect of crystallinity, degree of polymerization, entanglement and temperature on the conversion is lacking due to the difficulty in producing polyolefins with controlled chain length and dispersity. Here we synthesize PE by a controlled chain growth polymerization and study its degradation during ball milling at cryogenic conditions, at room temperature (RT), with and without air, and using either steel or zirconia grinding spheres. Resulting molecular weight distributions are fitted using a statistical chain cleavage model suggesting a statistical Gaussian distribution of chain cleavage probability around the middle of the chain. This is likely because if the chains are fixed in an entanglement or crystal at two points, they cannot slip out and force can act on them leading to cleavage. That the chain is fixed at both sides of a possible cleavage location is most likely if the cleavage location is in the center of the chain. Chain cleavage is also promoted when entangled domains exist that link crystalline regions. A micelle grown single crystal ultrahigh molecular weight PE without entanglements was milled and its molar mass decreased much less compared to the same sample that was annealed to create entanglements. However, in contrast to previous studies and common expectations, the initial molar mass of the polymer, the degree of crystallinity and the brittleness of the sample did not have a measurable influence on chain cleavage. While the decrease in molar mass was faster at cryogenic conditions compared to RT, nuclear magnetic resonance (NMR) results suggest that this is due to a suppression of radical recombination rather than the higher brittleness of the material below its glass transition temperature (∼−120 °C). Similarly, the number of permanent scissions increased by up to 2.6 times under air compared to nitrogen atmosphere, especially in combination with steel milling spheres. NMR spectra of the milled samples suggest that the reaction of mechanochemically formed chains with air suppresses recombination.