Investigation of the influence of substituents on the dielectric properties of polyethylene derivatives†

To systematically explore the influence of microscopic substituent structures on the macroscopic dielectric properties of polyethylene (PE), ten PE derivatives, incorporating 18 mol% of diverse functional groups such as halogens, azides, norbornene-based groups, and macrocyclic structures, were synthesized using post-functionalization reactions from the same poly(ethylene-co-vinyl acetate) precursors. Using linear low-density PE (LLDPE) as a reference, the experimental results reveal the effective modulation of the dielectric constants of PE derivatives by introducing various functional groups. The PE units in the molecular chain ensure excellent compatibility of PE derivatives with LLDPE to form homogeneous polymer blends in molten states. Blending with LLDPE effectively reduces the dielectric loss of PE derivatives and exhibits a higher dielectric constant than LLDPE at the frequencies below 10 Hz. Notably, these blends exhibited a more pronounced temperature dependence of the dielectric constants, indicating higher values at elevated temperatures. More importantly, the dielectric breakdown strength of the blends was effectively enhanced, reaching up to 1.4 times that of LLDPE. In addition, improvements in the mechanical properties of the blends were also observed with the strain-at-break exceeding 1000%. This research confirms that post-polymerization functionalization provides an excellent platform to systematically evaluate the influence of substituents on synthetic polymers, and it is expected to generate new insights into the mechanisms of enhancing polymer dielectric properties.