Photoactive fluorenyl-functionalized polypropylene for high-performance dielectrics

Reducing the impact of polyolefins on the environment is a difficult task not only because of its ubiquity in everyday items, but also because these polymers are used on a massive scale as high-performance materials, for example in electronics and automotive applications. This is the case for polypropylene that behaves as a robust and efficient dielectric in capacitors as a result of its high melting temperature and very low dielectric dissipation factor. The current challenge is to increase its low permittivity, in order to improve energy density and, at the same time, to contribute to the reduction of production, thus progressively increasing the miniaturization of devices.

Low content functionalisation has been found to be an efficient route to rise the polypropylene (PP) permittivity, without compromising the mechanical and thermal stability of the material. Recent work has shown that decreasing chain mobility in the amorphous phase is key to maintaining dielectric loss into the low level of PP. This has been achieved by inserting carbazole or N-alkyl pyrrole groups. The former increases the T_g due to their characteristic π-π stacking interactions and both enable tailored crosslinking, UV and thermally activated respectively.

The present study analyses the synthesis and thermal and dielectric stabilities of poly-(propylene-co-9-(undec-10-en-1-yl)-9H-fluorene), with fluorene contents up to 3 mol%, as another example of PP grade that has both enhanced T_g and crosslinking potential under UV radiation. Initial results reveal that these materials have a relative permittivity (ɛ_r) up to 3 and dielectric losses under 0.005. As in the case of carbazole, the strong π-π stacking between fluorene units improves the thermal, mechanical and dielectric responses and also, that further improvement in stability would be possible under appropriate post-processing irradiation conditions.