Alkyl-side-chains induced improvement of dielectric properties of polymers. 2. Polymers based on alkyl benzenes with crosslinkable benzocyclobutene groups

We previously discovered that crosslinkable fluorene derivatives with alkyl side chains exhibit excellent dielectric properties at a high frequency of 10 GHz. To further explore the impact of alkyl side chains on the dielectric properties of aromatic polymers, we have designed and synthesized four alkyl benzenes incorporating crosslinkable benzocyclobutene groups. The properties of their cured products are presented in this study. The results indicate that the cured resins exhibit the dielectric constant (D_k) ranging from 2.44 to 2.52 with the dielectric loss (D_f) from 5.0 × 10⁻⁴ to 7.8 × 10⁻⁴ at a frequency of 10 GHz, meaning that alkyl side chains have a big effect on the improvement of dielectric properties of aromatic polymers. Moreover, the D_k values decrease with increasing of the length of alkyl side chains. A comparable investigation indicates that a cured polymer exhibits the improvement of the glass transition temperature (T_g) and the expansion coefficient (CTE) when a butyl and a cyclohexyl are served as the asymmetric side chains. It is also found that the alkyloxy side chains make the dielectric properties of the cured polymers deterioration, suggesting that total hydrocarbon chemical structure are beneficial for the improvement of the dielectric properties of the polymers. This study provides a valuable insight into the designing of the new high-frequency low dielectric materials.