Methyl-substitution affects dielectric, thermal, mechanical properties, and shrinkage of fluorinated epoxy

Fluorination reduces dielectric constant and dielectric loss of epoxy resins, expanding their applications in high-speed communication. The methyl group (-CH3) is a non-polar and space-occupying group, and it can reduce dielectric constant (Dk) in some situations. However, it is not well understood how the methyl groups affect dielectric properties, as well as thermal and mechanical properties of fluorinated epoxy resin. To address this question, we specifically select benzene and m-xylene as the reaction monomer for comparison. We synthesize two fluorinated epoxides, diglycidyl ether of 1,3-bis(1,1,1,3,3,3-hexafluoroisopropyl)-benzene (DGE-bHFIP-B) and diglycidyl ether of 4,6-bis(1,1,1,3,3,3-hexafluoroisopropyl)-m-xylene (DGE-bHFIP-MX). We then thermally cure these two epoxides catalyzed with boron trifluoride ethylamine (BF3·MEA) to obtain EP-bHFIP-B and EP-bHFIP-MX. EP-bHFIP-MX with –CH3 has Dk of 2.49–2.51, slightly lower than EP-bHFIP-B with Dk of 2.51–2.54 at a high frequency range of 8.2–12.4 GHz. Their Dk is much lower than a commercial bisphenol A type epoxy resin (EP-51) with Dk of 2.80–2.83. Additionally, EP-bHFIP-MX shrinks less than EP-bHFIP-B after curing, and exhibits higher glass transition temperature with Tg−DMA of 181 °C, higher tensile strength of 39.76 ± 5.51 MPa, compared to 129 °C and 34.73 ± 5.10 MPa for EP-bHFIP-B. EP-bHFIP-B and EP-bHFIP-MX both have very low water absorption of 0.134% w/w and 0.147% w/w after being immersed in water for 24 h at 25 °C. Therefore, incorporating –CH3 groups into phenylene rings of fluorinated epoxies, we can enhance thermal resistance, increase mechanical strength, and reduce shrinkage without compromising dielectric performance.