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 (-CH 3 ) is a non-polar and space-occupying group, and it can reduce dielectric constant ( D k ) 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 (BF 3 ·MEA) to obtain EP-bHFIP-B and EP-bHFIP-MX. EP-bHFIP-MX with –CH 3 has D k of 2.49–2.51, slightly lower than EP-bHFIP-B with D k of 2.51–2.54 at a high frequency range of 8.2–12.4 GHz. Their D k is much lower than a commercial bisphenol A type epoxy resin (EP-51) with D k of 2.80–2.83. Additionally, EP-bHFIP-MX shrinks less than EP-bHFIP-B after curing, and exhibits higher glass transition temperature with T g − D M A 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 –CH 3 groups into phenylene rings of fluorinated epoxies, we can enhance thermal resistance , increase mechanical strength , and reduce shrinkage without compromising dielectric performance.