Polarity Dilution and Restricted Local Dipolar Motion in Silyl-Modified Low-Dielectric Epoxy Thermosets

Silyl substituents provide a versatile molecular handle for regulating the polarity and relaxation dynamics of epoxy thermosets. Herein, three silyl-functionalized epoxy resins grafted with triethylsilyl (Et3Si–), dimethylphenylsilyl (Ph1Si–), and triphenylsilyl (Ph3Si–) side chains were synthesized via platinum-catalyzed hydrosilylation and crosslinked with 4,4′-diaminodiphenylmethane (DDM). Nonisothermal differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), broadband dielectric spectroscopy, and atomistic molecular dynamics (MD) simulations were combined to systematically clarify how silyl architecture modulates curing kinetics, thermomechanical relaxation, thermal decomposition behavior, and dielectric performance. Curing reactivity is collectively determined by the steric volume, intrinsic polarity, and chain mobility of pendant silyl groups. Relative to both neat resorcinol diglycidyl ether (RDE)/DDM and the allyl-functionalized precursor A1E2PG/DDM thermosets, all silyl-modified networks deliver lower dielectric permittivity and suppressed dielectric loss. The reduced dielectric response is associated with two coupled effects: a lower volumetric density of hydroxyl-containing polar segments and restricted local dipolar motion. Phenyl-enriched silyl moieties, particularly triphenylsilyl, create a rigid and sterically crowded local environment that suppresses sub-glass β-relaxation and limits dipolar reorientation under alternating electric fields. MD simulations further support the stronger restriction of local molecular motion induced by bulky silyl pendants. Multiphenyl silyl side chains also increase char yield during thermal decomposition, indicating enhanced char-forming ability at elevated temperatures. These results establish a systematic structure–property relationship between silyl substituent architecture and epoxy performance and provide a molecular basis for designing intrinsically low-dielectric thermosetting resins through simultaneous reduction of polar-group density and restriction of local dipolar mobility.