Simulation of the Thermodynamic Properties and Hydrophobicity of Polydimethylsiloxane Modified by Grafting Nano-SiO2 with Different Silane Coupling Agents

Polydimethylsiloxane (PDMS) with good hydrophobicity and nano-SiO2 with excellent thermal stability and mechanical properties are used as a composite coating for cellulose insulating paper in oil-immersed transformers, which effectively reduces the moisture generated by the thermal aging process, thus prolonging each transformer’s service life. This study employed molecular dynamics simulations to investigate the effects of surface-modified nano-SiO2 with different silane coupling agents (KH570 and KH151) on the thermodynamic properties and hydrophobicity of PDMS. Four groups of anhydrous models were constructed, namely, PDMS, P-SiO2, P-570, and P-151, as well as four corresponding groups of water-containing models: PDMS/H2O, P-SiO2/H2O, P-570/H2O, and P-151/H2O. The results demonstrate that incorporating silane-coupled nano-SiO2 into PDMS enhances mechanical properties, FFV, CED, MSD, diffusion coefficient, interaction energy, and hydrogen bond count, with KH570-grafted composites exhibiting optimal thermomechanical performance and hydrophobicity. At a temperature of 343 K, KH570 modification increased the bulk modulus and CED by 26.5% and 31.0%, respectively, while reducing the water molecular diffusion coefficient by 24.7% compared to that of unmodified PDMS/SiO2 composites. The extended KH570 chains occupy additional free volume, forming a larger steric hindrance layer, restricting molecular chain mobility, suppressing hydrogen bond formation, and establishing a low energy surface.