Cross-Linking-Regulated Phonon Transport in Polydimethylsiloxane: From Network Conformation Tuning to Intrinsic Thermal Conductivity Enhancement

Polydimethylsiloxane (PDMS) is promising for thermal management but suffers from intrinsically low thermal conductivity. While cross-linking is known to modify polymer properties, its specific role in regulating thermal transport at the molecular level remains elusive. This study employs molecular dynamics (MD) simulations to unravel the effect of cross-linking degree (0% to 80%) on the thermal and mechanical properties of PDMS. We find that increased cross-linking enhances mass density, elastic modulus, and glass transition temperature, consistent with a more rigid and compact network structure with reduced free volume. Notably, the thermal conductivity is boosted by up to 28% at the highest cross-linking degree. We attribute this enhancement to the formation of a continuous, rigid backbone that facilitates efficient phonon transport. Phonon density of states analysis reveals a suppression of low-frequency, localized flexible motions and a concomitant enhancement of medium-to-high-frequency bonded vibrations. This signifies a fundamental shift in phonon behavior from localization to delocalized cooperative propagation. This interpretation is further supported by an increase in the phonon participation ratio and a spatial redistribution of vibrational energy toward the Si-O backbone atoms upon cross-linking. Moreover, the extended phonon relaxation times in the medium-to-high-frequency range indicate reduced phonon scattering, underpinning the improved thermal conductivity. Our work establishes a clear structure-property relationship, demonstrating how cross-linking topology engineers phonon transport to enhance thermal conductivity in PDMS, providing fundamental theoretical guidance for the rational design of high-thermal-conductivity polymers.