Molecular Weight Effect of Poly(dimethylsiloxane) on Its Interfaces with Water and Octane: A Molecular Dynamics...

We investigate how the molecular weight of poly-(dimethylsiloxane) (PDMS) governs the interfacial behavior in contact with polar and nonpolar liquids using the molecular dynamics simulation method. PDMS chains with various degrees of polymerization (n = 9, 18, and 27) are simulated to quantify molecular-weight effects on density profile, interfacial tension, mobility, and segmental orientation. While the interfacial tensions for PDMS-water (40 dyn/cm) and PDMS-octane (10-12 dyn/cm) remain nearly independent of molecular weight, the microscopic structural responses exhibit clear chain-length dependence. Longer PDMS chains develop enhanced density fluctuations and layered packing near interfaces, reflecting reduced conformational freedom and stronger intrachain correlation. At PDMS-water interfaces, all systems form sharp boundaries, which is attributed to the hydrogen bonding network of the water phase, and PDMS adopts predominantly horizontal orientations to minimize unfavorable polar-nonpolar interactions. In contrast, PDMS-octane interfaces show broad, compositionally mixed regions where shorter PDMS chains more easily penetrate the hydrocarbon phase, consistent with the low interfacial tension and chemical compatibility. Mean-squared displacement analyses reveal a monotonic decrease in chain mobility with increasing molecular weight, with strong suppression near water and enhanced mixing at PDMS-octane interfaces. These results demonstrate that molecular weight crucially modulates interfacial structuring, conformational ordering, and chain dynamics, even when macroscopic thermodynamic properties remain unchanged. This molecular-level insight provides a predictive basis for engineering PDMS-based coatings, adhesives, and liquid-contacting surfaces with tunable interfacial performance across diverse chemical environments.