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

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.