The Role of Ring Geometry and Rigidity in the Glassy Dynamics of Cyclic Dimethylsila Crowns under Elevated Pressure

Abstract

The interplay between molecular topology, rigidity, and cooperative relaxation remains central to understanding glassy dynamics in complex liquids. Here, broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC) were employed to investigate the relaxation behavior of three cyclic dimethylsila crown ethers (DMS11C4, DMS14C5, and DMS17C6) under ambient and high-pressure conditions. The systems exhibit distinct α- and β-relaxations, where the secondary (β)-process displays typical Johari–Goldstein features at ambient pressure and follows nearly the same trend upon compression, showing only slight shifts within the studied range of thermodynamic conditions. The pressure coefficients of the glass transition temperature (90–120 K GPa⁻¹) and activation volumes (0.2–0.3 nm³) reveal moderate compressibility and enhanced structural rigidity arising from the cyclic molecular framework. Notably, the smallest cyclic crown ether is characterized by the lowest sensitivity of global dynamics to applied pressure, which can be attributed to its increased rigidity resulting from the reduced ring size. Furthermore, calorimetric and dielectric analyses consistently yield nanometric cooperative length scales (≈ 1.9–2.0 nm), reflecting the spatial extent of cooperative rearrangements in the supercooled state. Additionally, a clear linear correlation between activation volume and cooperative length is observed for the data obtained at ambient pressure. Overall, the results demonstrate that molecular ring geometry and internal constraints play a decisive role in governing dynamic heterogeneity and pressure sensitivity in van der Waals glass formers. This work provides a molecular-level framework for tuning glassy dynamics through targeted structural design.