Observing density-dependent formation of a fragile glass in surface-bound molecular chains

Dynamics within a monolayer collection of surface-bound substituted-alkyl chains are studied with narrow-band dielectric spectroscopy. A transition from independent (intra-molecular) motion to complex, glassy (inter-molecular) motion is observed as the surface density increases. At high density, both the glassy mode [1,2] and the sub-T$_{g}$ relaxation [3] have a direct analogy to the equivalent relaxations in polyethylene. Thus, this experimental approach enables observation of the formation of a fragile glass as an explicit function of density. Addition of a strong terminal dipole shows the transition occurring at lower density, dipole-mediated interacting dynamics in the low density regime, and increased dominance of the sub-T$_{g}$ local mode. We will discuss results from monolayers and an analogous siloxane-based substrate where alkyl chain-chain distance can be similarly controlled. [1] M. C. Scott et al. \textit{ACS Nano} \textbf{2}, 2392 (2008). [2] M. Beiner and H. Huth, \textit{Nat. Mater.} \textbf{2}, 595 (2003). [3] Q. Zhang et al., \textit{J. Phys. Chem. B} \textbf{110}, 4924 (2006).