Modifying Hydrogen-Bonded Structures by Physical Vapor Deposition: 4-methyl-3-heptanol

Unlike many glass forming monohydric alcohols the equilibrium state of 4-methyl-3-heptanol (4M3H, Tg =162 K) is believed to be characterized by ring-like hydrogen-bonded structure. As a result, this alcohol is very non-polar and has a low dielectric constant. To test this connection between hydrogen bonded rings and the absence of polarity, we study vapor-deposited films of 4M3H for conditions which are meant to prohibit the formation of supra-molecular structures. Using deposition rates between 0.9 and 6.0 nm/s, we prepared films and measured the dielectric properties via an interdigitated electrode cell. Films deposited at temperatures below T_g show a high dielectric loss upon heating, which decreases by a factor of about 12 by annealing. This change is consistent with a drop of the Kirkwood correlation factor, g_K, by a factor of about 10, indicative of changes towards ring-like hydrogen-bonded structures characteristic of the ordinary liquid. We rationalize the high dielectric relaxation amplitude in the vapor deposited glass by suggesting that depositions at low temperature provide insufficient time for molecules to form ring-like supramolecular structures for which dipole moments cancel.