Modifying hydrogen-bonded structures by physical vapor deposition: 4-methyl-3-heptanol

We prepared films of 4-methyl-3-heptanol by vapor depositing onto substrates held at temperatures between T_dep = 0.6T_g and T_g, where T_g is the glass transition temperature. Using deposition rates between 0.9 and 6.0 nm/s, we prepared films about 5 μm thick and measured the dielectric properties via an interdigitated electrode cell onto which films were deposited. Samples prepared at T_dep = T_g display the dielectric behavior of the ordinary supercooled liquid. Films deposited at lower deposition temperatures show a high dielectric loss upon heating toward T_g, which decreases by a factor of about 12 by annealing at T_g = 162 K. This change is consistent with either a drop of the Kirkwood correlation factor, g_k, by a factor of about 10, or an increase in the dielectric relaxation times, both being indicative of changes toward ring-like hydrogen-bonded structure 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. Surprisingly, above T_g of the ordinary liquid, these vapor deposited films fail to completely recover the dielectric properties of the liquid obtained by supercooling. Instead, the dielectric relaxation remains slower and its amplitude much higher than that of the equilibrium liquid state, indicative of a structure that differs from the equilibrium liquid up to at least T_g + 40 K.