School of Molecular Sciences, Arizona State University, Tempe, Arizona 85287-1604, USA.
Department of Biomedical Engineering, The Dwight Look College of Engineering, Texas A&M University, College Station, Texas 77843, USA.
J Chem Phys. 2017 Nov 21;147(19):194504. doi: 10.1063/1.4999300.
We prepared films of 4-methyl-3-heptanol by vapor depositing onto substrates held at temperatures between T = 0.6T and T, where T 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 = T display the dielectric behavior of the ordinary supercooled liquid. Films deposited at lower deposition temperatures show a high dielectric loss upon heating toward T, which decreases by a factor of about 12 by annealing at T = 162 K. This change is consistent with either a drop of the Kirkwood correlation factor, g, 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 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 + 40 K.
我们通过在温度为 T = 0.6T 到 T 之间的基底上蒸镀 4-甲基-3-庚醇来制备薄膜，其中 T 是玻璃化转变温度。通过在 0.9 和 6.0 nm/s 之间的沉积速率，我们制备了约 5μm 厚的薄膜，并通过沉积在其上的叉指电极电池测量了介电性能。在 T = T 下制备的样品显示出普通过冷液体的介电行为。在较低沉积温度下沉积的薄膜在加热到 T 时表现出高介电损耗，通过在 162 K 下退火，介电损耗降低了约 12 倍。这一变化与 Kirkwood 相关因子 g 的降低因子约为 10 或介电弛豫时间的增加一致，这两者都表明结构向普通液体的环状氢键结构变化。我们通过提出在低温下沉积提供的时间不足以使分子形成环状超分子结构，从而使偶极矩相互抵消，来解释蒸镀玻璃中高介电弛豫幅度。令人惊讶的是，在普通液体的 T 以上，这些蒸镀薄膜未能完全恢复通过过冷获得的液体的介电性质。相反，介电弛豫仍然较慢，其幅度远高于平衡液态的幅度，表明结构与平衡液态至少在 T + 40 K 以上不同。
