Behavior of Glass and Supercooled Liquid Alkylbenzenes Vapor-Deposited on Cold Substrates: Toward the Understanding of the Curious Light Scattering Observed in Some Supercooled Liquid States

Abstract

Ethylbenzene (EB) films prepared on cold metal substrates by vapor deposition in vacuum show a curious light scattering in the supercooled liquid (SCL) state when the temperature is raised at a constant rate. To investigate the cause of this phenomenon, we examined the behavior of similarly prepared samples of a series of alkylbenzenes. We found that the vapor-deposited glass of propylbenzene (PB) and isopropylbenzene (IPB) showed a deposition-temperature (T d ) dependence of the initial molar volume (V m ) on deposition similar to EB glass. Interestingly, the samples of the three compounds, which were formed initially as glasses with V m much larger than that of SCL at the same temperature, exhibited the curious light scattering when they transformed to SCL states after the temperature elevation to above their glass-transition temperature, while the corresponding samples with initial V m smaller than that of SCL did not exhibit this light scattering. On the basis of these observations, a hypothesis on the cause of the light scattering in the SCL state is proposed in relation to structural transformation between different SCL states. A microscopic mechanism of the formation of a high-density glass with V m smaller than that of SCL is briefly discussed by referring to the observations of toluene samples which showed a slightly different T d dependence of V m and did not show the curious light scattering in the SCL state.We have studied the structure and relaxation behavior of amorphous molecular systems prepared by vapor deposition on cold metal substrates. 16 The samples were films with a thickness of about 10¯m. Such amorphous samples usually tend to crystallize when they are annealed with temperature elevation. 16 However, vapor-deposited amorphous samples of certain compounds undergo a glass transition and transform into supercooled liquid (SCL) states by temperature elevation without experiencing crystalline states. 710 Similar phenomena have been known for a variety of bulk amorphous materials prepared by liquid quenching.1113 Such special amorphous states are called glasses by analogy with conventional glass. A common structural feature of these glass-forming molecular compounds is their flexibility which permits various intra-and intermolecular conformations and hinders crystallization.We found that glass samples of ethylbenzene (EB) and related alkylbenzenes could be prepared by vapor-deposition with good reproducibility. We studied the properties of these materials using laser light interference in film samples.14,15 One of the remarkable observations during these studies was that the density and relaxation behavior of the glass state systematically depended upon the vapor-deposition temperature (T d ). 15 Glass samples deposited at a T d much lower than the glass-transition temperature (T g ) had a lower density than that estimated for the SCL state at the same temperature. In this paper, we call such a glass "low-density glass" (LDG). In contrast, glass samples deposited at a T d close to T g sometim...

Liquid–Liquid Relaxation in the Supercooled Liquid State of Ethylbenzene: Thermal Studies Using a Prototype DTA Sensor for the Study of Vapor-deposited Samples

“…We have recently come to consider that the depression is caused by light scattering due to the refractive index inhomogeneity accompanying the structural change in the SCL state of the sample originally made as LDG. 3 The two kinds of SCL did crystallize, however, at almost the same temperature (see the decreases of the light intensity at about 129 K) into the same crystalline state as revealed by X-ray analysis, 10 which suggests that the structures of the two kinds of SCL were the same just before the crystallization.…”

“…We have recently come to consider that the depression is caused by light scattering due to the refractive index inhomogeneity accompanying the structural change in the SCL state of the sample originally made as LDG. 3 The two kinds of SCL did crystallize, however, at almost the same temperature (see the decreases of the light intensity at about 129 K) into the same crystalline state as revealed by X-ray analysis, 10 which suggests that the structures of the two kinds of SCL were the same just before the crystallization.To explore further the cause of the characteristic depression of the reflected-light intensity observed for some of the vapordeposited samples, we fabricated a DTA sensor that worked as the substrate for the vapor deposition. A schematic cross section is displayed in the graphical abstract.…”

“…We recently found that EB and related compounds form glasses with different densities depending on the temperature of the vapor deposition (T d ). 2,3 Glasses, deposited at a T d much lower than the glass-transition temperature (T g ) of the compound, had densities less than those estimated for the SCL at the corresponding temperature, whereas glasses deposited at a T d only slightly lower than the T g had densities larger than the SCL. We abbreviate hereafter the lowdensity glasses as LDG and the high-density glasses as HDG.…”

“…16 We observed indications of liquidliquid structural transformation in the SCL states of EB and related compounds for samples obtained through the glass transition of vapor-deposited glasses. 3 These indications were determined through study of light scattering in the samples. The results of the light-scattering study, however, did not provide conclusive proof of the structural change in the bulk of the samples.…”

“…Figure 1 compares the typical intensity evolutions of the laser light reflected from EB samples originally prepared as LDG and HDG, respectively. 3 The rate of temperature elevation was 0.28 K min…”