No 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...

No abstract

Change in the light interference in film samples of isopropylbenzene was studied in the supercooled liquid (SCL) state. Samples were originally formed as glass by vapor deposition on a cold substrate and the intensity of laser light reflection from them was monitored as the temperature was raised with a constant rate up to the region of the SCL state above the glass-transition temperature. Two types of periodic changes in the light intensity were observed in the SCL state. One was attributed to the interference condition change accompanying the structural relaxation from the low-density SCL to the equilibrium SCL state, and the other was due to the gradual expansion of the high-density SCL. Analysis of the latter change revealed that the initial density of the high-density glass was larger than that estimated in our previous paper.

A prototype sensor for differential thermal analysis (DTA) was developed for in situ studies of the thermal behavior of molecular films vapor-deposited onto a cold substrate in a vacuum. The system was applied to the study of ethylbenzene (EB). By raising the temperature at a constant rate, we observed in the supercooled liquid (SCL) state a small exothermic peak simultaneously with the characteristic light scattering reported previously. These observations and additional optical experiments on the irreversibility of the phenomena indicated that a structural relaxation occurred from a less stable SCL to a more stable SCL.The behavior of vapor-deposited molecular glasses has attracted growing attention in recent years. 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. 7 We thus planned to make in situ thermal measurement on vapor-deposited samples while simultaneously monitoring the scattering of laser light.Sample films were vapor-deposited on cold metal substrates with a thickness of approximately ten micrometers. In our earlier studies on EB, we found a curious intensity depression of the laser light reflected from the sample in the SCL state when we raised the temperature continuously.8 This result was due to temporal light scattering. 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.Interestingly, the samples originally made as LDG exhibited the above-described reflected light depression when the temperature was raised to the region of the SCL state. However, the samples obtained from HDG did not show such a behavior. 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 ¹1. The patterns of the light intensity evolution were analyzed using our previously reported method, 9 and we determined the temperature region where each sample existed as a SCL as indicated in the figure. Note that the sample originally made as LDG exhibited a strong intensity depression (see the arrow) in the SCL state, while the sample originally made as HDG showed no such depression. We have recently come to consider that the depression is caused by light scatteri...