In situ wide angle X-ray diffraction of vapor-deposited glasses of small organic molecules

Structural evolution of vapor-deposited ethylbenzene (EB) and isopropylbenzene (IPB) were studied by in situ wide-angle X-ray (WAX) diffraction as the temperature was raised stepwise. The as-deposited glasses of both compounds showed a broad hump in the diffraction pattern indicating their amorphousness. Low-density glasses deposited at temperatures much lower than the glass transition temperature (T-g), the hump contained a low-angle component. It diminished accompanying the structural relaxation preceding the glass transition. These results are in harmony with our previous observations that such glasses showed shrinkage in advance of the glass transition. As examples of high-density glasses deposited at temperatures close to T-g, EB glass deposited at 105 K and IPB glass deposited at 121 K were studied. At the glass transition, the former EB glass showed an unexpected stepwise shift of the hump to the high-angle direction, while the latter IPB glass showed almost no change in the diffraction pattern. These observations are briefly discussed referring to our recent results obtained other experimental methods.