Excess Volume of Vapor-Deposited Molecular Glass and Its Change Due to Structural Relaxation: Studies of Light Interference in Film Samples

Abstract

A novel method for estimating the refractive index and density of vacuum-deposited molecular glasses was invented. It is based on the analysis of the interference of the light transmitted through film samples. Molar volumes of toluene, ethylbenzene, and propylbenzene in the glass states were thus elucidated. The volume just after the sample deposition at 78 K was larger by 4−10%, depending on the size of the substituent on the phenyl ring of each compound, than the fictitious value for the supercooled liquid estimated at the same temperature. By temperature elevation at a constant rate, the volume increases first by the thermal expansion of the glass and next decreases rapidly in a narrow temperature region just below the glass-transition temperature. This decrease is attributed to the structural relaxation in the glass, and its magnitude is related to the size of the substituent. Excess volumes included at the initial sample deposition are discussed in relation to the excess enthalpies reported for similarly prepared molecular glasses.