Existence of an intrinsic glass transition in a silicon rubber: Hypersonicversus calorimetric properties

Time-domain temperature-modulated differential scanning calorimetry and hig h-performance Brillouin spectroscopy have been used as experimental techniq ues in order to investigate the effect of the thermal glass transition on t he specific heat as well as on the acoustic properties, quantities which ar e intimately related by thermodynamics, but measured at completely differen t frequencies. Using a siloxane as a model substance, we demonstrate the ef ficiency of hypersonic relaxations until thermal freezing of the material. Introducing the optoacoustic dispersion function as a sensitive measure for structural relaxations, we show that the high-frequency cu relaxations are truncated by the thermal glass transition instead of dying out at much hig her temperatures. Moreover, the extrapolated static compliance of the rubbe ry state goes to zero close to but below the thermal glass transition tempe rature T-g, indicating the existence of an acoustic instability which is re miniscent of the so-called Kauzmann paradox. [S0163-1829(94)07837-6].